Role of preoperative midsagittal magnetic resonance imaging in endoscopic third ventriculostomy.

We measured the distance between the infundibular recess and the tip of the basilar artery (BA) on midsagittal magnetic resonance (MR) images in 217 subjects (mean age, 54.7 years) without intracranial disorders and 8 patients with hydrocephalus treated neuroendoscopically. The mean distance measured in the 217 subjects without intracranial disorders was 10.5 +/- 2.3 mm. There were no significant differences in this distance among age groups. In 28 of these 217 subjects (12.9%), the tip of the BA had shifted anterior to the mamillary bodies on MR images. Twenty-six of these 28 subjects (93%) were 60 years of age or older. The mean distance between the infundibular recess and the tip of the BA was 12 +/- 3.7 mm in 8 patients with hydrocephalus. There was no significant difference between the subjects without intracranial disorders and the patients with hydrocephalus in this distance. In an elderly patient with aqueductal stenosis for whom the distance between the infundibular recess and the tip of the BA was 6 mm, the tip of the BA had shifted anterior to the mamillary bodies and indented the floor of the third ventricle on preoperative midsagittal MR images, and perforation of the floor of the third ventricle therefore had to be carefully performed in order to avoid injury of the BA. Preoperative midsagittal MR images can clearly reveal neurovascular structures beneath the floor of the third ventricle and increase the safety of endoscopic third ventriculostomy.