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Introduction: Postural instability is a major source of disability in idiopathic Parkinson's disease ( IPD ). Deep brain stimulation of the globus pallidus internus ( GPI-DBS ) improves clinician-rated balance control but there have been few quantitative studies of its interactive effects with levodopa ( L-DOPA ). The purpose of this study was to compare the short-term and interactive effects of GPI-DBS and L-DOPA on objective measures of postural stability in patients with longstanding IPD. Methods: Static and dynamic posturography during a whole-body leaning task were performed in 10 IPD patients with bilateral GPI stimulators under the following conditions: untreated ( OFF ); L-DOPA alone; DBS alone; DBS + L-DOPA, and in 9 healthy Control subjects. Clinical status was assessed using the UPDRS and AIMS Dyskinesia Scale. Results: Static sway was greater in IPD patients in the OFF state compared to the Control subjects and was further increased by L-DOPA and reduced by GPI-DBS. In the dynamic task, L-DOPA had a greater effect than GPI-DBS on improving Start Time, but reduced the spatial accuracy and directional control of the task. When the two therapies were combined, GPI-DBS prevented the L-DOPA induced increase in static sway and improved the accuracy of the dynamic task. Conclusion: The findings demonstrate GPI-DBS and L-DOPA have differential effects on temporal and spatial aspects of postural control in IPD and that GPI-DBS counteracts some of the adverse effects of L-DOPA. Further studies on larger numbers of patients with GPI stimulators are required to confirm these findings and to clarify the contribution of dyskinesias to impaired dynamic postural control.

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