Table 8.2  Infantile nystagmus associated with sensory visual loss

Table 8.3  Clinical signs of congenital retinal dystrophy in infantile nystagmus

Photophobia

Paradoxical pupillary phenomenon

High myopia

Oculodigital reflex

reaction consists of an initial pupillary constriction (rather than dilation) when the room lights are turned off. This phenomenon is best observed by holding a penlight laterally to the globe to dimly illuminate the pupil while turning off the room lights.50 When a paradoxical pupillary reaction is observed, there is usually a lag time of approximately 1 s before the initial pupillary constriction is seen. It is now recognized that the paradoxical pupillary phenomenon can rarely be seen in a congenital optic nerve disorders and even in normal patients.89,90,206,456 In the setting of infantile nystagmus, however, the paradoxical pupillary phenomenon remains highly suggestive of a congenital retinal dystrophy. The paradoxical pupillary phenomenon appears to be agerelated in that it is difficult to see in adults or infants younger than 6 months of age.50 The paradoxical pupillary phenomenon has recently been linked to the intrinsic responses of melanopsin-containing ganglion cells, which continue to function when the photoreceptors are disabled.212

4. The oculodigital sign refers to a repetitive pushing on the eyes with the thumbs or fists. It is speculated that repetitive eye rubbing is a self-stimulatory behavior that elicits phosphenes (entoptic flashes of light) in infants with extremely poor vision. The oculodigital sign is commonly described in infants with Leber congenital amaurosis, in whom the effects of chronic eye rubbing may contribute to the associated findings of enophthalmos, keratoconus, and cataract.

ERG

If any of the foregoing four signs is present, a congenital retinal dystrophy should be suspected and ERG should be obtained to further characterize it. When ordering ERG in the child with nystagmus, it is important to recognize that the ERG is often subnormal in infancy.

Unless the clinician has access to an electrophysiology laboratory with normative values for infants of different ages, it is best to wait until at least 1 year of age (at which time the amplitude, sensitivity, and latency of the electroretinographic waveform approach adult values) before obtaining this study.209 If, at that time, the ERG is normal, the diagnosis of a congenital retinal dystrophy can be effectively ruled out. An abnormal ERG obtained after 1 year of age confirms the presence of an underlying congenital retinal dystrophy and, in some cases, establishes a specific diagnosis.

Hemispheric Visual Evoked Potentials

The ERG may be supernormal in tyrosinase negative albinism, but is usually normal in other forms of albinism and may be subnormal in the Hermansky Pudlak syndrome. In all patients with infantile nystagmus, a careful search for iris transillumination, macular hypoplasia, and chorioretinal hypopigmentation should again be undertaken because these signs may be difficult to detect in infancy. If these abnormalities are mild or equivicol, recording of VEPs to detect hemispheric asymmetry can be used to confirm the diagnosis. Simonsz and Kommerell503 studied patients with infantile nystagmus and found a high incidence of subtle oculocutaneous albinism when careful iris transillumination and retinal examinations were performed. Apkarian et al30 have demonstrated that hemispheric VEPs provide the most sensitive and specific means to establish the diagnosis of albinism. Because we see little prognostic value in distinguishing clinically occult albinism from idiopathic infantile nystagmus, we rarely find occasion to measure hemispheric VEPs in children with infantile nystagmus.