Anomalous foramen transversarium of C2 simulating erosion of bone

A case is reported of a patient with nocturnal myoclonus and weakness of the left arm in whom a round defect was seen in the arch of C2 on the left side. Vertebral angiogram revealed that this was caused by an anomalous course of the vertebral artery and not by an aneurysm. A similar location of this foramen was found in nine of 100 cases reviewed.