Anomalous vertebral artery not passing through the transverse foramen of the atlas

Ssually the vertebral artery （VA） arises from the first part of the subclavian artery and enters the transverse foramen of the C-6 vertebra （first segment） and then ascends through the transverse foramen from C-6 to the atlas （second segment）. In its third segment, the artery emerges on the superior surface of the atlas and curves horizontally over the lateral and posterior surfaces of the superior articular process of C-1 and then passes medially in front of the posterior atlanto-occipital membrane. In its fourth segment, the artery pierces the dura mater and arachnoid between the occipital bone and atlas then enters the cranial cavity through the foramen magnum. To identify and preserve the VA is vital in the surgery of the craniaocervical and cervical region. However, the variations of the VA complicate the procedures of VA identification. It is valuable to know the course of VA preoperatively. Some anatomical variations of the VA occur due to its abnormal development process.