Vertical displacement of the brain and the target area during open stereotaxic neurosurgery

During stereotaxic thalamotomies, we observed that the brain surface was sinking. The study was carried out to investigate to what extent the target area also was displaced and how this would affect the accuracy of the stereotaxic procedure. In 12 thalamotomies, with the patients operated on in the sitting position, we found that the cortical surface sank 0-9 (mean 5) mm during the operation. The vertical co-ordinate of the thalamic target was consequently adjusted per-operatively, and the electrodes were advanced an additional distance of 1-5.5 (mean 3.5) mm in an attempt to compensate for the assumed sinking of the target. This per-operative adjustment was based on the surgeon's experience and the results of macro-stimulation studies. The exact location of the thalamotomy lesion, and thereby the accuracy of the adjustment, was evaluated on 3 months postoperative CT scans. These showed that the intended target was hit with a sufficient degree of accuracy in all the patients, although the vertical co-ordinate had been slightly over-adjusted, as the center of the lesion on the average was located 1 mm below the intended location. Thus, if the vertical position had not been adjusted, the lesion would on the average have been located 2.5 mm too high compared with the intended target. Patients undergoing thalamotomy and other stereotaxic procedures, where a high degree of accuracy is needed, should be operated on in the sitting position. At the thalamic level, the vertical displacement of the target should be adjusted for by additional advancement of the stereotaxic probe. On average, this compensatory adjustment should be about half the per-operative sinking of the cortical surface.