Transcript الشريحة 1

STRABISMUS
BY
DR.
AMER ISMAIL ABU IMARA
JORDANIAN BOARD OF OPHTHALMOLOGY
I.C.O.
PALESTINIAN BOARD IN OPHTHALMOLOGY
‫بسم هللا الرحمن الرحيم‬
‫ علم‬‫ فن‬‫ تخيل‬‫ قرار‬‫ متابعة‬‫” ال يوجد علم دون تعب ‪ ...‬ودراسة مستمرة ‪ ...‬و متابعة‬
‫حثيثة ”‬
‫قال رسول هللا صلى هللا عليه و سلم ‪ ( :‬ان هللا يحب اذا عمل‬
‫احدكم عمال ان يتقنه ) ‪.‬‬
DEFENITIONS
• VISUAL AXIS (line of vision ) :
passes from the fovea through the nodal
point of the eye to the point of fixation (
object of regard ) .
In normal binocular single vision (BSV) the
two visual axes intersect at the point of
fixation , with the images from the two
eyes being aligned by the fusion reflex
and combined by binocular responsive
cells in the visual cortex to give BSV .
ORTHOPHORIA
Implies perfect ocular alignment in the
absence of any stimulus for fusion which is
uncommon.
HETEROPHORIA ( PHORIA )
Implies a tendency of the eyes to deviate
when fusion is blocked (latent squint ).
Slight phoria is present in most normal
individuals and is overcome by the fusion
reflex.
It can be either esophoria or exophoria .
When fusion is insufficient to control the
imbalance the phoria is described as
decompensating and is often associated
with symptoms of binocular discomfort or
double vision ( diplopia ).
HETEROTROPIA
It implies a manifest deviation in which the
visual axes do not intersect at the the
point of fixation .
The images from the two eyes are
misaligned so that either double vision is
present or more commonly in children ,
the image from the deviating eye is
suppressed at cortical level .
Why squint in childhood ?
1- failure of the normal development of
binocular fusion mechanisms.
2- oculomotor imbalance secondary to
differences in refraction of the two eyes
Why squint in adult ?
1- failure of fusion , for example secondary
to poor vision in one eye .
2- weakness of muscles .
3- restriction of muscles .
4- damage to nerve supply .
-
-
Horizontal deviation ( latent or manifest )
is the most common form of strabismus .
- A vertical deviation almost invariably
reflects abnormal ocular motility .
Upward displacement of one eye relative
to the other is termed hypertropia and a
controlled upward imbalance a
hyperphoria .
- Downward displacement is termed a
hypotropia and controlled imbalance a
hypophoria .
ANATOMICAL AXIS
Is a line passing from the posterior pole
through the center of the cornea .
Because the fovea is usually slightly
temporal to the anatomical center of the
posterior pole of the eye , the visual axis
does not usually correspond to the
anatomical axis of the eye .
ANGLE KAPPA
Is the angle subtended by the visual and the
anatomical axes and is usually about 5
degrees .
The angle is positive when the fovea is
temporal to the center of the posterior
pole resulting in a nasal displacement of
the corneal reflex and negative when the
converse applies .
A large angle kappa may give the
appearance of a squint when none is
present ( pseudo- squint ) and is seen
most commonly as a pseudo- exotropia
following displacement of the macula in
ROP where the angle may significantly
exceed + 5 degrees .
ANATOMY OF EXTRAOCULAR MUSCLES
The lateral and medial orbital walls are at
angle of 45 degrees with each other.
The orbital axis therefore forms an angle of
22.5 degrees with both the lateral and
medial walls .
~ 23 degrees
When the eye is looking straight a head at a
fixed point on the horizon with the head
erect ( primary position of gaze ), the
visual axis forms an angle of 23 deg. With
the orbital axis .
The action of the extraocular muscles
depend on the position of the globe at the
time of muscle contraction .
Primary action : of a muscle is it’s major
effect when the eye is in the primary
position .
