Anatomical variations of the V2 segment of the vertebral artery

OBJECTIVE: Our goal was to evaluate the incidence of anatomic variations of the V2 segment (from its entrance into the transverse canal to C2) of the vertebral artery. Ignoring such variations during anterior or lateral approach to the cervical spine can lead to inadvertent injury and potentially serious complications. METHODS: We studied the course of 500 vertebral arteries on 200 magnetic resonance imaging and 50 contrast-enhanced computed tomographic scans. RESULTS: The vertebral artery entered the C6 transverse foramen in 93.0% of all specimens. An abnormal level of entrance was observed in 7.0% of specimens (35 courses), with a level of entrance into the C3, C4, C5, or C7 transverse foramen, respectively, in 0.2% (n = 1; 2.9% of all anomalies), 1.0% (n = 5; 14.3% of all anomalies), 5.0% (n = 25; 71.4% of all anomalies), and 0.8% (n = 4; 11.4% of all anomalies) of all specimens. Seventeen (48.6%) abnormalities were right-sided and 18 (51.4%) were left-sided. Thirty-one out of 250 patients (12.4%) had a unilateral anomaly and two had a bilateral anomaly (0.8%). In cases of abnormal entrance into the transverse foramen on computed tomographic images (n = 6), the area of the unfilled transverse foramens was significantly smaller than the contralateral filled foramen (P < 0.0001) and was significantly smaller than the filled foramen of all patients at the same level (P < 0.0001). In five patients (2.0%), the vertebral artery formed a medial loop either into an unusually large transverse foramen whose internal border was medial to the uncovertebral joint or into the intervertebral foramen. CONCLUSION: The incidence of anatomic variations of the vertebral artery V2 segment is high. Potentially dangerous conditions can be detected on preoperative imaging.