2. *
*Visual Acuity is a measure of spatial resolution
of the eye, or in other words, an estimation of
its ability to discriminate between two nodal
points.
3. *
*A variety of different tests of visual performance
measure some aspect of the limits of the visual
system’s ability to discern detail or to recognize
detailed targets.
*Minimum Detectable Resolution
*Minimum Separable Resolution
*Recognition Resolution
-Landlot rings
-Letter Optotypes
-Tumbling E
4. *
*Refraction and prescribing Decisions
*Monitoring Ocular Health
*Visual Acuity for Normalcy
*VA Measurement applied to vision standards
5. *
*Snellen fraction
Visual acuity= (test distance)/(distance at which letters
. subtend 5min of arc)
*Decimal Notation
VA 20/20= 1.0
*Minimum Angle of Resolution
VA 20/20= 1min of arc
*Logarithm of the Minimum Angle of Resolution
VA 20/20= 1min of arc = log10(1.0)= 0.0
7. *
*Snellen Chart
*Bailey Lovie Design Principles
*Design features for VA charts
-Logarithmic Size Progression
-Letter Legibility
-Number of optotypes at each size level
10. *
*Snellen’s Test Types
*Landolt’s Test Types
*Tumbling E Chart
*Allen Picture Chart
*Sheridan Gardiner HOTV Chart
*Ivory Balls
*Hiding Heidi
*Ffooks’ Test
*Stycar Test
*Light House Cards
*Lea Symbols
11. *
*Visual acuity expresses the angular size of the
SMALLEST target that can just be resolved by
the patient
*Snellen fraction is the most common notation of
acuity
*Snellen fraction is an expression of angular size
of an optotype at the type
*Assume 5*5 grid for letters with detail
separation of 1/5 of letter size.
12.
13.
14. * Visual acuity = (test distance)/(distance at which
letters subtend 5min of arc)
*Place patient at 20ft or 6m from snellen chart( 6m
because at this distance it is assumed that the rays
are almost parallel and patient exert minimum
accommodation)
*The chart should be properly illuminated 9100 foot
candles)
*Instruct Patient to cover eye not press the eyes
*OD then OS
*Pinhole if <6/12
15. *Ask the patient to read with one eye from the top
letter while the contralateral eye is closed gently with
the patient palm or with the occluder in the trial
frame.
*Now patient is asked to read the snellen’s chart and
depending upon the smallest line which the patient can
read from distance of 6m his vision is recorded as 6/6.
6/9, 6/18, 6/24, 6/36, 6/60.
*But if patient is not able to see the top line from 6m he
is asked to come towards Snellen’s chart step by step
and vision recorded at 5,4,3,2,1 meter and noted as
5/60. 4/60, 3/60, 2/6-, 1/60 resp.
*If <1/60
*CF (include distance), CF3FT, CF 2FT, CF 1FT OR CF
close to face
*HM+ then PL and PR then NPL
16. *When the patient cannot distinguish hand
movements the examiner notes whether the
patient can perceive light (PL) or not. If he
perceive light it is noted as PL +ve otherwise as
PL-ve.
*Also examiner then throw the light from four
directions ( nasal, superior, temporal, inferior)
and record accordingly. If present patient
perceive light from all directions it is marled as
PR ( projection of rays) present or else mark as
absent or defective. The test is repeated for the
other eye in similar fashion.
17.
18.
19. *
*Letters not of equally legibility e.g. O and E
*Non-uniform progression of letter sizes
*Unequal number of letters on each line
*Irregular spacing between letters and lines
*Ability to recognize target ( letters) is influenced
by literacy and past experience
*Inaccurate results specially in those with low
vision
20. *
*Similar to Snellen’s except that instead of letters the
broken circles are used
*Each broken ring substends an angle of 5min at the
nodal point
*Ability to recognize target(letter) is influenced by
literacy and past experience and hence Landolt’s
rings were designed to eliminate these factors and
present a more objective test
*End point is detection of the orientation of break in
the circle
21.
22. *
*A test performed on a person who has diminished
visual acuity to distinguish a refractive error from
organic disease
*The patient looks through it with one eye at a
time, without correction.
*Light passes only through the centre of eye’s lens,
& errors of refraction have no effect while the
occluder is used.
*Pinhole blocks the peripheral rays, only letting
those rays which pass through the central portion
of the pupil.
*If the visual acuity is improved, the defect is
refractive, if not organic
23.
24. *
*The Tumbling E Test is similar to Snellen in that
it is performed at 20 feet(6m). The child must
tell the orientation of the legs of the letter ‘E’
(up, down ,left , right).
*Very useful for non-verbal children.
*Disadvantage- right-left disorientation is
common in this range
25.
26. *
*Four letters (H,O,T,V) are used in the chart.
