Ординатура / Офтальмология / Английские материалы / Handbook of Pediatric Neuro-Ophthalmology_Wright, Spiegel, Thompson_2006

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system damage at birth, special areas to explore include maternal infections, prematurity, birth trauma, and suspected brain damage such as hydrocephalus and cerebral palsy. Special emphasis should be placed on visual and neurological development and details of similar problems in other family members. Information about previous evaluations and the results of past neuroradiologic studies (MRI/CT) are also helpful in understanding the current issues.

VISUAL ACUITY

Parental assessment of the child’s visual activity is helpful in determining the degree of impairment. Ask such questions as: “Does the child appear to see?” “Does the child respond to light, faces, and toys?” and “Does the child fix on the parent’s face, bottle, hands, and feet?” Does the child smile appropriately? If the child does display visually guided behavior, an effort should be made to get some idea as to the degree of visual function. How far away can the child see? Does the child follow movement across the room, pick up small objects, or reach for things? This approach is helpful not only in determining the etiology of visual loss but also in predicting future visual performance.

Rarely, the parent will indicate that no visual function is present. In this circumstance the parent is usually correct because the normal tendency is to be optimistic and overestimate visual function. Caution must be exercised in dismissing parental observations of visual function in a child who is apparently blind from cortical damage. In this circumstance, visual function can fluctuate significantly and can be intermittently present.

Visual Acuity Measurement

The accurate assessment of visual function is an important part of the neuro-ophthalmic exam. Many of the entities in subsequent chapters have decreased visual acuity as the presenting symptom. Special techniques are required to obtain an objective assessment of visual function in the young child. Table 2-1 contains a summary of the methods applicable to the various age groups.

Binocular visual acuity should be evaluated first because it is often superior to monocular function and the manipulation

TABLE 2-1. Visual Acuity Testing by Age.

OKN, optokinetic nystagmus; VER, visual evoked response; HOTV, matching of Snellen characters.

required to test the vision monocularly may result in loss of cooperation. Near visual function testing should also be attempted because in most neuro-ophthalmic disorders visual acuity will be better at near than at distance. This point is extremely important when trying to assess whether the child will be able to attend regular schools and whether large-print books or other visual aids will be necessary.

OKN Testing

Optokinetic nystagmus (OKN) testing is especially useful in assessing visual function in infants. The optokinetic response elicits a jerk nystagmus, which, if present, is indicative of visual function. This nystagmus was first described by Helmholtz, who observed the eyes of passengers gazing out of train windows.9 He noted a slow following phase in the direction of target movement and a rapid jerk return in the opposite direction to pick up

FIGURE 2-2A,B. (A) Child with lights on. (B) Same child immediately after lights are turned down. Note marked retraction of the upper lids. This reflex indicates some visual function.

visual function include visual evoked potentials and pattern electroretinography.

Very young children (1–3 months of age) will exhibit marked lid retraction when the lights are suddenly turned off (Fig. 2-2). This primitive reflex can be used to assess function in the apparently blind infant. If little or no visual function is present, this reflex will not occur. However, if visual acuity is 20/400 or better, marked lid retraction will occur immediately after the room lights are turned off. The lids may stay retracted for 20 to 30 s but will eventually return to the normal position. This reflex will occur repeatedly every time the lights are turned off. The presence of this reflex offers reassurance that some visual function exists.

Visual Evoked Potential

With the advent of computer technology, the ability to analyze brain electrical activity (EEG) during repetitive visual stimulation has evolved. By averaging 200 to 300 such tracings, a waveform results that has been termed the visual evoked potential (VEP). The VEP is commonly used to detect disorders of the visual pathway, and the two main characteristics are the wave amplitude and latency. The latency is a measure of optic nerve con-

duction and is delayed in such entities as optic neuritis. When using a patterned visual stimulus the amplitude can be a measure of visual acuity. A recent modification called the sweep VEP may prove promising in assessing visual acuity in preverbal infants.

PUPILLARY EXAMINATION

A wealth of information can be learned from examination of the pupils. An attempt should first be made to determine whether the pupils react to light and, if so, how briskly? A brisk constriction to light usually indicates good ocular and optic nerve function. A sluggish response, or no response at all, implies retina, optic nerve, or third nerve dysfunction. This assessment of pupil response to light can be helpful in localizing the site of visual loss, which is especially helpful in the child with possible cortical visual impairment (Fig. 2-3). Brisk pupillary reactions in a child with profound visual loss indicate a cortical etiology (Table 2-2).

Afferent Pupillary Defect (Marcus Gunn Pupil)

A specific sign for optic nerve dysfunction is the presence of an afferent pupillary defect (Marcus Gunn pupil). In testing for an afferent pupillary defect, room illumination should be decreased, the patient should be fixating at a distant target, and a bright light source should be used. The test compares the pupillary constriction, and hence optic nerve function, of the two eyes. The eye with the optic nerve dysfunction perceives the light source as being less bright and the pupils constrict less as compared to when the same light is shown in the “normal” eye. Therefore, the direct response to light (light shown into the involved eye) is less than the consensual response (light shown into the uninvolved eye). Clinically, the test is performed by swinging a flashlight between the two eyes, resting approximately 2 s at each eye.12 During the swinging flashlight test it appears to the observer as if the involved eye is exhibiting pupillary dilation to the light source (see Fig. 2-3). This seemingly “paradoxical dilatation to light” is the hallmark of an afferent pupillary defect, and the eye with the pupil that “dilates” to light has either an optic nerve or retinal abnormality.

Performing the swinging flashlight test on young children is often troublesome because distance fixation cannot be

FIGURE 2-3. Testing for an afferent pupillary defect. The “swinging flashlight test” is performed by shining a light in the right eye (top right) and noting the pupil size of both eyes. The light is then “swung” to the left eye and both pupils observed (top left). The poorer response when shining the light in the right eye indicates a right afferent pupillary defect. The test can also be performed using room illumination by alternatively covering each eye (middle left and right). The larger pupil on the right eye indicates a right afferent defect. Bottom: testing for an afferent defect with a fixed right pupil. Compare the size of the left pupil when light is shone in the right eye (bottom left) as compared to the left eye (bottom right). The better response from the left eye indicates a right afferent defect.

TABLE 2-2. Pupil Evaluation in Low-Vision Infant.

TABLE 2-3. Paradoxical Pupillary Phenomena.

Common causes

Congenital stationary night blindness

Congenital achromatopsia

Optic nerve hypoplasia

Rare causes

Leber’s congenital amaurosis

Best’s disease

Albinism

Retinitis pigmentosa

momentarily constrict. After 2 to 10 s the pupils will undergo slow dilatation; this is best seen by providing some low-level side illumination as the lights are dimmed. This “paradoxical pupillary constriction to darkness” is indicative of optic nerve or retinal disease or both (Table 2-3).

Anisocoria

Occasionally the patient will present with one pupil larger than the other. The diagnostic dilemma is deciding whether the abnormality is the large pupil or the small pupil, or whether no abnormality exists at all. Usually, this can be accomplished with a few simple clinical observations (outlined in Table 2-4). It must be remembered that anisocoria is NOT a sign of severe unilateral retinal or optic nerve dysfunction. In an otherwise normal patient, a blind eye will not result in anisocoria.

TABLE 2-4. Anisocoria.