Computed tomography of the trapped fourth ventricle.

A dilated fourth ventricle due to outlet obstruction is a clinical-radiologic entity with symptoms similar to those of a posterior fossa space-occupying lesion. Computed tomography reveals cystic dilatation of the fourth ventricle and hydrocephalus supratentorially. Frequently the symptoms resolve completely and the fourth ventricle returns to normal following lateral ventricular shunting. If there is actual obliteration of the aqueduct, the fourth ventricle must be shunted directly. While the exact mechanism by which the fourth ventricle becomes trapped is not fully known, outlet obstruction must be present. It is most frequently related to previous episodes of meningitis or subarachnoid hemorrhage. In a review of 48 cases of an abnormally enlarged fourth ventricle demonstrated on CT, six were found to have a trapped fourth ventricle. Careful attention to densities and configurations on CT allows its differentiation from other lesions such as cystic tumors and cysts.