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Visual function in Children with Developmental Disabilities
THE CHILD WITH DEVELOPMENTAL DISABILITIES
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VISUAL FUNCTION IN CHILDREN WITH DEVELOPMENTAL DISABILITIES Sheryl J. Menacker, MD
Abnormalities of the visual system frequently are among the many obstacles children with developmental disabilities face. The visual problems encountered are often related to underlying disabilities and may range from minor to severe, transient to permanent, stable to progressive, and ocular to cortical in nature. Although the type and extent of visual anomalies differ according to diagnosis, children with developmental disabilities as a group are at higher risk for visual impairment than those without disabilities. The literature abounds with reports associating specific disabilities with their related visual disorders and vice versa. Rather than address individual entities, the goal of this article is to provide the reader with an approach to understanding visual function in any child with a developmental disability. Because an understanding of visual development in early childhood is the foundation for understanding visual function in children with developmental disabilities, our discussion begins at this point. The etiology of visual problems will then be related to types of disability as an approach to individualizing the evaluation of each child. Finally, assessment of visual function in children with disabilities is addressed from the perspective of both pediatrician and pediatric ophthalmologist. Methods by which the pediatrician may evaluate visual function are discussed as well as the major techniques used by ophthalmologists to assess vision in nonverbal children and the most common pediatric ophthalmic interventions. POSTNATAL VISUAL DEVELOPMENT Neuronal and Vascular Development
The newborn eye is not a miniature adult eye. Although there is good optical clarity within days of birth,8, 12 the visual system remains immature in From the Division of Ophthalmology, University of Pennsylvania School of Medicine; and The Children's Hospital of Philadelphia, Philadelphia, Pennsylvania
PEDIATRIC CLINICS OF NORTH AMERICA VOLUME 40 • NUMBER 3 • JUNE 1993
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many respects. The fovea, which is responsible for fine central visual acuity, is not well developed at birth. Photoreceptors in this region undergo a process of growth and organization that is nearly adult-like by approximately 4 years of age. B Similar organization occurs all along the visual pathway. Increased myelination of the optic nerve and tract and rapid growth of lateral geniculate nucleus neurons occur in the first 24 postnatal months. B Differentiation of the striate visual cortex also continues after birth. Due to these ongoing processes, maturation occurs over several years, with behavioral measures of visual acuity estimated to be 20/400 in the full-term neonate 59 and reaching adult levels of 20/20 by 3 to 5 years of age. B In most newborn infants, fixation and following motions occur by approximately age 3 months,16 and binocular function develops between ages 3 and 7 months. B, 9 Visual function in premature infants has been found to be related more to maturation of the visual system than visual experience and therefore is best evaluated in terms of postconceptual rather than postnatal age for at least the first 9 months of life. 40, 54 Also, maturation of retinal vasculature is incomplete in premature infants. Retinal blood vessels begin their growth course at the optic disc during the fourth month of gestation, attaining maturity in the ninth month of gestation when they have reached the most distal extent of the retina. 62 Therefore, the retinal blood vessels of premature infants terminate somewhere between the optic disc and the ora serrata at birth. Postnatal growth of retinal vasculature in these infants may lead to the development of retinopathy of prematurity, which, in turn, may cause long-term ocular problems ranging from myopia to retinal detachment. 7,47-49,52 Special Considerations for Visual Development in Children with Disabilities
Maturation of the visual system in newborns with developmental disabilities may be affected by congenital, genetic, metabolic, infectious, traumatic, or hypoxic influences, Processes governing eye motions, alignment, acuity, and visual perception may mature slowly, partially, or abnormally. The prevalence of significant ocular disorders among individuals with developmental disabilities has been reported to range between 48% and 75% .14, 19, 29, 32, 35, 44 Refractive errors, strabismus, and motility disorders are especially frequent. 14, 19,44 Conversely, there is an increased prevalence of developmental disability among visually impaired individuals. Demographic surveys indicate that a significant proportion of visually impaired children have other disabilities caused by genetic, infectious, traumatic, or congenital processes. 43, 50, 66 The Importance of Amblyopia Related to Visual Development
No discussion of visual development is complete without recognizing the significance of amblyopia in early childhood, Because the visual system is immature, it has a degree of plasticity that allows compensatory neural adaptations to be made, B, 23, 24, 65 which can be either forgiving or unforgiving of abnormal conditions. When the visual cortex has been damaged, such as with perinatal asphyxia, it is the flexibility of the young visual system that frequently allows improvement of visual function, 23, 24, 37, 58 The occurrence of visual deprivation or disparity in early childhood may result in a lifetime of poor vision due to amblyopia, however. Defined as subnormal vhmal acuity without
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a visible organic lesion, amblyopia has been more succinctly described by one ophthalmologist as "the condition in which the observer saw nothing and the patient very little."·3 It has been estimated that amblyopia affects more than 2% of the general population and causes loss of vision in more people under age 45 than all ocular diseases and trauma combined. 63 Thus, early recognition and prompt treatment are imperative. The extent of visual deficit in amblyopia ranges from mild to profound and often depends on the underlying problem, how early in life it has occurred, and how promptly it has been treated. When pattern vision is prevented due to factors such as congenital ptosis, congenital cataract, or even patching for as little as 1 week in infancy, severe, lifelong visual loss may result. Significant visual acuity deficits also can occur when disparity of the visual acuity or images between the two eyes exists, such as with asymmetrical refractive errors and strabismus. Treatment of the underlying cause of amblyopia combined with occlusion of the better eye in early childhood has been shown to improve and even normalize visual acuity in amblyopic eyes. When the visual system has matured, amblyopia can no longer be caused or cured. This age of visual maturation has not been definitively determined in humans but is generally believed to occur between 6 and 7 years old. Children are most sensitive to developing amblyopia during the first 2 to 3 years of life, after which this sensitivity gradually decreases until visual maturity is reached. 63 After age 9, it is unusual for treatment to significantly improve visual acuity in an amblyopic eye. IS
ETIOLOGY OF VISUAL PROBLEMS
Because there is an increased prevalence of visual disorders among children with developmental disabilities, awareness of ophthalmic problems known to be associated with certain types of disability is instrumental in assuring early recognition and treatment of amblyopia, remediation of other treatable disorders, and awareness of conditions with a poor visual prognosis. It is impractical, however, if not impossible, to memorize the visual problems related to myriad specific syndromes. A more realistic approach might be to associate related groups of developmental disabilities with specific types of visual deficits. One way to accomplish this is to consider the etiology of the disorder and how it relates to visual function. By grouping disorders into the broad categories of congenital, inherited, metabolic, infectious, vascular, tumor, traumatic, and idiopathic causes, general implications for visual function may be more easily understood. To illustrate how this may be approached, a brief discussion of each of these categories is offered along with some representative examples. Congenital Abnormalities
Congenital causes of developmental disability often feature abnormal embryologic development. Anomalies of the eye or visual pathways also may be associated with such disorders. Whether development has been disrupted from toxic or idiopathic causes, the resulting deficits are usually long-lasting. Ocular colobomas may range from isolated iris involvement (without visual impairment) to clinical anophthalmos. Colobomas involving the optic nerve or retina cause visual deficits, including visual field anomalies and poor visual acuity, depending on the extent of involvement. CHARGE syndrome must be
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considered in infants with ocular Colobomas, with great care taken to identify Heart defects, Atresia choanae, Retardation (growth and developmental), Genital hypoplasia in males, and Ear anomalies. 45 One important example of ocular and developmental problems due to toxic effects during gestation is fetal alcohol syndrome (FAS). Up to 90% of children with this disorder have eye abnormalities. 57 Optic nerve hypoplasia has been reported to occur in 48%; an additional 12% have other disc malformations. Retinal vascular tortuosity has been found in 49% of children with FAS, and strabismus is found in up to 55%. Other ocular manifestations include corneal opacities, iris defects, anterior chamber angle abnormalities, and refractive errors (especially myopia). Exposure of the fetus to cocaine and other drugs of abuse during pregnancy also has been reported to cause many ophthalmic abnormalities, including strabismus, nystagmus, optic nerve hypoplasia, and cortical visual impairment. 13 Genetic and Metabolic Disorders
Genetic and metabolic causes of visual impairment and developmental delay vary. Depending on the disorder, ophthalmic findings may be static or progressive and range from minimal to severe. Retinitis pigmentosa results in a progressive loss of vision, as do metabolic storage diseases such as Tay-Sachs disease. At the other end of the spectrum are tuberous sclerosis and cystinosis, in which ocular abnormalities are common but usually cause little to no visual impairment. Perhaps the best known chromosomal disorder featuring both visual and developmental disabilities is Down syndrome. In addition to characteristic lid and iris anomalies unassociated with impairment, children with Down syndrome are at increased risk for strabismus, high refractive errors, nystagmus, cataracts, keratoconus, and blepharitis. 55 In light of the many treatable causes of visual impairment in Down syndrome, ophthalmic evaluation in early childhood is highly recommended. Infectious Diseases
Both congenital and acquired infectious processes can cause visual and developmental disability. The most familiar intrauterine infections are those comprising the TORCH complex and syphilis, all of which may result in a wide variety of ophthalmic pathology. 53 Corneal involvement, both primary and recurrent, is well known to be associated with congenital herpetic and luetic infection. Chorioretinal scarring is a common manifestation of congenital toxoplasmosis, cytomegalovirus, and herpes simplex. Such scarring is permanent and may cause significant visual impairment, especially when the fovea is involved. In addition, cortical visual impairment may result from congenital infections affecting the central nervous system. Reactivation at the site of old toxoplasmosis scarring may occur later in life, resulting in new or increased ophthalmic problems. When acquired after birth, these infectious processes may similarly cause visual impairment. Human immunodeficiency virus (HIV) is another cause of visual impairment and developmental disability. Children with HIV have an increased risk of retinitis from a number of agents, especially cytomegalovirus. Early recognition of this retinitis is essential due to the availability of antiviral treatment, which may decrease visual morbidity.
