Primary Position Upbeat Nystagmus; A Report of Three Cases.

Primary position upbeat nystagmus (PPUN) is an extremely rare disease, and there is no general agreement about its etiology. Most investigators believe, however, that it is associated with disorders in areas extending from the brain stem to the cerebellum. We studied the etiology of and lesions responsible for this disease from the results of various tests and examinations conducted in 3 patients who had different clinical courses. Finding 1: Horizontal and vertical eye movements showed dissociation in Case 2, suggesting that different nerve tracts are involved in these movements. Finding 2: All 3 patients showed evidence of disorders in the brain stem and cerebellum, suggesting that the conduction route between the brain stem and the cerebellum was impaired.