Transcript of refractive error distributions

Department of optometry
Refraction III
By Dr. MOHAMED
A.ETARSHAWI
2014
1
Incidence and Distribution of Refractive Anomalies 4-8
FACTORS THAT AFFECT REFRACTIVE ERROR DISTRIBUTIONS 9-42
Development of the Ametropias43-136
Accommodation, the Pupil, and
Presbyopia 181-209
Infants, Toddlers‫طفل صغير‬, and Children-210-276
2
Chapter one
Incidence and Distribution of
Refractive Anomalies
3
Incidence and Distribution of
Refractive Anomalies
The eye's refractive error has been studied for decades.
Many of these investigations have focused on associations
between the distribution of refractive error and a wide variety of
factors.
These factors include-but are not limited to-age, gender, ethnicity,
geographical location, diet, intelligence, socioeconomic status,
performance of near work, and genetic factors.
Many of these associations are statistically strong and
have led to intellectually compelling ‫قهري‬theories on the etiology
of myopia as groups of people with markedly different
distributions of refractive error are compared.
4
.
CLASSICAL NOTIONS ‫ المفاهيم‬OF REFRACTIVE
ERROR DISTRIBUTIONS
A fascinating ‫ساحر‬process occurs in ocular development
between birth and puberty to produce a true distribution that
favors emmetropia and is skewed ‫ االنحراف‬toward myopia, with
more moderate to high myopes than moderate to high
hyperopes.
As seen in Figure 2-1, the distribution of refractive errors :
1-at birth closely resembles a normal distribution, with some
skew toward hyperopia.
2- Between infancy and childhood , the eye grows in such a way
that the distribution of refractive errors shifts toward
emmetropia, narrows considerably and shows a shift in skew
toward myopia.
5
Incidence and Distribution of
Refractive Anomalies
This process-whereby the average refractive error shifts toward
emmetropia and the entire distribution of refractive errors
decreases its variability-is termed emmetropization: Recent cross
sectional and longitudinal studies‫ الدراسات القطاعية والطولية‬agree that
the vast majority of emmetropization is completed rapidly in
infancy during the first year of life.':"
When does emmetropization stop? Between the ages of 5 and 15
years, ocular component development slows. During this decade,
1-anterior chamber depth increases by only 0.10 to 0.20 mm and
2- vitreous chamber depth and axial length by about 1.0 mm.':
3-Lens thinning seems to continue its earlier trend by continuing
to thin another 0.15 to 0.20 mm.
6
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTIONS
7
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTIONS
Age :Age is the single most important determinant of the
distribution of refractive error in a given group . The onset and
development of myopia occurs in well-established yet poorly
understood patterns . Only a very small proportion of infants are
myopic at birth, and much of this neonatal myopia is associated
with prematurity. Likewise, babies and toddlers ‫األطفال الصغار‬
exhibit a low prevalence of myopia. Even by the time they enter
formal schooling at age 6 years, children are generally not myopic.
During the ensuing 6 to 8 years ‫السنين التالية‬, however, low to
moderate myopia is first observed and progresses.
8
For juvenile-onset myopia (14-16 years), onset is typically
between the ages of 7 and 14 years." the rate of
progression is ±0.40 in average." and the age of cessation
is 14 to 15 years for females and 15 to 16 years for males.
The prevalence of myopia in older age groups increases
18.0-23 years to as high as 25% of the U.S. adult
population."
9
10
11
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTIONS
12
‫ المجندين‬recruits
‫ طالب‬Cadets
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTIONS
13
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTIONS
14
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTIONS
Gender
The trends in refractive error distribution seen with
gender, as opposed to age, are not as well defined and may in fact
be confounded ‫ متحير‬by age. A large sample of children from the
United Kingdom yielded (gave) no significant differences in
refractive error between boys and girls." In other studies, the
trend has gone both ways and it is therefore probably
inconclusive. Hirsch" found a more myopic mean refraction in
boys than in girls among 5- to 6 year-olds but more myopia
among girls by age 14 years, and Alsbirk" reported a similar
trend in adults.
15
Gender cont.
Myopia is more prevalent in Danish school-age girls than in
Danish boys of all ages." but both myopia and hyperopia are
more prevalent in Finnish‫ فنلندية‬school children than in
Goldschmidt's Danes. ‫ " غولدشميت في الدنماركيين‬Both groups of
investigators questioned the influence of puberty and earlier
maturation typically found in girls. Further, it is possible that all
the above results favoring females are due to their greater
participation in studies of this type.
16
Ethnicity‫العرق‬
Contemporary data supporting the general clinical impression that the
prevalence of myopia differs with race are relatively sparse (Table 2-8).
Few studies have simultaneously compared different races, and it is
problematic at best to compare data across races in different geographic
areas, different cultures, and samples with different socioeconomic and
educational bases. are reported.
Across all age groups, the prevalence of myopia in whites (26%) was twice
that in African-Americans (13%) in the NHANES survey.
That difference was the most marked among 18 to 24 year-olds and least
evident among 45 to 54 year olds .
The claim that there is a high prevalence of myopia among Asians is
difficult to document. Oft-cited ‫كثيرا ما يستشهد بها‬high prevalence rates among
Singapore medical students'?
17
18
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTIONS
Geography
Geographical differences in myopia prevalence can be seen in Table 2-10,
although differences among studies with regard to sampling and confounding by
other factors such as diet, education, and time spent in reading are difficult to
sort out from true differences based on geography. Certainly, no clear
geographical differences emerge beyond those identified in the preceding section
on racial differences in myopia prevalence in the United States.
19
Diet
Table 2-11 shows some of the various dietary insufficiencies that
have been associated with myopia. Gardiner“ conducted a
nonrandomized clinical trial of the effect of animal protein
supplement on the progression of myopia in children.
The treated group either altered diet or took protein supplements
to make 10% of the intake of calories come from animal protein.
The control group followed their regular, unmonitored diet. At
one year, the treated group showed less myopic progression (by 0.25 to -0.500 per year) compared with the control group.
20
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTION
Diet cont. : The effect of dietary protein on myopic progression
displayed some dose-response effect, in that myopic progression was less in
those who achieved the highest levels of intake of animal protein.
Unfortunately, this trial was not randomized. ‫العشوائية‬The treated children
were studied at the hospital clinic by their parents, whereas the control
children were seen at their school clinic. Without randomization, unknown
sources of bias-factors related to myopia but unrelated to the treatmentmay create differences between treated and control children and give false
results. For example,
1- differences in family income,
2- parents' educational level,
3-or whether one or both parents were myopic could create differences in
myopic progression unrelated to protein intake.
21
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTION
Diet cont. : Interestingly, the treated children differed from
controls in refractive error, but they were initially more myopic rather than
less. Edwards et al." examined the nutrition and diet of 1027- Hong Kong
children. Compared with children who did not become myopic, 34 children
who were myopic by age 10 years had a lower intake of several items,
including: protein; fat; vitamins BI , B2, and C; phosphorus; iron; and
cholesterol.
None of these children were malnourished. Malnourishment ‫سوء التغذية‬
appears to be associated with a lower amount of hyperopia in infancy, perhaps
because of some effect it might have on the crystalline Iens'" or because it
slows ocular growth .
22
23
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTION
TABLE 2-11 Dietary Problems Associated with Myopia Study
mentation appeared to have little effect on slowing myopia
progression in a case series reported by Feldman." Blood levels of
total, bound, and ionic calcium were not appreciably different
between myopes and hyperopes.
Until the multitude ‫ تعدد‬of confounding factors that influence diet
and refractive error is untangled , ‫ غير مرتبطة‬the precise role of
nutrition in the etiology of myopia will remain unclear .
Demonstrating that alteration of diet can affect the onset or
progression of myopia will require a formal , randomized clinical
trial potentially fraught with ethical dilemma.‫معضلة أخالقية‬
24
25
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTION
Systemic Conditions : Numerous systemic disorders have
an effect on the development of the eye and therefore affect its
refractive state. A more complete discussion of the following
conditions has been provided by Curtin.“
1- Albinism is the inability to produce the pigment melanin,
resulting in a lack of pigmentation of the hair, skin, and eyes.
The prevalence of the general form of albinism is 1 in 10,000.
In both the generalized and ocular forms, albinism has been
associated with myopia and high astigmatism. Its prevalence is
estimated to be between 2 and 34 in 10,000.
2- Several connective tissue and skeletal disorders, such as
Marfan's syndrome, Ehlers-Danlos syndrome, and Stickler's
syndrome, are also associated with myopia.
26
27
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTION
Personality
The conventional wisdom on associations between personality
traits and refractive error is that:
1- myopia is associated with introversion ‫االنطواء‬." Further, myopes
have been shown to exhibit an inhibited disposition ,‫التصرف ضعيف‬
a disinclination ‫ نفور‬for motor activity and social leadership.
2-hyperopes are carefree, ‫ كثير الهم‬impulsive‫ مندفع‬, hyperactive, and
socially passive.‫سلبية اجتماعيا‬
28
Age groups
: Researchers working in different decades
used different measurement methods and different criteria for
defining the types of refractive error . Different age groups
cannot be directly compared. Comparisons among groups of
different ethnic origins and from different parts of the world do
not lead to conclusions.
In a study designed to determine whether the prevalence of
myopia might be increasing, Fledelius" looked at a Danish
hospital-based sample and found the same trend toward
decreasing prevalence of myopia with increasing age into the
elderly age that was found in the Beaver Dam Eye Study in
Wisconsin" and the oft-cited study of Alaskan Eskimos." It is
still unknown whether these trends reflect true increases in the
prevalence of myopia in recent decades .
29
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTION
Ocular Diseases
It is clear from both animal and human data that clear visual input is
necessary for normal emmetropization to occur. binocular and
monocular disruptions of normal vision from sources such as :
1- cataract, retrolental fibroplasia, and ptosis. The distribution of
refractive errors of these groups was shifted toward more myopia or
less hyperopia compared with patients who experienced normal
development. Numerous conditions that interfere with normal vision
have been reported to affect refractive error through the induction of
a deprivation-like myopia.
2-These include corneal opacification." eyelid closure," vitreous
hemorrhage." and congenital cataract.
3-Astigmatism is also reported to increase after the surgical correction
of congenital
ptosis.
30
31
FACTORS ASSOCIATED WITH REFRACTIVE ERROR
Heredity
Genetic factors also play a significant role in the incidence of myopia.
A recent study demonstrated similarities between siblings ‫ األشقاء‬that
were not observed between parent and offspring ,‫ ذرية‬due to the
interaction of dominant genes , the visual environment , or a
combination of the two. Alsbirk" found an apparently lower
heritability ‫ التوريث‬for refractive error than for axial length, anterior
chamber depth, or corneal curvature.
Previous studies on the familial patterns of ocular component and
refractive error development differ on how large a role genetics plays
in myopia and whether the role of heredity differs for classical
juvenile-onset myopia and high myopia.'
32
Heredity
Studies of the heritability of refractive error and the ocular
components consist of two types: 1- those based on correlations
between parents and children and
2- those based on monozygotic and dizygotic twin comparisons.
Generally , in studies of parents and offspring, higher heritabilities
have been found for axial length and corneal power than for the
other ocular components or for refractive error. In studies of twins,
the heritabilities for corneal power, axial length, and refractive error
have all been high , and the differences in refractive error and the
ocular components have been smaller for monozygotic than for
dizygotic twins. Monozygotic twins resemble each other more
closely than dizygotic twins.
33
Heredity
Sorsby et al.124 measured refraction, corneal curvature, anterior
chamber depth, lens power and thickness, and axial length in :
1. 78 monozygotic twin pairs,
2. 40 pairs of dizygotic same-sex twins, and
3. 48 unrelated pairs.
The unrelated pairs were included for comparison because they
shared neither genes nor the effects of a common familial
environment.
A- 90% of monozygotic twins were within ± 1.25 D
of
each other. (great similarity)
B- Only 55% of dizygotic twins and
C- 52% of unrelated pairs were within ± 1.25 D.
Monozygotic twins were within ± 1.650 of each other 95% of the time. Only
62% of dizygotic twins and 60% of unrelated pairs were within ± 1.65 D.
The analysis for refraction and the other ocular components is summarized in
34 2-13.
Table
35
36
Teikari et al. reported on similarities between refractions in 6314
pairs of twins from the Finnish Twin Cohort Study. Correlations
in liability for refractive error between monozygotic twins were
81% compared to 30 % to 40 % for dizygotic twins. From these
data, Teikari et al. obtained an estimate of heritability, or the
proportion of variability in refractive error that can be
accounted for by variability in genetics. Heritabilities were 0.82
for males and 1.02 for females.
A recent estimate of heritability from 506 female twin pairs in
the United Kingdom also found high heritabilities for refractive
error across the spectrum of myopia to hyperopia, 0.84 to 0.86
due to additive genetic effects.!"
37
Near Work
Human Near-work Theories and Evidence
A body of work on the nurture theory of myopia development
indicts‫ يتهم‬excessive reading during childhood as the cause of
abnormal eye growth. Examples include an increased prevalence
of myopia among the first school-educated Eskimos": a
decreased prevalence of myopia during World War II in Iapan":
the association between myopia, intelligence, and near work'"?':
and the observation of adult-onset myopia in college populations.
38
Near Work
Experimental and epidemiological lines of evidence have
indicated that schooling, study, reading, and other near work
are associated with excessive axial elongation and myopia, but
evidence that near work directly causes myopia is difficult to
obtain from purely observational studies.
The characteristic of accommodation that has most
consistently been associated with refractive error is tonic
accommodation (T.A), or the accommodative state in the
absence of an accommodative stimulus.
39
Table 2-14 summarizes findings on T.A and other accommodative functions in
various refractive error groups.
First,
1. Myopes show the lowest levels of T.A . whereas
2.
Hyperopes have higher levels of T.A.
The level of T.A also appears to be related to refractive error type in
schoolchildren.
Second, it has been shown that near work, performed for both the short term
and the long term.!" can alter T.A. Such studies of adults have led to a growing
belief among some researchers that T.A is either a causative agent or a
predictive risk factor for myopia.
Longitudinal evaluation of this hypothesis has shown that T.A is not a
predictive risk factor for myopia. Although T.A was lower in myopic children in
either of two test conditions, lower values of T.A were not predictive of later
onset of myopia in an evaluation of over 700 children.
40
41
FACTORS THAT AFFECT REFRACTIVE
ERROR DISTRIBUTION
Intelligence
Numerous studies have documented associations between intelligence, school
achievement, and myopia.
Myopes tend to have higher scores on tests of intelligence and cognitive ‫المعرفي‬
ability , and better grades than do other refractive error groups.
Hyperopes, on the other hand, tend to show poorer reading skill and other
perceptual ‫اإلدراك الحسي‬anomalies more frequently.
Despite the finding of an association between myopia and near work for well
over a century, the relationship between these factors and intelligence has not
been adequately investigated. Only one investigator has attempted to analyze all
three factors in the same children, obtaining uncertain results.
Ashton used self-reported grades in school and results from cognitive tests
‫االختبارات اإلدراكية‬used in the Hawaii Family Study of Cognition as measures of
aptitude ‫الموهبة‬and achievement. Numbers of books and magazines read, hours
spent doing homework and watching television, and years of education served
as 42
measures of near work.
The progression of myopia with age , however, was not related to
near work after correction for aptitude ‫موهبه الذكاء‬
and achievement, whereas myopic progression remained
associated with aptitude and achievement even after correction for
near work. Two recent studies have also attempted to unravel ‫كشف‬
the interrelationships between :
1-near work,
2-aptitude, and
3-parental history of myopia.
Each found an association between near work and the aptitude
variables tested, Each study also found an association between
parental history of myopia and children's myopia. When all three
variables were placed in models testing the relative strength of
their associations with myopia, the study conducted in the United
States found that all three variables were independently related to
myopia with parental history of myopia had the strongest
43
association.
Socioeconomic Status
The association between refractive error and socioeconomic status has been
documented in two reports from large population-based studies: cycle III of
the Health Examination Survey (1966 to 1970) and the National Health and
Nutrition Examination Survey (NHANES) from 1971 to 1972.
In each study, myopes tended to be overrepresented among the higher
socioeconomic strata and underrepresented among lower income levels.
Angle and Wissmann!" categorized 15,536 subjects ages 12 to 17 years old by
refractive status and 10 levels of family income. Myopia was least frequent in
the lowest income group (16.8% of those with incomes below $500) and
increased steadily with increasing income to ( 35.1% of those with incomes
over $15,000) . Sperduto et al." analyzed data from 5282 subjects ages 12 to
54 years. Once again, myopia was least frequent among the families with the
lowest incomes (10.0% to 26.6% of those with incomes below $5000) and
most common in the highest income families .
44
SUMMARY
After reading this chapter, it might seem that the threats to emmetropia
are so numerous and that so many factors are needed to produce it
,that emmetropia should be a rarity. What is remarkable is not that
emmetropia happens at all, but that it is actually the rule, occurring so
commonly that the distribution of refractive error is highly peaked near
emmetropia and far from normal. Clinically, however, the emmetrope
is an infrequent visitor for refractive vision care before presbyopia
occurs.
Ametropia is encountered as a rule in clinical practice. Information in
this chapter should further the clinician's understanding of the
frequency with which a patient's refractive error occurs in the
population and what demographic variables may possibly influence it.
45
Chapter Two
Development of the Ametropias
46
REFRACTIVE CHANGES FROM BIRTH TO 5
YEARS OF AGE
Mohindra and Held reported a study of 400 full-term infants in Massachusetts,
in whom refractive error was measured in a dark room by manifest retinoscopy.'
There was a wide distribution of refractive errors in the first month of life, from
more than -10.0D of myopia to more than +5.0 D of hyperopia.
The distributions of refractive errors in seven age groups are given in Figure 3-1.
1- In the birth to 4-week age group, the mean refractive error was -0.70 D.
2-The mean shifted toward hyperopia with increasing age, being +0.59 D for the
129- to 256-week (roughly 2.5 to 5 years) age group.
3-The standard deviation decreased from +3.20 D in the birth to 4-week age
group to + 0.85 D in the 129- to 256 week age group. This can be observed in the
narrowing of the distribution of refractive errors with age in Figure )1 (
3-These changes could be explained by both myopic and hyperopic infant
shifting toward emmetropia.
47
48
REFRACTIVE CHANGES FROM BIRTH TO 5 YEARS OF AGE
Gwiazda ‫جويزدة‬et al." presented data for 72 children seen at regular
intervals from before 6 months of age for periods of 9 to 16 years.
Refractions were performed by the Mohindra" dark-room
retinoscopy procedure for children up to 3 years of age and by
standard manifest retinoscopy procedures for children older than 3
years.
1-Thirty-one infants had negative spherical equivalent refractions in
the first 6 months of life. The mean spherical equivalent refractive
error for this group moved toward emmetropia in the first year of
life and crossed to the hyperopic side by about 2.5 years of age.
2-Twenty subjects in their study had refractive errors of +0.50 D or
more plus before 6 months of age. The mean refractive error
decreased from over +1.50 D before 1 year of age, reaching about
+1.0 D at about age 2 years and staying around +0.75 to +1.00 D to
past 5 years of age.
49
REFRACTIVE CHANGES FROM BIRTH TO 5
YEARS OF AGE
Like Mohindra , Gwiazda et a found that the standard deviation
of refractive error decreased over the first 12 years.
The data used for analysis were the means of the spherical
equivalents obtained for the right eye and left eye for each subject
at each screening session.
Hirsch found that in most cases the change in refractive error
from 6 or 7 years of age to 11 or 12 years of age was linear.
For the children whose changes in refractive error were linear,
the mean slope was -0.07 per year.
There was a negative skew in the distribution of slopes of
refractive error change, because of the higher negative slopes for
children with myopia.
50
REFRACTIVE ASTIGMATIC CHANGES FROM
BIRTH TO 5 YEARS OF AGE
There is a higher prevalence of astigmatism in infants than in
older children and adults. 1012 samples of primarily white
infants have been reported to have a high prevalence of againstthe-rule astigmatism, which decreases over the first few months
and years of life.
Dobson reviewed cycloplegic refraction records of consecutive
patients in a Boston hospital medical center.
1- In 85 children under 3.5 years of age, against the-rule
astigmatism was found 2.5 times as often as with-the-rule
astigmatism.
2-In children between 5.5 and 9.5 years of age, with-the-rule
astigmatism was 3 times as common as against-the-rule
51
astigmatism.
REFRACTIVE astigmatic CHANGES FROM
BIRTH TO 5 YEARS OF AGE
Against-the-rule astigmatism was more common than with-therule astigmatism before 4.5 years of age, but after that with therule astigmatism was more common.
3-There were 29 children who had 1.0 DC or more of
astigmatism at 6 months of age . Of these children, 16 had
against-the rule, 8 had with-the-rule, and 5 had oblique
astigmatism.
All 29 had reductions in astigmatism by the time they were 4 to 6
years old. Some of the infants who had against-the-rule
astigmatism shifted to with-the-rule astigmatism by 6 years of
age. Howland and Sayles“ took photorefraction measurements of
312 infants and young children in New York State.
52
REFRACTIVE astigmatic CHANGES FROM
BIRTH TO 5 YEARS OF AGE
4-The prevalence of astigmatism of 1.0 DC or more was
approximately 7 times greater in children 1 year old or younger
than in children 4 years old or older.
5-For children up to 2 years old , the ratio of types of astigmatism
was 15 against the-rule to 9 oblique to 1 with-the-rule.
By 2 years of age, one-sixth of the subjects had spherical refractions.
By 4 years of age, one-third of the subjects had spherical
refractions.(without astigmatism)
53
REFRACTIVE CHANGES FROM
BIRTH TO 5 YEARS OF AGE
Most of the subjects who lost their astigmatism had started with 1.0
DC of astigmatism, but some lost as much as 2.50 DC.
Of the 299 subjects, 272 (91%) had against-the-rule astigmatism.
The cylinder axis usually did not change significantly during the 3
years of the study. During the 3 years, there was a shift in the
distribution of amount of astigmatism to lower values.
There were 49 subjects who started with at least 2.0 DC of
astigmatism. Most of them (40) experienced a decrease in
astigmatism, but 9 (18%) experienced an increase. Of these 9, a
disproportionate number (5) had with-the-rule astigmatism.
54
REFRACTIVE CHANGES FROM
BIRTH TO 5 YEARS OF AGE
Premature infants, especially those with very low birth weights, are
often found to have a high degree of myopia . Usually , this myopia
decreases with maturity over the first few months of life. Many of
these infants are emmetropic by 1 year of age if no other ocular
anomalies develop.":" In a longitudinal study conducted in Israel,
Scharf et al." found that 42% to 45% of 134 eyes of premature
babies were myopic shortly after birth . Of these myopic eyes, 46%
were emmetropic at 7 years of age.
Of the eyes that were hyperopic at birth , 77% were emmetropic at
7 years of age . The retinopathy of prematurity (ROP) often occurs
as a consequence of the high oxygen concentration necessary to keep
low birth weight infants alive."
55
Development of the Ametropias
Changes in Hyperopes Compared with Myopes
Both Hirsch and Langer noted that, among schoolchildren, the
greatest changes in refractive error occurred in those with myopia.
Hofstetter" had shown that myopes between the ages of 10 and 20
years, almost all of the changes were toward increased myopia. The
distribution of refractive change for the hyperopes was normal .
Hostettler's conclusion" that refractive change is faster when a child
crosses from hyperopia into myopia was supported by
Mantyjarvi ‫ منتيجارفي‬who’s analysis was based on the right eye
spherical equivalents of cycloplegic refraction in children 7 to 15
years of age.
56
Forty-six (46) hyperopic children and ( 133) myopic children were
followed for at least 5 and up to 8 years.
1. The children with hyperopia had a mean rate of refractive
error change of -0.12 D per year .
2. The children with myopia had a mean rate of -0.55 D per year.
3. There were also 30 children who were initially hyperopic and
became myopic during the period of observation.
57
‫أعذب األشعار‬
‫ولما قسا قلبي وضاقت مذاهبي‬
‫جعلت رجائي نحو عفوك سلما‬
‫تعاظمني ذنبي فلما قرنته‬
‫بعفوك ربّي كان عفوك أعظما‬
‫فما زلت ذا عفو عن الذنب لم‬
‫وتكرما‬
‫تزل تجود وتعفو منّة‬
‫ّ‬
‫ولدتك إذ ولدتك أمك باكيا‬
‫والقوم حولك يضحكون سرورا‬
‫فاعمل ليوم أن تكون إذا بكوا‬
‫في يوم موتك ضاحكا مسرورا‬
‫‪58‬‬
Onset of Myopia in Youth
Using data from several studies in different location,
Grosvenor‘‫ غروزفينور‬proposed a system for the classification of
myopia based on its age-related prevalence and age of onset .
The four types of myopia in this system are:
1. congenital,
2. youth-onset.
3. early adult-onset . and
4.late adult-onset. Youth-onset myopia has its onset
in the school-age years and is the most common type of myopia.
Most studies give a prevalence of myopia of about :
a) 2 % at 5 or 6 years of age and
b) 20
59 % to 25 % at 15 to 16 years of age.
It may be inferred ‫ يستدل‬that the most common ages of myopia
onset in girls precede those in boys by about 2 years. Further
support for a tendency toward later incidence for boys comes from
refraction data from 3000 Ohio schoolchildren." as shown in
Figure 3-2. and from a study of several thousand schoolchildren in
Hong Kong."
The incidence of myopia increased:
1)In boys from 9% at 6 years of age or less to nearly 20% at 10 years of age.
2)In girls , incidence increased from about 12% at 6 years of age or
less to over 27% at 11 years of age. Figure 3-2
60
61
Progression of Childhood Myopia
Once myopia appears in childhood , it increases until the middle to
late teens." Typical patterns of childhood myopia progression are
shown in Figures 3-3 and 3-4. Additional examples of patterns of
childhood myopia progression, shown in Figure 3-5, include some
points before the onset of myopia and show that the refractive
change accelerates at the onset of myopia. Perusal of the examples
in these figures suggests that the change in refractive error is
largely linear from the beginning to the end of childhood myopia
progression. GOSS studied the linearity of the change in refractive
error with age from ages 6 to 15 years in the optometric practice
records of 198 children with myopia.
Of the cases, 90% to 94% were found to fit a linear model by an F
test for linearity and the statistical significance of the correlation
coefficient,
62
63
158
64
148
65
Mantyjarvi"‫ منتيجارفي‬studied rates of progression among myopic
children examined at a community health center in Finland.
Refractive data were the spherical equivalents of the right eye from
retinoscopy after instillation of cyclopentolate. Children were
followed from age 5 to 8 years up to the age of 15 years.
For 133 children (75 girls and 58 boys) , the mean annual rate of
change was -0.55 per year ( range = 0 to -1.63 per year).
The standard deviation and the range were similar to those found
by Goss and COX but the mean rate was more negative.
Usually childhood myopia progression slows or stops in the middle
to late teens. Goss and Winkler“ used four methods to derive an
index of the age at which childhood myopia progression stops or
slows.
66
Ocular Optical Component Changes
in Progression of Childhood Myopia
The ocular optical component change responsible for childhood
myopia progression is:1- axial elongation of the vitreous chamber of the eye. Some of the
first evidence for this came from longitudinal studies conducted in
England by Sorsby. Sorsby et al." measured refractive error by
cycloplegic retinoscopy.
2-crystalline lens power by phakometry .
3 -calculated axial length from the refractive error,
4- corneal power by keratometry .They observed that during
childhood, axial length increased and the refractive power of the eye
decreased , the latter change resulting from decreases in crystalline
lens power .
67
Emmetropic children between the ages of 6 and 14 years show
increase in axial length, decrease in crystalline lens thickness, and
decrease in crystalline lens power. Pledelius ‫ بلدلويس‬provided
additional data showing axial elongation to be the component
mechanism of childhood myopia progression lesser extent,
decreases in corneal power . Occurring by themselves, axial length
increases resulting from:
1- increases in vitreous depth would cause refractive error changes
toward myopia, and 2- decreases in ocular refractive power would
cause refractive error changes toward hyperopia.
Sorsby and Leary" published longitudinal data for 129 children,
25 of whom were myopic. They noted that myopia developed when
the refractive effects of axial elongation exceeded the effect of
decreased ocular refractive power.
68
Ocular Optical Component Changes
in Progression of Childhood Myopia
Fledelius ‫ فليدلويس‬reported on changes in refractive error and the
ocular components in Danish children between 10 and 18 years
of age. The subjects were 70 children who had been born with:
1- low birth weight 3 in number «2000 g) and
2- full-term 67 children who had been born full-term in a
Copenhagen hospital . As a result of selection criteria, there
were more myopic children in this sample than in a random
sample of the population. As the summary of the data in Table 35 shows, the mean changes in anterior corneal radius, anterior
chamber depth, and crystalline lens thickness were not
significantly different from zero.
69
Ocular Optical Component Changes
in Progression of Childhood Myopia
Most of the increases in axial length could be attributed to
increases in vitreous depth. Fledelius" reported statistically
significant correlations between:
1- change in vitreous depth and
2- change in refractive error between the ages of 10 and 18
years .The earlier in life the onset of myopia occurs, the
greater the amount of myopia developed by the late teens to
young adulthood.
70
Factors That Affect the Rate of Childhood Myopia
Progression
I- ) Onset of myopia:
A higher rate of childhood myopia progression is associated with
earlier onset of myopia. For example, Rosen-berg ‫ روسنبيرج‬and
Cold-schmidr" ‫ كولدشمدر‬found a mean annual increase in myopia
of:
1.- 0.47 D for 30 girls with onset of myopia at 9 to 10 years.
