The Dandy-Walker complex is a continuum of posterior fossa cystic anomalies and consists of the Dandy-Walker malformation, vermian hypoplasia, and mega cisterna magna. Some authors utilize the term Dandy- Walker variant, which leads to confusion in some authors may use this to refer to a hypoplastic cerebellar vermis and a large cistern magna whereas others may use it to refer to the Dandy-Walker malformation in which one or more of the fourth ventricular outflow foramina are patent. It has therefore been suggested that the term “Dandy-Walker” variant be discarded. (Barkovich)

The imaging findings of Dandy-Walker malformation include hypoplasia or absence of the cerebellar vermis, hypoplasia of the cerebellar hemispheres, and enlargement of the posterior fossa and fourth ventricle, and are well-defined by prenatal ultrasound. The enlargement of the posterior fossa results in an elevation of the torcula that is nicely demonstrated on the sagittal images, as is associated hypoplasia of the brain stem. Prenatal ultrasound nicely demonstrates the findings of the Dandy-Walker malformation, but is limited in its ability to assess vermian hypoplasia. In addition Dandy- Walker malformation has a high incidence of other CNS anomalies, such as agenesis of the corpus callosum, holoprosencephaly, schizencephaly, and heterotopia. The presence of other anomalies has been shown to be associated with a worse prognosis, and these may be better evaluated by fetal MRI.

Vermian hypoplasia is identified by incomplete covering of the fourth ventricle, and a normally located tentorium on MR imaging. The outcome with children who have inferior vermian hypoplasia has been variable with some studies suggesting a favorable outcome and other studies revealing a more guarded prognosis.

Mega Cisterna Magna consists of an enlarged posterior fossa resulting from an enlarged cisterna magna. The cerebellar vermis and 4th ventricle are normal.