Congenital defects of the posterior arch of the atlas: a report of seven cases including an affected mother and son.

PURPOSE To describe our experience with congenital anomalies of the posterior arch of the atlas, with a review and classification of these defects and a note on their clinical significance. METHODS We report six children and one adult, the mother of one of the children, with an anomalous posterior arch of the atlas. The diagnosis was made on lateral films of the neck. Three patients also had axial CT of the cervical spine. RESULTS The anomalies encountered in the seven patients were absence of the posterior arch of the atlas (four patients), bilateral clefts (two patients), and unilateral cleft (one patient). In three patients the anomaly was discovered as an incidental asymptomatic finding; three other patients presented with transient neck pain or transient neurologic symptoms after head and neck trauma, and one patient (an adult woman) described neck symptoms of 1-year duration. CONCLUSIONS On the basis of these seven cases we conclude that congenital defects of the posterior arch of the atlas may be discovered as incidental asymptomatic findings, but symptoms occurring after trauma to the head and neck or spontaneously also may be encountered.