Preverbalchildren 22-4-14
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- 1. Methods of visual acuity testing in preverbal children
Dr.Yajuvendra Singh Rathore
- 2. DEFINITION: Visual acuity, in preverbal infants, is defined
as a motor or sensory response to a threshold stimulus of known
size at known testing distance. 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.
- 3. DEVELOPMENT AND MATURATION OF VISUAL ACUITY
- 4. 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.
- 5. 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 pathways. Foveal cones do not attain adult
appearance until 4months after term birth, and visual pathway
myelination continues until 2 years of age
- 6. 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
- 7. REFLEX DEVELOPMENT:
- 8. Why to record visual acuity in children ?
- 9. Most eye problems can be treated if detected early. Useful
in decision making. To know if visual development is normal. Helps
decide eligibility for low vision and rehabilitation services.
- 10. Types : 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 recognised. Resolution acuity is the smallest
separation between dots or between bars in a grating that can be
resolved.
- 11. TECHNIQUES FOR VISUAL ACUITY QUANTITATION
- 12. 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) Bocks candy test (100s and
1000s test) g) Worth ivory ball test h) others
- 13. 3) Tests for resolution acuity : a) Opticokinetic
nystagmus, b) Preferential looking test, c) Cardiff acuity cards,
d) Visual evoked potentials
- 14. 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.
- 15. Tests for indirect assessment of vision - Historical and
observational tests, - Binocular fixation preference and fixation
targets, - CSM method.
- 16. HISTORICAL AND OBSERVATIONAL TECHNIQUES : Parents or
caretakers are asked routinely whether the child responds to a
silent smile, enjoys silent mobiles, and follows objects around the
environment.
- 17. 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 strabismic eye. In the presence of a
constant, alternating strabismus, visual acuity is likely to be
normal in both eyes.
- 18. 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 sphincter. 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.
- 19. 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 5months. *when testing, care must be
taken not to brush air against the childs cornea and elicit a blink
by that mechanism.
- 20. 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 infants 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".
- 21. 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 illuminated room.
- 22. 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.
- 23. Binocular fixation preference : Behavioral evidence of
decreased vision in right eye. (A) A small toy is used to get the
childs 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 examiners hand. (C) When the right eye is
covered, the child does not object and tracks the object.
- 24. Some children object to having either eye covered, simply
because they do not like having the examiners hand near their face.
If this is the case, this test cannot accurately determine whether
there is a difference in vision between the eyes.
- 25. 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 examiners 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 two
eyes.
- 26. Evaluation : CSM 6/9 6/6 CSNM 6/36 6/60 Unsteady central
fixation < 6/60
- 27. Tests for recognition acuity Dot visual acuity Coin test
Miniature toy test Marble game test Worth ivory ball test Bocks
candy test Kay pictures LEA symbols Ffooks symbols Sheridan gardner
single letter optotypes Sonksen Silver acuity system
- 28. 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.
- 29. 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
childs 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. Bocks 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.
- 30. Examples of recognition acuity. A. Kay pictures B. LEA
symbols.
- 31. Tests for resolution acuity Opticokinetic nystagmus
Preferential looking test, Cardiff acuity cards, Visual evoked
potentials
- 32. OPTICOKINETIC NYSTAGMUS : Evaluation of the presence or
absence of opticokinetic nystagmus was the first technologic
approach to acuity measurement in preverbal children.
- 33. 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 infants 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.
- 34. Advantages : As the testing drums are reasonably priced,
portable, and rarely break, this technique remains in use as a
quick and easy method to evaluate infant acuity.
- 35. 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 opticokinetic responses. 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 visual
environment. 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 rigid
requirements.
- 36. 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 patients
failure to see the stimuli
- 37. FORCED CHOICE PREFERENTIAL LOOKING : 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.
- 38. 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 homogeneous field. The observer monitors the
direction of the childs eyes and head during stimulus presentation.
The position and width of the stripes are varied on each
trial.
- 39. 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 childs head and eye
movements, where the stimulus is located.
- 40. 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.
- 41. 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 childs 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. (vanishing optotypes)
- 42. Advantages : Testing cards are simple, portable, and cannot
lose calibration; in a typical child, the testing of both eyes
often takes less than 20min.
- 43. 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
infants. 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.
- 44. 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 this
technique. 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.
- 45. 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 infants.
- 46. Types : 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.
- 47. Procedure : 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 conductive paste. Infants are positioned on a
parents 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 equipment.
Testing is performed monocularly, using an adhesive occluder over
the fellow eye.
- 48. Test stimulus, showing a cartoon figure (top), which
appears before the sequence of gratings (sample at bottom).
- 49. Evaluation : 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
6/186/24 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.
- 50. METHODS OF VEP RECORDING : Time-locked responses to abrupt
presentations are referred to as transient VEPs. 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
- 51. 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
response.
- 52. LIMITATIONS : Cumbersome process of attaching and
standardizing electrodes, much time required for testing, expensive
instruments, 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
- 53. 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.
- 54. THANK YOU