{{ ::superior_vermian_cistern.jpg?400|}}Massive ventricular dilatation causes stretching and dehiscence of the [[fornix]] with formation of unilateral or bilateral pial pulsion diverticula of the inferior medial wall of the [[atrium]]. Enlargement of the pial pouch creates a dramatic sub[[arachnoid cyst]] that may herniate downward through the incisura into the lateral mesencephalic, precentral cerebellar, and [[superior vermian cistern]]s where it displaces the brain stem, vermis, and fourth ventricle.