Jumin Sunde and Mauricio A. Moreno Abstract
The neck anatomy covered in this chapter is from the perspective of the surgeon. The organization is unique in that the anatomy is compartmentalized by neck level. Each level of the neck is approached with a discussion of pertinent boundaries followed by a detailed discussion of the anatomical contents highlighting critical relationships. Special effort has been taken to discuss important anatomical variations when present.
Keywords: neck anatomy, lateral neck, central neck, posterior neck, cervical fascia
3.1 Introduction
3.1.1 Division of the Neck by Triangles
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 neck is divided in six main levels and additional sublevels. We will discuss each level in order with a description of relevant boundaries, followed by a detailed description of their contents. There are common boundaries to all neck levels; these include the prevertebral fascia—or deep layer of the deep cervical fascia (DLDCF)—which serves as the deep boundary, and the investing—or superficial—layer of the DCF, which serves as a superficial boundary. For this reason, we will begin with a description of cervical fascia layers.
3.2 Facial Layers of the Neck
3.2.1 Introduction
The facial layers of the neck are of critical importance to a fundamental understanding of surgical neck anatomy. They are utilized for surgical access, as they provide generally clean and avascular planes of dissection. They can also serve as natural barriers to the spread of disease processes within the neck, whether neoplastic or infectious. A keen understanding of these layers helps the surgeon to compartmentalize the neck anatomy.
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 re- ports.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 pectora- lis 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 jugu- losubclavian 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 plas- tination 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 This maneuver is performed, 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.)
3.3.1 Level I
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 posteroinfe- rior 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 ante- rosuperiorly (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 Posterior Boundary of the Lateral Neck
Cervical Plexus
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.
Sternocleidomastoid Muscle
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.
Fig. 3.16 Lateral view of the neck at the depth of the investing layer of the deep cervical fascia with superficial venous system and cutaneous sensory nerves depicted.
(Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)
The IJV is the major venous structure in level IV and is in the carotid sheath lateral and occasionally anterior to the common carotid inferiorly. Deep cervical nodes in this level are found anterior and lateral to the IJV. The IJV is crossed anteriorly by the intermediate tendon or superior belly of the omohyoid approximating the superior border of level IV. It is covered by the strap muscles, followed by the SCM more superficially. Anteroinferiorly, it is crossed by the anterior jugular veins, which are superficial to the strap muscles. The anterior jugular and EJV may enter the IJV directly, but more commonly they join the subclavian vein. The subclavian vein joins the IJV posterior to the medial head of the clavicle and forms the brachiocephalic vein, anterior to the first part of the subclavian artery. The confluence of these great veins is a landmark for critical lymphatic drainage.
The lymphatic drainage and major lymphovenous connections are highly variable but of paramount importance in level IV neck anatomy. The lymphatic system, in addition to its immunologic role, is responsible for transporting lymph, a fluid produced from interstitial fluid that permeates the walls of lymph capillaries throughout the body, as well as lipids and proteins from the intestine known as chyle. The thoracic duct is the largest lymphatic vessel in the body, draining lymph from all areas except for the right upper extremity, right head and neck, and right upper hemithorax, which is the domain of the right lymphatic duct. The thoracic duct ascends from the thorax, anterior to the spine and medial to the aorta, to enter the base of the neck. The thoracic duct empties into the great veins in the left neck in up to 95% of cases, with the remaining cases having a right-sided or rarely bilateral drainage pattern.18
Fig. 3.17 The sternocleidomastoid muscle reflected and contents of the lateral neck exposed to the level of the deep layer of the deep cervical fascia to better demonstrate the cervical plexus (continued).
The thoracic duct enters the neck after its ascent through the superior mediastinum, along the left lateral aspect of the esophagus being crossed anteriorly by the aortic arch. Below C6, it is found posterior to the common carotid. In this location, it courses laterally, posterior to the carotid sheath, and arches 3 to 5 cm above the clavicle. Once lateral to the IJV, it courses an- teroinferiorly to the subclavian artery on its way to the confluence of the IJV and subclavian vein, which is its most common point of drainage. The location immediately lateral to the inferior carotid sheath is the most likely site of injury. In this location, the thoracic duct is joined by lymphatic branches from the jugular and subclavian trunks, and terminates as one or multiple vessels. The termination is most commonly the venous angle of the IJV and subclavian, or the IJV itself with termination usually occurring within 15 mm of this angle (Fig. 3.19).18,19
The lymphatics in the right neck do not contain chyle except in the cases of right-sided or duplicate thoracic ducts, in which case chyle leaks have been reported. The lymphatic terminus in the right neck as in the left is centered around the confluence of the IJV and subclavian veins. It is formed by the confluence of jugular, subclavian, and bronchomediastinal lymphatic trunks. It is more common for these trunks to individually terminate into the venous circulation but on occasion they coalesce toward the right lymphatic trunk, which may approach a centimeter in length.14,19
Critical neural structures of level IV have been mentioned and include, from lateral to medial, the phrenic and vagus nerves as well as the cervical sympathetic trunk. These neural structures are protected by fascia: the phrenic below the DLDCF, the vagus housed within the carotid sheath, and the sympathetic trunk posterior to the carotid sheath on the surface of the prevertebral fascia. They all have a vertical course in the neck, and can be found immediately anterior to the first part of the subclavian artery when traced inferiorly. It is in this location in the right neck that the recurrent laryngeal nerve (RLN) branches from the vagus nerve. The RLN then loops around the subclavian artery inferiorly and ascends obliquely behind the common carotid to enter level VI. On the left, the RLN branches from the vagus near the anterior aortic arch, and ascends after looping under the arch to assume a more medial position in the tracheoesophageal (TE) groove without traversing the lateral neck.
Fig. 3.17 (Continued) Schema of the cervical plexus.
