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["Disproportionately large, communicating fourth ventricle" due to membranous obstruction of Magendie's foramen].

A case of disproportionately large, communicating fourth ventricle (DLCFV) with the entire ventricular system dilated symmetrically due to membranous obstruction of Magendie's foramen is reported. A 20-year-old female complained of headache and nausea. Slight papilledema was found in both eyes. CT scan and MRI showed that the entire ventricular system was dilated symmetrically, but we could not locate the cause of this disease, for example, tumor, arachnoid cyst, cerebellar malformation and so on. Although cinemode-MRI showed CSF pulsation as if CSF flowed through Magendie's foramen, we were not convinced that CSF flowed through it. We speculated that something obstructed CSF flow at Magendie's or Luschka's foramen, so surgery was performed by suboccipital craniectomy. As expected, a membranous obstruction was found at Magendie's foramen. After excision of the membrane, all symptoms improved. Postoperative cinemode-MRI clearly demonstrated CSF flow through Magendie's foramen. Ventriculoperitoneal shunting is generally effective for DLCFV, but we do not consider it the optimal treatment. The cause of DLCFV must be confirmed by suboccipital craniectomy if evidence of NPH, mass lesion or cerebellar malformation cannot be found anywhere especially around Magendie's foramen.