Pathological findings considered to account for dementia in PD include severe pathology in monoaminergic and cholinergic nuclei that project to the cortex producing a "subcortical dementia" (39%), coexistent Alzheimer's disease (AD) (29%), and diffuse cortical Lewy bodies (26%) (22). The basal forebrain cholinergic system is the subcortical region most often implicated in dementia, and neurons in this region are damaged in both AD and Lewy body dementia. Neuronal loss in the basal nucleus is consistently found in PD, especially PD with dementia (23). Cholinergic deficits are common in PD (24) and they may contribute to dementia in PD in those cases that do not have concurrent AD or cortical Lewy bodies.
While virtually all PD brains have a few cortical Lewy bodies (22), they are usually neither widespread nor numerous in PD patients who were not demented. Several studies have shown, however, that cortical Lewy bodies are numerous and widespread in PD with dementia (25-27) and that the density of cortical Lewy bodies and Lewy neurites, especially in the medial temporal lobe (28), correlates with the severity of dementia (29). There are exceptions to this rule with occasional reports of patients with many Lewy bodies who were clinically normal (30).
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