3.3.4 Level III
Middle jugular lymph nodes are contained within level III, which extends form the inferior border of the cricoid cartilage to the inferior border of the hyoid bone (Fig. 3.2). For the sake of organization, we will utilize the carotid bifurcation as the superior limit of this discussion and reserve our discussion of the external carotid artery (ECA) and its branches to level II.
The use of radiographically identifiable landmarks has served as a major driving force to modifications in cervical level boundaries.3,12 The initial work by Som et al was critical in the adoption of the revised superior, inferior, and medial borders of level III.20 The superior and inferior borders were revised in 2002: the superior border changed to the inferior border of the hyoid rather than the carotid bifurcation, and the inferior border changed to the inferior cricoid cartilage rather than the omohyoid.3 In 2008, the medial boundary of the common carotid was accepted by the AHNS as an alternate landmark to the sternohyoid muscle.12
The common carotid, with rare exceptions, is devoid of branches and bifurcates, in most instances, superior to the inferior aspect of the hyoid in level IIA. The IJV, which lies lateral to the artery within the carotid sheath, has the superior and middle thyroid vein as its only major tributaries at this level. The thyroid veins cross anterior to the common carotid. The vagus nerve is found posterior to the carotid artery and IJV within the carotid sheath. In this location, the anterior aspect of the carotid sheath is draped by the ansa cervicalis, which innervates the strap muscles immediately adjacent to it. The lymphatics of the midjugular chain include the jugulo-omohyoid node, located immediately superior to the superior belly of the omohyoid, near its junction at the superficial of the carotid sheath.
The superficial boundary of level III is the SLDCF, which invests the SCM. It extends medially to the MLDCF investing the lateral aspect of the sternohyoid muscle. In the superior reaches of level III, near the hyoid bone, the carotid has a more superficial course emerging from behind the medial aspect of the SCM, immediately deep to the SLDCF (Fig. 3.12).
The floor of level III is formed by the DLDCF. The muscles deep to the fascial floor of level III are the anterior scalene, lateral to the transverse process of the cervical spine, and the longus muscles medial to this structure (Fig. 3.8, Fig. 3.18). To better understand the contents surrounding the floor of level III, it is helpful to know which cervical vertebrae are likely to be present deep to this level. Using superficial radiographically identifiable landmarks like those used in delineating cervical level boundaries, Shen et al have made critical correlations to deeper structures based on radiographic data from 108 patients in consistent anatomical position. Using the plane of the superior thyroid cartilage, which approximates the inferior hyoid bone, and therefore the superior plane of level III, the authors determined that in more than 90% of cases this fell in the C4-C5 level. The carotid bifurcated greater than 1 cm above the level of this plane in 81.5% of cases, and therefore in the inferior aspect of level IIA. The inferior cricoid plane, which delineates the inferior boundary of level III, was most commonly at C7 (41.5%) ranging from C5 to T1.16 It is worthwhile noting that the reported location of the carotid bifurcation has been quite variable, with other prominent sources siting its location to be at the plane of the upper thyroid cartilage in the plane C3-C4 disc space.14
Fig. 3.18 Paraspinal muscles.
(Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)
Based on these data, the contents of the floor of level III are estimated to be related to C4-C7. The critical structure deep to the floor of the neck below the DLDCF would be the phrenic nerve, over the anterior scalene posterior lateral to the carotid sheath. The sympathetic trunk is also found in the floor of the neck posteromedial to the carotid sheath. The sympathetic trunk is generally found on the DLDCF covering the longus muscles; however, this can be variable, with some sources describing it deep to the fascia, and yet others reporting it within the carotid shealth.21 It takes a more medial course near the anterior tubercle of C6, which is recognized as the most prominent tubercle in the neck, and as previously mentioned, the level at which the vertebral artery enters the transvers foramina. In 30 cadaver dissections, the sympathetic trunk was found just over 1 cm lateral to the medial border of the longus colli muscle at C6.21 In the same study, the intermediate ganglia of the sympathetic trunk was found most commonly in the C5-C6 level. The critical structures found in proximity to C6, and discussed in detail in the preceding section on level IV, should be kept in mind when dissecting deep in the floor of level III.
3.3.5 Level II
The upper jugular chain lymph nodes are contained in level II, which extends from the skull base inferiorly to the plane of the inferior aspect of the hyoid bone (Fig. 3.2). Its anterior margin is the stylohyoid muscle, or radiographically the vertical plane posterior to the SMG, which was formally accepted by the AHNS as the anterior boundary in 2008.3,12 Posteriorly, as with other lateral neck levels, the posterior border of the SCM demarcates this level from level V. Level II is further divided into sublevels IIA and IIB, with the latter being superior and posterior to the oblique plane of the SAN (CN XI). The floor of level II is formed by the DLDCF covering the paraspinal muscles including the superior extent of the anterior and middle scalene, splenius capitis, and the superior portion of the levator scapulae near its insertion on the lateral process of C1, which is a prominent palpable landmark. The SAN traverses superficial to the levator scapulae throughout its course in the neck. Anteriorly, the floor is formed by the buccopharyngeal fascia covering the middle pharyngeal constrictor as well as muscular fascia over the posterior inferior hyoglossus muscle (Fig. 3.8). The roof of level II is the SLDCF.
Fig. 3.19 Root of the neck.
(Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)
Arterial Anatomy
External Carotid System (Netter’s Plate 30, 65)5
The ECA originates at the carotid bifurcation. As discussed previously, this is thought to occur within level IIA in the majority of cases based on radiographic data.16 After bifurcating from the common carotid, the ECA is initially found anterior and medial to the internal carotid. As it ascends with the internal carotid, it takes a gentle spiraling course to an anterior and then lateral position relative to the internal carotid. The ECA exits perfuse the midportion of the SCM. Beyond this, the STA is a central neck structure that has a critical relationship with the superior laryngeal nerve, and will be discussed further in level VI anatomy.14
Fig. 3.20 Arterial anatomy of the root of the neck.
(Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)
The lingual artery is found above the STA, branching anteriorly from the ECA in proximity to the greater cornu of the hyoid14 (Fig. 3.12). As its name suggests, it provides the major blood supply to the tongue. According to data from 41 cadavers, the location of lingual artery branching is approximately 1 cm above the carotid bifurcation. In approximately 80% of cases, the artery arises an independent branch, while in approximately 20% of cases the artery arises as a common trunk, known as the linguofacial trunk with the facial artery (Fig. 3.21). In such cases, the trunk has an average length of less than 1 cm prior to bifurcating into its respective arteries.23 It has also been described in rare cases arising with the STA (thyrolingual trunk) or as a common trunk with all three vessels (thyrolingual facial trunk). After branching, the artery has an initially ascending course with an anterior and medial trajectory before descending parallel to the hyoid. The middle pharyngeal constrictor is found immediately deep to the artery at this level. The artery is more superficial in its initial trajectory through level IIA, on its path deep to level IB where it courses medial to the hyoglossus muscle. Rare variations have been described with the artery coursing lateral to the hyoglossus muscle.24 The lingual artery is crossed laterally, in level IIA—at its most superior point—by the HN, which courses lateral to the external and internal carotid, and medial to the IJV and branches of the facial artery.14
The facial artery is the most superior of the anterior branches of the ECA in level IIA, and as described, has a variable pattern of origin not infrequently occurring from a common trunk particularly with the lingual artery. The artery is usually found immediately level IIA midway between the styloid process and the angle of the mandible as it courses deep to the stylohyoid and closely associated posterior digastric muscle. It is located lateral to the more superior stylopharyngeus and styloglossus muscles, and stylohyoid ligament, which separates the external carotid from the internal carotid, which lies medial to these styloid based structures (Fig. 3.8).14
Reflecting on the embryology of the branchial apparatus provides a road map to critical relationships within level IIA. The third arch derivatives include the common and proximal internal carotid, the glossopharyngeal nerve, and the stylopharyng- eus muscle. The styloid process and stylopharyngeus muscle, as well as the glossopharyngeal nerve, cross the immediate lateral surface of their third arch artery, the internal carotid, and intervene between it and the more lateral ECA.
Our discussion of ECA anatomy will be limited to branches found inferior to the stylohyoid muscle. Superior to this the artery enters the deep lobe of the parotid and travels superiorly in the parapharyngeal space before terminally branching into the internal maxillary and superficial temporal arteries. We will categorize ECA branches based on the orientation of branching: anterior, posterior, or medial.
The anterior branches found in level IIA include the superior thyroid, lingual, and facial arteries, which typically branch from the ECA in that order (Fig. 3.21). The superior thyroid artery (STA) is the first branch of the external carotid. After branching from the anterior surface of the ECA, near the carotid bifurcation, it descends toward the superior pole of the ipsilateral thyroid lobe paralleling the lateral surface of the thyrohyoid muscle. In very rare cases, it may branch from the ECA as a common trunk with the lingual artery.22 The STA has one branch of limited clinical significance in the lateral neck, the SCM artery, which transverses the carotid sheath laterally to above the superior cornu of the hyoid, although the distance from the carotid bifurcation can be quite variable (8-50 mm).1425 Like the lingual, the facial artery is superficial in level IIA, immediately deep to platysma and SLDCF and anterior to the SMC in the carotid triangle (Fig. 3.12). It takes an anterosuperior course deep to the stylohyoid and posterior digastric muscle, entering level IB. On its deep surface is the middle pharyngeal constrictor, and on the lateral surface the HN. There are two branches of the facial artery, each with a vertical trajectory in this level: the ascending palatine and tonsillar arteries.
Fig. 3.21 Anterior branches of the external carotid artery. (a) Thyrolingual, (b) linguofacial, (c) separate branching pattern, (d) and a representation of various branching patterns. CC, common carotid artery; ECA, external carotid artery; F, facial artery; L, lingual artery; LFT, linguofacial trunk; TLFT, thyrolinguofacial trunk; TLT, thyrolingual trunk; STh, superior thyroid artery. (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.)
The ascending pharyngeal artery is the lone medial branch of the ECA arising from its deep surface (Fig. 3.20). It originates near the STA close to the carotid bifurcation. It has a vertical trajectory and a deep course medial to the internal carotid and the stylopharyngeus and styloglossus muscles, supplying deep neuromuscular structures on its course to the skull base.14
The only posterior branch of the ECA within level IIA is the occipital artery (Fig. 3.22).5 The occipital artery branches from the ECA and courses posterosuperiorly, lateral to the contents of the carotid sheath and following the deep surface of the digastric muscle. Near its origin, it is crossed laterally by the HN. It courses between the lateral process of C1 inferiorly and the mastoid process superiorly, exiting level IIB deep to the SCM and digastric muscles. Its two major branches are to the SCM. The superior branch follows the SAN.14 The relationship between the superior branch and the accessory nerve was investigated by Rafferty et al in 33 neck dissections. The branch was consistently located inferior and lateral to the entry point of the nerve into the medial SCM with an average of 6.2 mm (1- to 11-mm range).26
The Internal Carotid
The internal carotid has a vertical trajectory from the carotid bifurcation to the skull base, except for rare cases in which it may take a medial and retropharyngeal course. It does not contribute branches in the neck. Many of the critical relationships of the internal carotid have been previously discussed. It leaves the carotid triangle deep to the digastric, and exits level IIA deep to the styloid process and its associated muscles: the stylohyoid, styloglossus, and stylopharyngeus. At its lateral surface is the IJV and vagus nerve more posteriorly. As the skull base is approached, the IJV takes a more posterior position with neural structures intervening between the great vessels (CN IX—XII). It is bordered medially by the middle and superior pharyngeal constrictors, with the superior laryngeal nerve coursing between (Fig. 3.22).5 Posteriorly is the longus capitis muscle and the transverse processes of the upper cervical vertebrae, and as has been the case throughout the neck, the sympathetic chain can be found in this space between the deep fascial and the posterior surface of the artery. As the artery courses superiorly, it is crossed laterally in sequence by the lingual and facial veins, the HN (which contributes the superior root of the ansa in this location), the digastric muscle, occipital artery, the glossopharyngeal nerve, the stylohyoid muscle, and the styloid process.
