Objective: Our aim is to evaluate the long-term benefit of unilateral VIM DBS.

Background: Essential tremor (ET) is the most common movement disorder, and in up to 50% of patients, medications fail to provide meaningful benefit. Deep Brain Stimulation (DBS) of the Ventral intermediate nucleus of the thalamus (VIM) is an established therapeutic option. Previous literature shows marked improvement in tremor with unilateral and/or bilateral procedures. Nonetheless, data on long-term outcomes in patients with unilateral DBS are limited.

Method: This retrospective chart review utilized data from the University of Florida INFORM database. We included ET patients who underwent unilateral VIM DBS surgery from January 2003 to September 2020. The Wilcoxon signed-rank test was employed to compare baseline contralateral Fahn-Tolosa-Marin Tremor Rating Scale (FTMTRS-B) scores with those observed under on stimulation conditions (FTMTRS-ON) and off stimulation conditions (FTMTRS-OFF) at each follow-up time point.

Results: Our analysis includes 74 patients (49 men / 25 women) with long-term follow-up extending up to 10 years post-DBS, with both On and Off DBS assessments. The average patient age (SD) at time of surgery was 70 (8.68) years. Average (SD) baseline total motor FTMTRS was 34.58 (9.38), and average (SD) contralateral motor FTMTRS was 15.82 (4.46). Unilateral VIM DBS improved contralateral FTMTRS motor scores in the short and long-term, with percentage improvement ranging from a 69.9 ± 23% (p<0.001) at 6 months post-surgery to 37.7 ± 26.51 (p=0.017) at the 10-year mark. The improvement remained statistically significant up to 10 years post-surgery. Comparing contralateral tremor scores in the DBS-OFF and DBS-ON state over time, there is significant improvement in tremor ranging from 58.77 ± 27.84% at 6 months post-surgery to 33.2 ± 22% after 10 years. However, a diminishing difference between ON and OFF states was observed over time.

Conclusion: Unilateral Vim DBS effectively improves contralateral tremor scores up to 10 years post-surgery. However, there is a gradual decrease in the percentage improvement over time, indicating a potential loss of efficacy and suggesting disease progression.

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