Double lesion MRgFUS thalamotomy for essential tremor: 4.5-year outcomes and framework for assessing loss of efficacy and tremor progression.

Background: The essential tremor (ET) course to 54 months post-unilateral VIM/PSA magnetic resonance-guided focused ultrasound (MRgFUS) in the treated arm (TA) and non-treated arm (NTA) of 12 patients is reported.

Methods: Tremor severity was rated using Bain Findley spirography (BFS) scores in the TA and NTA. We divided follow-up into 'Early' (0-6 months) and 'Late' (6-54 months) phases, to minimise the effect of peri-lesion oedema resolution on the latter.

Results: The mean baseline BFS score was 6.2 in TA and 5.7 in the NTA. After unilateral VIM/PSA MRgFUS, mean BFS improved in TA at all subsequent time points (p < 0.001), with no significant differences between BFS scores at consecutive assessments or between 1 and 54 months, while the NTA BFS scores worsened between 12 and 24 months (p < 0.003). Three patients showed worsening of their TA BFS scores and an increasing NTA-TA BFS difference, indicating slower tremor worsening in TA compared to NTA, whilst one patient showed a greater rate of worsening in the TA compared to NTA BFS.

Conclusion: After 54 months, the beneficial effect of MRgFUS is usually maintained with any worsening of BFS scores in TA slower than in NTA. Loss of treatment benefit is rare.