Lewy bodies in Alzheimer disease--one or two diseases?

To clarify the significance of the Lewy body (LB) in Alzheimer disease (AD), we determined the incidence and distribution of LB in 150 cases of AD and 75 controls. We also examined the pathological changes in the substantia nigra and quantified neocortical alterations, including the density of neuritic plaques, neurofibrillary tangles, and LB. LBs were present in 25% of AD cases, but in only 5% of controls. The presence of LB was associated with significantly lower numbers of neurofibrillary tangles as compared with cases of AD without LB, whereas the mean density of neuritic plaques remained unchanged. Two possibilities are discussed to explain these findings. First, the high frequency of LB in AD may reflect a predisposition of AD patients to develop idiopathic Parkinson disease. Second, the LB may represent a nonspecific cytoskeletal change in selected vulnerable neuronal populations in some subjects with AD, rather than the presence of a second pathological process.