Cognitive decline in Parkinson's disease: the complex picture.

Mild cognitive impairment (PD-MCI) and dementia (PDD) are among the most frequent non-motor symptoms in Parkinson's disease (PD). PD-MCI is six times more likely than age-matched controls to develop dementia and the PDD prevalence is 80% after 15-20 years of disease. Therefore, research has focused on the identification of early dementia biomarkers including specific cognitive at-risk profiles hoping to implement therapeutic interventions when they are most likely to be efficacious. However, given the heterogeneous neuropathological, neurochemical, and neuropsychological nature of cognitive deficits, definition of a comprehensive cognitive model of PDD is a challenge. Evidence from neuroimaging studies using different methods and techniques suggests that in addition to degeneration of the dopaminergic system, other mechanisms have a role including β-amyloid and tau deposition, and that specific cognitive scales could help identifying a malignant profile. Prospective studies combining neuroimaging techniques and specific cognitive tests are required to define the interplay between the various neurodegenerative processes and the contribution of structural disconnection in brain functional networks, heralding the development of dementia in PD.