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Vascular Abnormalities Hypoxia is the primary cause of cortical visual impairment in children. 24 , 65 Most affected children do recover some visual function, although it may not be adequate for a sighted education.23 Children with cortical visual impairment unassociated with anterior visual pathway disease usually have normal pupillary reflexes and no nystagmus. Nystagmus results from visual loss due to ocular or anterior pathway disorders and does not occur with more posterior visual pathology.6s Children with cortical visual impairment tend to have extremely variable visual function. Most do not bump into objects when walking and lack the visual self-stimulatory behavior, such as eye pressing, that is commonly associated with blindness from anterior pathology.6s
Tumors Intracranial tumors can affect the visual pathways through direct involvement or indirect compression, altering visual acuity, visual fields, and oculomotor function. Tumors causing hydrocephalus may induce optic atrophy through stretching of the optic nerves or compression of the optic chiasm by a distended third ventricleY Infiltration of ocular structures also may occur.
Traumatic Injury Traumatic brain Injury is a leading cause of developmental and visual disability in children (see the article by Michaud et al elsewhere in this issue). Damage may be caused by direct injury to ocular or cortical structures, indirect involvement from compression and edema, or hypoxia. Therefore, the type of visual impairment depends on the extent of the injury and whether the eyes, visual pathways, or a combination of both have been involved. As previously mentioned, children suffering cortical visual impairment often have gradual recovery of visual function. Impairment due to loss of an eye or transection of an optic nerve is obviously not recoverable. Children with developmental disabilities also may suffer traumatic visual impairment that is induced by themselves, their caregivers, or peers. Selfinjurious behavior such as eye poking and head banging may be a source of significant and repeated trauma to the eye (see the article by Mauk elsewhere in this issue). In addition, ocular autostimulatory mannerisms such as eye pressing in blind children have been reported to cause corneal ulceration and other ocular injury. 27
Idiopathic Finally, there are those disorders causing developmental disabilities for which a cause has not been recognized. One such example is learning disability, which may manifest as an idiopathic reading disability (dyslexia) in children with otherwise normal intelligence (see the article by Shapiro and Gallico elsewhere in this issue). Despite the obvious association between the eyes and reading, there is no evidence to support ophthalmic factors as playing a significant role in the etiology of this primarily language-based disorder. The joint policy statement from the American Academy of Pediatrics, American
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Association for Pediatric Ophthalmology and Strabismus, and American Academy of Ophthalmology supports early diagnosis and remediation of learning disability through educational procedures of proven value, demonstrated by valid research. There is no known scientific evidence supporting claims for improving reading ability in children with learning disabilities through treatment based on visual training, neurologic organizational training, or glasses (with or without tinted lenses, prisms, or bifocals).2
ASSESSING VISUAL FUNCTION The Pediatrician's Perspective Evaluating Visual Acuity
The pediatrician is often first to formally assess a child's visual function. This can be especially difficult in children with developmental disabilities. For the verbal child who can cooperate by identifying characters on a distance or near chart, determination of visual acuity is relatively easy. Keep in mind that the presence of amblyopia can be missed when characters are presented singly rather than in groups because acuity improves for single symbols compared with linear presentations (a phenomenon called the crowding effect).36 Visual acuity in nonverbal children can be successfully evaluated with character recognition eye charts, however. Quite often, a child who cannot (or will not) verbally identify a picture or character correctly points to a figure on a handheld near card, upon request. It is appreciably easier to attain cooperation using this approach56 and, although this may not accurately reflect the distance visual acuity, it is a good indication of function within the range where most visual tasks occur. Thus, it is wise to have a character recognition near card on hand, such as one utilizing Allen pictures.! One recent method of assessing visual acuity in nonverbal children incorporates a distance acuity chart using only the optotypes H,O,T, and V.3! The presence of black bars beside the letters controls for the crowding effect, allowing individual presentation of each character. A card containing one large H,O,T, and V is placed on the child's lap. As each letter is presented on the distance chart, the child is asked to point to the appropriate character on the lap card. This is a good nonverbal method to obtain a distance visual acuity. It does not require familiarity with the alphabet but does entail understanding, coordination, and cooperation on the part of the child. Whereas ophthalmic evaluation of the verbal, cooperative child may be relatively easy, a good assessment can be achieved even with children who are minimally interactive due to age or disability. It is important to concentrate on which visual problems should be anticipated as well as what findings Signal their presence. By observation alone, much information can be ascertained. It is, of course, important to make the assessments when the child is alert and calm. Does the child gaze at the surrounding environment and fixate on objects or people? Or do the eyes wander, have unusual motions, or remain in tonic gaze? Making good eye contact with the examiner as well as steadily fixating and following a face or toy indicate the presence of some functional vision. On the other hand, wandering eye motions, nystagmus, and tonic gaze preferences are warning flags for possible visual dysfunction. Comprehensive ophthalmic evaluation should be obtained for all such children.
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Evaluating Ocular Alignment
Ocular alignment also can be assessed by observation. While misalignment of the eyes often is seen in infants before binocular vision is established, the presence of strabismus after 3 to 4 months of age is usually abnormal' and requires further. investigation. Evaluation of corneal light reflexes generally indicates straight alignment when they are symmetrically centered. A centered light reflex in one eye coupled with one that is not centered in the other suggests the presence of strabismus. Because of individual anatomical variation, however, assessment of ocular alignment by light reflex testing alone may be misleading. Because the optic nerve, rather than the fovea, is anatomically centered in the visual axis, the eye must turn slightly to orient the fovea centrally and allow fixation. 64 The distance between the fovea and the optic nerve determines how far the eye must turn to achieve fixation. This slight turning to align the visual axes is most often barely, if at all, discernible. One is more likely to incorrectly suspect exotropia (out-turning) than esotropia (inturning) on the basis of corneal light reflex testing, however, because the eyes usually must turn slightly outward for fixation. In cases such as cicatricial retinopathy of prematurity, in which the eye may turn more than just slightly outward in order to fixate because fibrosis has literally dragged the fovea temporally, the visual axes may be perfectly aligned, despite the appearance of exotropia. The appearance of pseudoesotropia is more often caused by prominent or asymmetrical inner canthal skin folds. In these cases, symmetrical corneal light reflexes may be used to verify straight alignment. Because corneal light reflexes can be misleading in evaluating ocular alignment, it is better to rely on cover testing for more accurate assessment. Cover testing is performed by covering each eye and observing the fellow eye for movement. If the fellow eye is misaligned, and assuming that it has sufficient vision, it makes a quick movement centrally in order to establish fixation. Although cover testing provides a more reliable assessment of ocular alignment, it requires good fixation and cooperation on the part of the child, both of which may not be obtainable. Much can be learned about visual anomalies and ocular alignment by evaluating the red reflexes of the eyes. It is well known that the normally bright red reflex viewed through the ophthalmoscope may be dimmed due to abnormalities of the cornea, lens, vitreous, or retina. In addition, a relative difference in brightness between the red reflex of each eye may be caused by differences in pupillary size, refractive error, or alignment. 51. 61 Evaluating Ocular Anomalies
Even when visual acuity and alignment appear to be normal, ocular anomalies may exist. Therefore, it is always important to evaluate each eye anatomically. Looking at the child, any asymmetry of the eyes or face should be noted. Abnormal skull configuration or alignment of the orbits may indicate craniofacial anomalies. Does one eye appear "bigger" than the other? If so, is this due to lid asymmetry? A droopy lid, ptosis, which partially or completely covers the pupillary axis, can quickly cause dense amblyopia and should be promptly evaluated. If the lids are symmetrical, then is one cornea larger than the other? A relatively good estimate of corneal diameter can be ascertained by measurement with a pocket ruler held just in front of the opened eye. The corneal diameter normally measures approximately 10 mm in the full-term newborn, 6, 22 with 95% of adult proportions of 11 to 12 mm attained by age l,59 A corneal diameter measuring less than 9 mm indicates microcornea, whereas
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a measurement more than 12 mm signifies megalocornea, raising the suspicion of glaucoma! If the corneas are normal in size, are they normal in clarity? A cloudy cornea requires prompt evaluation because it may signify anything from birth trauma to glaucoma to a mucopolysaccharidosis and is a source of amblyopia. Abnormalities of the sclera, iris, and pupil can be appreciated with a penlight. In addition, fundus examination is important in order to note any obvious anomalies of the optic nerve or retina. Optic nerve malformations and colobomas usually can be observed with little difficulty using the ophthalmoscope.
