Intraoperative asymmetry of thalamic stimulation mapping in Essential Tremor: specificity of first hemisphere implantation and transitory lesioning effect

Deep Brain Stimulation (DBS) effectively reduces symptoms in Essential Tremor (ET). Clinical benefit depends on modulation of distributed neural circuits with still undeciphered mechanisms. Furthermore, the lesioning effect observable intraoperatively following the implantation of the DBS lead in the first brain hemisphere has received little attention. We hypothesize that hemispheric differences exist in the spatial organization of improvement and adverse effects, related to the sequential implantation of electrodes and a transitory lesioning effect.

We investigated this hemispheric asymmetry with a data-driven, probabilistic mapping approach, linking simulated stimulation volumes [Volumes of Tissue Activated (VTAs)] to symptom improvement (imp) and side effects (se) for each hemisphere. Intraoperative data from 23 ET patients undergoing DBS lead implantation in the ventrointermediate nucleus of thalamus (Vim) were retrospectively analyzed. The left hemisphere was systematically implanted first. Stimulation spots (positions) were categorized using therapeutic window values and side effect occurrence. Hemisphere-specific probabilistic effect volumes of imp (PVimp) and se (PVse) derived from VTAs, and clusters of stimulation spots grouped by category were computed. They were topographically analyzed in relation to structures of the Vim region, and compared within hemispheres and across hemispheres.

In the first implanted hemisphere, PVimp was in the infero-posterior part of Vim reaching the subthalamic tegmental fields, whereas the PVse was above, posterior and lateral to the PVimp, spanning Vim and the ventrocaudal nucleus of thalamus (VC). In the right one, PVimp and PVse overlapped, intersecting Vim and VC, in the intermediate part of both. The stimulation spots clusters, in the left hemisphere, were located around the intermediate part, slightly lateral of Vim and VC to a lesser extent. In the right one, they were located in the intermediate and superior parts of Vim.

Significant hemispheric asymmetries were observed, revealing a lesioning effect subsequent to the first DBS lead implantation in the left hemisphere. This led to a broader distribution of optimal positions with a larger PVimp largely overlapping with the PVse in the contralateral right hemisphere. The combined mapping and categorization framework yields hemisphere-specific insights on the transitory lesioning and bi-hemispheric modulation which could guide future DBS strategies.