192 Feasibility of Magnetic Resonance-Guided Focused Ultrasound Thalamotomy for Essential Tremor Patients With Low Skull Density Ratio

INTRODUCTION: Magnetic resonance-guided focused ultrasound (MRgFUS) is a safe, effective treatment for essential tremor (ET). Skull density ratio (SDR), the ratio between the densities of cancellous bone and cortical bone, affects energy penetration of the skull. Major clinical trials and guidelines have excluded patients with low skull density ratio due to concerns of disrupted treatment, though these patients represent nearly 40% of ET patients. METHODS: In this single-surgeon cohort study of prospectively collected and retrospectively analyzed data, we report the characteristics and clinical outcomes of all ET patients with low SDR (<0.40) vs. high SDR (=0.40) treated at Brigham and Women’s Hospital between June 2019 and February 2023. Clinical outcomes were determined by Fahn-Tolosa-Marin (FTM) scores in the affected limb and percentage of patients with clinical improvements. RESULTS: Thirty-one patients had low SDR. 239 patients had high SDR. Mean (±SD) SDRs in the low SDR and high SDR groups were 0.36 (±0.02) and 0.51 (±0.08), respectively. There were no significant differences between the two groups in the following characteristics: sex, handedness, duration of tremor, total preoperative Fahn-Tolosa-Marin (FTM) score, intention + posture preoperative FTM score, age at treatment, treatment laterality, and rate of intraoperative complications. While there were significant differences in the clinical outcomes of low SDR vs. high SDR patients on postoperative day 1 (POD1), the clinical outcomes between the two groups were not significantly different at every follow-up time point, including up to three years post-treatment. CONCLUSIONS: While clinical outcomes were significantly different on POD1, low SDR and high SDR patients had similar long-term clinical outcomes, suggesting that MRgFUS of ET patients with SDR < 0.40 is feasible and effective.