Fig. 3.22 Lateral view of the neck with the straps and sternocleidomastoid muscle removed highlighting the arterial system.
The Venous System of Level II
The IJV leaves the skull base as a continuation of the sigmoid sinus and jugular bulb, and it is found in the posterior aspect of the jugular foramen known as the pars vascularis (Fig. 3.23). The IJV is closely associated with CNs IX through XI exiting the foramen as will be discussed in the following section. The IJV is found posterior to the internal carotid at the skull base, but assumes a lateral position in the carotid sheath as it descends. The IJV exits the skull base deep and posterior to the styloid and its associated muscles. In level II, it is crossed on its lateral surface in descending order by CN XI (this is a variable relationship), the occipital artery with its SCM branches, the digastric muscle, and the ansa cervicalis (Fig. 3.13).5 Medial to the vein, at this level, is the transverse processes of the upper cervical vertebra, the paraspinal muscles (splenius capitis, levator scapulae, and anterior and middle scalene muscles) with associated DLDCF, and the cervical plexus.
Fig. 3.23 Venous drainage of the neck.
(Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)
The major tributaries of the IJV include the superior thyroid and lingual and facial veins. The location of the confluence of these vessels with the IJV can be approximated by the greater cornu of the hyoid (Fig. 3.23).14 The distal lingual and facial veins were discussed in more detail with level IB, and the superior thyroid will be discussed with level VI. The facial vein enters level IIA superficial to the digastric muscle traveling in a posterior trajectory. It is soon joined by the anterior division of the retromandibular vein, which descends through the parotid to enter level IIA traveling in the same plane as the facial vein (Fig. 3.12). This confluence forms the common facial vein. The common facial vein is superficial to the HN, lingual artery, and external and internal carotid arteries on its poster- oinferior course to the IJV.14 The lingual vein enters level IIA in plane with the lingual artery, deep in the hyoglossus muscle traveling superficially in plane with the facial vein to confluence in the IJV. Additional venous drainage from the deep and sublingual territories coalesces to form the vena comitans of the HN, which also terminates into the IJV directly or indirectly (Fig. 3.13).5
In 58 unilateral cadaver dissections, Shima et al described 5 variations in the pattern of confluence of the 3 major venous tributaries (facial, lingual and superior thyroid) with the IJV.
The thyrolinguofacial type, with all three vessels merging as a common trunk, was seen in 46.6% of dissections, followed by the linguofacial type in which the lingual and facial entered as a common trunk with the superior thyroid entering separately in 22.4% of dissections (Fig. 3.23). In the least common variant (6.9%), the lingual and superior thyroid entered as a common trunk below the facial. In the remaining 24.1% of specimens, there were lingual and superior thyroid tributaries without a facial vein contribution to the IJV, but no further mention was made regarding the whereabouts of the facial drainage.27 It is interesting to note that in a series of 178 unilateral cadaver dissections, 9% of facial veins drained superficially, directly into the EJV and did not pierce the SLDCF which could explain the finding from the above series. In these cases, the EJV was formed by the retromandibular vein, which did not divide into anterior and posterior divisions, joining the posterior auricular vein. The various branching patterns with the facial vein are described in detail by Gupta et al.28
Neural Anatomy of Level II
Taken in numerical order, we will discuss CN IX-XII, the sympathetic chain and pertinent branches of each. CN IX (glossopharyngeal nerve) exits the skull base in the pars nervosa of the jugular foramen anterior to the IJV, interposed between the internal carotid artery and the IJV. This is the common foramen for CN IX-XI with CN X and XI found just posterior to CN IX and associated with the medial wall of the IJV (pars vascularis). From here, CN IX descends medial to the styloid process and its associated muscles along the posterior border of the stylophar- yngeus, which is the only muscle it innervates (Fig. 3.15).14 As mentioned, embryologically it is a third arch derivative, and as is courses along the stylopharyngeus it curves anteriorly along its lateral surface, and travels toward the superior constrictor dividing the internal and external carotid as does the stylophar- yngeus. As the nerve continues anteriorly, it travels deep into the stylohyoid muscle and out of level IIA into the parapharyngeal space (Fig. 3.24).5
CN X (vagus nerve) descends in the carotid sheath, between the carotid and IJV, as in lower levels in the neck. It originates from the medial aspect of the jugular foramen. It is medial to the styloid process, and may be found in a plane between the styloid anteriorly and the transverse process of C1 posteriorly, in close association with CN IX to XII near the skull base. There are two enlargements in this level of the vagus, the superior and larger inferior (nodose) ganglia, which contain cell bodies for sensory fibers.14 The superior ganglia lie at the level of the jugular foramen, and the nodose lies just below it. The superior laryngeal nerve is a major branch from the vagus and originates at the inferior ganglia. It descends medial to the internal carotid, and lateral to the middle pharyngeal constrictor as it travels anteroinferiorly. It divides into internal and external branches near the medial aspect of the external carotid, as it continues its descent.29 The anatomy of these terminal branches will be discussed with level VI.
