Age distribution of patients with Parkinsonism.

The modal age at onset of the parkinsonian syndrome during the past thrity years is less than a decade higher than it was in the late 19th and early 20th centuries, suggesting that the same disease entity is affecting parkinsonian patients now as then. The evidence points to the existence of two distinct clinical entities: 1) parkinsonism secondary to encephalitis lethargica, which had its greatest influence on the epidemiology of parkinsonism between 1920 and 1945; and 2) classic parkinsonism, which has undergone little change in the past hundred years.