Background: The ventro-lateral thalamus is the stereotactic target of choice for severe intention tremor. Nevertheless, the optimal target area has remained controversial, and targeting of the subthalamic area has been suggested to be superior.
Patients and methods: Eleven patients with disabling intention tremor of different etiology (essential tremor (n = 8), multiple sclerosis (n = 2) and one with, spinocerebellar ataxia) were implanted bilaterally with DBS electrodes targeted to the ventro-lateral thalamus using micro-recording and micro-stimulation. Among five tracks explored in parallel optimal tracks were chosen for permanent electrode implantation. Postoperative tremor suppression elicited by individual electrode contacts was quantified using a lateralised tremor rating scale at least 3 months (in most patients >1 year) after implantation. The position of electrode contacts was determined retrospectively from stereotactic X-ray exams and by correlation of pre- and postoperative MRI.
Results: In all patients, DBS suppressed intention tremor markedly. On average, tremor on the left and right side of the body was improved by 68% (+/-19; standard deviation) and 73% (+/-21), respectively. In most patients, distal electrode contacts located in the subthalamic area proved to be more effective than proximal contacts in the ventro-lateral thalamus. In stereotactic coordinates, the optimal site was located 12.7 mm (+/-1.4; mean +/- standard deviation) lateral, 7.0 (+/-1.6) mm posterior, and 1.5 (+/-2.0) mm ventral to the mid-commissural point. In general, the best contacts could be selected for permanent stimulation. Nevertheless, in some instances, more proximal contacts had to be chosen because of adverse effects (paraesthesiae, dysarthria, gait ataxia) which were more pronounced with bilateral stimulation resulting in slightly less tremor suppression on the left and right side of body (63 +/- 18 and 68 +/- 19%, respectively).
Conclusion: Direct comparison of different stimulation sites in individual patients revealed that DBS in the subthalamic area is more effective in suppressing pharmacoresistant intention tremor than the ventro-lateral thalamus proper. Anatomical structures possibly involved in tremor suppression include cerebello-thalamic projections, the prelemniscal radiation, and the zona incerta.