The Anatomy of the Carotid Artery

A major branch of the common carotid artery in the cervical (upper) spine, the internal carotid artery is one of a pair that run along each side of the neck and access the inside of the skull through an opening called the foramen lacerum. Once inside, this artery is closely associated with a number of important nerves and brain regions before breaking into the anterior and middle cerebral arteries. This makes it a major source of blood for the brain and parts of the head.

Since it serves such an essential function in the body, medical conditions or trauma to the internal carotid artery can be very dangerous. Among the major health risks due to disorders here are stroke, the build-up of plaques in the artery, as well as dissection of the carotid artery.

Anatomy

One of a pair found on each side of the neck, the internal carotid artery branches off from the common carotid artery and works its way up into the cranium. Its path places it right alongside brain regions associated with visual and sensory processing and, at its end, it splits into the two cerebral arteries.

This vessel can be divided into seven sections:

• Cervical segment: The artery usually arises between the third and fourth vertebrae of the neck (C3 and C4). Alongside other major structures like the common carotid, internal jugular vein, vagus nerve, deep cervical lymph nodes, and sympathetic nerve fibers, it crosses the transverse processes (bone protrusions) of the upper vertebrae before reaching the carotid canal at the temporal bone at the base of the skull.   
• Petrous segment: Inside the carotid canal, the artery turns anteromedially (up towards the front and the middle), before progressing superomedially (above and towards the midline) toward the foramen lacerum.  
• Lacerum segment: This short segment travels over cartilage that covers the foramen lacerum, terminating at the petrolingual ligament there.
• Cavernous segment: Crossing into the cranium, the internal carotid artery travels just above the back part of the sphenoid bone (a major bone in the middle of the skull), making its way through the roof of the cavernous sinus, which is basically a gap that stretches out towards the eye. Here, it’s in close proximity to the abducens, oculomotor, trochlear, and ophthalmic nerves, as well as parts of the trigeminal nerves, which are all involved in the control of the eyes as well as sensory perception.   
• Clinoid segment: After exiting the cavernous sinus, the internal carotid artery crosses from the proximal to the distal dural ring. The latter of these rings is an anatomic marker indicating divisions in the internal carotid artery. 
• Ophthalmic segment: After passing through the distal dural ring, the artery travels below but parallel to the optic nerve (the nerve that delivers visual information to the brain for processing).
• Communicating segment: The final segment of the artery, the communicating segment gives rise to the posterior communicating and anterior choroidal arteries before splitting into the anterior and middle cerebral arteries.

Anatomic Variations

The most common variation seen in the internal carotid artery is asymmetry of the origin of the left and right arteries. In addition, while it usually originates between the third and fifth neck vertebrae, it sometimes begins higher or lower.

A couple of other abnormalities have also been observed:

• Congenital absence: A very rare anomaly present at birth in less than 0.1% of people, this is when the artery never develops. While other artery systems are able to compensate for this absence—and most cases are asymptomatic—this condition can be associated with certain types of brain aneurysm and can impact surgical decision making.
• Aberrant internal carotid artery: This variation is characterized by an abnormally small radius of the artery at its point of origin in the neck. To compensate, other portions may be larger than normal. If not enough blood reaches parts of the ear as a result, patients may experience tinnitus (a ringing in the ears). 
• Kissing carotids: This is when the right and left carotid arteries touch in the midline and are elongated.
• Lateralized artery: An abnormality at the petrous segment of the artery, lateralized artery affects where the artery accesses the skull and can also lead to tinnitus. 
• Persistent carotid-vertebrobasilar anastomoses: A congenital condition in which there are irregularities in the connections between the front and more rear sections of the artery.

Function

The primary role of the internal carotid artery is to deliver blood to the forebrain: the front part of the brain that houses the cerebral hemispheres (which are involved higher-level cognition, language, as well as visual processing), the thalamus (associated with visual, sensory, and auditory processing, sleep, and consciousness), and the hypothalamus (regulating metabolism and the release of hormones, among other functions).

Through its branches, this artery also delivers blood to the eyes and their related structures, the forehead, as well as the nose.

Clinical Significance

Disorders or injury to the internal carotid artery can cause inadequate blood flow to important brain regions. This can then lead to infarction—cell and tissue death due to a lack of nutrients and oxygen. When occurring in the brain, this leads to stroke.

As with any part of the circulatory system, this artery can also be subject to a build-up of atherosclerotic plaques. This causes narrowing of the artery (stenosis), significantly raising the risk of infarction. A specific kind of surgery, called carotid endarterectomy, is needed to correct this issue. Additionally, a carotid artery stent can be placed to improve blood flow in an area of the vessel narrowed by plaque build-up.

Finally, due to its location, the internal carotid artery can be injured in cases of skull fracture. If the artery rips as a result of such a trauma, it is called a dissection. Basically, this is a disruption of healthy circulation, which can lead to stroke and other issues.