At the conclusion of the Screening/Baseline Phase, subjects meeting all inclusion/exclusion criteria will be randomized to one of two treatment groups: vigabatrin or placebo. Subjects will proceed to a 12 week Treatment Phase and then undergo a 12 week follow-up period. Subjects will be scheduled for clinic visits 3 times per week for efficacy and/or safety assessments during the Screening/Baseline Phase and the 12 week Treatment Phase. Subjects will return for follow up visits at Weeks 13, 16, 20 and 24.