2 The Rationale and Anatomical Basis for Functional and Selective Neck Dissection
2.1 Introduction
Functional neck dissection, as described by Osvaldo Suárez, is based on the existence of a fascial barrier between the lymphatic tissue and the muscular, glandular, neural, and vascular structures of the neck. This anatomical separation allows the creation of a surgical plane of dissection. The fascial layer invests muscles and organs in the neck, forming planes and spaces where many important structures are crowded together. This fact, known as fascial compartmentalization, holds the rationale for functional neck dissection.
This chapter describes the anatomical bases of functional neck dissection from a practical and surgical standpoint.
2.2 Rationale for Functional and Selective Neck Dissection
2.2.1 Fascial Anatomy of the Neck
The anatomical description of the fascial layers of the neck has suffered a number of different descriptions. For practical reasons we will consider two distinct fascial layers in the neck, the superficial cervical fascia and the deep cervical fascia. The superficial cervical fascia corresponds to the subcutaneous tissue. The deep cervical fascia is the key element for functional and selective neck dissection.
The superficial cervical fascia extends from the zygoma down to the clavicle, enveloping the platysma muscle and the muscles of facial expression. The vascularization of the skin forms a net superficial to the platysma muscle, within the superficial cervical fascia, with few branches connecting this net to the underlying vascular supply. There is a potential space between the superficial fascia and the deep fascia that allows free movement of the skin and superficial fascia on deeper structures. This plane, located underneath the platysma muscle, is the cleavage plane that should be followed to properly elevate the cutaneous flaps in functional and selective neck dissection as well as other surgical procedures in the neck. By elevating the cutaneous flaps on this plane, the net that vascularize the skin is preserved, while the dissection is carried out on a relatively avascular plane (Fig. 2‑1).
The deep cervical fascia (Fig. 2‑2) surrounds the neck, enveloping its different structures. For teaching purposes, three different layers are considered within the deep cervical fascia: a superficial, a middle, and a deep or prevertebral layer. The carotid sheath, which is an important structure from the surgical standpoint, is located between these layers of the deep cervical fascia.
The superficial layer of the deep cervical fascia, also known as investing or anterior fascia, completely envelops the neck with the exception of the skin, platysma muscle, and superficial fascia. Superiorly, it is attached to the occipital protuberance, mastoid process, capsule of the parotid gland, and body of the mandible. As it descends, it passes anteriorly from the mandible to the hyoid bone and from here down to the sternum, and posteriorly it passes across the spinal processes of the cervical vertebrae and the ligamentum nuchae. Inferiorly, it attaches to the sternum, upper edge of the clavicle, acromion, and spine of the scapula. At the inferior border, in the midline, the superficial layer splits in two different layers just superior to the manubrium of the sternum, enclosing the suprasternal space of Burns. From posterior to anterior, the superficial layer splits to enclose the trapezius, the inferior body of the omohyoid muscle as it crosses the posterior triangle of the neck, and the sternocleidomastoid muscle. The superficial veins of the neck lie on or within this superficial layer of the deep cervical fascia.
The middle layer, or visceral layer, envelops the upper aerodigestive tract. Superiorly, it is known as buccopharyngeal fascia, which inserts in the skull base and surrounds the posterior and lateral aspects of the nasopharynx and the oropharynx. Further down, the visceral layer is attached to the hyoid bone and encircles the strap muscles (except for the inferior body of the omohyoid muscle), the larynx and the trachea along with the hypopharynx and the esophagus. The thyroid and the parathyroid glands, as well as the recurrent laryngeal nerves and the central compartment lymph nodes are also surrounded by the visceral layer. Inferiorly, it merges with the pericardium inside the mediastinum.
