Objective: Deep brain stimulation (DBS) of the thalamic ventrointermedius nucleus (Vim) is an accepted treatment for severe, drug-resistant essential tremor. There is an ongoing debate weather long term increase of tremor in Vim operated patients is due to disease progression or habituation.

Background: Vim DBS has been shown to be highly effective in tremor control in short term periods. Long term data are rare and reports show heterogeneous results.

Methods: 21 patients with severe essential tremor underwent bilateral Vim-DBS and were assessed up to 11 years postoperatively using the Fahn-Tolosa-Marin scale (TRS) at different time points (medium term: 3-35 month, long term: 24-120 month). Patients were tested with the Vim stimulation on and off. The rebound effect was assessed in detail using the TRS scale, Bain spiral score rating and quantitative accelerometry.

Results: Tremor severity worsened considerably over time both for the non-stimulated condition (TRS baseline: 55.0 ± 3.7(mean ± SD); long-term stim-OFF: 74.1 ± 5.0) and the stimulated condition (short-term: 20.9 ± 2.5; long-term: 42.0 ± 5.2). Vim-DBS improved the TRS from 55.0 ± 3.7 at baseline to 20.9 ± 2.5 points at short-term and from 74.1 ± 5.0 in the long-term stim-OFF to 42.0 ± 5.2 in the stim-ON condition. The Bain spiral score improved by 48% at short term but only 26% at long-term. Functional measures showed similar improvements. All changes were highly significant. The stimulation effect was negatively correlated with the time since surgery (Z=-3.542, p<0.001). Only one third of the patients had a rebound effect which was terminated 60 min after switching off the stimulator. Long-term worsening of the TRS was more profound in the stim-ON than in the stim-OFF condition indicating habituation to stimulation. 79% of the worsening is due to disease progression and 21% to habituation to stimulation.

Conclusions: ET patients with VIM-DBS suffer a profound worsening of their tremor severity and their effect of Vim-DBS over a decade. This is mainly due to disease progression and less than a quarter of this worsening is due to habituation. Efforts are needed to improve the long-term efficacy of Vim-DBS.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/long-term-habituation-of-deep-brain-stimulation-for-essential-tremor-an-observer-blinded-study/