Cognitive impairment and dementia in Parkinson's disease.

Relatively subtle cognitive disturbances may be present from the initial stages of Parkinson's disease (PD) that progress in many patients to a more severe cognitive impairment and dementia. Several of the initial deficits are ascribed to failure in the frontal-striatal basal ganglia circuits and involve executive defects in planning, initiation, monitoring of goal-directed behaviors and working-memory. Other non-demented PD patients also exhibit visuospatial and memory deficits more representative of posterior cortical functioning and fail performing naming or copying tasks. Major differences in the overall rate of cognitive decline among PD patients support the co-existence of at least two patterns of involution, differentiating a relatively slow decline of fronto-striatal deficits from a more rapid decline of posterior-cortical deficits, with different pathophysiological substrates, genetics, prognosis and response to drugs used to treat the motor symptoms of PD.