Thalamic spatial variability and the surgical results of lesions placed with neurophysiologic control

One hundred stereotactic thalamotomies performed for upper extremity tremor were reviewed and the spatial positions of the anterior limit of ventralis posterior (VP), the thalamic-internal capsular boundary and somatotopic areas within VP plotted with respect to the midline and the place of the anterior comissure. Considerable variability was noted as to the location of these points. The surgical results following 97 of the 100 thalamotomies were evaluated at least three months after the procedure. Fifty-six of 70 patients with Parkinsonian tremor had complete abolition of their upper extremity tremor, as did 20 out of 27 patients with intention tremon. Plotting the spatial positions of the lesions in those cases in which the tremor was abolished and those cases in which tremor continued revealed little difference. However, in all of the cases in which upper extremity tremor was totally abolished, the lesion site was situated in ventralis lateralis anterior to the VP representation of the buccal commissure, thumb, or index finger. This study not only demonstrates the wide range of individual spatial variability in thalamic structures, and the necessity for neurophysiologic corroboration of radiologic information during stereotactic surgery if optimal results are to be obtained, but also suggests a somatotopic organization within ventralis lateralis.