Congenital, latent and manifest latent nystagmus--similarities, differences and relation to strabismus.

Patients with congenital types of nystagmus, including congenital nystagmus (CN), latent/manifest latent nystagmus (LMLN) and combinations of the two, can be identified by waveform analysis and classified into three unambiguous groups. This categorization by waveform is supported by different clinical signs, including the relationship to strabismus. Strabismus is essential for LMLN but incidental to CN; most CN patients do not have strabismus. Seventy-seven percent of CN patients have a convergence null, 57% a gaze angle null and nearly half have both; only 14% have neither. Also supporting this patient grouping by waveform is the high incidence of patients in each of the two major groups (ie, they had either only CN waveforms or only the LMLN waveform). Comparing the incidence of each CN waveform, or combinations of waveforms, in families with that in the general CN population, reveals that heredity plays a role in determining waveform; heredity also affects other characteristics, such as gaze angle or convergence nulls. The nystagmus blockage syndrome has at least two mechanisms and the patients, therefore, can belong to either of two groups. Spasmus nutans is hypothesized to be an oscillation of the vergence system and therefore, unrelated to CN or LMLN.