We report a case of non communicating hydrocephalus due to membranous obstruction of Magendie's foramen. A 37-year-old woman presented with intracranial hypertension symptoms caused by the occlusion of Magendie's foramen by a membrane probably due to arachnoiditis. As far as the patient's past medical history is concerned, an Epstein-Barr virus infectious mononucleosis was described. Fundoscopic examination revealed bilateral papilledema. Brain magnetic resonance imaging demonstrated a significant ventricular dilatation of all ventricles and turbulent flow of cerebelospinal fluid (CSF) in the fourth ventricle as well as back flow of CSF through the Monro's foramen to the lateral ventricles. The patient underwent a suboccipital craniotomy with C1 laminectomy. An occlusion of Magendie's foramen by a thickened membrane was recognized and it was incised and removed. We confirm the existence of hydrocephalus caused by fourth ventricle outflow obstruction by a membrane. The nature of this rare entity is difficult to demonstrate because of the complex morphology of the fourth ventricle. Treatment with surgical exploration and incision of the thickened membrane proved to be a reliable method of treatment without the necessity of endoscopic third ventriculostomy or catheter placement.

Pseudomeningocele formation following posterior fossa surgery is a well-recognised complication, occurring in up to 33% of operated cases in some series. Ossification of a cranial pseudomeningocele is, however, an exceptionally rare event with only three prior reported cases. We present the unique case of a paediatric patient who developed rapid ossification of a giant occipital pseudomeningocele following posterior fossa surgery. An 8-year-old female patient underwent a midline posterior fossa craniotomy for resection of an exophytic brainstem low-grade glioma. Post-surgery, the patient developed pan-ventricular hydrocephalus and a large occipital pseudomeningocele, which initially increased in size despite a successful endoscopic third ventriculostomy (ETV) being performed. At approximately 3 months post-surgery, reduction of the pseudomeningocele was observed with associated prominent ossification of the pseudomeningocele wall on computed tomography (CT) imaging. Surgical excision was subsequently undertaken, and intra-operatively, a large ossified pseudomeningocele was found. Follow-up MRI 1 month later demonstrated almost complete resolution of the pseudomeningocele with an associated reduction in the degree of pan-ventricular ventriculomegaly. This case highlights that ossification of even giant pseudomeningoceles can occur over a time period of just a few months and clinicians should consider ossification whenever a change in size or consistency of a post-operative pseudomeningocele is encountered.

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