Computed tomographic signs of the Chiari II malformation. III: Ventricles and cisterns.

In patients with Chiari II malformations, the fourth ventricle is usually not visualized or appears small; the third ventricle is relatively small, typically has a large massa intermedia, and only occasionally exhibits parasellar and/or posterior third ventricular diverticula. The lateral ventricles are usually asymmetrically dilated, show medial pointing of the floor of the body near the foramen of Monro, flattening of the superolateral angles, and frequent absence of the septum pellucidum. Prior to and after shunting, the interhemispheric fissure may be either obliterated, or widely open with serrations corresponding to the interdigitated gyri of the cerebral hemispheres. Prominent confluent cisterns at the hind end of the third ventricle in patients with ventricular collapse may represent the CT equivalent of the dilated pericallosal, ambient and retropulvinaric cisterns seen in patients with hydrocephalus and poor ventricular filling at pneumography.