Objective: To investigate risk factors and the rate of progression of motor symptoms and disability in a population-based cohort of patients with Parkinson disease (PD).

Methods: In all, 232 patients with PD, derived from a community-based prevalence study, were followed prospectively over an 8-year period. Follow-up examinations were done 4 and 8 years after baseline, and 144 patients participated in at least one follow-up examination. Information on motor function and disability was obtained using the Unified Parkinson Disease Rating Scale (UPDRS), the Hoehn and Yahr staging, and the Schwab and England score. Population-averaged logistic regression models were used to describe annual disease progression and to analyze the influence of potential risk factors on functional decline.

Results: We found a similar mean annual decline in the UPDRS motor score and the Hoehn and Yahr staging of 3.1% and 3.2%, respectively. Also the UPDRS Activity of Daily Living (ADL) score and the Schwab and England scale changed similarly, with 3.5% and 3.6% per year, respectively. Age, age at onset, disease duration, and excessive daytime somnolence at baseline were strong and independent predictors of greater impairment in motor function and disability. Cognitive impairment at baseline predicted higher disability and higher Hoehn and Yahr scores. Time by age-at-onset interactions were found for the UPDRS motor score and the Hoehn and Yahr staging.

Conclusions: Motor function and disability worsened significantly with time, and to a similar extent. Age, age at onset and disease duration, as well as symptoms thought to be due to involvement of non-dopaminergic brain structures, are predictors of more impaired motor function and disability. However, age at disease onset was the main predictor of motor decline in our cohort, indicating a slower and more restricted pathologic disease process in patients with young-onset PD.