There are four defined posterior fossa malformations: Dandy Walker malformation, Dandy Walker variant, mega cisterna magna, and arachnoid cyst. Dandy Walker malformation is an enlarged posterior fossa with complete or partial agenesis of the cerebellar vermis in conjunction with the presence of a posterior fossa cyst that communicates with the fourth ventricle. Hydrocephalus is common and there are often supratentorial abnormalities. Dandy Walker variant is considered present when not all the criteria for the classic malformation are present. Most commonly, the cerebellar vermis is hypoplastic but present and the posterior fossa is not enlarged.

When an enlarged posterior fossa CSF cyst is present in the presence of a fully developed cerebellar vermis, a mega cisterna magna or arachnoid cyst are the two possibilities. If there is no mass effect on the cerebellum then it is considered a mega cisterna magna, whereas mass effect is considered to be from an arachnoid cyst.