Symmetrical optokinetic after-nystagmus loss in Wallenberg's syndrome and multiple sclerosis.

We have demonstrated abolition of OKAN in either horizontal direction with retention of caloric responses in 3 patients with unilateral brain stem infarcts. In one further patient with diffuse brain stem disease, we have been able to show bilateral loss of horizontal OKAN in the acute phase, with recovery during remission. These findings support the conclusion that the velocity storage mechanism or connections to it, are permanently disabled by unilateral medullary infarcts and may be temporarily disabled by demyelinating disease. Retention of caloric responses in the absence of OKAN would appear to be of assistance in the differential diagnosis between peripheral and central vestibular pathway lesions.