visual acuity testing various methods in children and adults

1. Dr RAKHI DCRUZ

2. Resolving power of the eye or
the ability to see two separate
objects as separate.

3.  The angle subtended at the nodal point of
the eye by the physical dimensions of an
object in the visual field.

4.  Receptor theory:
If the images fell on two cones
separated by an unilluminated cone,
then the points of light would be
perceived as two distinct sources.
Width of foveal cones-2.5microns
Means that two distinct points only can
be recognized as separate when they
subtend an angle of 1’ of arc.

5. When light from a point source passes through a small aperture ,it doe not produce a
bright dot as an image,rather it forms a difffuse circular disc known as AIRY DISC
due to diffraction property of light. Airy dics is surrounded by much fainter
concentric rings
Two “point” (unresolved) sources are resolved from
each other when separated by at least the radius of
the airy disk.
According to reyleigh,2 closely spaced airy discs are
distinct if they are farther apart than the distance at
which principal maximum of 1 coincides with first
minimum of 2nd airy disc

7.  Minimum Detectable ( visible)
 Minimum Separable (resolution)
 Minimum cognizable (recognition)
 Minimum Discriminable ( hyperacuity)

8. MINIMUM DETECTABLE:
Ability to determine whether or not an object is
present in a visual field.
The limit for this kind of acuity is ~ 1 arc second.

9.  Discrimination of 2 spatially separated targets.
 Assessment of function of fovea centralis
 Distance between 2 targets is specified by angle
subtended at nodal Point Of eye. Normal
angular threshold of discrimination for resolution
30-60 seconds of an arc. (minimum angle of
resolution).

10.  : not only discrimination of the spatial characteristics of
the test pattern but also the pattern with which
PERSON has previous experience
(Identification of faces, letters, symbols, pictures etc.)

11.  Ability to determine whether or not two parallel
and straight lines are aligned in the frontal plane.
 The threshold value of verneir acuity
is only 2-10 seconds of an arc.

12.  Criteria:
 Normal testing distance (distance VA): 6m
 For near : 33cm/ 40cm.
 Illuminated visual acuity chart.
 Proper room illumination.
 Steps:
 Unaided VA
 Aided VA
 Pinhole VA

13. 6m distance

14. Overall
 Illumination of test object.
 Contrast of letters.
 Pupil size
 Cognitive ability
 Ocular health
 Refractive Status

15. Stimulus related
 Luminance of test object
 Geometrical configuration
 Contrast difference of stimulus from background
 Influence of wavelength
 Exposure duration of stimulus
 Interaction effects of 2 targets
Observer related
 Retinal locus of stimulation
 Pupil size
 Accommodation
 Effect of eye movement
 Optical elements of eye

16.  DOT VISUAL ACUITY TEST
 CATFORD DRUM TEST
 STYCAR GRADED BALLS TEST
 RECOGNITION ACUITY TEST
 SNELLENS CHART
 LIPMANS HOTV CHART
 SNELLEN E CHART
 LANDOLTS C CHART
 ALLEN PICTURE CARD TEST
 LIGHT HOUSE TEST
 CARDIFF ACUITY
 BAILEY HALL CEREAL TEST

17.  OPTOKINETIC NYSTAGMUS TEST
 PREFERENTIAL LOOKING TEST
 VISUALLY EVOKED RESPONSE

19. Snellen Fraction is the most common notation of
acuity.
 The distant acuity is usually tested by snellen chart.
 Consist of a series of black capital letters on a white
board,arranged in lines,each progressively diminishing
in size.
 Each letters fits in a square,, sides of which are 5
times the breadth of constituent lines
 Assume 5x5 grid for letters with detail separation of
1/5 of letter size.

21.  Log of the MAR used to notate the acuity
 Used in research and low vision
◦ Their chart and its derivatives follow the
principles:
 Geometric progression of size & spacing by
0.1 Log unit or 4/5
◦ Found that the relative spacing between letters
and the number of letters per row can cause
substantial variation in acuity scores

22.  It is flat and typical “V” shape.
 Testing distance 4m.
 Every line has 5 letters .
 Point one (0.1) logarithmic progression. (5:4)
 Between two letter space is the width of a single
letter.
 Between two line space is the height of the lower
line letter.
 Acuity becomes worse as the log MAR value
increases
 Decrease brightness line by line

23.  Letter to letter Acuity assessment
 Every letter that is correctly read deducts 0.02 from
that line
 Example:
 If the patient reads all the letters of the logMAR 0.40 line
(20/50) and two letters from the 0.30 line (20/40), it is scored
as 0.36
 Snellen Equivalent to this would be 20/50+2

