Fig. 8.9  Comparison of ERG responses to scotopic blue, scotopic white, and photopic flicker (30/s) stimulation of 3-year-old with Leber congenital amaurosis, 5-year-old boy with X-linked incomplete achromatopsia,

5-year-old boy with X-linked CSNB, and 4-year-old normal control. ERG responses were recorded using corneal electrodes and Ganzfeld stimulator. Adapted, with permission, from Lambert SR et al355

Rarely, the nystagmus associated with CSNB mimics spasmus nutans.356 Lambert and Newman356 have therefore recommended that patients with spasmus nutans who are myopic undergo ERG to rule out CSNB. Figure 8.9 summarizes the electroretinographic features that distinguish the more common congenital retinal dystrophies.

When to Obtain Neuroimaging Studies in Children with Nystagmus

We have emphasized that it is a common mistake to obtain neuroimaging studies in a neurologically normal infant or child with infantile nystagmus because brain tumors and other compressive CNS lesions generally do not cause infantile nystagmus. Nevertheless, there are three clinical situations in which neuroimaging is warranted:

1. In infants with infantile nystagmus and optic nerve hypoplasia, we obtain MR imaging to evaluate the structural status of the pituitary infundibulum, cerebral hemispheres, and midline intracranial structures (septum pellucidum, corpus callosum). Because it is understood that the

nystagmus­ is sensory in nature (resulting from optic nerve hypoplasia), the purpose of neuroimaging is not to delineate the cause of the nystagmus, but rather to search for associated CNS anomalies that commonly coexist with optic nerve hypoplasia. It should be parenthetically noted that congenital suprasellar tumors (e.g., craniopharyngioma, chiasmal glioma) rarely disrupt optic axonal migration during embryogenesis and present with optic nerve hypoplasia, tilting of the optic discs, or other disc anomalies.529 Routine MR imaging of infants with optic nerve hypoplasia insures that this rare association is overlooked.

2. We obtain MR imaging in any infant with infantile nystagmus and optic atrophy to rule out a congenital suprasellar tumor (e.g., chiasmal glioma, craniopharyngioma) or hydrocephalus. In our experience, there are few noncompressive causes of congenital or early infantile optic atrophy (see Chap. 4).

3. We routinely obtain MR imaging in children in whom the diagnosis of infantile nystagmus is uncertain and the possibility of spasmus nutans exists to rule out chiasmal gliomas or other suprasellar tumors.

4. When infantile nystagmus is accompanied by seesaw nystagmus, we obtain neuroimaging to look for achiasmia.281