Subsidiary actions : are the additional effect
which depend on the position of the eye .
Listing plane : is an imaginary coronal plane
passing through the center of rotation of
the globe .the globe rotates on the X and
Z axes of Fick, which intersect in Listing
plane .
The globe rotates left and right on the
vertical Z axis .
- The globe moves up and down on the
horizontal X axis .
Torsional movements ( wheel rotation ) on
the Y ( sagittal) axis which traverses the
globe from front to back ( similar to the
anatomical axis of the eye .
Intorsion occurs when the superior limbus
rotates nasally and extorsion on temporal
rotation .
-
-
-
HORIZONTAL RECTUS MUSCLES
When the eye is in the primary position , the
horizontal recti are purely horizontal
movers on the vertical Z axis and have
only primary actions .
MEDIAL RECTUS : originates at the annulus
of Zinn at the orbital apex and inserts 5.5
mm behind the nasal limbus . it’s sole
primary action is adduction .
LATERAL RECTUS : originates at the annulus
of Zinn and inserts 6.9 mm
Behind the temporal limbus .it’s sole primary
action is abduction .
VERTICAL RECTUS MUSCLES
The vertical recti run in line with the orbital
axis and are inserted in front of the
equator . They therefore form an angle of
23 deg. With the visual axis .
SUPERIOR RECTUS : originates from the
upper part of the annulus of Zinn and
inserts 7.7mm behind the superior limbus.
The primary action is elevation , secondary
actions are adduction and intorsion .
when the globe is abducted 23 deg. The
visual and orbital axes coincide . In this
position it has no subsidiary actions and
can only act as elevator .
This is therefore the optimal position of the
globe for testing the function of the
superior rectus muscle .
If the globe was adducted 67 deg. , the
angle between the visual axis and orbital
axis would be 90 deg.
In this position the superior rectus could
only act as intorter .
INFERIOR RECTUS : originates at the lower
part of the annulus of Zinn and inserts 6.5
mm behind the inferior limbus .
The primary action is depression , secondary
actions are adduction and extorsion .
When the globe is abducted 23 deg. The
inferior rectus acts purely as a depressor.
As for superior rectus , this is the optimal
position of the globe for testing the
function of the inferior rectus muscle .
If the globe was adducted 67 deg. The
inferior rectus could only act as an
extortor .
SPIRAL OF TILLAUX
The spiral is an imaginary line joining the
insertions of the four recti and is an
important anatomical land mark when
performing surgery .
The insertions get further away from the
Limbus and make a spiral pattern .
Med.rectus 5.5 mm
Inf.rectus 6.5 mm
Lat. Rectus 6.9mm
Superior rectus 7.7 mm
OBLIQUE MUSCLES
The obliques are inserted behind the
equator and form an angle of 51 deg with
the visual axis .
SUPERIOR OBLIQUE
Originates superomedial to the optic
foramen. It passes forward through the
trochlea at the angle between the superior
and medial walls and is then reflected
backwards and laterally to insert in the
posterior upper temporal quadrant of the
globe .
The primary action of the superior oblique is
intorsion , secondary actions are
depression and abduction .
The anterior fibers of the superior oblique
tendon are primarily responsible for
intorsion and the posterior fibers for
depression , allowing separate surgical
manipulation of these two actions .
When the globe is adducted 51 deg. The
visual axis coincide with the line of pull of
the muscle . In this position it can only act
as a depressor . This is therefore , the
best position of the globe for testing the
action of the superior oblique muscle.
Thus , although the superior oblique has an
abducting action in primary position , the
main effect of the superior oblique
weakness is seen as failure of depression
in adduction .
When the eye is abducted 39 deg. The
visual axis and the superior oblique make
an angle of 90 deg. With each other .
In this position the superior oblique can only
cause intorsion .
INFERIOR OBLIQUE
Originates from a small depression just
behind the orbital rim lateral to the
lacrimal sac .
It passes backward and laterally to insert in
the posterior lower temporal quadrant of
the globe , close to the macula .
