This is a multicenter, randomized, double-blind, placebo-controlled trial of memantine 10 mg twice daily versus placebo, at a ratio of 1:1, to receive active drug or placebo. Screening and enrollment is planned to last approximately one year. A Data and Safety Monitoring Board, consisting of a clinical pharmacist and 3 neurologists will review all AE reports approximately every 3 months after study initiation. The DSMB will notify the principal investigator, the study sponsor and the CHR if significant concerns are raised by their review of the AE data. An interim analysis of efficacy data will be conducted after 50% of the targeted enrollment population has completed 26 weeks of drug treatment.