Effect of 3,4-diaminopyridine on the gravity dependence of ocular drift in downbeat nystagmus

The pathomechanism of downbeat nystagmus (DBN) remains controversial but each mechanism has to account for 1) its gaze-evoked vertical centripetal component which increases on down and lateral gaze,1 and 2) the vertical bias component of the upward slow phase velocity (SPV) in gaze straight ahead. The vertical velocity bias of DBN has a gravity-dependent component which leads to maximal drift velocity when patients lie in prone position and minimal in supine position.2 Recently, 3,4-diaminopyridine (3,4-DAP)3 has been shown to be effective in reducing DBN in patients with their heads upright. However, DBN of several of those patients3 showed only small changes. One reason might be that gravity-dependent mechanisms were not considered.

Patients and methods.

A 63-year-old woman had a 4-year history of vertical oscillopsia, diplopia, blurred vision, and postural instability. Symptoms increased on downward gaze and considerably more when she bent her head forward. Clinically she showed DBN. In the head forward bending position there was a marked increase of DBN. Except for the tendency to fall backward, the neurologic examination was unremarkable. MRI, CSF, and blood screening were normal. Electronystagmography was normal except for mildly impaired horizontal smooth pursuit. DBN was superimposed on downward vertical smooth pursuit …