CN XI (SAN) has a complicated intracranial origin with both cranial and spinal roots. For the purposes of level II neck anatomy, the main interest lies with the spinal root, as the cranial root joins the vagus nerve near the skull base above the inferior ganglion. The spinal root continues to provide motor innervation to the SCM and TPZ, and is the structure identified during neck dissection. The SAN descends from the jugular foramen near the medial wall of the IJV, deep and posterior to the plane of the styloid. It travels in a posterior and lateral trajectory with a variable relationship to the IJV, which it crosses near the superior border of the digastric (Fig. 3.8). The literature is surprisingly varied on this topic, with the nerve most commonly reported lateral to the IJV (39.8-96%).30 31 In very rare cases (< 3%), the nerve may pierce the artery. The nerve has a consistent relationship with the superior branch of the occipital artery, which is found inferior and lateral to the SAN. The occipital artery continues after the division of its superior branch to cross lateral to the nerve.
The SAN nerve traverses the contents of the lateral neck dividing the posterosuperior contents into level IIB and the anteroinferior contents into level IIA. Deep to the nerve, and below the DLDCF, is the levator scapulae muscle that the nerve travels superficial to through its course in levels II and V. The SAN is associated with the anteromedial surface of the SCM in its upper third. The nerve has been classified by Shiozaki et al as having three anatomical variants (types A, B, and C) at the SCM: not penetrating, partially penetrating, and completely penetrating, respectively.32 The nonpenetrating variant (A) is less common than the combined penetrating subtypes (B + C), but have been observed as frequently as 45.9% in one series.30 The cervical plexus has variable contributions to the SAN from the C2-C3 level, joining the nerve within the substance of the SCM. From the lateral margin of the SCM, the nerve enters level V and will be discussed later.
CN XII (HN) originates from the hypoglossal canal medial and anterior to the jugular foramen. In its descent, it initially courses laterally and is found posterior to the internal carotid and CN IX and is closely associated with the inferior vagal ganglia. From here, it assumes a vertical trajectory between the IJV and internal carotid.14 At a location approximated by the vertical plane of the mandibular angle, the HN curves anteriorly to assume a horizontal trajectory, and in doing so it courses between the IJV laterally and the internal carotid medially. It is in this location that the ansa hypoglossi divides and descends along the internal carotid. In 46 cadaver dissections, this point was on average 2.8 cm posterior to the angle of the mandible. The HN passed superiorly above the carotid bifurcation at an average of 15 mm.29 As the nerve coursed lateral to the external carotid, it crossed over the lingual artery in 72% of the cases, and it was inferior to its origin in the remaining 28%. The nerve continues anteriorly, superior to the hyoid, to enter level IB medial to the digastric and stylohyoid, and lateral to the hyoglos- sus muscles (Fig. 3.8). The HN has a consistent relationship with the posterior course of the occipital artery, which courses lateral to the vertical segment of the nerve.29
The cervical sympathetic chain (CSC) has been discussed in lower levels of the neck, and its position is consistent at this level, medial to the carotid sheath and in close relationship with the DLDCF associated with the longus muscles, specifically the longus capitis. The superior cervical ganglia is the largest of the ganglia, and readily identified in the C2-C4 level (Fig. 3.8).21 From this ganglia, postganglionic fibers are distributed broadly throughout the head and neck as well as intracranially via the carotid.14 Injury to the nerve produces Horner’s syndrome with the triad of ipsilateral ptosis, miosis, and anhidrosis.
Fig. 3.24 Lateral view of the neck with the straps and sternocleidomastoid muscle removed highlighting the neural and arterial anatomy.
3.3.6 Level V: The Posterior Triangle
Otherwise referred to as the posterior neck or posterior triangle, level V boundaries are as follows: the posterior SCM border and cervical plexus sensory branches anteriorly, the anterior border of the TPZ posteriorly, the junction of the SCM and TPZ superiorly, and the clavicle inferiorly. This level is further subdivided into VA and VB by an imaginary plane extending from the inferior border of the cricoid, which also demarcates levels III from IV.
The contents of level V are bound superficially by the SLDCF— or investing fascia—which invests the SCM and TPZ muscles (Fig. 3.25). This layer is immediately deep to the platysma, which is present superficially to the inferior aspect of level V (Fig. 3.4). The SLDCF is more poorly defined over the posterior triangle than anterior to the SCM, and is described as an areolar layer in this location in Gray’s Anatomy.1 The deep boundary of level V is the DLDCF, which covers the surface of the paraspinal muscles making up the floor of this level (Fig. 3.17).5 This includes from superior to inferior the splenius capitis, levator scapulae, and the middle and anterior scalene muscles (Fig. 3.26).
Fig. 3.25 Posterior cervical triangle (level V) investing fascia present (a) and removed (b).
(Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)
Critical structures that traverse level V will be discussed from superior to inferior. The SAN previously discussed in level II is of high importance in level V, as it has a superficial course and is particularly prone to injury. It travels across level V in a posterior inferior trajectory superficial to the levator scapulae in the fibrofatty tissue of level V. It can be found entering level V at the posterior border of the SCM. In 50 ipsilateral dissections, this point was found 6.13 cm below the mastoid tip, near the junction of the upper and middle third of the SCM.33 In a series of 80 ipsilateral dissections, the relationship of the SAN was found to be consistently superior to the nerve point (also referred to as Erb’s point). The nerve point signifies the location of emerging superficial sensory branches of the cervical plexus near the midpoint of the SCM, and includes from superior to inferior the lesser occipital, great auricular, transverse cervical, and supraclavicular nerves (Fig. 3.25). The SAN was found within 2 cm superior to this point and at an average of 0.97 cm from it. This relationship has been supported by numerous authors, and is perhaps the most consistent means of locating CN XI in the posterior triangle.34
After traversing level V, CN XI reaches the anterior border of the TPZ and innervates the muscle on its deep surface. This point on the TPZ is approximately 4 cm from the clavicle.1,33 CN XI frequently branches prior to reaching the TPZ, with up to five branches reported, with the majority having fewer than three (Fig. 3.19).32,33
The TCA, also known as the superficial cervical artery, has a variable origin, but most commonly branches from the TCT, which originates from the first part of the subclavian artery in the base of level IV. The TCA has a posterolateral trajectory after entering level V, at a point approximated by the lateral border of the anterior scalene (Fig. 3.26). It is located within the fi- brofatty tissue, superficial to the deep fascia and muscular floor of the neck and brachial plexus, and courses deep to the omohyoid and TPZ. The TCA can be found approximately 2 cm superior to the clavicle and is found paralleling the traverse cervical vein in a deep and superior plane. The vein drains into either the EJV or the subclavian vein, and has a variable course relative to the omohyoid with a deep position predominating (75%).35
Fig. 3.26 Posterior cervical triangle (level V) muscular pretracheal fascia or middle layer of the deep cervical fascia removed (a) and prevertebral or deep layer of the deep cervical fascia removed (b).
(Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)
The posterior belly of the omohyoid muscle is a prominent structure low in level V (Fig. 3.26). It travels through the fi- brofatty tissue of the level in a posteroinferior trajectory toward the scapular notch, in a plane superficial to the TCA. It runs superficial to the deep fascia covering the brachial plexus and muscular floor of the neck, and deep to the TPZ, and is covered by muscular pretracheal fascia.
The brachial plexus is derived from the ventral rami of C5-T1, and supplies motor and sensory innervation to the upper limb. It is found in the anterior-inferior-posterior triangle between with anterior and middle scalene and remains covered by deep fascia, forming the axillary sheath. It is associated with the third part of the subclavian artery, which emerges posterior to the lateral border of the anterior scalene (Fig. 3.26). The branchial plexus exits the posterior triangle in a plane posterior to the artery. The subclavian artery may extend partially above the clavicle, and can be up to 4 cm above the clavicle in the inferomedial aspect of level V, near the lateral border of the SCM.1 The subclavian vein is anterior and inferior to the artery, and may also extend into the supraclavicular fossa and into the base of level V.
The suprascapular artery is mentioned because it may be encountered in the inferior reaches of level V, originating from the TCT in approximately 50% of cases, with the remaining cases originating from the subclavian or internal thoracic artery. It shares a common destination with the omohyoid, the suprascapular notch. It enters level V crossing anterior scalene, approximated by the lateral border of the SCM, and travels laterally parallel to the clavicle, superficial to the subclavian artery and brachial plexus (Fig. 3.26).1,15
The dorsal scapular artery is found in the inferior aspect of level V, originating from the TCT in 76% of cases, with 16% of these cases arising as a common trunk with the TCA in a series of nearly 500 dissections. In most remaining cases, the origin was more lateral as a trunk from the third part of the subclavian artery. The dorsal scapular artery may pass either through the brachial plexus or superficial to it, as it courses posterolaterally toward the superior angle of the scapula, deep to the levator scapulae muscle.15
3.3.7 Level VI: The Central Neck
The boundaries of the central neck extend vertically from the hyoid bone to the sternal notch, and laterally to the common carotid arteries. The roof of level VI is formed by the muscular fascia investing the strap muscles, referred to as the MLDCF or muscular pretracheal fascia. The floor is formed by the DLDCF. The visceral structures enveloped by the fine visceral facial layer are critical to the intricate anatomy of the central neck, and include the laryngotracheal complex, hypopharynx, esophagus, thyroid, and parathyroid glands. We will discuss anatomy pertinent to central neck dissection for each of these structures in addition to discussing the neurovascular and muscular anatomy of level VI.
The laryngotracheal complex and hyoid form the central ridged framework of level VI anatomy (Fig. 3.27). The hyoid bone is near the C3-C4 level and separates level I superiorly from level VI inferiorly.1,16 It is a U-shaped bone with a central rectangular body attached laterally to the greater cornu bilaterally (Fig. 3.28). Near this typically ossified junction is small superior projection, the lesser cornu. The hyoid is suspended superiorly by multiple muscles (Fig. 3.15, Fig. 3.29). Those inserting into the body include the geniohyoid and mylohyoid. Near the junction of the body and greater cornu are the stylohyoid and digastric tendon, and along the length of the greater cornu is the hyoglossus. The lesser cornu is the insertion of the stylohyoid ligament.1
The middle pharyngeal constrictor is a fan-shaped muscle that attaches to the lesser cornu, and the upper aspect of the greater cornu of the hyoid (Fig. 3.30).5 Its transverse and inferior oblique fibers, which project deep to the inferior constrictor muscle, meet at the posterior midline raphe and are within the confines of level VI. These form the superior portion of the muscular wall of the hypopharynx. The hypopharynx serves as a conduit between the oropharynx and cervical esophagus and extends from the plane of the hyoid bone to the inferior border of the cricoid. The hypopharyngeal wall is closely associated with both the middle and inferior pharyngeal constrictors, which are covered by buccopharyngeal fascia.
The hyoid serves as the superior attachment for numerous structures (Fig. 3.27, Fig. 3.29). From medial to lateral, the sternohyoid and omohyoid are found inserting into the body of the hyoid bilaterally, with the thyrohyoid attaching laterally along the greater cornu. The sternohyoid originates from the posteromedial clavicle and superior manubrium with a near vertical course to the hyoid. The superior belly of the omohyoid courses vertically, adjacent to the lateral surface of the sternohyoid. It transitions from the posterior belly (discussed in levels III and V) and is at the intermediate tendon, which is formed by a condensation of deep fascia over the IJV near the level of the cricoid cartilage. The thyrohyoid has a short vertical course attaching to the oblique line of the thyroid cartilage.1 These muscles are covered by muscular fascia referred to previously as the MLDCF. Deep to the strap muscles in the superior central neck is the thyrohyoid membrane. This fascial layer attaches the medial aspect of the body and greater cornu of the hyoid with the superior surface of the thyroid cartilage.
