Huntington disease is characterized by difficulties in movement and thinking. Psychological disturbances including irritability, aggression, loss of interest, depressed mood, obsessions and compulsions, also represent common symptoms of HD. These symptoms are distressing both for HD patients and their caregivers, contribute to the loss of ability to carry out activities of daily living, and present a major treatment challenge for physicians. The goal of this study is to determine the effect of memantine on movement, thinking and emotional difficulties in HD patients. Memantine is a medication originally approved for the treatment of aggression and agitation in patients with moderate-to-severe Alzheimer's disease (AD), which has also recently been shown to improve the behavioural and neuropathological symptoms in a mouse model of Huntington Disease (HD).