3 Surgical Anatomy of the Neck

In discussing anatomy of the neck, it is helpful to have some scheme of subdividing the neck. This has been traditionally accomplished by describing cervical triangles that topographically divide the neck by means of clinically identifiable muscles ( Fig. 3.1). This is a simple system that aids the clinician in communicating general locations more effectively.

Two muscles, the sternocleidomastoid muscle (SCM) and the trapezius (TPZ), divide the neck into anterior and posterior triangles. These muscles are clear by visual inspection and palpation. The anterior triangle is limited posteriorly by the anterior border of the SCM, anteriorly by the anatomic midline, superiorly by the lower aspect of the mandible, and a line connecting the angle of the mandible to the mastoid tip. The anterior triangle can further be subdivided into the following triangles:

• Submental triangle: It is the midline structure defined by the anterior belly of the digastric (ABD) bilaterally and the hyoid bone at its base. The mylohyoid muscles form its floor. It contains the submental lymph nodes and the distal aspect of the submental branch of the facial artery.

• Submandibular (digastric) triangle: It is defined between the anterior and posterior bellies of the digastric, and superiorly by the inferior border of the mandible, and a line that connects the mandibular angle to the mastoid. Its floor is defined by the mylohyoid, hyoglossus, and superior pharyngeal constrictor muscles. It contains the facial vessels and their branches, the submandibular gland (SMG), marginal mandibular and cervical branches of the facial nerve, and lymph nodes.

• Carotid triangle: It is bounded by the posterior belly of the digastric (PBD) superiorly, the SCM posteriorly, and the superior belly of the omohyoid anteriorly. Its floor is defined by the middle and inferior pharyngeal constrictors. It contains the carotid artery and its branches, jugular vein, cranial nerves (CN) X and XII, and lymph nodes.

• Muscular (visceral) triangle: Its boundaries are anatomical midline anteriorly, the superior belly of the omohyoid muscle superiorly, and anterior border of the SCM inferiorly. It contains the infrahyoid (strap) muscles: sternohyoid, sternothyroid, and thyrohyoid and viscera (thyroid, parathyroid).

The posterior triangle is bound anteriorly by the posterior edge of the SCM, posteriorly by the anterior edge of the TPZ, its apex in the occiput at the junction of the SCM and TPZ, and its base is the middle third of the clavicle.¹ The posterior triangle as illustrated can further be divided by the crossing of the posterior belly of the omohyoid into the occipital triangle superiorly, and the supraclavicular triangle anteriorly. As described, the critical area covered by the SCM is not technically a part of either triangle.

3.1.2 Division of the Neck by Levels

Another common method for subdividing the neck is the use of the level system. This was first described by the Sloan Kettering Group in 1981² and has since been adopted by the American Head and Neck Society (AHNS) for the classification of neck dissection with various modifications ( Fig. 3.2).³ There are several obvious reasons to favor this classification as a template to describing neck anatomy systematically in a text devoted to neck dissection. As a major objective, this classification allows for consistent communication of pathology radiographically and clinically, therefore providing a framework for conceptualizing the neck. It also serves as the foundation for describing various selective neck dissections as will be discussed in future chapters. Each level can be thought of as a compartment unto itself that may or may not be dissected. As such, it is beneficial for surgeons to be familiarized with the cervical anatomy as defined by the confines of each level.

Fig. 3.1 Regions of the neck divided into triangles. Anterior triangle is green and the posterior triangle is blue separated by the sternocleidomastoid muscle. These triangles are further subdivided in the image. (Reproduced with permission of Watanabe K, Shoja MM, Loukas M, Tubbs RS, eds. Anatomy of Plastic Surgery of the Face, Head, and Neck. 1st ed. New York, NY: Theime; 2016.)

Fig. 3.2 Neck with the sternocleidomastoid muscle removed divided into cervical levels I to VI. (Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. Thieme 2005. Illustrations by Karl Wesker.)

