Precise stimulation location optimizes speech outcomes in essential tremor

Background Deep brain stimulation for essential arm tremor is often complicated by dysarthria and persistent voice tremor. Objective To determine the relationship of stimulation location to speech outcomes following bilateral thalamic deep brain stimulation (DBS) for essential tremor (ET). Methods Eighteen patients undergoing bilateral DBS for ET were prospectively studied. Speech pathologists grouped patients by final speech outcome (normal speech, voice tremor, or dysarthria). Locations of the active leads were calculated by normalizing the segmented thalamic volumes to those in the Morel atlas. Stimulation volumes within thalamic nuclei, error distances from target, and measures of accuracy were calculated and differences in measures between outcome groups tested. Results At optimal stimulation, 8 patients had normal speech, 6 had voice tremor, and 4 had mild dysarthria. Stimulation volumes were statistically concentrated within the ventral lateral posterior nucleus (VLp). The percentage of stimulation volume outside the VLp was higher in patients with dysarthria (60% vs. 24%, p = 0.02) or voice tremor (55% vs. 24%, p = 0.03) compared to patients with normal speech outcomes. The error distance from the center of VLp was greater for patients with dysarthria than those with normal speech (12.6 vs. 7.6 mm, p = 0.02). Electrodes with lower efficiency for VLp stimulation were more frequent with poor speech outcomes and in patients with persistent voice tremor. Conclusions Following bilateral DBS for ET, 22% of patients develop a non-disabling dysarthria. Optimal speech outcomes were achieved in 44% of patients and correlated with precise stimulation location within and not outside of the VLp.