To the Editor:
I read with interest the article by Sandvik et al.1 The authors performed a retrospective nonrandomized study on 2 groups of patients (17 ventral intermediate nucleus [Vim]/19 posterior subthalamic area [PSA]) undergoing deep brain stimulation for Essential Tremor. The Vim was the target for patients undergoing surgery from 1997 to 2004. The PSA was the target for those patients operated on from 2004 to 2007. The effect of optimal deep brain stimulation on tremor was judged by a >90% reduction in tremor. The contacts yielding the best results were in the PSA (Zi/Raprl/Forels field H) in 42.5% and in Vim in 17.5%. This was determined by postoperative computed tomography coregistered to the Schaltenbrand and Wahren atlas. The averaged co-ordinates were given for the >90% reduction of tremor in the PSA and Vim. Specifically, the latter target's y coordinate behind the midpoint of the intercommissural line seems somewhat anterior The authors offered the belief (and justifiably so) that the clinical effect may be generated by an overlap of stimulation of more than 1 electrode contact.
The authors in their discussion state there are few studies published on the relationship of efficacy to electrode location, but they failed to give credit to the most seminal article on this technology by Velasco et al.2 Those authors were acknowledged in 1 nondefinitive sentence with their choice of Raprl as the optimal target. The Velasco article was published in 1972 in the Journal of Neurosurgery. The authors used cessation of tremor with passage of the electrode into the target as the gold standard for the efficacy of location. This phenomenon occurred in 30 of the 90 patients. Forty-seven of these 90 patients had parkinsonian tremor, and 43 patients had tremor of attitude. The intercommissural line was divided into 10 equal parts whatever its length. One tenth of the line was used as a unit not only in the x coordinate, but also in the y and z. This proportional division of the reference lines was traced on the ventriculogram and coregistered to the Schaltenbrand-Bailey atlas, revealing consistent points where tremor was stopped, or sensory and motor responses were obtained in all 90 cases. Although the tremorgenic portion of the corticobasal ganglia circuit is quite extensive, a very focal area or funnel of this circuit can be found in the posterior subthalamic region. This work has remained a foundation for the anatomy and pathophysiology of the tremorgenic area over the past 40 years. All neurosurgeons performing movement disorder surgery would be well advised to read this article.
The author has no personal financial or institutional interests in any drugs, materials, or devices described in this correspondence.
1. Sandvik U, Koskinen LO, Lundquist A, Blomstedt P. Thalamic and subthalamic deep brain stimulation for essential tremor: where is the optimal target? Neurosurgery. 2012;70(4):840–846.
2. Velasco FC, Molina-Negro P, Bertrand C, Hardy J. Further definition of the subthalamic target for arrest of tremor. J Neurosurg. 1972;36(2):184–191.