Introduction: Transcranial magnetic resonance-guided focused ultrasound (MRgFUS) is an evolving treatment option for eligible patients with essential tremor (ET). Producing thermal lesions in the thalamic ventral intermediate nucleus results in clinical improvement of tremor. Skull density ratio (SDR), the ratio of cortical to cancellous bone, is a limiting factor in successfully producing a thermal lesion. This study sought to delineate the relationship between SDR and treatment outcomes in these patients.

Methods: Radiographic, treatment, and clinical outcome data of patients enrolled in our clinical trial (ET002) for the treatment of unilateral upper extremity ET with MRgFUS were reviewed. The relationships between SDR and time of procedure, maximal temperature recorded, and total Clinical Rating Scale for Tremor (CRST) score at one year of follow up were determined.

Results: A total of 20 patients, 18 men and two women, were included in this study. The mean age was 72.4 years (S.E. 1.1), and mean SDR was 0.60 (S.E. 0.03, range 0.41–0.82). The average treatment time was 106.3 minutes (S.E. 10.4, range 53.0 – 228.0) with a peak temperature achieved of 46.8C–66.4C, and a mean of 60.2C (S.E. 0.96). Total CRST score improved on average from 54.0 to 30.7 at one year of follow up (t-test, p<0.001). A higher SDR was associated with a shorter time of procedure (p=0.020, R2=0.267) (Figure 1). In addition, a higher SDR trended toward association with a higher peak temperature achieved (p=0.109, R2=0.137) (Figure 2). SDR was not associated with percent improvement in total CRST score (p=0.199, R2=0.090) (Figure 3).

Conclusions: SDR continues to be a limiting factor in eligibility for treatment of unilateral upper extremity ET with transcranial MRgFUS. In this small cohort of trial patients, a higher SDR portends a shorter procedure time, however it is not associated with clinical outcome. Future studies could investigate lowering the threshold for treatment eligibility.

Patient Care: Otherwise eligible patients who suffer from essential tremor who have a low or borderline skull density ratio (SDR) cannot currently benefit from focused ultrasound thalamotomy. This research attempts to explore the role of SDR in treatment outcome.

Learning Objectives: -Attendees will learn the eligibility for transcranial MRgFUS treatment among ET patients -Attendees will learn the role of skull density ratio in eligibility, treatment, and clinical outcomes in these patients

References: 1. Elias WJ, Lipsman N, Ondo WG, et al. A randomized trial of focused ultrasound thalamotomy for essential tremor. N Engl J Med. 2016;375(8):730-739. 2.Chang WS, Jung HH, Zadicario E, et al. Factors associated with successful magnetic resonance-guided focused ultrasound treatment: efficiency of acoustic energy delivery through the skull. Journal of Neurosurgery. 2016;124(2):411-416.