The cervical facial layers, although simple in concept, have been varied in their description throughout history. A nice summary of landmark historical descriptions highlighting this variability is provided by Natale et al.⁴ Modern descriptions of cervical fascia typically organize the layers into a superficial cervical fascia (SCF) and a DCF, which is then further subdivided into three layers of muscular or visceral fascia.⁵ Since the muscular and visceral fascial layers contained within the deep space are morphologically distinct, we will approach these separately, as in previous reports.⁴ The fascial layers will be described from superficial to deep, in the same order as they would be encountered during a typical cervical approach. Of note, although debated, muscular layers are generally considered to form concentric layers that circumscribe the neck ( Fig. 3.3).

3.2.2 Superficial Cervical Fascia

Between the dermis and the deep fascia is a region of loose connective tissue joined to both layers known as the SCF. This layer is present in some form throughout the body.^1,4 In the anterior neck, this thin layer envelops the platysma, and is continuous with the superficial musculoaponeurotic system (SMAS) investing the muscles of facial expression anteriorly, and with the galea capitis posteriorly ( Fig. 3.4).

The platysma originates at the level of the upper thorax, anterior to the clavicle. It has attachments to the subcutaneous tissues of the subclavicular and acromial regions, as well as the pectoralis and deltoid fascia. It projects superiorly in an upward and medial direction, and has a variable midline dehiscence with decussating fibers submentally. The platysma has numerous insertions, all of which are above the neck including the skin of the cheek, perioral muscles and SMAS, parotid fascia, zygoma, and mandible. The platysma is most commonly innervated by the cervical branch of the facial nerve, which descends to enter the deep surface of the muscle near the angle of the mandible.^1,6

Fig. 3.3 Cervical fascia layers. (a) Transverse section taken at the level of the glottis larynx. (b) Anterior view of the neck with the fascia layers exposed. (Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations (a) by Markus Voll and (b) by Karl Wesker.)

Fig. 3.4 Lateral view of the neck with investing fascia exposed showing the cutaneous sensory branches of the cervical plexus piercing posterior to the sternocleidomastoid muscle. The platysma is found superficial to this fascia layer. (Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)

There are several superficial veins with associated lymphatics that exist within the plane between the platysma and the DCF. The external jugular vein (EJV) is formed by the posterior branch of the retromandibular, and the postauricular vein in close association with the angle of the mandible, and drains portions of the face and scalp ( Fig. 3.5, Fig. 3.6). It courses in on oblique fashion crossing the SMC and transverse cervical nerves toward the midclavicle.¹ In the supraclavicular fossa, it pierces the deep fascia and drains into the subclavian, the jugulosubclavian confluence, or the internal jugular vein (IJV).⁷ The course of the EJV is most commonly found either at the border, or posterior to the border of the platysma, and in its superior half it courses anterior to the great auricular nerve.⁸

The anterior jugular vein is formed from superficial submandibular vessels near the level of the hyoid, and descends in the neck typically as a bilateral structure between the midline and the anterior border of the SCM. It traverses laterally inferiorly piercing the superficial layer of the DCF (SLDCF) and courses deep to the SCM and superficial to the infrahyoid straps to confluence with the EJV or subclavian vein directly.¹ The anatomy of these superficial veins can be duplicate and quite variable in their course.⁷

3.2.3 Deep Cervical Fascia

Deep to the platysma and the SCF lies the three layers of muscular DCF and the visceral fascia ( Fig. 3.3, Fig. 3.7).⁴ The first of these muscular fascial layers encountered is the SLDCF, also known as the investing layer of DCF, which invests the SCM and TPZ muscles, and is continuous posteriorly into the nuchal ligament. Anteriorly, it joins the opposing sheet as well as the hyoid. Superiorly, it has attachments to the superior nuchal line, mastoid, and the lower aspect of the mandible, extending deep to the parotid. Inferiorly, it merges with the periosteum of the manubrium, clavicle, and acromion.¹ Techniques utilizing plastination have recently brought into question the anatomic integrity of the SLDCF in the anterior cervical triangle, between the medial borders of the SCM, and in the posterior triangle between the lateral borders of the SCM and the TPZ.^9,10 These findings have not been widely accepted, and this layer is still considered the superficial boundary of cervical levels I to V.