2.- 0.37 D for 36 girls with onset at 11 to 12 years.
The refractive data used for analysis were right eye
autorefraction findings after cyclopentolate cycloplegia.
Intraocular pressure was measured with a Goldmann
applanation tonometer.
71
Ocular Optical Component Changes
in Progression of Childhood Myopia )IOP)
II-)Intraocular pressure:
1- Twenty-seven (27) children with intraocular pressure over 16
mm.Hg had myopia progression that averaged -1.32 D over the
next 2 years.
2- The mean increase in myopia in 2 years for 20 children with
intraocular pressure of 16 mm.Hg or less was -0.86 D .
Even though childhood myopia progression rates have been
reported for higher intraocular pressures , progression rates did
not decrease when intraocular pressure was lowered by the regular
application of timolol eye drops .
72
III-) pigment crescents at the optic nerve head :
Jensen also reported that children with pigment crescents at
the optic nerve head had greater myopia progression than
did children with no such fundus changes. Twenty-eight
children(28) with crescents had a mean myopia progression
of -1.39D over the next 2 years.
73
Myopia Control with Rigid Contact Lenses
IV-) Rigid contact lenses can slow the rate of childhood myopia
1-progression by flattening the cornea even though axial
elongation of the eye continues. Nolan'?‘ reported that 44
patients with myopia who started wearing contact lenses before
the age of 14 years had a mean change in refractive error of
+0.03 D in 1 year.
2-In comparison, 64 myopic patients who wore spectacle
lenses after the age of 14 years had a mean change of -0.42 D in
1 year.
In England, Stone and Powell Cullingford':“ found mean
refractive changes in 5 years of +0.09 D in myopic contact lens
wearers and -1.84 D in myopic spectacle lens wearers.
74
Tahe attempt to slow the progression of myopia is often
referred to as myopia control , and numerous methods have
been used to try to achieve it. Most have involved different types of
lens treatment regimens or the application of pharmaceutical
agents. After a brief discussion of pharmaceutical techniques , the
two most common methods of myopia control :1- rigid contact lenses and
2-bifocal spectacle lenses, are discussed in detail.
V-)Pharmaceutical techniques for myopia control have included
daily application of atropine, which has been successful in slowing
myopia progression. However, this treatment is unpopular with
patients, and myopia progression may accelerate upon cessation of
the drug treatment. The use of atropine has several disadvantages
and side effects, including complete cycloplegia, photophobia
from pupillary mydriasis,
75
and the possibility of allergic or idiosyncratic drug reactions or the
potential for systemic toxicity if the atropine is not applied properly.
It has been suggested that the effect of atropine in slowing myopia
progression may come from effects on the retina, rather than from
the cycloplegia . Chronic atropinization may be detrimental to the
development of retinal ganglion cells. Because of the various
problems associated with daily application of atropine, it appears
wise to look for other alternatives for myopia control.
One pharmaceutical agent that has received some attention is
pirenzepine; ‫ بيرنزبين‬studies are underway to assess its effectiveness
for myopia control.
76
Myopia Control with Bifocal Spectacle Lenses
VI-)Bifocal lenses: allow the patient to have one lens power for
distance viewing and another lens power for near viewing.
1-This is advantageous in certain accommodation and vergence
disorders, such as accommodative insufficiency and convergence excess .
2-It also appears to provide some degree of myopia control in some
cases. The findings of studies on the effect of bifocals on childhood.
Myopia progression are summarized in Tables 3-7 to 3-9.
Forty-eight patients wore single-vision spectacle lenses and then
switched to bifocal lenses .TABLE 3-7
77
78
79
Myopia Control with Bifocal Spectacle Lenses
Amount of refractive change was some what less with BF
than with SV but was not statistically significant; for
children with intraocular pressure mmHg, change was less
with BF than with SV. Amount of progression in 30 months
was significantly less in the bifocal group than in the single
vision group when progression adjusted for age.
80
Myopia Control with Bifocal Spectacle Lenses
• Miles reported that myopia increased at a rate of -0.75D per year in patients
wore single-vision lenses over age spans of 6 to 14 years.
• While the patients wore bifocals over age spans of 8 to 16 years, myopia
progressed an average of -0.40 D per year. The difference in rates may have
been due in part to the older ages when the bifocals were worn.
• However, inspection of Miles's composite graphs of myopia progression
suggests that over common age spans, myopia progression was less with
bifocals than with single vision lenses. Roberts and Banford":" reported on
the results of bifocal control of myopia in their optometry practices in New
York State. Included were myopic patients refracted in their practices at least
twice before 17 years of age. Three hundred ninety-six patients (231 girls and
165 boys) wore single-vision lenses over the entire observation period, and 85
patients (47 girls and 38 boys) wore bifocals over the entire observation
period. Most bifocal add powers were +0.75 to +1.50 D of rate and age.
81
Myopia Control with Bifocal Spectacle Lenses
The mean refractive error for the two eyes from the manifest
subjective refraction spherical equivalents was used for analysis.
The mean rate of progression for the bifocal wearers was -0.31 D
per year, and that for the single-vision lens wearers was -0.41 D
per year.The bifocal lenses usually consisted of a - 0.50 D
undercorrection for distance vision and a flat-top reading segment
with +1.50 to +2.00 add.
82
When a normal phoria was observed, mean yearly rates were -0.43 D
for 20 subjects wearing single vision lenses and -0.42 D for 41 subjects
wearing bifocals. When esophoria at near was observed, the mean
yearly rates were -0.51 D for 7 persons wearing single-vision lenses
and -0.31 D per year for 18 bifocal wearers. In all three studies,
esophores had about 0.20 D per year less myopia progression with
bifocals than with single-vision lenses.
83
Most subjects had exophoria. For 31 exophoric children who wore
single-vision lenses, the mean myopia progression in 2 years was -1.11 D
, compared with -0.88 D for 28 exophoric children who wore bifocals.
In summary, the results of the bifocal myopia control studies have
varied from sizable reduction in progression rate to no effect. Some
studies have indicated that myopia control with bifocals is more likely
when certain findings are present, including esophoria at near , and
higher intraocular pressure. None of the studies have suggested that
myopia progression can be stopped with bifocals. However, if the
myopia progression rate can be lowered 0.20 D per year, the amount of
myopia reached at the cessation of childhood myopia progression will be
about 1.0 D less if myopia progression occurred over a 5-year period, or
as much as 2.0 D if the onset of myopia was 10 years before cessation of
progression.
84
Changes in Astigmatism
Hirsch's study" in California, may be the only longitudinal
study on changes in astigmatism in an unselected sample of school
children. Refractive error was determined by manifest retinoscopy
performed twice a year for 8 years on 167 children. For each child, the
16 tests were divided into four groups of four tests each, and the child's
average astigmatism in each grouping was determined. The average
ages of the four groups were 6.5, 8.5, 10.5, and 12.5 years:1. At 6.5 years of age, 81% of the children had less than 0.25 DC of
astigmatism.
2. At 12.5 years of age this had decreased to 72% .
3. The percentage of children who had 0.25 DC or more with-the-rule
astigmatism remained fairly constant in the range of 10% to 14%
in each of the age groups,
4. 85with 4% to 6% having 0.75 DC of with-the-rule astigmatism.
Changes in Astigmatism
5. The percentage of children with 0.25 DC or more against-the-rule
astigmatism increased from 3% at 6.5 years of age to 11% at
12.5 years of age.
6. 58% of the children had changes in astigmatism of less than 0.02
DC per year.
7. 11% percent of the children changed in the with-the-rule direction
between 0.03 and 0.07 DC per year,
8. 23% changed in the against-the-rule direction at those rates (0.03
and 0.07 DC)
9. 2% percent had astigmatism change rates of 0.08 to 0.22 DC per
year in the with-the-rule direction.
10. 5% had against the- rule changes at those rates.( 0.08 to 0.22 DC
per year )
86
Changes in Astigmatism
Rates of astigmatism change were calculated by linear regression
‫ ;تراجع‬with-the-rule astigmatism assigned a positive (+) value and
against the-rule astigmatism a negative( - ) number.
Against-the-rule astigmatism was more common in this group of
myopes than in Hirsch's unselected sample.“
1- Children who had with-the rule astigmatism at their first
examination had a mean astigmatism change rate of 0.06 DC per year
in the with the rule direction .
2- Those who had against-the-rule astigmatism had a mean
astigmatism change rate of 0.03 DC per year in the against-the-rule
direction .
87
Changes in Astigmatism
3. Parssinen ‫ بارسونن‬found with-the-rule astigmatism in 10% of the
right eyes at the beginning of the study and 18% of the right
eyes at the end of the 3 years.
4. Against-the-rule astigmatism was present in 33% of the subjects
at the beginning of the study and 44% at the end of the study.
5. Oblique astigmatism (principal meridians between 31 and 59
degrees and between 121 and 149 degrees) was uncommon,
being present in less than 1% of the children at the beginning of
the study and 2.5% at the end of the 3 years.
6. The mean amount of astigmatism, irrespective of whether it
was with or against the rule, increased from 0.26 DC to 0.45
DC(= 0.19DC ) over the 3 years, for a mean rate of increase of
0.06 DC per year =0.18 DC.
88
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
General Trends
In this section, refraction changes from the late teens or early 20s to
about 40 years of age are discussed. For most persons, the young
adulthood years are a time of relatively stable refraction.
However, the onset or progression of myopia is not uncommon. Some
people also experience a small shift in the hyperopic direction.
Brown calculated the mean annual changes in refraction for clinic and
private practice patients in Chicago. Refractions were performed by
retinoscopy under atropine cycloplegia.
The mean annual change in the 20- to 34-year-old age span was -0.05 D per year,
based on 2971 computations. The mean annual change in the 35- to 43-year-old age
period was +0.03 D per year, based on 597 computations.
89
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
Slataper! ‫ “سال تبير‬reported similar data based on cycloplegic
refractions performed in his private practice in Houston.
Calculating the mean spherical equivalent refractive error for each
age, he determined that:
1- the mean refractive error was plus (hyperopic) each year from 20
to 40 years of age,
2- with the amount of plus decreasing from 20 to 31
3- and plus increasing from 31 to 40 years.
4- From 20 to 40 years, the magnitudes of all but one of the yearly
changes in the mean refractive error were less than 0.10 D and most of
the differences from 1 year to another were less than 0.04 D.
90
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
Myopia Onset and Progression
5- The vast majority of patients in the 21- to 34-year-old age range had
refractive change rates of zero or very close to zero.
6- When myopes had refractive changes, they were usually in the
myopic direction. The most negative change was not quite -0.03 per
month, perhaps about -0.35 per year.
7- The refractive change rates for the myopes in the 21- to 34year-old
age range were lower in magnitude than the rates for the myopes in the
10- to 20 year old age range.
8- When hyperopes showed refractive changes, they were most often
in the hyperopic direction. The largest change in the hyperopic
direction was between +0.02 and +0.03 per month, the annual change
rate being about +0.30 per year.
91
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
9- The mean spherical equivalent refractive error for the females was +0.09 D
at age 13 years and -0.13 D at age 33 years.
10- The mean refractive error for males was +0.39 D at age 13 and +0.35 D at
age 33.
11- Of the 95 subjects had changes of :
a) 44 (46%) less than ± 0.75 D over the 20-year period,
b) 28 (29%) changed -0.75 or more in the myopic direction, and
c) 23 (24%) changed +0.750 or more in the hyperopic direction.
12- Among the females, the mean horizontal meridian refractive error was
0.12 D more plus than the vertical at age 13 years and 0.24 D less minus than
the vertical at age 33 years.
13- In the males, the vertical meridian was 0.05 more plus than the
horizontal at age 13 and 0.08 more plus than the horizontal at 33 years.
92
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
There was an increase in the number of persons who were myopic
and a decrease in the number of persons near emmetropia
(-0.25 to +1.25 D). Sixty-five (59%) of the respondents had ± 0.50
or less change in refractive correction from 20 to 40 years of age.
1- Individuals with myopia at age 20 often became more myopic,
but the myopia increase was usually less than 1.0 D.
The largest myopia increase was -2.0 D.
2- Individuals with +1.0 D or more hyperopia at age 20 tended to
become more hyperopic. The largest increase in plus prescription
was +1.50 D.
3- Of the 111 respondents in Grosvenor's study ‫ غروزفينور‬65 (59%)
had no astigmatism at 20 years of age, and 29 (26%) had a withthe-rule correction, and 17 (15%) had an against-the-rule
correction.
93
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
astigmatism
At 40 years of age,
1.
2.
3.
42 (38%) had no astigmatism,
44 (40%) had with-the-rule cylinder, and
20 (18%) had against-the-rule cylinder.
At both 20 and 40 years of age,
There were 5 respondents with oblique astigmatism. The change in
astigmatism was zero in 61 (55%) persons,
1.in the with-the-rule direction in 31 (28%) persons,
2.in the against-the-rule direction in 18 (16%) persons, and toward
3. oblique astigmatism in 1 (1.6 )person. The mean change in astigmatism
over the 20 year period from 20 to 40 years of age was 0.10 DC in the
direction of with-the-rule astigmatism.
94
REFRACTIVE -ASTIGMATICCHANGES IN YOUNG ADULTHOOD
Mean amounts of astigmatism were calculated as a function
of age, with with-the-rule astigmatism given a plus sign and
against-the-rule astigmatism a minus sign.
1. at 15 to 19 years of age, the mean astigmatism was zero
2. at 20 to 24 years, it was +0.05 DC
3. at 25 to 29 years, it was +0.27 DC
4. at 30 to 34 years, it was +0.15 DC; and
5. at 35 to 39 years, it was +0.32 DC .
Thus there was a slight shift in the with-the-rule direction.
The number of persons who had with-the-rule astigmatism
increased over this age span.
95
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
Grosvenor estimated that the prevalence of myopia of 0.50 DS or
more increased from about 20% at 20 years to about 30% at 40
years of age. Grosvenor's use of 20 years as the start of the age
range for early adult-onset myopia is somewhat arbitrary‫; افتراضي‬
age of physical maturity is more likely a better distinction between
the youth-onset and early adult-onset myopias. Both youth-onset
myopia and early adult-onset myopia can progress in young
adulthood. The rate of early adult-onset myopia progression is
usually less than the rate of youth-onset progression.
Comparing childhood myopia progression and young adulthood
myopia progression, Goss et al. identified three basic patterns of
the change in refractive error with age.
96
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
They called these patterns adult stabilization, adult continuation,
and adult acceleration.
1. In adult stabilization (Figure 3-12), childhood myopia
progression is followed by stabilization of refractive error in
young adulthood.
2. Adult continuation (Figure 3-13) is characterized by childhood
myopia progression followed by a generally slower progression
of myopia in young adulthood.
3. In adult acceleration (Figure 3-14),
Refractive change in the myopic direction. If emmetropia or
hyperopia is present when the accelerated refractive change
occurs and myopia develops, the patient would fit into
97Crosvenor's early adult-onset myopia classification.
adult stabilization
98
Adult continuation
99
adult acceleration
100
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
Using data from five optometry practices in the Midwest, Goss et
al. subjectively classified patients with a sufficient number of data
points into one of three categories.
1-The majority of cases-68% of the males and 87% of the femalesfit into the adult stabilization category.
2-Adult continuation was found in 25% of the males and 13% of
the females.
3-The least common pattern was adult acceleration, which was
found in 6% of the males and none (zero )of the females. Among
males who had three or more refractions after the age of 18 years,
the mean rate of refractive change based on linear regression was
-0.07
D per year.
101
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
Synthesizing the results from the above studies and unpublished
studies in military academies and other academic settings, the
National Academy of Sciences Working Group on Myopia
Prevalence and Progression and Baldwin et al. reached the
following conclusions:
1. In populations not containing a large number of persons in
college, less than 10% of emmetropes and low hyperopes will
develop myopia before 40 years of age.
2. In contrast, in populations containing a large number of
persons as many as 20% to 40% of low hyperopes and
emmetropes who enter colleges and military academies are
likely to become myopic.
102
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
3. In young adults, the shift toward myopia experienced by
low hyperopes and emmetropes is generally less common
and lower in amount than the increase in myopia
experienced by persons already myopic.
4. It is unlikely that a person who enters college with a
noncycloplegic refraction of +1.00 or greater in either
principal meridian will become myopic by the end of 4
years of study.
5. Persons with low hyperopia at 17 or 18 years of age
appear to be more likely to become myopic in heavy nearwork
103 situations than are older persons with low hyperopia.
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
Three studies compared ocular optical components of one-time
measurements of small groups of early adult-onset myopes and
emmetropes. McBrien Millodot"" found
1. greater vitreous depth,
2. greater anterior chamber depth, and
3. less crystalline lens thickness in early adult-onset myopes
than in emmetropes.
A-Grosvenor and Scott!“ found greater corneal power in early
adult-onset myopes.
B-Bullimore et al. reported greater vitreous depth in early adultonset myopes than in emmetropes.
C-Longitudinal data have shown both vitreous depth increases
and corneal power increases in young adults with onset and
progression of myopia.
104
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
Of 20 patients with adult stabilization of myopia progression :1.Eleven (11)had corneal flattening,
2.Eight (8)had corneal steepening,
3.One(1) experienced no change.
Goss and Erickson used the same set of data to calculate
correlation coefficients ‫ معامالت االرتباط‬of rate of refractive
error change with rates of keratometer power between the
ages of 18 and 40 years. Increases in myopia were
associated with increases in corneal power.
105
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
Adams presented a case report of a higher than usual amount
of early adult-onset myopia. Spherical equivalent refractive
errors went from
1. -0.25 D at 19 years of age to
2. -1.25 D at 24 years of age to
3. -4.75 D at 42 years.
4. From 24 to 42 years of age, keratometer powers increased :
a)0.75 D (horizontal OD), 0.25 D (vertical OD),
b)1.0 D (horizontal, OS), and 0.370 (vertical, OS). Because the
amount of corneal power increase would not account entirely for
the myopia progression and ocular axial lengths were 25.8 mm
at 42 years of age, Adams suggested that this myopia
progression was due primarily to axial length increase.
106
REFRACTIVE CHANGES IN YOUNG ADULTHOOD
Adams and McBrien presented 2-year results of a
longitudinal study in a group of clinical microscopists .
Sixteen ( 16 ) of those who were initially emmetropic
developed myopia during the study. The mean amount
of myopia developed was -0.64 D.
Vitreous depth increased an average of 0.19 mm, and
corneal steepening occurred, with a mean decrease in
corneal radius of 0.05 mm .
107
(‫)أخيرا‬REFRACTIVE CHANGES FROM
AGE 40 ON Trend Toward Hyperopia
In cycloplegic refraction findings from his private practice in
Houston, Slataper!" observed that the mean spherical equivalent
refractive error increased in plus from 40 years of age to the mid60s, after which it decreased. The mean refractive error was:1. +0.73 D at 40 years of age and
2. +1.97 D at 64 years of age,
3. The mean subsequently changed from +1.72 D at 65 years of age
to +1.21 D at age 74.
4. For patients who had more than +2.00 D of hyperopia before 40,
the mode rate of change was about +0.06 D per year.
108
REFRACTIVE CHANGES FROM
AGE 40 ON Trend Toward Hyperopia
Hirsch analyzed data from his practice in California from 460
women and 360 men aged 45 years or older. Refractive errors in
the analysis were the means of the spherical equivalents of the two
eyes from the manifest subjective refraction.
1. The median refractive error for patients aged 45 to 49 years
was +0.18 D.
2. The median refractive error increased in hyperopia to +1.02 D
at age 75 or older.
3. The range increased from +1.00 D at ages 45 to 49 years to
+ 2.27 D at age 75 years or older.
109
REFRACTIVE CHANGES FROM
AGE 40 ON Trend Toward Hyperopia
The prevalences of different levels of refractive error
found by Hirsch!" are given in Table 3-10. There was a decrease
in the prevalence of refractive error within ±1.12 D of
emmetropia and an increase in the prevalence of hyperopia after
the age of 45 years. Beginning in the 60s, there was also an
increase in the prevalence of myopia.
Hirsch pointed out that because almost everyone over 45 years
of age needs optical correction for distance vision, near vision,
or both, these prevalences may be fairly representative of a non
visually selected population.
110
REFRACTIVE CHANGES FROM
AGE 40 ON Trend Toward Hyperopia
In summary, the overall trend in refractive
changes after 45 years of age is a shift in the
hyperopic direction, with some myopic persons
showing increases in myopia. If age-related
nuclear cataracts develop (usually after 60
years of age), there is often a shift toward
myopia.
111
Trend Toward Against-the-Rule
Astigmatism- Over the age of 40
Hirsch 180 studied the changes in subjectively determined
astigmatism in his practice patients over the age of 40.
Minus-cylinder axes within 20 degrees of 180 degrees were
considered with-the-rule astigmatism, and those within 20 degrees
of 90 degrees were considered against the- rule astigmatism.
Cases of oblique astigmatism and astigmatism in excess of 4.00 DC
were omitted. With the- rule astigmatism was given a positive sign
and against-the-rule astigmatism a negative sign.
Astigmatism gradually shifted in the against-the-rule direction,
the mean going from +0.27 DC in the 40 to 44-year old age group to
-0.81 DC in the over 80 age group.
112
Trend Toward Against-the-Rule
Astigmatism- Over the age of 40
In the same time span, the median astigmatism went from +0.09 DC
to -0.91 DC. The change in both the mean and median in this 40-year
period was about 1.00 DC, or about 0.25 DC per decade.
1-The prevalence of with the- rule astigmatism of more than 0.25 DC
decreased from 29.0% in the 40- to 44-year-old age group to 6.8%
in the over-80 group.
2- Against-the-rule astigmatism of more than 0.25 DC was observed
in 9.5% of patients 40 to 44 years old and in 65.1% of those over 80
years old.
3-Against-the-rule astigmatism had become more common than
with-the-rule astigmatism by 45 to 49 years of age (23% vs. 16% of
patients).
113
Trend Toward Against-the-Rule Astigmatism
Anstice!" studied the changes in astigmatism in patients seen by an
optometrist in New Zealand:-
1- Patients over 40 had experienced changes in corneal
astigmatism in the preceding decade that ranged from + 0.62 DC
in the with-the-rule direction to -1.75 DC in the against-the-rule
direction.
2- There were more against-the rule than with-the-rule changes,
with the majority of points being in the range of about 0 to 0.62
DC against the- rule.
3- The prevalence of with-the-rule corneal astigmatism of 0.50 DC
or more was 84% for 41- to 50-year-olds and 41% among those 61
years or older.
4- The prevalence of against-the-rule corneal astigmatism of 0.50 DC or more
was 2% at 41 to 50 years of age and 18% at 61 years or older.
114
Trend Toward Against-the-Rule Astigmatism
Further evidence for the cornea as the source of the shift toward
against-the-rule astigmatism comes from Baldwin and Mills, who
collected data from an optometry practice that had been in existence
for more than 60 years. (1)- Data were selected for patients who had
had keratometric measurements taken periodically over at least 40
years, the first time at or before 30 years of age and the last time at 70
years or older. (2)- Eyes with refractive and corneal principal
meridians within 10 degrees of horizontal and vertical were selected
for study; (3)- 34 eyes fulfilled these criteria and were used for
analysis. Discerning no trend of change in either corneal or
refractive astigmatism before 40 years of age, Baldwin and Mills
used the first examination after 40 years as the initial datum point
115
‫ نقطة ارتكاز‬for each patient.
Trend Toward Against-the-Rule Astigmatism
1-The mean vertical meridian refractive error increased
in hyperopia by +0.30 D, and the horizontal meridian
refractive error increased in hyperopia by +0.82 D,
resulting in a +0.52 DC shift in the against-the-rule
direction.
2-The mean vertical meridian keratometer power
increased 0.07 D, and the horizontal meridian
keratometer power increased 0.38 D, representing a
change in corneal astigmatism of 0.31 DC toward
against-the
rule astigmatism.
116
ETIOLOGICAL CONSIDERATIONS Emmetropization
Distributions of refractive errors in the general population
are variable and skewed toward myopia . The peak of refractive
error occurs at emmetropia and low hyperopia. The term
emmetropization is often used to describe the process theorized to
explain why there are more people with emmetropia and near
emmetropia than ametropia
It appears that emmetropization can be accounted for:
1. in part by coordinated growth of the eye .
2. and in part by some form of vision-dependent feedback
system for ocular refractive development.
117
ETIOLOGICAL CONSIDERATIONS Emmetropization
During childhood,
1) vitreous depth increases,
2) crystalline lens power and thickness decrease,
3) and anterior chamber depth increases.
By itself, an increase in vitreous depth would cause a change toward
myopia, By itself, a decrease in crystalline lens power would cause a
change toward hyperopia, Because they occur simultaneously, the net
change in refractive error is thus reduced. One study found a
significant correlation of vitreous depth with posterior crystalline lens
radius but not with anterior crystalline lens radius."
118
ETIOLOGICAL CONSIDERATIONS Emmetropization
In addition, when humans do not have normal ocular imagery, ‫الصور‬large
refractive errors usually develop. Conditions such as :
1.
2.
3.
4.
lid hemangiomas,
ptosis,
neonatal eyelid closure,
retrolental fibroplasia associated with retinopathy of prematurity,
5. vitreous hemorrhage in infants and children lead to high myopia.
6.Axial length was found to be greater than normal in patients with neonatal
eyelid closure, juvenile corneal opacification, and congenital cataracts.
Distributions of corneal power, crystalline lens power, and anterior chamber
depth are normal, but distributions of axial length have been found to be
different.
119
Lid haemangioma
120
PTOSIS
121
neonatal eyelid closure
122
retrolental fibroplasia
123
vitreous hemorrhage
124
ETIOLOGICAL CONSIDERATIONS Emmetropization
On this basis, it can be suggested that the visual
feedback that directs ocular refractive development
directs axial elongation of the eye.
Because emmetropization appears to be a vision
dependent phenomenon, and because it tends to
reduce either myopic or hyperopic refractive error,
visual input ‫ النتيجة‬must direct the visual system in
sensing the presence and direction of refractive error.
125
ETIOLOGICAL CONSIDERATIONS Emmetropization
Because spectacles provide a compensation for refractive
error, it could be asked whether they could disrupt
‫تعطيل‬or affect emmetropization in some way.
The fact that bifocals and PALS (Progressive-addition
lenses) slow myopia progression in some cases suggests
that there is an effect. The influence of spectacle
correction on emmetropization is in infants and small
children. Because uncorrected hyperopia can be
associated with esotropia and anisometropia is associated
with amblyopia.?
126
ETIOLOGICAL CONSIDERATIONS
Emmetropization
The mean refractive error in the most hyperopic meridian decreased
from:• +4.6 to + 3.4 D in the spectacle wearers and from
• +4.3 to + 3.1 D in those who did not wear spectacles.
Thus, the reduction in hyperopia was the same in both groups.
The mean amount of astigmatism decreased from:• 1.9 to 1.0 D in the spectacle wearers and from
• 1.7 to 0.7 D in those who did not wear spectacles.
An additional analysis showed no difference in the results for subjects
who were compliant ‫ متوافق مع‬with spectacle lens wear and those that
were not. Ingram et al. followed 6-month-old infants for variable
times ranging from about 2 to 4 years. Included in the study were
infants with more than +5.25 D hyperopia in one meridian by
127
retinoscopy after instillation of cyclopentolate.
ETIOLOGICAL CONSIDERATIONS
A) Emmetropization
Data were analyzed for three groups by treatment:
(1) no spectacles,
(2) spectacles with +2.0 D undercorrection of hyperopia in all meridians
and later judged to have worn spectacles consistently , and
(3) spectacles with +2.0 D undercorrection in all meridians, but later
judged to not have worn spectacles consistently.
• The 89 nonstrabismic infants who did not wear spectacles had a mean
decrease in hyperopia of 1.34 D (SE = 0.13).
• The 45 nonstrabismic infants who wore spectacles consistently had a
mean decrease in hyperopia of 0.89 D (SE = 0.18).
• The 55 without strabismus who wore spectacles inconsistently had a
mean decrease in hyperopia of 1.35 D (SE = 0.15). Thus, it appeared that
inconsistent wear of undercorrected spectacles for hyperopia did not
affect emmetropization in either study and that consistent wear did not
128 affect emmetropization in one study but may have in the other.
B) Prevailing Theories of Myopia Development
Pedigree ‫ نسل‬studies have identified modes of inheritance for some
uncommon forms of high myopia. There is a resemblance of errors in
parents and offspring. Furthermore, a tendency toward myopia may be
present in the children of myopic parents before they actually become
myopic, as evidenced by the fact that they have greater vitreous depths.
However :
1- common lifestyle and
2- environmental factors as well as
3- genetic inheritance can contribute to family resemblance in refractive
error. Statistical studies of the relative contribution of genetics to
variability in refractive error have shown that refractive error cannot
be attributed entirely to genetic inheritance causes and that, in some
populations, apparently little of the variability in refractive error can be
attributed
to genetic inheritance.
129
B) Etiology of myopia
Did near work causes myopia development ?
Several theories of myopia etiology have attempted to identify
mechanisms by which near work causes myopia development.
