Abstract:The severity of intellectual impairment of 89 patients with Parkinson's disease was evaluated with the Osaka Intelligence Scale for the Aged (OISA). Based on their intelligence levels, the patients were divided into three groups: normal, slight dementia and dementia. EEGs, psychiatric complications such as hallucination, degree of motor disability, history of medication, relationship between changes in intelligence and total amount of administered drugs were also examined. There was no significant correlation between the length of duration of the illness and the degree of intellectual impairment. Two characteristic subgroups were found among our subjects: a group of patients who rapidly became demented after the onset of Parkinsonism, and a group of patients whose intelligence was preserved for a longer period. The mean age of the onset of the disease in the former group was older than that of the latter. Patients of the former group exhibited psychiatric complications and EEG abnormalities more frequently. The severity of motor disability and medications administered at the time of the OISA examination did not differ between the two groups. Deterioration of the intelligence of the patients with Parkinson's disease did not correlate with the total amount of the administered antiparkinsonian drugs.