Expansion of central arachnoid pouches.

Three case of central arachnoid pouches or cysts which are freely in communication with the subarachnoid space and cause hydrocephalus are reported. One case died; the second recovered following transtentorial opening of a huge cystic cavity located between the tentorium and the cerebellum, which opened into the cisterna magna. The patient was well although mentally retarded 6 years later. The third case was treated with a ventriculoatrial shunt and has fully recovered. In these cases, the shape and situation of the pouches are consistent with a processes of upward movement of CSF. In the CSF waves, originating from constant variations of venous pressure, there is an obvious aetiological factor. Even if the hydrocephalus were primary and the pouch secondary, the pouch might contribute to the progression of the former by occasional compression of the aqueduct. This can be another example of the way hydrocephalus leads to its own exacerbation.