Published on: Mar 4, 2016
Transcripts - Preverbalchildren 22-4-14
Methods of visual acuity testing
in preverbal children
Dr.Yajuvendra Singh Rathore
Visual acuity, in preverbal infants, is defined as
a motor or sensory response to a threshold
stimulus of known size at known testing
In preliterate but verbal children, visual acuity
is defined as the smallest target of known size
at known testing distance correctly verbally
identified by a child.
MATURATION OF VISUAL
In order for a visual system to develop normally, several
components are required.
To receive visual stimulation the anatomical structures
must be present, the two eyes must be positioned
correctly and have clear media.
The neurological connections of the visual pathway to the
visual cortex must also be functional.
Compared with the relatively dark environment within the
uterus, the newborn is bombarded with visual stimuli of
differing light intensity and contours within the first few
months of life. This encourages the development of the
lateral geniculate nucleus and striate cortex.
Structural development is largely complete by 2-3 yrs of
life but functional changes continues throughout life.
Visual acuity improves rapidly during the first year of life and
then matures more gradually to adult levels at approximately
5-6 years of age.
Although the central cones function by term birth, acuity as
measured by the different techniques does not approach
20/20 (6/6) until from 6 to 30 months (depending upon the
examination technique used).
Reasons for this delay include the incomplete development
and specialization of photoreceptors, maturation of synapses
in the inner retinal layers, and myelination of the upper visual
Foveal cones do not attain adult appearance until 4months
after term birth, and visual pathway myelination continues
until 2 years of age
VISUAL MILESTONES :
Very soon after birth - Can fix and follow a light source, face or
large, colorful toy.
1 months - Fixation is central, steady and maintained, can follow a
slow target, and converge, preference of looking at face.
3 months - binocular vision and eye cordination, eyes follow a
moving light or face, responsive smile.
6 months - Reaches out accurately for toys.
9 months – look for hidden toys.
2 years - Picture matching
3 years - Letter matching of single letters (e.g., Sheridan Gardiner)
5 years - Snellen chart by matching or naming
Why to record visual acuity in
Most eye problems can be treated if
Useful in decision making.
To know if visual development is normal.
Helps decide eligibility for low vision and
There are two types of visual acuity :-
1. Recognition acuity and
2. Resolution acuity.
Recognition acuity relates to the detail in the
smallest letter, number or other shape that can be
Resolution acuity is the smallest separation
between dots or between bars in a grating that can
TECHNIQUES FOR VISUAL
1. Tests for indirect assessment of vision :
a) Historical and observational tests,
b) Binocular fixation preference and fixation targets,
c) CSM method.
2. Tests for recognition acuity :
a) Dot visual acuity,
b) Coin test
c) Miniature toy test
d) Marble game test
e) Sheridans ball test
f) Bock’s candy test (100’s and 1000’s test)
g) Worth ivory ball test
3) Tests for resolution acuity :
a) Opticokinetic nystagmus,
b) Preferential looking test,
c) Cardiff acuity cards,
d) Visual evoked potentials
Children in this age group generally perform best if
the examination takes place when they are alert.
Examination early in the morning or after an infant's
nap is usually most effective.
Because infants tend to be more cooperative and
alert when feeding, it is also helpful to suggest that
the parent bring a bottle for the child.
Tests for indirect assessment
- Historical and observational tests,
- Binocular fixation preference and fixation
- CSM method.
HISTORICAL AND OBSERVATIONAL
Parents or caretakers are asked routinely
whether the child responds to a silent smile,
enjoys silent mobiles, and follows objects around
Pertinent observations include strabismus, nystagmus,
persistent staring, and inattention to objects
For example, when a unilateral, constant strabismus is
present, visual acuity is presumed to be reduced in the
In the presence of a constant, alternating strabismus,
visual acuity is likely to be normal in both eyes.
