Surgical treatment of a rare congenital anomaly of the vertebral artery: case report and review of the literature. A case of an abnormal loop of the vertebral artery compressing both the cervicomedullary junction and the accessory nerve is reported. The embryological development of the vertebrobasilar system may explain this anatomical anomaly. The possibility of an abnormal location of the vertebral artery may complicate the lateral C1-C2 puncture for myelography. Only five similar cases have been reported previously, but none of them presented any clinical symptomatology, and surgical treatment was never required. The present patient was cured by microvascular decompression. The pathogenetic and surgical implications are discussed in light of the literature. 