Monocular downbeat nystagmus.

A patient with sporadic pontocerebellar degeneration and downbeat nystagmus limited to the left eye is described. The nystagmus was not modified by head movements, but was associated with a purely tonic upgaze paresis in the same eye. Absence of internuclear ophthalmoplegia indicated sparing of the medial longitudinal fasciculus. It is suggested that the vertical oculomotor abnormalities are due to dysfunction of the ipsilateral brachium conjunctivum.