The deep or prevertebral layer encloses the vertebral column and the paraspinal and prevertebral muscles. It attaches posteriorly to the spinous processes of the cervical vertebrae and ligamentum nuchae, and laterally to the anterior tubercles of the transverse process of the cervical vertebrae. At its superior limit, it goes to the skull base at the jugular foramen and carotid canal, then passes across the basilar process to the opposite side. In the upper part of the neck, this fascial layer covers the muscles of the back that enter into the lateral neck immediately deep to the trapezius muscle (splenius and levator scapula). At this level, there is a potential space between superficial and deep layers that is crossed by the spinal accessory nerve, along with some lymph nodes. At the lower end, both fascial layers further separate, the deep layer covering the scalene muscles, whereas the superficial layer remains attached to the trapezius muscle and the clavicle. The phrenic nerve runs inferiorly on the anterior aspect of the scalene group, covered by the deep fascial layer.
The carotid sheath or vascular sheath lies between the different layers of the deep cervical fascia and is composed by contributions of the three of them (Fig. 2‑3). This vascular sheath runs from the base of the skull to the root of the neck. It may be regarded as a cylinder-like structure with independent compartments for the internal jugular vein, the carotid artery, the vagus nerve, and the ansa cervicalis. The cervical portion of the sympathetic trunk runs posterior to the carotid sheath, within the prevertebral layer.
Fascial compartmentalization allows the removal of cervical lymphatic tissue by separating and removing the fascial walls of these “containers” along with their contents from the underlying vascular, glandular, neural, and muscular structures.
2.2.2 Lymph Node Distribution: Lymphatic Chains
The lymphatic system of the neck consists of a network of lymph nodes intimately connected by lymphatic channels. For teaching purposes, two major lymphatic networks may be considered in the neck, a superficial and a deep web.
Superficial Lymphatics
The superficial lymphatics of the head and neck drain the skin into the superficial lymph nodes located around the neck and along the external and anterior jugular veins. Superficial lymphatics include the submental, submandibular, external jugular, anterior jugular, occipital, mastoid, and parotid groups (Fig. 2‑4).
The submental nodes, usually two or three in number, lie in a midline triangular space bounded by the anterior bellies of the digastric muscles and the hyoid bone. They drain the skin of the chin, the skin and mucous membrane of the central portion of the lower lip and jaw, the floor of the mouth, and the tip of the tongue. These nodes drain into the submandibular chain or directly into the deep cervical chains.
The submandibular nodes are located along the inferior border of the horizontal ramus of the mandible. They usually lie over the submandibular gland although intracapsular nodes are also possible. The submandibular chain, along with some inconstant small facial nodes, drain the skin and mucous membrane of the nose, medial portion of the eyelid, cheek, upper lip, lateral part of the lower lip, gums, and anterior third of the lateral border of the tongue. These nodes drain into the transverse cervical and deep cervical chains.
The external jugular nodes are located between the lower parotid nodes and the midportion of the sternocleidomastoid muscle, along the external jugular vein. They drain the lower part of the ear and the parotid gland into the superior deep cervical chain.
The anterior jugular nodes are located on the anteroinferior portion of the neck, parallel to the anterior jugular vein. They drain the infrahyoid area toward the inferior deep jugular chain.
The occipital nodes drain the skin of the occipital region and part of the superficial and deep lymphatics of the nape.
The mastoid nodes are located over the mastoid process and drain the ear, external auditory canal, and skin of the temporal region.
The parotid group includes both superficial and deep nodes. The superficial nodes are located over the external surface of the parotid gland, whereas the deep nodes are intraglandular and accompany the intraparotid course of the retromandibular and external jugular. The parotid nodes drain the skin of the temporal and frontal area, eyelid, auricle, middle ear, parotid, and the mucous surface of the nasal cavity.
Deep Lymphatics
The deep lymphatics drain the mucous membranes of the upper aerodigestive tract, along with organs such as the thyroid and larynx, into the deep cervical lymph node chains. These include the internal jugular, spinal accessory, transverse cervical, retropharyngeal, and deep anterior lymphatic chains (Fig. 2‑5).