The Ophthalmologist's Perspective The ophthalmologist is often consulted to answer the question of whether or not a child with disabilities can see and, if so, what the quality of that vision might be. This information is critical in helping to determine the best habilitative program for an individual child. For a child with poor vision, therapy based on visual stimuli can be a frustrating endeavor. On the other hand, a child may have much better visual function than anticipated, allowing for a more comprehensive approach to meeting therapeutic and educational objectives. In recent years, the methods of assessing vision in infants and nonverbal children have improved. Several different, techniques are available that test visual function without relying on verbal responses or character recognition. These techniques are well suited to assessing vision in children with developmental disabilities and include optokinetic nystagmus, visual evoked potentials, and preferential looking. In addition, visual fields can be assessed by either confrontation or kinetic perimetry in both infants and nonverbal children. Optokinetic Nystagmus
Nystagmus may be elicited soon after birth by moving a drum or tape with alternating black and white stripes in front of a newborn's eyes. B, 34 An involuntary optokinetic nystagmus is induced that consists of the eyes slowly following a stripe in the direction of movement until it passes beyond the range of fixation, at which time a fast eye motion in the opposite direction occurs to begin the process of slowly following stripes in the direction of movement once again. lO The slow phase of this jerk nystagmus is believed to be controlled by a neural pathway originating in the occipital lobe of the brain, while control of the fast phase is thought to originate in the parietal lobe." 25 It is estimated that the vision necessary for optokinetic nystagmus to be induced must at least allow the perception of fingers held in front of the eyes. to Optokinetic nystagmus also may be used to assess quality of visual acuity by using various stripe widths and ascertaining the thinnest that produces a response. Infant optokinetic nystagmus responses are not identical to those in adults because they are asymmetrical until a corrected age of 3 to 6 months. B, 16, 34 When one eye is patched, the optokinetic nystagmus in the uncovered eye is much more pronounced if the stripes are moved in a temporal-to-nasal direction than if they are moved in a nasal-to-temporal fashion. Asymmetrical optokinetic nystagmus responses have been demonstrated to persist, however, in older children who have had significant visual deprivation in one or both eyes before age 6 months. Interestingly, this asymmetry has been found not only in children left with poor visual acuity but also in those whose ocular problem had been treated aggressively at an early age and who had subsequently
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achieved good visual acuity." Thus, abnormal optokinetic nystagmus responses may not correlate reliably with visual acuity. Although the presence of an optokinetic nystagmus response is reassuring when determining whether a child can see, its absence does not always indicate poor visual function. Lack of interest in the target, inappropriate rate or distance of stripe movement, and the status of the patient's oculomotor system may each preclude a positive response. lb , 25," Anticipating such obstacles is espeCially important when testing children with developmental disabilities, in whom attention deficits and oculomotor problems are common. For example, it is often difficult to assess optokinetic nystagmus responses in children with nystagmus when the stripes are moved horizontally. Rotating the stripes vertically instead can produce eye movements that are much more easily interpreted. Despite its shortcomings, optokinetic nystagmus testing is a quick, easy, inexpensive, and noninvasive method of evaluating visual function in infants, nonverbal children, and those with disabilities. Electrophysiological Testing: The Electroretinogram and Visual Evoked Potential
Electrophysiological testing of the visual system is an important part of the clinical investigation when visual disability is suspected. Such testing enables the determination of whether the system is intact and, if not, whether the dysfunction is primarily ocular or cortical in nature. The electroretinogram tests retinal function, whereas the visual evoked potential analyzes the pathway between the eye and the occipital cortex. Both tests may be administered to infants and young children, often during the same appointment. Electroretinogram testing requires modified contact lenses to be placed in the eyes after instillation of topical anesthetic drops. Depending on the type of equipment used, one to three electrodes are also affixed to the face or body. A computer analyzes the information received via leads from the contact lenses and electrodes after lights are momentarily flashed in the patient's eyes under different conditions. In order for the electroretinogram to evaluate retinal photoreceptors that function in the dark (rods) as well as those that function in conditions of light (cones), patients are adapted to the dark prior to testing by wearing an opaque black blindfold for approximately 30 minutes. Sedation may be necessary in some children for optimal cooperation and results. The electroretinogram is particularly useful in demonstrating disorders of the retinal photoreceptors. Fundus examination of young children with congenital and inherited retinal disorders may be normal, yet the electroretinogram may be markedly abnormal or nonrecordable. This is especially so in retinitis pigmentosa and its related syndromes, several of which are associated with developmental disabilities (e.g., Laurence-Moon-Biedl syndrome, neuronal ceroidlipofuscinoses, and infantile phytanic acid storage disease). Therefore, the electroretinogram may not only indicate that retinal dysfunction is the cause of visual impairment but also implicate a specific disorder. This is also true in the evaluation of nystagmus, in which the electroretinogram can detect the presence of certain retinal conditions known to be associated with this disorder. In the electrophysiologic evaluation of a patient with poor visual function, visual evoked potential testing is the next step once a normal electroretinogram is ascertained. The visual evoked potential has been used to determine the overall integrity of the neural pathway between the eye and the brain, as well as to estimate visual acuity. A normal visual evoked potential in such cases is
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reassuring but may not predict normal visual function in the future. Conversely, whereas an abnormal visual evoked potential may point to a defect along the visual pathway, it does not necessarily indicate that abnormal visual function will persist. Visual evoked potential responses in children with cortical visual impairment may range from normal to undetectable. 25 Many infants with normal electroretinograms and abnormal visual evoked potentials have gone on to have significantly improved or even normal visual evoked potentials and visual function. I7, 24, 25, 37, 38, 41 When evaluating infants who have' suffered perinatal hypoxia, some investigators believe that serial visual evoked potential recordings in the early postnatal period are a good prognostic indicator of long-term visual outcome,37 whereas others maintain that visual evoked potentials have little value in this regard. 24 , 25 Visual evoked potential responses also have been used to estimate visual acuity in nonverbal children. A child is seated in front of a television screen on which checkerboard patterns of various sizes are rapidly alternated at various rates. Scalp electrodes affixed over the occiput receive impulses as the child watches the screen, Computer analyzed results reflect activity in the visual cortex and are used to obtain an estimate of visual acuity. The advantages of visual evoked potential estimation of visual function include lack of voluntary movement, speech, or other behavioral activity needed to perform testing, One disadvantage is that it may be inaccurate when evaluating children with cortical visual impairment, seizure disorders, or nystagmus as well as those infants who are sedated, under anesthesia, or have critical electrolyte imbalances. 26 Other disadvantages include the need for electrode placement, expensive equipment, trained technicians, and, most important, good attention to the screen, 16, 26 Fixation is not a significant factor when visual evoked potentials are used to assess overall integrity of the visual system because this can be accomplished by flashing a bright light into the child's eyes (flash visual evoked potential), rather than presenting checkerboard patterns (pattern visual evoked potential), once the scalp electrodes have been affixed. For accurate visual evoked potential estimation of visual acuity, however, fixation is essential. Uncorrected refractive errors may also result in deficient fixation during testing. Preferential Looking Techniques
One of the exciting recent advances in visual acuity testing has been the advent of preferential looking techniques. Previously limited to research centers by the need for expensive, specially constructed, cumbersome equipment, the emergence of the acuity card method has brought preferential looking to the clinical forefron t. Preferential looking relies on the fact that an infant preferentially fixates on a boldly patterned target when it is paired with a blank target of equal luminance. 6o During testing with the acuity card method, the child is shown a series of cards containing a pattern of black and white stripes, or gratings, on one side and a blank gray target of equal luminance on the other side (Fig. 1). The stripe widths become progressively thinner on successive cards, creating finer gratings that require better visual resolution. Each card has a central peephole through which the tester observes the child's fixation. Upon presentation of an acuity card, the tester cannot see whether the stripes on front are located on the right or left side of the card. Observation of the child's eyes moving from midline towards the right or left allows the tester to guess that location. If the child looks in the opposite direction when the card is turned
s
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Figure 1. Assessment of visual acuity by preferential looking. The acuity card method is depicted. The child is looking toward the stripes, while the examiner observes through the peephole in the center of the card.
