Downbeat nystagmus: a type of central vestibular nystagmus.

We used electrooculography to study horizontal and vertical eye movements in 17 patients with downbeat nystagmus (Chiari malformation, 9; multiple sclerosis, 2; olivopontocerebellar atrophy, 2; brainstem infarction, 2; and idiopathic nystagmus, 2). In all patients the velocity of slow components of the spontaneous nystagmus increased on lateral gaze, remained relatively unchanged with loss of fixation, and increased markedly after rapid positional changes in the sagittal plane. Horizontal and vertical smooth pursuit was impaired in all patients. Vertical pursuit recordings at multiple ramp velocities suggested superimposition of the spontaneous nystagmus on attempted pursuit rather than selective impairment of downward pursuit. Finally, all patients had impaired fixation suppression of vestibular nystagmus when they were rotated with a fixation point moving with their head. We conclude that downbeat nystagmus is a type of central vestibular nystagmus resulting from an imbalance in the central vertical vestibuloocular pathways.