An Acute Discovery of Fourth Ventricle Outflow Obstruction in Previously Idiopathic Chronic Hydrocephalus: A Case Report.

Hydrocephalus is the disruption of cerebral spinal fluid homeostasis, representing a common neurosurgical illness. Up to 10% have no identifiable cause, with fourth ventricle outflow obstruction (FVOO) being an extremely rare subtype. A 31-year-old male with a history of idiopathic hydrocephalus for over 10 years with the need for a ventriculoperitoneal shunt had shown progressively enlarged tetraventricular ventriculomegaly. There was no history of meningitis, traumatic injuries, or surgeries besides the shunt placement. Given clinical worsening with diplopia and gait impairment and imaging suggestive of shunt failure, the patient underwent surgery for a replacement. In the following days, the patient worsened and became lethargic. Bilateral frontal external ventricular drainages were placed for acute hydrocephalus due to tetraventricular haemorrhage, although ventricular enlargement kept progressing. Brain magnetic resonance imaging (MRI) showed a membrane in the Magendie foramen region with an apparently unobstructed aqueduct and no transependymal edema. The surgical team opted for a suboccipital craniectomy for membrane fenestration. Post-operative computed tomography (CT) scans showed a stark reduction of the ventricles. Within one week, the patient had an almost complete recovery and was discharged. One year later, the patient is asymptomatic with no need for acetazolamide or shunt. Hydrocephalus is a rather common neurosurgical pathology presenting serious comorbidity. It is amenable to very effective and potentially curative treatment, and therefore efforts should be made to find its cause. Here, we reported a rather challenging referral of an acute decompensation of a chronic hydrocephalus with no previous cause identified. Ultimately, in the midst of this chronically altered cerebrospinal fluid (CSF) dynamics, the use of advanced high-resolution imaging proved crucial and allowed adequate treatment for the true cause of the hydrocephalus. We hope to raise awareness of the possible existence of membranes in the foramen of Magendie and its unusual presentation.