[Disproportionately large communicating fourth ventricle--report of 2 cases].

A term of 'disproportionately large, communicating fourth ventricle' (DLCFV) was first proposed by in Harwood-Nash in 1980. It is somewhat different from the well known clinical entity of 'isolated or trapped fourth ventricle', because of apparent patency of aqueductal canal. Two cases of typical DLCFV encountered in our clinic were described. First patient was a 24 year old man in whom this condition developed following operations for lumber disc and second patient was 22 year old woman in whom the disease developed after subarachnoid hemorrhage. In both cases, main symptoms were attributable to hydrocephalus but three posterior fossa symptoms, nystagmus, Parinaud' sign and truncal ataxia were also characteristic. On the CT scan, the fourth ventricle was extraordinarily enlarged. Patency of the aqueductal canal was demonstrated by air study or Conray and Metrizamide ventriculography. On the other hand, occlusion was demonstrated or highly suspected in or near the foramina Magendie and Luschka. After a routine ventriculo-peritoneal shunt operation, the fourth ventricle decreased in size and the symptoms were immediately relieved. Plausible explanation for mechanism involved in occurrence of DLCFV were (1) occlusion process in or near the fourth ventricle outlets seems to be crucial in this pathologic condition. Collision of CSF pulse waves against the obstruction may yield a water hammer effect on the fourth ventricle. (2) abnormal weakness of the brain stem parenchyma around the fourth ventricle to CSF pressure may be another contributory factor.