Dandy Walker malformation consists of a group of anomalies where there is a posterior fossa cyst that communicates with the fourth ventricle, as well as abnormal development of the cerebellar vermis.

In its classical presentation, Dandy Walker malformation consists of the triad of:

1. Hypoplasia of the vermis and cephalad rotation of the vermian remnant.
2. Cystic dilatation of the fourth ventricle extending posteriorly. Usually, the cerebellar hemispheres are displaced anterolaterally, but with a normal size and morphology.
3. Enlarged posterior fossa with torcular-lambdoid inversion.