The Effect of Donepezil on Gait and Balance in Parkinson's Disease

This study involves Parkinson's disease (PD). Symptoms include slow movement, tremor, and muscle rigidity. Current medications for the treatment of PD do not improve gait and balance difficulties in individuals with PD. Donepezil (study drug) has been found to reduce falls in individuals with PD. The mechanism in which this reduction of falls occurs is unclear. The investigators study will look at what aspects of gait and balance are improved by the study drug. The study drug is not approved to treat PD in the United States or other countries because we do not know enough about it.

Parkinson's disease (PD) is a common neuro-degenerative disease affecting about 2% of the adult population in the United States over the age of 65. Some of the most disabling symptoms of Parkinson's disease are balance and gait dysfunction, leading to falls. These symptoms do not respond to current dopamine directed therapies. Evidence from both pathologic studies and advanced imaging has demonstrated that a cholinergic deficiency in the thalamus and basal ganglia is found in individuals with PD who fall compared to non-fallers. The central acting acetylcholine esterase inhibitor, donepezil, has been demonstrated to decrease falls in individuals with PD. The mechanism by which falls decreased is unknown. Our open label pilot data indicates that donepezil can improve quantitative measures of balance in individuals with PD. Suggesting that improvements in balance in the mechanism by which donepezil reduces falls. Our goal is to determine whether donepezil will: * Improve quantitative measures of balance in subjects with Parkinson's disease compared to placebo. * Improve quantitative measures of gait in subjects with Parkinson's disease compared to placebo. * Improve cognitive measures in non-demented subjects with Parkinson's disease.