a progressive neurodegenerative disease caused by the death of dopamine-producing neurons in the pars compacta region of the substantia nigra of the brain, which controls balance and coordinates muscle movement.

Symptoms typically begin late in life with mild tremors (see resting tremor), increasing rigidity of the limbs, and slowness of voluntary movements. Later symptoms include postural instability, impaired balance, and difficulty walking (see festinating gait). Dementia occurs in some 20% to 60% of patients, usually in older patients in whom the disease is far advanced, and depression and anxiety are often observed as well. [first described in 1817 by James Parkinson (1755–1824), British physician]