Muscles of the lateral neck

The sternocleidomastoid muscle (SCM) is the most prominent muscle of the neck. Near its origin, it is comprised by a sternal and a clavicular head separated by the lesser supraclavicular fossa. Its manubrial attachment is a round tendon just inferior and medial to the ipsilateral manubrial cartilage. Its clavicular head attaches to the superior surface of the clavicle with a broad tendon located 1 cm lateral to the superior articular margin of the clavicle. As it ascends posterolaterally, the clavicular head wraps around its sternal counterpart, merging into a thick, rounded muscle ( Figure 1 ). The SCM measures approximately 18-20 cm in length on its course to the mastoid process and lateral portion of the superior nuchal line of the occipital bone. ^, A study by Sato et al. with 354 specimens found 96% of these to share mastoid and superior nuchal line insertions, underscoring a consistent attachment on the skull’s surface.

Left lateral oblique anatomic illustration of relevant structures and relationships seen in the lateral neck. Illustration demonstrating the relationship of the external jugular vein (EJV) lying superficial to the sternocleidomastoid muscle (SCM). Removal of the SCM (dashed line) reveals structures deep to it zooms in to highlight anatomic relationship of structures in levels III and IV. The include the following: the omohyoid muscle (OMH) coursing superficial to the IJV, and the thoracic duct (TD, green) entering the IJV posteriorly. The anterior jugular vein (AJV) and the jugular venous arch (JVA) can be seen traveling superficially to the strap musculature (grey). The transverse cervical artery (TCA) can be seen originating from the thyrocervical trunk (TcT). The middle thyroid vein (MTV) is shown draining into the IJV.

The SCM receives vascular supply from the SCM branch of the occipital artery at its superior third, from the suprascapular artery at its inferior third, and most commonly receives contributions from the superior thyroid artery at its middle segment. ^, When contracted unilaterally, the SCM aids in ipsilateral lateral neck flexion and contralateral neck rotation. ^, Bilateral contraction results in cervical flexion, especially in the supine position, and elevates the thorax in deep inspiration. ^, Two to 4 branches from the spinal accessory nerve (CN XI) innervate the SCM. ^, Although they do not contribute to somatic neuromuscular innervation, contributions of the cervical spinal nerves to CN XI have been a matter of great debate. Our review suggests that CN XI receives branches from the second and third cervical spinal nerves in approximately 90% of the cases just after SCM branches take off on CN XI’s deep surface .

Posterior to the posterior border of the SCM lie 2 distinct landmarks: Erb’s point and the nerve point. Wilhelm Erb described a point 2-3 cm above the clavicle and posterior to the posterior border of the SCM at the level of the carotid tubercule that, when stimulated, results in the contraction of upper extremity muscles. This is the true definition of Erb’s point, commonly mistaken for the nerve point or punctum nervosum. The latter refers to the general region where the cutaneous branches of the cervical plexus, namely the lesser occipital, greater auricular, transverse cervical, and supraclavicular nerves loop around the posterior border of the SCM emerging into the subcutaneous tissues of the neck, roughly 6 cm inferior to the tip of the mastoid process. Historically, there have been conflicting reports where the nerve point has been referred to incorrectly as Erb’s point. Therefore, what is commonly called Erb’s point should be called nerve point or punctum nervosum.

Grouped with the lateral vertebral muscles, the anterior scalene (ASM) aids in cervical flexion when contracted bilaterally and lateral flexion when acting unilaterally. ^, Additionally, it contributes to first rib elevation against a fixed cervical spine during deep inspiration. ^, Although a minor contribution, cadaveric studies also suggest a role for the anterior scalene in ipsilateral rotation of the cervical spine. ^, It originates on the transverse process of cervical vertebrae C3-C6. ^, Fibers from each tendinous attachment merge inferolaterally on its course to the scalene tubercle located on the upper surface of the first rib. ^, The ascending cervical artery, a branch of the thyrocervical trunk, supplies 3 fine branches to the ASM as it ascends on its anterior surface before penetrating the intervertebral foramina of the fourth or fifth cervical spinal nerves. ^, Branches of C4-C7 ventral rami innervate the ASM on its dorsal surface as they course between the vertebral origins of the muscle. ^{, ,}

