Preverbalchildren 22-4-14

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

Preverbalchildren 22-4-14

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