Anatomy, Head and Neck, Posterior Cervical Region

Structure and Function

Anatomic Boundaries 

The posterior triangle occupies the posterolateral aspect of the posterior cervical region on each side of the neck. (see Image. Surface Anatomy of the Anterolateral Cervical Region). Distinct anatomic borders define this area. The anterior border is bounded by the posterior aspect of the sternocleidomastoid (SCM) muscle. The posterior border is delineated by the anterior border of the trapezius muscle. The union of the SCM and the trapezius at the superior nuchal line of the occipital bone forms the apex of the posterior triangle. The superior aspect of the middle 1/3 of the clavicle defines the inferior boundary, which forms the base of the posterior triangle (see Image. Anatomical Structures of the Lateral Neck).

The inferior belly of the omohyoid muscle further subdivides the posterior triangle of the neck into 2 smaller triangles. The omohyoid consists of superior and inferior bellies joined by an intermediate tendon, which lies just posterior to the inferior attachment of the SCM. The superior belly extends from the body of the hyoid bone to the intermediate tendon and lies within the anterior triangle of the neck. The inferior belly originates from the lateral end of the superior border of the scapula and inserts on the intermediate tendon. The inferior belly traverses the posterior triangle of the neck. The area superior to the omohyoid is the occipital triangle, whereas the area inferior to the muscle is the subclavian triangle.[1]

The superficial border, or "roof," of the posterior triangle of the neck consists of the investing layer of the deep cervical fascia. The deep cervical fascia subdivides into several distinct layers that further organize the structures of the neck. The investing layer of the deep cervical fascia lies just deep to the superficial fascia, a sheet of loose connective tissue containing the platysma. The investing layer of the deep cervical fascia forms a single layer over the roof of the posterior triangle. However, the investing layer splits into 2 layers to enclose the SCM and infrahyoid muscles anteriorly and the trapezius posteriorly, thereby "investing" these muscles. Several structures originating deeper in the neck pierce the investing layer of the deep cervical fascia within the posterior triangle to reach more superficial structures, including the external jugular vein and the transverse cervical, supraclavicular, lesser occipital, and greater auricular nerves.[2]

The deep border, or "floor," of the posterior triangle of the neck consists of the prevertebral layer of the deep cervical fascia. The prevertebral layer surrounds the cervical vertebrae and the deep prevertebral muscles of the neck. The space between the investing and prevertebral layers of the deep cervical fascia contains the contents of the posterior triangle of the neck (see Image. Cross-Section of the Human Neck).[3]

Function

The posterior cervical region contains the posterior triangle of the neck, one of the major anatomic subdivisions of the cervical area. Muscles and fascial layers of the posterior cervical region form natural boundaries that organize the neck and its contents (see Image. Musculature of the Posterior Neck). The posterior cervical region also contains muscles, nerves, blood vessels, and lymphatics.

Embryology

The pharyngeal apparatus gives rise to the structures of the head and neck. The pharyngeal apparatus consists of 6 branchial arches that develop in a cranial-to-caudal direction. Each arch contains mesoderm and neural crest cells that form cartilage, nerves, fascia, and muscles. Branchial clefts of ectodermal origin line the superficial surface of the arches. Endodermal branchial pouches line the deep surface of the arches.[4]

The SCM and the trapezius, which occupy a large portion of the posterior cervical region and form the borders of the posterior triangle, develop from somites and neural crest cells located just caudal to the 6th branchial arch. A case study describes a congenital absence of the posterior triangle resulting from fusion of the SCM and the trapezius. Embryologically, this anomaly results from failure of the mesoderm of the 6th branchial arch to separate and subsequently degenerate.[5]

Blood Supply and Lymphatics

Blood Vessels

Posterior triangle vessels are located within the subclavian triangle, inferior to the omohyoid. The subclavian artery divides anatomically into 3 parts. The 1st part is the most proximal segment. The 2nd part passes posterior to the anterior scalene muscle. The 3rd part is the most distal segment and enters the posterior triangle after emerging from behind the anterior scalene. The subclavian artery becomes the axillary artery after passing over the 1st rib.