Immediately medial to the ASM lies the triangle of the vertebral artery. The vertebral artery is centered within this space, traveling superiorly after it branches from the first part of the subclavian artery (SA), which comprises the triangle’s base. The lateral and medial boundaries of this triangle are the medial border of the ASM and the lateral border of the longus cervicis muscle, respectively, while the apex is at the anterior tubercule of the transverse process of the sixth cervical vertebrae. This landmark is often called the carotid or Chassaignac’s tubercule. A systematic review and meta-analysis found the vertebral artery to originate from the SA in 94% and 99% of cases on left and right sides, respectively. Additionally, the vertebral artery entered the sixth cervical transverse foramina in 92% of cases, regardless of laterality. In addition to the artery, the triangle consistently houses the ventral rami of the eighth cervical spinal nerve. The second portion of the SA and the brachial plexus sit, in that order, posterior to the ASM near its insertion to the first rib, while the subclavian vein is anterior to the muscle’s surface at the same level. 1845, ^, The suprascapular and transverse cervical (TCA) arteries, branches of the thyrocervical trunk, cross the muscle laterally, superficial to the phrenic nerve.

Neurovascular structures of the lateral neck

Arteries

On the right, the first portion of the SA arises from the innominate artery behind the upper part of the sternoclavicular joint, and from the aortic arch on the left. ^, The second portion of the SA lies behind the ASM bilaterally ( Figure 2 ). ^, Here, the middle scalene sits posterior to the vessel, the brachial plexus superiorly, and the pleura inferiorly. ^, The subclavian vein sits in front of and slightly inferior to the SA, separated by the ASM. ^, Just lateral and superior are the upper trunks of the brachial plexus and the omohyoid muscle. ^, Distally, the third portion of the SA runs in a rather lateral course, adopting a mildly arced trajectory from the lateral margin of the ASM to the outer border of the first rib, where it becomes the axillary artery.

Anatomic illustration of the arterial system originating from the aortic arch, including the innominate/brachiocephalic artery (IA), from which the right common carotid (RCC) and right subclavian artery (RSA) originate, and the left common carotid (LCC) and left subclavian artery (LSA). The anterior and middle scalene muscles are shown (grey) to demonstrate the relationship of these muscles to the 3 parts of the subclavian arteries, and the location of the vertebral artery (VA), thyrocervical trunk (TcT), transverse cervical artery (TCA), inferior thyroid artery (ITA), internal thoracic artery (IThA), and costocervical trunk (CcT).

The first branch of the SA is the vertebral artery, as discussed previously. Next, the internal thoracic artery branches inferiorly from the SA and heads vertically into the thorax. Immediately medial to the ASM, the thyrocervical trunk branches from the superior surface of the SA and gives off the suprascapular and TCA. Finally, the right costocervical trunk arises from the second portion of the SA, while its origin remains from the first portion of the vessel on the left.

The course of the TCA is particularly relevant for the H&N surgeon as it marks the inferior limit of level IV dissection. ^, The TCA’s position can be defined as 3 cm from midline and approximately 1 cm posterior to the clavicle. Its length varies, ranging between 4 and 7 cm and averaging a 0.25 cm diameter. These vessels are routinely identified lateral to the SCM’s attachment to the clavicle in a triangle bounded by the lateral edge of the SCM medially, the external jugular vein (EJV) laterally, and the clavicle at its base. ^, At the SCM’s medial border, the TCA can be found approximately 1.5 cm superior to the clavicle and 2 cm above the same level at the muscle’s lateral edge.

Veins

The origin of the IJV is marked by the jugular bulb, found within the jugular foramen as it exits the skull accompanied by CN IX, X, and XI. It descends vertically within the carotid sheath, lateral to the internal, or, when inferior to the bifurcation, common carotid arteries. , At the level of the superior margin of the posterior belly of the digastric (PBD) and lateral to the styloid process and stylohyoid muscle, CN XI lies lateral to the IJV ( Figure 3 ). ^, CN XI crosses the IJV anteriorly in 60% of cases and posteriorly in the remaining 40%. Although uncommon, cases of CN XI piercing the IJV are reported in the literature. Along its descent, the IJV collects tributaries from the facial, superior thyroid, and middle thyroid veins, emptying into the jugulosubclavian confluence posterior to the sternal end of the clavicle and merging into right and left brachiocephalic veins. ^{, ,}

Left lateral anatomic illustration of relevant structures and relationships seen in the lateral neck. The top right illustration highlights several key relationships of level II structures, including the posterior belly of the digastric muscle superficial to the internal jugular vein (IJV), spinal accessory nerve (CN XI), and external carotid artery (ECA). Additionally, CN XI is shown in its most common variant, traveling anterior to the IJV. The IJV is also shown originating deep to the styloid process. The bottom right illustration highlights the important relationships between the ECA, the IJV, and the hypoglossal nerve (CN XII). CN XII is deep to the plane of the removed digastric muscle. Overlying it are ranine veins commonly encountered during surgery. As CN XII is traced laterally towards the IJV, it turns superiorly deep to the occipital artery, which tethers it down. The superior root of the ansa cervicalis can also be seen branching from CN XII.