The primary action is extorsion , secondary
actions are elevation and abduction .
When the globe is adducted 51 deg. , the
inferior oblique acts only as an elevator .
When the eye is abducted 39 deg. , it’s
main action is extorsion .
MUSCLE PULLEYS
The four rectus muscles pass through
condensations of connective tissue and
smooth muscle just posterior to the
equator .
These condensations act as pulleys and
minimize upward and downward
movements of the of the bellies of the
medial and lateral rectus muscles during
up-gaze and down-gaze … and
horizontal movements of the superior
rectus and inferior rectus bellies in
Left and right gaze .
Pulleys are the effective origins Of the
rectus muscles and play an important role
in the coordination of eye movements by
reducing the effect of horizontal
movements on vertical muscle actions and
vice versa .
Displacement of the pulleys can be one
cause of abnormalities of eye movements
such as V and A patterns .
NERVE SUPPLY
LATERAL RECTUS :is supplied by the 6th
cranial nerve .
( abducent nerve – abducting muscle ).
SUPERIOR OBLIQUE : is supplied by the
fourth cranial nerve .
( trochlear nerve – muscle associated with
the trochlea ) .
OTHER MUSCLES : together with the levator
muscle of the upper lid and the ciliary
and
Sphincter pupillae muscles are supplied by
the 3rd ( oculomotor ) nerve .
OCULAR MOVEMENTS
DUCTIONS
Ductions are monocular movements around the
axes of Fick . They consist of adduction ,
abduction , elevation , depression , intorsion and
extorsion .
They are tested by occluding the fellow eye and
asking the patient to follow a target in each
direction of gaze .
Torsional ductions are mainly observed in
association with other abnormal eye movements
.
VERSIONS
Versions are binocular, simultaneous ,
conjugate movements ( in the same
direction ) .
Dextroversion and laevoversion ( gaze right
, gaze left ) , elevation ( up-gaze ) and
depression ( down-gaze ).
These four movements bring the globe into
the secondary positions of gaze by
rotation around either a vertical Z or a
horizontal X axis of Fick .
Dextroelevation and dextrodepression ( gaze
up and right , gaze down and right ) and
laevoelevation and laevodepression ( gaze
up and left and gaze down and left ).
These four oblique movements bring the
eyes into the tertiary positions of gaze by
rotation around oblique axes lying in
Listing plane , equivalent to simultaneous
movement about both horizontal and
vertical axes .
Torsional movements to maintain upright
images occur on tilting of the head .
These are known as the righting reflexes .
On head tilt to the right the superior limbi
of the two eye rotate to the left , causing
intorsion of the right globe and extorsion
of the left .
VERGENCES
Vergences are binocular , simultaneous ,
disjugate or disjunctive movements ( in
opposite directions .
Convergence is simultaneous adduction (
inward turning ) , divergence is outwards
movement from a convergent position.
Convergence may be voluntary or reflex .
Reflex convergence has four components :
- Tonic convergence : which implies
inherent innervational tone to the medial
recti , when the patient is awake .
- Proximal convergence : is induced by
psychological awareness of a near object.
- Fusional convergence : is an optomotor
reflex , which maintains BSV by insuring
that similar images are projected onto
corresponding retinal areas of each eye .
-
It is initiated by bitemporal retinal image
disparity .
Accommodative convergence : is induced
by the act of accommodation as part of
the synkinetic - near reflex .
Each dioptre of accommodation is
accompanied by a constant increment in
accommodative convergence , giving the
accommodative convergence by
accommodation ( AC/A ) ratio .
This is the amount of convergence in prism
dioptres per dioptre change in
accommodation .
The normal value is 3-5 prism dioptre .
this means that one dioptre of
accommodation is associated with 3-5
prism dioptres of accommodative
convergence .
It will be shown later that abnormalities of
the AC/A ratio play an important role in
the aetiology of strabismus .
