Epidemiology, diagnosis, and correlates of mild cognitive impairment in Parkinson's disease
Elise Caccappolo and Karen Marder
Earlier work on mild cognitive impairment (MCI) in Parkinson’s disease (PD) was constrained by a lack of standardized diagnostic criteria, and frequently applied established MCI criteria typically used in Alzheimer’s populations [1, 2]. In the clinical context of memory impairment, the construct of MCI has been conceived as representing the prodromal stage of early Alzheimer’s disease (AD)  given that a high proportion of people with amnestic MCI progress to AD. Petersen’s  criteria for amnestic MCI require the presence of a memory complaint and objective memory impairment for age with all other cognitive domains being within normal limits, as well as preserved activities of daily living. As the criteria for MCI expanded to incorporate non-amnestic MCI as a separate clinical phenotype of MCI , the identification of multiple-domain MCI and single-domain non-memory MCI as constructs acknowledged the possibility of impaired performance in other cognitive domains, i.e. with no memory deficit and intact or minimally impaired functioning.
As the construct of MCI has been applied to PD, PD-MCI has been described as the earliest stage of cognitive decline  as well as a risk factor for dementia [6, 7], in much the same way as amnestic MCI may progress to AD. The application of the construct of MCI in PD differs from its use in predicting AD in that PD is, by definition, already diagnosed when cognitive deficits may be demonstrated by the patient . In fact, subtle cognitive impairment may already be present at the time of diagnosis of PD [9-12]. Although PD patients are at a higher risk of developing cognitive impairment than controls, with a more rapid decline in those who do [5, 6, 13], the timing of cognitive decline to dementia is variable , as is the profile and pattern of impairment [15-17]. It is unclear whether all PD-MCI patients will eventually progress to dementia or whether other factors such as genetic mutations or concomitant disease may contribute to the development or more rapid progression of cognitive impairment. Longitudinal studies have identified a more significant rate of cognitive decline across domains in PD patients compared with age-matched healthy controls  and have found dementia in 78% of PD patients 8 years after diagnosis  and in as many as 83% after 20 years .