Objective: To assess current spread to the corticospinal tract (CST) in patients with Parkinson’s disease (PD) treated with bilateral Deep Brain Stimulation (DBS) of the Subthalamic Nucleus (STN).

Background: Proximity to the CST can limit therapeutic STN-DBS. Neurophysiological assessment might be able to detect affection of the CST at a low threshold.

Methods: We recruited 21 patients with advanced Parkinson’s disease one year after a MRI-guided and MRI-verified STN-DBS implantation. All patients had electrodes of 1.27mm in width, with contacts of 1.5mm in length, separated by 0.5mm, numbered 0–3 (left hemisphere) and 8–11 (right hemisphere). Patients were assessed with surface EMG placed on the Orbicularis Oris (OOr) and on the first dorsal interosseous (FDI) muscle to detect MEPs in a resting state elicited by the impulse generator used for chronic DBS. Stimulation at 3Hz and 60μs was increased up to 6.0 mA or until bothersome side affects appeared (maximum of 96 parameters). Thirty sweeps of EMG triggered by the stimulation artifact were averaged per condition to detect the threshold of CST activation and measure the amplitude of the motor response. EMG data was correlated with postoperative stereotactic MRI by modeling the Volume of Tissue Activated (VTA) using SureTune beta version for all 96 parameters. VTA images were fused to atlas-derived CSTs, obtaining volumes of overlap for all parameters in all patients.

Results: A total of 150 contacts in the 21 patients were assessed by EMG. The mean threshold for contralateral FDI activation was 3.9±1.1mA of stimulation amplitude with a non-significant side difference (3.8±1.1mA L electrode; 4.1±1.0mA R electrode; p=.11). For the contralateral OOr the mean thresholds were lower at 3.4±1.1mA (paired p<.001) with a significant side difference (3.2±1.1mA L electrode; 3.7±1.0mA R electrode; p=.017). The volumes of overlap between VTA and CST correlated with MEP amplitudes of the FDI (e.g. r=.42, p<.001 for all measurements obtained at 6.0mA) and inversely with the thresholds of FDI activation (r=-.47, p<.001).

Conclusions: EMG assessment is able to sensitively detect affection of the CST by subthalamic DBS. The present results will be complemented by clinical data to further elucidate the usefulness of this tool as troubleshooting aid in patients with unsatisfactory response to stimulation.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/neurophysiological-assessment-of-corticospinal-tract-activation-due-to-subthalamic-deep-brain-stimulation-in-patients-with-parkinsons-disease/