The EJV emerges just below the parotid gland from the confluence of the posterior division of the retromandibular and posterior auricular veins. From here, the EJV crosses the SCM superficially as it begins its slightly oblique trajectory, adopting a lateral position to the SCM at its lower third before crossing the ASM anteriorly and joining the jugulosubclavian venous confluence. Along its descent, it collects tributaries from the transverse cervical, suprascapular, and anterior jugular veins. The mean diameter of the EJV is 0.9 cm and may be inversely correlated to the diameter of the IJV. However, subtle changes in vascular hemodynamics lead to rapid increases in EJV diameter.

Nerves

The ventral rami of the first 4 cervical spinal nerves arise deep in the neck from the intervertebral foramina. From these, small branches may extend superiorly and travel with CN XII or inferiorly to contribute to the brachial plexus. However, nerves arising solely from these 4 cervical levels form the cervical plexus deep to the IJV and anterior to the middle scalene and levator scapulae muscles. As they exit the vertebral column, each ramus unites with neighboring rootlets through ascending or descending loops. These rootlets are contained within the prevertebral fascia and exit as they course laterally posterior to the IJV. Muscular branches of the cervical plexus supply the prevertebral cervical muscles, the strap muscles via the ansa cervicalis, the diaphragm via the phrenic nerve, or join CN XI on its way to the trapezius in the posterior neck. Cutaneous branches include the lesser occipital, great auricular (GAN), supraclavicular, and transverse cervical nerves (TCN).

The GAN appears as it pierces the deep investing fascia of the neck and loops around the posterior aspect of the SCM, deep to the superficial musculoaponeurotic system. ^, This point is called the nerve point as described above. The nerve point can be predictably found 6 cm from the mastoid process or near the midpoint of the posterior border of the SCM, immediately lateral to the EJV as it crosses the muscle inferiorly. ^, Raikos et al. reproduced this technique and found the ratio to consistently lie between 33-57% of total SCM length, predicting a nerve point within the middle third of the SCM in all cases. This point’s importance lies in its utility in identifying CN XI, as these 2 have a constant relationship where CN XI sits approximately 1 cm superior to the GAN along the posterior edge of the SCM. ^{, ,} The distal trajectory of the GAN was described previously by Peraza et al. At this nerve point, the TCN also emerges from the posterior border of the SCM. Contributions from the second and third cervical spinal nerves comprise the TCN, which courses superficially to the anterior neck and splits into ascending and descending branches.

The ventral rami of the first 3 cervical spinal nerves form the ansa cervicalis. Ansa is the Latin term for “handle” or “loop”. In the H&N, it accurately describes the plexus’ morphology as it forms a loop of nerves supplying the infrahyoid muscles. It is comprised of 2 main trunks, each with distinct contributions and courses. Ventral rami from the first and second cervical nerves emerge laterally from the vertebral column, coursing posterior to the second portion of the vertebral artery. From here, branches from each rami join CN XII just medial to the IJV. Several millimeters above the intermediate tendon of the digastric, medial to the PBD and lateral to the internal and external carotid arteries (ICA and ECA, respectively), the superior root of the ansa cervicalis branches from CN XII as the latter loops around the occipital artery and transitions towards the oral cavity. ^, The superior root of the ansa cervicalis descends over the ECA’s anterior surface. Along its descent, one branch extends medially to reach the superior belly of the omohyoid. As it approaches the ansa cervicalis, the superior root has traveled approximately 7 cm inferiorly to merge with the inferior root, anterior and medial to the anterior surface of the IJV. ^,

Contributions to the inferior root arise from the ventral rami of the second and third cervical spinal nerves, which join the inferior root anywhere from 1.7 to 4.4 cm superior to the ansa cervicalis. ^{, ,} Initially, the inferior root descends posterior to the IJV to then wrap around the vein. From the IJV’s lateral surface, it initiates an anteromedial descent towards the ansa cervicalis that extends 5 cm inferiorly. The ansa cervicalis often forms deep to the tendon of the omohyoid as the muscle crosses superficial to the great vessels. Here, branches to the sternohyoid and sternothyroid muscles leave the ansa as a common trunk, and a branch to the inferior belly of the omohyoid runs within a fascial sheet inferior to the tendon. ^, Altogether, nerves arising from the ansa supply all strap muscles except the ThH.

