Objectives: To determine the overall rate of functional decline and to assess the progression of different signs of Parkinson disease (PD).
Patients and methods: Patients with clinically diagnosed PD followed up for at least 3 years were included in this study. Demographic and clinical data (including the Unified Parkinson's Disease Rating Scale [UPDRS]) were analyzed by the multivariate unbalanced repeated-measurements design using the mixed-effects model to study the association between different symptoms and various demographic variables. Regression models helped estimate the rates of progression of the disease in relationship to the various components of the UPDRS. Patients were categorized as having tremor-dominant or postural instability-gait difficulty-dominant PD and the 2 categories were compared for progression of their total UPDRS scores.
Design: A multivariate mixed-effects model was used to study the relationship between the different symptoms and various demographic variables. Nonparametric statistical tests were used to compare the progression of symptoms in the "on" (good function) state and the "off" (poor function) state groups for 2 age-at-onset categories (< or =57 and >57 years).
Results: Data from 1731 visits on 297 patients (181 men) followed up for an average of 6.36 years (range, 3-17 years) were analyzed. The annual rate of decline in the total UPDRS scores was 1.34 when assessed during the on state and 1.58 when assessed during the off state. Patients with an older age at onset had more rapid progression of PD than those with a younger age at onset. Furthermore, the older-onset group had statistically significantly more progression in mentation, freezing, and parts I (mentation) and II (activities of daily living) UPDRS subscores. Handwriting was the only component of the UPDRS score that did not notably deteriorate during the observation period. Regression analysis of 108 patients whose symptoms were rated during their off state showed a faster rate of cognitive decline as age at onset increased. The slopes (ie, the annual rates of decline) of progression in the UPDRS scores, when adjusted for age at the initial visit, were steeper for the postural instability--gait difficulty--dominant group compared with the tremor-dominant group.
Conclusion: Based on longitudinal follow-up data, our findings provide evidence for a variable course of progression of the different PD symptoms, thus implying different biochemical or degenerative mechanisms for the various clinical features associated with PD.