Dilated fourth ventricle in Arnold-Chiari malformation type II: isolated fourth ventricle as sequelae of shunt?--Case report.

A female infant with Arnold-Chiari malformation type II developed cystic dilatation of the fourth ventricle at age 15 months. She received shunt emplacement into the dilated fourth ventricle to restore communication to the subarachnoid space and achieved improved symptoms and decreased ventricular size. Such dilatation is a typical feature of this malformation, and the isolation of the fourth ventricle is supposed to result from the cerebrospinal fluid shunting procedure. In this patient, the cause appeared to be external compression of the aqueduct by the beaking deformity of the mesencephalic spur, a characteristic of Arnold-Chiari malformation type II.