The thyroid cartilage is a prominent shield-shaped structure with a superior border approximated by the C4-C5 level in close proximity to the height of the carotid bifurcation.16 Its two laminae are joined in the midline with an intervening superior thyroid notch. On the lateral aspect of the lamina is a ridge called the oblique line, which connects the superior and inferior thyroid tubercles and is directed from anterior to posterior in an oblique superolateral trajectory (Fig. 3.27). As mentioned earlier, this is the attachment of the thyrohyoid muscle as well as its associated sternothyroid muscle, which originates from the manubrium and first rib below the sternohyoid. The oblique line also serves as the attachment of the thy- ropharyngeus, which is the upper portion of the inferior constrictor muscle, which attaches at the posterior midline raphe to the contralateral muscle. The superior and inferior cornua are located at the posterior aspect of thyroid cartilage. The superior cornu is attached to the greater cornu of the hyoid via a condensation of thyrohyoid membrane fascia. The shorter inferior cornu articulates with the lateral aspect of the cricoid forming a synovial joint.1
Fig. 3.27 Lateral view of the skeletal components for the laryngotracheal complex.
(Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)
Fig. 3.28 Hyoid bone depicted from (a) anterior, (b) posterior, and (c) lateral oblique views.
(Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)
Fig. 3.29 Muscular attachments to the hyoid.
(Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)
The cricoid cartilage is found inferior to the thyroid cartilage with which it articulates. It is the only complete ring in the laryngotracheal complex (Fig. 3.30).5 The inferior plane of the cricoid is most commonly found at C7 in adults.16 The cricoid is composed of an arch anteriorly that is narrow relative to the lamina posteriorly (Fig. 3.27). The superior border is oblique, sloping superiorly to the wider posterior lamina, and the inferior border is horizontal attaching to the first tracheal ring via the cricotracheal ligament. The cricoid attaches to the thyroid cartilage via the previously mentioned joint, the cricothyroid ligament, the cricothyroid muscle, and the inferior constrictor muscle. The cricothyroid muscle attaches anteriorly to the arch of the cricoid, and posteriorly to the inferior tubercle and inferior cornu of the thyroid lamina. The inferior constrictor is the most robust of the constrictors, and has attachments to the thyroid (mentioned above) as well as the cricoid. There is a tendinous arch connecting the lesser thyroid tubercle and the posterior cricoid lamina that loops over the cricothyroid muscle and provides additional attachment for the thyropharyngeus, the superior contribution to the inferior constrictor. The cricopharyngeus muscle is the lower contribution to the inferior constrictor, and attaches to the cricoid arch, posterior to the cricothyroid muscle. This muscle serves as the transition between the hypopharynx and the cervical esophagus, and is the functional upper esophageal sphincter. The buccopharyngeal fascia, a thickening of epimysium, envelops the constrictor muscles. There is an intervening loose areolar space separating this fascia from the prevertebral fascia covering the vertebral bodies, and longus muscles.1
Fig. 3.30 Lateral view of the neck demonstrating the laryngotracheal anatomy with associated musculoskeletal structures (straps absent).
The esophagus and trachea continue inferiorly in the central neck before exiting posterior to the manubrium. The esophagus deviates toward the left of midline as it descends in the neck toward the superior mediastinum posterior to the trachea and anterior the prevertebral fascia.
The thyroid gland is composed of an isthmus, situated anterior to the cervical trachea approximating rings 2 through 4, and joining the right and left thyroid lobes. The lobes are roughly conical in shape, with a taped superior pole and rounded inferior pole (Fig. 3.10). Medially, the thyroid lobes are related to the inferior constrictor, posterior cricothyroid muscle, larynx, trachea, esophagus, and the contents of the TE groove including the RLN. Posterolaterally, the thyroid is related to the carotid sheath and the common carotid artery. The anterior and lateral aspects of the gland are related to the deep surface of the strap muscles. These muscles include the sternohyoid medially, which joins its contralateral muscle pair at the midline linea alba (Fig. 3.7). Laterally, it is associated with the sternothyroid, found deep to the omohyoid, inserting into the oblique line of the thyroid cartilage. This muscular insertion into the oblique line serves as an anterosuperior limit for the superior pole of the thyroid. The thyroid is tethered to the cricoid and upper two tracheal rings at the ligament of Berry. This is a condensation of thyroid capsule commonly referred to as the posterosu- perior suspensory ligament and is responsible for the elevation of the thyroid with swallowing.
The embryology of thyroid formation results in common anatomic variations. The pyramidal lobe—a remnant of the thyroglossal tract—extends superiorly from the isthmus and can be identified in over half of patients. The tubercle of Zuckerkandl is a posterolateral projection of thyroid that may be variably positioned laterally, or rarely medial to the RLN near its laryngeal entry point. It is thought to represent a remnant of thyroid near the fusion of its median and lateral anlage. Thyroid rests may also be found below the inferior pole, with variable connection with the thyroid within the thyrothymic tract.36
The parathyroid glands are discrete organs located within the perithyroidal soft tissues, adjacent to the posterior thyroid capsule. They are approximately 5 mm in greatest dimension, and receive blood supply through a laterally based vascular pedicle. They are derivatives of the branchial pouches, which provide insight into their anatomical location. The superior glands originate from fourth pouch with the parafollicular C-cells, which form the lateral thyroid anlage and have a shorter path of migration relative to the inferior parathyroids. Their location is more consistent and generally found in proximity to the posterolateral aspect of the superior thyroid pole, within 1 cm of the cricothyroid joint posterior to the plane of the RLN. They are most commonly perfused by the ITA but may also receive contributions from the STA. The inferior parathyroid glands are derived from the third pouch with the thymus. It has a longer path of migration and notoriously more variable location. It is most commonly found within the loose perithyroidal fatty tissue, off the inferior or lateral aspect of the inferior thyroid pole, and superficial to the plane of the RLN. This can be highly variable with ectopic locations described from the mandible to the mediastinum. It is also perfused by the ITA.37
The RLN is a sixth arch derivative, branching from the vagus nerve to loop under the lowest persisting aortic arch derivative on each side (the fourth arch vessel). With the rare exception of a nonrecurrent right RLN (0.5-1%), in the case of an anomalous right subclavian artery, the right RLN has a more lateral trajectory to the central neck as it loops under the subclavian artery, while the left RLN loops under the aortic arch below the ligamen- tum arteriosum (Fig. 3.31). On the right, the RLN enters the central neck after passing posterior to the origin or the common carotid artery. The right RLN tends to take a more oblique course through the central neck en route to the laryngeal entry point. This distal segment of the nerve is the most anatomically consistent. The nerve enters the larynx deep to the lowest fibers of the inferior constrictor muscle posterior to the cricothyroid joint and muscle. The nerve commonly bifurcates, and rarely trifurcates prior to this point with approximately one quarter remaining un- branched.3839 Immediately proximal to the laryngeal entry point, the nerve is most commonly posterior to Berry’s ligament but may be found within its posterior fibers.
