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Mystery Case: Acute hydrocephalus caused by radiographically occult fourth ventricular outlet obstruction

A 19-year-old woman with no history of CNS inflammatory pathology or hemorrhage presented with 5 days of diplopia and headache. Examination revealed papilledema and bilateral sixth nerve palsies. Imaging demonstrated panventricular enlargement and marked 4th ventricular dilation (figure 1). Cine MRI revealed turbulent fourth ventricle CSF flow suggesting outflow obstruction, which was confirmed with contrast ventriculography. A suboccipital craniotomy was then performed, which revealed an arachnoid web (figure 2). Membranous occlusion of the fourth ventricular outlet is a rare cause of obstructive hydrocephalus usually associated with a history of inflammatory conditions or hemorrhage.1 A small number of idiopathic cases have been reported.2