A controlled study of dementia in Parkinson's disease.

Tests of cognitive functions were carried out in a group of patients with Parkinson's disease and repeated after a three-year interval. Comparison was made with a control group drawn from a population of psychiatric patients, matched for age and sex. No differences in cognitive functions were found between the groups, either initially, or between those surviving for three years. Deaths among the index group included a high proportion of patients with cognitive impairment and there was an increasing prevalence and severity of dementia in the index group which exceeded that observed in the control group. Requirements for a methodologically sound study of dementia in Parkinson's disease are discussed.