Neuromodulation of the subthalamic nucleus in Parkinson's disease: the effect of fiber tract stimulation on tremor control

Background Therapeutic effects of deep brain stimulation (DBS) of the subthalamic nucleus (STN) in Parkinson's disease (PD) may in parts be attributed to the stimulation of white matter near the targeted structure. The dentato-rubro-thalamic (DRT) tract supposed to improve tremor control in patients with essential tremor could be one candidate structure. The aim of this study was to investigate the effect of stimulation proximity to the DRT on tremor control in PD patients treated with STN-DBS. Methods For this retrospective analysis, we included 36 consecutive patients (median age 65.5 years) treated with STN-DBS for disabling motor symptoms including tremor. Stereotactic implantation of DBS electrodes into the motor area of the STN was performed using direct MRI-based targeting and intraoperative microelectrode recording. Tremor severity was assessed preoperatively and at regular intervals postoperatively (Unified Parkinson's Disease Rating Scale III). The DRT was visualized in 60 hemispheres after probabilistic fiber tracking (3-T MRI). The position of active electrode contacts was verified on intraoperative stereotactic X-rays and postoperative CT images after co-registration with 3D treatment planning MRI/CT images. We determined the shortest distance of active contacts to the ipsilateral DRT tracts on perpendicular view slices and correlated this value with tremor change percentage. Results Twelve patients had unilateral tremor only, and accordingly, 12 hemispheres were excluded from further imaging analysis. The remaining 60 hemispheres were associated with contralateral resting tremor. Active brain electrode contacts leading to resting tremor improvement (46 hemispheres) had a significantly shorter distance to the DRT (1.6 mm (0.9-2.1) [median (25th-75th percentiles)]) compared with contacts of non-responders (14 hemispheres, distance: 2.8 mm (2-4.6), p < 0.001). Conclusion This retrospective analysis suggests that in STN-DBS, better tremor control in PD patients correlates with the distance of active electrode contacts to the DRT. Tractography may optimize both individually DBS targeting and postoperative adjustment of stimulation parameters.