Objective:Deep brain stimulation of the subthalamic nucleus (STN-DBS) is a useful therapy to improve motor functions and reduce dependence on medication in patients with Parkinson's disease (PD). The purpose of the study is to assess the long-term clinical outcomes of STN-DBS and to determine the optimal placement of electrodes that for the most positive outcomes. Methods:A consecutive series of 42 PD patients were evaluated using the Unified Parkinson's Disease Rating Scale (UPDRS) before and after STN-DBS lead implantation. Postoperatively, patients were evaluated during both the medication 'ON' period (medication suppressed symptoms) and the medication 'OFF' period (when medication failed to suppress symptoms), and the results were compared to the baseline values prior to surgery. Follow-up assessments, focusing on motor functions, were performed 1, 3, 5, and 8 years after the initial implantation surgery. The locations of electrodes were measured and compared against the clinical outcomes. Results:STN-DBS remarkably improved the UPDRS-II, -III, and -IV dyskinesia and motor fluctuation scores in the OFF-medication condition when compared to baseline values. In addition, the dose of levodopa needed to elicit an effect declined sharply in the OFF-medication condition. Over time, the axial signs progressively worsened even with continuous stimulation and a levodopa response. The location of electrodes correlated with the most beneficial outcomes was the dorsal STN margin. Conclusions:Our results confirm that overall, stimulation-induced motor improvement is still evident after 8 years. However, the primary best outcome declines with the progressive loss of favourable axial signs.