Intermittent downbeat nystagmus secondary to vermian arachnoid cyst with associated obstructive hydrocephalus.

A 27-year-old man presented with a history of dizziness and intermittent vertical oscillopsia after oblique extension of the head and neck, following a neck injury 2 years previously. He had no other symptoms of cerebellar or brainstem dysfunction. On examination, an intermittent downbeat nystagmus lasting about fifty seconds was elicited by head extension and rotation. Radiologic examination showed slight lateral instability of the odontoid on lateral rotation, and computed tomography brain scan with Iopamidol 300 (Niopam) contrast revealed a 5-cm vermian arachnoid cyst in the posterior fossa with obstructive hydrocephalus. Removal of the cyst cured the nystagmus.