The carotid sheath is a fibrous connective tissue that encircles four key structures within the neck. These structures include:
The carotid sheath is located posterior to the sternocleidomastoid muscle and is a part of the deep cervical fascia of the neck. It consists of all three fascial layers of the neck, which include the pretracheal fascia, the investing fascia, and the prevertebral fascia. The carotid sheath starts superior to the sternum, and the first rib then extends to the base of the skull. Within the carotid sheath, the internal jugular vein is lateral to the carotid artery with the vagus nerve posterior to both vessels in most individuals, though many different anatomic configurations exist.
The right common carotid artery originates from the bifurcation of the brachiocephalic artery, whereas the left common carotid artery arises from the aortic arch. Both common carotid arteries run within the carotid sheath until they bifurcate into the internal and external carotid arteries at the level of the upper border of the thyroid cartilage. The carotid body and carotid sinus are located at the bifurcation and function as important receptors within the body. The carotid body is a chemoreceptor that is sensitive to chemical changes including oxygen, carbon dioxide, and hydrogen ion concentration within the blood and helps control respiration. The carotid sinus is located just above the bifurcation at the origin of the internal carotid artery and functions as a baroreceptor helping detect and correct changes in blood pressure. Continuing from the bifurcation the external carotid artery exits the sheath and supplies blood to the various structures throughout the face and neck. The internal carotid artery continues within the carotid sheath and enters into the temporal bone through the carotid canal and gives rise to the ophthalmic artery as well as the anterior and middle cerebral arteries.
The internal jugular vein is a continuation of the sigmoid sinus and begins its descent towards the heart at the jugular foramen along with the vagus nerve. It travels within the carotid sheath and ends with it meeting the subclavian vein, thus forming the brachiocephalic vein. Wile moving posteriorly, the internal jugular receives venous blood from the facial, lingual, and superior and middle thyroid veins.
The vagus nerve has the longest course of any cranial nerve and begins its journey by passing through the jugular foramen with the internal jugular vein and makes it descent within the carotid sheath. Within the neck, it gives off the pharyngeal branch and the superior laryngeal nerve. The pharyngeal branch supplies motor fibers to the muscles of the pharynx with the exception of the stylopharyngeus. A lesion to the pharyngeal branch causes deviation of the uvula to the opposite side of the injury. The superior laryngeal nerve divides into the internal and external laryngeal nerves. These nerves supply sensory fibers to the larynx above the vocal cord, lower pharynx, and epiglottis as well as supplying taste fibers to the root of the tongue near the epiglottis.
The glossopharyngeal nerve (CN IX), accessory nerve (CN XI), and hypoglossal nerve (CN XII) pierce the superior section of the carotid sheath briefly and then exit. In addition, the ansa cervicalis is embedded in the anterior of the carotid sheath. It is important to visualize both of these structures when performing procedures in which the carotid sheath must be incised or manipulated.
The function of the carotid sheath is to separate and, possibly, help protect the vital structures within it.
The adventitia of the cervical great arteries becomes apparent by 15 weeks gestation and is one of the earliest components of the fetal deep cervical fascia. The carotid sheath does not appear until approximately 20 weeks gestation. At this time it is fused with the pretrachial fascia, but not yet integrated into the prevertebral lamina of the deep cervical fascia. 
A carotid endarterectomy is performed to excise atherosclerotic sclerotic thickening of the intima within the internal carotid artery in an effort to reduce strokes in patients with significant carotid artery stenosis. To begin, the opening incision is made and the sternocleidomastoid muscle is retracted. At this point it is important to visualize the carotid sheath for this is where the structure vital to the procedure is located. The carotid artery lies on the medial side of the internal jugular vein, and the vagus nerve is situated posteriorly. Superiorly, the carotid sheath may also contain the hypoglossal nerve, the glossopharyngeal nerve, and the accessory nerve. These structures pass in a horizontal fashion and cross the internal carotid artery. It is important to identify these structures before incising any structure. The surgeons open the carotid sheath to gain exposure to the common carotid bifurcation as this is the most common site for atherosclerosis. Once access is gained into the carotid sheath and the carotid bifurcation is located, the surgeon then removes any atherosclerotic plaque found and repairs the vessel. Possible complications include air embolism and laceration of the internal jugular vein or carotid artery.
Penetrating Neck Trauma
An ongoing debate amongst trauma surgeons is over a no-zone approach, which leans heavily on multidetector CT angiography versus a zoned approach to surgical exploration.
Zones of the Neck
The neck is divided into three zones. These become important when assessing and managing trauma in those with neck injuries.
Zone I - cricoid cartilage to the sternal notch: trachea, lung, esophagus, thoracic duct, vertebral arteries, origin of the common carotid artery, and subclavian vessels, spinal cord, thoracic duct, thyroid gland
Zone II - cricoid cartilage to angle of mandible: carotid sheath and its components (carotid artery, internal jugular vein, vagus nerve), trachea, esophagus, spinal cord, larynx, pharynx
Zone III - angle of mandible to base of skull: distal portion of internal carotid artery, vertebral arteries, jugular veins, pharynx, spinal cord, sympathetic chain, CN IX,X,XI,XII
The decision to explore these areas is based on hard and soft signs. Regardless of the zone, if the patient becomes unstable, surgical exploration is the best course of management.
If the patient presents with hard signs emergent management is needed, whereas if soft signs are present close observation and reevaluation may be appropriate.
Internal Jugular Central Venous Line
An internal jugular central line is performed to gain access to push medication or fluids into the systemic vasculature. The procedure begins by placing the patient in slight Trendelenburg and locating the internal jugular vein within the apex of the triangular interval between the clavicular and sternal heads of the sternocleidomastoid muscle. It may help to have the patient turn the head to the contralateral side to better visualize these anatomical landmarks. If the patient is conscious, begin by applying a local anesthetic. Next, insert the needle at a 45-degree angle posteriorly while applying negative pressure. The needle will pass through the carotid sheath and the internal jugular vein will produce blood within the syringe. An ultrasound can be used to help assist and ensure proper placement of the guide needle into the vein. While keeping the guide needle in place, begin threading the guide wire. Once in place, remove the needle and introduce the dilator and then the catheter. The catheter should now be within the superior vena cava.
It is important to understand the relationships of the structures within the carotid sheath to properly perform an internal central venous line.
It is uncommon for any pathological conditions to directly affect the carotid sheath itself. Instead, it is more common to see pathological changes to the structures within the sheath and thus to affect the sheath secondarily. However, one scenario that could involve the sheath is a disseminated dental infection. This could lead to cellulitis within the head and neck with the carotid sheath becoming involved. The disease process could further progress and involve the mediastinum causing mediastinitis and endocarditis.