In the anterior triangle, immediately deep to the SLDCF, lies the second muscular layer of DCF, referred to here as the middle layer of DCF (MLDCF), also referred to as the muscular pretracheal fascia.⁵ This layer is composed of muscular fascia surrounding the infrahyoid strap muscles, as well as the mylohyoid and geniohyoid muscles above the hyoid bone ( Fig. 3.3, Fig. 3.7). There is evidence that this fascia communicates posteriorly with the levator scapula muscle, thus forming a concentric lamina with the DLDCF encircling the visceral space.⁴ This fascial layer serves as the superficial boundary for the level VI (central neck) compartment and the deep boundary for level I.

Fig. 3.5 Neurovascular structures found between the superficial fascia and the deep cervical fascia. (Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)

The third muscular fascial layer is the DLDCF commonly known as the prevertebral fascia. This fascial layer is thickest near the midline covering the vertebral bodies and extends laterally to cover the longus muscles, the anterior and middle scalene muscles, and the levator scapulae. This fascia extends along the brachial plexus and subclavian artery, which emerges behind the anterior scalene and forms the axillary sheet. The superficial sensory branches of the C1–C4 ventral rami forming the cervical plexus, which will be discussed later, pierce this fascia, while the phrenic nerve remains posterior throughout its course in the neck ( Fig. 3.8, Fig. 3.9).¹ The superior attachment of the DLDCF is the skull base and continues into the superior mediastinum. Laterally there is a loose areolar attachment to the carotid sheath. The DLDCF serves as the floor, or deep boundary for cervical levels II to VI.

The final fascial layer of the neck is the visceral fascia, a very thin layer that envelops the viscera of the neck, which is analogous to the subperitoneal and subpleural fascia of the abdomen and thorax, respectively.⁴ Visceral structures include the laryngotracheal complex, thyroid and parathyroids, and esophagus. The buccopharyngeal fascia is a condensation of the epimysium of the pharyngeal constrictors and esophagus and separates the visceral compartment from the DLDCF with an intervening loose alveolar space ( Fig. 3.7). Some authors consider the carotid sheath to be included in this visceral classification, while Gray’s Anatomy describes the carotid sheath simply as a condensation of all surrounding DCF layers, one being the pretracheal or visceral fascia.^1,4 The carotid sheath houses the common and internal carotid artery, IJV, and the vagus nerve.

3.3 Levels of the Neck

The neck is divided in six levels (I–VI) with additional sublevels (A, B) in levels I, II, and V ( Fig. 3.2).

Fig. 3.6 Superficial lymphatics of the neck. (Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)

This level is subdivided into sublevels IA and IB, which correlate with the submental and digastric triangles, respectively. We will discuss these sublevels including their boundaries and contents separately ( Fig. 3.10).

Level IA

Level IA is an unpaired central triangle bound by the ABD and the body of the hyoid bone inferiorly. Its apex is the symphysis of the mandible superiorly. The roof of this triangle is the SLDCF immediately deep to the superficial fascia investing the platysma and the floor is the MLDCF, which is the muscular fascia investing the mylohyoid.

The ABD receives its blood supply from the submental artery. The course of this artery and associated vein has been well described in the literature as it pertains to the submental flap. It is a branch of the facial artery and courses along the inferior border of the mandible toward the lateral aspect of the ABD. It has a terminal but variable cutaneous perforator near the apex of the submental triangle on either side of the ABD.¹¹ The ABD is continuous with the PBD by an intermediate tendon attached to the hyoid via a fibrous sling. It is innervated by the nerve to the mylohyoid, a branch off the lingual distribution of V3.¹

Level IB

The boundaries of this triangle are the ABD anteroinferiorly, the stylohyoid muscle posteroinferiorly, and the base of the mandible superiorly. To make the posterior boundary of level 1B more recognizable on clinical examination and radiographically, the AHNS modified the boundary to be the posterior plane of the posterior edge of the SMG rather than the stylohyoid muscle.¹² The floor of this triangle is the mylohyoid and hyoglossus muscles. The roof of this triangle is the SLDCF.

