Objective: To investigate tremor improvement but also side effects of deep brain stimulation (DBS) for essential tremor (ET) and their relationship to the volumes of tissue activated (VTA).

Background: DBS in ET is effective and safe but there are still controversies regarding the location of the sweet spot that some authors see in the VIM and others in the posterior subthalamic area or zona incerta.

Method: 30 patients with ET (12 women and 18 men, mean age 65) who received bilateral DBS between 11/2011 – 08/2017 at the Charité Berlin with clinical tremor assessment before and ON and OFF DBS at 12 months follow up (12 moFU) on video were included. Tremor intensity was retrospectively rated by two blinded movement disorder specialists using the Fahn-Tolosa-Marin Tremor Rating Scale (Fahn et al., 1993). Patients also completed the ADL questionnaire (Bain et al., 1993)  asking for independent functioning in daily life. Using the Lead-DBS software (www.lead-dbs.org) and the thalamic DBS connectivity atlas (Akram et al., 2018)  we localized the electrodes on the postoperative CT or MRI and modeled VTAs in MNI space for each lead with the stimulation parameters from the 12 mFU. VTAs of all patients were weighted by the respective side effects binary score. All VTAs were then averaged and thresholded for more than 50% side effects frequency and more than 5 patients in each voxel.

Results: Tremor improvement was 57% on average both when comparing the total FTMTRS before DBS and at 12 moFU and DBS ON vs. OFF at 12 moFU.  As side effects, ataxia was present in 9, dysarthria in 6 and dyskinesia in two patients. 16 patients did not show any side effects at 12 moFU. ADL scores improved by 54% in average. The improvement in ADLs correlated with improvement in tremor (r=0.404, p=0.045). Clusters of side effects involved VTAs with more dorsal and anterior locations in relation to the VIM.

Conclusion: Our results demonstrate the effectiveness of DBS for ET in our cohort for both tremor reduction and independent functioning in everyday life. Side effects at 12 moFU were associated with a more dorsal and anterior location of the VTAs. These results prompt further characterization of the sweet spot for DBS in ET in order to obtain minimal side effects but an optimal tremor reduction.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/searching-for-the-optimal-functional-target-for-deep-brain-stimulation-in-essential-tremor/