In reviewing the PD literature, Lieberman (29) reported that 17% to 53% of treated and untreated PD patients without dementia demonstrate cognitive dysfunction. Unfortunately, few of the studies reported formal criteria for determining what did or did not constitute dementia, thus making it difficult to determine whether patients were in the early stages of dementia. As noted earlier, more recent studies suggest that formal neuropsychological testing may uncover mild cognitive deficits in 25% to 36% of PD patients at the time of diagnosis (22,23). When present in early PD, cognitive dysfunction is typically mild and most commonly involves bradyphre-nia (a slowness of thought) and subtle deficits in executive functions, recall, and/or visuoperceptual/spatial functions (30).
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