Specific Imaging Findings

Ventricular diverticula are focal dilations of the ventricular system resulting from severe chronic obstructive hydrocephalus. Despite their different origins, they are typically unilateral and similarly located in the quadrigeminal and superior cerebellar cisterns. Most of them arise from the inferomedial atrium of the lateral ventricles and frequently extend into the posterior fossa through the tentorial incisura, displacing the brain stem, vermis, and fourth ventricle. Detection of an ostium of a diverticulum or a communication between the cyst and the ventricular system is the key for diagnosis and is usually best seen on sagittal or coronal MRI. The communication may be confirmed with a phase-contrast study demonstrating uninterrupted CSF flow. Lateral ventricular diverticula may be identified and distinguished from other cystic lesions when all of the following signs are present on CT: (1) marked unilateral atrial dilation; (2) focal dehiscence of the medial atrial wall; (3) ipsilateral shortening of the tentorial band on axial images; (4) focal defect in the tentorial band in coronal section; (5) draping of the medial atrial wall over the free margin of tentorium, with continuity of CSF density around the edge of tentorium; (6) bowing of the crus of fornix; (7) separation of fornix from splenium, with visualization of the hernia ostium; (8) asymmetrical position of the choroid plexi (which attach to and define the lateral borders of the fornices); (9) contralateral displacement of the internal cerebral veins; and (10) septa separating diverticulum from the third ventricle.