1-One hypothesis is that sustained accommodation causes an
increase in intraocular pressure, which in turn leads to a
stretching of the posterior segment of the eye and axial
elongation. This hypothesis appears to be at odds with the fact
‫ على خالف مع حقيقة‬that intraocular pressure as measured at the
cornea decreases with accommodation , proposed that an
intraocular pressure gradient ‫تدرج متزايد‬between the anterior
and vitreous chambers occurs during accommodation.
130
2- A recent hypothesis is that myopia results from a defocus ‫يزيل‬
‫ التباؤر‬of the ocular imagery‫ صور‬so that the point of clearest focus
would be significantly behind the retina, as would occur when a
person who does a lot of near work has deficient accommodative
function. One way to view the defocus hypothesis from a functional
standpoint ‫وجهة نظر‬is that accommodation is a short-term
mechanism for clear near-point vision and myopia is a long-term
mechanism for clear near-point vision. As mentioned earlier ,
altered visual input can result in myopia.
131
C) Etiology of Astigmatism
The etiology of most cases of astigmatism is unknown.
A)1- One common hypothesis is that eyelid tension steepens the
vertical corneal meridian and causes with-the-rule astigmatism.
2-Most corneas have decreases in corneal with-the-rule
astigmatism when the eyelids are lifted away from the eye.‘
3- In addition, most eyes show an increase in refractive withthe-rule astigmatism as measured by autorefractor when the
palpebral aperture is narrowed.?
4- Case reports indicate that chalazia and lid masses can induce
changes in corneal astigmatism .
5- However, in two studies a relationship was not found between
lid tension and corneal astigmatism.
132
6 -It has been suggested that the shift toward against-the-rule
astigmatism experienced by persons over 40 years of age is due to
decreased lid tension.
Vihlen and Wilsorr'" did find a decrease in lid tension with age.
B) Another force that might affect corneal astigmatism is
contraction of the extraocular muscles ; small changes in
corneal curvature have been observed to occur with
convergence.
C) racial causes :
• American Indians have a high prevalence of with-the-rule
astigmatism, but the cause of this increased prevalence is
unknown ( racial causes).
• The presence of with-the-rule astigmatism appears to be related
to the degree of American Indian ancestry.‫أصل‬
• Wilson found lower intraocular pressure in Navajos and
Cherokees ‫ النافاخوس والشيروكي‬with higher amounts of with-therule corneal astigmatism.
133
SUMMARY
1.
2.
3.
4.
5.
6.
7.
8.
Newborn babies have a wide range of refractive errors.
Infants with myopia shift toward emmetropia.
Hyperopic infants shift toward emmetropia as well,
By 5 or 6 years of age the vast majority of children are
emmetropic.
White newborns have a high prevalence of against-the-rule
astigmatism, which decreases over the first few years of life.
The school-age years are the most common time of myopia
onset.
Once myopia appears in childhood, it increases in amount until
the middle to late teens.
The major ocular optical component change associated with
childhood myopia progression is increased vitreous depth.
134
SUMMARY
Several factors have been related to higher rates
of childhood myopia progression. These include:
1.
2.
3.
4.
5.
6.
earlier myopia onset age,
near-point esophoria,
temporal crescents
other myopic fundus changes,
higher intraocular pressure,
greater amount of time spent reading and doing near work,
and less time spent on outdoor activities.
The most common methods for trying to control childhood
myopia progression are rigid contact lenses
135
CHAPTER FIVE
Accommodation, the Pupil, and
Presbyopia
136
Accommodation, the Pupil, and
Presbyopia
THE ACCOMMODATIVE PROCESS
Accommodation refers to the process whereby changes in the
dioptric power of the crystalline lens occur so that an in-focus
retinal image of an object of regard is obtained and maintained at
the high-resolution fovea. Although lenticular-based focusing was
first proposed by Descartes , it was Thomas Young' who initially
demonstrated that changes in the crystalline lens itself were
responsible for such focusing changes, and Hermann von
Helmholtz, considered the father of physiological optics," who
advanced the first basic but reasonably accurate explanation of the
accommodative process.
137
A variety of ideas have been proposed regarding how we see clearly
at different distances. Some of them are as follows:
1 . There is no need for an active form of focusing. This radical
notion ‫ فكرة راديكالية‬was accounted for by judicious use of the interval
of Sturm in uncorrected astigmatism and appropriate placement of
the depth of focus in all other situations.
Clearly, this is insufficient to explain the clarity of vision and large
range of accommodation found in children and young adults.
2. Pupil size changes with the effort to see clearly at near. However,
the depth of focus for the smallest normal physiological pupil
diameter (approximately 2.0 mm) in prepresbyopes again can
account for only a small portion of their accommodative amplitude.
138
The range of image distances over which the image of an improperly
focused object is acceptably sharp is called the depth of focus
139
Accommodation
140
Accommodation
3.Role of corneal curvature changes with a change in focal point.
As Thomas Young' demonstrated over 200 years ago, however,
when he immersed his cornea into a beaker ‫ فنجان‬of water and
thus neutralized its power, accommodation was still possible.
Therefore, the cornea is not a factor in the accommodative process.
4. The anteroposterior position of the lens changes with variation
in focal point. This theory has been discounted ‫تم نقضها‬by a variety
of techniques, including biomicroscopy and ultrasonography.
Moreover, given the small range over which the lens could
theoretically shift in the human eye, the changes in power would
be rather small and, again, could not begin to equal the 15.0 D or
so amplitude found in young children.
141
Accommodation
5. Changes in the axial length of the eyeball itself account for shifts
in the position of the retinal image for objects at various distances.
Again, Young,' with his large, protruding eyeballs and considerable
commitment, ‫ التزام‬provided the disconfirming ‫عدم تأييد‬evidence for
this theory. Placing a clamp near the anteroposterior axis of his
own eye, Young demonstrated that the size and intensity of the
mechanical-pressure-generated did not change with
accommodation. Thus, the eye did not change in axial length. This
theory has also been discounted for the most part with clinical
ultrasound. However, minute changes in axial length «0.04D
equivalent with accommodation have been found."
142
Accommodation
6. Changes in the shape, and therefore power, of the crystalline
lens allow objects at various distances to be focused on the
retina. By both default ‫ االفتراضي‬and available evidence and
logic, this is clearly the correct mechanism of the human
accommodative process. Some of the processes listed above do
occur in other species, however.“ The basic sequential
biomechanical and anatomical changes that occur during
accommodation are shown in Figures 4-1, 4-2.
143
Steps in the Biomechanics of the Near
Accommodative Process Change in a Young Adult
1. A step input increase occurs in the firing frequency ‫اطالق التردد‬of
neural innervation to the ciliary muscle.(with parasympathetic
stimulation).
2. The contraction force of the ciliary muscle increases.
3. The ciliary muscle moves inward and anteriorly.
4. The ciliary ring advances approximately 0.5 mm along with the
ciliary muscle.
5. The choroid and posterior zonules stretch approximately
0.5mm.
6. The anterior zonular tension decreases. and the zonules relax.
144
ciliary ring
145
Steps in the Biomechanics of the Near
Accommodative Process Change in a Young Adult
7. The elastic forces of the lens capsule and the viscoelastic
properties of the lens cause the lens to become more spherical. Thus
the overall power of the lens increases , with the following changes:
a.The equatorial diameter decreases by 0.4 mm (from 10 to 9.6 mm).
b. The anterior lens pole moves forward 0.3 mm.
c.The central anterior radius of curvature changes from 11 to 5.5mm
d. The posterior lens pole may move back 0.15 mm.
e. The central posterior radius of curvature decreases from 5.18 to
5.05 mm.
f. The central thickness increases by 0.36 to 0.58 mm.
g. The lens sinks 0.3 mm as a result of gravity respect to the anterior
zonules, lens capsule, and crystalline lens.
146
Steps in the Biomechanics of the Near
Accommodative Process Change in a Young Adult
1-With increased ciliary muscle contraction , the
anterior zonules reduce their tension and "relax,“
allowing the inherent ‫متأصل‬forces of the lens capsule and
lens itself to interact appropriately.
2-With subsequent reduced contraction of the ciliary
muscle, the anterior zonules exhibit increased tension,
thus pulling on the lens capsule and lens. Again, all of
these changes represent passive biomechanical
alterations. However, recent evidence suggests an
important role for the cerebellum .
147
Steps in the Biomechanics of the Near
Accommodative Process Change in a Young Adult
Parasympathetic pathway to the ciliary muscle originates in the
Edinger-Westphal nucleus and courses with the third nerve, where
the fibers travel to and synapse in the ciliary ganglion.
The majority of the postganglionic parasympathetic fibers travel to
the ciliary muscle via the short ciliary nerves, but some of them also
travel with the long ciliary nerves. There is also evidence for a direct
pathway of uncertain functional significance to the internal eye
structures from the Edinger-Westphal nucleus.
148
149
150
151
Steps in the Biomechanics of the Near
Accommodative Process Change in a Young Adult
The sympathetic supply to the ciliary muscle originates in the
diencephalon ‫الدماغ البيني‬and travels down the spinal cord to the lower
cervical and upper thoracic segments, to synapse in the spinociliary center
of Budge in the intermediolateral tract of the cord.
From there, second-order nerves leave the cord by the last cervical and first
two thoracic ventral roots; these preganglionic fibers run up the cervical
sympathetic chain to synapse in the superior cervical ganglion.
The third-order fibers continue up the sympathetic carotid plexus and enter
the orbit, either with the first division of the trigeminal nerve (following the
nasociliary division) or independently, where they join the long and short
ciliary nerves, in the latter instance passing through the ciliary ganglion
without synapsing.
152
sympathetic supply to the ciliary muscle
153
Types of Accommodation
Accommodation like convergence may be divided into the following
components parts as described by von Noorden1:
1. Reflex accommodation is an automatic adjustment response to blur which
is made to maintain a clear and sharp retinal image0
2. Convergence-accommodation is the amount of accommodation stimulated
or relaxed associated with convergence. Clinically this is noted as the blur
value when performing base out prism (BO) and base in prism (BI) fusional
reserves at near and BO at distance. The link between accommodation and
convergence is known as accommodative convergence and is expressed
clinically as the AC/A ratio
3-Proximal accommodation is also known as psychic accommodation and is
induced by the awareness of the nearness of a target. This is independent of
the actual dioptric stimulus.
4. Tonic accommodation, also know as dark accommodation is the passive
state of accommodation. It is due to the tonus of the ciliary muscle and is
active in the absence of a stimulus. The resting state of accommodation is not
at infinity but rather at an intermediate distance. The value obtained varies
between individuals and usually is between +0.75 to +1.50D.
154
Components of Accommodation
There are four components System of accommodation include
reflex, vergence, proximal, and tonic accommodation.
1- Reflex Accommodation
Reflex accommodation is the automatic adjustment of refractive state to
obtain and maintain a sharply defined and focused retinal image in response
to a blur input, . This occurs for relatively small amounts of blur, perhaps up
to 2.0D or beyond that, voluntary accommodative effort is required.
Small scanning eye movements, assist in the process," possibly by producing
multiple retinal-image luminance gradients about the fovea from which the
blur information can be more easily extracted." Reflex accommodation is
probably the largest and most important component of accommodation under
both monocular and binocular viewing conditions."
155
2- Vergence Accommodation
Vergence accommodation is the accommodation induced by the
innate ‫فطري‬neurological linking and action of disparity‫تفاوت‬
(fusional) vergence." This gives rise to the convergence
accommodation/convergence (AC/A) ratio, which is approximately
0.40 D per meter angle (MA) in young adults. Vergence
accommodation is probably the second major component of
accommodation.
156
3- Proximal Accommodation
Proximal accommodation is the accommodation due to the
influence or knowledge of apparent (or perceived) nearness of an
object." It is stimulated by targets located within 3 meters of the
individual," hence its name.
Its open-loop contribution can become quite large with near
viewing, providing up to 80% of the total near response, that is,
the combined proximal and tonic outputs, thus the proximal
contribution becomes quite small (around 4%) , with a maximum
of 10% . Proximal accommodation is stimulated by perceptual
stimuli. ‫إدراكي حسي‬
157
4- Tonic Accommodation
Tonic accommodation is revealed in the absence of blur, disparity,
and proximal inputs." as well as any voluntary or unusual learned
aspects. There is no stimulus per se for tonic accommodation, as
there is for the other three components. Rather, it presumably
‫ يفترض‬reflects baseline neural innervation from the midbrain and
thus represents a relatively stable input.
Tonic accommodation can be measured in many ways, all of which
involve removal of the other three inputs.
158
Tonic Accommodation
Perhaps the best way to measure tonic accommodation is to
place the individual in the center of a totally darkened room
whose walls are at least 3 meters away from the person, with
the accommodative measuring device ‫ جهاز‬also away from and
not visible to the person so that proximity and propinquity
‫تشابه‬effects, which will inflate the true tonic value."
Under such conditions, the mean tonic accommodative level in
young adults is approximately + 1.0 D, with a range from nearly
+1.0 to +2.0 D.
159
Development of Accommodation
Considerable insight ‫ تبصر ودراسة‬has been gained in the past 30 years
or so into the accommodative ability of young infants. In the classic
study by Haynes et al.." dynamic retinoscopy was used to assess
steady state accommodation at various near distances.
Accommodative stimulus-response profiles were determined in
infants whose ages ranged from 6 days to 4 months.
1-Accommodation during the first month appeared to be relatively
fixed at approximately 5.0 D.
2- Whereas in the subsequent 3 months it progressively became more
accurate and approached adult-like behavior.
160
Development of Accommodation
Only tonic accommodation did not appear to exhibit an early
developmental trend": it was the same (approximately 1.40 D) in
infants and young adults.
3- Finally, accommodative dynamics in response to steps of blur
input appear to have adult like velocities by the age of 3 months.
4- Vergence accommodation, AC/A is present in infants 3 to 6
months of age. It develops concurrently‫ في وقت واحد‬with both blurdriven accommodation and vergence accommodation .
161
Development of Accommodation
5-There have been no carefully controlled studies of
accommodation in young children between the ages of 1 and 4.5
years. Children at these ages are difficult to assess properly
because it is :
a. not easy to ensure that one has their full attention.
b. not easy that they understand the test procedures and criteria.
c-and that they are in need for maximal effort for measurement
of accommodative amplitude and facility.
However, it should be possible to obtain reasonable estimates of
accommodative accuracy and sustaining ability by using
dynamic retinoscopy with targets of high attentional value, for
example in a gamelike environment.
162
Development of Accommodation
6- In one study in children ages 2 to 14 years the amplitude of
accommodation decreased with age, thus being consistent with
overall age-related trends for older individuals.
Chen and O'Leary" found that the slope (decrease) of the
accommodative stimulus/response function remained relatively
constant and normal with age in young emmetropic children (3 to
14 years old) using objective methods.
163
Steady-State (Static) Accommodative Stimulus-Response
Function
7- Within the age range from 5 to 10 years , a number of studies
have been conducted, several of which assessed .
One of the most important relations to understand in this area is the
accommodative stimulus-response function, or the profile of
accommodative response .
It provides an accurate, quantitative description of the
accommodative response over a full range of accommodative stimuli,
allowing practitioners to understand several fundamental principles
regarding neurological control of accommodation. This profile can
be divided into the following six zones or regions (one linear and five
nonlinear), each with specific response characteristics (Figure 4-4).
164
Linear manifest zone
The linear manifest zone is the response midregion
over which a change in the accommodative stimulus
produces a relatively large and proportional change in
the accommodative response . Thus, as the near
accommodative stimulus increases, the steady-state
accommodative change increases by a fixed
proportion." The slope of the linear response region
ranges from 0.7D to nearly 1.0D .
165
ACCOMMODATIVE STIMULUS (D)
Initial Nonlinear Zone
The initial nonlinear region extends from 0 to 1.50 D or
so. It might be slightly more with a physically close ,
closed-field optical stimulation and measurement system
in which there is much proximal drive, and it might be
somewhat less in a more distant, open-field system in
proximal input is minimal or even absent. This results
from its presumed proportional neural control": that is,
the system change is proportional to the system input.
‫مدخالت النظام‬
166
Figure 4-4
Static accommodative stimulus-response function profile (curved line).
Equivalence of stimulus and response is represented by
the diagonal line‫خط قطري‬.
Numbers represent zones of the accommodative profile.‫البيانات الشخصية‬
167
Nonlinear Transitional Zone
The region in which further increases in the
accommodative stimulus level (just beyond the upper
linear manifest zone) produce progressively smaller
changes in accommodative response (i.e., soft saturation)
is the nonlinear transitional zone. In this zone,
progressively greater increases in accommodative change
are evident. This is due to initial crystalline lens
biomechanical limitations in responsivity that occur near
the168upper limit of the amplitude, regardless of age .
Nonlinear Latent Zone
The region in which yet further increases in the
accommodative stimulus level fail to produce any
additional change in accommodative response (i.e., hard
saturation) is the nonlinear latent zone. This region
extends approximately 2.0 D beyond the nonlinear
transitional zone, with its initial portion defining the
amplitude of accommodation. This total lack of
responsivity is due to further biomechanical limits in lens
responsivity. This zone has also been referred to as the
age-independent "functional presbyopic region ‫ هام‬.
169
Factors that Affect Accommodation
A multitude of factors affect the accommodative response to varying
degrees. These factors may be categorized as the stimulus to and the
influence on accommodation.‘ There is considerable evidence and
consensus ‫إجماع‬that :
1- blur is the stimulus to accommodation ' Our eyes see a blur
pattern and respond accordingly to produce an in-focus retinal
image. Thus, there is little justification ‫ التبرير‬to propose ‫يقترح‬
anything more complicated for the basic accommodative blur
detection and reduction process. In contrast, cues ‫الدروس‬for
accommodation provide the requisite ‫ شرطا‬directional information
regarding the blur pattern.
170
Factors that Affect Accommodation
2- Voluntary effort , mood." and target luminance
Finally, there are factors that can influence the accommodative
response. These include such diverse entities ‫كيانات متنوعة‬
as voluntary effort , mood." and target luminance." As an
example, if a high-contrast target, initially placed at some
intermediate distance from an observer, is suddenly and rapidly
displaced either farther or closer along the midline, a retinal blur
pattern would be produced that would serve as the stimulus to
accommodation.
171
Training the Accommodative System
A variety of studies over the past 45 years have demonstrated
that the normal human accommodative system can be trained
to improve response accuracy and time optimality.‫الوقت المثالي‬
Such training primarily involves relatively :
1- rapid motor learning , as well as much
2- slower perceptual learning ‫ التعلم اإلدراكي‬to assess and respond
appropriately to the blur stimulus, .
Marg'" showed that steady-state accommodation could be
varied easily by volitional ‫ارادي‬control in the presence of a
target and related blur feedback.
172
Training the Accommodative System
It was even found that such responses could be elicited in total
darkness." The dynamics of voluntary accommodation are the same
as those of the more reflexive accommodation described by
Fincharn. ‫ “فنشارن‬suggesting similar basic neuromotor control
despite very different modes and probable sites of initiation (i.e.,
retinal defocus versus higher level cortical control processes).
The accommodative stimulus-response function showed
improvement after 8 to 16 weeks of basic accommodative therapy in
a group of college students.
This normalization was maintained when patients were retested 6 to
9 months later. Together, results of the foregoing studies clearly
demonstrate that symptoms related to near focusing were correlated
with173the clinical accommodative lens movement rate.
Training the Accommodative System
Several studies have demonstrated that it is also possible to train and
improve accommodation in symptomatic patients manifesting
slowed dynamics. The first study was that of Liu et al., who treated
three optometry students with symptoms related to focusing
difficulties at near using standard vision training procedures,
including :
1-jump focus,
2-plus-and-minus lens flippers, and
3-pencil push-ups.':" Subjects trained themselves at home for 20
minutes each day for 4.5 to 7 weeks, and objective measurements of
dynamic accommodation were made each week.
174
Vision Training with Focusing Flippers
What is this exercise for?
Focusing flippers help to train our eyes in accommodation, which is changing our
focusing from distance to near. Specifically, flippers help improve accommodative
facility - the ability to increase and decrease our focusing on demand. A variety of
binocular vision problems cause a decrease in our ability to accurately focus: some
people focus too much and cannot relax, other focus too little and experience
blurry vision for close tasks. The flippers you are given have lenses on one side
that require you to relax your focus (magnifying lenses), and the other side
requires you to focus more (minifying lenses).
What will you need?
Focusing flippers (powers range from +/-2.00D down to +/-1.00D) from
Innovative Eye Care
Innovative Eye Care near vision chart appropriate for your reading level
Habitual reading material with normal sized print for reading (eg a magazine,
novel, school reading homework)
175
176
Instructions:
Read one line of the Innovative Eye Care reading chart through one side of
the flippers. Try to keep each word clear as you read it aloud.
Turn the flipper over so you are looking through the other side of the lenses.
Read the next line aloud.
Flip back to the first set of lenses and read the next line.
Repeat this process until you reach the bottom or cannot focus well enough
to read the words.
Try to do 1 minute of flipper reading, then give yourself 30 seconds rest.
Repeat this cycle 5 times. Find time to do this 3 times a day.
If the bold side becomes easy to read, try the low-contrast chart on the
reverse side.
If you learn all the words on your Innovative Eye Care chart ask your
optometrist if you can use normal reading material on a book/magazine or
computer in the same way - flipping the lenses over at the end of each line.
Make sure the print is small enough to challenge you to focus on it clearly.
Remember: For these exercises to be effective, the words you are looking at
need to be clear and single. Ask us if you have any questions about the
exercises.
177
Training the Accommodative System
Initially, these measurements showed prolongation of the time
constant and latency of accommodation.
During treatment, the patients exhibited significant reductions
in these two parameters that correlated well with reduction of
symptoms. Flipper rates increased and symptoms were either
markedly diminished or no longer present at the termination
of therapy.
These results clearly demonstrate that vision training in this
small sample of young adult patients resulted in objective
improvement of accommodation.
178
Training the Accommodative System
The reduction in time constant suggested revision and improvement
in the neuromotor control program. leading to a more efficient, time
optimal response. This might involve greater synchronization‫تزامن‬of
neural signals related to the improved blur information processing.
The reduced latency also suggested more efficient signal processing
of blur information.
The adult results were also later confirmed and extended by Bobier
and Sivak':" using a different objective recording technique
(photorefraction). They found no regression of improvement 4.5
months after cessation of training.
179
Effects of Disease on Accommodation
DISORDERS OF ACCOMMODATION
Abnormalities of accommodation usually are acquired and occur
most frequently as part of the normal aging process (presbyopia).
However, disturbances of accommodation also may occur in
otherwise healthy persons, in persons with
generalized systemic and neurologic disorders, and in persons with
lesions that produce a focal interruption of the parasympathetic
(and rarely the sympathetic) innervation of the ciliary body. Also,
disrupted.accommodative function can be voluntarily
180
PARALYSIS
I- Congenital and Hereditary Accommodation
Insufficiency and Paralysis
a-Congenital defects are a rare cause of isolated accommodation
insufficiency. The ciliary body is defective in a number of
congenital ocular anomalies, but in most cases vision
is so defective that an inability to accommodate is never
noted by either the patient or the physician.
181
b.Aniridia and choroidal coloboma cause obvious defects of the
ciliary body. Ciliary aplasia can occur in well-formed eyes in
which the iris is intact and reacts normally to light.
Se´dan and Roux described three brothers who could see
normally for distance without glasses but required spherical
lenses for reading defect. .
182
In another family of 10 affected members, an accommodative
defect was present in infancy and thereafter nonprogressive by
history . Pharmacologic assessment with various topical agents
suggested a difficulty with either the ciliary musculature or the lens
of the affected eyes.
c.Congenital absence of accommodation has been noted in
combination with congenital mydriasis. In three such cases,
was an associated ‫القناة الشريانية السالكة‬patent ductus arteriosus
anomaly.
183
d. Isolated Accommodation Insufficiency
Accommodation insufficiency refers to an accommodative ability
that measures below the minimum for the age of the patient.
Most clinicians use the near point of accommodation as their
diagnostic criterion for accommodation insufficiency
(accommodative amplitude that is 2 diopters or more below the
age-appropriate minimum). Isolated accommodation insufficiency
occurring in otherwise healthy eyes can be divided into two
groups:
184
(a) static insufficiency
and
(b) dynamic insufficiency
Static accommodation insufficiency is an inadequate
response of either the lens or the ciliary muscle, despite
normal ciliary body innervation and neural function. It
usually occurs gradually from changes occurring in
either the lens or the ciliary body. The most common
cause of isolated static insufficiency is presbyopia.
Treatment consists of appropriate spectacle correction.
185
Dynamic accommodation insufficiency occurs in patients
who have inadequate parasympathetic impulses required
to stimulate the ciliary musculature but have normal pupil
size and reactivity. Such patients usually are asthenopic
persons who become ill, often hospitalized, with some
unrelated condition. Dynamic accommodation
insufficiency also may occur in otherwise healthy young
individuals, particularly in children with nonspecific viral
illnesses .
186
The transient loss of accommodation that can occur just before
or after childbirth may be another example of this
phenomenon .Raskind listed various systemic disorders
associated with an acquired accommodation insufficiency .In all
these cases, it is likely that the accommodation insufficiency
represents a nonspecific manifestation of the systemic disorder.
187
The symptoms of dynamic accommodation insufficiency
are asthenopia, tiring of the eyes sometimes associated with:brow ache, irritation and burning of the eyes, blurred vision
particularly for near work, inability to concentrate, and
photophobia. As a general rule, symptoms resolve and
accommodation recovers following treatment of the underlying
illness and restitution of the patient’s former state of health.
If accommodation insufficiency remains, the prescription of
convex (plus) lenses is indicated, regardless of the patient’s age.
188
Accommodation Insufficiency Associated with Primary
II. Ocular Disease Iridocyclitis
may cause profound dysfunction of the ciliary body.
1- In the acute stage, there may be ciliary spasm and spasm of
accommodation.
2- In the chronic stage, atrophy of the ciliary body results in accommodation
insufficiency.
The more severe the uveitis, the more commonly mydriasis and cycloplegia
(internal ophthalmoplegia) are associated with it.
189
3-In addition, viruses such as herpes zoster may produce a uveitis associated
with a ciliary ganglionitis, resulting in atonic pupil syndrome .Atonic pupil,
sometimes referred to as internal ophthalmoplegia, is the result of
damage to the parasympathetic innervation to the eye, resulting in
decreased
function of the iris sphincter as well as the ciliary bod.
5-Glaucoma in children or young adults causes accommodation insufficiency
from secondary atrophy of the ciliary body.
6-The drugs used in the management of glaucoma affect the ciliary body as
well as the iris.
In patients who still are able to accommodate, miotic drugs frequently
produce ciliary spasm with symptoms of blurred vision.
190
7-Immediately following a concussion injury to the globe ,
the pupil is small and accommodation is spastic.
Subsequently, the pupil dilates and the ciliary muscle
becomes paretic. Accommodation usually returns in 1–2
months.
The traumatic etiology of the accommodation paresis may be
suspected by slit-lamp biomicroscopic observation of tears in
the iris sphincter, tears at the root of the iris, or recession of
the angle of the anterior chamber with posterior
displacement of the ciliary attachment.
191
8-Following trauma to the globe, rupture of zonular fibers with
partial subluxation of the lens also may produce loss of
accommodation.
192
9-Iatrogenic trauma to the eye, such as that
which occurs during retinal reattachment surgery,
cryotherapy, or panretinal photocoagulation, may injure
the ciliary nerves, producing accommodation paresis
and mydriasis.
193
10-Laser applications at or anterior to the equator and long
exposure times are important factors in the development of
accommodation paresis following photocoagulation .
Optic nerve sheath fenestration performed from the lateral
approach can damage the short ciliary nerves that penetrate the
temporal sclera, causing a postoperative tonic pupil.
Sector palsy of the iris sphincter has been reported after argon laser
trabeculoplasty .
11-Transient internal ophthalmoplegia also can result from local
anesthesia injected into the lids or gums that inadvertently enters
into the orbit .
194
12- Accommodation Insufficiency associated with Neuromuscular
Disorders . Some diseases produce myopathic changes in the
smooth muscle fibers of the ciliary body, but isolated ocular
involvement of this type is rare.
Myasthenia gravis (MG ) ‫الوهن العضلي الوبيل‬may cause defective
accommodation .Manson and Stern studied patients with MG and
patients with unexplained accommodation disturbances.
All patients were questioned about diplopia, ptosis, and the effects
of sustained reading or close work upon their vision.
