Intermittent downbeat nystagmus and oscillopsia reversed by suboccipital craniectomy.

We describe an unusual case of vertical oscillopsia and primary position downbeat nystagmus associated with an adult Arnold-Chiari malformation. A unique aspect of the ocular motor abnormality was the intermittent nature of the nystagmus, which displayed no relationship to head position, emotion, or the Valsalva maneuver. Conventional computed tomography with posterior fossa views was normal, but metrizamide cisternography demonstrated an extramedullary mass originally believed to be a meningioma. There was a dramatic diminution of the visual complaints and downbeat nystagmus after suboccipital craniectomy for an Arnold-Chiari malformation. Nystagmus characteristics were documented by serial quantitative eye movement recordings.