BACKGROUND AND IMPORTANCECerebral ventricular noncommunication has been described in the setting of infection and acutely in the setting of intracranial hemorrhage. We describe the first adult case series of individuals who developed delayed isolated fourth ventricles after rupture of intracranial posterior circulation aneurysms and define treatment modality.CLINICAL PRESENTATIONA retrospective review was performed of all patients with aneurysms treated at a single institution from 2005 to 2009. Both microsurgical obliteration and endovascular cases were queried. Of 1044 aneurysms treated in this period, 3 patients were identified who required fourth ventricular shunting, for the treatment of the isolated ventricle. All 3 patients underwent microsurgical clip obliteration of their aneurysms and had subsequent frontal approach ventriculoperitoneal cerebrospinal fluid diversion. These patients had no evidence of infection of the cerebrospinal fluid as measured by serial cultures. Subsequently, all 3 patients presented in a delayed fashion with symptoms attributable to a dilated fourth ventricle and syringomyelia or syringobulbia. Either exploration or percutaneous tapping confirmed the function of the supratentorial shunt. These patients then underwent fourth ventriculoperitoneal cerebrospinal fluid diversion by the use of a low-pressure shunt system. The symptoms attributable to the isolated fourth ventricle resolved rapidly in all 3 patients after shunting. This clinical improvement correlated with the fourth ventricular size.CONCLUSIONIsolated fourth ventricle, in an adult, is a rare phenomenon associated with intracranial posterior circulation aneurysm rupture treated with microsurgical clip obliteration. Fourth ventriculoperitoneal cerebrospinal fluid diversion is effective at resolving the symptoms attributed to the trapped ventricle and associated syrinx.