The Anatomy, Function, and Significance of the Basilar Artery

A major branch of the circulatory system providing blood to the brain and central nervous system, the basilar artery arises at the meeting of the vertebral arteries at the base of the skull, where the head meets the neck. It ensures that oxygen and nutrients are delivered to several essential regions, including the cerebellum, brainstem, and occipital lobes. Disruptions or damage to it can lead to an aneurysm, stroke, as well as a number of other neurological conditions.

Anatomy

The basilar artery represents the rear (posterior) side of the circle of Willis, an artery ring that supplies blood to various parts of the brain as well as the posterior cranial fossa—the floor of the cranium. This artery originates where the neck’s two vertebral arteries meet right at an area called the medullo-pontine junction. This is where the medulla and pons—two major parts of the brainstem—meet at the base of the skull.

The artery travels inside the skull along the central groove of the pons to the midbrain in the pontine cistern, a space below this structure. It runs adjacent to the abducens nerve and the origin of the oculomotor nerve, which regulates aspects of eye movement.

The basilar artery terminates into two branches called the posterior cerebral arteries, which supply the posterior portion of the cerebral hemispheres.

Along its course, the basilar artery breaks off into several important branches.

• Anterior Inferior Cerebellar Artery (AICA): One of three major sources of blood for the cerebellum, the AICA arises near the medullo-pontine junction.
• Labyrinthine Artery: In some cases, this artery arises directly from the basilar artery, though it’s also been observed to branch off the AICA. Also known as the auditory artery, this is a long, thin artery that delivers blood to structures in the inner ear.
• Pontine Arteries: These are a series of smaller vessels that branch off the basilar artery to supply the pons.
• Superior Cerebellar Artery: The final major branch before the basilar artery terminates, this vessel works its way upwards to help supply the cerebellum—a brain region that regulates motor coordination and balance.

Anatomical Variations

There are a number of variations to the anatomy of the basilar artery. These include:

• Carotid-Basilar Artery Anastomosis: This rare case, occurring in less than 0.5% of cases, is characterized by there being connections between the basilar and carotid artery (which runs from the neck vertebrae to the skull). This junction can occur in several different locations along the course of these vessels.
• Fenestrated Basilar Artery: A condition in which the artery has openings, leading to parts of it being duplicated to compensate. This disruption in blood flow might increase risk of aneurysm, where there is bulging or ballooning in a section of the vessel.
• Differential Origin of the Labyrinthine Artery: As noted above, in about 15% of cases, the labyrinthine artery arises directly from the basilar artery, rather than the AICA.  
• Hypoplastic Basilar Artery: Often associated with carotid-basilar artery anastomosis, this is a case where there is an incomplete development of the basilar artery.
• Differential Origin of Posterior Inferior Cerebellar Artery: Usually one of the terminal branches of the neck’s vertebral artery, in about 10% of cases, doctors have observed this artery arising from the basilar artery.

Function

Primarily, the basilar artery delivers blood to several important brain and brainstem regions.

It also delivers to the cerebellum, which regulates balance, posture, coordination, and is associated with speech.

Furthermore, this artery courses to the occipital lobes, which are the primary areas of visual processes. Through its branches, this artery also is closely involved with the delivery of blood to the temporal lobes (involved with auditory processing).

Clinical Significance

Damage or disorder to the basilar artery can have a very severe impact because of its essential function in providing blood to brain and brainstem areas.

Basilar artery aneurysm, which represents about 3-5% of all intracranial aneurysms, can lead to headaches, disruptions in vision, nausea, vomiting, and loss of consciousness. Depending on the location of the issue, this condition leads to an extremely painful, “thunderclap” headache.

Another major issue that can arise here is basilar artery thrombosis, in which stroke or other issues arise due to clotting of the artery. Depending on the exact location of the issue, this can be divided into three clinical syndromes:

• Top-of-the-Basilar Syndrome: When clotting occurs towards the front of the basilar artery, leading to ischemia (inadequate blood supply) of the upper brainstem and thalamus. Manifestations of this issue include hallucinations, drowsiness, visual disruptions, an inability to move eyes properly, as well as behavioral shifts.
• Locked-In Syndrome: When the occlusion occurs closer to the more central parts of the basilar artery, there’s a disruption to parts of the brain, though the pons is spared. This means that the patient remains conscious and is able to move his or her eyes, but the voluntary motion is severely affected. Motion and speech are impossible, despite the person being responsive.
• Pontine Warning Syndrome: A condition that cycles through more intense and less intense periods, this is a disruption of motor and speech disturbances. Weakness on one side of the body and dysarthria, an inability to properly articulate in speech, can also arise.

Finally, vertebrobasilar insufficiency can arise when there is occlusion of the artery closer to its origin. This condition is usually temporary and leads to vertigo, double vision, disruption of speech patterns, impaired coordination, general confusion, and sudden falls. This issue often arises due to the narrowing of the artery due to plaque build-up and can be aggravated by hyperextension of the neck, placing strain on the basilar artery.