The transverse cervical and suprascapular arteries also course through the subclavian triangle anterior to the anterior scalene. Both arteries arise from the thyrocervical trunk, a major branch of the 1st part of the subclavian artery that originates immediately medial to the anterior scalene.

The subclavian, transverse cervical, and suprascapular veins accompany their corresponding arteries through the posterior triangle. However, the subclavian vein passes anterior to the anterior scalene, unlike the subclavian artery. The external jugular vein courses superficial to the posterior triangle before piercing the investing layer of the deep cervical fascia to enter the posterior triangle and drain into the subclavian vein.[6]

Lymphatics 

Several lymph node groups are located within the posterior cervical region. The supraclavicular lymph nodes lie along the superior border of the clavicle within the posterior triangle. The posterior cervical lymph nodes course along the external jugular vein within the posterior triangle. The superficial cervical lymph nodes lie along the anterior surface of the SCM, whereas the deep cervical lymph nodes follow the posterior surface of the same muscle. The posterior auricular and occipital lymph node groups are situated near the apex of the posterior triangle, superficial to the superior portions of the SCM and the trapezius, respectively (see Image. Lymph Nodes of the Head and Neck).[7]

The lymphatic and circulatory systems converge near the posterior cervical region. On the right side of the body, the right lymphatic duct empties into the junction of the right internal jugular and right subclavian veins. The right lymphatic duct drains lymph from the right side of the head and neck, the right upper extremity, and the right hemithorax. The thoracic duct drains lymph from the remainder of the body. The thoracic duct empties into the junction of the left internal jugular and left subclavian veins.[8]

Nerves

The spinal accessory nerve (cranial nerve XI) provides motor innervation to the SCM and the trapezius.[9] After exiting the skull through the jugular foramen, the spinal accessory nerve courses along the deep surface of the SCM. The nerve descends obliquely through the posterior triangle. Investing fascia encloses the spinal accessory nerve as it crosses the posterior triangle and continues to the deep surface of the trapezius.[10]

The superficial branches of the cervical plexus supplying cutaneous sensation to the cervical region emerge from the posterior aspect of the SCM near its midpoint at the nerve point of the neck, or the Erb point.[11] The anterior rami of the cervical spinal nerves pierce the investing fascia and divide into 4 cutaneous nerves that course deep to the platysma.

The lesser occipital nerve arises from the 2nd cervical spinal nerve and ascends posterior to the SCM to innervate the scalp and neck posterior to the ear.[12] The greater auricular nerve arises from the 2nd and 3rd cervical spinal nerves and ascends along the anterior surface of the SCM to innervate the skin over the mastoid and parotid regions inferior to the ear.[13] The transverse cervical nerve arises from the 2nd and 3rd cervical spinal nerves and courses horizontally across the SCM to innervate the anterior neck. The supraclavicular nerve arises from the 3rd and 4th cervical spinal nerves and descends across the investing fascia over the posterior triangle before branching extensively to innervate the skin over the clavicle and first 2 ribs.[14]

The phrenic nerve lies deep to the posterior triangle within the prevertebral fascia as it courses along the anterior surface of the anterior scalene. The anterior rami of the 5th cervical through the 1st thoracic spinal nerves form the roots and trunks of the brachial plexus, which also lie within the prevertebral fascia between the anterior and middle scalenes.[15]

Muscles

The largest muscles of the posterior cervical region are the SCM and the trapezius. The margins of these muscles form the anterior and posterior borders of the posterior triangle, respectively. The SCM originates from the anterior surface of the manubrium and the medial 1/3 of the clavicle. This muscle inserts on the mastoid process and the lateral aspect of the superior nuchal line. The SCM rotates the head contralaterally and flexes the neck ipsilaterally. The trapezius is a broad muscle with an extensive origin from the superior nuchal line and the spinous processes of the C2 to T12 vertebrae. This muscle inserts on the posterior aspect of the lateral 1/3 of the clavicle, the acromion, and the spine of the scapula. The trapezius elevates, adducts, and depresses the scapula. The muscle also assists scapular rotation during arm abduction above the horizontal.[16][17]

