[Cystic dilatation of the fourth ventricle--case report].

The patient, a 25-year-old male, was admitted with a 2-year history of double vision and recurrent suboccipital headache. Neurological findings on admission revealed nystagmus, double vision in the far field, bilateral abducense palsy, weakness in the bilateral legs, hypalgesia and hypesthesia under the L5 level, gait disturbance and hypotonia of the anal sphincter. Plain skull X-ray showed marked digital impression and disappearance of the posterior clinoid process and the dorsum sella. Myelography disclosed a filling defect dorsal to upper and midcervical cord. Intraventricular injection of metrizamide outline cystic dilatation and caudal migration of the fourth ventricle. Occipital craniectomy and laminectomy from C1 to C6 was performed. Operative findings showed thickened arachnoid membranes and cystic expansion of the fourth ventricle lying dorsal to the cervical cord. Caudal portion of the ventricle was removed, and V-P shunt was placed into the fourth ventricle to maintain decompression of the cystic part of the fourth ventricle. This patient is considered to represent a case of cystic dilatation (or ventricular diverticulum) of the fourth ventricle in Arnold-Chiari malformation. We stress that posterior decompression with V-P shunting procedure is recommended as the treatment of choice for such Arnold-Chiari type II malformation.