The dementias of Parkinson's disease: prevalence, characteristics, neurobiology, and comparison with dementia of the Alzheimer type.

Overt dementia occurs in 40% of Parkinson's disease (PD) patients, but neuropsychological testing indicates that mental status changes are ubiquitous. The characteristics of the intellectual changes in PD are those of subcortical dementia and include impaired recall, visuospatial disturbances, executive deficits, bradyphrenia, and depression. Language function is largely uninvolved. These features contrast with the neuropsychological abnormalities of dementia of the Alzheimer type (DAT) where language and memory encoding are prominently affected and depression is rare. The dopaminergic deficit of PD is complicated in some patients by a cholinergic deficit and in others by DAT. Several dementia syndromes exist within PD, reflecting the superimposition of different types of pathologies and multiple neurotransmitter deficits.