6.1.11\ Third Ventriculocisternostomy

6.1.11.1\ Discussion

Endoscopic fenestration of the anterior floor of the third ventricle (Liliequist’s membrane) creates an alternative route for cerebrospinal fluid into the subarachnoid space via the prepontine cistern. This procedure bypasses obstruction at the level of the Sylvian aqueduct and restores cerebrospinal fluid flow out of the ventricular system. Third ventriculocisternostomy is a minimally invasive alternative to shunt implantation and is indicated for patients with aqueductal stenosis, obstructive tumors, and obstructive cysts.

a

c

The hemodynamic changes induced by third ventriculocisternostomy can be detected on MRI (Fig. 6.23). In particular, the sagittal phase-­ contrast cine sequence is the gold standard for evaluating cerebrospinal fluid flow dynamics. Following successful third ventriculostomy, a flame-shaped jet of high signal (flow) intensity across the floor of the third ventricle is characteristic. A jet of cerebrospinal fluid can also manifest on T2-weighted sequences. With highresolution heavily T2-weighted cisternogramtype sequences, the normal Liliequist’s membrane­ can be identified. Following third ventriculostomy, the membrane will appear disrupted.

b

Fig. 6.23  Third ventriculocisternostomy. Preoperative sagittal FIESTA (a) shows enlargement of the third and lateral ventricles. Postoperative sagittal FIESTA (b) shows a defect in Liliequist’s membrane (encircled) and

decrease in size of the third and lateral ventricles. The postoperative sagittal phase-contrast MRI (c) shows the a strong jet of cerebrospinal fluid (arrow) across the third ventriculostomy

6.1.12\ Endoscopic Septum

Pellucidum

and ­Intraventricular Cyst

Fenestration

6.1.12.1\ Discussion

Endoscopic fenestration can be performed to treat symptomatic septum pellucidum cysts. The procedure consists of creating a burr hole, introducing a cannula and endoscope into the lateral ventricles, and coagulating the septum pellucidum to allow communication between the right

a

and left lateral ventricles. Postoperative MRI can show the disrupted membranes of the septum pellucidum and decrease in size of the cyst after successful fenestration (Fig. 6.24). Arachnoid and porencephalic cysts can be successfully fenestrated to create a communication with the ventricular system or cisterns, averting the need for a shunt catheter. Rarely, tumor cysts are decompressed into the ventricular system as a last resort (Fig. 6.25). This approach is generally avoided due to the risk of subsequent hydrocephalus secondary to malabsorption from the cyst contents.

b

Fig. 6.24  Endoscopic septum pellucidum cyst fenestration. Preoperative T2-weighted MRI (a) shows a dilated cavum septum pellucidum cyst. Postoperative T2-weighted

MRI (b) shows bilateral defects in the septum pellucidum (arrows) resulting in decompression of the cyst