Voice Tremor Response to Deep Brain Stimulation in Relation to Electrode Location in the Posterior Subthalamic Area

Abstract

Background: Deep brain stimulation of the motor thalamus or the posterior subthalamic area (PSA) shows promising results for patients with voice tremor, although only for about 50% of patients. There are indications that voice tremor requires more focused stimulation within the target area compared with hand tremor. The objective of the present study was to determine the most efficient location for reducing voice tremor within the PSA.

Methods: Thirty-seven patients with essential tremor were evaluated off stimulation and in a set of experimental conditions with unilateral stimulation at increasing amplitude levels. Two listeners performed blinded assessments of voice tremor from recordings of sustained vowel productions.

Results: Twenty-five patients (68%) had voice tremor. Unilateral stimulation reduced voice tremor for the majority of patients, and only 6 patients had poor outcomes. Contacts yielding efficient voice tremor reduction were deeper relative to the midcommissural point (MCP) and more posterior relative to the posterior tip of the subthalamic nucleus (pSTN) (z^MCP = -3.1, y^pSTN = -0.2) compared with poor contacts (z^MCP = -0.7, y^pSTN = 1.0). High-amplitude stimulation worsened voice tremor for 7 patients and induced voice tremor in 2 patients. Hand tremor improved to a greater extent than voice tremor, and improvements could be seen throughout the target area.

Conclusions: Our results indicate that efficient voice tremor reduction can be achieved by stimulating contacts located in the inferior part of the PSA, close or slightly posterior to the pSTN. We observed cases in which voice tremor was induced by high-amplitude stimulation.

Keywords: AC-PC, Anterior and posterior commissures; Caudal zona incerta; DBS, Deep brain stimulation; Deep brain stimulation; ET, Essential tremor; Electrode location; Essential tremor; ICC, Intraclass correlation coefficient; M, Mean; MCP, Mid-commissural point; PSA, Posterior subthalamic area; Posterior subthalamic area; SD, Standard deviation; Vim, Ventral intermediate nucleus of the thalamus; Voice tremor; cZi, Caudal zona incerta; pSTN, Posterior tail of the subthalamic nucleus.

Figure 1

T2 magnetic resonance imaging and schematic view of the target area in the axial plane, at the level of the maximal diameter of the red nucleus. The target point is visually identified in relation to the posterior tip of the subthalamic nucleus (STN) and the red nucleus. In relation to the anterior and posterior commissures, it will typically be located at a laterality of 12 mm, 7.5 mm behind, and 4 mm below the midcommissural point.

Figure 2

Distribution of efficient and poor contacts relative to the midcommissural point (MCP). Upper row: for voice tremor on the axial plane (A) and the sagittal plane (B). Lower row: for hand tremor on the axial plane (C) and the sagittal plane (D).

Figure 3

Distribution of efficient and poor contacts relative to the posterior tip of the subthalamic nucleus (pSTN). Upper row: for voice tremor on the axial plane (A) and the sagittal plane (B). Lower row: for hand tremor on the axial plane (C) and the sagittal plane (D).

Figure 4

Contact locations relative to the midcommissural point (MCP). (A and C) Axial plane. (B and D) Sagittal plane. Contacts are labeled according to their effect on voice tremor (A, B) and hand tremor (C, D).