[Dissociated adduction nystagmus in a patient with hereditary spinocerebellar degeneration]

A 66-year-old woman with a 10 years' history of autosomal dominant hereditary spinocerebellar degeneration developed an atypical nystagmus. Neurological examination revealed cerebellar ataxia, absent tendon reflex, supranuclear upward gaze palsy, saccadic pursuit and nystagmus. Nystagmus was a dissociated type and appeared only in lateral gaze and the amplitude was prominent in adducting eyes. Brain MRI showed cerebellar hemispheric and vermal atrophy, and pontine atrophy. Neuro-otological examination revealed normal caloric test, but impaired optokinetic nystagmus and visual suppression test. The dissociated adduction nystagmus of this case may correspond to gaze evoked nystagmus and latent hypofunction of the abducens motor neurons.