The internal jugular chain is formed by a variable number of lymph nodes—between 30 and 60—located around the internal jugular vein. The most posterior and smaller nodes are located over the splenius, levator scapulae, and scalene muscles, whereas the anterior nodes are in close relation with the anterior wall of the internal jugular vein. The posterior nodes drain the skin of the back of the head and receive efferent vessels from the occipital and mastoid nodes, as well as cutaneous and muscular tributaries from the neck. The anterior nodes drain the superficial and deep structures of the anterior part of the head and neck, both directly and indirectly.
At the intersection between the digastric muscle and the internal jugular vein there is a constant prominent node, known as the jugulodigastric or Küttner node. It drains the base of the tongue and the palatine tonsil. Another prominent node, the juguloomohyoid or Poirier node, is located further down at the crossing of the omohyoid muscle with the internal jugular vein. It receives lymph flow coming from the tongue and submental region.
For practical purposes, the internal jugular chain may be divided into upper, middle, and lower parts, with the dividing lines located at the jugulodigastric and juguloomohyoid nodes. The nodes of the lower part of the internal jugular chain are less constant and participate also in the drainage of noncervical adjacent structures.
The spinal accessory chain follows the spinal accessory nerve in the upper part of the posterior triangle and merges with the transverse cervical chain beneath the trapezius muscle. It receives lymph from the occipital and mastoid areas.
The transverse cervical chain runs along the transverse cervical vessels. It receives efferent vessels from the spinal accessory chain and from the lateral part of the neck.
The retropharyngeal nodes are located at the lateral portion of the parapharyngeal space. They drain the nasal cavity, soft palate, paranasal sinuses, middle ear, nasopharynx, and oropharynx.
The deep anterior chain includes the prelaryngeal (Delphian) node, the pretracheal, and the paratracheal nodes. They drain the subglottis, the trachea, and the thyroid gland. This chain is connected with the internal jugular chain.
Major Lymph Ducts
Both, the superficial and the deep lymphatic system initially drain in the nearest lymph nodes and then proceed to more central nodes to finally form lymphatic trunks. At the base of the right side of the neck, the jugular trunk (which collects most of the lymph from one side of the head and neck), the transverse cervical trunk, and the subclavian trunk frequently join to form the right lymphatic duct or great lymphatic vein. This large collector courses along the medial border of the scalene muscle and empties into the venous system at the junction of the right internal jugular vein and the right subclavian vein.
The thoracic duct begins in the abdomen, passes through the thoracic region, and emerges in the root of the left side of the neck between the common carotid and subclavian arteries. It then arches above the subclavian artery and in front of the vertebral artery and thyrocervical trunk, to pass behind the carotid sheath between the internal jugular vein and the anterior scalene muscle. The thoracic duct empties laterally into the venous system at the junction of the left subclavian and internal jugular veins. The left jugular trunk, left transverse cervical trunk, and left subclavian trunk drain in the thoracic duct or directly in the jugulo-subclavian junction.
2.2.3 Lymph Node Distribution: Nodal Groups
For practical reasons, the neck may be artificially divided into different lymph node regions. This does not mean that there is a true anatomical or physiological separation within the lymphatic system of the neck. On the contrary, a widespread interconnection exists between the different nodal chains, as already described. Thus, the regional lymph node classification should be regarded only as a schematic representation of the lymphatic system of the neck, and not as an anatomical transcription of the reality. As often happens in medicine, nature is much more complex than we would like it to be.
The most popular terminology for subdividing the lymph node groups was proposed by the Memorial Sloan Kettering group in the 1980s, further developed in 1991 by the Committee for Head and Neck Surgery and Oncology of the American Academy of Otolaryngology—Head and Neck Surgery, and reviewed in 2002 and 2008 by a collaboration of the Neck Dissection Committee of the American Head and Neck Society. According to this classification, the neck is divided into six different levels (Fig. 2‑6, Fig. 2‑7), three of them additionally subdivided into two sublevels:
Sublevel IA: Submental nodes. This group includes the lymph nodes located within the submental triangle, bounded by the anterior belly of both digastric muscles and by the hyoid bone.
