To evaluate the importance of intraoperative assessment in deep brain stimulation (DBS) of the ventral intermediate (Vim) nucleus of the thalamus.

Vim DBS is a well-established modality for treatment of tremor. It reduces the amplitude and decreases the regularity of tremor. The procedure is traditionally performed without general anesthesia, based on coordinates derived from stereotactic anatomic atlases, with the option of electrode repositioning based on intraoperative testing. Recent literature suggests equivalence in effectiveness and safety in procedures done with (with the patient asleep) and without general anesthesia (with the patient awake).

We retrospectively reviewed intraoperative documentation in Vim DBS procedures from 2004 to 2018 at the University of Cincinnati. All procedures were performed under local anesthesia.  Data on encounters in which electrode repositioning was needed to a) prevent stimulation-induced side effects, and/ or b) optimize efficacy was reviewed. Change in parallel trajectory was classified as major repositioning, whereas change in depth of electrode was classified as minor repositioning.

Vim DBS was performed in 124 patients with essential tremor, 11 with dystonic tremor, 2 with Parkinson’s disease, 4 with other forms of tremor.  A total of 145 encounters (each unilateral lead placement counted as one encounter) were reviewed, out of which 60.7% (88 encounters) involved repositioning of the electrode: 49 encounters involved minor repositioning, 10 required major repositioning and 29 involved both.  The reason for repositioning was identified as efficacy in 34 encounters, side effects in 26 encounters and both in 28 encounters. During these 88 encounters, the electrode was re-positioned a total of 207 times. In the 95 alterations done to ameliorate side effects, 34 lead to complete resolution and 28 showed partial improvement.

Electrode repositioning was required for the majority of Vim DBS cases for optimal tremor control and reduction of side effects, demonstrating the importance of intraoperative assessment.