24.  With toddlers and slightly older 'verbal' children, simple
pictures constructed on Snellen's principles may be used.
A very effective test is the 'E-test' in which the examiner
holds cards on which the letter E is printed, in various
positions and in various sizes.
 If the test is treated as a game, the child standing 6 m
away will readily respond on request by indicating the
direction of the letter with his hand or by holding a similar
card in the same position so long as he sees it.
 A similar test is the Landolt C chart.
 This is the chart containing a series of broken rings, with each
gap subtending an angle of 1 minute at nodal point at a given
distance
 SHERIDAN GARDINER HOTV TEST-Child is handed charts
with H,O,T,V and are asked to match with letters in distant chart

26.  Dot visual acuity test-
 child is shown illuminated box with black dots of different
size.smallest dot identified denotes visual acuity of child.
 Miniature toy test-
 Child is shown a miniature toy from distance 10 ft and asked
to pick the pair from assortment.
 CHILDREN 1-2 YRS
 MARBLE GAME TEST-child asked to put marbles into a box
with each eye seperately with other occluded, vision of the
eye is then noted as USEFUL OR LESS USEFUL
 SHERIDAN BALL TEST-Rolling ball on white background
asking child to pick it up. Smallest sized ball to which child
gives good response is rough way to estimate VA.

27.  OPTOKINETC NYSTAGMUS
 Nystagmus is elicited by passing succession of black and white
stripes through subjects field of action.the visual angle
subtended by a smallest strip width that still elicits an eye
movement is measure of VA.OKN reaches level of 6/30 at 6
months and 6/6 by 20-30months
 PREFERENTIAL LOOKING TEST
 Infant prefers to look at stripped pattern more than
homogenous fields,in this examiner is hidden behind a screen in
which 1 projects a homogenous surface on 1 side and black &
white stripes on other .the 2 stimuli are alternated randomly
 The location of striped pattern is varied randomly from left to
right and fineness of stripes is gradually reduced until there is no
longer any correlation between judged direction of infants gaze
and location of stripe.
 VA determined by this method ranges from approximately 6/240
in newborns to 6/60 at 3 months and 6/6 at 36 months of age.

30.  EEG recording made from occipital lobe in response to
visual stimuli.
 FLASH VER-integrity if macula and visual pathway
 PATTERN REVERSAL VER-Rough estimate of VA. VER
studys show that VA in infants 6/120 at 1 month,6/60
2 months,6/6-6/12 at 6months to 1 year.
 CATFORD DRUM TEST
 DETECTION ACUITY TEST
 Child observes an oscillating drum with black dots of
varying size.the smallest dot that evokes pendular eye
movement denotes level of visual acuity.

32.  Based on preferential fixation of cards which
have a picture optotype and a blank located
vertically.the picture optotypes are of same
sizes but have been specially drawn with 2
dark lines with a white space in between
such that picture is visible only at a particular
distance or closer, these are called vanishing
optotype as they vanish at farther distance
 The child can identify the picture by
verbalizing,pointing or fixation preference.

34. DENSE CATARACT
The LOTMAR RODENSTOCK laser interferometer forms a
diffraction pattern of parallel lines on the retina even
through a moderate cataract. Utilizes coherent white
light/helium neon lazer generated interface fringes that
are projected into retina through ocular media,width of
fringes correspond to VA.The patient is asked to identify
the orientation of progressively finer lines, to establish the
visual acuity likely to be regained after surgery. REF ERR
need not be corrected as projn of fringes occur over 3
dimensions and is not affected by ammetropia
 Potential acuity meter ( GUYTON MINKOUSKI)
This is attached to a slit lamp & projects a tiny Snellen
chart through pinhole clear area onto the retina around a
lens opacity and the patient is required to read the
alphabets.ref errors are compensated by apparatus

35.  Visual acuity for near Near vision is tested by asking the patient
to read the near vision chart, kept at a distance of 35 cm in good
illumination, with each eye separately. In near vision charts, a
series of different sizes of printer type are arranged in increasing
order and marked accordingly
 Commonly used near vision charts are as follows:
 1. Jaeger’s chart. In this chart, prints are marked from 1 to 7
and accordingly patient’s acuity is labelled as J1 to J7 depending
upon the print he can read.
 2. Roman test types. According to this chart, the near vision is
recorded as N6, N8, N10, N12 and N18 (Printer’s point system).
 3. Snellen’s near vision-graded thickness of different lines is
about 1/17th of distant vision .in this event the letter equivalent
to 6/6 will subtend an angle of 5 minutes at an average reading
distance.

37. Snellen near vision chart

38.  DUKE ELDERS PRACTICE OF REFRACTION
 KHURANA OPTICS AND REFRACTION
 PRADEEP SHARMA STRABISMUS SIMPLIFIED
 VARIOUS INTERNET SOURCES