Proprioceptive and Postural Outcomes Following Magnetic Resonance Guided Focused Ultrasound Thalamotomy for Essential Tremor: A Case Report

Introduction: Magnetic resonance guided focused ultrasound (MRgFUS) thalamotomy has been demonstrated to be an effective technique for management of medically-refractory essential tremor (ET). Three-dimensional magnetic resonance tractography has emerged as a mechanism for thalamic lesion targeting. We report a case where imperfect targeting resulted in apparent proprioceptive changes following left-sided MRgFUS thalamotomy.

Methods: MRgFUS thalamotomy was performed on a 76-year-old right-handed male with medically-refractory ET. Four-tract tractography (4TT) was used to target the intersection of the decussating and non-decussating dentato-rubro-thalamic tracts (DRTTc & DRTTi) while avoiding the corticospinal tract (CST) and medial lemniscus (ML).

Results: 4TT in conjunction with preoperative MR imaging yielded an initial target 1.5 mm superior to the Anterior Commissure(AC)-Posterior Commissure (PC) plane, 15mm lateral and 6mm anterior to the PC. Intraoperatively, a second target was identified and ablated, 15.5mm lateral and 3mm superior to the AC-PC plane, and 6mm anterior to PC. The intervention resulted in an approximate 80% reduction in qualitative tremor severity as well as a clinical rating scale for tremor (CRST) score reduction from 52.5 preoperatively to 34 on postoperative day 5. However, the patient developed proprioceptive deficits, involuntary posturing of the right arm, and contracture of the right-lower face within days of the procedure. A postoperative fluid-attenuated inversion recovery (FLAIR) MRI was merged with the preoperative FLAIR MRI sequence using BrainLab surgical navigation software. Preoperative target tracts were superimposed on postoperative imaging, and a region of interest was constructed to represent the MRgFUS lesion. In addition to the expected targeting of the DRTTc and DRTTi, this revealed that the MRgFUS lesion partially overlapped with the antero-medial corticospinal tract.

Conclusion : Proprioceptive deficits following corticospinal tract disruption represent a novel observation. Further research is required to understand the corticospinal tract's role in proprioception and its relevance to thalamotomy targeting.