The pupillary light response is not equivalent to visual
ability, but its presence indicates intact afferent visual
neurologic pathways to the level of the brachium of the
superior colliculus and efferent pathways to the iris
This reflex is present in premature babies over 29–
31weeks of gestational age.
Visualization in very young children sometimes requires a
magnifying glass, as their pupils are smaller than those of
older children (because of decreased sympathetic tone)
and the light responses are of small amplitude.
The blink to a bright light is a behavior learned by 30
weeks of gestational age and occasionally is present
in decorticate infants. The blink to a threatening
gesture is another learned reflex, usually present by
*when testing, care must be taken not to brush air against
the child’s cornea and elicit a blink by that mechanism.
Another behavior that is unique to babies is “eye
popping.” Sometimes, for a variety of reasons, very
young infants don't show any distinguishable visual
behavior at all. In this case, the eye popping reflex
indicates at least the infant’s ability to detect changes
in room illumination.
When the room lights are suddenly dimmed, the
baby's upper eye lids should pop open wide for a
moment. The baby will often close its eyes when the
lights are brought back up, but will again pop its eyes
open when the lights are dimmed. This behavior is
documented as "positive eye popping".
FIXATION TARGETS (fix and follow) :
If appropriate targets are used, this reflex can be
demonstrated by about 6 wk of age.
The test is performed by seating the child comfortably in the
caretaker's lap. The object of visual interest, usually a bright-
colored toy, is slowly moved to the right and to the left. The
examiner observes whether the infant's eyes turn toward the
object and follow its movements (fix and follow behavior) .
The examiner can use a thumb to occlude one of the infant's
eyes in order to test each eye separately.
If the child has a f/f behaviour then it is assumed that the
patient could see a small target or toy in a normally
The human face is a better target than test objects. If
the appropriate following movements are not elicited,
the test should be repeated with the caretaker's face
as the test stimulus.
It should be remembered that even children with poor
vision may follow a large object without apparent
difficulty, especially if only one eye is affected.
Binocular fixation preference :
Behavioral evidence of decreased vision in right eye.
(A) A small toy is used to get the child’s attention, and the examiner covers the right eye to monitor fixation of the left
eye. The child fixates on the toy without objecting.
(B) When the left eye is covered, the child objects and tries to move the examiner’s hand.
(C) When the right eye is covered, the child does not object and tracks the object.
Some children object to having either eye covered,
simply because they do not like having the examiner’s
hand near their face. If this is the case, this test cannot
accurately determine whether there is a difference in
vision between the eyes.
CSM METHOD :
It is done with one eye fixating on an accommodative target
held at 40cm
‘C’ refers to the location of corneal light reflex as the patient
fixates the examiner’s light under monocular conditions.
Normally light is reflected on the centre of the cornea and it
should be positioned symmetrically in both eyes. If fixation
target is viewed eccentrically, fixation is termed uncentral.
‘S’ refers to steadiness of fixation on examiners light as it is
held motionless and also as it is slowly moved about.
‘M’ refers to the ability of the patient to maintain alignment
first with one eye, then with the other. Maintenance of fixation
is evaluation under binocular conditions. Inability to maintain
fixation with either eye, with opposite eye uncovered is
presumptive evidence of a difference in acuity between the
CSM – 6/9 – 6/6
CSNM –6/36 – 6/60
Unsteady central fixation < 6/60
Tests for recognition acuity
•Dot visual acuity
•Miniature toy test
•Marble game test
•Worth ivory ball test
•Bock’s candy test
•Sheridan gardner single
•Sonksen Silver acuity
Dot visual acuity test : child is shown an
illuminated box with black dots of different sizes printed
on it. The smallest dot identified denotes the visual
acuity of the child.
Coin test : Child is asked to identify two faces of
coins of different size held at different distance.
Miniature toy test :Child is shown a miniature toy
from a distance of 10 feet and asked to name / pick the
pair from assortment.
Marble game test : The child is asked to place marbles in holes of
a card or in a box. It compares the functioning of the child’s eye when one
or the other is closed and vision is noted as useful or less useful.
