Hydrocephalus and large posterior fossa arachnoid cyst

There is marked hydrocephalus with dilatation of the lateral and third ventricles associated with volume reduction of the adjacent brain parenchyma and severe corpus callosal thinning. A large retrocerebellar CSF density cyst is present. 

There is marked hydrocephalus with dilatation of the lateral and third ventricles associated with volume reduction of the adjacent brain parenchyma and severe corpus callosal thinning. The septum pellucidum demonstrates areas of fenestration. No evidence of transependymal flow of CSF.

CSF flow through the cerebral aqueduct is present at VENC 7 (although no flow was seen at VENC 10 or 25), indicating that the aqueduct is patent albeit with slow flow. No evidence of aqueduct stenosis or a web.

Fourth ventricle has normal morphology and size. A large retrocerebellar arachnoid cyst is associated with upward displacement of the tentorium and straight sinus. The arachnoid cyst exerts mass effect on the underlying cerebellum which is displaced anteriorly and distorted, to a greater extent on the left.

The cerebellum itself appears completely formed, with no communication between the fourth ventricle and cyst. The cyst appears walled-off. No cerebellar tonsillar descent.

Conclusion: Severe longstanding hydrocephalus. There is slow flow of CSF through the cerebral aqueduct, with no evidence of aqueduct stenosis or obstruction. Large retrocerebellar arachnoid cyst remodeling the posterior fossa and distorting cerebellum.