Fig. 3.7 Cervical fascia layers transverse cut at the level of the thyroid. (Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)

The contents of level IB will be discussed from superficial to deep as encountered beneath the platysma ( Fig. 3.11).⁵ The marginal branch of the facial nerve (MBFN) and cervical branch of the facial nerve are encountered superficial to level IB on the SLDCF fascia, immediately deep to the platysma ( Fig. 3.12). The angle and base of the mandible have been the most common landmarks used to describe the location of the MBFN. The neck position affects the location of the nerve relative to the mandible. The nerve is most commonly described as having two branches, but it may have multiple. It has a downward trajectory after leaving the parotid parenchyma coursing over the angle of the mandible. In 78 neck dissections, the lowest branch of the nerve was found at average of 1.25 cm below the margin of the mandible in the location lateral to the facial vein overlying the fascia of the SMG with the neck extended.¹³

The facial vein and artery lie deep to the MBFN, with the vein consistently posterolateral to the artery. The vein is found in the fascia superficial to the SMG and courses in a posteroinferior trajectory as it descends lateral to the PBD as it leaves level IB. It is joined by the anterior division of retromandibular vein, sometimes referred to as the posterior facial vein, in this location and will be discussed later in level II ( Fig. 3.13).⁵

The facial artery enters level IB deep to the stylohyoid muscle and PBD after branching off the external carotid as will be discussed with the level II anatomy. The facial artery courses anterior and superior along the lateral surface of the hyoglossus muscle. It indents the posterior and lateral surfaces of the SMG taking a inferolateral trajectory along the lingual surface of the mandible, exiting the triangle at the facial notch of the mandible. At the inferior mandible, it is immediately anterior to the facial vein. The artery is notable for its tortuous appearance throughout its course. The submental artery leaves the facial artery near the inferior border of the mandible.

The SMG is found immediately deep to the facial vein. It occupies much of the volume of level 1B and is partially cradled by support from myofascial attachments to the hyoid bone from the mylohyoid medially and SLDCF laterally, both extending from the hyoid to the mandible. It has an irregular shape imparted by straddling the posterior free edge of the mylohyoid dividing the gland into a deep and superficial lobe.

The superficial lobe constitutes the bulk of the gland and defines the posterior border of level IB. Superiorly, the gland extends medially to the body of the mandible and inferiorly the gland may drape the digastric tendon. It is found deep to the SLDCF, which courses deep to the tail of parotid. A condensation of SLDCF called the stylomandibular ligament separates the two glands posteriorly.¹

The deep lobe lies in the space between the mylohyoid laterally and the hyoglossus muscle medially. It extends anteriorly deep to the floor of the mouth approximating the sublingual gland anterosuperiorly ( Fig. 3.11).⁵ This space between the mylohyoid and hyoglossus muscles is traversed by the lingual nerve, the submandibular duct, and the hypoglossal nerve (HN; Fig. 3.14).⁵

The lingual nerve, which originates as a branch of V3 in the infratemporal fossa, is closely associated with the mandibular periosteum near the third molar. It descends into level IB lateral to the hyoglossus muscle, along the superior aspect of the floor of level 1B ( Fig. 3.15). As the lingual nerve travels anteriorly, it courses medial to the mylohyoid muscle superior to the HN. The lingual nerve descends lateral to the submandibular duct near the anterior border of the hyoglossus, and cradles the undersurface of the duct before projecting medially to innervate the anterior oral tongue.¹ The lingual nerve is immediately associated with the deep surface of the SMG and is in continuity with the submandibular ganglia; this is the site of postsynaptic parasympathetic fibers that innervate the gland ( Fig. 3.11).⁵

Fig. 3.8 Lateral view of the neck exposed to the level of the deep fascia and paraspinal muscles. (Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)

Fig. 3.9 Cervical plexus with superficial sensory branches transected. (Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)

Fig. 3.10 Anterior view of the neck with level I outlined. (Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)

The HN enters the triangle deep to the stylohyoid muscle associated with the inferior and deep surfaces of the gland medial to the digastric tendon and lateral to the hyoglossus muscle ( Fig. 3.15). It courses anteriorly with associated vena comitans, which drain into the lingual vein. The nerve courses deep to the mylohyoid and inferior to the lingual nerve and submandibular duct.

