Air-ventriculography provokes an anterior displacement of the third ventricle during functional stereotactic procedures.

The width of the third ventricle, the length of the anterior commissure-posterior commissure line (AC-PC line), the spatial position of the midplane of the third ventricle, and the co-ordinates of the AC, the PC, and of 17 brain targets in the thalamus, hypothalamus and pallidum, were assessed on a pre-operative stereotactic computed-tomography (CT) study and compared to measurements on intra-operative air-ventriculography, using a non-invasive relocatable stereotactic frame. There were no significant differences in the length of the AC-PC line, in the position of the midsagittal plane of the third ventricle, or in the vertical or lateral co-ordinates of the AC, the PC and the cerebral targets, between measurements on CT and on air-ventriculography. However, the width of the third ventricle was significantly larger, and the spatial positions of both AC and PC were significantly more anterior on air-ventriculography than on the CT study. This anterior dislocation of the commissures was presumably due to the insufflation of air into the ventricles of patients being in the supine position during surgery.