Intellectual impairment in Parkinson's disease: clinical, pathologic, and biochemical correlates.

The prevalence of overt dementia in 27 studies representing 4,336 Parkinson's disease (PD) patients was 39.9%. The studies reporting the highest incidence of intellectual impairment (69.9%) used psychologic assessment techniques, whereas studies identifying the lowest prevalence of dementia (30.2%) depended on nonstandardized clinical examinations. Neuropsychologic investigations reveal that PD patients manifest impairment in memory, visuospatial skills, and set aptitude. Language function is largely spared. Intellectual deterioration in PD correlates with age, akinesia, duration, and treatment status. Neuropathologic and neurochemical observations demonstrate that PD is a heterogeneous disorder: the classic subcortical pathology with dopamine deficiency may be complicated by atrophy of nucleus basalis and superimposed cortical cholinergic deficits, and a few patients have the histopathologic hallmarks of Alzheimer's disease. Mild intellectual loss occurs with the classic pathology, and the more severe dementia syndromes have cholinergic alterations or Alzheimer's disease. Thus, PD includes several syndromes of intellectual impairment with variable pathologic and neurochemical correlates.