Worth Ivory ball tests : Ivory balls 0.5 to 2.5" in diameter are
rolled on the floor in front of the child and he is asked to retrieve each.
Acuity is estimated on the basis of smallest size for the test distance.
Bock’s candy bead test : Snellen equivalent of 6/60 is
estimated by this method. The child is asked to match pick up beads 1mm
size at 40 cm.
Examples of recognition acuity. A. Kay
pictures B. LEA symbols.
Tests for resolution
Preferential looking test,
Cardiff acuity cards,
Visual evoked potentials
Evaluation of the presence or absence of
opticokinetic nystagmus was the first
“technologic” approach to acuity measurement
in preverbal children.
Acuity was measured binocularly while the infant was positioned on
his or her back in a crib looking up at a canopy of black and white
stripes. During testing, the stripes moved in an arc across 180 O of
the infant’s visual field.
Patient follows the stripe
with a slow motion and as it
disappears, suddenly picks up
a new stripe.
An assessment of visual
acuity is made by varying the
width of stripes or the distance
from the drum.
As the testing drums are reasonably priced,
portable, and rarely break, this technique remains
in use as a quick and easy method to evaluate
Disadvantages / limitations :
Disturbing fact is the realization that normal responses may
occur in the occasional decorticate infant, which indicate that
subcortical areas of the occipital cortex may generate
If one uses the readily available handheld opticokinetic
nystagmus drum or tape, it is difficult to keep the infant fixated
on this stimulus which takes up only a small portion of his or her
Smudges, distortions, unequal stripe widths or any imperfection
in the stimulus that the eye can resolve may elicit false
opticokinetic nystagmus responses in a testing circumstance.
Moreover, it is essential that the stimuli have uniform space-
average luminance and be moved at a uniform rate across the
visual field. Obviously, most testing devices available in
ophthalmology clinics and examining rooms do not meet these
Eye movements evoked by dot stimuli can be suppressed
and poor correlation has been found between the acuity
measurements obtained by this technique and those
obtained by a standard Snellen assessment.
Finally, it is important to note that the use of opticokinetic
nystagmus to assess visual acuity may lead to errors of
interpretation, because one is evaluating a motor
response in an attempt to assess sensory function.
The absence of opticokinetic nystagmus may be due to
some alteration in the ocular motor systems necessary to
generate this eye movement and not to the patient’s
failure to “see” the stimuli
FORCED CHOICE PREFERENTIAL
The FPL technique was conceived by David Teller.
This testing technique is based on the observation that
infants demonstrate a greater tendency to fix a pattern
stimulus than a homogeneous field.
They measure resolution acuity, using either a grating target
as with the Teller cards or the vanishing optotype principle, as
with the more recently Developed Cardiff Acuity Cards.
Preferential looking involves showing the infant two stimuli, a grating
composed of black and white stripes (or other quantitated patterns), and a
grey screen of equal space-average luminance.
An observer, unaware of the location of the patterned stimuli, is positioned
behind a peephole located centrally between the grating and the
The observer monitors the direction of the child’s eyes and head during
stimulus presentation. The position and width of the stripes are varied on
Acuity is estimated by determining the smallest
striped width to which the infant will show differential
fixation of the grating as opposed to the
homogeneous field i.e The frequency of the line
spacing determines the visual acuity.
The threshold is usually defined as when the
observer is correct 75% of the time.
This technique becomes a “forced choice” method
when the observer has to decide, based on their
observation of the child’s head and eye movements,
where the stimulus is located.
The Cardiff Test is good for slightly older children
(18 - 60 months). It consists of different cards, which
are held in front of the child at 50cm. Each has a
picture in the upper or the lower part of the card. If
the child looks towards the picture on the card, you
note the size as detected.
In Cardiff Acuity Card , the targets are pictures drawn with a white
band bordered by two black bands, all on a neutral grey
background. The average brightness of the picture is equal to that
of the grey background. If the child’s vision is good enough to
resolve the white and black bands, the picture will be visible but if
the bands are too narrow for the child to resolve them, the picture
merges with the grey background, and simply becomes invisible.
