A sporadic neurodegenerative disease with onset in middle-age characterized clinically by Parkinsonian features (e.g., muscle rigidity; hypokinesia; stooped posture) and hypotension. This condition is considered a clinical variant of multiple system atrophy. Pathologic features include a prominent loss of neurons in the zona compacta of the substantia nigra and putamen. (From Adams et al., Principles of neurology, 6th ed, p1075-6)