These changes in accommodation ,
convergence and pupil size , which occur
in response to a change in the distance of
viewing are known as the near triad and
occur in response to both image blur and
temporal image disparity .
POSITIONS OF GAZE
SIX CARDINAL :positions of gaze are those
in which one muscle in each eye has
moved the eye into that position as
follows :
- Dextroversion ( right lateral rectus and left
medial rectus ).
- Laevoversion ( left lateral rectus and right
medial rectus ).
- Dextroelevation ( right superior rectus and
left inferior oblique ).
- Laevoelevation ( left superior rectus and
right inferior oblique ).
- Dextrodepression ( right inferior rectus
and left superior oblique ).
- Laevodepression ( left inferior rectus and
right superior oblique ).
NINE DIAGNOSTIC positions of gaze are
those in which deviations are measured .
They consist of the six cardinal positions ,
the primary position, elevation and
depression ).
LAWS OF OCULAR MOTILITY
1- AGONIST- ANTAGONIST : are muscles of
the same eye that move the eye in
opposite directions . The agonist is the
primary muscle moving the eye in a given
direction . The antagonist acts in the
opposite direction to the agonist .
Example : the right lateral rectus is the
antagonist to the right medial rectus .
2- SYNERGISTS :are muscles of the same
eye that move the eye in the same
direction .
Example : the right superior rectus and the
right inferior oblique act synergistically in
elevation .
3- YOKE MUSCLES :( contralateral synergists
) are pairs of muscles , one in each eye ,
that produce conjugate ocular movements
Example : left superior oblique – right
inferior rectus .
SHERRINGTON LAW of reciprocal
innervation ( inhibition ) states that
increased innervation to an extraocular
muscle ( e.g. right medial rectus ) is
accompanied by a reciprocal decrease in
innervation to its antagonist ( e.g. right
lateral rectus ).
This means that when the medial rectus
contracts the lateral rectus automatically
relaxes and vice versa .
Sherrington law applies to both versions and
vergences .
HERING LAW of equal innervation states
that during any conjugate eye movement,
equal and simultaneous innervation flows
to the yoke muscles .
In the case of a paretic squint , the amount
of innervation to both eyes is symmetrical
, and always determined by the fixating
eye , so that the angle of deviation will
vary according to which eye is used for
fixation .
For example , if in the case of left lateral
rectus palsy , the right normal eye is used
for fixation , there will be an inward
deviation of the left eye due to the
unopposed action of the antagonist of the
paretic left lateral rectus ( left medial
rectus ).
The amount of misalignment of the two
eyes in this situation is called the primary
deviation .
If the paretic left eye is now used for
fixation , additional innervation will flow
To the left lateral rectus , in order to establish this.
However , according to Hering law , an equal
amount of innervation will also flow to the right
medial rectus ( yoke muscle ).
This will result in an over action of the right medial
rectus and an excessive amount of adduction of
the right eye .
The amount of misalignment between the two
eyes in this situation is called the secondary
deviation . In a paretic squint the secondary
deviation exceeds the primary deviation .
MUSCLE SEQUELAE are the effects of the
interactions described by these laws. They
are of prime importance in diagnosing
ocular motility disorders and in particular
in distinguishing a recently acquired palsy
from a longstanding one .
The full pattern of changes takes time to
develop and can be summarized as follows
:
- Primary under action ( e.g. Left superior
oblique )
-
-
Secondary contracture of the unopposed
direct antagonist ( left inferior oblique ) .
Secondary contracture of the contralateral
synergist or yoke muscle ( right inferior
rectus , Hering law ).
- Secondary inhibitional palsy ( right
superior rectus , Sherrington law .
SECONDARY CONSIDERATIONS
BASIC ASPECTS
NORMAL BSV involves the simultaneous use
of both eyes with bifoveal fixation, so that
each eye contributes to a common single
perception of the object of regard .
This represents the highest form of
binocular cooperation .
Conditions necessary for normal BSV :
1- normal routing of visual pathways with
overlapping visual fields .
