Approach to the differential diagnosis of posterior fossa cysts:
- Arachnoid cyst can occur anywhere in the posterior fossa and often causes mass effect on an otherwise normal cerebellum and may cause scalloping of the inner table of the skull
- Dandy Walker malformation results from agenesis of the cerebellar vermis and a massive dilation of the fourth ventricle, resulting in an enlarged posterior fossa and superior displacement of the superior cerebellum
- Dandy Walker variant results from hypoplasia of the cerebellar vermis and a mild dilation of the fourth ventricle, resulting in a normal sized fossa and normal position of the superior cerebellum
- Mega cisterna magna is the mildest form of the Dandy Walker spectrum with an intact cerebellar vermis and usually manifests as a mild-moderate in size cystic structure posterior to the midline of the cerebellum
Pediatric Imaging 