Surgical procedure and stimulation parameters
The surgical procedure followed a predefined protocol in all study centers. Treatment planning for stereotactic bilateral implantation of quadripolar DBS-brain electrodes (3387, Medtronic, Minneapolis, MN, USA) was based on high-resolution pre- or intraoperative MRI images (1.5T or 3.0T MRI-scanner). The target for electrode implantation (translating to the inferior border of the most distal electrode contact) was defined by a combination of indirect coordinate-based and direct anatomical targeting. Standard coordinates in relation to the midsagittal plane were: Both, frame-based stereotactic implantation of the brain electrodes and implantation of the non-rechargeable pulse generator (ACTIVA®PC, Medtronic; Minneapolis, MN, USA) in the subclavicular area were performed as a single surgery under general anesthesia. All patients underwent post- or intraoperative CT- or MRI scans for verification and documentation of final electrode placement (see Fig. 