180 deg, it can be assumed that the child is visually attracted by the stripes. The tester then verifies this assumption by checking the position of the stripes on the front of the card. The finest set of stripes for which a child reliably produces this behavior is the grating acuity. Acuity card testing has been shown to correlate well with traditional estimates of visual acuity and, despite the fact that it relies on the clinical judgment and possible bias of the tester, there is good interobserver reliability.21. 39. 60 Individual results for infants and young children are best evaluated in the context of whether they fall within corrected-age established norms rather than concentrating on Snellen acuity equivalents, which may be calculated. 60 The overall success rate of preferential looking acuity testing among subjects with developmental disabilities is quite high, ranging from 82% to 99% in preterm and full-term infants at risk for neurologic complications; pediatric neurologic patients; patients with multiple disabilities; and those with cortical visual impairment, cerebral palsy, and Down syndrome. 2o• 21, 39 In general, children with more severe disabilities tend to have worse results, perhaps reflecting poorer visual acuity or responses that are less easily interpreted. 2o, 21, 39 Advantages of the acuity card method of preferential looking testing are that it can be accomplished in minutes, it requires no devices to be attached to the child (other than a patch for monocular testing), it does not require speech or recognition behavior, and it is relatively inexpensive. Disadvantages are that it may overestimate visual acuity in cases of amblyopia,I6, 39, 60 underestimate visual acuity in children with oculomotor abnormalities, and it also requires some level of cooperation. Despite these limitations, preferential looking is an easily performed method of evaluating acuity in infants and children with developmental disabilities. In addition, preferential looking has been shown to be a more successful method of testing than visual evoked potential for visually impaired children, with and without neurologic disorders. 5
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COMMON INTERVENTIONS BY PEDIATRIC OPHTHALMOLOGISTS
Once identified, treatable ophthalmic conditions in children with developmental disabilities are usually managed as they would be in those without disabilities. Whether management entails patching for amblyopia, prescription of glasses, or surgery, the goal is to optimize visual function. Two of the most common pediatric ophthalmic problems requiring intervention are amblyopia and strabismus. As previously discussed, early recognition of amblyopia is essential. Management usually consists of patching the better eye coupled with treatment of the underlying cause of amblyopia. This may require glasses, correction of strabismus, cataract extraction, ptosis repair, or other appropriate intervention. Strabismus, when constant, is usually treated with glasses or surgical intervention. Surgery to correct strabismus should be considered in those children for whom glasses are not indicated or who have failed to improve alignment. Measurements of the angle of strabismus should be stable before surgery is undertaken. The optimal timing of surgery for strabismus is a controversial issue, especially in the case of congenital esotropia. Manyophthalmologists recommend early intervention because of evidence that suggests that surgical alignment before age 24 months produces the highest yield of binocular function in congenital esotropia. 2s Commonly, surgery is perfonned as soon as alignment is stable after 6 months of age. 18, 42 Other ophthalmologists prefer to wait until the child is somewhat older. 18, 33 Controversy also surrounds the timing of surgery for exotropia, especially when present intermittently. 30 Regardless of how or when straight ocular alignment is obtained, it is extremely important for physicians and parents to recognize that the risk of amblyopia is present until the visual system reaches maturity!· In most instances of strabismus, one eye or the other is used for central fixation, even after straight alignment has been attained. Therefore, unless fixation is alternated between the two eyes, amblyopia is prone to occur. Danger lies in a false sense of security that all is well once good ocular alignment has been achieved. There is no cosmetically obvious clue that one eye is preferred and the other ignored for fixation. Therefore, careful follow-up of all children with a history of strabismus is essential. One other common therapeutic intervention in pediatric ophthalmology is the prescription of glasses. Glasses are usually prescribed for children under the following conditions: 1. Hyperopia, myopia, or astigmatism are present to such a degree that functional visual acuity is poor without correction. The eye is able to use its power of accommodation to correct mild to moderate hyperopia. Therefore, in the absence of strabismus, glasses are not usually prescribed to correct hyperopic refractive errors unless the level of hyperopia exceeds the amount the eye can comfortably accommodate. Unlike hyperopia, the eye is not able to compensate for myopia. But glasses may not be necessary when only mild myopia is present because good visual acuity should be present within a comfortable range at near, and the visual demands required for sustained viewing conditions at distance (e.g., driving, looking across the room at a blackboard) are not required in the child's daily routine. Similarly, correction of mild astigmatic errors may not be necessary if visual acuity without correction is adequate for the child's visual needs. 2. There is asymmetry of refractive error between the two eyes. When there is a significant difference in refractive errors between eyes (anisometropia),
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the potential for amblyopia is increased. In such cases, glasses are prescribed to provide equal clarity of the images that are focused on the retina of each eye. 3. Correction of refractive error is indicated for the treatment of strabismus. When esotropia is present, correction of hyperopia commonly results in improved alignment. Eyeglasses must be worn full time, and the eyes usually turn inward when the glasses are removed. In time, titration of the hyperopic prescription may allow good alignment even without glasses. Glasses are less commonly prescribed in an effort to correct exotropia. Correction or intentional overcorrection of myopia may help to decrease the angle or frequency of outturning, however. 11 4. Special tinting is required because of photophobia. Children with albinism, aniridia, corneal pathology, or other conditions which cause significant photophobia often benefit from glasses containing special tinting or filtering. 5. Protection of the eyes. Glasses are highly recommended for those individuals who are blind or have poor vision in one eye in order to provide protection of the fellow eye. 6. Accommodation is deficient. Occasionally, neurologic deficits impair the ability of the eyes to accommodate, making near vision difficult. In these cases, children function like older adults who need reading glasses and similarly benefit from bifocals or full vision lenses for near.
SUMMARY Vision involves a process of maturation that occurs in early childhood. Early recognition of visual disorders such as amblyopia is necessary to assure prompt treatment and optimize visual potential. This is especially important in children with developmental disabilities, in whom there is a high prevalence of such problems. Certainly, the type of ocular disorder varies according to the type of disability. But individual ophthalmic assessment is important, regardless of diagnosis, to identify treatable conditions that may improve or maximize visual function. In addition, ophthalmic examination may reveal distinctive anomalies in cases in which the etiology of disability has been difficult to establish. The present level of expertise and technology allows a comprehensive ophthalmic assessment to be performed, regardless of a child's level of impairment or ability to cooperate.
References 1. Allen HF: A new picture series for preschool vision testing. Am J Ophthalmol 44:38,
1957 2. American Academy of Pediatrics Committee on Children with Disabilities, American Association for Pediatric Ophthalmology and Strabismus, American Academy of Ophthalmology: Learning disabilities, dyslexia, and vision. Pediatrics 90:124, 1992 3. Archer SM, Sondhi N, Helveston EM: Strabismus in infancy. Ophthalmology 96:133, 1989 4. Bajandas FJ: Supranuclear and internuclear gaze pathways. In Neuroophthalmology Board Review Manual. Thorofare, Slack Inc, 1984, p 45 5. Bane MC, Birch EE: VEP acuity, FPL acuity, and visual behavior of visually impaired children. J Pediatr Ophthalmol Strabismus 29:202, 1992 6. Beauchamp GR, Varley GA: Corneal abnormalities. In Isenberg SJ (ed): The Eye in Infancy. Chicago, Year Book Medical Publishers, 1989, p 238
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7. Birch EE, Spencer R: Visual outcome in infants with cicatricial retinopathy of prematurity. Invest Ophthalmol Vis Sci 32:410, 1991 8. Boothe RG, Dobson V, Teller DY: Postnatal development of vision in human and nonhuman primates. Ann Rev Neurosci 8:495, 1985 9. Braddick 0, Atkinson J: The development of binocular function in infancy. Acta Ophthalmol 157:27, 1982 10. Burde RM, Savino pJ, Trobe JD: Nystagmus and other periodic eye disorders. In Clinical Decisions in Neuro-ophthalmology. St. Louis, CV Mosby, 1985, P 197 11. Caltrider N, Jampolsky A: Overcorrecting minus lens therapy for treatment of intermittent exotropia. Ophthalmology 90:1160, 1983 12. Dobson V, Teller DY: Visual acuity in human infants: a review and comparison of behavioral and electrophysiological studies. Vision Res 18:1469, 1978 13. Dominguez R, Vila-Coro AA, Slopis JM, et al: Brain and ocular abnormalities in infants with in utero exposure to cocaine and other street drugs. Am J Dis Child 145:688, 1991 14. Edwards WC, Price WD, Weisskopf B: Ocular findings in developmentally handicapped children. J Pediatr Ophthalmol Strabismus 9:162, 1972 15. France TO: Amblyopia. In Isenberg SJ (ed): The Eye in Infancy. Chicago, Year Book Medical Publishers, 1989, p 100 16. Friendly OS: Visual acuity assessment of the preverbal patient. In Isenberg SJ (ed): The Eye in Infancy. Chicago, Year Book Medical Publishers, 1989, p 48 17. Granet DB, Hertle RW, Breton ME, et al: The visual evoked response in infants with central visual impairment (Poster). The American Association for Pediatric Ophthalmology and Strabismus. Maui, Hawaii, February 9-13, 1992 18. Helveston EM: Esotropia in the first year of life. In Transactions of the New Orleans Academy of Ophthalmology, Pediatric Ophthalmology and Strabismus. New York, Raven Press, 1986, p 419 19. Henry JG: The ophthalmological assessment of the severely retarded child. Australian Journal of Ophthalmology 8:1, 1980 20. Hertz BG, Rosenberg J, Sjo 0, et al: Acuity card testing of patients with cerebral visual impairment. Dev Med Child NeuroI30:632, 1988 21. Hertz BG, Rosenberg J: Effect of mental retardation and motor disability on testing with visual acuity cards. Dev Med Child Neurol 34:115, 1992 22. Hirst LW: Congenital corneal problems. Int Ophthalmol Clin 24:55, 1984 23. Hoyt CS: Cortical Blindness in Infancy. In Pediatric Ophthalmology and Strabismus: Transactions of the New Orleans Academy of Ophthalmology. New York, Raven Press, 1986, p 235 24. Hoyt CS: Neurovisual adaptations to subnormal vision in children. Aust N Z J Ophthalmol 15:57, 1987 25. Hoyt CS: Objective techniques of visual acuity assessment in infancy. Aust N Z J OphthalmoI14:205, 1986 26. Hoyt CS: The clinical usefulness of the visual evoked response. J Pediatr Ophthalmol Strabismus 21:231, 1984 27. Hyasaka S, Yamamoto Y, Setogawa T: Ocular injuries by autostimulation in mentally retarded and nearly blind children. Ophthalmic Paediatr Genet 9:157, 1988 28. Ing MR: Early surgical alignment for congenital esotropia. Ophthalmology 90:132, 1983 29. Jacobson L: Ophthalmology in mentally retarded adults. Acta Ophthalmol 66:457, 1988 30. Jampolsky A: Treatment of Exodeviations. In Transactions of the New Orleans' Academy of Ophthalmology, Pediatric Ophthalmology and Strabismus. New York, Raven Press, 1986, p 201 31. Jenkins PF, Prager Pc, Mazow ML, et al: Preliterate vision screening: A comparative study. Am Orthop J 33:91, 1983 32. Landau L, Berson 0: Cerebral palsy and mental retardation: Ocular findings. J Pediatr Ophthalmol Strabismus 8:245, 1971 33. Lang JL: The optimum time for surgical alignment in congenital esotropia. J Pediatr Ophthalmol Strabismus 21:74, 1984 34. Lewis TL, Maurer 0, Brent HP: Optokinetic nystagmus in normal and visually deprived children: Implications for cortical development. Can J Psychol 43:121, 1989
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35. Maino OM, Maino JH, Maino SA: Mental retardation syndromes with associated ocular defects. JAm Optom Assoc 61:707, 1990 36. Mayer DL, Gross RD: Modified Allen pictures to assess amblyopia in young children. Ophthalmology 97:827, 1990 37. McCulloch DL, Taylor MJ, Whyte HE: Visual evoked potentials and visual prognosis following perinatal asphyxia. Arch Ophthalmol 109:229, 1991 38. Mellor DH, Fielder AR: Dissociated visual development: Electrodiagnostic studies in infants who are 'slow to see.' Dev Med Child Neurol 22:327, 1980 39. Mohn G, van Hof-van Duin J, Fetter WP, et al: Acuity assessment of non-verbal 40. 41. 42. 43. 44. 45. 46. 47.
