A number of cross-sectional or prevalence studies of dementia in Parkinson's disease have been carried out. The frequency of dementia reported ranges from zero to 81 per cent. In a review of 17 studies Brown and Marsden found that, overall, 35 per cent of subjects were regarded as demented. (14> However, if more stringent criteria for dementia were applied then the proportions demented fell to between 15 and 20 per cent. The authors regarded these figures to be more realistic, and this level has proved to be in keeping with more recent cross-sectional studies.
Follow-up studies have great advantages in studying the frequency of dementia in Parkinson's disease: they allow the diagnosis of Parkinson's disease to be checked; repeated assessment reduces errors in the recognition of dementia; the pattern of evolution of dementia may be followed; the underestimation of dementia by selective loss through death is avoided; and they reveal the incidence rather than the prevalence of the condition. The problems of follow-up studies include the difficulties in the choice of those methods of diagnosis and assessment that will remain appropriate throughout the period of the follow-up.
No prospective controlled study of the incidence of dementia in Parkinson's disease has been entirely satisfactory in its methodology. Probably the most satisfactory is that reported by Biggins et a/.,(l5> and subsequently after a longer period of follow-up by Hughes et a/.(16) Although this study employed satisfactory methods in most respects, its greatest weakness was in the selection of the original samples of both patients and controls. Biggins et a/.(!5) reported an incidence of dementia of 19 per cent after 4.5 years observation, or 48 per 1000 person-years of observation. A later report on the same cohorts of subjects showed an incidence of dementia of 47.8 per cent after 10 years of observation, or 46.9 cases per 1000 person-years of observation. The study shows a substantial incidence of dementia in Parkinson's disease increasing with the passage of the years. The control group showed cases of cognitive impairment but none amounting to dementia, thereby demonstrating a substantial excess risk in those subjects with Parkinson's disease.
The findings shown in Fig, l are broadly in keeping with the few other reported prospective studies.(!, ,,18)
Fig. 1 Comparison of the incidence of dementia in Parkinson's disease with a control group of healthy subjects matched for age and sex. Note that the figure shows the percentage not demented. (Data from Biggins et a/A5) and Hughes et a/A6))
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