Thermal Neuromodulation Technique Optimization of Focused Ultrasound Ablation for Essential Tremor: A Single-institution Experience

Introduction: Ventral intermediate nucleus (VIM) focused ultrasound ablation (FUSA) is increasingly utilized to treat essential tremor (ET). While effective, long-term ataxia occurs in 25-33% of patients(1), attributed to off-target ablation in the white matter tracts adjacent to VIM(2). Following the recent description of optimal thermal neuromodulation (TN) parameters (3), we present our initial experience with the prospective TN implementation in consecutive ET patients undergoing VIM FUSA, specifically evaluating the impact on ataxia risk.

Methods: The optimized TN technique included 30-33 second sonications at 43-45 degrees Celsius (°C) with target adjustment based on tremor improvement and any adverse events. VIM targeting utilized tractography with FDA-approved software (Elements, Brainlab, Inc). We analyzed treatment parameters (i.e., treatment time, sonication count, and thermal maps) and outcomes. A movement disorders neurologist assessed tremor and ataxia using The Essential Tremor Rating Assessment Scale (range:0-32) and Scale for Assessment and Rating of Ataxia (SARA, range:0-40), with modified versions intraoperatively (tremor range: 0-24, SARA range: 0-16).

Results: Sixteen patients (11 males, mean age 74.1±7.6 years) underwent VIM FUSA. The mean procedure time was 34.4 minutes (SD:14.0) with 4.75 (SD:1.0) sonications per treatment, including 2.4±0.6 TN sonications for subthreshold testing. Mean prescribed power (124.6 Watts) yielded 44.7°C, achieving optimal TN in 14 patients out of 16 subjects. Initial TN sonication reduced tremor by 1.4 points, with similar improvement (1.6 points) after the second TN sonication. SARA scores remained stable during TN (≤1-point change in six subjects). Post-procedure tremor improved by 68.3% initially and 53.1% at six months. SARA scores worsened in four subjects initially (1-6 points) and three at 6 months (1,2,4 points respectively). Interestingly, SARA scores also improved in 6 subjects at 6 months.

Conclusion : This standardized TN protocol demonstrates efficient physiological target verification prior to permanent ablation, contributing to the ongoing efforts to optimize the VIM FUSA technique.