Background: The risk for dementia in Parkinson disease (PD) is high, with important clinical consequences for patients with PD. However, the absolute risk of dementia and how it affects survival in PD are not known. Such questions are important for patients, their families, and service providers but require long-term studies.
Methods: This study is a prospective longitudinal cohort study with patients from a prevalence study of PD in Norway. Patients were reassessed 4, 8, 9, 10, 11, and 12 years after prevalence day. A dementia diagnosis according to Diagnostic and Statistical Manual of Mental Disorders, Third Edition, Revised, criteria was based on a semistructured caregiver interview, cognitive rating scales, and neuropsychological tests. Progression from PD to PD with dementia and death was modeled using a continuous-time three-state irreversible Markov model.
Results: A total of 233 PD patients were included, and 140 patients (60%, 95% CI 54% to 66%) had developed dementia by the end of the study period. The cumulative incidence of dementia steadily increases with age and duration of PD and, conditional on survival, increases to 80% to 90% by age 90 years. Women live with PD longer than men and spend more years with dementia. At age 70 years, a man with PD but no dementia has a life expectancy of 8 years, of which 5 years would be expected to be dementia free and 3 years would be expected to be with dementia.
Conclusion: Dementia is a key part of survival in Parkinson disease and must be planned for in services for this condition.