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Anatomy of the Vertebrobasilar System

Understanding vertebrobasilar anatomy is essential for accurate diagnosis and management of posterior circulation ischemia and stroke. The brachiocephalic trunk (also called the innominate artery) ascends within the thorax and divides into the right common carotid and the right subclavian at the level of the sternoclavicular joint, while the left subclavian artery arises directly from the arch of the aorta. The subclavian artery crosses the dome of the cervical pleura below its summit, immediately superior to the subclavian vein.

The right vertebral artery arises from the subclavian artery and tracks posteriorly toward the cervical vertebrae. On the left side, the left vertebral artery is the first branch of the left subclavian artery. Less commonly, in 5% of the population, a nondominant left vertebral artery can arise directly from the aortic arch, between the origin of the left common carotid artery and the left subclavian artery (Figure 35-1).


Catheter is positioned in the innominate artery. This roadmap demonstrates the right common carotid most medially, followed by the vertebral artery arising from the superior-anterior aspect of the subclavian artery.

Vertebral artery usually arises from the posterior superior aspect of the subclavian artery; other anatomic variants include direct origin from the superior aspect, the anterior superior aspect, or the proximal subclavian artery.

The vertebral artery has four named segments (Figure 35-2). V1 begins at the takeoff from the subclavian. The V2 segment begins when the artery enters the first transverse foramen at the C6 cervical vertebra. V2 continues to ascend cephalad throughout the foramina and becomes V3 upon exiting C2. Medial and lateral segmental arteries arise from V2 at the level of each vertebral body. Medial segmental arteries can supply the cervical cord in a watershed distribution, and the lateral segmental arteries supply the soft tissue. Several anastomoses to this segment can become prominent in cases of cervical artery occlusive disease, with deep cervical artery and occipital artery anastomoses being the most common. The V3 segment is the last extradural segment. It passes through the C1 foramen and courses posteriorly in its groove along the C1 spinous process before looping around the occipital condyle. Posterior meningeal artery arises from this segment and supplies the dura around the foramen magnum. The V4 segment begins when the vessel pierces the dura. It passes through the foramen magnum and into the skull. The V4 segment gives rise to the posterior inferior cerebellar artery (PICA) and the anterior spinal artery (ASA). The left and right vertebral arteries meet at the lower border of the pons to form the basilar artery. It is not unusual for one vertebral artery (typically the left) to be larger and, therefore, the dominant supply to the basilar. The size of the ...

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