195
‫األعراض الظاهرة على المريض ‪:‬‬
‫* تعب شديد في حالة المجهود ويزول مع الراحة ‪.‬‬
‫* يكون للمرض فترات نشاط وأخرى فترات خمول ‪.‬‬
‫* تُصاب جميع العضالت ماعدا عضلة القلب ‪.‬‬
‫* تُصاب العضالت العليا باألطراف أكثر من‬
‫العضالت السفلى ‪..‬‬
‫* التعب العام وصعوبة الحركة ‪.‬‬
‫* عدم القدرة على تحريك األطراف ‪.‬‬
‫* عدم القدرة على البلع ومضغ الطعام ‪.‬‬
‫* عدم القدرة على المشي الكثير ‪.‬‬
‫* التعب عند صعود الساللم والوقوع احيانا ً ‪.‬‬
‫* ازدواج الرؤية واحوالل العينين وسقوط جفن العين‬
‫العلوي ‪.‬‬
‫* يحدث المرض في أي عضلة إرادية في الجسم‬
‫* يختلف مقدار الضعف في العضالت من مريض‬
‫آلخر‪.‬‬
‫* االختناق وعدم القدرة على التنفس ‪.‬‬
‫* عدم القدرة على حمل األشياء ‪.‬‬
‫* في الحاالت المتقدمة الدخول في حالة إغماء ‪.‬‬
‫* تتحسن الحالة مع مرور الوقت ‪.‬‬
‫* تتحسن حالة المريض بعد الراحة وبعد االستيقاظ‬
‫من النوم‪.‬‬
‫ماهو مرض الوهن العضلي ‪Myasthenia Gravis‬‬
‫هو مرض يحدث نتيجة اختالل في جهاز المناعة‪،‬‬
‫ينتج من وجود أجسام مضادة للوصالت العصبية بالعضالت‬
‫وعليه تنقص أعداد مستقبالت األسيتايلكولين بالعضالت‬
‫‪196‬‬
13-The toxins produced by C. botulinum interference with the
release of acetylcholine at the neuromuscular junction and in the
cholinergic autonomic nervous system. Although several different
subtypes of toxin exist and routes of human infection can vary
(food-borne, wound, gastrointestinal tract colonization in infants),
clinical manifestations are similar.
These include progressive descending skeletal muscle weakness,
ocular motor palsies, and bulbar paralysis. Food-borne botulism
produces prominent gastrointestinal symptoms as well.
Almost 90% of patients with botulism ‫تسمم من أكل اللحم‬of any type
complain of blurred vision. Paralysis of accommodation and
impairment of the pupillary light reflex are common and early signs
of botulism, often appearing suddenly about the 4th or 5th day of the
illness.
197
14-Tetanus ‫الكزاز‬
can produce accommodation paralysis. In
most cases, the accommodation paralysis
occurs in the setting of generalized
ophthalmoparesis; however, one patient
described had normal eye movements and
normally reactive pupils to light stimulation.
198
Eyes with Myotonic
dystrophy .
lens opacities in a 52
ys patient with
myotonic dystrophy
15- Myotonic dystrophy frequently produces
degenerative changes in the lens, the region of the ora
serrata, and the anterior chamber angle. It also may be
associated with ocular hypotension. It is reasonable to
assume that the ciliary muscle also is affected, because
other smooth muscle dysfunction occurs in such
patients leading to accommodation insufficiency.
199
16 -Accommodation paresis may be caused by both focal and
generalized neurologic disorders that interrupt the innervation of
the ciliary body. Supranuclear lesions can influence the signal
inputs to the parasympathetic midbrain nuclei for accommodation,
resulting in paresis or even paralysis of accommodation.
Such lesions include damage to the cerebral cortex, midbrain,
superior colliculus, and possibly the cerebellum .
In these cases, convergence insufficiency usually accompanies the
accommodation insufficiency because of cross-coupling between
these two systems.
200
17-Acute infectious or epidemic encephalitis as well as
postinfectious acute disseminated encephalomyelitis
(ADEM (such as that which is associated with or follows
measles , chickenpox , or other viral infections can cause
accommodative paralysis . Typically, patients have loss of
the near triad (convergence palsy, absence of pupillary
near response (miosis) ,failure of accommodation) and
normal pupil responses to light.
201
18-Wilson disease is an hereditary disorder of copper metabolism
characterized by progressive degeneration of the CNS associated
with hepatic cirrhosis.
a)The general neurologic syndrome frequently includes :
rigidity, difficulty speaking and swallowing, and a characteristic
tremor of the wrists and shoulders.
b) Ocular findings include :
1/ a peripheral corneal ring of copper deposition involving
Descemet’s membrane (Kayser Fleischer ring).
2 / copper pigment under the lens capsule, and
3/ various ocular motor disturbances, including jerky oscillations of
the eyes, involuntary upgaze, paresis of upgaze, an slowed saccadic
‫رمشي‬movements (flickering ).
202
17-Wilson disease
203
204
19 - Lesion of the ciliary ganglion or short ciliary nerves:
An isolated accommodation paralysis—accommodation
paralysis without mydriasis—theoretically can be caused by
a lesion of the ciliary ganglion or short ciliary nerves. We are
unaware of any well-documented cases of this phenomenon.
205
III -Accommodation Insufficiency Associated with
Systemic Disease
Children and adults may develop transient accommodation
paresis following various systemic illnesses. In such cases, the
accommodation paresis often appears to occur as an indirect
complication of the systemic disorder rather than from direct
damage to the ciliary body or its innervation.
There are, however, certain systemic diseases that produce
accommodation insufficiency through direct effects on the ciliary
body and lens or on their innervation.
1- In patients with diphtheria, accommodative paralysis usually is
bilateral and occurs during or after the 3rd week following the
onset of infection.
206
III -Accommodation Insufficiency Associated with
Systemic Disease
2- Loss of accommodation may occur in patients with diabetes
mellitus from several mechanisms . For example ,accommodation
paresis may develop in young patients with previously uncontrolled
diabetes who have just begun treatment. Hyperopia and
accommodation weakness develop concurrently within a few days
after the patient’s blood glucose has been lowered and then
gradually return to normal over 2–6 weeks.
The mechanisms for the refractive and accommodative changes in
diabetes are poorly understood.
Sorbitol (a sweet-tasting crystalline compound found in some fruit)
accumulates in the lens during periods of hyperglycemia, causing it
to swell, and the lens appears responsible for the shifts in refraction
207
because these shifts do not occur in aphakic or pseudophakic eyes.
III -Accommodation Insufficiency Associated with
Systemic Disease
Persistent loss of accommodation can occur in patients
with both controlled and uncontrolled diabetes mellitus
from damage to the parasympathetic innervation to the
eye.
In this setting , accommodation paresis and mydriasis are
due to denervation injury rather than to effects on the
lens. Thus, either metabolic or neurologic mechanisms
can be responsible for reduced accommodation in
patients with this disease.
208
III -Accommodation Insufficiency Associated with
Systemic Disease
3- In a series of 161 patients with head injury (average age 29
years), Kowal found that :
•
16% had poor accommodation ,
•
19 %had over-accommodation (pseudomyopia), and
•
14% had convergence insufficiency .
In about half of these patients, near vision complaints improved
or resolved within the first year after their injury . Similarly,
among 39 patients with whiplash (injury caused by a severe jerk
to the head, typically in a motor-vehicle accident) or indirect
injury to the neck, Burke et al . found decreased accommodation
and convergence in nine ( 23% .
209
III -Accommodation Insufficiency Associated with
Systemic Disease
6% ,)six of whom were symptomatic . Five of
these six patients recovered accommodation
after 9 months.
210
VI -Accommodation Insufficiency and Paralysis from
Pharmacologic Agents
Most topical pharmacologic agents that produce pupillary
mydriasis also produce cycloplegia, including atropine, scopolamine,
homatropine, eucatropine, tropicamide, and cyclopentolate. Various
investigators have compared the duration and effectiveness of
cycloplegia produced by these agents when used as ocular solutions .
None of these agents causes persistent paralysis of accommodation
after discontinuation, although there may be some confusion when
loss of accommodation occurs after treatment of a severe viral uveitis
(e.g., herpes zoster, varicella )with a cycloplegic agent).
211
VI -Accommodation Insufficiency and Paralysis
from Pharmacologic Agents
In such cases, the accommodation paralysis occurs from the
effects of the virus on the ciliary ganglion and not from the
cycloplegic drug.
When cycloplegic agents or related substances are incorporated
in medications that are taken internally or applied to the skin as
ointments or plasters, there may be sufficient absorption to
produce paresis of accommodation.
In such cases, the accommodation deficit is partial and recovery
begins shortly after the medication is discontinued.
212
V -Sympathetic Paralysis
Lesions of the cervical sympathetic outflow may produce
a defect that prevents the patient from accommodating fully
from near to far, but most reports describe an increase in
accommodative amplitude on the side of the Horner syndrome .
Cogan described an ipsilateral increase in near accommodation in
five patients with Horner syndrome and noted an apparent
paresis of accommodation in one patient.
213
General Considerations
Accommodation spasm is due to excessive activity of the
ciliary muscle that results in an abnormally close point of
focus. Clinically, there is an apparent or increased myopia
that disappears following cycloplegia (pseudomyopia).
Accommodation spasm typically affects both eyes, but
unilateral cases have been reported .
214
It can occur in isolation as pseudomyopia or in association with
convergence spasm and excessive pupillary miosis in varying
combinations and degrees, all of which probably represent the
spectrum of clinical presentations of spasm of the near reflex.
Symptoms of isolated accommodation spasm are :
1-Blurry vision, especially at distance, 2-fluctuating vision, 3asthenopia ,4-eyestrain, 5-poor concentration, 6-brow ache, and
7- headaches.
The diagnostic finding is a greater myopia on manifest refraction
compared with cycloplegic refraction ( less or no myopia) , the
difference ranging from 1 to 10 diopters.
215
The management of most patients with nonorganic spasm
of the near reflex begins with:
1- simple reassurance that they have no irreversible visual or
neurologic disorder.
2- In other instances, referral for psychiatric counseling is
appropriate.
3- Symptomatic relief may be necessary with a cycloplegic
agent and bifocal spectacles or reading glasses.
4- Glasses with an opaque inner third of the lens to occlude vision
when the eyes are esotropic have been proposed for the
convergence spasm.
216
CHAPTER SIX
Presbyopia
217
Overview of Presbyopia
Definition: Presbyopia ("aged eye") refers to the slow, normal,
naturally occurring, age-related, irreversible reduction in
maximal accommodative amplitude (i.e. recession of the near
point) sufficient to cause symptoms of blur and ocular
discomfort or asthenopia at the customary near working
distance. Essentially , the near point goes away and then
becomes coincident with the far point.
Age of onset: Presbyopia is generally first reported clinically
between 40 and 45 years of age, with its peak onset between ages
42 and 44 years, although its onset may occur any time from 38
to 48 years of age, depending on a variety of factors. From
approximately age 52 years on, the prevalence of presbyopia is
considered to be essentially 100%; however, its prevalence across
all218ages in the population is 31%.
Overview of Presbyopia-risk factors
Although there are a variety of potential risk factors for the
development of presbyopia two are particularly important:
1. Refractive error: because accommodative demand at the corneal
plane in spectacle-corrected hyperopes is greater than that in
myopes for the same accommodative stimulus and degree of
ametropia at the spectacle plane , spectacle-corrected hyperopes
exhibit apparent relatively reduced accommodative amplitudes and
thus effectively become presbyopic a few years earlier than either
myopes or emmetropes .
219
Overview of Presbyopia- risk factors
potential risk factors for the development of presbyopia
2. Ambient temperature: with the eyeball being peripheral to the
body core, it may exhibit considerable surface temperature
variations because of the influence of ambient temperature.
There is an inverse relation between ambient temperature
and age of onset of presbyopia . ‫تكثر في المناخ الحار والضد صحيح‬
3. Clinically, when the near-work distance in meterrs dioptrically
equals half of an individual's residual accommodative amplitude,
which occurs, on average, at 40 years of age , the gradual onset of
symptoms will become manifest.
220
Presbyopia- symptoms
These symptoms are as follows:
1) blurred vision at the customary near-work distance or can be
sustained only with excessive accommodative effort and some
ocular discomfort.
2) Drowsiness ‫ نعاس‬after a short period of reading or near work.
3) Reading material must be held farther away (e.g. closer to the
receding near point ) to be seen more clearly.
Thus, on average, smaller individuals with proportionally shorter
arms develop presbyopic symptoms at an earlier age than do agematched but proportionally taller persons. Some patients may
actually complain, "My arms aren't long enough to see up
close anymore .
221
Presbyopia- symptoms
What they are describing is the fact that they can no longer keep
the object of interest within the proximal edge of the depth of
focus of their progressively receding near point.
4) Asthenopia : occasionally, especially in very early or incipient
presbyopia, asthenopia related to attempts at excessive
accommodative effort is reported. It may even lead to an
accommodative spasm and pseudomyopia.
5) Transient diplopia and variable esophoria may be experienced
as a result of the increased accommodative response/effort and
the consequent synkinetically ‫ مرافقة‬over driven accommodative
convergence .
222
Analysis of the Biological Components of Presbyopia
A variety of biomechanical, biochemical, and physiological
factors contribute to the age-related loss of accommodation and
the onset of clinical presbyopia (Figure 4-32). The following
findings on the biological components of age-related changes in
accommodation are summarized in Table 4-5. Various aspects of
the lens and capsule alone produce most of these changes.
223
Biological Components of Presbyopia
224
225
226
The three primary factors of presbyopia onset
Three factors have traditionally been considered the primary
factors contributing to the age-related loss of accommodation.
1. The modulus ‫ معامل المرونة‬of the elasticity (i.e., springiness) of
the lens capsule decreases from youth to old age. It becomes
progressively more stiff and less compliant, ‫ مطاوع‬like an old,
stretched-out rubber band. Thus, the energy it can store and then
release to deform mold ‫ قالب‬the underlying lens substance
decreases with advancing age. The effective capsular energy is
proportional to the loss of accommodation up to age 45 years.?"
227
The Three Primary Factors of presbyopia onset
2. The modulus ‫ معامل‬of the elasticity of the lens substance increases
slightly from youth to age 40 years and less precipitously ‫اندفاعا‬
thereafter."? The lens substance becomes stiffer , less plastic ‫لين‬,
and more sclerotic-like with advancing age.
However, it should be noted that no true sclerosis in the
physiological sense occurs, because the lens' overall water content
remains constant with age. Thus, the energy required to deform the
lens substance itself increases with age, with this energy being
proportional ‫ تناسب عكسي‬to the modulus of elasticity. Fisher"?
asserted that this factor contributed to 44% of the loss of
accommodation.
.
228
The Three Primary Factors of presbyopia onset
3. The lens size/volume increases progressively with
age. This makes the lens capsule function less
effectively. It is more difficult to deform a larger body
than to deform a smaller body, especially with the
concomitant change in shape. Pisher"“maintained that the
increased lens size/volume and the decreased elasticity of
the lens capsule contributed to 55% of the loss of
accommodation.
229
Other Factors of presbyopia onset
Other factors may also contribute to the age-related loss
of accommodative ability.
4-An anterior shift of the equatorial fibers"? occurs because of the
passive movement of the lens capsule with increased lens growth.
Because the zonules are fixed within the lens capsule, as the lens
increases in size and volume, it places force on the capsule that
passively pulls the zonules forward. This change in
zonule/capsule/lens geometry reduces the mechanical advantage of
the zonular suspensory system. A vector ‫سهم التوجيه‬force applied
perpendicularly to a load is more effective than the same force
applied tangentially .‫ تماسي‬However, this becomes a factor only in
the mid-40s, when there is relatively little residual accommodation.
230
Factors of presbyopia onset
5-The equatorial zonular fibers decrease in number , become less
dense, and appear to be more fragmented, which reduces their
biomechanical advantage somewhat. This, too, occurs relatively
late in life.
6-There is an increase in the number of disulfide bridges in the
lens capsule and lens.!" These bridges stabilize the collagen
molecules within the capsule and lens by the process of
crosslinking. However, this produces more resistance between the
lens fibers themselves during accommodation, with the result
that the lens becomes more rigid and difficult to deform. It also
makes the capsule less elastic .
231
Factors of presbyopia onset
7- Changes in the ciliary muscle anatomy, including :
a. decreased length of the ciliary muscle, may contribute as
much as 20% to 33% of the accommodative loss from age 30
to 50 years . However, other aspects remain constant (the
width of the ciliary muscle) or even increase (the area of the
circular portion of the ciliary muscle).
b. gross ‫ترى بالعين‬anatomical and mechanical changes in the
ciliary muscle. Age related factors .
c. There is also some reduction of inward and forward
movement of the entire ciliary muscle and ciliary muscle ring
with age.?
232
Factors of presbyopia onset
Such a static shift in effect reduces the amount of
accommodation that can occur , because it reduces the residual
amount of ciliary body movement that is allowed to take place..
8-The change in lens curvature may be expected to produce
myopia at far . However, a concurrent ‫ متزامن‬decrease in the
effective index of refraction compensates for any curvaturerelated power change (e.g., the "lens paradox“ hypermetropia
versus myopia), making the distance refractive error with
increased age relatively stable.?“
233
Factors of presbyopia onset
9- finally, choroidal elasticity progressively stiffens up
to age 35 years, with a slower rate of increase in its modulus
of elasticity thereafter . The choroid may be thought of as a
spring that acts against the inward and forward pulling of the
ciliary muscle during increased accommodation.
Thus , the ciliary muscle needs to exert a slightly greater force
with age to produce the same resultant dioptric change, which
would be expected to produce a small increase in the response
AC/A ratio with age as most recent studies demonstrate .
234
Components That Do Not Contribute in presbyopia
The three components that do not seem to contribute to the agerelated loss of accommodation are described below.
1-The zonular elasticity remains constant.?" Thus, as the ciliary
muscle contracts and moves both forward and inward with
increased accommodation, releasing zonular tension, the forces
per se transmitted from the zonules to the capsule/lens body
remain the same throughout life.
2-The force/contractile power of the ciliary muscle increases
(a maximum of 50%) from youth to age 45 years and exhibits a
slight decline from age 45 to 60 years the period in which the
accommodative loss is 100%.
235
Components That Do Not Contribute in presbyopia
3- Presumably, the increased muscle force is necessary to overcome
the increased choroidal antagonistic restoring force. Such an
increase in force/innervation to the ciliary muscle should produce
an increase in the response AC/A ratio with age in this time period.
Indeed, the response AC/A ratio appears to increase by up to 50%
from age 20 to age 45 years, after which it is too unreliable ‫غير جدير‬
to measure because of the small or even absent changes in it with
increased age.
236
Analysis of the Model Components of Presbyopia
1- Static Components
The statuses of the seven static components (see Figure 4-8) with
age are as follows.
a. Tonic Accommodation. Tonic accommodation decreases
approximately 0.04/year , from around 1.80 D at age 20 years to
0.90D at age 50 years, for a decrease of 50%. This is probably
the result of subtle biomechanical limitations of the lens and lens
capsule in response to the constant (i.e., age independent) neural
innervation to the ciliary muscle. The aging biomechanics
progressively restrict the amount of "rounding up" ‫ التكوير‬of the
lens in response to this constant tonic neural signal.
237
Analysis of the Model Components of Presbyopia
b. Concurrent age-related reduction in pupil size
could only account for 30% of this effect. This
suggests that we become subjectively more tolerant of
slight blur and accept it as the normal with increased
age, perhaps as a blur-adaptive phenomenon.
238
c. Accommodative adaptation:
Previous studies have found that subjects can increase the
velocity of accommodation using visual exercises such as
pencil push ups, flippers, Brock strings and the like and myriad
papers have shown improvement in accommodation facility
(speed) and sufficiency (amplitude) using subjective tests
following vision training but few have objectively measured
accommodation before and after training in either normal
subjects or in patients diagnosed with accommodative
infacility (abnormally slow dynamics)...
239
Accommodation is driven either directly by blur or
indirectly by way of neural crosslinks from the vergence
system. Until now, no study has objectively measured
both accommodation and accommodative-vergence
before and after vision training and the role vergence
.modifying the speed of accommodation.might play in
240
c. Accommodative adaptation
In the present study, accommodation and accommodative-vergence
were measured with a Purkinje Eye Tracker/ optometer before and after
normal subjects trained in a flipper-like task in which the stimulus
stepped between 0 and 2.5 diopters and back for over 200 cycles. Most
subjects increased their speed of accommodation as well as their
speed of accommodative vergence. Accommodative vergence led the
accommodation response by approximately 77 ms before training and
100 ms after training and the vergence lead was most prominent in
subjects with high accommodation and vergence velocities and the
vergence leads tended to increase in conjunction with increases in
accommodation velocity.
241
Analysis of the Model Components of Presbyopia
c. Accommodative adaptation: accommodative adaptation
decreases progressively with age (0.034 per year),
becoming zero by age 55 years.". This reduction may be due to
decreased effort to accommodate , which may explain why the
stimulus AC/A ratio has been found by some to decrease slightly
with age. An alternative explanation is that it may be due to
subtle biomechanical limitations of the lens and lens capsule. The
stimulus AC/A ratio either remains relatively constant':"? or
decreases slightly with age.
242
Analysis of the Model Components of Presbyopia
As mentioned, this decrease is probably due to reduced effort to
accommodate, thus driving less accommodative convergence.
In contrast, recent results show that the response AC/A ratio
increases slightly over this same time period, changing from
approximately 3:1 at age 20 years to 5:1 at age 45 .
Beyond this age, this measurement develops a serious signal-tonoise problem and cannot be assessed reliably.
243
Analysis of the Model Components of Presbyopia
This progressive and modest ‫متواضع‬AC/A increase may reflect:
(1) a true adaptive gain change (i.e., increase) in the crosslink gain
from accommodation to vergence to compensate for the lens
system's age related reduced responsivity .
(2) an age-related increase in ciliary muscle force to compensate
for the increased stiffness of the choroid .
244
Analysis of the Model Components of Presbyopia
(3) Dynamic Contributions
The 2 dynamic accommodative components with age are as follows.
a-Latency: The latencies (i.e., reaction times) for both increasing and decreasing
accommodation slightly (around 2.5 msec /year) from 20 to 50 years of age, consistent
with other age-related reaction time measures.!" This reflects aging of the underlying
neurology, probably related to:
i- Slowing
of the decision process of the person itself
ii-or to the more basic nerve conduction velocity and efficiency .
b -Time constant: The time constant (i.e., the time to reach 63% of the final
response amplitude) remains unchanged for both increasing and decreasing
accommodation in the linear response region over this same time period.
245
Analysis of the Model Components of Presbyopia
This ‫ ما سبق‬suggests an absence of biomechanical limitations and
changes in the newer (i.e., outermost lens cortex) and still
normally responsive portions of the lens . In contrast, the
innermost cortical and nuclear layers become progressively less
responsive as they age and become compressed , the time
constant is prolonged and the peak velocity decreases because of
normal biomechanical limitations producing response saturation
effects.
246
CHAPTER SEVEN
Infants ‫الرضع‬, Toddlers ‫األطفال‬
‫الصغار‬, and Children
247
SENSITIVE PERIODS OF VISION DEVELOPMENT
I- Refractive blur can have negative effects on neural development
during the "sensitive periods of visual development.
These are ages during which abnormal sensory input causes
abnormal neurological development.
Unilateral blur produces its main effects on the parvocellular
‫صغير الخاليا‬visual system, evident anatomically at the lateral
geniculate nucleus (LGN) and cortical levels and evident
physiologically at cortical levels.
Cortical cells driven by retinal ganglion cells in eyes treated with
atropine during the first 7 months of life display reduced contrast
sensitivity, acuity, and acuity at peak sensitivity.
Animals with more severe amblyopia also had more severe
anisometropia , greater acuity and sensitivity losses , and a greater
loss of binocular neurons in the cortex.
248
parvocellular and magnocellular systems
249
The LGN has:6 layers. Cells have monocular input.
Layers alternate inputs from each of the two eyes.
The top four are parvocellular layers, two layers from each eye. Parvo (small)
LGN cells receive inputs from (small) midget ganglion cells.
The bottom two are magnocellular layers, one layer from each eye. Magno
(large) LGN cells receive inputs from (large) ganglion cells.
250
SENSITIVE PERIODS OF VISION
DEVELOPMENT
II- The effects of strabismus are similar in kind but
exaggerated in degree:
1-Fewer cells in the cortex respond to stimulation of the
deviating eye, their acuity is worse, and few binocular
neurons exist.
2-The effects of unilateral deprivation are most drastic
‫ عنيف‬and affect both parvocellular and magnocellular
systems, resulting in an elimination of binocular neurons
and marked loss of responsiveness in cortical cells
responding to stimulation of the deprived eye at all spatial
frequencies.
251
SENSITIVE PERIODS OF VISION
DEVELOPMENT
Clinicians and scientists alike have attempted to define the age
limits of :
(1) normal development of vision .
(2) amblyopia development: susceptibility of the developing
brain to damage from abnormal sensory input .
(3) reversing damage ‫العودة إلى الوضع الطبيعي‬by normalizing
the sensory input.
In general , conditions that prevent image formation produce the
earliest and most severe amblyopia with peak induction effects
during the first 18 months and waning ‫ تراجع‬sensitivity until age
8 to 10 years. Short term delays in treating unilateral, congenital
cataracts significantly reduce the prognosis to recover acuity or
binocularity.
252
SENSITIVE PERIODS OF VISION
DEVELOPMENT
III-Hanisometropia
• Surprisingly , clinicians have not demonstrated improved
outcome with earlier treatment of unilateral (anisometropic)
refractive errors .
• Clinicians suspect that children with higher anisometropia,
abnormal binocular interaction, and lower acuity who are
treated late will not recover.
• There is no consensus ‫ استنتاج‬from current clinical research that
any of these factors, alone or in combination, can predict final
acuity outcome .
• At present, there is no firm evidence to withhold ‫ منع‬treatment
for older anisometropes because the sensitive period fact, the
sensitive period for blur is poorly understood.
253
Emmetropization
Figure 30-2 shows:
1- That infants have a greater range of refractive errors than
adults.
2-Various studies have shown a gradual loss of refractive error
with age.
3- After the first few months of life , the vast majority of eyes
are emmetropic and remain so throughout middle adulthood.
254
255
Depth of focus in infants
Depth of focus, eye size , and visual acuity are interrelated so that
the perception of blur is reduced if (negative results of V.A):
1- the eye is small ,
2- visual acuity is low, and
3- the pupils are small.
Figure 30-4 is a geometrical illustration of the depth of focus.
Depth of focus = 7.03 (pupil diameter (mm) x visual acuity in
cycles per degree) Table 30-1 summarizes the depth of focus
using this equation, acuity estimates from Mayer et aI., and
estimates of pupil size from Banks. Estimates range from 2.08 D
at age 1 month to 0.27 D at age 6 months.
256
Depth of focus in infants
F=focus
PUPIL
Image
Target
Circle of blur
257
258
Spherical power in children
259
D/month
260
‫يؤجل‬
‫‪261‬‬
Area of HM
N
A
E
Plus
262
‫يؤجل‬
‫‪263‬‬
‫يؤجل‬
‫‪264‬‬
265
266
267
‫يؤجل‬
‫‪268‬‬
‫يؤجل‬
‫يؤجل‬
‫‪269‬‬
‫يؤجل‬
‫‪270‬‬
Emmetropization
The improvement in acuity is the most influential factor in the
reduction of the estimated depth of focus from birth to age 6
months. Later, guidelines are presented that suggest corrections
for refractive errors that are much greater than those predicted to
produce a just-noticeable blur .
In other words, refractive blur that occurs normally is
perceptible‫ ملموس‬to infants but not necessarily corrected by
clinicians.
271
Measurement Error in Small Eyes
The distance from the optic disk to the macula remains constant
throughout life, even during early growth when axial length is
increasing markedly. The pupillary axis is directed to a retinal point
between the fovea and the optic disk. The eye turns slightly temporal to
locate the object of regard on the fovea. With the slight temporal
deviation of the eye, the corneal reflex is displaced slightly nasally
(positive angle kappa) with respect to the center of the pupil. Early in
life, the infant’s eye may look exotropic because this angular
difference is greater for the shorter eye (Figure 30-5) With axial
elongation, the angular difference between the pupillary axis and the
visual axis of infants' eyes gradually reduces.
272
child
Angle Kappa
infant
273
Astigmatic errors may be expected when the infant is fixating the
retinoscope light, because the visual axis cuts obliquely through
the refracting surfaces of the eye. Some of the astigmatism
measured in infant up to age 18 months may naturally decrease as
the angular difference between these two axes reduces with
growth.
Retinoscopy and autorefraction often overestimate the magnitude
of hyperopia in young patients.
274
Cycloplegic Retinoscopy
Values obtained with cycloplegic retinoscopy are affected by :
1.the choice of the cycloplegic agent,
2. the depth of cycloplegia produced, and
3.the examiner's ability to neutralize only the on-axis,
4. central reflexes observed in the large pupil.
Cycloplegic measures are the best to describe the refractive
state of the eye without accommodative fluctuation, and they are
an important clinical measure. Actual measurement should only
be attempted while fixation is under control and the corneal
reflex is centered in the retinoscopy reflex.
However, these measures do not describe the actual refractive
status the child experiences in daily activities with intact
accommodation.
Additional measures should be taken prior to cycloplegia to
assess the child's ability to accommodate..
275
Near Retinoscopy
Near retinoscopy is a special noncycloplegic technique in which an
infant or child:
1. fixates a dimmed retinoscope light,
2. 50 cm distant, with
3. the fellow eye patched,
4. in a totally darkened room .
5. not the working distance, is subtracted
"Near retinoscopy" for infants and children should not be confused
with dynamic retinoscopy . The objective of "near retinoscopy" is
to give an estimate of the distance refraction and it is distinguished
from dynamic retinoscopy , the retinoscopy light by use of a nonaccommodative target. The retinoscopist achieves neutrality with
the patient fixating.
276
Near Retinoscopy
Owens et al. demonstrated that the dim beam of the retinoscope
viewed monocularly is not an effective stimulus to
accommodation, producing similar refractive estimates for a
correction factor, not the working distance, is subtracted from the
neutralizing lens to create best agreement with cycloplegic values.
Mohindra originally suggested a correction factor of 1.25 D ,
Saunders and Westall suggest 0.75D for infants and 1.0 D after age
2 years, and they report that accuracy may be improved by using
a combination of spherical and cylindrical lenses rather than
neutralizing each meridian independently using spherical lenses
only.