infants and children: Clinical experience with the acuity card procedure. Dev Med Child Neurol 30:232, 1988 Morante A Dubowitz LM, Levene M, et al: The development of visual function in normal and neurologically abnormal preterm and fullterm infants. Dev Med Child Neurol 24:771, 1982 Nawratzki 1, Landau L, Auerbach E: Belated visual maturation in a mentally retarded child. J Pediatr Ophthalmol Strabismus 6:39, 1969 Nelson LB, Wagner RS, Simon JW, et al: Congenital esotropia. Surv Ophthalmol 31:363, 1987 O'Keefe M, Burke JP, Bowell R, et al: Childhood blindness. Ir Med J 83:57, 1990 Orel-Bixler 0, Haegerstrom-Portnoy G, Hall A: Visual assessment of the multiply handicapped patient. Optom Vis Sci 66:530, 1989 Pagon RA, Graham JM, Zonana J, et al: Coloboma, congenital heart disease, and choanal atresia with multiple anomalies: CHARGE association. J Pediatr 99:223, 1981 Parks MM: After the eyes are straightened what is the ophthalmologist's responsibility? Ophthalmology 93:1020, 1986 Quinn GE: Retinopathy of prematurity: natural history and classification. In Flynn JT, Tasman W (eds): Retinopathy of Prematurity. New York, Springer-Verlag, 1992,
p7 48. Quinn GE, Dobson V, Barr Cc, et al: Visual acuity in infants after vitrectomy for severe retinopathy of prematurity. Ophthalmology 98:5, 1991 49. Quinn GE, Dobson V, Repka MX, et al: Development of myopia in infants with birth weights less than 1251 grams. Ophthalmology 99:329, 1992 50. Robinson Gc, Jan JE, Kinnis C: Congenital ocular blindness in children, 1945 to 1984. Am J Dis Child 141:1321, 1987 51. Roe LD, Guyton DL: The light that leaks: Bruckner and the red reflex. Surv Ophthalmol 28:665, 1984 52. Schaffer DB, Quinn GE, Johnson L: Sequelae of arrested mild retinopathy of prematurity. Arch Ophthalmol 102:373, 1984 53. Schaffer DB: Eye findings in intrauterine infections. Clin Perinatol 8:415, 1981 54. Searle CM, Horne SM, Bourne KM: Visual acuity development: A study of preterm and full-term infants. Aust N Z J Ophthalmol 17:23, 1989 55. Shapiro MB, France TO: The ocular features of Down's syndrome. Am J Ophthalmol 99:659, 1985 56. Simons K: Visual acuity norms in young children. Surv Ophthalmol 28:84, 1983 57. Stromland K: Ocular involvement in the fetal alcohol syndrome. Surv Ophthalmol 31:277, 1986 58. Summers CG, MacDonald JT: Vision despite tomographic absence of the occipital cortex. Surv Ophthalmol 35:188, 1990 59. Teland V, Dweck HS: The eye of the newborn: A neonatologist's perspective. In Isenberg SJ (ed): The Eye in Infancy. Chicago, Year Book Medical Publishers, 1989,
p3 60. Teller DY, McDonald M, Preston K, et al: Assessment of visual acuity in infants and children: the acuity card procedure. Dev Med Child Neurol 28:779, 1986 61. Tongue AC, Cibis GW: Bruckner Test. Ophthalmology 88:1041, 1981 62. Urrea PT, Rosenbaum AL: Retinopathy of prematurity: An ophthalmologist's perspective. In Isenberg SJ (ed): The Eye in Infancy. Chicago, Year Book Medical Publishers, 1989, p 428 63. von Noorden GK: Examination of patient-III, sensory signs, symptoms, and adaptations in strabismus. In Binocular Vision and Ocular Motility, ed 4. St. Louis, CV Mosby, 1990, P 200
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64. von Noorden GK: Pseudostrabismus. In Atlas of Strabismus, ed 4. St Louis, CV Mosby, 1983, P 32 65. Whiting S, Jan JE, Wong PK, et al: Permanent cortical visual impairment in children. Dev Med Child NeuroI27:730, 1985 66. Williamson WD, Desmond MM, Andrew LP, et al: Visually impaired infants in the 1980s. Clin Pediatr 26:241, 1987 67. Wybar K: Symposium: The visually handicapped child, clinical assessment. Proceedings of the Royal Society of Medicine 62:555, 1969
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VISUAL FUNCTION IN CHILDREN WITH DEVELOPMENTAL DISABILITIES Sheryl J. Menacker, MD
Abnormalities of the visual system frequently are among the many obstacles children with developmental disabilities face. The visual problems encountered are often related to underlying disabilities and may range from minor to severe, transient to permanent, stable to progressive, and ocular to cortical in nature. Although the type and extent of visual anomalies differ according to diagnosis, children with developmental disabilities as a group are at higher risk for visual impairment than those without disabilities. The literature abounds with reports associating specific disabilities with their related visual disorders and vice versa. Rather than address individual entities, the goal of this article is to provide the reader with an approach to understanding visual function in any child with a developmental disability. Because an understanding of visual development in early childhood is the foundation for understanding visual function in children with developmental disabilities, our discussion begins at this point. The etiology of visual problems will then be related to types of disability as an approach to individualizing the evaluation of each child. Finally, assessment of visual function in children with disabilities is addressed from the perspective of both pediatrician and pediatric ophthalmologist. Methods by which the pediatrician may evaluate visual function are discussed as well as the major techniques used by ophthalmologists to assess vision in nonverbal children and the most common pediatric ophthalmic interventions. POSTNATAL VISUAL DEVELOPMENT Neuronal and Vascular Development
The newborn eye is not a miniature adult eye. Although there is good optical clarity within days of birth,8, 12 the visual system remains immature in From the Division of Ophthalmology, University of Pennsylvania School of Medicine; and The Children's Hospital of Philadelphia, Philadelphia, Pennsylvania
PEDIATRIC CLINICS OF NORTH AMERICA VOLUME 40 • NUMBER 3 • JUNE 1993
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many respects. The fovea, which is responsible for fine central visual acuity, is not well developed at birth. Photoreceptors in this region undergo a process of growth and organization that is nearly adult-like by approximately 4 years of age. B Similar organization occurs all along the visual pathway. Increased myelination of the optic nerve and tract and rapid growth of lateral geniculate nucleus neurons occur in the first 24 postnatal months. B Differentiation of the striate visual cortex also continues after birth. Due to these ongoing processes, maturation occurs over several years, with behavioral measures of visual acuity estimated to be 20/400 in the full-term neonate 59 and reaching adult levels of 20/20 by 3 to 5 years of age. B In most newborn infants, fixation and following motions occur by approximately age 3 months,16 and binocular function develops between ages 3 and 7 months. B, 9 Visual function in premature infants has been found to be related more to maturation of the visual system than visual experience and therefore is best evaluated in terms of postconceptual rather than postnatal age for at least the first 9 months of life. 40, 54 Also, maturation of retinal vasculature is incomplete in premature infants. Retinal blood vessels begin their growth course at the optic disc during the fourth month of gestation, attaining maturity in the ninth month of gestation when they have reached the most distal extent of the retina. 62 Therefore, the retinal blood vessels of premature infants terminate somewhere between the optic disc and the ora serrata at birth. Postnatal growth of retinal vasculature in these infants may lead to the development of retinopathy of prematurity, which, in turn, may cause long-term ocular problems ranging from myopia to retinal detachment. 7,47-49,52 Special Considerations for Visual Development in Children with Disabilities
Maturation of the visual system in newborns with developmental disabilities may be affected by congenital, genetic, metabolic, infectious, traumatic, or hypoxic influences, Processes governing eye motions, alignment, acuity, and visual perception may mature slowly, partially, or abnormally. The prevalence of significant ocular disorders among individuals with developmental disabilities has been reported to range between 48% and 75% .14, 19, 29, 32, 35, 44 Refractive errors, strabismus, and motility disorders are especially frequent. 14, 19,44 Conversely, there is an increased prevalence of developmental disability among visually impaired individuals. Demographic surveys indicate that a significant proportion of visually impaired children have other disabilities caused by genetic, infectious, traumatic, or congenital processes. 43, 50, 66 The Importance of Amblyopia Related to Visual Development
No discussion of visual development is complete without recognizing the significance of amblyopia in early childhood, Because the visual system is immature, it has a degree of plasticity that allows compensatory neural adaptations to be made, B, 23, 24, 65 which can be either forgiving or unforgiving of abnormal conditions. When the visual cortex has been damaged, such as with perinatal asphyxia, it is the flexibility of the young visual system that frequently allows improvement of visual function, 23, 24, 37, 58 The occurrence of visual deprivation or disparity in early childhood may result in a lifetime of poor vision due to amblyopia, however. Defined as subnormal vhmal acuity without
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a visible organic lesion, amblyopia has been more succinctly described by one ophthalmologist as "the condition in which the observer saw nothing and the patient very little."·3 It has been estimated that amblyopia affects more than 2% of the general population and causes loss of vision in more people under age 45 than all ocular diseases and trauma combined. 63 Thus, early recognition and prompt treatment are imperative. The extent of visual deficit in amblyopia ranges from mild to profound and often depends on the underlying problem, how early in life it has occurred, and how promptly it has been treated. When pattern vision is prevented due to factors such as congenital ptosis, congenital cataract, or even patching for as little as 1 week in infancy, severe, lifelong visual loss may result. Significant visual acuity deficits also can occur when disparity of the visual acuity or images between the two eyes exists, such as with asymmetrical refractive errors and strabismus. Treatment of the underlying cause of amblyopia combined with occlusion of the better eye in early childhood has been shown to improve and even normalize visual acuity in amblyopic eyes. When the visual system has matured, amblyopia can no longer be caused or cured. This age of visual maturation has not been definitively determined in humans but is generally believed to occur between 6 and 7 years old. Children are most sensitive to developing amblyopia during the first 2 to 3 years of life, after which this sensitivity gradually decreases until visual maturity is reached. 63 After age 9, it is unusual for treatment to significantly improve visual acuity in an amblyopic eye. IS
ETIOLOGY OF VISUAL PROBLEMS
