Clinical features and pathogenesis of acquired forms of nystagmus.

Nystagmus is a common finding in patients with disease affecting the brain stem and cerebellum. Basic research into mechanisms that normally control eye movements has led to a better understanding of the pathogenesis of different types of acquired nystagmus. Nystagmus is caused by disorders of the mechanisms that normally function to hold gaze steady: the vestibular system, the gaze-holding mechanism, the visual stabilization system and the smooth pursuit system. Thus, evaluation of a patient's nystagmus requires a systematic examination of each functional class of eye movements. In addition, it is important to determine the direction of the nystagmus for different angles of gaze and head positions, and the effects of monocular and binocular visual fixation upon the nystagmus. Measurement of the nystagmus waveform, using reliable methodology, is often helpful in securing a diagnosis. Such measurements help differentiate acquired nystagmus from congenital forms of nystagmus and from saccadic disorders that lead to instability of gaze.