anatomical structure.

Sluggish or absent response to light or an afferent defect indicates an anterior visual pathway abnormality such as optic nerve or retinal dysfunction. Responses can be normal in mild abnormalities, such as foveal hypoplasia, achromatopsia, and primary motor nystagmus. The normal response to darkness is the immediate dilation of the pupil. If, instead of dilating, the pupils paradoxically constrict, optic nerve or retinal disease is present.

In addition to pupillary responses, iris structure should be assessed. Defects such as iris colobomas suggest that optic nerve or retinal colobomas may also be present. Iris transillumination can be a sign of albinism. Aniridia and albinism are associated with foveal hypoplasia, poor vision, and nystagmus.

Ocular motility

Patients with nystagmus may have strabismus for several reasons: (1) as a result of poor vision, if that is the cause of the nystagmus; (2) due to damping the nystagmus by converging; or (3) because of latent nystagmus secondary to strabismus. Children with manifest latent nystagmus (fusion maldevelopment nystagmus syndrome) often fixate with the preferred eye in adduction to maximize their visual acuity and turn their heads to cross-fixate (look to the right with the left eye, to the left with the right eye).

Fundus

Optic nerve hypoplasia and foveal hypoplasia are common causes of nystagmus. Although many of the retinal disorders associated with nystagmus exhibit visible abnormalities, in some, the fundus appears normal or there are only subtle retinal pigmentary changes (Table 13-2). In patients with nystagmus and normal-appearing fundi, electrophysiologic testing may be necessary to identify the cause.

Table 13-2

Types of Childhood Nystagmus

Many forms of nystagmus can occur as variants of congenital nystagmus in some patients, or they can be acquired in others. In this chapter, these forms are discussed according to the category in which they are most commonly seen in children. While the characteristics of the nystagmus may be indistinguishable, the distinction between congenital and acquired is important because of the greater implications for associated neuropathology in acquired forms.

Congenital Nystagmus

Congenital motor nystagmus

Congenital motor nystagmus (CMN; infantile nystagmus syndrome) is a binocular conjugate nystagmus with several distinctive features (Table 13-3). It is not associated with central nervous system abnormalities. Patients have nearly normal visual function. The plane of nystagmus is most often horizontal, and it remains so on upgaze and downgaze (uniplanar). When CMN has a jerk waveform, it shows an exponential increase in velocity of the slow phase (Fig 13-1). A null point may be present. If one is present but not in primary position, the patient may adopt an abnormal head position to improve vision by placing the eyes near the null point. This head position becomes more

the first 3 months of life. Its severity is somewhat correlated with the degree of vision loss. The waveform of sensory nystagmus can be pendular or jerk and cannot be distinguished from that of congenital motor nystagmus.

Searching, slow, wandering conjugate eye movements may also be observed. Searching nystagmus —defined as a roving or drifting, typically horizontal movement of the eyes without fixation—is usually seen in children whose visual acuity is worse than 20/200. Pendular nystagmus occurs in patients with visual acuity of better than 20/200 in at least 1 eye. Jerk nystagmus is often associated with visual acuity between 20/60 and 20/100.

The associated bilateral pregeniculate afferent abnormality may be obvious, such as in children with bilateral cataracts or corneal opacities. The ocular abnormality may be more subtle, such as in children with optic nerve or foveal hypoplasia. A child with congenital sensory nystagmus as a result of a retinal dystrophy may have mild vascular attenuation, optic disc pallor, or a completely normal retinal appearance. An electroretinogram may be required for diagnosis. See Table 13-1 for a list of some ocular conditions that cause sensory nystagmus.

Periodic alternating nystagmus

Periodic alternating nystagmus (PAN; central vestibular instability nystagmus) is an unusual form of jerk nystagmus that can be congenital or acquired, especially with Arnold-Chiari malformation. The cause of congenital PAN is unknown, but it has been associated with oculocutaneous albinism. The nystagmus periodically changes direction. The motion typically starts with a jerk nystagmus in 1 direction; then the nystagmus slowly begins to damp, eventually leading to a period of no nystagmus lasting from 10 to 20 seconds. The nystagmus then jerks in the opposite direction in a cycle that repeats approximately every 4 minutes. Some children adopt an alternating head turn to take advantage of the changing null point.

Latent nystagmus

Another form of congenital nystagmus, latent nystagmus (fusion maldevelopment nystagmus syndrome) is a conjugate, horizontal jerk nystagmus that is a marker of fusion maldevelopment. Latent nystagmus occurs in children with decreased fusion, which results from either early-onset strabismus (infantile esotropia, most commonly) or decreased vision in 1 or both eyes. When 1 eye is occluded, a conjugate jerk nystagmus develops, with the fast phase directed toward the uncovered eye. Thus, a left jerk nystagmus occurs when the right eye is covered; a right jerk nystagmus, when the left eye is covered. This nystagmus is the only form that reverses direction with a change in fixation. Also, because the nystagmus damps when the fixating eye is in adduction, the head turn reverses with a change in fixation in order to use the fixating eye in adduction. Occlusion of the right eye induces a left jerk nystagmus with a left head turn, and occlusion of the left eye induces a right jerk nystagmus with a right head turn (Fig 13-2). Asymmetries in amplitude, frequency, and velocity of the nystagmus can also be present, depending on which eye is covered.