Visual acuity by Abdulwahab Usman

TOPIC: VISUAL ACUITY
PRESENTER: ABDULWAHAB USMAN
30TH SEPTEMBER,2016
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 INTRODUCTION
 DEFINITION
 PRINCIPLES OF VISUAL ACUITY TESTING
 CLASSIFICATION
 MEASUREMENT OF VISUAL ACUITY
 CONDUCT OF VISUAL ACUITY
 BARRIERS TO GOOD VISUAL ACUITY TESTING
 ASSESMENT OF VISUAL ACUITY IN CHILDREN
 REFERENCES
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 Visual acuity is an essential assessment tool
in ophthalmic practice. It gives clue to the
functional state of the eyes as well as the
extent of visual loss in ophthalmic patients.
 Its role in the diagnosis and management of
ophthalmic patients cannot be over
emphasized.
 It knowledge and conduct MUST be
acquainted by every eye care provider.
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 Visual acuity is the ability to distinguished
one object from the other and appreciate
the details of the visible object.
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Visual Acuity estimation (Snellen VA testing) is
based on the principle of estimating the visual
angle and the visual angle is usually equal to
the angle subtended by retinal image at the
nodal point.
The visual angle is therefore defined as the
angle subtended by an object at the nodal
point.
Two distinct point can only be recognized as
separate when they subtend an angle of 1
minute of arc.
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AGE
 At birth 6/120
 4 months 6/60
 6 months 6/36
 1 year 6/18
 2 years 6/6
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 Visual acuity is broadly classified into four
groups based on:
1. Task required
2. Techniques of conducting the visual acuity
3. Types of VA
4. Degree of target resolution
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A. Based on the task-required
. Distant VA
. Near VA
. Contrast VA
B. Based on the technique
.Unaided VA
. Aided VA -Pin hole VA
-VA with glasses
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C. Based on the type of VA
. Static VA -Absolute VA
-Relative VA
-Natural VA
.Dynamic VA
D. Based on the degree of target resolution!
. Minimum Detectable
. Minimum Separable
. Minimum cognizable
. Vernier Acuity!
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A. DISTANCE VISUAL ACUITY
 Subjective methods:
. Snellen’s chart
. Tumbling E-chart
. Landolt’s ring test
objects
.Picture chart
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 Objective method
. Optokinetic nystagmus test
. Visual evoked response (VER)
. Auto-refractors
.Lasers
. Automated visual acuity
estimation.
13

B. NEAR VISUAL ACUITY
 Jeager’s chart
 Snellen near vision
 Roman test
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 Explain the procedure to the patient and
obtain consent.
 Position patient 6 metres from the Snellen
chart, in a well lit area, seated if possible. If
6 metres is not available, a reduced chart
can be used at 3 metres or a mirror can be
positioned at 3 metres and used with a chart
with reversed letters.
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 Using an occluder, cover the patients left
eye.
 Ask the patient to start at the top of the
chart and read the letters out, to the
smallest size they can manage.
 Repeat by occluding the patients’ right eye.
 Record level of acuity achieved for each eye
as a fraction, with the distance the test was
performed at on the top and the size of the
letters managed on the bottom
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 If the patient is unable to see the largest
letter on the chart, then move the chart
towards the patient, one metre at a time,
until they can manage it or until you are at 1
metre.
 Record in the same way as above, the
distance the test was performed at on the
top and the size of the letters managed on
the bottom.
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 If the patient is still unable to read the letter
at 1 metre you should hold up several fingers
at about 1/2 to 1/3 metre and see if they
can count them. If they can, this should be
recorded as Counts Fingers.
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 If the patient is unable to count your fingers,
then move you hand across their visual field
at about 1/2 to 1/3 metre to see if they can
see the movement. If they can this is
recorded as Hand Movements .
 If the patient is unable to see hand
movements, see if they are aware of a pen
torch light. This would be recorded as
Perception of Light.
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 If unable to make out the pen torch then the
vision would be recorded as No Perception of
Light (or NPL).
 If the patients’ visual acuity is below 6/9
and/or the patient does not have their
distance prescription available, then a
pinhole should be used.
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PATIENT’S FACTOR
.Communication gap
Inability to communicate
Organic defects – deafness, dumbness
. Language barrier
. Literacy level (especially illiteracy)
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.Poor compliance/cooperation
. Previous knowledge/experience of the test
(e.g. Knowing the VA chart letters by heart) .
. Patient piping through the fellow eye
. Presence of ocular conditions (acute/chronic)
. Contrast sensitivity defect of the patient
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EXAMINER’S FACTOR
Lack of skill/knowledge of VA testing
. Poor recording techniques
.Interpretation problems
.Attitude/behaviours to patient
. Contrast sensitivity defect of the examiner
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ENVIRONMENTAL FACTOR
Illumination problem- poor lighting system
. Size of the test objects
. Distant from the test object
. Contrast of the background of the test chart
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 OBJECTIVE TEST
Preverbal children (< 2years)
 SUBJECTIVE TEST
Verbal children (> 2years)
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OBJECTIVE TEST
 Fixation and following test
 Preferential looking test (PLT)
 Optokinetic Nystagmus test (OKN)
 Visual evoked response (VER)
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 Bright colored objects
with high contrast are
used.
 Binocular fixation is
assessed first .
 Monocular fixation
reveals the defective
vision in one eye.
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QUALITY OF FIXATION BEHAVIOUR
 C-Central
 S-Steady
 M-Maintained
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 This is based on the natural preference
for children to look at patterns rather
than blank background.
 Example is the Teller acuity cards with
stripes on one side which will elicit head
and eye movement towards the side with
the stripes
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Nystagmus is elicited by passing a
succession of black and white stripes
(OKN drum) through the patients
field of vision.
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 Refers to EEG recordings made from the
occipital lobe in response to visual stimuli
 It is the only clinically objective
techniques available to assess the
functional state of visual system beyond
the retinal ganglion cells
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SUBJECTIVE TEST
 These tests are employed in verbal children.
 Optotype –
symbols which identifications
implies the visual acuity.
Examples are Coin test, candy test, ball test
Lea's symbol, Sheridan Gardiner test ,
Snellens chart, Landolt-C chart and E chart
Allen picture test.
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 LEA’S SYMBOL
 The test is conducted
at the distance of 3
metres.
 4 shapes are included
:Circle, Square ,hut
and Apple.
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 SHERIDAN GARDINER
 Done at a distance of
6 metres.
 It includes simple
alphabets such as
H,O,T,V,X,A,U
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 Textbook of Ophthalmology ,Volume 1 By
Sunita Agarwal, Athiya Agarwal, David J.
Apple.
 Harley’s Pediatrics Ophthalmology edited by
Robinson D. Harley, Leonard B. Nelson, Scott
E. Olitsky.
 A.A Khurana Theory and practice of Optics
and refraction
 School of post basic ophthalmic nursing
National eye centre Kaduna procedure book.
 http://www.ophthalmictechnician.org/123-
visual-acuity-testing
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Visual acuity by Abdulwahab Usman

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