To determine the prevalence of neuropsychiatric symptoms among patients with Parkinson's disease (PD), Parkinson’s disease with dementia (PDD), and dementia with Lewy bodies (DLB), and to evaluate their impact on mortality in a population-based incident cohort from 1991 to 2020 in Olmsted County, MN.

Neuropsychiatric symptoms are non-motor complications, occurring years after diagnosis of PD. Incidence rates vary across studies, highlighting a need for long-term observation and clinical definition.

Patients with Parkinsonism onset between 1991 and 2020 were identified using the Rochester Epidemiology Project. Psychosis was diagnosed according to the NINDS of Mental Health Work Group criteria. Neuropsychiatric symptoms included visual and auditory hallucinations, delusions, confusion, delirium, and illusions. A movement disorder specialist reviewed all medical records and confirmed the diagnosis.

A total of 1,358 cases of Parkinsonism were included, of whom 380 (28%) developed neuropsychiatric symptoms and 98 (7.2%) developed psychosis. Among PD patients, 102 (26.8%) presented with neuropsychiatric symptoms and 16 (16.3%) with psychosis. Compared with PD, DLB had the highest hazard for neuropsychiatric symptoms (HR 11.54, p < 0.001) and psychosis (HR 9.09, p < 0.001), followed by PDD (HR 3.21 and 6.26, both p < 0.001). Both neuropsychiatric symptoms and psychosis were associated with increased mortality (HR 3.18 and 2.86), with the greatest risk observed when dementia coexisted with either of them (HR 13.50 and 19.46).

In our population-based study, patients with DLB or PDD presented an increased risk of developing psychosis compared to patients with PD without dementia. DLB patients exhibited the highest prevalence of neuropsychiatric symptoms, which was associated with an increased risk of mortality and progression to dementia.