277
Near Retinoscopy
Figure 30-7 shows their data points from infants (mean
age 13.6 months) and children (mean age 71 months)
from one examiner using near retinoscopy compared
with another examiner using cycloplegic retinoscopy.
Acceptable agreement was found in the majority of
children.
Young age and poor confidence in the near result were
related to poor agreement between techniques. The
authors felt that near retinoscopy was a useful
procedure.
278
Near Retinoscopy
279
Dynamic Retinoscopy
Dynamic retinoscopy is recommended for all children to obtain an
objective measure of the degree of blur accepted at near and to aid
in the interpretation of accommodative and vergence skills.
Dynamic retinoscopy is also performed at near under non
cyclopleged conditions, but it differs significantly from the near
retinoscopy technique just discussed. This testing is performed
under ambient illumination ,‫ اإلضاءة المحيطة خافتة نسبيا‬using a highcontrast, detailed target ( picture ), under binocular conditions.
The goal is to quantify the child's accommodative response to near
targets.
The clinician must work hard to obtain a valid accommodative
response, using a target that the child will attend and controlling
attention by eliciting verbal (reading letters, describing picture) or
kinesthetic ("touch the picture") feedback.
280
Dynamic Retinoscopy
1-In the monocular estimate method (M.E.M), the clinician
estimates the power of the lens needed to neutralize the "with" or
"against" motion and then quickly introduces a neutralizing lens in
front of one eye. The clinician must be able to arrive at the correct
lens power needed for neutralization of the reflex using a series of
lenses introduced so briefly that the child's accommodation is not
disturbed or changed during the measure.
2-In the Nott (dynamic retinoscopy) method, neutrality is
achieved by moving the retinoscope aperture to the near point with
the patient fixating a near object .
Although the purpose of cycloplegic retinoscopy is to estimate the
refractive error obtained under cycloplegia , the purpose of
dynamic retinoscopy is to quantify the error in accommodation
that the child accepts for a near target.
281
Static Retinoscopy
Static retinoscopy differs from the rest because the child
is asked to fixate a high-contrast , detailed target presented
at distance under binocular conditions. Children with either
myopia or large amounts of astigmatism usually respond with true
"resting" levels of accommodation, and there is little difference
between a value obtained with or without cycloplegia.
Most hyperopes respond with a "habitual" level of
accommodation that is greater than zero, creating differences
between static and cycloplegic retinoscopic measures.
Static retinoscopy is the preferred technique for older children who
are capable of stable, distance fixation is easier, because:
1- stable accommodative responses are hidden by cycloplegia, and
2- children who are ready for static refraction are also likely to be
able to participate in subjective techniques and respond better
without cycloplegia, because the pupils are smaller and accurate
282
retinoscopy
.
REFRACTIVE MANAGEMENT OF INFANTS AND
TODDLERS (BIRTH TO 3 YEARS) Normal Limits
Certain types and amounts of refractive error are
normal and may be transient or unstable in infant
eyes. Refractive correction should only be considered
for stable refractive errors of abnormal degree.
1- The first year of life is a period of rapid change in
growth and refraction. Saunders et al. have shown that
emmetropization occurred more rapidly in infants and
toddlers with initially higher refractive errors .
Significant refractive errors should be monitored at
least every 3 months during the first year and
generally should not be corrected until proven stable.
283
REFRACTIVE MANAGEMENT OF INFANTS AND
TODDLERS (BIRTH TO 3 YEARS) Normal Limits
2- By age 12 months, infants average no more
than 2.0D of hyperopia, and 95% of infants
have spherical equivalent values between +3 and
more than +8 D.
284
REFRACTIVE MANAGEMENT OF INFANTS AND
TODDLERS (BIRTH TO 3 YEARS) Normal Limits
3- between 1 and 3 years :
a - Astigmatism is common and gradually reduces.
b - Anisometropia of 1.0 to 2.0 D can be transient during
the early years unless the refractive error of the more
ametropic eye is at least 3.0 D.
c - Myopia may be present in infants and toddlers, but it
disappears by the preschool years in the great majority of
normal children. Myopia is rare in preschool or early
school-age children unless there is:
i. a history of premature birth,
ii. neurodevelopmental delay,
iii. or a family history of degenerative myopia.
285
Common Reasons to Prescribe Glasses
During Infancy
1-Most babies and toddlers that wear spectacles do so to
alleviate esotropia.
2-Other babies may require refractive correction
secondary to an ocular problem affecting refractive
development, such as
(a)congenital cataract or Leber's congenital amaurosis,
(b) a neurodevelopmental disorder, such as retinopathy of
prematurity or Down’s syndrome that may be
accompanied by abnormal refractive error .
(c) inherited refractive errors that present early.
3- Otherwise, correction could be established for an
abnormal refractive error that is detected early in life .
286
Retinoscopy in Infants
Static refraction (that refractive error measured with
accommodation at rest ) cannot be ensured by efforts to
sustain fixation to distant targets when the patient is an
infant or toddler. The ideal measurement comes from a
well-cyclopleged , nonsleepy baby who quietly fixates
the retinoscope light. Worst is a crying, squirming
‫يرتبكون‬infants with poor cycloplegia. One of the
strongest argument for complete eye examinations at
age 6 months versus 12 or 18 months is that the younger
baby's behavior is easier to control.
Older babies are more distractible ‫متعب‬- ‫ مشتت‬and more
frightened by the drops and examination procedures.
287
Retinoscopy in Infants
Two major methods of retinoscopy have been discussed
for use in determining the distance refraction of
infants.
They are:
1- near retinoscopy and
2- cycloplegic retinoscopy.
Accommodative responses observed in the clinical
population may be more variable because of the
increased prevalence of ocular and developmental
disorders in babies who are brought for eye
examinations.
288
Retinoscopy in Infants
In clinical settings, near retinoscopy is best suited to
screen for :
1- higher refractive errors that should be quantified with
cycloplegic methods .
2- for follow-up examinations in children whose
cycloplegic value is already known .
3- or in those unusual circumstances in which parents
refuse cycloplegia and no other option exists.
Prescription based on near-retinoscopy measures is not
currently advised .
289
Cycloplegia
Cycloplegia is recommended on a routine basis to
determine the refractive state in infants and toddlers.
Administration of drops is easier in younger infants,
who are not initially frightened and who seem to have
lower corneal sensitivity.
Initial use of an anesthetic drop is not a routine
procedure at this age. Cyclopentolate is currently the
drug of first choice for routine cycloplegia in infants and
children, using the 0.5% concentration for infants 12
months or less, and the 1% concentration thereafter .
290
Cycloplegia
Cyclopentolate results in only a moderate and
variable dilation, so it should be combined with another
agent, such as tropicamide or phenylephrine, to
promote examination of the retina.
Full-term infants under 12 months of age can be
dilated and cyclopleged with a combination drop
yielding concentrations of 0.5% tropicamide, 0.5%
cyclopentolate, and 2.5% phenylephrine. One drop of
this combination produces dilation equivalent to each of
the medications delivered individually (six drops to each
infant) .
291
Cycloplegia
The combination drop did not cause tachycardia or
increased blood pressure in neonates weighing 1769 to
3800 g. , postconceptional age ‫ بعد الوالدة‬36 to 38 weeks.
In this study of neonates, the average pupil size was
• 4 mm following 1% cyclopentolate alone,
• 5 mm using 1% tropicamide alone, and
• 7 mm following one drop of the combination drop.
The upset and risk to the infant is obviously
minimized by the one-drop encounter .‫في المرة‬
292
How to avoid overdosing infants?
1-Depression of the punctum to reduce systemic
adsorption of the medications .
2- Another option is to tip ‫ يميل‬the infant's head so that
excess medication flows laterally.
3- and to wipe away ‫ تنشيف‬the excess with a tissue.
The most common error in administration is using too
many drops, because too little seems to contact the
ocular surface in an upset infant or toddler . With
proper rapport ‫عالقة‬and gentle restraint , ‫ كبح‬the first
eye can usually be dropped without difficulty, and the
second eye should be dropped quickly .
293
How to avoid overdosing infants?
Only one drop should be expressed per encounter, and one
or two encounters are preferred.
Infants, especially darkly pigmented infants, may often
not be completely cyclopleged with this conservative ‫محافظ‬
approach. The clinician must decide when it is both
safe and necessary to deviate from this rule of thumb ‫حكم‬
‫التجربة‬
294
Cycloplegia in children
Babies aged 12 months and older can be dilated and
cyclopleged with a combination solution yielding 1%
tropicamide and 1% cyclopentolate prepared by a
compounding pharmacy.
This can be repeated with another drop for additional
cycloplegia, especially for babies with dark irides, or
boosted with 2.5% phenylephrine if additional dilation
than cycloplegia . The is clinically more important
efficacy of spacing the drops by 5 minutes has recently
been questioned, yielded equivalent dilation and
retinoscopy results across a broad pediatric age range.
295
Partial Refractive Corrections
Partial corrections of refractive error are generally
recommended for infants and toddlers because of the
greater range of refractive error that is normally present
, because there may be harm associated with removing
blur completely, and because measurements are often less
valid and reliable.
A good clinical guide for undercorrection is to leave
residual error that is within normal limits for same-aged
children.
296
REFRACTIVE MANAGEMENT OF PRESCHOOL
CHILDREN ( 3 TO 5 YEARS) Normal Limits
Three years is the approximate age limit by which most of the
large and transient refractive errors of infancy have disappeared.
1- Astigmatism : there is a striking decrease in the prevalence of
astigmatism. By this age:
a. astigmatism of 1.50 D or greater should generally be corrected .
b. lesser amounts may be considered for correction based on the
child's function and any accompanying refractive error .
2- Anisometropia : detected during the preschool years may be
stable even if the refractive error is low . Stable anisometropia of
1.0 D or more should be corrected, unless equal, normal corrected
acuity can be demonstrated in the office. .
297
REFRACTIVE MANAGEMENT OF PRESCHOOL
CHILDREN ( 3 TO 5 YEARS) Normal Limits
3- Myopia is rare and should raise suspicion of
another neurodevelopmental or ocular anomaly in the
absence of a positive family history of degenerative
myopia.
By this age, parents can be told that there is little
likelihood that the child will outgrow an abnormal
refractive error, although there is still some hope that
it may decrease .
298
Common Reasons to Prescribe Glasses at Preschool Age
1-The most common problem that initiates a prescription
for spectacles around age 3 years is accommodative
esotropia .
2- Other children may have a refractive error that is first
detected by some type of vision screening . Those errors
including moderate to high myopia or myopic
astigmatism have the priority of correction.
Parents may suspect a vision problem because of :
1- unusual visual behavior or
2- delayed development of fine motor skills.
Preschool children may begin to wear corrections for lesser
degrees
of refractive error to enhance their visual performance
299
.
Detection and Vision Screening
Almost 50% of children with amblyopia (most without apparent
strabismus) are detected after school entry . The reasons are
varied, but studies suggest that underscreening and under referral
in the primary care setting are common.
Wasserman et al. studied the vision screening and referral
practices for preschoolers in 102 pediatric practices throughout
the United States and Puerto Rico. During the study, vision
screening was only attempted on 38% of 3-year-old children.
The most common outcome of a failed vision screening was a
repeat screen in the pediatrician's office 1 year later.
Only 26% of children failing vision screening (uncooperative)
were referred to an eye professional, and only 50% of parents
knew the results of the vision screening performed in the
pediatrician's office when contacted by telephone 2 months later.
300
Reliability of ‫دقة‬Retinoscopy (3.5 to 5 years)
By this age, the eye is larger, angle kappa is decreased,
and there is less possibility that astigmatic errors can be
attributed to measurement error retest repeatability of
retinoscopy in the horizontal meridian without
cycloplegia in Asian kindergarten ‫ روضة أطفال‬children
aged 3.5 to 5 years who wore +1.50D fogging lenses is
high for experienced retinoscopists .
The largest difference was ±0.50 D, and 95% of
measures lie between +0.53 and -0.50 D. Children aged
3.5 to 5 years were recruited ‫جندت‬from a preschool, and
refractive errors ranged from + 1.25D to -1. 50 D.
301
Retinoscopy ‫دقة‬of Reliability
Fixation is easier to control in preschool children who have
developed some language skills, and stable fixation reduces
measurement error from off-axis sampling .
A switch-operated toy, a projected picture slide, or cartoon
movies are all in common use for distance fixation.
Traditional static retinoscopy and even cycloplegic retinoscopy
is still the dominant technique for refractive measurement of
clinic patients. The well-cyclopleged child should fixate the
retinoscope light for good, on-axis measures, and distant targets
should only be used if cycloplegia is incomplete refraction may
be possible for some preschoolers .
302
Retinoscopy without cycloplegia
Measures without cycloplegia should generally be used as
complementary information ‫ معلومات تكميلية‬. Retinoscopy without
cycloplegia may be indicated for:1.screening,
2.follow-up visits,
3.to assess accommodative responses (Dynamic).
4.and as the method of choice once an abnormal error has
been ruled out ‫ استبعد‬with cycloplegia.
303
Retinoscopy without cycloplegia
Although some preschool children may be capable of
subjective testing , the clinician should use the most
sophisticated responses the child can supply and be
prepared to go to alternative procedures if needed .
At this age, the clinician may be able to demonstrate
improved acuity resulting from a refractive correction
immediately .
304
Cycloplegia in children
Children of preschool age are sufficiently large that
one to two drops of 1% cyclopentolate may be used
.
A single encounter combination drop containing 1%
cyclopentolate and 1% tropicamide can be prepared
at a compounding pharmacy, delivered in drop or
spray form, and administered once or twice.
Spray administration is helpful at this age, when the
child is too large for the restraint sometimes
required for drops.
305
Partial Corrections
Although the rate of development of the anterior
structures has slowed, the axial elongation is still occurring in all
eyes between 3 and 5 years. Partial corrections should still be
considered at this age, so that emmetropization is not
unintentionally ‫ ال تعوق‬with later age.
The intended outcome of spectacle wear is:
•-to alleviate amblyopia: at (3-5 years) dominated by efforts,
•-amblyopia prevention: compared with the infants'
intended outcome of amblyopia prevention.
306
Benefits and Prognosis of Spectacle Correction
Refractive amblyopia can be detected by acuity measures
at this age, appearing sometime between ages 12 months
and 4 years.
1- By preschool age, the response to acuity testing using
the "good" eye serves as an indicator of the child's
readiness for testing using a particular method,
2- an acuity deficit in the amblyopic eye can be
appreciated by the clinician.
3- Treatment of refractive errors in children at age 4
years cuts the risk of permanently reduced vision
(amblyopia) in half compared with children first treated
307
)1: 2 ) at age 7 years
Benefits and Prognosis of Spectacle
Correction
4- Late detection of defective vision children ,
2.86% had reduced acuity (amblyopia), compared
with 1.33% ( 1:2.2 ) of children who were detected
early detection at age 4 years has better results and
received all possible treatment.
5- Anisometropia detection andtreatment
(strabismics excluded) was seen in the majority of
cases with permanently reduced acuity (amblyopia)
in both age groups, despite the prescription of
spectacles and patching.
308
REFRACTIVE MANAGEMENT OFYOUNG CHILDREN
(5 TO 7 YEARS)
Normal Limits
Studies of the refractive status of entire pediatric populations
show a striking preponderance ‫ رجحان‬of emmetropic
eyes at this age, with very few children having refractive
errors greater than 1D hyperopia, and very few with
myopia of any degree.
Common Reasons to Prescribe Glasses
The most common reason children are first prescribed glasses at
this age is for an "old" problem that is first detected at vision
screening performed in elementary school . Often, glasses are for
hyperopia, astigmatism, or anisometropia that has been present
for some time. Studies of Asian children, who start formal
schooling at age 3 years, show that 12% have already
developed"school myopia" by age 6 years. In contrast, about 1%
to 2% of American children are myopic at school entry.
309
Detection and Vision Screening
It is ironic ‫ ساخر‬that this age of peak refractive development
is the age at which most amblyopiagenic refractive errors are
first detected. This is because elementary school is the first
opportunity for mass screening in the United States.
The quality of school-based vision screening is highly variable
and often depends on the commitment ‫التزام‬of volunteer parent
organizations to organize and provide acuity screening.
310
Behavioral Management
Younger children are usually eager to please ‫ متلهف إلرضاء االخرين‬,
but they may not know what to expect. Anxiety is sensed in a
younger child, the clinician should briefly explain that the eye
examination involves looking at letters and pictures and letting the
doctor shine lights in the eyes.
If the child is still anxious, the clinician should ask what the child
thinks the eye examination involves. Some younger children may
have been "prepared for the worst" by their peers ‫أقرانهم‬or siblings,
for example, having been told that their eyes would be removed
and then fixed. Other children enter with memories of past painful
or upsetting procedures and expect the same from you .
311
The clinician will obtain better compliance in the initial and
subsequent examinations if some effort is spent determining
the source of any fear and addressing it honestly.
The child enters fearful about drops, tell the child "We may not
‫صلة‬need drops" and get to work on tests that gain rapport
such as stereopsis , color vision , and acuity . Consider using a
spray or prescribing atropine for home use.
Anesthetic drops are recommended at this age. Younger
children will definitely remember how dilating drops felt without
anesthesia , and follow-up examinations may be unnecessarily
difficult if they associate the eye examination with pain.
312
Behavioral Management
Younger children are at a transition between requiring simplified
instructions and vocabulary ‫ مفردات اللغة‬and being able to respond
in adult-like manner to routine instructions.
Work with the child on the most sophisticated level possible for
testing purposes. Require that the child proves" that he or she is
paying attention to your distant or near target by insisting on
feedback such as reading letters or providing a verbal description
of a symbol or a sticker used for near fixation..
This is an age ( 5-7 years ) when full correction of a refractive
error determined with or without cycloplegia could be given.
313
Full Correction
Refractive errors that remain by t his age5-7 years, when
the vast majority of children have achieved emmetropia,
are not likely to go away naturally.(not removed without
correction)
1-There is less concern‫ أقل قلق‬that full correction for each meridian
and eye will interfere with any beneficial process.
2-Full correction of a hyperopic spherical component is usually
unnecessary without an esodeviation.
3- There is no known contraindication ‫ مانع‬of full correction for
children aged 5 to 7 years.
4- Adaptation to full correction of any refractive error, including
astigmatism or anisometropia, is rarely problematic at this age.
314
P1413
Benefits and Prognosis of Spectacle Correction
Clinically, the end of the critical period is signaled when the
new appearance of an amblyopiagenic factor no longer results
in amblyopia. Keech and studied acuity outcome following
primarily deprivation (46 cases) or strabismus (18 cases).
Their results show that , during this age period (5-7 years), an
important transition takes place whereby the child's sensitivity to
amblyopia development rapidly declines to zero (Figure 30-11).
Given normal vision during the critical period, normal corrected
vision is expected for strabismus or high refractive errors that
first present at age 7 years or later.
Failure to achieve normal vision following treatment for
conditions that first present after age 6 or 7 years should lead to
a search for an organic cause for reduced vision.
315
Improvement in visual responses on acuity and binocular
measures can usually be documented during the initial
examination.
1-Young myopes should demonstrate improved distance acuity
with a myopic correction in the trial frame.
2- Hyperopic children with esophoria should respond with less
esophoria through plus lenses in the trial frame. The effects of
refractive interventions are more readily demonstrable because
newly acquired refractive problems do not cause regression
of visual functions that have already developed.
Figure 30-11
Number of patients and their ages at the onset of deprivation or
strabismus. The risk of amblyopia is nearly absolute up to age
40 months and is negligible by age7 years. Newly acquired
refractive problems do not cause regression of visual functions
that have already developed.
316
REFRACTIVE MANAGEMENT OF OLDER CHILDREN (8 TO 12 YEARS(
Refractive Characteristics of Older Children
1- After the early elementary school years, most children remain
emmetropic;
2- however, ‫ مذهل‬a staggering 25% of school children in U.S
develop myopia before the end of their formal schooling. ‫التعليم‬
‫ الرسمي‬In Asian countries, the vast majority of children develop
myopia during this same period.
3- New astigmatism and anisometropia are usually related to the
development of myopia.
4- Hyperopia, hyperopic anisometropia, and hyperopic
astigmatism do not develop anew ‫ من جديد‬at this age.
5-Smaller refractive errors may be corrected for the first time,
because they are associated with symptoms of asthenopia,
focusing problems, and visual fatigue as visual demands
increase.
317
Refractive Techniques In children (8 TO 12 YEARS)
Technical determination of refractive corrections hould undergo a
major shift in older children. Behavioral technique are preferred to
cycloplegia for accurate refractive correction in older children.
Cycloplegic values may still be important but play a complementary
role. This is the opposite of what has been suggested for younger
‫اسباب سهولة فحص البصريات في هذا السن‬children. In part, this shift occurs
because :
(a) control of attention and accommodation to a distant target can
now be obtained,
(b) the accuracy of retinoscopy is improved with smaller pupils,
(c) myopia, now the most common refractive error, is more accurately
determined by skillful optometrist through static refraction.
(d) "habitual" or ‫"الكميات المعتادة‬favored" amounts of accommodation are
revealed by skillful static refraction, and
(e) adaptation problems ‫التعود على النظارة‬occur if habitual or preferred
amounts of accommodation are ignored in older children.
318
Many clinicians have heard that myopic children may
be "overminussed" during subjective procedures.
New data show this is more likely due to insensitivity
to blur produced by negative lenses rather than overaccommodation.
This view is consistent with data showing that similar
refractive values are obtained in young, myopic
children with subjective procedures prior to
cycloplegia or autorefraction after cycloplegia.
319
Excessive or unstable accommodation during refraction
can be suspected in anisometropes , and in uncorrected
hyperopes, following lengthy refractive procedures, or as
an undesired response to fogging lenses. Excessive or
unstable accommodation ‫أسباب‬
1-One common reason for unstable accommodative levels
is fatigue. ‫التعب من طول مدة الفحص‬Clinicians need to develop
quick retinoscopy and subjective techniques, so that all
measures are obtained in both eyes before fatigue affects
the measures.
2-Another common reason that accommodative responses
differ from one eye to the next is anisometropia.
320
If anisometropia is detected during retinoscopy, the clinician
should decide first whether unstable accommodation is a
problem before attempting subjective procedures.
A number of techniques are possible, including :‫كيفية االكتشاف‬
1- Watching for changes in the retinoscopic reflex or
pupil size,
2- or sweeping the beam across both eyes and
observing the stability of the refractive difference.
3-Sometimes, retinoscopy should be repeated with
lower degrees of initial plus. Some children react to
"fogging lenses with powerful lenses" with excessive
and unstable accommodation.
321
Solution For Accommodative Imbalance
1- Once a believable retinoscopy result is obtained, binocular
refractive techniques should be used whenever accommodative
imbalance is suspected.
Performing a binocular technique is always a good option and
could be used routinely if the clinician sets up an efficient
system to dissociate the images between the two eyes.
In a binocular refraction, the accommodative level is
equalized between the eyes by simultaneous view of a target,
which is seen by both eyes or which appears whole when both
eyes are opened.
Usually, this is a border around a group of letters or symbols.
The test targets, individual letters or symbols, are seen
in only one eye. Separation of targets is most commonly
achieved with a septum.
322
Solution For Accommodative Imbalance
323
Solution For Accommodative Imbalance
2- An alternative to binocular retinoscopy and refraction is to
perform monocular refraction with each eye occluded in turn.
This technique should be followed by a balance procedure.
3- Older children can respond to routine procedures in which
vertical prism in the phoropter is used to dissociate the
images of the two eyes. Starting with the image slightly in
front of the retina (fogged by about 0.25 D), attention is
directed to each image in turn, and small lens changes are
introduced to blur the clearer image until images in each eye
are perceived as equally blurry .
324
Solution For Accommodative Imbalance
In this case, the clinician could choose to:(a) recheck the retinoscopy and reattempt the balance using
the sphere revealed by retinoscopy as the starting point,
(b) recheck the final subjective sphere in each eye, making
sure that an additional 0.25 D plus reduces acuity or
(c) repeat retinoscopy after cycloplegia and use the
difference between the eyes determined with pharmacological
control of focusing to determine the difference in the
prescription between left and right spectacle lenses.
325
Cycloplegic Retinoscopy
Cycloplegic retinoscopy should occasionally be performed in
selected older patients. Patients with :
1. anisometropia,
2. latent hyperopia,
3. higher hyperopia, or
4. accommodative esotropia.
All may exhibit clinically significant differences between
measures taken with and without cycloplegia.(Pre and Post).
Cyclopentolate (1%) is still the best suggested for older, lightly
pigmented children. The 0.5% preparation is ineffective in
many hyperopic African Americans, so it is not recommended
for routine use in these patients.
326
Cycloplegic Retinoscopy
An ideal option for many older children is to perform a retinoscopic
measure following instillation of one or two drops of 1% tropicamide
used primarily for dilation, provided that the older child does not
exhibit the indications given earlier for a stronger cycloplegic drops.
Egashira et al. have shown that equivalent distance autorefraction
measures(Results) are obtained in cooperative 6 to 12-year-old
children whether they receive 1% cyclopentolate or 1% tropicamide.
Equivalence in results is found for children with as much as 4.50 D
hyperopia.This information would be used as baseline information or
to compare against the results of static refraction to identify children in
whom cyclopentolate might be necessary.
327
Partial Correction for Adaptation Purposes)8-12 years old (
1- Little change is occurring in the refracting elements of the
eye in older children except myopes, whose axial length is still
increasing. Undercorrecting for the purpose of maintaining
emmetropization mechanisms intact is no longer indicated..
2- Older hyperopic children without esodeviations rarely require
prescription of the full hyperopic error correction using
cycloplegia and vise versa.
3- Often, the clinician can rely on the static refraction and
prescribe the maximum plus that allows best distance acuity
without cycloplegia.
4-Static refraction performed at follow-up often reveals more
hyperopia. This may indicate a change in the resting or
preferred amount of accommodation, and the child may
function better if the prescription is changed.
328
Building up plus, )Plus Overcorrection (
Gradually increasing the amount of hyperopic prescription,
should usually be reserved for children in these cases :
1. with persisting symptoms of asthenopia,
2. decreased binocular function,
3. improved acuity with a stronger plus prescription.
4. If an esodeviation is present, the correction usually is
determined as that amount which provides optimal
binocular performance at distance and near. This may
require a bifocal prescription .
329
Astigmatic prescriptions refractive correction of
astigmatic blur
This must be obtained optically, because compensatory
changes in viewing distance or accommodation by the child
does not clear astigmatic focus.
1-The first issue to decide is whether the child has amblyopia.
2- If so, the clinician has little choice, but to prescribe best
sensory correction because precise optical correction provides
the basis for any possible acuity improvement. Unfortunately,
meridional amblyopes are generally without complaint until they
are told to wear glasses.
330
Astigmatic prescriptions refractive correction of
astigmatic blur
3- They may not perceive much visual benefit from the optical
correction, and the full astigmatic correction may cause
complaints of:
1)asthenopia,
2)vestibular symptoms, of body equilibrium ‫أعراض الدهليزي‬
3)or headaches in a previously asymptomatic child.
Ideally, the clinician can obtain compliance with the full
astigmatic correction by forewarning ‫ إنذار مسبق‬the parent and
child about possible difficulties with adaptation, reassuring both
that vision with the glasses will be comfortable within hours to
days of full-time wear, and letting them know that no alternative
therapy will improve vision. This can be a difficult situation,
because the drawbacks ‫عيوب تظهر بسرعة والمنافع تظهر ببطء‬are
apparent immediately but the benefits are not.
331
4- Children who cannot comply with spectacle wear may
fare better ‫ يحقق نتائج أفضل‬with contact lenses if this is a
practical option for the family.
5- Otherwise, the astigmatic correction may have to be
reduced to obtain compliance (undercorrected).
The goal should be eventual wear if the child does not
have amblyopia, he or she will not acquire it at this age, so
there is no harm associated with wearing less than full
astigmatic correction. However, most children prefer sharp
vision and are not of the full correction in amblyopic
children.
332
Nonstrabismic Binocular Vision Disorders Significance
Older children usually present to an eye examination
because they are conscious of visual difficulty in the
classroom, or because a parent or teacher wants to rule
out an ‫ استبعاد عين‬or vision problem as a possible cause of
poor acadmic performance. Children of this age spend
more than 50% of their school day performing tasks at
near (mostly reading and writing at the desk) and an
additional 25% on tasks requiring alternation of near and
distance fixation, such as copying from the board or
watching demonstrations. Common complaints in older
children with vergence or accommodative problems are:
1-intermittent blur, 2- headache,3- asthenopia,4- fatigue,
5-poor attention, or 6- avoidance of near activities.
333
• Benefits of Refractive Prescription
The visual benefits of spectacle prescription are easily
demonstrated at this age by :
1- improved acuity,
2- resolution of symptoms (asthenopia, focusing problems,
excessive fatigue at near), or
3- objective measures showing improved accommodative
and vergence skills.
334
•
‫تقييم النتائج‬Outcome Issues
Alarge -scale study of school age children in the United Kingdom
showed that :
1- Roughly half of 10 year-old children with reduced distance or near
acuity had spectacles .
2- On average , two thirds of children who had been prescribed
spectacles could produce them ‫حصلوا عليها أثناء الفحص المدرسي‬at the
"school medical examination.
3- This percentage varied with social class, from 70% in the highest
income group to 48% in the lowest.
