Importance: Mild cognitive impairment (MCI) is common in Parkinson disease (PD), but the prognostic value of MCI in early PD is unknown.
Objective: To examine the course of MCI and its progression to dementia in an incident PD cohort.
Design: Prospective longitudinal cohort study.
Setting: The Norwegian ParkWest study, an ongoing population-based study of the incidence, neurobiology, and prognosis of PD in western and southern Norway.
Participants: A population-based cohort of 182 patients with incident PD monitored for 3 years.
Main outcomes and measures: Serial neuropsychological tests of attention, executive function, verbal memory, and visuospatial skills were administered at baseline, 1 year, and 3 years. Patients were classified as having MCI and received a diagnosis of dementia according to published consensus criteria.
Results: Significantly more patients with MCI than without MCI at baseline (10 of 37 [27.0%] vs 1 of 145 [0.7%]; relative risk, 39.2 [95% CI, 5.2-296.5]; P < .001) progressed to dementia during follow-up. Of those with MCI at baseline, 8 of 37 (21.6%) had MCI that reverted to normal cognition during follow-up. Mild cognitive impairment at the 1-year visit was associated with a similar progression rate to dementia (10 of 36 patients [27.8%]) and reversion rate to normal cognition (7 of 36 [19.4%]). However, among the 22 patients with persistent MCI at baseline and the 1-year visit, 10 (45.5%) developed dementia and only 2 (9.1%) had MCI that reverted to normal cognition by the end of study.
Conclusions and relevance: Mild cognitive impairment at PD diagnosis predicts a highly increased risk for early dementia. Repeated neuropsychological testing increases the prognostic accuracy of MCI with respect to early dementia development in PD.