Object. The goal of this study was to relate the degree of clinical improvement and that of energy consumption to the anatomical position of electrode poles used for long-term stimulation.
Methods. The authors conducted a retrospective analysis of 15 consecutive patients in whom targeting of the subthalamic nucleus (STN) had been performed using ventriculography, three-dimensional (3D) magnetic resonance (MR) imaging, and 3D computerized tomography, together with macrostimulation and teleradiographic control of the electrode position. In these patients the follow-up period ranged from 6 to 12 months. Postoperative improvement in contralateral motor symptoms, which was assessed by assigning a lateralized motor subscore of the Unified Parkinson's Disease Rating Scale (UPDRS), and stimulus intensity required for optimal treatment results were correlated with the intracerebral position of the active electrode pole.
Bilateral high-frequency stimulation of the STN improved the UPDRS motor score during the medication-off period by an average of 60.5% compared with that at baseline. Repeated transfer of stereotactic coordinates from postoperative teleradiography to treatment-planning MR images documented the proper localization of the most distal electrode pole (pole 0) in the targeted STN. Nevertheless, in most cases the best clinical improvement was achieved using electrode poles that were located several millimeters above the electrode tip. If the relative improvement in motor symptoms was correlated with the required electrical energy for chronic stimulation, the best coefficient was observed for active electrode poles projecting onto white matter dorsal to the STN.
Conclusions. This observation makes blocking or activation of large fiber connections arising in the STN or running nearby more likely than electrical interference with cell bodies inside the STN. Anatomical correlates may be the pallidothalamic bundle (including Field H of Forel and the thalamic fascicle), the pallidosubthalamic tract, and/or the zona incerta.