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Twelve members of a family with hereditary cerebellar ataxia of late onset were examined and, in 5, quantitative recording of eye movements were obtained. The initial and most severe symptom in all patients was ataxia of gait, followed by dysarthria and later by dysmetria of the limbs. Clinical examination did not reveal involvement of structures other than the cerebellum. Ocular motor examination showed: (1) inability to hold eccentric gaze resulting in gaze-paretic nystagmus; (2) downward beating nystagmus, accentuated on lateral gaze; (3) defective smooth pursuit, with relative preservation of optokinetic nystagmus induced by full-field stimulation; (4) rebound nystagmus; (5) enhanced gain (eye velocity/head velocity) of the vestibulo-ocular reflex during rotation in darkness; (6) decreased ability to suppress the vestibulo-ocular reflex during fixation of an object rotating with the patient; (7) saccadic dysmetria, especially downward overshoot; and (8) square wave-jerks. Although each of these signs can probably occur with lesions elsewhere in the brain, in combination they are highly suggestive of cerebellar involvement. With the reservation that we do not yet have pathological confirmation of the location of our patients' lesions, our results support the suggestion that the cerebellum specifically: (1) helps maintain eccentric gaze; (2) produces smooth pursuit eye movements; and (3) modulates the amplitude of saccadic eye movements. Many of the characteristics of the altered vestibulo-ocular responses and rebound nystagmus could be explained by the underlying anomaly in the smooth pursuit system.

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