Because there is an increased prevalence of visual disorders among children with developmental disabilities, awareness of ophthalmic problems known to be associated with certain types of disability is instrumental in assuring early recognition and treatment of amblyopia, remediation of other treatable disorders, and awareness of conditions with a poor visual prognosis. It is impractical, however, if not impossible, to memorize the visual problems related to myriad specific syndromes. A more realistic approach might be to associate related groups of developmental disabilities with specific types of visual deficits. One way to accomplish this is to consider the etiology of the disorder and how it relates to visual function. By grouping disorders into the broad categories of congenital, inherited, metabolic, infectious, vascular, tumor, traumatic, and idiopathic causes, general implications for visual function may be more easily understood. To illustrate how this may be approached, a brief discussion of each of these categories is offered along with some representative examples. Congenital Abnormalities
Congenital causes of developmental disability often feature abnormal embryologic development. Anomalies of the eye or visual pathways also may be associated with such disorders. Whether development has been disrupted from toxic or idiopathic causes, the resulting deficits are usually long-lasting. Ocular colobomas may range from isolated iris involvement (without visual impairment) to clinical anophthalmos. Colobomas involving the optic nerve or retina cause visual deficits, including visual field anomalies and poor visual acuity, depending on the extent of involvement. CHARGE syndrome must be
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considered in infants with ocular Colobomas, with great care taken to identify Heart defects, Atresia choanae, Retardation (growth and developmental), Genital hypoplasia in males, and Ear anomalies. 45 One important example of ocular and developmental problems due to toxic effects during gestation is fetal alcohol syndrome (FAS). Up to 90% of children with this disorder have eye abnormalities. 57 Optic nerve hypoplasia has been reported to occur in 48%; an additional 12% have other disc malformations. Retinal vascular tortuosity has been found in 49% of children with FAS, and strabismus is found in up to 55%. Other ocular manifestations include corneal opacities, iris defects, anterior chamber angle abnormalities, and refractive errors (especially myopia). Exposure of the fetus to cocaine and other drugs of abuse during pregnancy also has been reported to cause many ophthalmic abnormalities, including strabismus, nystagmus, optic nerve hypoplasia, and cortical visual impairment. 13 Genetic and Metabolic Disorders
Genetic and metabolic causes of visual impairment and developmental delay vary. Depending on the disorder, ophthalmic findings may be static or progressive and range from minimal to severe. Retinitis pigmentosa results in a progressive loss of vision, as do metabolic storage diseases such as Tay-Sachs disease. At the other end of the spectrum are tuberous sclerosis and cystinosis, in which ocular abnormalities are common but usually cause little to no visual impairment. Perhaps the best known chromosomal disorder featuring both visual and developmental disabilities is Down syndrome. In addition to characteristic lid and iris anomalies unassociated with impairment, children with Down syndrome are at increased risk for strabismus, high refractive errors, nystagmus, cataracts, keratoconus, and blepharitis. 55 In light of the many treatable causes of visual impairment in Down syndrome, ophthalmic evaluation in early childhood is highly recommended. Infectious Diseases
Both congenital and acquired infectious processes can cause visual and developmental disability. The most familiar intrauterine infections are those comprising the TORCH complex and syphilis, all of which may result in a wide variety of ophthalmic pathology. 53 Corneal involvement, both primary and recurrent, is well known to be associated with congenital herpetic and luetic infection. Chorioretinal scarring is a common manifestation of congenital toxoplasmosis, cytomegalovirus, and herpes simplex. Such scarring is permanent and may cause significant visual impairment, especially when the fovea is involved. In addition, cortical visual impairment may result from congenital infections affecting the central nervous system. Reactivation at the site of old toxoplasmosis scarring may occur later in life, resulting in new or increased ophthalmic problems. When acquired after birth, these infectious processes may similarly cause visual impairment. Human immunodeficiency virus (HIV) is another cause of visual impairment and developmental disability. Children with HIV have an increased risk of retinitis from a number of agents, especially cytomegalovirus. Early recognition of this retinitis is essential due to the availability of antiviral treatment, which may decrease visual morbidity.
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Vascular Abnormalities Hypoxia is the primary cause of cortical visual impairment in children. 24 , 65 Most affected children do recover some visual function, although it may not be adequate for a sighted education.23 Children with cortical visual impairment unassociated with anterior visual pathway disease usually have normal pupillary reflexes and no nystagmus. Nystagmus results from visual loss due to ocular or anterior pathway disorders and does not occur with more posterior visual pathology.6s Children with cortical visual impairment tend to have extremely variable visual function. Most do not bump into objects when walking and lack the visual self-stimulatory behavior, such as eye pressing, that is commonly associated with blindness from anterior pathology.6s
Tumors Intracranial tumors can affect the visual pathways through direct involvement or indirect compression, altering visual acuity, visual fields, and oculomotor function. Tumors causing hydrocephalus may induce optic atrophy through stretching of the optic nerves or compression of the optic chiasm by a distended third ventricleY Infiltration of ocular structures also may occur.
Traumatic Injury Traumatic brain Injury is a leading cause of developmental and visual disability in children (see the article by Michaud et al elsewhere in this issue). Damage may be caused by direct injury to ocular or cortical structures, indirect involvement from compression and edema, or hypoxia. Therefore, the type of visual impairment depends on the extent of the injury and whether the eyes, visual pathways, or a combination of both have been involved. As previously mentioned, children suffering cortical visual impairment often have gradual recovery of visual function. Impairment due to loss of an eye or transection of an optic nerve is obviously not recoverable. Children with developmental disabilities also may suffer traumatic visual impairment that is induced by themselves, their caregivers, or peers. Selfinjurious behavior such as eye poking and head banging may be a source of significant and repeated trauma to the eye (see the article by Mauk elsewhere in this issue). In addition, ocular autostimulatory mannerisms such as eye pressing in blind children have been reported to cause corneal ulceration and other ocular injury. 27
Idiopathic Finally, there are those disorders causing developmental disabilities for which a cause has not been recognized. One such example is learning disability, which may manifest as an idiopathic reading disability (dyslexia) in children with otherwise normal intelligence (see the article by Shapiro and Gallico elsewhere in this issue). Despite the obvious association between the eyes and reading, there is no evidence to support ophthalmic factors as playing a significant role in the etiology of this primarily language-based disorder. The joint policy statement from the American Academy of Pediatrics, American
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Association for Pediatric Ophthalmology and Strabismus, and American Academy of Ophthalmology supports early diagnosis and remediation of learning disability through educational procedures of proven value, demonstrated by valid research. There is no known scientific evidence supporting claims for improving reading ability in children with learning disabilities through treatment based on visual training, neurologic organizational training, or glasses (with or without tinted lenses, prisms, or bifocals).2
ASSESSING VISUAL FUNCTION The Pediatrician's Perspective Evaluating Visual Acuity
The pediatrician is often first to formally assess a child's visual function. This can be especially difficult in children with developmental disabilities. For the verbal child who can cooperate by identifying characters on a distance or near chart, determination of visual acuity is relatively easy. Keep in mind that the presence of amblyopia can be missed when characters are presented singly rather than in groups because acuity improves for single symbols compared with linear presentations (a phenomenon called the crowding effect).36 Visual acuity in nonverbal children can be successfully evaluated with character recognition eye charts, however. Quite often, a child who cannot (or will not) verbally identify a picture or character correctly points to a figure on a handheld near card, upon request. It is appreciably easier to attain cooperation using this approach56 and, although this may not accurately reflect the distance visual acuity, it is a good indication of function within the range where most visual tasks occur. Thus, it is wise to have a character recognition near card on hand, such as one utilizing Allen pictures.! One recent method of assessing visual acuity in nonverbal children incorporates a distance acuity chart using only the optotypes H,O,T, and V.3! The presence of black bars beside the letters controls for the crowding effect, allowing individual presentation of each character. A card containing one large H,O,T, and V is placed on the child's lap. As each letter is presented on the distance chart, the child is asked to point to the appropriate character on the lap card. This is a good nonverbal method to obtain a distance visual acuity. It does not require familiarity with the alphabet but does entail understanding, coordination, and cooperation on the part of the child. Whereas ophthalmic evaluation of the verbal, cooperative child may be relatively easy, a good assessment can be achieved even with children who are minimally interactive due to age or disability. It is important to concentrate on which visual problems should be anticipated as well as what findings Signal their presence. By observation alone, much information can be ascertained. It is, of course, important to make the assessments when the child is alert and calm. Does the child gaze at the surrounding environment and fixate on objects or people? Or do the eyes wander, have unusual motions, or remain in tonic gaze? Making good eye contact with the examiner as well as steadily fixating and following a face or toy indicate the presence of some functional vision. On the other hand, wandering eye motions, nystagmus, and tonic gaze preferences are warning flags for possible visual dysfunction. Comprehensive ophthalmic evaluation should be obtained for all such children.
