註釋 [Truncated abstract] One of the classical and more debilitating symptoms of Parkinson's disease is postural instability. As a result of this impairment in balance, patients with Parkinson's disease experience restricted mobility, are prone to falls, display a reduced capacity to perform activities of daily living and have a reduced quality of life. This thesis reviews our current knowledge of Parkinson's disease in the context of the mechanisms regulating postural control and how impairments to these lead to postural instability. The thesis then examines the characteristics and assessment of patients with Parkinson's disease, with a history of falls or near falls compared to non-fallers and controls. Our first objective was to explore the circumstances surrounding fall and near fall events in a cohort of 130 patients with idiopathic Parkinson's disease using an approach based on a 16-item questionnaire. This survey the largest of its kind undertaken to date showed that over 50% of patients experienced one or more falls over the previous two years with over 80% experiencing near falls over the same period. Our study suggests that fear of falling is more prevalent in patients than previously thought and that a fear of falling, restrictions on activities and reduced participation in exercise correlate with falling and the number of falls experienced. From this initial cohort of patients, a smaller cohort of 48 patients was selected comprising two sub-groups, fallers and non-fallers. These were used to address the question of whether a multi-dimensional approach to assessing postural instability based on a combination of established clinical and functional measures, provides a more accurate and quantifiable assessment of an individual's balance and mobility compared to any single methodology. Fallers had longer disease duration compared to non-fallers but importantly could be discriminated from non-fallers by most of the individual assessment methodologies. Importantly, a combined analysis of assessments showed significant linear trends between measured parameters and the number of fall events over the previous two years. To explore this issue further, we used posturography a technique widely recognised as superior to established clinical and functional measures of postural stability in terms of its quantifiability and reproducibility to measure and quantify balance in fallers and non-fallers with Parkinson's disease compared to controls. Of the static posturographic variables measured, only two differed significantly between fallers and controls. By contrast, several of the dynamic posturographic variables differed significantly between the groups (viz. fallers, non-fallers and controls). A correlation analysis showed significant correlations between the static and dynamic posturographic measures and clinical and functional measures. Consistent with the fact that the postural instability associated with Parkinson's disease represents a continuum from slightly impaired balance through to functional impairment typified by falls and near falls, our findings suggest that using a combination of clinical and functional measures of balance and mobility coupled with posturography provides the best approach to assessing subtle changes in balance in patients, particularly in a research setting ... 