The annual incidence of clinically diagnosed dementia in PD (PDD) is about 3% for individuals younger than 60 years and 15% or less for those 80 years and older (66,67). Estimates of dementia prevalence in patients with PD vary between 9% and 93%, depending on which diagnostic criteria, ascertainment methods, and sampling methods are implemented (24). The methodologically soundest studies yield prevalence estimates of about 25% (68). Dementia is very rarely present early in the disease course; moreover, dementia that precedes or accompanies the evolution of motor symptoms should raise concern that the dementia might be related to factors other than PD, for example, AD, LBD, or depression. Recently revised diagnostic criteria for LBD (69) propose that the clinical diagnostic term "PD with dementia" be reserved for individuals who have a clinical diagnosis of PD and have had only motor symptoms for at least 12 months before developing fluctuating cognition and other neuropsychiatric symptoms such as hallucinations. When the neuropsychi-atric presentation precedes any extrapyramidal signs, the differential diagnoses include LBD, AD, and vascular dementia. Whether PDD and LBD turn out to be neu-ropathologically distinct entities remains to be resolved, though neuropsychological studies have generally failed to distinguish between these two putative entities (30).
Dementia in PD, like other dementias, involves multiple cognitive impairments and a related decline in day-to-day functioning. Cummings' (70) categorization of dementia as "cortical" and "subcortical" on the basis of neurobehavioral features has been criticized on neuroanatomical grounds, but nevertheless remains a useful clinical heuristic. While recent work suggests that the cognitive profile of dementia in PD is likely heterogeneous (perhaps reflecting variability in neuropathological findings) at the group level, the neuropsychological deficits evident in PDD most often resemble those of the "subcortical" dementias (71). Perhaps the most striking features of the "subcortical" dementias, including PDD, are bradyphrenia, memory retrieval deficits, executive dysfunction, diminished spontaneity, and depression. Features of the "cortical" dementias such as AD (e.g., aphasia, agnosia, and apraxia) are typically absent in PDD, even later in the course of dementia. Recently published "practice parameters" concerning diagnosis of dementia (72) provide little guidance in the evaluation of patients with PD and dementia, and fail to address neuropsychological evaluation methods.
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