Why do central arachnoid pouches expand?

Three cases of progressive hydrocephalus, two occurring in infants and one in a 12 year old girl who also exhibited precocious puberty, were found to be associated with large arachnoid pouches originating within the posterior fossa. The pathogenesis of such cysts is discussed with special reference to the possibility that their progressive distension results from CSF pulsations of venous origin. Both a direct method of treatment (opening the cyst into the adjacent subarachnoid space) and an indirect one (insertion of a ventriculo-atrial shunt) have been used with success.