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)