[On-off mechanism of shunt system due to slit ventricle (author's transl)].

We report two cases of high shunt dependency, which were first thought to be shunt independent arrested hydrocephalus. Though their shunt systems didn't seem to work, symptoms of rapid increasing intracranial pressure were observed after obstruction or replacement of shunt tube. Their ventricles looked so small like a slit on CT scan and PVG that the apex of the ventricular tube were easily obstructed by a ventricle wall. This is the reason why we misjudged them to be shunt independent arrested hydrocephalus. The cause of slit-like ventricles was overflow of CSF fluid due to the low pressure valve and the siphon effect. In general, after the shunt operation, most of the cases with thickening of cerebral mantle show the shunt dependency. Especially the cases showing rapid and marked thickening of the cerebral mantle are highly shunt dependent. Therefore, we must observe such cases carefully, in which the ventricle becomes small. Short interval follow-ups by CT scan after the shunt operation are quite necessary in order to observe the ventricle size. Easy and reliable judging method to know whether the shunt system is working or not is required to be developed.