Relation between clinical characteristics of Parkinson's disease and cognitive decline.

This study examined how cognitive impairments in Parkinson's disease (PD) are related to clinical characteristics such as age at testing, duration of illness, motor impairment, and especially age at disease onset. To address these issues, we administered 14 tests of memory, language, visuospatial, and frontal lobe capacities to 104 PD patients and 60 healthy volunteers of comparable age and education. The participants completed 1-9 test sessions over 1-10 years. Duration of PD was associated with deteriorating performance on most cognitive tests, independent of age-related decline. Severity of motor impairment, indexed by Hoehn and Yahr stages, was positively related to impairment on almost all cognitive tests, holding age and duration constant. For some tests, especially those that were motorically demanding and those that assessed language skills, cognitive deficits appeared earlier in the disease course for late-onset than for early-onset PD patients. These late onset deficits were synergistic effects beyond the additive contributions of disease duration and normal aging. These findings may assist physicians in advising PD patients and their families about the future course of the illness.