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Animal experiments have demonstrated that aminopyridines increase Purkinje cell excitability, and in clinical studies, 4-aminopyridine (4-AP) and 3,4-diaminopyridine (3,4-DAP) improved downbeat nystagmus. In this double-blind, prospective, crossover study, the effects of equivalent doses of 4-AP and 3,4-DAP on the slow-phase velocity (SPV) of downbeat nystagmus were compared.Eight patients with downbeat nystagmus due to different etiologies (cerebellar degeneration [n = 1], bilateral vestibulopathy [n = 1], bilateral vestibulopathy and cerebellar degeneration [n = 1], Arnold-Chiari I malformation and cerebellar ataxia [n = 1], cryptogenic cerebellar ataxia [n = 4]) were included. They were randomly assigned to receiving a single capsule of 10 mg of 3,4-DAP or 4-AP followed by 6 days with no medication. One week later, the treatment was switched, that is, 1 single capsule (10 mg) of the other agent. Recordings with 3-dimensional video-oculography were performed before and 45 and 90 minutes after drug administration.Both medications had a significant effect throughout time (pre vs post 45 vs post 90) (F(2,14) = 8.876; P < 0.01). Following the administration of 3,4-DAP, mean slow velocity decreased from −5.68°/s (pre) to −3.29°/s (post 45) to −2.96°/s (post 90) (pre vs post 45/post 90 P < 0.01). In 4-AP, the mean SPV decreased from −6.04°/s (pre) to −1.58°/s (post 45) to −1.21°/s (post 90) (pre vs post 45/post 90 P < 0.00001). Both after 45 and after 90, the mean SPVs were significantly lower for 4-AP than for 3,4-DAP (P < 0.05). None of the patients reported serious side effects.Based on these results, 10-mg doses of 4-AP lead to a more pronounced decrease of the SPV of downbeat nystagmus than do equivalent doses of 3,4-DAP.