*The test is performed at 10feet (3m), and is
administered similarly to the Snellen Acuity.
*It comes with a near card so patients can
match the letters at a distance by pointing to
the corresponding letter on the near card.
*Advantage: This test does not have a
directional component.
27.
28. *
*Grating Acuity Tests
Visually Evoked Potential
Prefential Looking Tests
Optokinetic Nystagmus
*Flash Card Tests
Letter Flash cards
*Picture or symbol charts
Letter charts for children
29. *
*With Visual Evoked Response (VER), a scalp
electrode is used to record electrical signals
from the visual cortex while the patient views
a grating or check board Stimulus.
*Objective technique available to assess visual
system beyond retinal ganglion.
*Flash VER determines the integrity of macula &
visual pathway function.
*Pattern VER depend on form sense & gives
rough estimate of the visual acuity.
30.
31. *
*PL is used to assess VA in infants & young children who are
unable to identify pictures or letters.
*Procedure
*The child is presented with two stimulus field.
*One with stripes and the other with a homogenous grey are of
the same average luminance as stripes randomly alternated.
*Typically, infants and children will look at the more
interesting stripes.
*A small peephole is centered between the two fields, for
observer.
*Observer judges the location of the strips based on the child’s
head & eye movements.
32. *If the child can see the stripes, he/she will
prefer to look them.
*If the child can’t see them, the child will not
show a preference.
*Visual acuity determined with this method.
*RECORDING
*New born – 6/240
*3 months- 6/60
*36 months- 6/6
33.
34. *
*OKN testing can be used to verify if the patient possess
a cortical visual response.
*The OKN drum contains black stripes that should be
oriented vertically in front of the patient. The drum is
spun slowly and the examiner observes the patient’s
eye movement as they follow the rotating drum. The
patient should exhibit a nystagmus movement because
their eyes should jump back to look at a new pattern,
as the one they originally followed rotates out of the
sight. The visual angle subtends by the smallest strip
width that still elicits eye movement is a measure of
visual acuity.
35.
36. *
*Contains line drawings of familiar objects
(birthday cake, hand, bird, house, rotary
telephone, jeep). The test distance is 20
feet(6m).
*Disadvantages- Pictures not constructed
accroding to snellens formula
*- Children may not be familiar to all the
images.
*-Minimum threshold acuity level on thte
picture chart is 6/9
37.
38. *
*It is based on preferential looking and Snellen
principle.
*The chart is placed at a distance of 1m from
the patient.
*It is usually used for the age group of 3-9
months.
*There are cards available of various thickness
of lines.
39. *At a time two cards are held in front of the
patient. The blank in front and the one with
lines i.e., held behind it.
*Then immediately the second card is flipped
out and we keep on changing the positions.
*The patient should appreciate the card with
lines.
*The test is done at same eye level and the eye
movement of patient is seen.
40.
41. *
*It is a detection acuity test.
*It is useful in infants & preschool children.
*In this test, the child is made to observe an
oscillating drum with black dots of varying
sizes.
*The smallest dot that evokes pendular eye
movements denotes the level of visual acuity.
42.
43. *
*The test is performed by placing two pictures
side by side. One picture has complete wheels
while the other picture has sections missing.
The child is asked to point to the car with
broken wheels. Pair of cars are kept is
progressively smaller sizes.
44.
45. *
*The principle of the target design is that of the
vanishing optotype.
*The targets are pictures drawn with a white
band border by 2 black bands, all on a neutral
grey background.
*The examiner simply observes the children
fixation.
46.
47. *
*If child able to pick up small sweets at 33cm,
visual acuity is at 6/24 or 20/80.
48. *
*Blink Response in response to sound.
*Menace reflex i.e.; closure of the eyes on the
approach of an object if vision is normal.
49. *
*Behaviour evidence of decreased vision in right
eye.
*A small toy is used to get the child’s attention
& the examiner covers the right eye to monitor
fixation of the left eye. The child fixates on
the toy without objecting.
*When the left eye is covered, the child objects
& tries to move the examiner’s hand.
*When the right eye is covered, the child does
not object & tracks the object.
50.
51. *
*Done with one eye fixating on an accommodative
target held at 40cm.
*‘C’ refers to the location of corneal light reflex
fixates the examiner light at monocular
conditions.
*Normally reflected light from cornea in near the
centre of cornea and it should be positioned
symmetrically in both eyes.
*If fixation target is viewed eccentrically, fixation is
termed uncentral.
52. *‘S’ refers to the steadiness of fixation at examiners
light and also as it slowly moved about.
*‘M’ refers to the ability of the patient to maintain
alignment first with one eye then the other as the
opposite eye is uncovered.
EVALUATION
*CSM-6/9-6/6
*CSNM- 6/36-6/60
*Unsteady central fixation <6/60
