Effect of 4-Aminopyridine on Upbeat and Downbeat Nystagmus Elucidates the Mechanism of Downbeat Nystagmus

The effects of the potassium-channel blocker 4-aminopyridine (4-AP) on upbeat nystagmus (UBN) and downbeat nystagmus (DBN) were investigated with the search-coil technique during fixation in different gaze positions and smooth pursuit in two patients before and after ingesting 10mg 4-AP. DBN of unknown etiology was reduced from -3.9 deg/s to 0.14 deg/s in light, and from -3.8 deg/s to 1.1 deg/s in darkness, thus changing to mild UBN. Impaired downward smooth pursuit was restored (gains: pre 0.42; post 0.68). In the second patient, UBN, possibly due to a small traumatic lesion, was reduced in light from 8.6 deg/s…