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Evaluating Ocular Alignment
Ocular alignment also can be assessed by observation. While misalignment of the eyes often is seen in infants before binocular vision is established, the presence of strabismus after 3 to 4 months of age is usually abnormal' and requires further. investigation. Evaluation of corneal light reflexes generally indicates straight alignment when they are symmetrically centered. A centered light reflex in one eye coupled with one that is not centered in the other suggests the presence of strabismus. Because of individual anatomical variation, however, assessment of ocular alignment by light reflex testing alone may be misleading. Because the optic nerve, rather than the fovea, is anatomically centered in the visual axis, the eye must turn slightly to orient the fovea centrally and allow fixation. 64 The distance between the fovea and the optic nerve determines how far the eye must turn to achieve fixation. This slight turning to align the visual axes is most often barely, if at all, discernible. One is more likely to incorrectly suspect exotropia (out-turning) than esotropia (inturning) on the basis of corneal light reflex testing, however, because the eyes usually must turn slightly outward for fixation. In cases such as cicatricial retinopathy of prematurity, in which the eye may turn more than just slightly outward in order to fixate because fibrosis has literally dragged the fovea temporally, the visual axes may be perfectly aligned, despite the appearance of exotropia. The appearance of pseudoesotropia is more often caused by prominent or asymmetrical inner canthal skin folds. In these cases, symmetrical corneal light reflexes may be used to verify straight alignment. Because corneal light reflexes can be misleading in evaluating ocular alignment, it is better to rely on cover testing for more accurate assessment. Cover testing is performed by covering each eye and observing the fellow eye for movement. If the fellow eye is misaligned, and assuming that it has sufficient vision, it makes a quick movement centrally in order to establish fixation. Although cover testing provides a more reliable assessment of ocular alignment, it requires good fixation and cooperation on the part of the child, both of which may not be obtainable. Much can be learned about visual anomalies and ocular alignment by evaluating the red reflexes of the eyes. It is well known that the normally bright red reflex viewed through the ophthalmoscope may be dimmed due to abnormalities of the cornea, lens, vitreous, or retina. In addition, a relative difference in brightness between the red reflex of each eye may be caused by differences in pupillary size, refractive error, or alignment. 51. 61 Evaluating Ocular Anomalies
Even when visual acuity and alignment appear to be normal, ocular anomalies may exist. Therefore, it is always important to evaluate each eye anatomically. Looking at the child, any asymmetry of the eyes or face should be noted. Abnormal skull configuration or alignment of the orbits may indicate craniofacial anomalies. Does one eye appear "bigger" than the other? If so, is this due to lid asymmetry? A droopy lid, ptosis, which partially or completely covers the pupillary axis, can quickly cause dense amblyopia and should be promptly evaluated. If the lids are symmetrical, then is one cornea larger than the other? A relatively good estimate of corneal diameter can be ascertained by measurement with a pocket ruler held just in front of the opened eye. The corneal diameter normally measures approximately 10 mm in the full-term newborn, 6, 22 with 95% of adult proportions of 11 to 12 mm attained by age l,59 A corneal diameter measuring less than 9 mm indicates microcornea, whereas
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a measurement more than 12 mm signifies megalocornea, raising the suspicion of glaucoma! If the corneas are normal in size, are they normal in clarity? A cloudy cornea requires prompt evaluation because it may signify anything from birth trauma to glaucoma to a mucopolysaccharidosis and is a source of amblyopia. Abnormalities of the sclera, iris, and pupil can be appreciated with a penlight. In addition, fundus examination is important in order to note any obvious anomalies of the optic nerve or retina. Optic nerve malformations and colobomas usually can be observed with little difficulty using the ophthalmoscope.
The Ophthalmologist's Perspective The ophthalmologist is often consulted to answer the question of whether or not a child with disabilities can see and, if so, what the quality of that vision might be. This information is critical in helping to determine the best habilitative program for an individual child. For a child with poor vision, therapy based on visual stimuli can be a frustrating endeavor. On the other hand, a child may have much better visual function than anticipated, allowing for a more comprehensive approach to meeting therapeutic and educational objectives. In recent years, the methods of assessing vision in infants and nonverbal children have improved. Several different, techniques are available that test visual function without relying on verbal responses or character recognition. These techniques are well suited to assessing vision in children with developmental disabilities and include optokinetic nystagmus, visual evoked potentials, and preferential looking. In addition, visual fields can be assessed by either confrontation or kinetic perimetry in both infants and nonverbal children. Optokinetic Nystagmus
Nystagmus may be elicited soon after birth by moving a drum or tape with alternating black and white stripes in front of a newborn's eyes. B, 34 An involuntary optokinetic nystagmus is induced that consists of the eyes slowly following a stripe in the direction of movement until it passes beyond the range of fixation, at which time a fast eye motion in the opposite direction occurs to begin the process of slowly following stripes in the direction of movement once again. lO The slow phase of this jerk nystagmus is believed to be controlled by a neural pathway originating in the occipital lobe of the brain, while control of the fast phase is thought to originate in the parietal lobe." 25 It is estimated that the vision necessary for optokinetic nystagmus to be induced must at least allow the perception of fingers held in front of the eyes. to Optokinetic nystagmus also may be used to assess quality of visual acuity by using various stripe widths and ascertaining the thinnest that produces a response. Infant optokinetic nystagmus responses are not identical to those in adults because they are asymmetrical until a corrected age of 3 to 6 months. B, 16, 34 When one eye is patched, the optokinetic nystagmus in the uncovered eye is much more pronounced if the stripes are moved in a temporal-to-nasal direction than if they are moved in a nasal-to-temporal fashion. Asymmetrical optokinetic nystagmus responses have been demonstrated to persist, however, in older children who have had significant visual deprivation in one or both eyes before age 6 months. Interestingly, this asymmetry has been found not only in children left with poor visual acuity but also in those whose ocular problem had been treated aggressively at an early age and who had subsequently
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achieved good visual acuity." Thus, abnormal optokinetic nystagmus responses may not correlate reliably with visual acuity. Although the presence of an optokinetic nystagmus response is reassuring when determining whether a child can see, its absence does not always indicate poor visual function. Lack of interest in the target, inappropriate rate or distance of stripe movement, and the status of the patient's oculomotor system may each preclude a positive response. lb , 25," Anticipating such obstacles is espeCially important when testing children with developmental disabilities, in whom attention deficits and oculomotor problems are common. For example, it is often difficult to assess optokinetic nystagmus responses in children with nystagmus when the stripes are moved horizontally. Rotating the stripes vertically instead can produce eye movements that are much more easily interpreted. Despite its shortcomings, optokinetic nystagmus testing is a quick, easy, inexpensive, and noninvasive method of evaluating visual function in infants, nonverbal children, and those with disabilities. Electrophysiological Testing: The Electroretinogram and Visual Evoked Potential
Electrophysiological testing of the visual system is an important part of the clinical investigation when visual disability is suspected. Such testing enables the determination of whether the system is intact and, if not, whether the dysfunction is primarily ocular or cortical in nature. The electroretinogram tests retinal function, whereas the visual evoked potential analyzes the pathway between the eye and the occipital cortex. Both tests may be administered to infants and young children, often during the same appointment. Electroretinogram testing requires modified contact lenses to be placed in the eyes after instillation of topical anesthetic drops. Depending on the type of equipment used, one to three electrodes are also affixed to the face or body. A computer analyzes the information received via leads from the contact lenses and electrodes after lights are momentarily flashed in the patient's eyes under different conditions. In order for the electroretinogram to evaluate retinal photoreceptors that function in the dark (rods) as well as those that function in conditions of light (cones), patients are adapted to the dark prior to testing by wearing an opaque black blindfold for approximately 30 minutes. Sedation may be necessary in some children for optimal cooperation and results. The electroretinogram is particularly useful in demonstrating disorders of the retinal photoreceptors. Fundus examination of young children with congenital and inherited retinal disorders may be normal, yet the electroretinogram may be markedly abnormal or nonrecordable. This is especially so in retinitis pigmentosa and its related syndromes, several of which are associated with developmental disabilities (e.g., Laurence-Moon-Biedl syndrome, neuronal ceroidlipofuscinoses, and infantile phytanic acid storage disease). Therefore, the electroretinogram may not only indicate that retinal dysfunction is the cause of visual impairment but also implicate a specific disorder. This is also true in the evaluation of nystagmus, in which the electroretinogram can detect the presence of certain retinal conditions known to be associated with this disorder. In the electrophysiologic evaluation of a patient with poor visual function, visual evoked potential testing is the next step once a normal electroretinogram is ascertained. The visual evoked potential has been used to determine the overall integrity of the neural pathway between the eye and the brain, as well as to estimate visual acuity. A normal visual evoked potential in such cases is
