Early cognitive changes and nondementing behavioral abnormalities in Parkinson's disease.

Early cognitive changes in patients with PD are often subtle and influenced by factors that interact with the disease process, including age of disease onset, medication, and the specific constellation of motor symptoms. These factors notwithstanding, ample evidence exists that specific cognitive changes occur early in the course of PD. This evidence does not imply that cognitive deficits are pervasive during the early stages. To the contrary, they are usually subtle and often difficult to detect without formal neuropsychological testing. Executive-function deficits are the most frequently reported cognitive problems and, given that executive skills are an integral part of many tasks, it follows that subtle difficulties may be seen on a wide range of cognitive measures, particularly in working memory and visuospatial dysfunction, two areas that rely heavily on executive skills. Whereas apraxia and language processing deficits occur infrequently, subtle changes in olfaction and contrast sensitivity have also been repeatedly observed. Finally, depressive symptoms are also common in the early stages of the disease. The significance of the early behavioral changes and their prognostic implications are largely unknown. Prospective studies are needed to understand the longitudinal course of early cognitive changes to determine whether they remain as circumscribed impairments or represent a precursor to a more widespread dementia.