4-Aminopyridine--therapeutic use Activities of Daily Living Adolescent Adult Aged Aged, 80 and over Ataxia--drug therapy Calcium Channels--genetics Child Double-Blind Method Female Follow-Up Studies Genetic Testing Humans Intracellular Signaling Peptides and Proteins--genetics Male Middle Aged Mutation--genetics Nystagmus, Pathologic--drug therapy Outcome Assessment, Health Care Potassium Channel Blockers--therapeutic use Quality of Life Young Adult