Effectiveness and safety of MR-guided focused ultrasound thalamotomy in patients with essential tremor and low skull density ratio: a study of 101 cases

OBJECTIVE The objective was to investigate the effectiveness and safety of MR-guided focused ultrasound (MRgFUS) treatment in patients with essential tremor, particularly those with low skull density ratio (SDR) and including those with very low SDR, and to identify the factors influencing treatment effectiveness and to provide insights into therapeutic ap- proaches for patients with lower SDR. METHODS Real-world data from 101 patients who underwent MRgFUS between July 2019 and March 2022 at a single institution were analyzed. Tremor severity was assessed using the Fahn-Tolosa-Marin Clinical Rating Scale for Tremor (CRST). The patients were categorized into quartile groups based on their mean SDR, and the characteristics, treatment effectiveness, treatment parameters, and adverse events were evaluated among these subgroups. RESULTS Patients were classified into 4 quartiles based on the mean SDR: quartile 1 (Q1) (SDR 0.26-0.37), Q2 (SDR 0.38-0.42), Q3 (SDR 0.43-0.49), and Q4 (SDR 0.50-0.75). MRgFUS significantly improved total CRST and tremor score across all SDR subgroups. Additionally, there were no significant differences in the improvement rates among the 4 subgroups. Analysis of the treatment parameters revealed that lower mean SDR was associated with lower target maximum temperature and smaller coagulation volume after focused ultrasound (FUS). Regarding adverse events, headache and nausea during FUS and facial and head edema on the day after surgery were more frequent in the Q1 subgroup (very low-SDR group). In contrast, numbness was more common in the Q4 subgroup. However, all these adverse events had resolved by the 3-month follow-up except numbness. CONCLUSIONS This study suggested that MRgFUS is effective and safe for patients with medication-resistant essential tremor, including those with very low mean SDR. However, the very low-SDR group had insufficient temperature elevation at the target site compared with the high-SDR group, suggesting the need for a different strategy. Notably, with careful adjustments and considerations, positive outcomes can still be achieved in patients with very low SDR. Therefore, very low SDR should not be considered an absolute exclusion criterion because it is expected to increase the number of patients who benefit from MRgFUS.