Isolated unilateral infarction of the cerebellar tonsil: ocular motor findings.

The oculomotor abnormalities with isolated infarction of the cerebellar tonsil are unknown. In a patient with acute infarction of the right tonsil, we found (1) nearly completely abolished ipsilateral smooth pursuit and impaired contralateral pursuit, (2) a low-amplitude ipsilesional right-beating nystagmus without fixation, (3) gaze-holding deficits, and (4) normal vestibulo-ocular reflex. These findings contrast with striking vestibular abnormalities reported with unilateral flocculus and anterior tonsil infarction. Taken together, these findings allow more diagnostic certainty in cerebellar patients, help resolve controversies about interpretation of experimental findings in monkeys, and clarify homologies between the monkey and human cerebellum.