3.3.2 The Lateral Neck (Levels II–IV)

The lateral neck contains the upper, middle, and lower jugular chain of lymph nodes and correlates with levels II, III, and IV, respectively. The lateral neck extends form the skull base superiorly to the clavicle inferiorly ( Fig. 3.2). Its superficial boundary or roof is the SLDCF, which invests the SCM ( Fig. 3.16). The deep boundary or floor is the DLDCF synonymous with the prevertebral fascia that covers the paraspinal muscles including the longus muscles, the anterior scalene, and levator scapulae, which contribute to the muscular floor of the lateral neck ( Fig. 3.3). The posterior boundary of these levels is defined by the posterior edge of the SCM as well and the superficial sensory branches of the cervical plexus, which is particularly useful intraoperatively.³ This separates the lateral neck from level V posteriorly, which will be reviewed later. The anterior boundary of lateral neck at levels III and IV is defined by the lateral border of the sternohyoid superficially, and the medial aspect of the common carotid in the deep plane ( Fig. 3.8).^3,12 More superiorly, the anterior boundary of level II is the vertical plane of the posterior border of SMG, which approximates the stylohyoid muscle but is more clinically and radiographically appreciated that muscle.¹² We will begin our review of the lateral neck by discussing the critical structures that form these boundaries and then proceed to discuss each level and its contents individually.

Fig. 3.11 Anterolateral view of submandibular triangle with the platysma and SLDCF (superficial layer of the deep cervical fascia) removed. The mandibulectomy defect allows visualization of the contents of the submandibular triangle deep to the mylohyoid muscle.

The superficial or sensory branches of the cervical plexus have a critical role in demarcating the division between the lateral (levels II–IV) and posterior (level V) neck and therefore the whole of the cervical plexus will be discussed here. This superficial plexus along with the posterior border of the SCM demarcates the posterior border of levels II to IV ( Fig. 3.16). The cervical plexus is formed by the ventral rami of C1–C4 ( Fig. 3.17).⁵ These rami function to innervate muscles of the neck and diaphragm as well as provide sensory innervation to the skin of the neck and chest ( Fig. 3.9). In general, the cervical ventral rami are larger than dorsal rami except for the first and second, which are smaller than their dorsal counterparts (the cervical dorsal rami provide cutaneous cervical innervation to the posterior neck as well as paraspinal muscular innervation). The cervical plexus enters the neck piercing the DLDCF between the anterior and middle scalene except for the phrenic nerve, which remains deep to the fascia. There are ascending and descending branches from each level except for C1, and multiple communicating branches between levels. Additionally, there are superficial and deep branches; superficial branches have cutaneous sensory functions and pierce the SLDCF, while the deep branches innervate muscle.¹⁴

Fig. 3.12 Anterior lateral neck with investing fascia removed from the carotid triangle. (Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)

The superficial branches provide the anatomic demarcation between the lateral neck compartment and the posterior compartment (level V). They radiate laterally until they reach the posterior border of the SCM, at which point they take either an ascending or descending trajectory piercing the SLDCF. They are found immediately deep to the superficial fascia and platysma in this location ( Fig. 3.16).

