Common Carotid Artery
The common carotid artery is a large elastic artery, which provides the main blood supply to the head and neck region. There is one common carotid artery on either side of the body and these arteries differ in their origin. The left common carotid artery arises from the aortic arch within the superior mediastinum whilst the right common carotid artery arises from the brachiocephalic artery posterior to the right sternoclavicular joint. The common carotid artery ascends lateral to the trachea and oesophagus within a deep cervical fascia, the carotid sheath, with the internal jugular vein and the vagus nerve. This article will discuss the anatomical relations and variations of the common carotid artery as well as briefly talking about its branches and specialised structures. This will be followed by any clinical pathology related to the common carotid artery.
Relationships of the common carotid artery to other structures in the neck can be important in clinical practice. Within the lower aspect of the neck, the two common carotid arteries are separated by the trachea as well as by the larynx, pharynx and thyroid gland more superiorly. The common carotid artery ascends within the carotid sheath medial to the internal jugular vein and anterior to the vagus nerve.
Inferior to the omohyoid muscle, the common carotid artery is covered by the skin, the superficial fascia, the deep cervical fascia and the platysma, sternohyoid, sternothyroid and sternocleidomastoid muscles. At the level of the cricoid cartilage, the artery is crossed anterolaterally by the intermediate tendon of the omohyoid muscle. The artery then ascends more superficially and is only covered by the skin, fascia layers, the platysma and the sternocleidomastoid. Superior to the omohyoid muscle it is crossed by the sternocleidomastoid branch of the superior thyroid artery. The anca cervicalis, a loop of nerves derived from the cervical plexus, is superficial to the common carotid artery within the carotid triangle.
Three veins cross the common carotid artery superficially. The superior thyroid vein crosses the artery at the upper aspect of the thyroid cartilage whilst the middle thyroid vein crosses at the level of the cricoid cartilage. The final vein, the anterior jugular, crosses the artery above the level of the clavicle.
The sympathetic trunk and ascending cervical branch of the inferior thyroid artery lie posterior to the common carotid artery. Deep to these structures are the longus colli, longus capitis and scalenus anterior muscles, which are attached to the transverse processes of C4-6. Below C6, the artery is located between the scalenus anterior and longus colli muscles and anterior to the sympathetic trunk, vertebral vessels, inferior thyroid artery and subclavian artery.
The left common carotid artery varies in its origin and can arise with the brachiocephalic artery. The right common carotid artery arises above the level of the sternoclavicular joint in 12% of cases. Occasionally, the common carotid artery bifurcates at a higher level near the hyoid bone. More rarely, it bifurcates lower than usual at the level of the larynx. In very rare cases, the common carotid artery does not bifurcate resulting in the absence of the external and internal carotid arteries. However, these may be replaced by arteries, which arise directly from the aorta. Apart from these two terminal branches, the common carotid artery usually gives off no other branches. However, it may occasionally give rise to superior thyroid, inferior thyroid, vertebral, occipital, ascending pharyngeal or superior laryngeal arteries.
At the level of the superior border of the laryngeal thyroid cartilage, the artery divides into two terminal branches referred to as:
- The external carotid artery
- The internal carotid artery
The external carotid artery arises at the level of the intervertebral disc between C3 and 4 and ascends slightly anteriorly before inclining posterolaterally. In the carotid triangle, it is anteromedial to the internal carotid artery. The external carotid gives off eight main branches, which supply regions of the head and neck.
The internal carotid artery runs from its origin at the carotid bifurcation to the anterior perforated substance where it bifurcates into the anterior and middle cerebral arteries at the Circle of Willis. It supplies the forehead, nose, eyes and the ipsilateral cerebral hemisphere.
Near its bifurcation, the common carotid artery forms two specialised structures referred to as the:
- The carotid sinus
- The carotid body
The carotid sinus is a dilation of the base of the internal carotid artery, which is involved in relaying information about the arterial blood pressure to the hypothalamus. It is therefore referred to as a baroreceptor and is innervated by the carotid branch of the glossopharyngeal nerve.
The carotid body is an oval structure, located posterior to the carotid bifurcation, involved in relaying information about the arterial chemical composition to respiratory centres in the brainstem. Like the carotid sinus, it is innervated by the carotid branch of the glossopharyngeal nerve. The carotid body is surrounded by a fibrous capsule and consists of multiple lobules divided by septa. Within each lobule, there are two types of cells: glomus (type I) cells and sustenacular (type II) cells.
The glomus cells are involved in storing peptides, such as neurotensin, and amines, such as adrenaline, noradrenaline and dopamine. The sustentacular cells separate the glomus cells from an extensive network of fenestrated sinusoids.
The carotid body is a chemoreceptor stimulated by hypercapnia, hypoxia and increased hydrogen ion concentration (low pH). In response to these changes, the carotid body changes the rate and volume of respiration via a reflex involving the respiratory centres in the brainstem.