Direct Targeting of the Ventrointermediate Nucleus During Magnetic Resonance Guided Focused Ultrasound Thalamotomy

Introduction: Essential tremor (ET) affects up to 8% of the population over the age of 60 and is disabling when severe. Magnetic resonance-guided focused ultrasound (MRgFUS) thalamotomy of the ventrointermediate nucleus (VIM) is an effective option for treating medically refractory ET. However, targeting remains a challenge. Current standard of practice uses stereotactic coordinates to indirectly approximate the target location, putting patients at risk of off-target effects or incomplete treatments. We report results from direct targeting using the commonly available MRI fast grey matter acquisition T1 inversion recovery (FGATIR) sequence.

Methods: A retrospective review compared the outcomes of patients who underwent MRgFUS thalamotomy using traditional indirect versus FGATIR direct targeting modalities. The target is selected 3.5 mm medial to the internal capsule, 1.5 mm superior to the commissural plane, and at the narrowing of a white matter tract between the red nucleus and the thalamus. Cohorts of 16 and 195 patients were used for the direct and indirect groups respectively, with primary outcomes including the number of sonications, total sonication time, total energy, and the incidence of side effects at 2-week follow-up.

Results: On average, the direct targeting cohort required 1.06 fewer sonications, had a 17.14 s reduction in total sonication time, and received 12478.78 fewer Joules of energy. The incidence of any side effects dropped from 83% (indirect cohort) to 69% (direct cohort). Of the side effects, the percentage of patients with ataxia dropped from 34% to 31%, weakness dropped from 16% to 13%, and sensory abnormalities dropped from 27% to 19%. There was no difference in the rates of dysphagia between the cohorts (6%).

Conclusion : These results show a significant improvement in patient outcomes associated with the use of FGATIR-based direct targeting of the VIM. A larger cohort of patients with a more methodical study will be required to validate these findings.