Posterior fossa ependymal cyst and atlantoaxial subluxation in a patient with Down syndrome: CT findings.

A 61-year-old woman with Down syndrome presented with progressive deterioration of gait over 9 months. Cranial CT without and with intravenous administration of contrast material demonstrated a posterior fossa cyst. The cyst did not communicate with the fourth ventricle or subarachnoid spaces as proven by CT following a metrizamide ventriculogram. Surgical fenestration of the cyst into the fourth ventricle was done. In addition, a moderate atlantoaxial subluxation with 2 mm movement from extension to flexion was present, which was thought not to be clinically significant, but which might require a spinal fusion at a future time. Ependymal cells were found as a result of a biopsy of the cyst wall.