Study to Assess the Clinical Benefit and Safety of Droxidopa in Parkinson's Disease

Since droxidopa has been approved in Japan for treating freezing of gait in Parkinson\'s disease patients, this is to confirm and further investigate the safety and efficacy using a similar dose. The possible beneficial effects on cognition in mildly cognitively impaired Parkinson\'s disease patients will also be tested, since this problem in Parkinson\'s disease may be associated with decreased brain synthesis of norepinephrine (a neurotransmitter associated with multiple brain functions). During this 11 week study, droxidopa will be slowly titrated up to 600 mg daily. Walking and freezing of gait will be evaluated and rated. Cognitive functions will be evaluated by a computer-based program.