Firstly, the complexity of PD, in which motor and non-motor features appear with a variable expression and severity over time. Although improvement of some of the PD features may be achieved with medication, a number of patients will ultimately experience changes in responsiveness to levodopa and develop psychiatric disturbances.
Most comorbid conditions are nonfatal, but add to the disease burden.
Thirdly, the evolution of clinimetric standards highlights the need for improved methods to evaluate patients. Traditionally, scales that assess patients with PD have focussed on the impairment and disability levels of WHO disease evaluation. Responsiveness of scales, required to ensure the usefulness in the longitudinal evaluation of PD patients, has never been addressed. PD assessment scales are generally developed for trial purposes. Due to applied inclusion criteria PD patients in trials lack serious comorbidity.