Effect of 4-aminopyridine on upbeat and downbeat nystagmus elucidates the mechanism of downbeat nystagmus.

The potassium channel blocker 4-aminopyridine (4-AP) restored vertical smooth pursuit and gaze holding in light in one patient with upbeat (UBN) and in one with downbeat nystagmus (DBN). Without a visible target, however, 4-AP had no effect on UBN, but DBN vanished. We hypothesize that this difference in the effects of 4-AP, which is known to increase the excitability of cerebellar Purkinje cells, can be attributed to the different lesion sites involved in UBN and DBN.