Carotid Sheath Anatomy

Carotid Sheath

• The lower part of the carotid sheath contains the common carotid artery, internal jugular vein, vagus nerve, and the ansa cervicalis.
• Posteriorly, it freely slides over the prevertebral fascia.
• Anteriorly, fibrous tissue connects it to the fascia on the deep surface of the sternocleidomastoid muscle.
• Pus from the pharynx tracks behind the carotid sheath, pointing in the posterior triangle.
• The sheath is thin over the internal jugular vein, enabling dilation for increased blood flow.
• The 'dead space' around the vein houses anterior and posterior groups of inferior deep cervical lymph nodes, with the latter extending into the posterior triangle as the supraclavicular group.
• The lower part of the carotid sheath extends from the sternoclavicular joint to where the common carotid artery bifurcates into the external and internal carotids.
• Carotid bifurcation typically occurs at the upper border of the thyroid cartilage (C4 vertebra) but can be higher, near the tip of the greater horn of the hyoid bone (C3 vertebra).
• A dilated portion, the carotid sinus, is often present at the terminal end of the common carotid artery, continuous with that at the start of the internal carotid artery.

Common Carotid Artery

• On the left, it originates from the aortic arch, positioned in front of the subclavian artery up to the sternoclavicular joint.
• On the right, it arises from the brachiocephalic trunk behind the sternoclavicular joint.
• Proximal to its bifurcation, the common carotid artery does not emit any branches.
• Within the medial portion of the carotid sheath, the common carotid artery lies lateral to the larynx and trachea.
• The internal jugular vein is lateral to the common carotid artery, and the vagus nerve is positioned deeply between the two vessels.
• The sympathetic trunk lies behind the artery, outside the sheath.
• The infrahyoid muscles and sternocleidomastoid muscle overlap the sheath superficially.
• The carotid pulse can be palpated by pressing the artery against the transverse process of the C6 vertebra, between the trachea and lower larynx medially, and the sternocleidomastoid laterally.
• A line from the sternoclavicular joint to the bifurcation near the greater horn of the hyoid bone marks the surface of the common carotid artery.
• Surgical exposure of the vessel involves retracting the lower sternocleidomastoid and incising the carotid sheath.

Ansa Cervicalis

• The ansa cervicalis is located on the front of the internal jugular vein and innervates the infrahyoid muscles.
• Lying within the anterior wall of the carotid sheath, it consists of superior and inferior roots.
• The superior root (descendens hypoglossi) branches off the hypoglossal nerve near the posterior belly of the digastric muscle containing only C1 fibers.
• The inferior root (descendens cervicalis) originates from C2 and C3 anterior rami in the cervical plexus.
• The inferior root spirals around the internal jugular vein to join the superior root.
• The two nerves may merge forming a loop, or join high up in a Y shape.
• Branches from the loop innervate the sternohyoid, sternothyroid, and omohyoid muscles segmentally from above down.

Hypoglossal Nerve

• In the submandibular region, the hypoglossal nerve emerges between the internal carotid artery and internal jugular vein, deep to the posterior belly of the digastric muscle.
• Looping around the occipital artery, it curves forward over the external carotid and lingual arteries.
• The nerve crosses these three arteries just below the digastric muscle, behind its tendon, this is where the superior root of the ansa cervicalis originates.
• It runs forward across the hyoglossus muscle, which separates it from the lingual artery.
• The hypoglossal nerve gives off a branch to the thyrohyoid muscle (C1 fibers) before running deep to the mylohyoid muscle to enter the mouth.

The carotid sheath contains vital structures like the common carotid artery, internal jugular vein, and vagus nerve. Infections can spread to the sheath affecting these structures. Key anatomical features include the carotid sinus and common carotid artery bifurcation at C6.