Skull Density Ratio and Treatment Outcomes in Essential Tremor Patients Treated With Transcranial Magnetic Resonance-Guided Focused Ultrasound Thalamotomy

Abstract

Abstract 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 a 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 1 yr of follow-up were determined. RESULTS A total of 20 patients, 18 men and 2 women, were included in this study. The mean age was 72.4 yr (SE 1.1) and mean SDR was 0.60 (SE 0.03, range 0.41-0.82). The average treatment time was 106.3 min (SE 10.4, range 53.0-228.0) with a peak temperature achieved of 46.8°C to 66.4°C, and a mean of 60.2°C (SE 0.96). Total CRST score improved on average from 54.0 to 30.7 at 1 yr of follow-up (t-test, P < .001). A higher SDR was associated with a shorter time of procedure (P = .020, R2 = 0.267). In addition, a higher SDR trended toward association with a higher peak temperature achieved (P = .109, R2 = 0.137). SDR was not associated with the percent improvement in total CRST score (P = .199, R2 = 0.090). CONCLUSION SDR continues to be a limiting factor in eligibility for the 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.