Cumulative incidence and risk factors of dementia in Parkinson's disease and parkinsonism: a population-based study.

Cognitive impairment is among the most common non-motor symptoms in parkinsonism and Parkinson's disease, ranging from subjective complaints to dementia. Although several predictive factors have been described, their prognostic relevance remains uncertain. This study aimed to examine the predictive association between clinical features and the risk of dementia and determine the incidence of dementia in a population-based cohort of incident Parkinsonism cases. Patients with parkinsonism onset between 1991 and 2020 were identified using the Rochester Epidemiology Project records-linkage system. Subjects with cognitive impairment preceding motor symptoms onset or concomitant Alzheimer's disease were excluded. Diagnoses of dementia and mild cognitive impairment were established using neuropsychological testing or brief cognitive screening. Development of dementia was assessed using a Cox proportional hazards regression model. Associations between dementia and each risk factor were individually estimated in adjusted Cox models. All risk factors were also included in a least absolute shrinkage and selection operator penalized multivariable Cox model. A total of 1104 patients were included: 153 (13.9%) developed mild cognitive impairment, and 543 (49.2%) developed dementia during follow-up. The cumulative incidence of dementia was 30.3% at 5 years, 48.6% at 10 years, 61.4% at 15 years and 69.3% at 20 years, indicating that a considerable portion of patients remained free of dementia after a prolonged follow-up. The majority of patients with dementia were males (61.5%), and older age at disease onset was associated with risk of dementia (hazard ratio [HR] = 1.32, P < 0.001 per 5-year increment). In the penalized model, dementia risk was significantly higher among individuals presenting with impaired postural reflexes (HR = 1.26, P = 0.014), consistent with a poorer prognosis in akinetic-rigid-predominant parkinsonism. Eye movement abnormalities, including reduced ocular range of motion (HR = 1.86, P < 0.001) and smooth pursuit alterations (HR = 1.34, P = 0.024), were also associated with dementia. Non-motor symptoms, including rapid eye movement sleep behaviour disorder (HR = 1.51, P < 0.001), orthostatic hypotension (HR = 1.30, P = 0.009) and constipation (HR = 1.24, P = 0.022), were linked to dementia risk, supporting a potential involvement of non-dopaminergic and central autonomic pathways in cognitive decline. These findings were largely confirmed in the Parkinson's disease/Parkinson's disease with dementia group, in which 77 (13.5%) developed mild cognitive impairment and 249 (43.5%) developed dementia. Among these patients, smooth pursuit alterations, reduced ocular range of motion, orthostatic hypotension and constipation were associated with dementia.