4- Children who had good binocular acuity without correction were
less likely to have their glasses (37%) than children with more severe
acuity deficit (73%).
335
•
‫تقييم النتائج‬Outcome Issues
5- Further study revealed that children with minor visual
defects had similar educational attainment scores whether
or not they had been prescribed spectacles.
6- Presumptive myopes (defined by distance acuity reduction)
had higher I.Q and higher reading scores even after adjusting
for I.Q, whether or not they had been prescribed spectacles.
7-Presumptive ‫ داع لإلفتراض‬hyperopes (reduced acuity at near
only) had lower scores than presumptive myopes, whether or
not spectacles had been prescribed.
8- No advantage in educational achievement could be found
following spectacle prescription in presumptive myopes whose
distance acuity without correction was 20/60 or better in the
better eye, or for presumptive hyperopes with normal distance
acuity and near acuity of 20/30 or worse in one or both eyes.
336
•
‫تقييم النتائج‬Outcome Issues
These authors concluded that "the clinical view that minor
degrees of myopia can interfere with learning to a significant
degree is therefore very hard to reconcile ‫ تقدر‬with test data.
It seems difficult to justify screening for these minor defects or
treating them on educational grounds. Clinicians who may
disagree with these conclusions have not yet countered this
argument with data ‫مشادة مع البيانات‬either refuting‫ دحض‬that
conclusion or detailing other benefits of spectacle wear.
Such studies may become essential in the future health care
environment.
337
Nonvisual Aspects of Compliance in Older Children
Older children usually enter with strong preformed opinions, either
for or against spectacle wear. At this age, compliance requires more
of a balanced responsibility between parent and child. The clinician
must involve both parties in discussions about spectacle wear.
There is no danger of amblyopia developing if new refractive errors
are not corrected. On the other hand, failure to wear glasses may
result in:
1- reduced acuity , 2- persistence of symptoms or
amblyopia, 3- avoidance of near or sports activities requiring good
vision, or
4- continued binocular dysfunction.
As older children become more self- conscious dysfunction ,
vision dysfunction about their physical appearance , contact lens
wear is frequently discussed.
338
Therapy for Refractive Amblyopia in Older Children
If amblyopia is newly detected (8-12 years old) this late, compliance
with patching regimens and spectacle corrections will prove difficult.
Individual doctors may deny treatment because they judge that
prognosis and compliance will be too poor. New information from a
national , multicenter clinical trial of amblyopia treatment in children
aged 7 to 17 years shows that :
A) Spectacles alone improved acuity in 25% of amblyopic children at all
ages studied, and
B) Approximately half (50%) of children aged 7 to 12 years benefit from
additional amblyopia treatment with included :
1- part-time patching,
2- near activities,
3- and atropine in sound eye.
Therapy for Refractive Amblyopia in Older Children
1- A surprisingly high percent of children (40%) were prescribed
glasses for the first time.
2- Older children who were treated with part-time patching were more
likely to improve if they had no previous treatment (47%)
3- And less to improve if they were treated previously (16%).
This information provides a strong evidence base to advise
parents and children that :
(1) refractive correction should be attempted for all ages.
(2) amblyopia therapy should be initiated at all ages, and
(3 ) amblyopia therapy could be continued or resumed until age 12
years, but the odds ‫ إحتمال‬of improving afterwards are low.
340
REFRACTIVE MANAGEMENT OF HYPEROPIA
Figure 30-12 summarizes the management of hyperopic refractive
error in Upper Normal Limits and Course
New cross-sectional data (Figure 30-13) taken in infants and preschool
children after cycloplegia using 1% cyclopentolate shows that
equivalent refractive estimates :spherical equivalent
(1) are most variable during the first 12 and
(2) decrease gradually from a mean value of approximately 1.50 D at
12 months to 1.0 D at 48 months.
(3) After age12 months, only 2.5% of children in this sample had
hyperopia exceeding 3.0 D and only 0.5% exceed 3.75D.
341
≥more or =
342
Figure 30-13
Spherical equivalent of the right eye in a cross sectional sample of 514
infants and children measured after cycloplegia with 1% cyclopentolate.
(Reproduced with permission from Mayer LD, Hansen RM, Moore BD, et aI,
2001. Cycloplegic refractions in healthy children aged 1 through 48 months.
Arch Ophthalmol
343
Therapy for Refractive Amblyopia in Older Children
Although older studies report slightly higher values of hyperopia, all
studies agree that after 12 months, low hyperopia is expected and
reduces very slowly during childhood . Newer studies summarized by
Mayer et al. indicate mean spherical equivalent hyperopia around :
• 0.75 D at age 6 years decreasing to
• about 0.50 D by age 10 years.
Exceptions to this rule of gradual reduction in hyperopia may occur if:1- the child is or becomes esotropic: Increasing hyperopia during
infancy is a stronger risk factor for strabismus and amblyopia (15
times the risk) than is a single measure of abnormal hyperopia.‫بدون تغيير‬
2- another exception to the expected gradual reduction occurs for
higher hyperopias: If the hyperopia of the most ametropic meridian is
2.50D
or greater by age 1 year, future changes are unpredictable.
344
Esotropia Correction (8-12 years):
A trial correction of hyperopic refractive error of +2.50 D or more is the
current standard of care for esotropes prior to considering surgery.
1-Spectacle correction of fully refractive and accommodative esotropia
is the standard of care.
2- Partially accommodative esotropes, those who exhibit an
esodeviation while wearing a maximum hyperopic correction, are
treated using a combined surgical and refractive approach.
Rethy has emphasized the importance of serial cycloplegic refraction,
the high frequency of increasing hyperopia gradually revealed in
esotropes, and the decreased need for surgery following an aggressive
(under atropine cycloplegia) refractive approach to ensure that a
maximum hyperopic prescription has been determined.
3- Recently, an analysis of claims data from two hospitals showed a
58% decline in muscle surgery for strabismus that was attributed to
increased prescription of full plus correction. )N.B(
4- New information suggests that if treatment for a constant )squint(
misalignment is delayed 4 months or more, the prognosis to recover
normal binocularity is greatly reduced , lending new urgency to develop
efficient referral and treatment networks.‫جراحيا‬
345
High Hyperopia Correction (8-12 years):
Several studies of clinical populations of isohyperopic children agree
that the risk for and depth of amblyopia increase with increasing
hyperopic blur, and that amounts of 5.0 D or more are usually
associated with amblyopia .
Isohyperopia is a bilateral hyperopia without anisometropia, also called
isometropic hyperopia. Most practitioners prescribe for an amount of
+5.0 D or more of hyperopia after infancy. Infants present a special case.
In the absence of an esodeviation, poor vision, or poor accommodation,
even higher amounts could be monitored at 3-month intervals for
reduction. If no reduction is seen, a partial correction may be
‫كافية‬prudent.
Surprisingly, the age at initial correction of isohyperopia is a weak
predictor of final acuity, measured months to years after its initial
correction.
346
347
Borderline Amounts
1- Correcting refractive errors less than 5D, or even less than the
upper normal limit (3D), is not always necessary.
Because borderline and lower amounts are frequently associated
with good visual function without intervention , the clinician usually
searches for other factors to decide whether or not a spectacle
prescription is warranted. ‫مرغوبة‬
2-The presence of amblyopia indicates a need for full-time wear of
best sensory correction. ‫استثناء‬
3-Older children are more likely to exhibit reduced distance or near
acuity that responds immediately to refractive correction. In these
situations, refractive correction is well advised .‫استثناء‬
348
Nonstrabismic Esodeviations Correction (8-12 years):
If a hyperopic correction is not given, the child must
display both appropriate accommodation and
convergence to maintain single, clear binocular vision at
distance and near. If no valid estimate of the near phoria
can be made because dynamic retinoscopy reveals
inadequate focusing, refractive correction may be
indicated.
349
• Strabismic Esodeviations Correction (8-12 years)
1. Esophorias of 6DP or more are usually significant in
older children, and lesser deviations may be significant
when divergence reserves are small.
2. The Prescription for a hyperopic refractive error, or of additional
plus power for near (Bifocal) , is usually a straightforward
treatment for esodeviations.
3. Some children who had uncorrected high hyperopia throughout the
critical period demonstrate no linkage between accommodation and
convergence. N.B
4. Amount of plus for comfortable vision can usually be predicted by
the AC/A ratio and refined in the office using the trial frame.
350
Exodeviations with Defective Accommodation.
Defective accommodation can be the cause of a manifest exodeviation
that remits with treatment of the underlying
accommodative problem.
1-Children with (smaller hyperopic refractive errors) who display
deficient accommodation may be best treated with:
a. Spectacle prescription if the accommodative deficiency is secondary
to neurodevelopmental delay, low vision, or anticholinergic medication
causing cycloplegia.
b. Children who do not have such problems may be good candidates for
vision training . ) Orthoptic (
c. Spectacle correction could be advised as a second line of defense if
vision training fails.
2-Children with (significant refractive error ) can be fully corrected first
and allowed to adapt to the spectacle correction for a few weeks before
final binocular diagnoses and treatment plans are formulated.
3- Reduced Stereopsis. Stereopsis can be reduced with hyperopia in the
presence of good visual acuity and alignment. Stereopsis may therefore
be a more sensitive indicator of the adverse effects of refractive defocus
on the developing visual system than traditional measures of visual
acuity. The presence of reduced stereopsis could be considered an
351
indication for spectacle correction of hyperopia .
4- Delayed Visual Skills. The need for a refractive prescription may be
signaled by poor development of fine motor skills or visual perceptual
performance rather than poor acuity or accommodation. The parent or
clinician may suspect that accommodation is not sustained during the
child's routine activities by behaviors such as avoidance of near
activities. Fine motor skills may be assessed in office using the Beery
Test for Visual Motor Integration (VMI), the Test of Visual Analysis Skills,
or another visual perceptual assessment tool.
Problems with overall development could be ascertained by interview
using the Profile II, by combining interview with observations of the
child's performance using the Denver Developmental Screening Test in
office, or by referral for early intervention evaluation services. Clinicians
should offer refractive correction as one option that may benefit some
children and suggest observations or measurements that will help the
family judge its effectiveness.
In general, refractive or other interventions are more likely to be
necessary in children with neurodevelopmental delay, because they are
less likely to compensate efficiently for visual or ocular problems, and
they are more likely to have such problems.
352
5- Associated Astigmatism or Anisometropia. Both of these
conditions greatly increase the need for spectacle correction,
because no compensatory accommodative response by the child can
result in clear vision for both meridians or both eyes simultaneously.
The accommodative amplitude naturally decreases with age so
that hyperopic errors that were fully compensated earlier in life may
cause symptoms by age 10 to 12 years.
Older children with a stable hyperopia may begin to complain of
asthenopia as the near demands of the upper grades
increase.
Generally, hyperopic corrections that are manifest
without cycloplegia should be :
a. prescribed to symptomatic older children, and
b. vision training should be reserved for symptoms that persist after
spectacle correction.
353
SM=Sphere myopia
SH= Sphere
hypermetropia
ARC=Anomalous retinal
correspondence
RTC=repeated
time call
354
Indications for Partial Corrections
1- Correction of the full amount of hyperopic refractive
error interferes with emmetropization and results in
higher final ametropia.
2- Recently, partial corrections for hyperopia have been
shown to allow emmetropization to proceed with final
refractive errors within normal ranges.
3- Partial corrections should be prescribed to infants
and toddlers unless they have or develop an esodeviation .
355
Exodeviations with Accurate Accommodation.
N . B : If accommodation is accurate, a hyperopic correction will
exacerbate an exodeviation . Most commonly, the clinician detects a
borderline amount of hyperopia and considers any exophoria as a
contraindication to spectacle prescription. Sometimes, an exophoric
child presents with a higher hyperopia and poor acuity or stereopsis
demanding refractive intervention.
1- A full correction may be necessary to improve acuity and
accommodative functioning and could be prescribed if the child
tolerates the correction in trial frame without exhibiting strabismus.‫نادرا‬
2- Otherwise, a partial correction along with vision training to increase
positive fusional vergences ( convergence ) and accommodative skills
would be a better initial approach. ‫أكثر احتماال‬
3-Older children often require a partial hyperopic correction to
facilitate adaptation . ‫أكثر احتماال‬
356
Indications for Full Correction of hypermetropia
1- Across the pediatric ages, prescribing the full hyperopic
error determined under cycloplegia is the most
common practice for esotropes. Possible interference with
emmetropization would be outweighed by the benefits of
binocular alignment and binocular sensory (visual)
development.
2- Many practitioners prefer to prescribe the full hyperopic
error determined under cycloplegia even without apparent
esodeviations.
3- This clinical practice would be most acceptable for
children of early school age, after emmetropization has
occurred but before problems of adaptation to hyperopic
prescriptions begin.
357
‫دواعي لبس النظارة بدوام كامل‬
Indications for Full-Time Wear
1- Full-time wear of a hyperopic correction is advised in amblyopes to
allow "catch-up" development ‫ اللحاق بالركب‬of the neural mechanisms
underlying acuity.
2- If other visually related behaviors such as fine motor skills are behind
,‫ متخلفة‬full-time wear might be advised so that the benefits of clear vision
are present during any possible learning experience.
3- This is especially important for children with low vision, in whom
refractive correction is advised as a mechanism to maximize residual
vision . )magnification(
4- If the child is developmentally delayed or mentally retarded, full-time
wear is best because compensatory mechanisms like accommodation are
often inefficient and because the indications for part-time wear cannot be
explained to the child.
5- Full-time wear is also advised if visual function is normal only when the
correction is worn. For instance, full-time wear is advised for
accommodative esotropes so that their strabismus is controlled.
6- Full-time wear would also be recommended for hyperopes with
significant esophorias .
7- Children with accommodative dysfunction that is corrected with spectacle wear
358
‫دواعي لبس النظارة بدوام جزئي‬
Indications for Part-Time Wear
Many older children wear hyperopic prescriptions part
time:
1- during the child school daily hours .
2- other periods of prolonged near work.
3- children who display normal, uncorrected acuity and
good compensatory focusing and vergence responses
over the short term.
359
‫متابعة لمضاعفات طول البصر‬Follow-up for Complicated Hyperopia
If hyperopia is complicated by another problem, the
problem causing most concern will set the recall ‫االستدعاء‬.
For instance, anisohyperopes with amblyopia would be
recalled according to their patching schedule.
1- Recall might be as often as weekly for toddlers or
preschoolers with esotropia and amblyopia .
2- or only annually ‫ سنويا‬for older children with high,
bilateral isohyperopia.
3- Esotropes should be recalled as indicated by the :
a- stability of their deviation.
b- vision-training procedures. or
c- the likelihood that their hyperopia might increase as
indicated by their age. All of these complications would
indicate the need for a shorter-than average recall interval.
4-The maximum recommended recall ‫المتابعة المتواصلة‬intervals
are360described next for simple hyperopia.
Follow-up for Simple Hyperopia
Recall for the child whose main problem is hyperopia generally is
by the age of the child. ‫ المتابعة تحدد حسب عمر الطفل‬set
1- Infants up to age 12 months show rapid changes in their hyperopia,
so any prescription should be rechecked at least every 3 months at
this age.
2- After age 12 months, the initial recall should be in 3 months, with
this interval maintained if the error has changed and lengthened if the
error is stable.
3- By 3 years of age, recall using similar logic, ‫باستخدام منطق مماثل‬with the
first interval of 3 to 6 months lengthened after stability is demonstrated.
4- After school age , hyperopia is stable, and annual intervals initially
followed with biannual intervals after several years of stability are
361
recommended
Prevention of Amblyopia and Strabismus with Early Correction of
Abnormal Hyperopia
Cross-sectional and longitudinal studies of infants and toddlers with
hyperopia have shown that a spherical equivalent hyperopia of 2.75 D or
more is strongly associated with the presence of strabismus and
amblyopia (Figure 30-14).
Ingram et al. compared cycloplegic retinoscopy at age 1 year in a
population of children living in the Kettering district of the United
Kingdom with the acuity and strabismus outcome obtained at age 3.5
years.
1- Of those with at least 3.50 D in any meridian, 48% developed
strabismus.
2- This prevalence drops ‫يقل‬with lesser hyperopia (33% develop
strabismus if 3.0 to 3.50 D hyperopia is present in any meridian) .
3- and is much less ‫ يقل أكثر‬4% develop strabismus for hyperopia of less
than 3D
4- Babies with 3.50 D or more of hyperopia were at 20 times the risk to
develop esotropia compared with children with lesser hyperopic errors.
362
REFRACTIVE MANAGEMENT OF ASTIGMATISM
Overview of Clinical Management
Figure 30-15 summarizes the management of astigmatic refractive error in
children.
1- During the first year of life, about 50% of infants have astigmatism of 1.0 D or
more according to noncycloplegic measurements, including near retinoscopy
and photorefraction. Previous studies using cycloplegia also showed high
percentages during the first 12 months that resolve ‫ بدون عالج‬by preschool age.
2- New data using keratometry and video phakometry showed the source to be
corneal and lenticular (posterior lens curvature).
3- Roughly 5% or less of healthy children with healthy eyes are astigmatic
throughout the remainder of childhood.
3- Throughout school age, most children have less than 0.50 DC astigmatism.
4- The most likely outcome of astigmatism present in infants or toddlers, even
if it is of high degree, is its disappearance before school age without
intervention.
363
PRN= permanantly
FTW=Full time
wear
RTC=recalling
364
5 - Astigmatism of 2.0 D or less can be considered normal in infants,
6- Astigmatism 0.50 D or less can be considered normal after age 3.5
years.
7-The axis of infantile astigmatism varies with the population studied.
Two recent studies (one of children in Cambridge, England and
another of American children living in California) agree that the
predominant axis of astigmatism prior to age 12 months is with the
rule (WTR). Earlier studies also showed WTR in Cambridge and in
Chinese infants.
Other studies using cycloplegic or noncycloplegic techniques have
reported predominantly against-the-rule (ATR) astigmatism.
8- All studies agree that oblique types are rare and likely to persist.
9- By school age and throughout much of adult life, WITH THE RULE
astigmatism predominates.
365
Association of Astigmatism with Other Refractive Errors
Anatomically, astigmatism results from asphericity in the anterior or refracting
structures of the eye (cornea and/or lens), whereas emmetropization occurs
from fine-tuned adjustments in axial length that may produce only spherical
errors. The association between an aspheric refracting system and an axial
length that is not matched to either major focus results in the clinical
observation that most astigmatism accompanies either myopia or hyperopia.
Some clinical researchers have speculated that persistent astigmatism may
act to derail ‫عرقلة‬the emmetropization mechanism by prohibiting the formation
of a clear image on the retina but new data in infants shows that astigmatism
is not related to changes in the spherical equivalent refractive error.
Some studies have shown more progression of myopia in astigmatic
children, but others have not.
Infants and toddlers with higher hyperopias who go on to develop strabismus
and amblyopia are more likely to have astigmatism .
366
Meridional Amblyopia
Around the same time that clinicians reported a surprisingly high prevalence
and degree of astigmatism in infants, basic researchers were beginning to
demonstrate the functional and neurological bases of amblyopia.
Because the infant cannot compensate for astigmatic blur by changes in
accommodation or viewing distance, a "natural experiment" was realized in
which the effects of blur during infancy on visual acuity development could be
investigated. Researchers were quick to note that astigmatic adults wearing full
correction, often since childhood, had reduced acuity for grating targets
presented at the anatomically blurred orientation, or "meridional amblyopia.
Clinicians do not make an axis-specific search for amblyopia, and
they usually only diagnose meridional amblyopia when Snellen acuity is
reduced and the child has astigmatism that can be assumed to have been
present during infancy and preschool years.
1-Hyperopic astigmatism can usually be assumed to be longstanding.
2-Myopic astigmatism is usually of recent development and cannot support a
diagnosis of amblyopia unless there is firm evidence the myopia or the
astigmatism was congenital and persisted throughout the early years.
367
Indications to monitor astigmatism without Intervention )without correction( :‫هام‬
1- Astigmatic blur is not a potent amblyopiagenic factor during the
first 12 months, when acuity for high spatial frequencies cannot be
demonstrated behaviorally.
2-The earliest documented case of meridional amblyopia occurred late in the
third year of life, coinciding roughly with the age at which infantile astigmatism
has disappeared in 95% of children.
3- Prescribing for stable astigmatism, in which the less ametropic meridian is
near normal, may be delayed until age 2 to 3 years, because the critical period
for refractive amblyopia is delayed compared with deprivation or strabismic
amblyopia.
4-Young age also implies that the astigmatism may be resolving naturally.
Longitudinal measures are necessary to differentiate those cases in which the
astigmatism will persist.
5- Even high amounts of astigmatism may disappear without intervention, so
the clinician must resist the tendency to prescribe based on the initial
discovery of an abnormal degree of astigmatism.
368
Indications to prescribe for astigmatism: ‫هام‬
1- First, the blur between the second and fourth year of life . Although the
visual system can detect blur within the first 12 months, the presence of blur
does not seem to result in amblyopia this early. Our current level of
knowledge, that meridional amblyopia develops between the second and
fourth year of life, leaves a wide margin of uncertainty regarding optimal
clinical management.
2- The second factor indicating a need to prescribe is the presence of oblique
astigmatism greater than 1.0 to 2.0DC that is present after the first year.
3- Third, longitudinal measures showing a failure to emmetropize (i.e.,
increasing or stable astigmatism during the first 3 years indicates an
increased risk of later amblyopia. ( In summary)Thus, one should correct
astigmatism greater than 1.50 to 2.0D, present after the first year, that is not
self correcting, or is present at an oblique axis.
369
Borderline amounts of astigmatism : ‫هام‬
1- may be prescribed if the spherical equivalent is not normal for the age of the
child. For instance, the spherical equivalent of plano /-1.00 x 180 is -0.50 D, and
most practitioners would prescribe the astigmatic correction for a school-aged
child. (cylindrical correction).
2- On the other hand, an error of +0.50 / -1.0 x180 would generate fewer
prescriptions because the spherical equivalent is plano and vision without
correction may be good.
3- If the least ametropic meridian of the eye is not in a normal range, a
prescription is usually given that incorporates the astigmatic correction. For
example, +3.0D / -1.00 x180 represents an abnormal amount of hyperopia in
both meridians, and the full astigmatic correction would usually be prescribed
provide best sensory correction. )spherocylidrical correction(
1428
370
Indications for Full Correction For Anisometropia : ‫هام‬
1- Standard of care is to fully prescribe the entire symptomatic anisometropic
difference in spherical refractive error, to create an equal accommodative
demand between the eyes.
2-Undercorrecting both eyes (sometimes) by the same amount, may be
advisable.
3- In the few reported cases wherein full anisometropic correction has been
given to nonsymptomatic adults, adaptation to spectacles was obtained within 1
week , and improvements in binocularity were measured and reported by the
patients
4- The exception would be hyperopia with esodeviation , in which case
maximum hyperopic prescription is recommended. Such children should be
given full correction and monitored every 3 months until the refractive error
has been demonstrated to be stable.
.
371
Indications for Full Correction For Anisometropia : ‫هام‬
5- The authors concluded that "these patients would have missed
the opportunity to judge for themselves the benefits of
binocularity" unless they tried refractive correction. In office
demonstrations of spectacle correction, using trial frames may not
be adequate to truly demonstrate the visual benefits and
drawbacks that will be experienced by anisometropic persons in
their daily activities.
6- When possible, correcting both eyes or only the more ametropic
eye with a disposable contact lens is a good option that reduces
the costs of the trial of refractive correction to both patients and
doctors.
372
REFRACTIVE MANAGEMENT OF MYOPIA:
Figure 30-22 summarizes the clinical management of nonpathologic myopia
in children. Children typically develop myopia during their school years and
retain good corrected vision throughout life.
1- Nonpathologic , or "simple," myopia has been characterized by normal
corrected acuity , onset during school age, progression that self-limits
around 6.0 D, and the absence of staphyloma formation or high refractive
error , or reduced vision.
2- Degenerative staphylloma myopia presents before school age with
staphyloma formation may be apparent immediately or may develop later in
life.
The major visual morbidity occurs around the fifth decade of life, with
chorioretinal degeneration in the area of staphyloma, appearance of lacquer
cracks , and loss of central vision from leakage of neovascular membranes in
the macula.
373
myopic neovascular membranes in the macula
374
lacquer cracks retina
375
Abnormal limits
M=myopia more or =
PRN =permanent use
FTW=Full time wear
RTC=recalling time
376
3- intermediate between simple and pathologic : Curtin uses a classification
system that emphasizes myopic crescent formation and includes other
ophthalmoscopic signs (supertraction ‫شد زائد على الشبكية‬, tessellation, ‫التغطية‬
‫ بالفسيفساء‬and pallor ‫ )شحوب رأس العصب‬of axial elongation to define a category of
myopia that is intermediate between simple and pathologic.
Curtin has suggested that the ophthalmoscopic signs are more reliable
indicators of excessive axial elongation than the degree of refractive error or
the axial length of the eye. If effective treatments are developed, future
clinical management may include use of ophthalmoscopic signs to predict
children who are at risk to develop ocular tissue compromise from myopia.
Or, as research efforts in various fields add to our understanding of myopia
development, clinicians may use new genetic, biochemical, or physiological
markers to guide the clinical management of myopia .
377
‫الخلل يعتمد على كمية‬
‫الفرق بين العينين‬
‫‪spherical‬‬
‫‪378‬‬
Factors Associated with the Progression of Myopia
Children may have a stable refraction for years and then develop myopia.
Sorsby cautions that axial elongation and compensatory changes in the
refracting components are probably occurring despite the apparent stability
suggested by an unchanging refractive error.
A recent study in children confirmed that ocular components measured with
ultrasound show that the eye is growing (axial length increases) while its
refraction appears stable.
Once myopia presents, higher rates of refractive change become apparent.
a. Whether this is because the limits of refractive compensation for axial
elongation have been exceeded,
b- Or whether a vision sensitive mechanism goes away , (is uncertain).
379
Factors Associated with the Progression of Myopia
1- Deficient Accommodation with myopia
Many myopic children have a higher accommodative lag for targets presented
at near, and for letters viewed monocularly through negative lenses.
Insufficient accommodation is most evident during myopia progression.
These findings, along with data from animal studies showing that eyes
exposed to chronic retinal defocus become myopic, provided the foundation
for a prospective, multicenter clinical trial comparing the progression of
myopia in 469 children wearing progressive addition lenses (PALs) versus
single vision lenses. Data from the Correction of Myopia Evaluation Trial
(COMET) show that :
A. children with good accommodation did myopia progress (Figure 30-27, B)
B. but that children with poor accommodation who wore single vision lenses
progressed significantly more (Figure 30-27,A).
C. Children with poor accommodation who wore PALs progressed at the
lower rate seen in children with good accommodation.
380
Factors Associated with the Progression of Myopia
R=radius
P=power
2- Near Work and Reading Distance
Evidence for a link between near work and myopia progression
continues to be reported in a wide range of populations.
Parssinen has shown that near work and short reading distances are
significantly correlated to increased myopia progression (r = 0.253, P= 0.0001
for near work and r =0.255, P =0.001 for reading distance), whether or not the
child accommodates at near through the full distance prescription, removes the
spectacles for near work, or wears bifocals.?'‫ه‬i.e all the same
3- Age
Children who develop myopia at young ages develop higher final amounts,
although progression among individual children varies considerably .
striking relationship between age and myopia progression
in 469 "COMET" children is illustrated in Figure (30-28, A).
381
382
Factors Associated with the Progression of Myopia
4- Gender
Onset, progression, and stabilization of myopia all appear to run
their course at younger ages in girls. This may contribute to the
frequent reports of increased myopia progression in girls
observed during the follow-up periods of many studies. Data
from COMET show that girls progressed significantly more than
boys during the first 3 years of follow-up (Figure 30-28, B ).
5- Ethnicity
• African-Americans progressed significantly less (-1.17 D over 3
years) ‫أقلهم جميعا في قصر النظر‬compared to children from other
ethnic groups
• [Caucasians (-1.35 D),
• Asians (-1.47 D) and
• Hispanics (-1.36) .
383
Factors Associated with the Progression of Myopia
6- Intraocular Pressure
Jensen designed a prospective, randomized trial to determine the effects of
bifocal wear and timolol on the progression of myopia in Danish children
attending second through fifth grade, with at least -1.25 D of myopia in one or
both eyes at recruitment. ‫التوظيف‬
1- Seventy four ( 47 ) children had baseline intraocular pressure of 17 mmHg or
greater .
2-Whereas (68 ) had intraocular pressure of 16 mmHg or less.
3- Myopia progression from baseline to follow-up at year 2 was greater in
control children (corrected with single-vision lenses) with higher intraocular
pressure (-1.32 D) versus lower intraocular pressure (- 0.86 D). However,
evidence for an association between lOP and myopia progression is mixed.
384
ctors Associated with the Progression of Myopia
7-Ophthalmoscopic Signs
Curtin suggested that eyes developing signs of :
1- Excessive axial elongation were at higher risk of later glaucoma and retinal
detachment and should be targeted for preventive measures. Jensen found
that children with :
2- Crescent formation at baseline had a higher initial myopia and a
significantly higher progression rate during a 2-year follow- up. Crescent
formation was the most common finding, with rare development of tessellation
or pallor . Because the ophthalmoscopic signs have received relatively little
attention, questions still remain regarding grading ophthalmoscopic signs,
possible variations with differing levels of fundus pigmentation, and the degree
to which ophthalmoscopic signs are correlated with myopia or its rate of
progression.