The left RLN is more acute in trajectory than the right and can more commonly found in the TE groove. Shindo et al classified the RLN by its angle of approach in its distal segment, near the laryngeal entry point relative to the coronal plane of the TE groove, and found the majority of right nerves approach at 15 to 45 degrees, whereas left nerves approached most commonly at 0 to 30 degrees.40 More proximally, in the midpolar location, the RLN has a close association with the ITA. The nerve is most commonly deep to the plane of the artery, but may also be found encompassed by distal branches, or less commonly anterior to the artery.38
The superior laryngeal nerve originates as one of the first branches of the vagus nerve at the inferior (nodose) ganglion, and its course within level II has been previously discussed (Fig. 3.31). It enters the central neck after first descending posterolateral to the internal carotid, then crossing medial to the internal and external carotid as it travels toward the larynx. It divides into external and internal branches approximately 1.5 cm inferior to the nodose ganglia.41 This point of division is located variably in relationship to the superior pole of the thyroid, but is typically within 3 cm. The internal branch provides sensory function to the supraglottic larynx and pharynx. It travels in proximity to the superior laryngeal branch of the STA, penetrating the thyrohyoid membrane posterior to the thyrohyoid muscle after ramifying into multiple branches.7 The external branch of the superior laryngeal nerve (EBSLN) provides the sole motor function to the cricothyroid muscle. It descends along the inferior constrictor muscle, and medial to the sternothyroid muscle, the STA, and occasionally the superior pole of the thyroid before reaching the cricothyroid muscle. The most widely used classification for this variable relationship with the superior pole of the thyroid was proposed by Cernea et al42 and constitutes the following:
• Type 1: EBSLN crossing the STA greater than 1 cm above the plane of the superior pole.
• Type 2a: EBSLN crossing the STA less than 1 cm above the plane of the superior pole.
• Type 2b: EBSLN crossing the STA below the plane of the superior thyroid.
The frequency of each type varies within the different reports, but approximately 20% may be considered high risk for injury (type 2b) and 60% low risk (type 1).
Fig. 3.31 The vagus, recurrent laryngeal, and superior laryngeal nerves depicted in relationship to vascular anatomy and visceral compartment of the neck and thorax. (Adapted from THIEME Atlas of Anatomy: General Anatomy and Musculoskeletal System. © Thieme 2005. Illustrations by Karl Wesker.)
The arterial anatomy of the central neck will be approached from superior to inferior, beginning with the STA (Fig. 3.20, Fig. 3.31). Reference the section on level II anatomy for a discussion of its origin from the ECA. The STA takes an anterior and then inferior course from the ECA along the lateral aspect of the inferior constrictor muscle posterior to the thyrohyoid muscle. It has a close relationship to the superior laryngeal nerve as previously mentioned. As it approaches the superior pole of the thyroid, it arborizes into terminal branches supplying the thyroid parenchyma; the posterior branch may anastomose with the ITA supplying the superior parathyroid. The superior laryngeal artery is a larger branch of the STA that accompanies the internal branch of the superior laryngeal nerve piercing the thyrohyoid membrane.
ITA is a terminal branch of the TCT and the proximal course was discussed in level IV anatomy (Fig. 3.8, Fig. 3.20, Fig. 3.31). The ITA enters the central neck deep to the internal carotid artery and sympathetic chain and superficial to the deep fascia covering the longus colli muscle. It arborizes prior to reaching the thyroid parenchyma near the midpolar level. As discussed, the artery has an intimate association with the RLN in this area. The ITA is the primary blood supply to the parathyroid glands via laterally based vascular pedicles.
The thyroid ima artery is a rare, unpaired artery, supplying the inferior pole of the thyroid and lying anterior to the trachea. It may originate for the brachiocephalic, common carotid, aortic arch, or internal thoracic artery, and is more commonly right sided. It may be associated with variable vascular anatomy including absent ITA.43 Another consideration in midline vascular anatomy is a high riding innominate (brachiocephalic) artery, which is the first branch of the aortic arch. It is found anterior to the trachea and in rare cases may extend as high as the inferior pole of the thyroid gland.44
Venous drainage of the thyroid and central neck includes the superior thyroid vein, which accompanies the artery in a superior pole vascular pedicle and is a tributary to the IJV (Fig. 3.10, Fig. 3.23).5 The middle thyroid vein is unaccompanied, and drains the lateral aspect of the thyroid, traversing anterior to common carotid on its lateral course to the IJV. The inferior thyroid veins may be varied and numerous, forming a plexus below the isthmus and anterior to the trachea, draining into the brachiocephalic veins.
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