Primary position upbeat nystagmus increased on downward gaze: clinicopathologic study of a patient with multiple sclerosis.

We describe a patient with multiple sclerosis who had primary position upbeat nystagmus that increased in amplitude on downward gaze. Pathologic examination revealed demyelinating foci with macrophage infiltration in the lower medulla, involving the perihypoglossal nuclei (the intercalatus nuclei). A lesion of this region may be responsible for upbeat nystagmus.