Neurophysiological evaluation of the optimum target in gamma thalamotomy: indirect evidence.

Gamma thalamotomy has been useful for the treatment of Parkinson's disease and other movement disorders, but it has disadvantages, such as a delayed clinical effect after irradiation and the inaccuracy of targeting because depth recording is not available. Therefore, we sought to determine the optimum target in gamma thalamotomy based on the results of conventional selective thalamotomy with reference to the AC-PC line. To obtain indirect support for the appropriateness of the optimum target, we performed depth recording around the region of the estimated optimum target based on the results of conventional thalamotomy. Four patients with tremor caused by Parkinson's disease or essential tremor were used as subjects after they gave their fully informed consent. The targets were determined as points 6-8 mm anterior to the posterior commissure, 4-6 mm dorsal to the level of the intercommissural line, and 15-17 mm lateral from the midline. Rhythmic discharge time-locked to tremor and/or kinesthetic neurons were found within the expected target area in all patients. Finally, in all cases, the tremor was abolished without complications after coagulative lesions were made with dual coagulation needles to cover the supposed Vim zone according to the depth recording. We considered that the target point determined in the same way as in gamma thalamotomy is suitable from an anatomophysiological perspective.