The inferior belly of the omohyoid is the only muscle located within the true boundaries of the posterior triangle.[18][19] The inferior belly passes through the triangle in an anterior-to-posterior direction approximately 2.5 cm above the clavicle and further subdivides the posterior triangle into the occipital and subclavian triangles. The infrahyoid layer of the pretracheal fascia encloses the omohyoid. The infrahyoid layer is another component of the deep cervical fascia, along with the investing and prevertebral layers. Several posterior cervical muscles lie just deep to the posterior triangle and are enclosed by the prevertebral layer of the deep cervical fascia, collectively forming the prevertebral muscles. The prevertebral fascia forms the deep border of the posterior triangle, and the underlying prevertebral muscles constitute the floor. The prevertebral muscles include the anterior, middle, and posterior scalenes and the levator scapulae.

Physiologic Variants

Anatomic variation of the posterior cervical region occurs most commonly at the nerve point of the neck. The terminal distributions of the cutaneous branches of the cervical plexus are relatively consistent, but the origins of these branches vary considerably. After a cervical spinal nerve pierces the investing layer of the deep cervical fascia, the nerve divides into 4 cutaneous branches. However, the branching point is often variable. A cervical spinal nerve may divide before emerging from the investing fascia, allowing 1 to 4 cutaneous nerves to pierce the investing fascia individually. The transverse cervical and supraclavicular nerves may also travel together for a greater distance before dividing, and the same pattern may occur in the lesser occipital and greater auricular nerves.

Surgical Considerations

Various surgical procedures rely on detailed knowledge of posterior cervical anatomy. For instance, cutaneous branches of the cervical plexus may be anesthetized with a superficial cervical plexus block. When performing this procedure, the anesthesia provider inserts a needle along the posterior aspect of the SCM through the skin and platysma. Local anesthetic is injected near the site where the cutaneous branches emerge from the investing fascia. Deep or unintended spread of the local anesthetic can increase the risk of phrenic nerve involvement, so the technique is typically avoided in patients with respiratory comorbidities, such as chronic obstructive pulmonary disease. Superficial cervical plexus blocks are often performed bilaterally with or without ultrasound guidance and are used for analgesia in thyroidectomy, parathyroidectomy, and carotid endarterectomy (see Image. Area Covered by a Superficial Cervical Plexus Block).[20]

The interscalene block is another regional anesthetic technique involving the posterior cervical region.[21] This procedure targets the roots and trunks of the brachial plexus to provide anesthesia for orthopedic procedures involving the clavicle or humerus. When performing an interscalene block, the anesthesia provider typically inserts a needle posterior to the clavicular head of the SCM through the prevertebral fascia and into the interscalene groove. Ultrasound guidance identifies the superior, middle, and inferior trunks of the brachial plexus during the procedure. The trunks appear on ultrasound as hollow circles arranged in a "stoplight" pattern (see Image. Interscalene Block Regional Landmarks).

Several major structures within the posterior cervical region are susceptible to inadvertent injury during surgery, including the spinal accessory nerve.[22] Intraoperative stimulation of the spinal accessory nerve helps map its course and motor distribution, reducing the risk of iatrogenic injury.[23]

Clinical Significance

Cervical lymphadenopathy is a common manifestation of various disease processes. Lymph node enlargement has numerous causes, and physical examination is essential for determining the underlying etiology. Soft, tender lymph nodes typically result from acute inflammatory processes, such as infectious mononucleosis or streptococcal pharyngitis. In contrast, large, firm, rubbery lymph nodes often indicate chronic conditions, such as lymphoma.[24]

Nerve injury within the posterior triangle is another important clinical consideration. Spinal accessory nerve injury can impair shoulder shrugging and arm elevation above the horizontal because of trapezius dysfunction. Damage to the spinal accessory nerve within the posterior triangle spares the SCM because motor branches arise proximal to the triangle.[25] Trauma and iatrogenic injury can also affect the cutaneous branches of the cervical plexus, resulting in sensory deficits over the cervical region. Recognition of the sensory distribution of the cervical plexus helps localize the affected nerve.