Testing cards are simple, portable, and cannot
lose calibration; in a typical child, the testing of
both eyes often takes less than 20min.
Disadvantages / limitations :
The child must be alert and able to generate neck and eye
movements, which disqualifies many whose hypotonia and
inattention prevent such purposeful movement – a significant
limitation in the evaluation of developmentally delayed
In addition, this test presents a resolution acuity task, not a
recognition acuity task, and thus may be less ideal for the
detection of amblyopia than the visual evoked response test.
As the cards can be presented with the stripes in one
orientation (vertical) only, the acuities of some optically
uncorrected astigmatic children may be estimated using
Children who have nystagmus may be unable to fixate on
the targets accurately, and those who have visual field
defects may have difficulty finding the targets.
False high acuities are detected in patients with
anisometropia and strabismic amblyopia as these patients
typically have better near visual acuity.
VISUAL EVOKED POTENTIALS :
Visual evoked potentials (VEPs) are electrical brain responses
that are triggered by the presentation of a visual stimulus.
VEP is the only clinical objective technique available to assess
the functional state of the visual system beyond the retinal
ganglion cells. It is quite usefull in assessing visual function in
1. Flash VEPs- just tells about the integrity of the macular
and visual pathway. The flash VEP is much less macula
dominated than the pattern VEP and can be recorded
through cataracts or corneal scars.
2. Pattern reversal VEPs- is recorded using some
patterned stimulus, as in the checkerboard. In it the
pattern of stimulus is changed with the overall
illumination remaining same.
A proprietary disposable headband with integrated electrodes
is used for recordings.
The headband aligned the occiput (Oz), the mid-forehead
(Fpz), and the temple (ground). Skin contact with the pre-
gelled electrodes is enhanced with a small amount of EEG
Infants are positioned on a parent’s lap and children are
seated in a comfortable chair at a measured distance of 57
cm from a 17-inch (43-cm) display monitor, so that the
stimulus subtended a total visual angle of 20o.
The room is darkened except for the light from the testing
Testing is performed monocularly, using an adhesive
occluder over the fellow eye.
Test stimulus, showing a
cartoon figure (top), which
before the sequence of gratings
(sample at bottom).
When the size of the checks is reduced to the point
where the contrast borders can no longer be
resolved, the cortical response disappears.
At a check size which subtends a visual angle of 15
min arc, a visual acuity of approximately 6/18–6/24 is
required for a clear cortical response.
At a check size of 60 min arc, a visual acuity of 3/60–
6/60 is required for a clear cortical response.
METHODS OF VEP RECORDING :
Time-locked responses to abrupt
presentations are referred to as transient
A second method of recording VEPs, the
steady-state method, uses temporally
periodic stimuli. For commonly used pattern
reversal stimuli, the frequency of the
repetition is often specified as the pattern
reversal rate in reversals per sec. This rate is
twice the stimulus fundamental frequency (in
Hz), which is more commonly used to
describe the temporal frequency of pattern
As the stimulus repetition rate increases, the
responses to successive stimuli begin to overlap. At
high stimulation rates, the response is comprised of
only a small number of components that occur at
exact integer multiples of the stimulus frequency.
Activity at each of the frequency components of the
steady-state response is characterized by its
amplitude and phase, where phase represents the
temporal delay between the stimulus and the evoked
Cumbersome process of attaching and standardizing
much time required for testing,
the relatively monotonous stimuli,
the complexity of the generated waveforms, hence
requires training ,
VEP are recorded even in absence of occipital cortex and
in cortical blindness due to contribution by sec. visual
Selecting the appropriate clinical test :
Because a child can vary significantly from expected age norms, it is important
not to rely solely upon chronological age when choosing testing procedures.
Appropriate test procedures need to be based on the child's developmental age
and specific capability.