[A rare case of trapped fourth ventricle: an unique symptom appeared after fourth ventriculo-peritoneal shunting]

We experienced a unique case of trapped fourth ventricle after shunting for post-meningitic hydrocephalus. A five-year-old infant was diagnosed as meningitis shortly after his birth, and secondarily suffered from hydrocephalus. He underwent lateral-ventriculo-peritoneal shunting, fourth-ventriculo-cisterna-magna shunting and so on, but bilateral abducens palsy appeared. The following head CT and MRI revealed "trapped fourth ventricle". Though there are several case reports of trapped fourth ventricle with abducens palsy, most of them followed enlargement of the fourth ventricle; nevertheless in our case, abducens palsy appeared when the fourth ventricle reduced in size and the symptom vanished when it enlarged. We thought that a traction force to the abducens nerve had occurred also in the condition of reduced fourth ventricle size, because there would have been a dense adhesion after meningitis in his subarachnoidal space. We tried to improve his symptom in one way or another by keeping the fourth ventricle in appropriate volume. His abducens palsy was controlled by switching the on-off valve between forth ventricle and peritoneum. We expect that a higher-pressure programmable shunt valve or a lower-flow-regulating shunt system be invented in order to cope with the cases like ours.