385
High myopic fundus
Fundus pigmentation
386
Chorioretinal degeneration
Myopic macular degeneration
with crescent formation
Factors Associated with the Progression of Myopia
8- Astigmatism
The evidence for increased myopia progression due to astigmatism is mixed.
Fulton et al. studied the relationship of astigmatism to myopia progression in
myopic children aged birth to 10 years.
Retrospective record review suggested that :
1) astigmatism, especially of the oblique axis , was related to higher amounts of
myopia.
2) Fulton et al. concluded that children with uncorrected astigmatism
experienced more progression of myopia than did children without astigmatism.
3) A weak correlation with initial astigmatism and final progression was found
for boys only and disappeared when corrected for the spherical equivalent.
4) Parssinen concluded that astigmatism was a symptom of deviation from
emmetropia
and did not play a causal role in myopia progression.
387
Conventional Treatment of Myopia
‫العالج التقليدي لقصر النظر‬
Although there has been intense effort spanning many years to
reduce the progression of myopia in children, no method is
currently accepted as being effective in preventing myopia
development or slowing its progression in the majority of
myopic children. Although a modified approach may eventually
be advised for myopia children with poor accommodation , the
current standard of care is full correction of myopic refractive
error in single vision lenses .
388
Conventional Treatment of Myopia
Clinically Significant Amounts of Myopia
During visual development, myopia is of less functional concern than all
other types of refractive error. Infants, toddlers, and preschoolers get clear
retinal images from near targets, and detailed vision should develop normally
unless the myopia is very high.
In striking contrast to the flood of information readily found on nearly any
other topic and its association with myopia, amblyopia is only mentioned in
connection with anisometropia.
Myopia of 5.0 D or greater is clinically assumed to be amblyopiagenic.
Because larger degrees of myopia that are associated with amblyopia may
also be associated with pathological changes in the fundus, the source of
acuity loss is not always clear.
Because amblyopia may develop and cause a secondary acuity loss in an eye
with pathology, some children with high, unilateral myopia may benefit from
a trial of patching of the best eye even in the presence of staphyloma
formation as a type of treatment .
389
Conventional Treatment of Myopia
1-The amount of myopia that might interfere with the child's
activities decreases from about -5.0 D during infancy to about -1.50
D at preschool age
2- Prior to school age, it is difficult to predict whether a given baby or
preschooler will benefit from correction of myopia.
Discuss the possible advantages (e.g., increased attention to the
environment, improved social interactions) that refractive correction
might allow, and advise a trial of spectacle wear with the parents.
3- By the later elementary school years ‫نهاية الدراسة االبتدائية‬, the
situation is more clear cut , because the myopic child will have
difficulty seeing the board.
4- Many practitioners offer a choice between glasses for corrections
of 1.0 D or less versus sitting near the board ‫أماما‬during the
elementary school years.
5- Future research is needed to clarify whether aggressive correction
‫مثال‬390‫العالج الجراحي‬of early myopia will reduce overall myopic progression.
Full Myopic Correction Using Spectacles
The standard of care for myopic children of at least school age is :
1- Full correction of the myopic error .
2- Care should be taken to avoid prescribing excess amounts of myopia
correction in children who are insensitive to blur produced by minus lenses.
3-Manifest refraction procedures are usually sufficient to determine the myopic
refractive error. Autorefraction with or without cycloplegia usually produces
similar values compared to subjective refraction .
Parssinen and Hemminki published a series of papers describing a trial to
evaluate the effect of full myopic correction :
a. worn continuously,
b. at distance only,
b. or combined with a near addition, on the progression of myopia in Finnish
schoolchildren recruited from third, fourth, and fifth grades with myopic
spherical equivalents from 0.35 to 3.0D. Progression was the same in all three
groups. There is no firm evidence in children that :
a. full-time wear of diverging lenses increases myopia progression,
b. nor is there evidence that reducing the stimulus to accommodate by
removing the spectacles or using a near addition reduces myopia
391
progression.
Clinical Attempts to Slow the Rate of Progression of Myopia
I- Bifocals ‫المحاوالت السريرية لتبطئ تقدم قصر النظر عند األطفال‬
The association of myopia with near work has caused much speculation
about
‫ تخمين‬a link between myopia development and accommodation.
Practitioners who reasoned that excess accommodation caused myopia
prescribed bifocals to reduce myopia progression. The weight of
evidence is for no effect of bifocal wear on myopic children overalI but
a suggestion that esophoric children might benefit.
A recent, prospective clinical trial of 82 myopic children with
esophoria who were randomly assigned to wear bifocals or singlevision lenses showed a small difference between groups after 30
months (0.99 D in the bifocal group versus 1.24 D in the single
vision group). The authors concluded that bifocals should not be
prescribed to esophoric children for purposes of reducing myopia
progression.
392
II- Progressive Addition Lenses (PAL)
1- One possible reason that bifocals do not slow the progression of
myopia could be that children avoid the near add or respond
inappropriately to the step change in power.
2- Progressive addition lenses offer a gradual reduction in power
which may result in better compliance, as well as the possibility of
clear vision for a variety of distances and accommodative
responses. 3- Previously, data from 469 COMET children aged 6
to 11 years at baseline were presented showing differences in
progression
according to accommodative response.
393
4- Later analyses showed that accommodative lag ( is the
dioptric value in which the accommodative stimulus
exceeds the accommodative response) is a more powerful
predictor of treatment success than esophoria.
A subgroup of children with accommodative lag and
esophoria had a clinically significant reduction in myopia
progression if they wore progressive addition (0.98 D)
versus single-vision lenses (1.75 D). (Figure 30- 29(
394
III- Undercorrection vs. Overcorrection of Myopia
Myopia progression is not slowed by prescribing more or less minus correction.
In fact, undercorrecting enhanced myopia progression.
IV- Monovision to Slow Progression in One Eye
1-Monovision correction of young myopic children has been attempted in response
to suggestions from animal studies that maintaining a myopic focus
(undercorrecting) should slow axial elongation.
2- Surprisingly, children used the eye with distance correction for near viewing,
resulting in constant, myopic focus in the undercorrected eye for distant and near
images.
3- A significant reduction in myopia progression was found in the undercorrected
eye along with an increase in anisometropia. This is interesting, but not
recommended clinically, due to the iatrogenic ‫ ثانوي‬anisometropia .‫ال يوصى به‬
395
'
.
V- Rigid Contact Lenses
Many practitioners have observed that children fit with rigid gas permeable (RGP)
contact lenses seem to have less myopia progression. Early studies have design errors
including high drop-out rates. ‫ارتفاع معدالت التسرب‬
A recent clinical trial recruited 147 children aged 8 to 11 years for a trial period of RGP
wear lasting 1 to 2 months, to prove readiness for a randomized controlled trial
comparing progression with RGP (alignment fit) versus soft contact lenses (SCLs).
One hundred fourteen children (114) who successfully wore RGPs enrolled.
A small significant difference in myopia progression was found 3 years later (-1.56 D in
RGP versus -2.19 D in SCL groups), which was attributed to corneal flattening. Since
axial elongation was not affected , this reduction in myopia was not expected to
persist after discontinuation of RGP wear.)‫(مؤقت‬
The authors recommended against RGPs fit according to standard clinical ‫المؤلف ال‬
‫يوصي باستعمال العدسات الصلبة‬protocols for purposes of slowing myopia progression.
396
Spherical
equivalent
397
VI- Orthokeratology
Orthokeratology (the temporary reshaping of the cornea (usually
overnight) with specially made rigid contact lenses, in order to
correct myopia)
(ortho -K) is a process of fitting rigid lenses that are intended to
mold the cornea. Ortho-K has seen a recent upsurge ‫ارتفاع مفاجئ‬in
interest fueled ‫تسبب ب‬
by new high OK materials and reverse-geometry lens designs.
Children undergoing Ortho-K sleep in the lenses, undergo a
gradual flattening of the central cornea, and eventually may have
good unaided vision during the day.
A recent small study enrolled 29 children aged 8 to 11 years, with
an average myopia of -2.44 ± 1.38 D at baseline. Children wore
reverse geometry lenses at night, without serious adverse events.
After 2 weeks, most children had good unaided visual acuity
during the day, and eventually wore the lenses on alternate days
only.
398
VI- Orthokeratology
For these young children with low myopia, a 98% reduction
in myopia was seen at the 6 month visit.
Despite this large effect, important questions about safety
and clinical benefit persist . Various instances of corneal
ulcers associated with ortho-K in young children, some of
whom suffered permanent, significant vision loss, have been
reported recently. This risk must be weighed against the
intangible ‫غير ملموسة‬and temporary benefits of ortho-K, as
well as the relative safety of wearing standard, daily-wear
contact lenses during the day.
399
VII- Drug Therapies
Atropine and Other Muscarinic Antagonists . Early clinical
studies using atropine to reduce the progression of myopia,
reviewed by GOSS and Curtin were controversial because none
had all the elements of proper research design, with frequent lack
of masking and randomization, high drop-out rates, and
inconsistent follow-up and treatment intervals.
Two recent studies have shown that nightly atropine reduces
myopia progression.
The first showed a dose response with concentrations of 0.5%
resulting in annual progression of 0.04 D/Year compared to 1.06
who used Tropicamide as a placebo.D/Year in control children.
400
Indications for Part-Time Wear For Myopia
Part-time wear may be indicated for children with lower amounts of myopia,
‫قصر نظر قليل لصغار األطفال‬who can keep up with ‫ يمكن مواكبة‬their glasses if worn
intermittently.
As discussed earlier , there is no evidence that removal of spectacles at near
reduces myopia progression.
Indications for Full-Time Wear For Myopia
Time will usually favor full-time wear of myopic prescriptions because most
myopias progress.
1- Older children have a more difficult time arranging to sit near the board
once they enter middle and high school . ‫لألطفال الكبار‬
2-Younger children ‫ لألطفال الصغار‬may have to be told to wear the glasses full time
because they cannot be expected to remove them for near tasks, and because
they401are more likely to lose them entirely if not worn.
Patients with
Amblyopia and
1460Strabismus
402
Amblyopia and strabismus are relatively common problems that affect
approximately 2% and 5% of the population in the United States,
respectively.
Assuming these estimates are still valid, this represents almost 6 million
amblyopes and 15 million strabismics using the population estimate of
nearly million Americans as of January, 2006.
The visual handicap of unilateral amblyopia or strabismus is generally
considered to be mild. Recent studies, however, suggest that even the better
eye in amblyopes and strabismics is not "normal" when it is carefully
measured using a variety of visual function tasks.
In addition, the amblyope may be at greater risk for becoming blind than is
normally recognized
403
Perhaps the most disturbing-yet the most hopeful aspect ,
‫تعادل بين القلق واألمل‬of amblyopia and strabismus is that their
development is usually confined to the first 6 to 8 years of life.
‫ناحية القلق‬Persons in this age group might not be aware of their
problem, thus delaying intervention.
‫ ناحية األمل‬On the positive side, these conditions tend to manifest
when there is still great plasticity in the visual system, so
rigorous ‫ صارم‬early intervention may eliminate or greatly reduce
their impact.
404
Generally, accurate determination of refractive status is
considered the starting point for effective intervention in cases of
amblyopia and strabismus. Because of the number of strabismic
and amblyopic patients, this becomes a clinical challenge.
The amblyopic eye is often insensitive to changes of spherical or
astigmatic power, which makes a subjective endpoint difficult to
determine . Strabismic eyes are turned, making on-axis
retinoscopy difficult. Because strabismus is commonly associated
with amblyopia, objective and subjective refractions can be
more complicated and of lesser accuracy than those for the
routine patient.
405
Thus, one clinical challenge is to determine the refractive error
and then to decide what the best optical correction should be ?
In all cases, the important refractive findings should be used to
determine a correction that allows a well defined retinal image to
stimulate receptor cells and permit neural development.
This will help prevent the development of amblyopia or, perhaps,
allow the amblyopia to regress .
The angle of deviation in strabismus may also be altered through
stimulation or inhibition of accommodative convergence.
In this chapter, the clinical influence that binocular anomalies
including amblyopia and prescription technique of optical
corrections is discussed.
406
AMBLYOPIA
Amblyopia is usually defined as :
1- A nonspecific loss of visual acuity ,
2- of at least two lines that is ,
3- not caused by pathology .
4- or correctable by ordinary refractive means .
Before the age of 45 years, amblyopia is responsible for vision
loss in more patients than are all ocular disease and trauma
combined to . Although most concern and attention in amblyopia
centers on visual acuity, the condition also manifests as
dysfunctions in many nonacuity factors that compound the total
visual handicap .
1-Accommodative control , 2-eye movement precision ,‫دقة‬
3-contrast sensitivity functions ,4- and spatial judgments are all
compromised in the amblyopic eye.
407
‫التصنيف‬Many classification schemes have been used to categorize
amblyopia, and they usually concentrate on the presumed
amblyogenic condition. The classifications in this chapter follow
this format. In all functional (as opposed to "organic") amblyopia,
the assumption is that the visual pathway fails to develop
normally because of inadequate stimulation.
‫السبب‬The underlying cause of all amblyopia is the inability of the
visual system to comfortably handle dissimilar images from the
two eyes as a result of abnormal competitive binocular
interactions.
A corollary ‫نتيجة مباشرة‬corollary to this assumption is that
rehabilitation of the visual pathway may be facilitated by
normalizing the stimulation.
408
Whether the dissimilarity results from :
1. blur (refractive amblyopia),
2.different scenes (strabismic), ‫مشاهد مختلفة‬
3.a totally degraded image (cataract), or
4.occlusion (by ptosis), the result is some form of
amblyopia.
Perhaps surprisingly, amblyopia resulting from diffusion
of the image(1,2&3) appears to be more difficult to treat
than )4))occlusion from ptosis).
409
REFRACTIVE AMBLYOPIA
In refractive amblyopia, the retinal image is degraded because of
optical blur. There are three main categories of refractive amblyopia:
(1) meridional,
(2) isometropic, and
(3) anisometropic (Table 31-1).
(1) Meridional Amblyopia
Meridional amblyopia, which is typically caused by uncorrected high
astigmatic errors in each eye, is demonstrated at a slightly later age
than are other blur-induced amblyopias. Depending on the orientation
of the uncorrected astigmatism, the amblyopia may escape detection
on some of the common visual acuity tasks used with young children.
A target oriented in the blurred meridian must often be used when
testing these patients. The condition is frequently bilateral and,
although demonstrable on monocular testing, the blur is somewhat
410
filled
‫يشغل‬in on binocular tests.
(2) Isoametropic Amblyopia
• A degraded image ‫صورة المتدهورة‬may result in isometropic amblyopia,
(sphere) a bilateral condition in which the refractive error in each eye
is so great that a clear retinal image cannot be obtained anywhere in
space. The result is bilaterally decreased visual acuity.
• Most often this is seen in patients with very high hyperopia.
For example, a 6 year - old patient with +8.0 D of uncorrected
hyperopia in each eye would have difficulty focusing to form a clear
image at distance or near (bilateral amblyopia) .
As a result, even after proper optical correction , visual acuity is
likely to initially remain reduced in each eye (see Table 31-1).
411
412
• It might be expected that patients with significant uncorrected
hyperopia in each eye would manifest a marked esotropia.
• However, these patients often do not have the experience of
clarity and, therefore, their visual systems do not recognize the
need to accommodate.
• Consequently , these patients frequently do not demonstrate
a strabismus. Fortunately, when neither eye is able to obtain a clear
retinal image, the child often behaves in a telltale manner, ‫طريقة‬
‫ متعثرة‬and the parents usually seek professional attention in time to
prevent lasting effects.‫يبحثون عن العالج مبكرا‬
413
(3) Anisometropic Amblyopia
More commonly seen in clinical practice is the child who has a
normal refractive error with appropriate vision in one eye and a
significant refractive error with substantially reduced corrected
acuity in the other. This is called anisometropic amblyopia.
Because the child has good vision in one eye, his or her behavior
may not reveal the problem. These children often become masters
at "cheating" ‫ خداع‬on vision tests by peeking around occluders to
read letters with the better eye.
The mechanism for the visual acuity loss is thought to be
partially the result of active inhibition of the amblyopic eye, which
results in changes throughout the visual pathway.
414
Visual pathway changes occur because the parvocellular layers of
the lateral geniculate nucleus, which respond to high spatial
frequency simulation, are ineffectively driven by the blurred eye.
These effects of blur occur during the "critical period" of
development in the visual system, usually during the first years of
life. Newborn infants have very small pupils that increase their
depths of field. The effect of blur is negligible for infants as a result
of their reduced resolution caused by an immature retina and a
rapidly changing angle lambda. (This angle lambda is referred to
clinically as angle kappa) .
However, some investigators suggest that, if a hyperopic
anisometropia of at least persists through the age of 2 years, a blurinduced amblyopia is likely to develop .
415
Hyperopic Anisometropia
Amblyopia resulting from hyperopic anisometropia is probably the
most common refractive amblyopia. As little as +1.0 DS of hyperopic
anisometropia can cause a breakdown of central fusion and result in
amblyopia of the more hyperopic eye .
1- Central vision of the more hyperopic eye is not used at either
distance or near.
When both eyes are hyperopic as well as anisometropic, an esotropia
may also develop, especially at near.
2- However, if the less ametropic eye ( the control eye) is not very
hyperopic, the eyes may remain perfectly straight and demonstrate
reasonable levels of sensory fusion. Although there are reports that
stereopsis on the Random Dot E test (presented at 2 m) is not
obtainable with amblyopia, many patients with anisometropic
amblyopia pass this test (see Table 31-1) .
416
Myopic Anisometropia
1- Mild to moderate amounts of myopic anisometropia «5.0 D) usually
do not result in amblyopia, especially if the less myopic eye is close to
emmetropic.
2- Unlike what is seen in cases of hyperopic anisometropia (in which
the more hyperopic eye has no advantage at any viewing distance), in
patients with myopic anisometropia,
3- The less myopic eye is often used for distance vision, and the more
myopic eye is used for near vision. Thus, amblyopia is avoided.
4- In cases of high unilateral myopia , the more ametropic eye again
has no advantage at any viewing distance , and a deep, recalcitrant
amblyopia often develops. In addition , there may be a posterior
staphyloma in many of the unilaterally high myopic eyes , which adds
another potential obstacle to remediation.
417
STRABISMUS AND STRABISMIC AMBLYOPIA
Strabismus is the condition in which the line of sight of one of the
two eyes is not coincident with the object of regard.
This condition affects up to 5% of the population of the United
States. In addition to the early onset and accommodative
heterotropias familiar to most clinicians , strabismus is a common
sequela to many pathological conditions such as :
1. stroke,‫السكتة الدماغية‬
2. thyroid myopathy, ‫مرض عضالت العين الخارجية الثيرودي‬
3. and myasthenia gravis. ‫الوهن العضلي الوبيل‬
418
Plus reflex deviation +0.5
419
minus reflex deviation - 1.5
Strabismus is classified by the direction of turn of the nonfixating
eye and by the frequency with which the turn is noted (Table 31-2).
1-The directions of deviation are esotropia, exotropia, hypertropia,
and hypotropia, with subclassifications based on the fixing eye.
2-Frequency is either intermittent or constant.
3-Distance and near serve to identify the location of the strabismus.
To these basic qualifiers, the magnitude of the deviation measured in
prism diopters is added. Therefore, a verbal description of a patient's
condition may be, for example, "an intermittent 20 prism diopter left
esotropia at near. (Table 31-3) .
420
421
Clinicians also are concerned about whether a strabismus was of
early onset (within 6 months of birth), accommodative, or
acquired. This information provides an indication of the patient's
likelihood of having binocular cells developed and consequently
helps estimate the prognosis for a total functional cure.
The more time that equal visual acuity and binocularity had to
develop before the disruption by strabismus, the more likely that
functional binocularity may be reestablished. ‫قاعدة ذهبية‬
Information about early occlusion programs, optical corrections,
and surgery should be elicited in detail, because this is also
important when developing a prognosis for functional cure.
422
Strabismic Amblyopia
Unlike refractive amblyopias, which primarily feature decreased
resolution and poor contrast sensitivity, strabismic amblyopia adds:
1-spatial uncertainty or difficulty with localization. This may result
from monocular adaptation to the anomalous correspondence
observed in many strabismic amblyopes.
2- In addition, fixation patterns and eye movement accuracy are
markedly affected in strabismic amblyopes.
3- Asymmetrical nasalward gaze drifts (esotropia) are often seen in
eyes with strabismic amblyopia. Nasal drifts may result in
anomalous oculomotor behavior in both eyes of strabismic
amblyopes.
423
4-Together, these deficits may cause letters in the middle of lines to
be skipped ‫ تخطي‬or read out of order. Often, only the first and last
letters are read correctly by the strabismic amblyopic eye.
5- Strabismic amblyopes fail the Random Dot Estereo test at 2 m,
and they do not obtain stereoacuity of better than 60 seconds of arc
on linear disparity targets.
6- A further consideration with strabismic amblyopes is eccentric
fixation. This is the condition in which a nonfoveal point assumes
the role of "straight ahead" (the principal visual direction).
424
Dot Estereo test
425
The monocular fixation of the amblyopic eye lines up with a
nonfoveal point that is then considered to point directly at the
target of regard. Many theories have been proposed as to why
this condition might develop , but clearly visual acuity and
other compromised visual functions begin to improve if the
patient is able to regain foveal fixation .
426
PROGNOSIS
1- Unilateral strabismus is certainly an amblyogenic condition.
2- However, an intermittent strabismus or a constant alternating
strabismus is less likely to result in amblyopia. Observation of the
fixation pattern of infants will suggest whether amblyopia is likely. If an
infant is not seen to alternate freely, a forced alternation may be elicited
by covering the fixating eye with the thumb. If the child maintains the
fixation beyond the next blink, it is likely that there is some alternation
throughout the day.
More desirable is the free, habitual alternation that would indicate no
fixation preference. In this condition, each eye receives equal
stimulation, and equal acuity is expected. The later the onset of a
strabismus, the better the chance that binocularity has been established
previously.
427
The earlier the intervention occurs after onset, the less chance of
troublesome ‫ مزعج‬sensory adaptations having developed .These
adaptations include:
a.amblyopia,
b.anomalous correspondence, and
c. suppression . The demonstration of binocularity on a Random Dot
stereotest or even a second-degree fusion improves the prognosis.
If the patient does not have intermittent strabismus, the clinician
should neutralize the objective angle with prism and remeasure
responses on binocular tasks. Notwithstanding ‫على الرغم من‬
possible adaptations, a trial treatment can be attempted at any age the
factors determining prognosis will be the conditions present .
428
Treatment of amblyopia
The determination of the optical error of the eye is the first step in
developing a total treatment plan for patients with amblyopia or
strabismus. That is not to say that the clinician is obligated to always
prescribe the full amount of refractive error revealed. However,
refractive error provides an important starting point for all therapeutic
decisions. Monocular retinoscopy on a strabismic eye may be less
accurate if marked eccentric fixation is present. Because the patient is
fixating with a nonfoveal point on the retina, off-axis retinoscopy may
not reveal the true refractive error.
429
As the patient proceeds through therapy and regains foveal fixation,
subsequent refractions may be more accurate. Before definitive
testing is begun, it is desirable to investigate whether any
meaningful‫ ذو معنى‬anisometropia is present. In infants, this may be
accomplished by photorefraction or by a screening retinoscopy such
as near retinoscopy . For toddlers and older children, a near dynamic
retinoscopy such as the monocular estimate method (MEM) will
quickly reveal an imbalance.
Cycloplegic refraction is the method of choice, especially with
young children. Patients with amblyopia are often insensitive to the
precise demands required of them during a subjective refraction. By
relaxing accommodation through cycloplegia, the clinician may
obtain the .........................
430
…………best objective information about the lens required to produce
a clear retinal image in the amblyopic eye. A standard cycloplegic
procedure is detailed in Table 31-5.
For children younger than 5 years old, a cycloplegic spray is often
easier to administer than drops or ointment.
One spray per eye repeated in 5 minutes is very effective, and it is best
to spray the open eye directly.
However, when that is not possible, spraying a closed lid usually
allows enough penetration through the lid openings or drips in the eye
from the residual mist ‫ الرذاذ‬on the eyelashes to provide adequate
cycloplegia.
431
When there is reason to suspect residual hyperopia after the
prescription of spectacles, atropine is the drug of choice. Atropine
ointment is less prone to causing systemic effects, and it does not sting
during application.
The recommended dosage (see Table 31-5) is applied as a lis-inch strip
into the inferior cul-de-sac three times a day for 3 days. Although this
may seem excessive, it is a procedure that seems to have held up well
over time. With infants, it is often difficult to judge whether a smallangle strabismus is present. The Hirschberg test (Figure 31-1) is
helpful when the binocular status is in doubt.
432
433
Under close observation , a bright pinpoint light reflex may be
observed within the red background; this is the reflex used for
the Hirschberg test. Comparison of the positions of the reflexes
suggests the placement of the visual axes of the eyes.
The expected Hirschberg reflex position is 0.5-mm nasal to the
center of the pupil. One millimeter of displacement difference
between the reflexes is equal to of misalignment.
434
GUIDELINES FOR THE PRESCRIPTION OF THE OPTICAL CORRECTION IN
PATIENTS WITH AMBLYOPIA
Even with the best refractive technique and the most cooperative
patient, the clinician is faced with the final dilemma:
1- How much of the true refractive error should be prescribed ?
2- Having arrived at a cycloplegic refraction, the clinician must make
optical correction decisions based on this finding and other relevant
data.
3- For instance, is the child young enough that the refractive error is
still changing?
4- Will prescribing the full prescription at this time interfere with
emmetropization?
435
5- Does the full optical correction do any more to eliminate the
amblyogenic cause than a partial optical correction?
The answers to these questions are obviously different for patients
with strabismus than for those with anisometropic amblyopia, and
they may also vary for children of different ages. The clear
determination of goals is the most important step for the clinician.
This not only allows for the modification of the optical correction,
but it also allows the clinician to explain the treatment strategy to
the parents.
436
The major goals when prescribing for amblyopia are:
1- to allow a clear, distinct retinal image to be formed for each eye
2- and to allow the accommodation of the two eyes to be balanced .
Binocular balancing becomes increasingly important if the patient
is to maintain the gains that have been achieved through:
a. occlusion and monocular therapy : if an appropriate balance
cannot be obtained, fusion is disrupted, and an amblyogenic
process remains active.
b.an optical correction that equalizes the input to each eye
maximizes the chances of obtaining fusion, and this, in turn,
provides the "glue" to hold the system together.
437
(Cont. of b.)The importance of the balance seems to increase as the
vision improves and the patient is able to make more accurate
discriminations on balancing tasks. Immediate Contrast Test can
provide the best results .
c. In cases of both anisometropic and strabismic amblyopia, it is
important to allow the patient to adapt to wearing the new spectacles
before deciding on further treatment.
d, Following the patient at monthly intervals until no further
improvement is found in visual acuity is a reasonable guideline.
438
Critical Periods
The early detection of amblyogenic conditions is encouraged, because
research evidence indicates that there is a critical period during which
the neurological sensory development of the visual system has not
been irreparably ‫ال يمكن إصالحه‬damaged. It is unclear what the exact
timeline of these critical periods is or what the best expected
remediation is after this major window of opportunity is closed.
However, the current level of understanding is that, during certain
sensitive time frames, afferent input stimulates the development of the
anatomy and physiology of the developing visual system.
439
Critical Periods
a. Abnormal input resulting in a blurred optical image has profound
effects on the visual potential.
b.Whereas the same stimulus conditions before or after the critical period
result in only slight, transient changes. Thus, the determination of the
critical periods will guide the need for aggressive testing and
intervention techniques.
c.The critical period for amblyogenesis may be different for different
types of refractive errors. This may be reflected in the changes of the
refractive condition with age. For instance, anisometropia is quite
common in premature and young infants, with the incidence dropping off
‫ تنقص‬significantly by the age of 1 year.
d. Too early a correction, it is believed, could upset the normal
emmetropization process , whereas correcting too late may allow
amblyopia to develop.
440
Critical Periods
Recalling that decreasing acuity over time in the affected eye indicates
a need for more aggressive intervention , two primary options are
available:
1- The first one is to provide a partial correction , reducing the blur
but leaving room for the normalization process to continue. Frequent
follow up appointments allow the clinician to stay ahead of the ‫استباق‬
‫ التغييرات‬changes.
2- The second option is to evaluate the patient at regular intervals
(e.g. every 3 months) to monitor the changes. Three successive visits
at these intervals that show stable refraction may indicate that
definitive action ( full correction ) can be taken without disrupting
emmetropization.
441
Nonacuity functions of the eye ‫الوظائف الغير بصرية للعين‬
Although considered by most clinicians to be a monocular dysfunction,
amblyopia has been shown in many studies to affect the presumed
‫ مفترض‬normal eye. These anomalies include :
1- fixation maintenance,
2- vertical optokinetic response, and, of course,
3- stereopsis. An amblyopic eye is an eye in a visual system that is in
need of complete rehabilitation. Therefore, attention must be paid to the
areas that are not involved in standard visual acuity measures but that are
very important in the daily functions of vision (nonacuity functions ).
These include :
1- contrast sensitivity,
2- eye movement control, ‫التحكم في حركات العين‬
3- accommodative control, 4- spatial uncertainty,
5- and fixation maintenance ‫استمرار التركيز‬.
