Horizontal nystagmus in methylmercury poisoned squirrel monkeys.

Chronic states of methylmercurialism were induced in squirrel monkey subjects. Principal neurological signs included ataxia, abnormal gait, incoordination and amaurosis. Although slight to moderate vacuolization occurred in supporting cell layers of the cristae and maculae, receptor cell function was essentially normal. Except for a lowered cold threshold, bithermal caloric-induced nystagmus was not significantly different from control values. Pre and postrotatory (Barany chair) tests revealed a reduction only in frequency related variables. The development of spontaneous and positional nystagmus (sometimes with eyes open) coupled with the behavioral signs and the evidence of normal receptor response suggested cerebellar dysfunction. Severe pathologic changes were present in the cerebral cortex, but no lesions were found in the cerebellar cortex. Substantial neuronal degeneration and gliosis, however, were observed in several subcortical nuclei, including cerebellar and vestibular nuclei.