The phrenic nerve receives contributions from the third, fourth, and fifth cervical spinal nerves during its relatively brief course in the neck. ^, Vertically, the nerve’s inception is approximately 6 cm superior to the upper edge of the clavicle. The nerve travels inferiorly in the neck towards the chest along the anterior surface of the ASM for about 2.5 cm and then descends into the thorax anterior to the SA and posterior to the subclavian vein. ^,

Neck dissection: levels I-IV (AHHS and AAO-HNS)

In the following sections, we discuss levels I through IV from the perspective of a lateral neck dissection. A summary of the boundaries for the levels of the lateral neck accompanies each of the following sections as defined by their surgical criteria and comparison to radiologic and radiation oncology deliniation is included for reference in Table 1 . For a detailed discussion on levels V and VI, please refer to the Central Neck and Posterior Neck submissions within this issue.

Level I

Level I is divided into Level IA and Level IB. Level IA is an unpaired level within the neck and extends from the symphysis of the mandible superiorly to the body of the hyoid bone inferiorly. The anterior bellies of the ipsilateral and contralateral digastric muscles are its lateral and medial boundaries, respectively. The mylohyoid muscle lies deep to level I and thus defines the floor for the level (Figure 4A). ^, The mylohyoid muscle serves as a reliable radiographic landmark to define level IA’s superior boundary. The presence of LNs in the submental triangle can vary and node distribution is dictated by the submental artery’s perforators within the level. A study on 40 vascularized submental island flaps found an average of 1.3 nodes along the distal course of the submental artery and its perforators, totaling up to 2.6 nodes within level IA. LNs in level IA measure approximately 0.5 cm in their largest dimension. Nodes here drain the chin and medial lower lip, the anterior mandibular alveolar ridge, the floor of mouth, and the anterior oral tongue. It is important to note that nodes encountered in this region can drain ipsilaterally or contralaterally.

Level IB lies just lateral to level IA, extending from the body of the mandible superiorly to the attachment of the digastric tendon to the hyoid bone inferiorly. Medially, it is defined by the medial edge of the anterior belly of the ipsilateral digastric muscle and laterally by the posterior border of the submandibular gland. In addition to the mylohyoid, the hyoglossus joins the floor of level IB, while the stylohyoid muscle serves as the posterior boundary of all of level I. ^, A transverse line drawn tangent to the posterior edge of the submandibular gland serves as its lateral boundary radiographically as shown in Table 1 .

Surgical dissection of level IB should proceed with caution. First, care should be taken to localize and preserve the marginal mandibular nerve (MMN) above the inferior border of the mandible as it courses anterior and superficial to the facial artery ( Figure 4B ). This can also be achieved by elevating the submandibular gland fascia and the posterior facial vein off the gland anterosuperiorly. This branch of the facial nerve is most reliably found within 1 cm of the facial artery and vein at the level of the facial notch. The surgeon then works anteriorly along the inferior border of the mandible to identify the ipsilateral anterior belly of the digastric and the mylohyoid muscle, which forms the medial floor of this level. The soft tissue overlying the mylohyoid between the anterior bellow of the digastric and the mandible must be elevated and retracted inferiorly, in continuity for the main neck dissection packet. While approaching this level on a plane superficial to the mylohyoid, localization of the neurovascular bundle comprised of the nerve to the mylohyoid and the submental artery is a critical step in avoiding postoperative hematomas and ensuring submental artery island flap viability.

Anatomic illustration of levels IA and IB in the left lateral neck. (A) Illustration showing the structures that make up the surgical borders of levels IA and IB, including the bilateral anterior bellies of the digastric muscles and the posterior border of the submandibular gland (SMG), as well as the position of SMG within the submandibular triangle. (B) Illustration showing the common locations of lymph nodes (green) within these surgical levels, including the marginal mandibular nerve (MMN), submental nodes and jugulodigastric nodes.

Next, the inferior border of the mylohyoid is identified, and the muscle is retracted superomedially (toward the mentum) to expose the neurovascular layer of level 1B. Typically, the first structures identified here are the lingual nerve, submandibular duct and ganglion just deep to the mylohyoid. The nerve sits in close contact with the mandible around the second and third mandibular molars, in the region where the ganglion takes off inferiorly. The submandibular duct arises from the medial aspect of the submandibular gland on the mylohyoid’s deep surface, taking anteromedial course superior to the lingual nerve. Before ligating the duct, care should be taken to identify and preserve CN XII. This is done by localizing CN XII at its entry to the submandibular triangle, sitting deep to the stylohyoid and PBD and lateral to the hyoglossus as it courses superiorly into the intrinsic tongue musculature. Of note, the ranine vein travels superficial to CN XII as they drain into the thyrolinguofacial trunk prior to draining into the IJV. Careful dissection over this plexus ensures visualization and protection of CN XII.

