Abstract

The possibility of an association between Parkinson's disease and Alzheimer's disease has been examined by studying the age-specific prevalence of Lewy bodies in the substantia nigra in a group of 273 control cases without Parkinson's disease and 121 cases of Alzheimer's disease. The substantia nigra was also studied in 14 cases of Down's syndrome, 13 of which had cortical Alzheimer pathology. Twelve (7.8%) of the controls aged over 60 years showed nigral Lewy bodies. There was mild nerve cell degeneration and/or an extranigral distribution of Lewy bodies, suggestive of presymptomatic Parkinson's disease. Twenty five (22.5%) of the Alzheimer's disease cases over 60 years showed Lewy bodies, but only 14 (14.0%) of these had mild nigral cell loss consistent with presymptomatic Parkinson's disease. No case of Down's syndrome had Lewy bodies. Counts of tangles and plaques in hippocampus, frontal and temporal cortex were lower in cases of Alzheimer's disease with Lewy bodies compared with those without, but cortical choline acetyltransferase (ChAT) activities were similar. This suggests that Lewy body degeneration in the nucleus basalis of Meynert contributes to the deficit of cortical ChAT, but not to the cortical Alzheimer pathology. The relatively small difference in the prevalence of Lewy bodies between controls and Alzheimer's disease could be explained by the additive effects of Lewy body and tangle pathology causing dementia, rather than a greater than chance association between Parkinson's disease and Alzheimer's disease.