Nystagmus alternans.

Alternating nystagmus is a spontaneous nystagmus which changes in direction without any external influence. In the literature this phenomenon has so far been described in 90 cases; in 12 of these, the nystagmus had a congenital origin. In contradistinction to the congenital type, the acquired type of alternating nystagmus is in most cases caused by a central nervous system disorder. We found the phenomenon of alternating nystagmus in 21 cases, in 15 of which it was congenital. The nystagmus was observed by means of electronystagmography. Differences were found between the two nystagmus types. The characteristics of alternating nystagmus are discussed.