2- binocularly driven neurons in the visual
cortex .
3- normal retinal ( retino - cortical (
correspondence ( NRC ) resulting in
cyclopean viewing .
4- accurate neuromuscular development and
coordination , so that the visual axes
directed at , and maintain fixation on , the
object of regard .
5- approximately equal image clarity and
size for both eyes .
BSV is based on NRC , which requires first
an understanding of uniocular visual
direction and projection .
VISUAL DIRECTION
Is the projection of a given retinal element in a
specific direction in the subjective space .
PRINCIPAL visual direction is the direction in
external space interpreted as the line of sight .
This is normally the visual direction of fovea and
is associated with a sense of direct viewing .
SECONDARY visual directions are the projecting
directions of extra- foveal points with respect to
the principal direction of the fovea , associated
with indirect ( eccentric ) viewing .
PROJECTION is the subjective interpretation
of the position of an object in space on
the basis of stimulated retinal elements .
* If a red object stimulates the right fovea (
F ) , and black object which lies in the
nasal field stimulates a temporal retinal
element (T) , the red object will be
interpreted by the brain as having
originated from the straight a head
position and the black object will be
interpreted as having originated in the
nasal field .
Similarly nasal retinal elements project into the
temporal field , upper retinal elements into the
lower field and vice versa .
* With both eyes open , the red fixation object is
now stimulating both foveae , which are
corresponding retinal points . The black object is
now not only stimulating the temporal retinal
elements in the right eye but also the nasal
elements of the left eye . The right eye therefore
projects the object into its nasal field and the
left eye projects the object into its temporal
field.
Because both of these retinal elements are
corresponding points , they will both
project the object into the same position
in space ( the left side ) and there will be
no double vision .
RETINO – MOTOR VALUES
The image of an object in the peripheral visual
field falls on an extrafoveal element. To establish
fixation on this object a saccadic version of
accurate amplitude is required . Each extrafoveal
retinal element therefore has a retino – motor
value proportional to its distance from the fovea
, which guides the amplitude of saccadic
movements required to ( look at it ) .
Retino – motor value , zero at the fovea ,
increases progressively towards the retinal
periphery .
CORRESPONDING POINTS
Are areas on each retina that share the same visual
direction ( for example : the foveae share the primary
visual direction ).
Points on the nasal retina of one eye have corresponding
points on the temporal retina of the other eye and vice
versa .
For example , an object producing images on the right
nasal retina and the left temporal retina will be projected
into the right side of visual space . This is the basis of
NRC .
This retino – topic organization is reflected back a long the
visual pathways , each eye maintaining separate images
until the visual pathways converge onto binocularly
responsive neurons in the primary visual cortex .
THE HOROPTER
Is an imaginary plane in the external space ,
all points on which stimulate
corresponding retinal elements and are
therefore seen singly and in the same
plane .
This plane passes through the intersection
of the visual axes and therefore includes
the point of fixation in BSV .
PANUM FUSIONAL SPACE
Is a zone in front of and behind the horopter
in which objects stimulate slightly non
corresponding retinal points ( retinal
disparity ).
Objects are seen singly and the disparity
information is used to produce a
perception of binocular depth ( stereopsis
).
Objects in front of and behind Panum space
appear double .
This is the basis of physiological diplopia .
Panum space is shallow at fixation ( 6 sec.
of arc ) and deeper towards the periphery
( 30 – 40 seconds of arc at 15 deg. From
the fovea ).
Therefore objects on the horopter are seen
singly and in one plane . Objects in Panum
fusional area are seen singly and
stereoscopically . Objects outside Panum
fusional area appear double .
Physiological diplopia is usually accompanied
by physiological suppression and many
subjects remain unaware of this
phenomenon .
The retinal areas stimulated by images
falling within Panum fusional space are
termed Panum fusional areas .
BSV
Is characterized by the ability to fuse the
images from the two eyes and to perceive
binocular depth .
