Transcript VISUAL ACUITY

VISUAL ACUITY
PRESENTED BY
T.Muthuramalingam
DEFINITION
Is an ability of eye to discriminate two
stimuli separated in space.
Is the resolving power of eye.
PURPOSE
For refraction and prescribing spectacles
For monitoring ocular health
PRINCIPLE
To see an object it must subtend 1min
angle at the nodal point.
That means to produce an image of the
minimum size of 0.004mm
NOTATION
 Visual acuity is noted in terms of Snellen’s
fraction.
 Near visual acuity is noted in terms of N
notation
 V/A = distance at which test is made
distance at which the smallest
letter read subtend angle of 5min.
 E.g. the distance between pt and test
object is 6m and he read the letter of size
for 9m for normal person, then V/A=6/9
 Print size of the letter is near visual acuity
 E.g letter size is 8point the V/A= N8
Equipments for
measurement
 V/A is measured by using different charts.
 Like snellen’s chart, ETDRS chart,logmar chart
etc
 Snellen’s chart is more popular using chart
 Equipments:
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Snellen’s chart
Pointer
Occluder
Pinhole
Near vision chart
Construction of snellen’s
chart
 Principle:
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Each letter is designed in a
(Fig:)square with sides 5 times
the width of letter strokes.
The breadth of black (red)
strokes and spaces are equal.
The breadth of line and spaces
produce 1° of angle at nodal
point when viewed at certain
distance.
Each letter subtends an angle
of 5° at the nodal point when
seen at a certain distance.
Construction
 The first line of the type is so constructed that the
angle is formed at a distance of 60 meters, the
sixth at 9, the seventh at 6,while additional lines
are usually inserted which subtend the same
angle at 5 and 4 meters .
 These letters should thus be read by a person with
standard vision at these distance away.
Contd.
Meters
6/6
6/9
6/12
6/18
6/24
6/36
6/60
Feet
20/20
20/30
20/40
20/60
20/80
20/120
20/200
Types of chart and
room setup
 Paper chart, projection chart, E-chart, alphabet
chart, picture chart
 Room’s length should be 6m as light rays are
parallel for practical purpose.
 If it is not possible then required distance is
achieved by using mirror in 3m.(for reflection of
reversed test types)
 Illumination should not less then 50lumen/sq.ft
 Chart should be more or less at the eye level of
the patient.
Distance visual acuity
 This measure the patient’s distance vision by
testing the ability to read distance characters at a
standard distance.
 Snellen’s chart is used with different test types.
 For literate number chart and alphabet chart and
for illiterate E-chart,broken C-chart are used
 The normal V/A is 6/6 in meter or 20/20 in feet
Near visual acuity
It is measured at distance with in arm’s
length usually it is 33to 40cm.
Test material is in the style of book or
newspaper but in series of unrelated words.
The size label ‘N8’indicates that the size of
test font is 8points and distance is
specified.
E.g. N8 at 40cm.
Pinhole acuity
A below – normal visual acuity recording
may be result of refractive error which can
be conformed with pinhole acuity
If use of pinhole improves a patients poor
uncorrected V/A means pt’s has refractive
errors.
If not improved then it is any other problem
other than refractive error.
Procedure of recording
V/A
Contd.
 Position the Patients 20 feet( 6 meters ) from an
illuminated snellen’s chart.
 Have the patient cover the left eye with an Occluder.
 Ask the patient to read the letters from left to right up to
last line.
 Note the smallest line in which more then half the
characters are read correctly.
 Record the corresponding acuity fraction as well as the
number of letters missed. Ex,6/18 –2
 Repeat Steps 2 to 4 with the right covered.
 Record the acuity value for each eye separately, with and
without correction.
Contd.
 If the subjects cannot read the largest letter he is
asked to walk towards the types.For example if he
sees the top letter at a distance of 2 meters,
then VA =2/60.
 If not possible – VA = CF(counts fingers) at 1meter
 If not possible – VA = HM(Hand movements).
 If not possible – VA = PL(perception of light)
 If not recorded - VA = No PL, this is total
blindness.
Vision assessment in
children
 Neonates - Follow a light, face object(ask mother)
Optokinetic nystagmus
► 3-6 months
- Fixation and following
► 6-18 months – Preferential looking (keeler card)
► 9-24 months - Cake decoration
► 2-3 years
- Cardiff vanishing card
► 21/2–5 years - picture cards & matching charts
(S.G.card)
► >5 years
- Snellen chart
Types of chart
 Keeler Card( fig.1)
 Cardiff Card( fig.2)
 Sheridan Gardiner test( fig.3)
 Cambridge Crowding Card( fig.4)
 Picture Chart( fig.5)
 Snellen’s Chart( fig.6)
1.
3.
2.
5.
4.
6.
Points to remember
 Carefully observed whether the patients is
completely covering the eye with cup of hand or
not
 Observed the position of head whether he is
trying to see from side
 Tell the patient to sit straight
 As V/A plays a vital role in eye examination it
should be recorded carefully.