There is a congenital posterior fossa malformation. The superior cerebellar peduncles are thick and horizontal. They do not decussate in the dorsal midbrain (see DTI). There is a "molar-tooth" morphology of the midbrain on axial imaging and a classic bat-wing appearance of the fourth ventricle. The vermis is high-riding and there is a coronal cleft further inferiorly. The midbrain is thinned and elongated, and the brachium pontis slightly small.
There is a cyst located above the cerebellum within the superior vermian cistern. This winds around the left side of the tentorium to abut and displace the posterior hippocampus and left temporal horn. Mass effect is pronounced. This cyst is not of CSF intensity being of lower signal than CSF on T2 and slighlty hyperintense on FLAIR and T1 (relative to CSF). Further cyst locules are seen more anteriorly within the posterosuperior aspect of the third ventricle (suprapineal recess)-these do follow CSF intensity. There are also cyst locules within the prepontine cistern. A small membrane is also seen inferiorly in the posterior fossa. There is no evidence of obstruction of CSF flow however. No gross hydrocephalus although the fourth ventricle is slightly enlarged.
The lateral ventricles are irregular in outline but there is no obvious loss of white matter bulk or signal change in the medial cerebellar hemispheres. The corpus callosum is present but shortened in AP diameter. Its splenium is displaced superiorly by the cysts. No obvious cortical malformation seen.]]>