The deep branches provide muscular innervation to the strap muscles, geniohyoid, diaphragm, and anterior paraspinal muscles ( Fig. 3.9). Additionally, via lateral muscular branches and contributions to the spinal accessory nerve (SAN), the cervical plexus innervates the SCM, TPZ, levator scapula, and scalene muscles. The ansa cervicalis is formed by a superior and inferior root supplied by cervical plexus contributions from C1 and C2–C4, respectively, which supplies all strap muscles with the exception of the thyrohyoid.¹ The superior root branches from the HN as it courses lateral the carotid sheath near the occipital artery. The superior root descends, coursing along or within the carotid sheath, to meet the descending inferior root in the mid neck on the lateral surface of the IJV.

The phrenic nerve innervates the diaphragm and is formed by contributions from C3–C5. It is formed in the floor of the lateral neck and descends on the surface of the anterior scalene and is unique in that it does not pierce the DLDCF.

Fig. 3.13 Lateral view of the neck with the straps and sternocleidomastoid muscle removed highlighting the venous system.

The posterior border of the SCM is an additional landmark for the posterior boundary of the lateral neck. It is invested by the SLDCF and forms the roof over a large majority of the lateral neck ( Fig. 3.16). The muscle is formed by two heads inferiorly, the sternal anteriorly, and the clavicular posteriorly, with a slight depression between the two, which terminates as the muscle bellies merge. It has an oblique course in the neck as it ascends toward the mastoid process and lateral nuchal line superiorly. It is innervated by the SAN and contributions from the lateral muscular branches of the cervical plexus (C2–C3), which descends obliquely to reach the muscle’s deep surface near the junction of the upper and middle third.

The Medial Boundary of the Lateral Neck

The Carotid Sheath

The medial boundary of levels III to IV is defined by the medial wall of the common carotid inferiorly and internal carotid artery more superiorly ( Fig. 3.12). This landmark is generally more useful radiographically as surgically the lateral border of the sternohyoid is more commonly used. As the carotid is the most medial structure within the carotid sheath, the contents of the sheath are in the domain of the lateral neck. The contents of the sheath are the common and internal carotid arteries, the IJV, the vagus nerve, and portions of the ansa cervicalis. Within the sheath, the carotid lies medial to the IJV, while the vagus nerve is situated between and posterior to both vessels.

Fig. 3.14 Cross section through level IB and the oral cavity allows demonstration of the neurovascular structures relative to the mylohyoid and hyoglossus muscles and the submandibular gland.

The common carotid is derived from the aortic arch in the left neck, and the brachiocephalic artery on the right with rare exceptions. From the deep plane to the sternoclavicular joint, the course of both arteries is similar, diverging laterally and coursing more superficially until their bifurcation.¹⁴ The bifurcation into external and internal carotids served as the demarcation between levels II and III prior to the revised classification, which adopted the inferior border of the hyoid bone (which approximates the carotid bifurcation). Above the level of the cricoid, the carotid is more superficial, emerging from the anterior medial border of the SCM. The common carotid generally does not branch prior to the bifurcation though rarely branches of the external carotid may arise from it ( Fig. 3.10).

The internal carotid continues within the carotid sheath to supply the anterior cerebral circulation, eye, nose, and portions of the forehead without branching in the neck. For a brief period, prior to ascending deep to the stylohyoid muscle, the internal carotid may serve as the anterior limit of level III below the plane of the hyoid bone.

3.3.3 Level IV

Level IV contains the lower jugular chain lymph nodes and occupies the space superior to the clavicle and inferior to the lower border of the cricoid cartilage ( Fig. 3.2). We will discuss the pertinent anatomy of this level relative to the major vascular structures.

The arterial supply to the head and neck enters through the root of the neck traversing level IV ( Fig. 3.8). These include the common carotid, and branches of the subclavian including the vertebral artery, and vessels classically associated with the thyrocervical trunk (TCT). The most medial of these vessels forms the medial boundary of level IV, the common carotid. Anterior to the common carotid artery are the sternohyoid and sternothyroid strap muscles as well as the SCM more superficially. The artery is crossed anteriorly by the middle thyroid vein and omohyoid muscle, both in proximity to the superior border of this level. The omohyoid approximates the position of the cricoid as it crosses the lateral surface of the carotid sheath, and was previously used as the distinction between levels III and IV ( Fig. 3.8).³

Medial to the artery lies level VI, which will be discussed in detail later. Laterally, the artery is bound by the IJV, which can rotate to a more anterior position low in the neck. The vagus nerve is posterolateral to the artery.

