The Chiari II malformation: lesions discovered within the fourth ventricle.

Structural lesions associated with the Chiari II malformation have been identified within the fourth ventricle in 8 patients. During the 42-month period encompassing the operations of the 7 patients treated surgically, only 9 other patients were explored without the discovery of some associated structural lesion. The patients ranged in age from 2 to 26 years. The following lesions were identified: glial or arachnoidal cysts (3 patients), glial or choroidal nodules (3 patients) and subependymoma (2 patients). These lesions were all situated in the roof of the fourth ventricle adjacent to or interspersed with the choroid plexus. Only the cystic lesions were identified by preoperative imaging. In only 1 case did the associated lesion, a 2-cm cyst, seem to contribute to the patient's clinical presentation. Structural lesions of the fourth ventricle associated with the Chiari II malformation are common in patients who are submitted to decompression. These lesions may be dysplasias of developmental origin, or they may be reactive lesions related to chronic compression and ischemia. They do not necessarily required biopsy or excision.