Mild cognitive impairment and cognitive reserve in Parkinson's disease.

Patients with Parkinson's disease (PD) typically present with motor symptoms, but non-motor symptoms, including cognitive impairment, autonomic dysfunction and neuropsychiatric symptoms, are usually also present, when looked for carefully. The objective of this paper is to provide an up-to-date, comprehensive review of two undecided issues about cognitive impairment in PD patients without dementia: the concept of Mild Cognitive Impairment (MCI) and the concept of Cognitive Reserve (CR). Empirical findings support the value of the concept of MCI in this population, from the early untreated stages onwards. Further studies are needed to establish 1) the clinical-neuroimaging characteristics of MCI subtypes in PD, in comparison to those MCI subtypes in patients without PD; 2) whether different types of MCI in PD are associated with different rates of cognitive decline during the progression of the disease. Preliminary empirical evidence also shows that education might exert a protective effect on cognitive decline in PD and that less educated subjects are at increased risk for developing dementia, lending support to the CR hypothesis, in this population as well. Further studies are necessary to investigate how CR modulates cognitive decline in PD and other frontal-subcortical disorders, e.g., by identifying possible differential effects of CR on different cognitive domains.