Parkinson disease: A slowly progressive neurologic disease characterized by a fixed inexpressive face, a tremor at rest, slowing of voluntary movements, a gait with short accelerating steps, peculiar posture and muscle weakness, caused by degeneration of an area of the brain called the basal ganglia, and by low production of the neurotransmitter dopamine. Most patients are over 50, but at least 10 percent are under 40. Also known as paralysis agitans and shaking palsy.
From a genetic viewpoint it is now clear that Parkinson disease is heterogeneous. It is not one, but a number of diseases. Genes appear to be involved in all forms of Parkinson disease. See also: Parkinson disease gene.