442
Nonacuity functions of the eye
During the normal course of amblyopia treatment, many of these
functions will improve. However, many of these functions continue
to improve even after Snellen vision acuity undergoes no further
change. The improvement of the nonacuity functions may allow the
patient to perform more comfortably in his or her daily environment;
it therefore becomes important to continue therapy beyond the level
at which Snellen acuity has apparently become "normal.
In fact, improvement in daily functioning may continue as a result of
improvement in these nonacuity factors. In many cases, clinicians
should continue to actively treat the amblyopic patient, even when
further Snellen acuity increase is unlikely until improvement in the
nonacuity factors plateau. ‫ ثبات‬,‫استقرار‬
443
Binocular Effects )Stereopsis(:
1-It has long been accepted that stereopsis is substantially decreased
and more likely absent in the presence of even a small magnitude
strabismus.
2-Stereopsis levels may be different in different types of amblyopia even those with the same visual acuity because of the component
dysfunction in different causes of amblyopia in different persons.
3-Some researchers have spent a great deal of effort demonstrating
that stereopsis is also markedly decreased even in mild anisometropic
amblyopia.
4-Much of the push for ‫تشجيع‬the use of stereopsis tests is to find the
perfect screener to be used by schools or at large screening clinics.
444
Binocular Effects )Stereopsis(
5-Clinically, it has been less convincing that this is a reliable detector
of anisometropic amblyopia. Anisometropic amblyopes that can pass
Random D or E or stereoacuity tests with normal scores are frequently
seen.
6- Although there is no question that stereopsis testing is an important
component of any serious screening for binocular anomalies, it must
be combined with several other tests to develop a clear picture of the
visual system.
445
Binocular Effects-Amblyopia Treatment
Amblyopic patients usually require therapy in addition to the best
optical correction. As visual acuity improves, the patient's sensitivity
to the subjective refraction increases, and better optical correction
may be obtained. Therefore, refraction at regular intervals is
recommended
1-Reevaluation at 3-month intervals during treatment for
amblyopia, with a wet (cycloplegic) refraction every other visit, is
recommended. 2-Particular attention should be paid to the
astigmatic axis, because the patient's judgment required to make
these decisions involves sensitivity that may have initially been
lacking.
3-In the case of hyperopic anisometropia, where the less hyperopic
eye is 0 to 2.0D and there is no strabismus the important
consideration is the balance between the eyes. If the refraction in
the better eye increases or strabismus manifests, remember that
relaxation of the controlling eye results in a change in the strabismic
446 because of the altered accommodative response.
angle
Binocular Effects-Amblyopia Treatment
3 ‫تابع بند‬
Thus, in a condition of +3.0D OD, +6.50 OS, an optical correction of
plano 0.0 DS , +3.50 DS would meet the requirement of making the
eyes equal in refractive balance. (Partial correction).
4- However, if there was eyestrain or if an esotropia was present,
correction up to the full refractive error might be required. It is usually
not efficacious to significantly overplus a patient, because this might
lead to the rejection of the correction and the paradoxical stimulation of
accommodation because of blur. However , maintaining accurate
balance between the eyes and precise binocular alignment are important
priorities.
5-Astigmatism appears to change more frequently than spherical
correction during infancy. Guidelines of when to prescribe-especially
the full amount-follow a somewhat different schedule than those
presented for spherical optical corrections.
447
The "3-3" rule is a good starting point: three successive visits
spaced 3 months apart showing a stable astigmatic error suggests
that it is time for aggressive intervention.
6- When the anisometropia is too great (more than 4 to 5 D) ,
contact lenses may be considered. Children usually adjust
readily to contact lens wear . The challenge is to have the
parents adjust to insertion, removal, and lens maintenance.
Contact lenses :a - minimize the prismatic changes in various
fields of gaze that result from anisometropic spectacle
correction, which allows for more optimal eye alignment,
in the presence of refractive anisometropia (different corneal
curvatures).
b- contact lenses reduce the image size difference between the
eyes and improve the prospects ‫ آفاق‬for the establishment of
excellent binocular vision .
448
GUIDELINES FOR THE PRESCRIPTION OF THE OPTICAL CORRECTION
IN PATIENTS WITH STRABISMUS
The determination of the amount of refractive error and prism
needed to obtain fusion is the starting point for effective optical
correction. In addition, these findings may be further modified
depending on:
1- the difference in the angle of deviation at distance and near.
2- and the type of sensory adaptation.
Certainly the desire is to eliminate any amblyogenic conditions
by using the optical correction guidelines listed above, in the
amblyopia section. Beyond these, the impact of refractive
correction on the binocular status must also be considered.
Decisions that must be resolved are discussed in the following
section. (Page1468)
449
Best Distance Visual Acuity Versus Binocular Alignment
Best distance visual acuity versus binocular alignment is often a
consideration with young esotropes who are :
1- ortho at distance with their refractive correction .
2- yet at near they have a slight residual esotropia.
Optical correction of esodeviation in infants and toddlers : ‫هام‬
1- In infants and young children : It may be possible to avoid bifocals by
simply slightly overplussing the patient with a single vision correction.
For infants , an even greater amount of overcorrection may often be
‫عالمه المحيط القريب‬accepted , because their visual world is primarily at near
point
2- Preschool age :Usually 0.50 to 0.75 D of overplussing is accepted.
Although ocular alignment may be achieved with this correction,
distance visual acuity is reduced because the overplus correction
makes the patient residually (artifitially) myopic.
3-School age :This acuity remains equal at near, and the slight reduction
of plus lenses in distance visual acuity ‫تقليل قوة العدسات الموجبة ال يؤثر في النظر‬
‫البعيد‬does not affect the child's schoolwork or play.
It is important to alert the parents ‫ تفهيم الوالدين‬of the likely reduction of
distance acuity with the new optical correction and to explain the
purpose and goal (i.e., straight eyes without bifocals).
Emphasize that the important factor is the development of neurological
integrity in the visual acuity pathway (as demonstrated at near) and the
binocular system.
4. later school age :more sophisticated optical corrections can be used
to allow for best vision and alignment at all distances by using PALs.
451
5. Contact lenses : When the refractive error is 3 D of hyperopia or greater,
another alternative to a bifocal is correction with contact lenses. This may
allow for alignment at distance and near , perhaps as a result of lens power
effectivity and the improved peripheral visual field .
6. At times, the amount of hyperopia revealed under cycloplegia appears to be
enough to eliminate an esotropia , yet the patient is unable to clear distance
vision and rejects the optical correction. When the patient is unable to fully
relax accommodation (so that the esotropia is manifest) , using cycloplegia may
assist the acceptance of plus power. A 1/8-inch strip of 1% atropine ointment
(once) placed in the inferior cul-de-sac in the office is usually adequate.
The patient will realize that the optical correction is needed to function daily.
By the time the cycloplegia wears off ‫تنتهي‬, the patient has become accustomed
to wearing the spectacles without overaccommodating.
452
Binocular Alignment versus Chance of Induced Myopia
1- One treatment used for divergence excess exotropia (i.e. the
angle of exotropia is greater at distance than at near) is to intentionally
overminus the optical correction to recruit accommodative
convergence to assist fusional vergence with the achievement of ocular
alignment and fusion. Some clinicians have expressed great concern
that this treatment option will result in a significant increase in
myopia, although research has not supported this contention.
However, even if it did , the question the clinician must face is whether
it is more desirable to maintain fusion or to avoid the possibility of
inducing myopia. When vision therapy is a viable option, these
questions may be irrelevant. However, in many practices and with
many patients, vision therapy may not be a viable option. The choice
may be between optical overcorrection or surgery.
453
Disruption of sensory adaptations
Optical corrections for strabismic patients are usually intended to
facilitate fusion. The prism components of the prescription are primarily
designed for patients with normal correspondence.
When a patient has anomalous correspondence, it may first be necessary
to disrupt (interrupt) this abnormal sensory adaptation before attempting
to establish normal fusion? One method of disruption is to overcorrect
the angle of deviation with prism . The amount of overcorrection is
determined by neutralizing the angle of deviation with prism and then
continuing to add prism until a reversal of movement is noted .
454
2-Thus, with an esotrope, BO prism is added during the
alternate cover test until a slight exo movement is seen. The prismatic
components may be applied for diagnostic purposes in the form of
reusable Fresnel or clip-on prisms, and an in-office prism adaptation
test may be conducted for 30 minutes. If, after this amount of time, the
patient still is exo through the prism, that amount of prism (+) plus 12
to 15PD of overcorrection could be prescribed. Thus, if a patient is
manifesting 15PD diopters of esotropia, an optical correction of 30PD
BO may be given. After the 30-minute test period, if the patient has
made an anomalous fusion movement and is no longer exo, continue
to add prism and continue performing the prism adaptation test until
no anomalous fusional movement is made; then, prescribe that
amount of prism plus a 12 to 15PD overcorrection.
455
The Fresnel prism in the example has the same deviating power as
a conventional prism of the same material; however, the Fresnel
prism is only 1 mm thick (0.5 mm prism base and 0.5 mm
platform), one tenth the thickness of the conventional
prism of the same power.
456
The Fresnel principle is a common clinical observation that the
smaller the spectacle frame, the thinner is the base of a
conventional prism. Figure 2 shows that reducing the base-apex
dimension of a prism from 40 mm to 20 mm reduces the base
thickness from 10 mm to 5 mm. Reducing the prism to the very
small size of only 4 mm would result in a base thickness of only 1
mm. In each case, the deviating power of the prism is the same
since the apex angle is constant . If the base-apex size were
reduced to only 2 mm, it is seen in the shaded area of Figure 2 that
the base of the prism would be only 0.5 mm thick. A Fresnel prism
can be imagined to be a series of small prisms lying adjacent to
each other on a platform creating a thin membrane.
457
Fig 2. A Fresnel prism, much thinner than a conventional
ophthalmic prism of the same power, can be imagined to be a
series of small plastic prisms (see shaded prisms) lying adjacent to
each other on a thin platform of plastic. The Fresnel prism in the
example has the same deviating power as a conventional prism of
the same material; however, the Fresnel prism is only 1 mm thick
(0.5 mm prism base and 0.5 mm platform), one tenth the
thickness of the conventional prism of the same power.
458
3-Treatment of monocularly induced symptoms:
Many patients with strabismus experience symptoms of asthenopia
after near work, even though they are not binocular. It must be
remembered that even truly monocular patients may be asthenopic
because of accommodative dysfunction.
If the symptoms can be attributed to accommodative factors, the use
of bifocals or separate single vision readers may be considered.
One author stated that bifocals should only be given to patients with
strabismus when they establish binocularity.
Although this viewpoint is often repeated, it overlooks functional
problems that occur in daily life when accommodative problems cause
symptoms.
It is important to explain to the patient (or parents) that you are
prescribing bifocal or progressive addition (PAL) lenses to assist
monocular difficulties rather than as treatment of the strabismus.
459
4- Optical Correction of Astigmatism )1468(
Cylindrical correction often takes on a special significance for
many patients with intermittent exotropia.
The better the refractive correction , the easier for the patient to
maintain binocularity. When an eye is exotropic, there is often an
extorsion of the eye when the strabismus is present. When fused,
the correcting cylinder axis may change by as much as 8 degrees.
This becomes quite significant as the cylindrical power increases.
460
"Optical Surgery"
Partial or complete correction of strabismus using prisms, added
lenses, or both can be considered during the refractive sequence.
Prism and added lens treatments have the goal of optically
changing the angle of strabismus. Use of prisms or added lenses
allows the clinician :
1-to change the angle of strabismus .
2-to establish fusion.
3- to provide an adjunct to vision therapy programs.
4- or to postpone vision therapy or surgery.
Prisms or lenses may also affect cosmetic enhancement when
there is a poor prognosis for functional treatment. In these cases,
their use has been loosely referred to as "optical surgery .
In most cases, decisions to use prism are typically made at the
outset. Any remaining strabismic angle is treated subsequently
using added lenses (e.g., bifocals). Treatment of the distance angle
with
prism allows for the use of smaller amounts of near additions.
461
Prism Optical Correction
After the determination of refractive error, the clinician should
determine if additional prism allows for bifixation.
This can often be done at the initial examination. However , if the :
1- patient has not worn a correction previously and
2- a rather high refractive error was revealed during cycloplegia, it
is usually best to provide the patient with full refractive correction
for 4 to 6 weeks and then to retest the deviation with prisms .
462
so they may not be well accepted by some patients.
1-When the patient is treated for mild amounts of amblyopia, placing
the Fresnel prism before the "better" eye may provide the
necessary reduction in acuity to serve as occlusion so that the
amblyopic eye can improve acuity without the use of a more obvious
occlusion.
2-When there is a marked difference in the visual acuity of the eyes
(anisometropia) and amblyopia will not respond to treatment, the
major portion of the Fresnel optical correction is usually prescribed
before the eye with poorer vision.
463
Advantages and disadvantages of
Fresnel prisms
Advantages
1-Fresnel prisms are very useful as trial prisms for prism
adaptation tests.
2- and they provide immediate relief for symptomatic patients.
Disadvantages
1-Fresnel prisms decrease visual acuity (up to one line for 5DP
and up to three lines for 15DP)
2-and decrease contrast sensitivity , who may later be
prescribed a ground-in prism.
464
In more elaborate
contrast sensitivity
tests, you may be
Contrast sensivity tests.
shown images such
The Pelli Robson contrast
as dots and bars with
sensitivity chart tests your
different contrasting
ability to detect letters that are
backgrounds under
gradually less contrasted with
varying conditions
the white background as your
such as glare.
eyes move down the chart
(Images
465
If a strabismic angle remains after refractive correction and
correspondence is suspected, the clinician would be ‫يفطن‬normal
astute to perform a prism adaptation test by prescribing prism for
a few weeks. Fresnel prism is especially useful in this regard, when
the attempt is made to obtain sensory fusion in the patient's
normal environment.
The amount of prism required to obtain fusion on a second-degree
target (i.e., red glass or Worth dot) at distance is prescribed in the
form of Fresnel press-on prisms. The patient is encouraged to wear
this optical correction full time for 1 to 2 weeks.
466
On recheck, the patient will either show a stable strabismic angle with
fusion or will have increased the size of the eye turn. In the
vernacular,‫ بالعامية‬this latter condition is known as "eating prism."
Should the angle increase, removal of the Fresnel prism will allow the
eyes to resume their previous position in a short period of time.
If the test is positive (i.e., if fusion is present), several options become
possible: the prism may be :
1-ground into the spectacles;
2- the Fresnel prisms may be "dribbled off," ‫ يقلل بالتدريج‬perhaps in
conjunction with vision therapy;
3- or a surgical referral may be appropriate.
The procedure for dribbling or weaning the patient off of the prism is
described in Box 31-1.
467
Procedure for Weaning (Dribbling Off ) Prism :
1. Prescribe the amount of prism required to produce fusion with the
"red lens test" in a dark room. Usually Fresnel prisms are prescribed
that split the total prism between the two eyes.
2. Have the patient return in 1 month.
3. Check fusion with a red lens in a lit room.
4. If the patient is fusing, attempt to reduce the compensating prism
by holding a horizontal prism (or 16 for vertical prism) in the
opposite direction. For example, if a patient is wearing BO prism,
hold up a BI prism over the optical correction.
5. If the patient is fused with the reduced amount of prism, prescribe
the new amount by reducing the Fresnel prism over one eye.
468
Red Lens, Short Handle Occluder is
molded from strong, lightweight plastic.
Perfect for measuring lateral and vertical
phoria, in both near and distant vision
469
Procedure for Weaning (Dribbling Off ) Prism :
6. If the patient is unable to fuse with the reduced amount , do not
modify the optical correction.
7. Have the patient return in 2 to 4-week intervals until no
further reduction is possible. Usually three successive visits
without change indicate that a plateau has been reached.
8-Vision therapy may improve chances for prism reduction.
9. When a plateau is reached (no change in 3 consecutive months),
the prism may be ground into the lenses for the permanent
optical prescription .
470
vertical deviation
An increase in the vertical deviation between primary gaze and
downgaze often produces symptoms at the reading position. These
patients may benefit from separate optical corrections for distance
and near work.
However, there are more viable options for varying the amount of
vertical prism between viewing distances in the vertical direction than
in the horizontal direction.
Of course, Fresnel prism segments may be used with the convenience
as well as the restraints listed above.
1- Traditional slab-off prism (BU) is prescribed to the more myopic
or less hyperopic eye, and prefabricated ‫ مبني مقدما‬reverse
2- slab-off prism (BD) can be prescribed in the opposing manner.
471
Use of Added (PLUS or Minus) Lenses
When the angle of deviation is different at distance and near,
modification of the optical correction may allow fusion at both
distances. Additional lenses (powers added to the manifest optical
correction) may be used for cases of :
1- exotropia (distance minus adds, especially for divergence excess)
2- or esotropia (near plus adds, especially for convergence excess).
472
press-on optics
Another form of press-on optics is available from Neoptx of
Redmond, WA. Called Optx 20/20, these press-on additional
segments are supplied in pairs with refractive powers in 0.50 DS
increments ‫ كسور‬between +1.0 DS and +3.0 DS. These press-on
segments are achieved with the use of conventional surface
curvatures that result in visual clarity much improved over that of
Fresnel press-on segments. Hence, the practitioner has a superior
device for diagnostic or therapeutic use (Figure 31-4).
473
Press-On™ Prism and Aspheric Lens
3M™ Press-On™ Prisms are a valuable tool used to treat
several ocular motility disorders, including strabismus. The
Press-On lenses are useful for some refractive corrections and
you can use both types of optics for either temporary or long
term corrections.
3M™ Press-On™ Prisms, also known as Fresnel Prisms, and
Press-On Aspherics, provide a system for creating prismatic
and spherical corrections is simple, inexpensive, and fast.
Press-on prism material is flexible static vinyl which can
easily be cut to shape with scissors to determine the
acceptance
of a proposed corrective prescription.
474
Adheres well to existing lenses, yet it can be repositioned easily.
You can apply a Press-On sphere or prism to the back surface of
one or both lenses of the patient`s eyeglasses with just water.
It can be applied to the entire lens or to any region of the lens.
A full range of seventeen powers from 1.0 to 40.0 prism diopters
allows you to broaden therapeutic uses and provides additional
treatment options in your practice.
475
Exodeviations (Added Minus(
Added minus can provide substantial clinical benefit (e.g. prolonged
fusion, improved stereopsis) for some patients with exotropia.
The exact mechanism for the increase in fusion seen with added
minus lenses has not known.
476
Constant Exotropia
Because the patient with constant exotropia is essentially monocular,
added minus stimulates accommodative convergence and thereby
reduces the angle of strabismus. For example, if there is a 35 degree
constant exotropia and the patient has an AC/A (accommodative
convergence/accommodation) ratio of 4.0 DP/1.0 DS, up to 9.0 DS of
minus (4.0DP/1.0 DS x 9.0 DS = 36 degree) may theoretically be
used to provide binocular alignment.
Thus, for the patient with constant exotropia, large amounts of added
minus lenses may theoretically be used. Usually a lower amount of
minus power is used to initiate a converge response through
accommodation then gradual lens power occurs .
477
Intermittent Exotropia
1- Minus lenses may be added to the distance optical correction to
enhance fusion in patients who have intermittent exotropia at
distance. The minus lenses stimulate accommodation to clear
distant targets and to induce accommodative convergence via the
AC/A ratio that will hopefully permit fusion at distance.
2- An alternative mechanism of action for those able to converge
to fuse without overcorrection is that the CA /C (convergence
accommodation/convergence) ratio can stimulate accommodation
for near and force the patient to become blurred at distance.
478
Adding minus will clear up the pseudomyopia , thereby allowing the
patient to see clearly at distance. For instance, a patient with 24DP
intermittent exotropia who has a CA/C ratio of 0.50 DS/6 DP will
require 20 DS of accommodation stimulated by convergence
(0.50DS/6.0DP x 24 DP = 2.0 DS). Because -2.0 DS lenses will
allow clear vision, they additionally will facilitate fusion. As is seen
with the added lenses prescribed to patients with constant exotropia,
these are presented to the patient and parent as "exercise lenses,"
because they are used to help maintain fusion rather than to improve
visual acuity.
Care must be taken not to cause a near eso deviation while restoring
distance fusion. Most eso deviations are noted immediately after
starting overminus therapy , and these are quickly eliminated after
full-time wear of the optical correction for a few weeks.
479
It is not necessary to determine the CA/C to ascertain the starting point
for overminus optical correction. The goal is simply to allow clear
vision rather than to attempt to neutralize the distance angle of
deviation. It is usually best to simply prescribe an appropriate
amount of overminus (a good starting point is -1.50 to -2.0 D) and to
bring the patient back in a month to measure the results.
The benefit can be seen when the patient can more rapidly regain
fusion on the unilateral cover test or when the added minus lenses
provide a substantial increase in binocular acuity when the patient
reads an acuity chart during times when the eyes are aligned .
480
On monthly follow-up visits, a decision may be made about the
modification of the optical correction. Because the clinician is not able
to monitor the child during daily activities, parental reports of the
frequency of the exodeviation are important. Objectively, the recovery
of bifixation after the removal of an occluder serves as a good clinical
guideline for the effectiveness of treatment. This may be rated on a 0
(no recovery) to a 4+ (very rapid) scale. Comparison should be made
between the normally fixating eye's recovery and the recovery of the
strabismic eye. A near esophoria should signal the consideration of a
bifocal spectacle prescription or the use of the Optx 20/20 press-on
segments described above. In addition, accommodative difficulties
resulting from overminus optical correction may be revealed by MEM
(Monocular estimate method) retinoscopy.
481
A large lag of accommodation indicates that the patient may be
having some difficulty with the minus overcorrection.
Again, a bifocal will eliminate the problem by canceling the effect
of the overminus at near. The parents should be told at the
beginning of therapy that the eye turn may manifest itself when the
child is very tired or ill (intermittent deviation) and that the real
goal of therapy is the reduction of the frequency of the turn.
482
Esodeviations
A more common use of added lenses is to eliminate near
esodeviations.
1- With infants and young toddlers having a near visual world, a
single vision optical correction may be given with an added plus
lens that is greater than the cycloplegic evaluation revealed.
The goal is to obtain binocular alignment through a range in space.
Distance visual acuity may be compromised, but it is rarely a problem
in the young age group. However, there is a fine line between
fostering ‫ تعزيز‬binocularity and disrupting ‫ إخالل‬visual acuity to the
point that the optical correction is rejected. Frequent follow-up visits
are needed to monitor the deviation and to ensure compliance with the
spectacle correction. Cycloplegic evaluation should be repeated every
4 to 6 months, with lens modifications performed as needed .
483
2-Children in preschool programs and older children: Bifocals are
best used with a starting point for near plus additions, in diopters,
may be determined by dividing the distance deviation in prism
diopters measured through the best distance optical correction by the
interpupillary distance measured in centimeters.
Theoretically, a target placed at the focal length of this plus lens
power should be bifixated (the centration point). For example, an 18
DP esotrope at distance with a 60-mm interpupillary distance should
be orthophoric with a +3.00 DS add when fixating a target at 33 cm.
It is often found that a lens of even less power than that predicted by
the rule of thumb is adequate to straighten the eyes.
rule of: ‫ حساب تقريبي‬,‫ حكم التجربة‬,‫ قاعدة ناجمة عن اإلختيار‬,‫ معتمد على الخبرة‬,‫حكم يعتمد على الخبرة‬
thumb
484
Regardless of the starting point, the patient should return for
reevaluation after 1 month of wear. MEM retinoscopy is very useful
for determining the potential for any further plus acceptance at near.
As long as a lag of accommodation is observed, the patient is not
overplussed. Bifixation may be determined by the unilateral cover test.
Sensory testing with third-degree (stereoscopic) targets provides the
best subjective information regarding fine ocular alignment.
An alternative to bifocals for patients with moderate to high hyperopia
and convergence excess is the use of contact lenses. Some have
theorized that, because of the proximity to the nodal point and the
movement of contact lenses with the eye, contact lenses provide more
relaxation of near eso than the same power in plus spectacles.
There is an increase in clarity of the peripheral visual field and little
or no prismatic effects such as are present with spectacles. Children
usually respond quite well to contact lenses if they are mature enough
to be compliant with lens maintenance.
485
WHEN FUSION IS NOT POSSIBLE
Equal visual acuity in the two eyes and bifixation are the goals that the
clinician strives to achieve with every patient. However, in many
cases, binocular fusion is simply not possible. In these cases, the goal
of the optical prescription is to allow comfortable, efficient monocular
visual skills to develop. This may be accomplished initially by simply
prescribing the refraction found in each eye without any attempt to
achieve binocularity.
However, there are times when a patient with strabismus does not have
the potential for fusion and when the strabismus is cosmetically
unacceptable. The common option for these cases has been surgical
intervention. When this is not desired by the patient or the surgeon
(e.g., because of patient health problems or the small size of the
deviation), a cosmetic optical option may be useful. London has
referred to this as "optical surgery," because the goal is not fusion but
rather cosmetic improvement by making the deviating eye appear to
align
486 with the fellow eye.
A common device used in these cases is a reverse prism that is used
for cosmesis.
In other words, a BI prism may be used to reduce the inward
appearance of an esotropic eye. Approximately 15DP is usually
needed to achieve the cosmetic improvement. This is best
accomplished by prescribing 8DP BI in front of the deviating eye and
7DP BO in front of the fixating eye (Figure 31-5). The edges of
lenses with large amounts of prism should be rolled and coated and
placed within a polymer frame, with the front of the lens flush with the
frame. In this manner, all of the prism will be positioned behind the
frame, and it will be as cosmetically neutral as possible.
487
Added Lenses
Bifocals or progressive addition lenses may also be used for cosmetic
purposes in patients with esotropia that is partially accommodative.
Although eye turns during near tasks are often not noticeable because
the near work hides the eyes, there are some patients who benefit
cosmetically from this intervention. These include patients who work on
video display terminals and those who have a time lag between looking
up from near and reestablishing acceptable ocular alignment.
When equal visual acuity is obtained in each eye, the patient may
develop an alternating strabismus. This helps to maintain the visual
acuity in each eye, but occasionally patients will have reading problems.
1- Words "jump" when the patient is reading,
2- or the patient tends to lose his or her place when the eyes alternate
fixation in the middle of reading a line of print.
488
For this reason, it is often better to train patients capable of alternation
to use one eye primarily for distance vision and the other eye for near
vision. Monovision is prescribed in this situation with spectacles or
contact lenses to stop the eyes from alternating between themselves
and competing at the same distance. For a large portion of patients,
monovision optical correction can alleviate difficulties with binocular
competition in cases of alternating strabismus. In addition to patients
with alternating strabismus, monovision often provides relief to
patients with acquired strabismus who are experiencing constant
diplopia. The blurring of the image in the deviating eye makes it
easier for the patient to suspend or suppress and, therefore, to ignore
the out-of-focus eye. Box 31-2 lists several steps that increase the
success of monovision in the case of alternating strabismus or
diplopia.
489
Guidelines for the Monovision Correction of Diplopia and Alternating Strabismus
1.Prescribe the distance optical correction to the less deviated eye if it
can look into primary gaze. If the eyes are equally deviated, use the
patient's preferred eye for distance correction.
2. Take advantage of the eye position. If one eye is hypotropic, use it for
the near optical correction.
3. If the patient is prepresbyopic, slightly overminus the distance optical
correction (by 0.50 DS) and slightly overplus the near optical correction
(by 0.50 DS) until the patient has adapted; then the
normal optical correction can be given,
4. Near optical correction may be attempted before the dominant eye to
help alternation.
5. Use of cycloplegia for the distance eye may assist the prepresbyopic
patient to alternate eyes.
6. Instruct the patient to simply look at and see the object of regard and
to not try to figure out ‫معرفة‬which eye is working for any given distance.
7-The
patient should enjoy seeing at all distances without overanalyzing
490
how the system works.
Occlusion Options
A last resort, sometimes required in cases of acquired
strabismus with complaints of diplopia, is the occlusion
of one eye. Occlusion options include decisions about
whether the entire visual field (whole eye) should be
occluded or only a sector of the visual field should be
occluded (partial occlusion). In addition, the clinician
must decide whether the occlusion should be opaque or
translucent. An opaque, total occluder blocks out light
most effectively. Fortunately, it is rarely necessary to
resort ‫ اللجوء إلى‬to an option with such poor cosmesis.
491
Translucent occluding can be performed with frosted lenses, ‫العدسات‬
‫بلوري‬clear nail polish, ‫طالء األظافر‬frosted tape, ‫الشريط بلوري‬
and Bangerter filters. Bangerter filters are translucent press-on lenses
that are available in various densities of translucence. They can
degrade visual acuity from 20/25 (6/7.5) to 20/200 (6/60), depending
on the density.
Clear contact paper ‫ الالصق الشفاف‬has been found to be effective
(visual acuity reduced to 20/400 or 6/120), inexpensive and easy for
the clinician and patient to use. Figure 31-6 (purchased in hardware
stores), and easy for the clinician and patient to use. It may be placed
over the whole lens, in a sector, or in a circular patch near the center
of the lens. A small circle is preferable whenever
possible, because it allows for vision
of a good portion of the peripheral field,
eliminates the diplopia , and is cosmetically
more acceptable (Figure 31-7).
492
493
494
495
496
497
‫شكرا لصبركم وانتباهكم‬
‫أمنياتي لكم بالتفوق‬
‫‪498‬‬