A dual-centre, double-blind, cross-over trial of 4-aminopyridine in the downbeat nystagmus syndrome-effects of the drug on slow-phase eye velocity by Kalla, R, Claassen, J, Spiegel, R, Foldon, M, Kennard, C, Danchaivijitr, C, Bardins, S, Dera, T, Rettinger, N, Schneider, E, Glasauer, S, Brandt, T, Bronstein, A, Strupp, M