Episodic vertical oscillopsia and downbeat nystagmus in a Chiari malformation.

A 36-year-old woman had intermittent downbeat nystagmus. Between episodes of downbeat nystagmus, eye movements were normal, except for stationary positional nystagmus and decreased vestibulo-ocular responses (VOR) to rotation. Smooth pursuit, optokinetic nystagmus, and visual-VOR interactions, which are usually impaired in patients with downbeat nystagmus and cerebellar dysfunction, were normal. Computed tomography and findings during a suboccipital craniotomy demonstrated a mild Chiari malformation, type 1. Episodic downbeat nystagmus that is not associated with other ocular motor abnormalities due to cerebellar dysfunction can be an initial sign of a Chiari malformation.