Acute distortion of the anatomy of the third ventricle during third ventriculostomy. Report of four cases.

The authors report on four third ventriculostomy procedures in which upward ballooning of the third ventricular floor occurred immediately after perforation of the floor and withdrawal of a Fogarty catheter. The floor herniated into the third ventricle, hindering the endoscopic view. Preoperative magnetic resonance imaging demonstrated a similar anatomy in all four cases, consisting of hydrocephalus, extreme dilation of the third ventricle, and disappearance of the interpeduncular cistern due to a very thin, membranous floor of the third ventricle, which herniated downward, draping over the basilar artery.