Sublevel IB: Submandibular nodes. The submandibular group includes the lymph nodes located within the boundaries of the anterior and posterior bellies of the digastric muscle, the stylohyoid muscle, and the body of the mandible. These nodes are located around the submandibular gland, which should be removed when this nodal group is included in the resection.
Level II: Upper jugular nodes. This group contains the lymph nodes located around the upper third of the internal jugular vein and the spinal accessory nerve. It goes from the level of the skull base superiorly to the level of the inferior border of the hyoid bone and/or carotid bifurcation inferiorly. The posterior boundary is the posterior border of the sternocleidomastoid muscle, and the anterior boundary is the lateral border of the stylohyoid muscle. This level is divided by the spinal accessory nerve into two sublevels:
Sublevel IIA: nodes located anterior-medial to the vertical plane defined by the spinal accessory nerve.
Sublevel IIB: nodes located posterior-lateral to the plane of the nerve.
Level III: Middle jugular nodes. The lymph nodes around the middle third of the internal jugular vein, between the limits of the levels II and IV. The posterior boundary is the posterior border of the sternocleidomastoid muscle and/or the plane defined by the sensory branches of the cervical plexus. The anterior boundary is the lateral border of the sternohyoid muscle and/or the common carotid artery.
Level IV: Lower jugular nodes. This nodal group contains the lymphatic structures located around the lower third of the internal jugular vein. Its upper limit is the inferior border of the cricoid cartilage and/or the point at which the superior belly of the omohyoid muscle crosses the internal jugular vein. The inferior boundary is the clavicle, while the posterior and anterior boundaries are the same as in level III.
Level V: Posterior triangle. The boundaries are the anterior border of the trapezius muscle posteriorly, the posterior border of the sternocleidomastoid muscle and/or the plane of the sensory branches of the cervical plexus anteriorly, and the clavicle inferiorly. This level is subdivided by a horizontal plane at the inferior level of the cricoid cartilage into two sublevels:
Sublevel VA: includes the lymph nodes located along the lower half of the spinal accessory nerve.
Sublevel VB: includes the nodes following the transverse cervical vessels, as well as the supraclavicular lymph nodes.
Level VI: Anterior or central compartment. This level contains the prelaryngeal (Delphian) node, and the pre- and paratracheal nodes, including perithyroidal nodes, and the lymph nodes along the recurrent laryngeal nerves. The boundaries are the hyoid bone superiorly, the suprasternal notch inferiorly, and the common carotid arteries laterally.
Level VII. Some authors consider this an additional area. It includes the upper mediastinal lymph nodes located below the suprasternal notch and above the innominate artery.
One of the main theoretical advantages of the nodal group classification is that every group of nodes may be related to different head-and-neck structures in order to assess the potential risk for metastasis for every primary location. Table 2‑1 shows the relationship between the location of the primary tumor and the nodal groups at greatest risk for harboring metastases.
A Final Comment on the Nodal Group Classification
The main use of the nodal group classification is to support the worthiness of selective neck dissections. However, the artificial nature of the division creates some inconsistencies that must be kept in mind to avoid falling into a “nodal group fundamentalism,” which often happens nowadays under the name of “super-selective neck dissections.” In our opinion, the following are the main weak points of the artificial division of the neck into nodal regions.
Despite the effort to correlate surgical and radiological landmarks in the nodal group classification, there is a notorious lack of consistency into this correlation. This makes it very difficult to compare results, even if we all use the same classification. At surgery, the theoretically well-defined anatomical and radiological boundaries of some of the various levels and sublevels are distorted by the operative maneuvers. It is not unusual to decide to stop the dissection at a given point to find later that more tissue than desired has been removed because too much traction has been used during the dissection. On the other hand, even in the ideal situation, one person’s upper level IV lymph node may easily be another’s lower level III node.
