Abstract

Ataxia with ocular motor apraxia type 2 (AOA2) is an autosomal recessive disorder that is characterized by adolescent-onset gait ataxia, peripheral neuropathy, ocular apraxia, and cerebellar atrophy. A 19 year-old male with AOA2 from a novel SETX mutation showed distinct oculomotor abnormalities that included spontaneous and gaze-induced downbeat nystagmus, impaired smooth pursuit, and reversed catch-up saccades during horizontal head impulse tests, as well as peripheral neuropathy involving the lower extremities and mild slowing of frontal processing. He also showed positional hemiseesaw nystagmus in the supine and straight head-hanging positions. Positional hemi-seesaw nystagmus is a new manifestation of hereditary cerebellar ataxia and may be explained by a gravity dependent position-induced error in estimating the tilt in the roll plane due to dysfunction of the tilt-estimator circuit.

• Keywords: hemiseesaw nystagmus; cerebellum; ataxia; downbeat nystagmus