Surgical Management of Isolated Fourth Ventricular Hydrocephalus Associated with Injury to the Guillain-Mollaret Triangle.

Background The occurrence of isolated fourth ventricle and injury to the Guillain-Mollaret triangle in the setting of posterior fossa ependymoma represents a new association. In this case report, we discuss the clinical, theoretical, and therapeutic aspects of this problem. We describe a lateral transcerebellar trajectory and shunt valve configuration for safe fourth ventricle shunting in a patient with prior posterior fossa surgery. Case Description A 45-year-old woman underwent subtotal resection of a fourth ventricle ependymoma (World Health Organization grade III) followed by radiation therapy to control the residual tumor. Her course was complicated by a cerebral abscess and subsequent communicating hydrocephalus, for which she received a lateral ventriculoperitoneal shunt. After placement of the lateral ventricle shunt, there was a progressive increase in the volume of the fourth ventricle over the next 2 years, from 2.5 to 12.0 mL. She developed palatal myoclonus, hand incoordination, bilateral foot numbness, and progressive ataxia. Neuroimaging also revealed hypertrophic degeneration of the inferior olivary nuclei bilaterally. The isolated fourth ventricle was treated by a separate fourth ventriculoperitoneal shunt inserted through a lateral transcerebellar trajectory. A programmable variable pressure valve was implemented. Conclusions Development of an isolated fourth ventricle and injury to the Guillain-Mollaret triangle in the setting of fourth ventricular ependymoma is a newly encountered complication. Choice of treatment modality and timing of intervention should be carefully considered on a case-by-case basis. The data presented in this report may assist in the selection of surgical treatment for isolated fourth ventricle.