Dementia is a frequent problem encountered in advanced stages of
Parkinson disease (PD). In recent years, research has focused on the pre-
dementia stages of
cognitive impairment in PD, including
mild cognitive impairment (MCI). Several longitudinal studies have shown that MCI is a harbinger of
dementia in PD, although the course is variable, and stabilization of cognition - or even reversal to normal cognition - is not uncommon. In addition to limbic and cortical spread of Lewy pathology, several other mechanisms are likely to contribute to
cognitive decline in PD, and a variety of
biomarker studies, some using novel structural and functional imaging techniques, have documented in vivo brain changes associated with
cognitive impairment. The evidence consistently suggests that low cerebrospinal fluid levels of amyloid-β42, a marker of comorbid
Alzheimer disease (AD), predict future
cognitive decline and
dementia in PD. Emerging genetic evidence indicates that in addition to the
APOE*ε4 allele (an established risk factor for AD), GBA mutations and SCNA mutations and triplications are associated with
cognitive decline in PD, whereas the findings are mixed for MAPT polymorphisms. Cognitive enhancing medications have some effect in PD
dementia, but no convincing evidence that progression from MCI to
dementia can be delayed or prevented is available, although
cognitive training has shown promising results.