- SENSORY FUSION :involves the
integration by the visual areas of the
cerebral cortex of two similar images , one
from each eye , into one image .
It may be central , which integrate the
image falling on the foveae , or peripheral
, which integrates parts of the image
falling outside the foveae .
It is possible to maintain fusion with a
central visual deficit in one eye, but
peripheral fusion is essential to BSV and
may be affected in patients with field loss
as in advanced glaucoma .
- MOTOR FUSION : involves the
maintenance of motor alignment of the
eyes to sustain bifoveal fixation . It is
driven by retinal image disparity ,which
stimulates fusional vergences .
FUSIONAL VERGENCE
Involves disjugate eye movements to
overcome retinal image disparity .
Fusional vergence amplitudes can be
measured with prisms or in the
synoptophore .
Normal values are :
Convergence : about 15-20 ∆ for distance
and 25 ∆ for near .
Divergence : about 6-10 ∆ for distance and
12-14 ∆ for near .
Vertical : 2-3 ∆
Cyclovergence : about 2-3 °
Fusional convergence helps to control an
exophoria whereas fusional divergence helps to
control an esophoria . The fusional vergence
mechanism may be decreased by fatigue or
illness , converting a phoria to a tropia . The
amplitude of fusional vergence mechanisms can
be improved by orthoptic exercises , particularly
in the case of near fusional convergence for the
relief of convergence insufficiency .
STEREOPSIS
Is the perception of depth ( the third dimension ,
the first two being the height and width ). It
arises when objects behind and in front of the
point of fixation ( but within Panum fusional
space ) stimulate horizontally disparate retinal
elements simultaneously .
The fusion of such disparate images results in a
single visual impression perceived in depth .
A solid object is seen stereoscopically ( in 3D )
because each eye sees a slightly different aspect
of the object .
SENSORY PERCEPTIONS
At the onset of a squint two sensory
perceptions arise based on the normal
projection of the retinal areas stimulated :
Confusion and pathological diplopia may
result .
These require simultaneous ( visual )
perception i.e. the ability to perceive
images from both eyes simultaneously .
CONFUSION
Is the simultaneous appreciation of two
superimposed but dissimilar images
caused by stimulation of corresponding
retinal points ( usually the foveae ) by
images of different objects .
PATHOLOGICAL DIPLOPIA
Is the simultaneous appreciation of two
images of the same object in different
positions and results from images of the
same object falling on non- corresponding
retinal points .
- In esotropia the diplopia is homonymous (
uncrossed ).
- In exotropia the diplopia is heteronymous (
crossed ).
SENSORY ADAPTATION TO STRABISMUS
The ocular sensory system in children has the
ability to adapt to anomalous states ( confusion
and diplopia ) by two mechanisms :
A- suppression
B- abnormal retinal correspondence .
These occur because of the plasticity of the
developing visual system in children under the
age of 7-8 years .
Occasional adults who develop sudden – onset
strabismus are able to ignore the second image
after a time and therefore do not complain of
diplopia .
SUPPRESSION
Involves active inhibition , in the visual cortex , of
an image from one eye when both eyes are
open .
Stimuli for suppression include diplopia , confusion
and a blurred image from one eye resulting from
astigmatism / anisometropia .
Clinically , suppression may be :
1- CENTRAL OR PERIPHERAL in central
suppression the image from the fovea of the
deviating eye is inhibited to avoid confusion .
Diplopia on the other hand , is eradicated by the
process of peripheral suppression , in which the
image from the peripheral retina of the deviating
eye is inhibited .
2-MONOCULAR OR ALTERNATING
suppression is monocular when the image
from the dominant eye always
predominate over the image from the
deviating eye ( or more ametropic ) eye ,
so that the image from the latter is
constantly suppressed . This type of
suppression leads to amblyopia .
When suppression alternates ( switches
from one eye to the other ) amblyopia
does not develop .
3- FACULTATIVE OR OBLIGATORY
facultative suppression occurs only when
the eyes are misaligned . Obligatory
suppression is present at all times ,
irrespective of whether the eyes are
deviated or straight .
