Vertical displacement of stereotaxic target in the cat brain after burr hole production of the skull.

In order to observe the postoperative vertical displacement caused by various open functional stereotaxic procedures in the coordinate of brain target, we produced a similar surgical procedure in 14 cats. On preoperative computed tomography (CT) scans, certain well defined brain structures (the cortical surface of the brain and the floor of the third ventricle) were chosen as anatomical landmarks and their relationship to the skull were determined with the cat in a prone position. Thereafter, a burr hole was opened on the right side of the skull of each animal and a small dural incision was made. All animals were re-examined with CT scanning using the same technique and the distances to the skull were determined. There was a cortical descent in postoperative coronal CT images (range 1.2-3.6, mean 2.1 mm). On the other hand, the vertical coordinate of the floor of the third ventricle was found to be lower than it had been preoperatively, with an average of 0.6 mm lower (range -1.2(-)+0.3 mm). These results indicate that there was a good correlation between the degree of cortical descent and displacement of the vertical coordinate of the floor of the third ventricle (r = 0.70 and r = 0.69, respectively; P < 0.01). Our results also suggest that thalamic targets can be calculated by compensatory adjustment of the vertical coordinate by an additional distance of about half of the vertical descent. However, further experimental and clinical studies will be needed to determine the validity of this result in human beings.