LNs in level IB are located within the preglandular, prevascular, retrovascular, retroglandular, intracapsular, and deep submandibular subgroups. ^, The pre- and retrovascular LNs, also known as the perifacial LNs, are found anterior and posterior to the anterior facial vein, respectively. ^, In a series of 63 neck dissections, at least one perifacial LN was found in 84% of dissections. Among these, those with 1 anterior and 1 posterior perifacial node were more common than those with a single node either anterior or posterior to the vein. Of note, only the anterior perifacial LNs were in direct contact with the MMN, present in 60% of dissections.

Nodes within level IB found anterior to the submandibular gland, superior to the level of the anterior belly of the digastric and superficial to the mylohyoid muscle, are referred to as the preglandular nodes. ^, A study of 42 dissections on 35 cadavers consistently identified 1 to 2 preglandular nodes in their specimens. The average largest diameter of these nodes is approximately 1 cm. Subgroups not frequently found are the retroglandular, intracapsular, and deep mandibular nodes. In a review of 50 oral cavity SCC patients, examination of 69 submandibular glands found no intracapsular nodes in any of the specimens. A similar study comprised of 20 dissections found no retroglandular, intracapsular, or deep submandibular nodes in their specimens. On average, the submandibular triangle, or level IB, contains 3 LNs.

Lateral neck

The SCM lies superficially in the lateral neck and is invested by the superficial layer of the deep cervical fascia. ^, The floor of the lateral neck, invested by the deep layer of the deep cervical fascia, is found deep to levels II-IV. From superior to inferior, this floor is comprised of the levator scapulae, splenius capitis, and the ASM, middle scalene, and the cervical rootlets invested by the deep layer of the deep cervical fascia. Along the external jugular vein’s (EJV) course on the anterior border of the SCM lie the external jugular LNs. Similarly, on a plane deep to the SCM, the deep chain of lateral cervical nodes are closely related to the internal jugular vein (IJV). ^{, ,} There are conflicting reports on the number of superficial jugular nodes, with averages ranging from 1 to 5 nodes. ^, However, absence of nodes in this region is not uncommon, as shown by one study describing the absence of nodes in 7 out of 20 dissections. These nodes collect afferents from the occipital and mastoid LNs, which in turn drain the lobule of the auricle, external acoustic meatus, scalp, and skin over the angle of the mandible. ^, Their efferents drain deep in the neck, piercing the superficial layer of the deep cervical fascia and emptying into the middle and lower jugular node groups. ^, The distribution and drainage of the upper, middle, and lower jugular nodes of the deep cervical chain is characterized by their relationship with the IJV and will be discussed in the following sections as levels II, III, and IV, respectively.

Level II

The superior jugular nodes are found within level II, defined by the skull base superiorly and the carotid bifurcation inferiorly. A vertical plane defined by CN XI divides level II into IIA and IIB. Level IIA’s anterior (or medial) boundary is defined by the posterior edge of the submandibular gland, while CN XI outlines its lateral and posterior boundaries. Due to difficulty identifying CN XI in axial images and variation of carotid bifurcation levels, the landmarks used on radiological images to define the inferior and lateral boundaries of level IIA are a horizontal plane through the inferior border of the hyoid and the posterior border if the IJV, respectively ( Table 2 ). Level IIA’s inferior deep boundary and Level IIB’s entire deep boundary is comprised by the splenius capitis and levator scapulae muscles ( Figure 5A ).

Anatomic illustration of levels II-IV in the left lateral neck. (A) Illustration of the structures that make up the surgical borders of levels II, III, and IV, including the skull base, carotid bifurcation, omohyoid muscle, transverse cervical vessels, and clavicle. (B) Lymph node distribution (green) in these levels, adding the submandibular gland and relevant nerves.

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

1. The pterygopalatine fossa in three dimensions: A digital anatomical model derived from radiographic imaging, refined with peer-reviewed literature, and optimized with medical illustration
2. Development of a 3-dimensional model of the infratemporal fossa
3. The central neck in 3 dimensions: a digital model derived from radiology, peer-reviewed literature, and medical illustration
4. Head and neck fascial layers and the spaces they create in 3-dimensions: an anatomic model created using peer-reviewed literature, radiographic imaging, and an experienced medical illustrator

Stay updated, free articles. Join our Telegram channel

Join