Posterior fossa cystic malformations

1-Is composed of four disorders

-Dandy-Walker malformation.

-Dandy-Walker variant.

-Mega cisterna magna.

-Arachnoid cyst.

2-Dandy-Walker malformation:

-There is a posterior cranial fossa cyst communicating with fourth ventricle through the defect made of agenesis of the cerebellar vermis.

-There is hypo plastic cerebellum resulting in enlarged posterior fossa.

-Associated with supra tentorial ventricular dilatation and / or agenesis of corpus callosum.

3- Dandy-Walker variant:

-In some cases, the vermis is hypo plastic and not completely agenetic, this result in formation of variant and not malformation.

-Mild vermian hypo plasia with large cisterna magna.

-Normal fourth ventricle.

-Normal posterior fossa and brain stem.
-Supra tentorial hydrocephalus.

4-Mega cisterna magna.

-Is just an enlargement of cisterna magna in the posterior aspect of the posterior fossa.

 -Vermis and cerebellum is okay.
-No supra tentorial hydrocephalus.

4-Retro cerebellar arachnoid cyst:

-A cyst behind cerebellum.

-It will compress the cerebellum and the fourth ventricle causing supra tentorial hydrocephalus.

-Vermis and cerebellum is okay.