Under normal conditions, the lymph flow follows a rather predictable course that is used as an argument to support the oncological safety of selective neck dissections. However, head-and-neck cancer patients do not fully satisfy the criteria of “normal conditions,” and the flow pattern may be modified by factors related to the tumor itself, the anatomical characteristics of the patient, and the influence of external factors such as previous treatment. Some operations have stood the test of time and can be considered positively safe from an oncological standpoint. Others still need documented proof of efficacy. Meanwhile, the use of super-selective operations should be cautiously recommended.
Finally, the ultimate rationale for selective operations should not be sought on the nodal region subdivision, but on the functional concept. If selective neck dissections are useful—and for some of them this is a fact—it is because the functional concept is a reality. We can remove the lymphatic tissue from the neck without the need to remove adjacent cervical structures. The exact limits of this removal for every single head-and-neck tumor have not been determined with certainty and require further studies and well-designed investigations.
2.3 Anatomical Basis for Functional and Selective Neck Dissection
2.3.1 Topographic Anatomy
The topographic description of the neck intends to serve as a guide in which the external and readily accessible superficial features of the neck provide essential landmarks for deep structures. This is a critical element in the examination and description of clinical findings.
From a topographic standpoint, the sternocleidomastoid muscle and the carotid sheath divide each side of the neck into two different spaces. Although pyramidal in shape, these spaces are known as the anterior and posterior triangles of the neck (Fig. 2‑8). The posterior-lateral space has a cranial apex at the level of the mastoid and a base at the level of the clavicle. It does not have a definite anatomical boundary, because it merges into the axilla through the cervicoaxillary canal. The apex of the anterior-medial space is located at the bottom of the neck and its base lies at the level of the submandibular gland and tail of the parotid gland. These spaces contain the lymph nodes that drain most cervical structures.
Anterior Triangle
The anterior triangle is bounded by the anterior midline of the neck, the anterior border of the sternocleidomastoid muscle, and the inferior border of the mandible. The jugular notch constitutes the apex, and the base is formed by the inferior border of the mandible. The posterior belly of the digastric muscle and the superior belly of the omohyoid further divide this space into several smaller triangles (i.e., submental, submandibular, carotid, and muscular; Fig. 2‑9).
The submental triangle is an unpaired space bounded on each side by the anterior belly of the digastric muscle, inferiorly by the body of the hyoid bone, and superiorly by the inferior border of the mandible. The floor of the submental triangle is formed by the mylohyoid muscles, which meet in a median fibrous raphe. This space is occupied by fat and lymph nodes.
The submandibular triangle is limited on each side by the inferior border of the mandible and the anterior and posterior bellies of the digastric muscle. The muscular floor of the submandibular triangle is formed, from anterior to posterior, by the mylohyoid, hyoglossus, and middle constrictor of the pharynx. The mylohyoid muscle further divides it into supramylohyoid and inframylohyoid spaces. The supramylohyoid space contains the sublingual gland. The submandibular gland and a variable number of lymph nodes are contained within the inframylohyoid space. The lingual nerve, the hypoglossal nerve, part of the facial artery and vein, and the submental artery pass through this triangle.
The carotid triangle (also known as the superior carotid triangle) is bounded superiorly by the posterior belly of the digastric muscle, inferiorly by the superior belly of the omohyoid muscle, and posteriorly by the anterior border of the sternocleidomastoid muscle. The carotid triangle provides an important surgical approach to the carotid system. Many important structures, such as the common carotid artery, internal jugular vein, vagus nerve, and sympathetic trunk, lie within the limits of this space. The common carotid artery divides into the internal and external branches at the level of the superior border of the thyroid cartilage. Many deep cervical lymph nodes lie along the internal jugular vein, and between the vein and the common carotid artery, within the carotid sheath.
The muscular triangle (or inferior carotid triangle) is bounded by the superior belly of the omohyoid muscle, the anterior border of the sternocleidomastoid muscle, and the midline of the neck. It contains the strap muscles, the thyroid and parathyroid glands, the larynx and the hypopharynx, the trachea, and the cervical esophagus.
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