Methods of visual acuity testing in preverbal children
METHODS OF VISUAL ACUITYTESTING IN PREVERBAL CHILDREN Presented by :- Dr. varsha varshney Resident, Dept. of ophthalmology.
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.
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.
THERE IS ALSO A QUESTION OF NATURE VERSUS NURTURE. Does the development of vision rely on the maturation of the visual system with growth or does it depend upon environmental stimuli? A study comparing premature babies with babies born at full term, showed that at a given age, the premature babies did not have better visual acuity(Brown & Yamamoto, 1986), even though they had had more months of visual stimulation. This suggests that the anatomical structures have to reach a certain growth andmaturity before they can all perform at similar levels.This favours the nature argument. Interestingly, in support of the importance of nurture, it has been shownin recent research by Smith et al (2007), the absence of a fovea in primates at 3 weeks did not affect refractive development and emmetropisation still occurred as long as visual stimulation was normal.
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 30months (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
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
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 .
Types : There are at least two types of visual acuity :- recognition acuity and 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.
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 infants 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 of vision- Historical and observational tests,- Binocular fixation preference and fixation targets,- CSM method.
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.
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.
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.
The blink to a bright light is a behavior learned by 30weeks 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 child’s corneas 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 dont 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 babys 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 caretakers 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 infants eyes turn toward the object and follow its movements (fix and follow behavior) . The examiner can use a thumb to occlude one of the infants 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.
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 caretakers 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’sattention, and the examiner covers the right eye to monitor fi xation of the left eye. The child fixates on the toy without objecting. (B) When the left eye is covered, the child objects and triesto move the examiner’s hand. (C) When the right eye is covered, the child does not object andtracks the object.
Some children object to having either eye covered, simply because theydo not like having the examiner’s hand near their face. If this is thecase, this test cannot accurately determine whether there is a differencein 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 reflected light from cornea in near 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, as the opposite eye in uncovered. 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.
Tests for recognition acuity Dot visual acuity Kay pictures Coin test LEA symbols Miniature toy test Ffooks symbols Marble game test Sheridan gardner single letterWorth ivory ball test optotypes Bock’s candy test Sonksen Silver acuity system
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. Kaypictures B. LEA symbols.
Tests for resolution acuity Opticokinetic nystagmus Preferential looking test Cardiff acuity cards Visual evoked potentials
OPTICOKINETIC NYSTAGMUS : 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 stripewith a Slow motion and As itdisappears,Suddenly picks upA new stripe. An assessment of visualAcuity is made by varying theWidth of stripes or the distanceFrom the drum.
Advantages : As the testing drums are reasonably priced, portable, and rarely break, this technique remains in use as a quick and easy method with which to evaluate infant acuity.
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 optokinetic 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 optokinetic 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.
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 optokinetic 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 optokinetic 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 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.
CAC were used from a distance of 50cm and LG were evaluated from 57cm with a luminance of 23 cd/m2 as recommended by the manufacturers. 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 child’s eyes and head during stimulus presentation. The position and width of the stripes are varied on each trial. 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. 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. (vanishing optotypes)
Advantages : 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 infants. Thus, as with the opticokinetic nystagmus technique, vision is evaluated by means of a motor response. 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. While Tellers card measures only grating acuity which is better in than resolution acuity in children.
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 erroneously 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. Lack of crowding phenomenon.
VISUAL EVOKED POTENTIALS : Visual evoked potentials (VEPs) are electrical brain responses that are triggered by the presentation of a visual stimulus. VEPs are distinguished from the spontaneous electroencephalogram (EEG) due to their consistent time of occurrence after the presentation of the stimulus (time-locking). The surface-recorded VEP reflects the activity of cortical visual areas, with contributions from subcortical generators being apparent only under highly specialized recording conditions evoked potentials (EPs) are so small compared to the background EEG activity that the responses to a number of stimuli have to be recorded and averaged with a computer in order to permit their recognition and definition. The background EEG activity, which has no fixed temporal relationship to the stimulus, is averaged out by this procedure.
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 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 equipment. Testing is performed monocularly, using an adhesive occluder over the fellow eye.
Test stimulus, showing a cartoonfigure (top), which appearsbefore the sequence of gratings(sample at bottom).
Flash VEP : It is possible to record a VEP to simple flashes of light. This is useful where the visual acuity is too poor to perceive even large checks or where fixation or concentration is poor. The flash VEP is much less macula-dominated than the pattern VEP andcan be recorded through cataracts orcorneal scars. Unfortunately, it is much more variable in appearance than thepattern VEP and, in general, it is used as a basic indication of the integrity ofThe visual pathway from the eye to theoccipital cortex. The most consistent feature is a positive component, designated P2, which usually occurs at around 125 ms after the flash, but identification of even this component is not always easy
PATTERN VEP : Pattern VEP can be used to measure VA by measuring the response amplitude of the VEP at each frequency, for a range of spatial frequencies. The amplitude is plotted against spatial frequency and a regression fit is used to determine the point at which the response would become zero, which is used as an estimate of VA.
Pattern reversal VEP is measured using checker board. In it the pattern of stimulus is changed ( black squares go white and white squares go black) with the overall illumination remaining same. It depends on form sense and thus gives a rough estimate of visual acuity.
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 6/18–6/24 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.
The component of major clinical importance is the so-called P100 response, a positive peak having a latency of approximately 100 ms. Its presence, latency, and symmetry over the two sides of the scalp are noted. Amplitude may also be measured, but changes in size are much less helpful for the recognition of pathology. VEPs are most useful in detecting dysfunction of the visual pathways anterior to the optic chiasma.
Sweep VEP : Sweep VEP essentially performs the same operation, but the spatial frequencies are varied very quickly over time and the amplitudes are immediately plotted with respect to spatial frequency (or time). For example, to measure VA, the spatial frequency changes from low to high in about 10-20 seconds. The regression line of the response amplitude is extrapolated to zero, which gives a measure of the VA.
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 onset–offset stimuli. 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.
It is important to recognise that the exact measure of VAobtained with sVEP or ssVEP depends on many parameters; whether data from electrodes is averaged or the best value is taken, whether a checkerboard or a grating is used, and the exact method for estimating the acuity threshold.
LIMITATIONS : cumbersome process of attaching and standardizing electrodes, the 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 cortices.
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 childs developmental age and specific capability.