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Object Trapped fourth ventricle (TFV) is a rare late complication of postinfectious or posthemorrhagic hydrocephalus. This entity is distinct from a large fourth ventricle because TFV entails pressure in the fourth ventricle and posterior fossa due to abnormal inflow and outflow of CSF, causing significant symptoms and signs. As TFV is mostly found in children who were born prematurely and have cerebral palsy, diagnosis and treatment options are a true challenge. Methods Between February 1998 and February 2007, 12 children were treated for TFV in Dana Children's Hospital by posterior fossa craniotomy/craniectomy and opening of the TFV into the spinal subarachnoid space. The authors performed a retrospective analysis of relevant data, including pre- and postoperative clinical characteristics, surgical management, and outcome. Results Thirteen fenestrations of trapped fourth ventricles (FTFVs) were performed in 12 patients. In 6 patients with prominent arachnoid thickening, a stent was left from the opened fourth ventricle into the spinal subarachnoid space. One patient underwent a second FTFV 21 months after the initial procedure. No perioperative complications were encountered. All 12 patients (100%) showed clinical improvement after FTFV. Radiological improvement was seen in only 9 (75%) of the 12 cases. The follow-up period ranged from 2 to 9.5 years (mean 6.11 ± 2.3 years) after FTFV. Conclusions Fenestration of a TFV via craniotomy is a safe and effective option with a very good long-term outcome and low rate of morbidity.

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We conclude that a negative pressure gradient in the posterior fossa, relative to the supratentorial compartment, is the etiology of the chronic uncal herniation in our patient. Comparing the case reports in the literature with our case, we postulate that chronic uncal herniation is a complication of shunting of a posterior fossa fluid cavity in children, many of them with Dandy-Walker syndrome and/or other cerebellar cystic formations. The treatment priority at presentation should be to rule out shunt malfunction. In the event of association with clinical over-drainage syndrome, there may be a role for changing the shunt system into a more conservative drainage combination.