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reassuring but may not predict normal visual function in the future. Conversely, whereas an abnormal visual evoked potential may point to a defect along the visual pathway, it does not necessarily indicate that abnormal visual function will persist. Visual evoked potential responses in children with cortical visual impairment may range from normal to undetectable. 25 Many infants with normal electroretinograms and abnormal visual evoked potentials have gone on to have significantly improved or even normal visual evoked potentials and visual function. I7, 24, 25, 37, 38, 41 When evaluating infants who have' suffered perinatal hypoxia, some investigators believe that serial visual evoked potential recordings in the early postnatal period are a good prognostic indicator of long-term visual outcome,37 whereas others maintain that visual evoked potentials have little value in this regard. 24 , 25 Visual evoked potential responses also have been used to estimate visual acuity in nonverbal children. A child is seated in front of a television screen on which checkerboard patterns of various sizes are rapidly alternated at various rates. Scalp electrodes affixed over the occiput receive impulses as the child watches the screen, Computer analyzed results reflect activity in the visual cortex and are used to obtain an estimate of visual acuity. The advantages of visual evoked potential estimation of visual function include lack of voluntary movement, speech, or other behavioral activity needed to perform testing, One disadvantage is that it may be inaccurate when evaluating children with cortical visual impairment, seizure disorders, or nystagmus as well as those infants who are sedated, under anesthesia, or have critical electrolyte imbalances. 26 Other disadvantages include the need for electrode placement, expensive equipment, trained technicians, and, most important, good attention to the screen, 16, 26 Fixation is not a significant factor when visual evoked potentials are used to assess overall integrity of the visual system because this can be accomplished by flashing a bright light into the child's eyes (flash visual evoked potential), rather than presenting checkerboard patterns (pattern visual evoked potential), once the scalp electrodes have been affixed. For accurate visual evoked potential estimation of visual acuity, however, fixation is essential. Uncorrected refractive errors may also result in deficient fixation during testing. Preferential Looking Techniques
One of the exciting recent advances in visual acuity testing has been the advent of preferential looking techniques. Previously limited to research centers by the need for expensive, specially constructed, cumbersome equipment, the emergence of the acuity card method has brought preferential looking to the clinical forefron t. Preferential looking relies on the fact that an infant preferentially fixates on a boldly patterned target when it is paired with a blank target of equal luminance. 6o During testing with the acuity card method, the child is shown a series of cards containing a pattern of black and white stripes, or gratings, on one side and a blank gray target of equal luminance on the other side (Fig. 1). The stripe widths become progressively thinner on successive cards, creating finer gratings that require better visual resolution. Each card has a central peephole through which the tester observes the child's fixation. Upon presentation of an acuity card, the tester cannot see whether the stripes on front are located on the right or left side of the card. Observation of the child's eyes moving from midline towards the right or left allows the tester to guess that location. If the child looks in the opposite direction when the card is turned
s
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Figure 1. Assessment of visual acuity by preferential looking. The acuity card method is depicted. The child is looking toward the stripes, while the examiner observes through the peephole in the center of the card.
180 deg, it can be assumed that the child is visually attracted by the stripes. The tester then verifies this assumption by checking the position of the stripes on the front of the card. The finest set of stripes for which a child reliably produces this behavior is the grating acuity. Acuity card testing has been shown to correlate well with traditional estimates of visual acuity and, despite the fact that it relies on the clinical judgment and possible bias of the tester, there is good interobserver reliability.21. 39. 60 Individual results for infants and young children are best evaluated in the context of whether they fall within corrected-age established norms rather than concentrating on Snellen acuity equivalents, which may be calculated. 60 The overall success rate of preferential looking acuity testing among subjects with developmental disabilities is quite high, ranging from 82% to 99% in preterm and full-term infants at risk for neurologic complications; pediatric neurologic patients; patients with multiple disabilities; and those with cortical visual impairment, cerebral palsy, and Down syndrome. 2o• 21, 39 In general, children with more severe disabilities tend to have worse results, perhaps reflecting poorer visual acuity or responses that are less easily interpreted. 2o, 21, 39 Advantages of the acuity card method of preferential looking testing are that it can be accomplished in minutes, it requires no devices to be attached to the child (other than a patch for monocular testing), it does not require speech or recognition behavior, and it is relatively inexpensive. Disadvantages are that it may overestimate visual acuity in cases of amblyopia,I6, 39, 60 underestimate visual acuity in children with oculomotor abnormalities, and it also requires some level of cooperation. Despite these limitations, preferential looking is an easily performed method of evaluating acuity in infants and children with developmental disabilities. In addition, preferential looking has been shown to be a more successful method of testing than visual evoked potential for visually impaired children, with and without neurologic disorders. 5
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COMMON INTERVENTIONS BY PEDIATRIC OPHTHALMOLOGISTS
Once identified, treatable ophthalmic conditions in children with developmental disabilities are usually managed as they would be in those without disabilities. Whether management entails patching for amblyopia, prescription of glasses, or surgery, the goal is to optimize visual function. Two of the most common pediatric ophthalmic problems requiring intervention are amblyopia and strabismus. As previously discussed, early recognition of amblyopia is essential. Management usually consists of patching the better eye coupled with treatment of the underlying cause of amblyopia. This may require glasses, correction of strabismus, cataract extraction, ptosis repair, or other appropriate intervention. Strabismus, when constant, is usually treated with glasses or surgical intervention. Surgery to correct strabismus should be considered in those children for whom glasses are not indicated or who have failed to improve alignment. Measurements of the angle of strabismus should be stable before surgery is undertaken. The optimal timing of surgery for strabismus is a controversial issue, especially in the case of congenital esotropia. Manyophthalmologists recommend early intervention because of evidence that suggests that surgical alignment before age 24 months produces the highest yield of binocular function in congenital esotropia. 2s Commonly, surgery is perfonned as soon as alignment is stable after 6 months of age. 18, 42 Other ophthalmologists prefer to wait until the child is somewhat older. 18, 33 Controversy also surrounds the timing of surgery for exotropia, especially when present intermittently. 30 Regardless of how or when straight ocular alignment is obtained, it is extremely important for physicians and parents to recognize that the risk of amblyopia is present until the visual system reaches maturity!· In most instances of strabismus, one eye or the other is used for central fixation, even after straight alignment has been attained. Therefore, unless fixation is alternated between the two eyes, amblyopia is prone to occur. Danger lies in a false sense of security that all is well once good ocular alignment has been achieved. There is no cosmetically obvious clue that one eye is preferred and the other ignored for fixation. Therefore, careful follow-up of all children with a history of strabismus is essential. One other common therapeutic intervention in pediatric ophthalmology is the prescription of glasses. Glasses are usually prescribed for children under the following conditions: 1. Hyperopia, myopia, or astigmatism are present to such a degree that functional visual acuity is poor without correction. The eye is able to use its power of accommodation to correct mild to moderate hyperopia. Therefore, in the absence of strabismus, glasses are not usually prescribed to correct hyperopic refractive errors unless the level of hyperopia exceeds the amount the eye can comfortably accommodate. Unlike hyperopia, the eye is not able to compensate for myopia. But glasses may not be necessary when only mild myopia is present because good visual acuity should be present within a comfortable range at near, and the visual demands required for sustained viewing conditions at distance (e.g., driving, looking across the room at a blackboard) are not required in the child's daily routine. Similarly, correction of mild astigmatic errors may not be necessary if visual acuity without correction is adequate for the child's visual needs. 2. There is asymmetry of refractive error between the two eyes. When there is a significant difference in refractive errors between eyes (anisometropia),
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the potential for amblyopia is increased. In such cases, glasses are prescribed to provide equal clarity of the images that are focused on the retina of each eye. 3. Correction of refractive error is indicated for the treatment of strabismus. When esotropia is present, correction of hyperopia commonly results in improved alignment. Eyeglasses must be worn full time, and the eyes usually turn inward when the glasses are removed. In time, titration of the hyperopic prescription may allow good alignment even without glasses. Glasses are less commonly prescribed in an effort to correct exotropia. Correction or intentional overcorrection of myopia may help to decrease the angle or frequency of outturning, however. 11 4. Special tinting is required because of photophobia. Children with albinism, aniridia, corneal pathology, or other conditions which cause significant photophobia often benefit from glasses containing special tinting or filtering. 5. Protection of the eyes. Glasses are highly recommended for those individuals who are blind or have poor vision in one eye in order to provide protection of the fellow eye. 6. Accommodation is deficient. Occasionally, neurologic deficits impair the ability of the eyes to accommodate, making near vision difficult. In these cases, children function like older adults who need reading glasses and similarly benefit from bifocals or full vision lenses for near.
SUMMARY Vision involves a process of maturation that occurs in early childhood. Early recognition of visual disorders such as amblyopia is necessary to assure prompt treatment and optimize visual potential. This is especially important in children with developmental disabilities, in whom there is a high prevalence of such problems. Certainly, the type of ocular disorder varies according to the type of disability. But individual ophthalmic assessment is important, regardless of diagnosis, to identify treatable conditions that may improve or maximize visual function. In addition, ophthalmic examination may reveal distinctive anomalies in cases in which the etiology of disability has been difficult to establish. The present level of expertise and technology allows a comprehensive ophthalmic assessment to be performed, regardless of a child's level of impairment or ability to cooperate.
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