Examples are seen in intermittent exotropia
and Duane syndrome .
ABNORMAL RETINAL CORRESPONDENCE
Is a condition in which non-corresponding retinal
elements acquire a common subjective visual
direction , i.e. fusion occurs in the presence of a
small angle manifest squint .
The fovea of the fixating eye is thus paired with a
non- foveal element of the deviated eye .
ARC is a positive sensory adaptation to strabismus
( as opposed to suppression ) , which allows
some anomalous binocular vision in the
presence of a heterotropia .
Binocular responses in ARC are never as good as
in normal bifoveal BSV .
ARC is most frequently present in small angle
esotropia ( microtropia ) associated with
anisometropia .
MICROTROPIA
Is a small angle squint ( < 10 ∆ ) in which
stereopsis is present but reduced and there is a
relative amblyopia of the more ametropic eye .
Microtropia has two forms :
A- in microtropia with identity the point used for
monocular fixation by the squinting eye also
corresponds with the fovea of the straight eye
under binocular viewing conditions .
Therefore on cover test there is no movement of
the squinting eye when it takes up monocular
fixation .
B- in microtropia without identity the monocular
fixation point of the squinting eye does not
correspond with the fovea of the straight eye in
binocular viewing . There is therefore a small
movement of the deviating eye when it takes up
monocular fixation on cover testing .
ARC is less common in accommodative esotropia
because of the variability of the angle of
deviation , or in large angle deviations , because
the separation of the images is too great .
CONSEQUENCES OF STRABISMUS
- The fovea of the squinting eye is
suppressed to avoid confusion.
- Diplopia will occur , since noncorresponding retinal elements receive the
same image .
- To avoid diplopia , the patient will develop
either peripheral suppression of the
squinting eye or ARC .
- If constant unilateral suppression occurs
this will subsequently lead to strabismic
amblyopia .
MOTOR ADAPTATION TO STRABISMUS
Motor adaptation involves the adoption of
an abnormal head posture (AHP ) and
occurs primarily in children with
congenitally abnormal eye movements
who use the AHP to maintain BSV.
In these children loss of an AHP may
indicate loss of binocular function and the
need for surgical intervention .
These patients may present in adult life with
symptoms of decompensation , often
unaware of their AHP .
Acquired paretic strabismus in adults may be
consciously controlled by an AHP provided the
deviation is neither too large nor too variable
with gaze ( incomitance ).
The AHP eliminates diplopia and helps to centralize
the binocular visual field .
The patient will turn the head into the direction of
the field of action of the weak muscle , so that
the eyes are then automatically turned the
opposite direction and as far as possible away
from its field of action ( i.e. the head will turn
where the eye cannot ).
An AHP is analyzed in terms of the following three
components :
- Face turn to right or left .
- Head tilt to right or left .
- Chin elevation or depression .
1- A face turn will be adopted to control a purely
horizontal deviation .
For example , if the left lateral rectus is paralyzed ,
diplopia will occur in left gaze ; the face will be
turned to the left which deviates the eyes to the
right , away from the field of action of the weak
muscle and area of diplopia .
A face turn may also be adopted in a paresis of a
vertically acting muscle to avoid the side where
The vertical deviation is greatest ( e.g. in a
right superior oblique weakness the face is
turned to the left ) .
2- a head tilt is adopted to compensate for
torsional and / or vertical diplopia .
In left superior oblique weakness the left
eye is relatively elevated and the head is
tilted to the right , toward the hypotropic
eye ; this reduces the vertical separation
of the diplopic images and permits fusion
to be regained .
If there is a significant torsional component
preventing fusion , tilting the head in the
same left direction will reduce this by
invoking the righting reflexes ( placing the
extorted right eye in a position which
requires extorsion ) .
3- chin elevation or depression may be used
to compensate for weakness of an
elevator or depressor muscle or to
minimize the horizontal deviation when A
or V pattern is present .