History: 26-year-old man with hydrocephalus since birth
Findings: Axial CT scans without contrast demonstrate agenesis of the vermis and hyposplasia of the cerebellum. A large posterior fossa cyst is present that is in direct communication with the fourth ventricle.
The cerebellar vermis (Latin for worm) is located in the medial, cortico-nuclear zone of the cerebellum, residing in the posterior fossa of the cranium. The primary fissure in the vermis curves ventrolaterally to the superior surface of the cerebellum, dividing it into anterior and posterior lobes.
Discussion: The Dandy-Walker syndrome, accounting for approximately 15% of cystic posterior fossa masses, consists of dysgenesis of the vermis, cystic dilatation of the fourth ventricle, absence of the foramina of Magendie and Luschka, absence of communication between the subarachnoid space and the fourth ventricle, and enlargement of the posterior fossa. The cerebellar hemispheres are hypoplastic, and dysgenesis of the vermis ranges from hypoplasia to complete agenesis. Obstructive hydrocephalus is present in approximately 80% of patients. An enlarged posterior fossa with the torcular herophili (where the sinuses come together) lying far above the lambdoid suture (torcular-lambdoid inversion) is a classic finding on plain films. Associated central nervous system anomalies and extracranial abnormalities are present in approximately 70 and 25% of patients, respectively. The Dandy-Walker variant, accounting for 30% of cystic posterior fossa masses, consists of mild vermin hypoplasia, slight to moderate fourth ventricle enlargement, and a normal or only mildly enlarged posterior fossa.