The Anatomy of the Vertebral Artery

The vertebral arteries work their way along both sides of the neck, connecting the subclavian arteries in the shoulder up to the cranium. Running through the gaps in the vertebrae (foramen), they access the posterior fossa, a small space at the base of the skull near the brainstem and cerebellum, as well as the occipital lobes of the brain. These arteries not only provide blood to the neck but, along with the carotid arteries, they’re the primary source of blood to the brain.

Because these arteries are so essential, serious problems can arise if they’re damaged due to blunt trauma or experience other disorders.

Anatomy

Working from the shoulders up, vertebral arteries begin in the subclavian arteries, which are the major sources of blood for each arm, working up each side of the neck vertebrae. Each artery, usually 3 to 5 millimeters in diameter, runs next to the internal carotid artery and behind the roots of the hypoglossal nerve through the transverse foramina—the holes in the vertebrae that allow vessels through.

Both vertebral arteries join to form the basilar artery at the junction between the medulla and pons regions of the brainstem, basically right where the brainstem and the brain itself meet. 

This artery is divided into four segments:

• V1 (preforaminal): The first, preforaminal segment arises in the subclavian artery of the shoulder and works itself behind the carotid artery (another major source of blood for the brain) to enter the lowest neck vertebrae (C6).
• V2 (foraminal): The next segment runs through the lower four vertebrae of the neck and is accompanied by nerves, veins (delivering blood back towards the heart), before twisting to form an L-shape to access the second neck vertebrae (C2).
• V3 (atlantic, extradural, or extraspinal): The artery emerges from the C2 vertebrae and sweeps across to pass through the foramen of the top neck vertebrae (C1). It then twists around to access the dura and arachnoid, the membranes that surround the brain. This curved shape ensures that the vessel can accommodate head and neck movement.
• V4 (intradural or intracranial): The final section of the vertebral artery ascends behind the hypoglossal nerve, joins up with its counterpart from the other side of the neck, to form the basilar artery at the base of the pons, the part of the brainstem that links the medulla oblongata and thalamus brain regions.

Throughout, spinal branches split off at the vertebral foramina to supply various parts of the vertebral bodies, the medulla brain region, and the posterior inferior cerebellar artery (PICA). The latter of these represents the largest branch and is one of the primary sources of blood to the cerebellum.

Anatomical Variations

Variations in the anatomy of the vertebral artery can be common and are congenital, meaning people are born with them. While they’re harmless in most cases, issues sometimes arise due to these abnormalities:

• Asymmetry: Irregular and insufficient development of arteries, a condition called “vertebral arterial hypoplasia,” can affect this region. In these cases, the vertebral artery either terminates into the PICA or is absent, which can affect the amount of blood that reaches the brain.
• Vertebral artery duplication: As the name implies, this a rare case when the vertebral artery has two sources rather than just one, affecting the course it takes through the neck. In most cases, though, this condition is asymptomatic.
• Vertebral artery fenestration: This is when the lumen, or inside of the artery, actually splits into two either inside or outside of the cranium. Also very rare—it’s only seen in between 0.23% and 1.95% of the population—it’s largely believed to be harmless, though some links have been found with certain kinds of brain aneurysms. 
• Variable origin: An abnormality at the point of the vertebral artery’s origin. In most cases, both arteries emerge on the left side, though this can also be seen on the right side or there may be variation in the shape of both sides.
• Variable orientation of ostium: This refers to differences in the positioning of the openings of the vertebral artery. In 47% of cases, these are seen in the cranial area, while in 44% of cases it’s seen in the rear of the artery.

Function

The vertebral artery delivers blood to the neck’s vertebrae, upper spinal column, the space around the outside of the skull. It also supplies blood to two very important regions of the brain: the posterior fossa and the occipital lobes.

The posterior fossa is the small space in the skull near the brain stem and cerebellum, which are associated with essential functions like breathing and keeping in balance. The occipital lobes are essential for visual processing and house the primary visual cortex.

Clinical Significance

Clearly an important part of the circulatory system, these arteries can face problems either due to injury or other endemic conditions in the body.

• Trauma: Falls, car accidents, and other issues can lead to injury and blunt trauma to the vertebral arteries and, often, damage to the neck’s vertebrae accompanies the issue. Depending on the injury, blood clotting could arise, or there may be internal bleeding. Though cases in which these vessels are damaged due to an accident are relatively rare, the disruption of proper blood flow can certainly impact neurological function.
• Cerebral ischemia: Arising due to damage in the vertebral artery—whether the result of injury or an endemic condition—cerebral ischemia is when not enough blood is getting to where it needs to go in the brain. The vertebral artery can become blocked off because of a blood clot, which can be potentially fatal and lead to stroke.
• Cervical artery dissection: A condition which arises spontaneously or as the result of trauma, where the walls of the artery are split, leading to internal bleeding and disruption of blood flow. Though more often occurring with carotid arteries (the other major ones supplying the brain through the neck), vertebral arteries can be impacted. Usually asymptomatic, in some cases cervical artery dissection can also lead to stroke.