Congenital nystagmus in identical twins: discordant features.

Identical twins with congenital nystagmus were examined and studied with eye movement recordings and videotaping. The nystagmus differed in its location of the null zone, its frequency and amplitude, the degree of persistence in the dark, and the degree of impairment of pursuit and optokinetic nystagmus. As the two boys have the identical genetic constitution, we postulate that the discordant features of the nystagmus represent variable expressivity or, alternatively, the influence of environmental factors.