Object: Tremors, including its proximal component, are often refractory to standard thalamic surgery. In the 1960s the posterior part of the subthalamic white matter was reported to be a promising target in treating various forms of tremor, but was also found to be associated with adverse effects. Advances involving a less invasive method, that is, deep brain stimulation (DBS), has led to a reappraisal of this target.
Methods: Eight patients with severe essential tremor involving the proximal arm were treated using unilateral stimulation of the posterior part of the subthalamic white matter. The tentative target was situated in the area lateral to the red nucleus and posteromedial to the subthalamic nucleus. Macrostimulation was used to find the optimal site to suppress tremor. Through a quadripolar DBS lead, somatosensory evoked potentials (SSEPs) were recorded. Improvement of tremor was evaluated based on a modified clinical tremor rating scale. Anatomical locations of all contacts were assessed using stereotactic guidance and represented on the Schaltenbrand-Wahren atlas.
Conclusions: A characteristic diphasic pattern of SSEPs reaffirmed the electrophysiological endorsement of this target. Tremors, both proximal and distal, were remarkably improved in all patients. The rate of improvement, as indicated by the total tremor score, was a mean of 81%. Axial tremors in the legs and head were also improved. Most of the contacts associated with remarkable improvement were located in the posterior part of the subthalamic white matter (the zona incerta and prelemniscal radiation). Neither major complications nor neurological deterioration was observed. The authors concluded that DBS of the posterior part of the subthalamic white matter together with SSEP recording is a safe and effective method to ameliorate severe intractable tremors.