Fig. 3.15 Left lateral view of the floor of level IB. (Adapted from THIEME Head and Neck Anatomy for Dental Medicine. © Thieme 2010. Illustrations by Karl Wesker.)

Deep to the common carotid lies the muscular floor of the neck composed of paraspinal muscles covered by prevertebral fascia, otherwise known here as the DLDCF. In level IV, these muscles include the longus colli medially the anterior scalene laterally ( Fig. 3.19). The anterior scalene has attachments to the transverse processes of the lower cervical vertebra (C3–C6), which lie immediately posterior to the common carotid. The muscular fibers of the anterior scalene join in an oblique course leaving the neck to insert on the first rib. The phrenic nerve is a very critical structure that runs over the anterior surface of the muscle deep to the DLDCF. At the lateral border of the anterior scalene, the ventral rami of the cervical plexus (C2–C4) pierce the DLDCF demarcating the lateral extent of the lateral neck levels. As the lateral border of the anterior scalene is followed inferiorly, the brachial plexus is encountered emerging between the anterior and middle scalene muscle covered by an extension of the DLDCF. However, being posterior to the posterior border of the SCM, the brachial plexus lies deep to the floor of level V.

Deep to the common carotid and inferior to the transverse process of C6, an angle is formed as the anterior scalene diverges from the more medial longus colli muscle ( Fig. 3.18). In this angle, the vertebral artery, inferior thyroid artery (ITA), sympathetic trunk, and thoracic duct (on the left) can be found ( Fig. 3.19).¹⁴

The ITA is a terminal branch of the TCT. The vertebral artery and TCT originate from the first part of the subclavian artery, which is the segment lateral to the anterior scalene. These are consistently the first and second superior branches of the first part of the subclavian as demonstrated in 498 neck dissections ( Fig. 3.19).¹⁵

The vertebral artery ascends in a posterior and medial trajectory crossing behind the common carotid in a lateral to medial direction before piercing the angle between the longus colli and anterior scalene to ascend in the vertebral canal of the transverse process (C6–C1). The artery enters at C6 in more than 80% of cases ( Fig. 3.20).¹⁶ The sympathetic truck is in proximity to the medial aspect of the vertebral artery below C6 and posterior to the common carotid in this location. The middle cervical ganglion of the sympathetic trunk may be found in this location, below C6, and anterior and inferior to the ITA ( Fig. 3.19).

The TCT, which originates from the subclavian artery lateral to the vertebral artery near the medial border of the anterior scalene, branches almost immediately into the inferior thyroid, the transverse cervical, and the suprascapular arteries in its classical description. However, as we will discuss, this anatomy can be quite variable. The ITA, which is considered the terminal branch of the TCT, initially ascends over the medial aspect of the anterior scalene before turning medially. The ascending cervical artery consistently branches superiorly from the ITA to supply the paraspinal muscles. In its medial trajectory, the ITA crosses over the vertebral artery and deep to the common carotid and sympathetic trunk before entering level VI.^14,15

The other branches of the TCT include the transverse cervical (also referred to as the superficial cervical artery), the suprascapular, and dorsal scapular arteries. There is a high degree of variability in the origin of these vessels. The transverse cervical artery (TCA) originated from the TCT (75%) or the subclavian artery (21%), with the remaining cases originating from the internal thoracic artery in a series of 498 dissections.¹⁵ The TCA originates from the TCT at a mean of 17 mm superior to the clavicle at the anterior border of the SCM.¹⁷ The TCA traverses level IV deep to the IJV and SCM, coursing from medial to lateral superficial to the anterior scalene and DLDCF in level V. The suprascapular artery has a more inferior course descending posterolateral anterior to the subclavian artery paralleling the omohyoid toward the suprascapular notch.

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