Key Points
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Head and neck anatomy is divided into fascial layers and triangles or levels, which serve as organizational units to manage the volume of anatomic detail in the neck.
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An understanding of the fascial relationships in the neck is important, not only because of the boundary relationships but also because fasciae form planes that provide routes of surgical access or pathways for hemorrhage and infection.
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At approximately 4 to 5 weeks of gestation, the area of the embryo’s future face and neck consists of five or six pairs of fingerlike masses of tissue called the branchial arches.
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The outer surfaces of the arches, as well as the clefts, are lined by ectoderm; the substance of the branchial arches contains mesoderm and neural crest cells. The pharyngeal pouches are outpouchings from the foregut region and are composed of endoderm.
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Second branchial cleft anomalies are the most common. The fistula tract starts with the external opening in the mid-lower neck at the anterior border of the sternocleidomastoid muscle, and it heads superiorly between the internal and external carotid arteries, passes over cranial nerves IX and XII, and ends in the palatine tonsil bed.
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Third branchial cleft anomalies travel from the medial edge of the sternocleidomastoid muscle, lateral to the common carotid artery, medial and posterior to the internal carotid artery, over cranial nerve XII, under cranial nerve IX, and through the thyrohyoid membrane into the piriform sinus.
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The paired maxillary and mandibular prominences from the first branchial arch along with the unpaired frontonasal prominence grow and come together to form the structures of the face during weeks 4 to 10 of embryonic development.
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Branchial arches 1 through 4 all contribute to the development of the tongue with the anterior two thirds of the tongue formed from the lateral lingual swelling and medial lingual swelling or tuberculum impar from the first branchial arch; the posterior one third of the tongue develops from the copula or hypobranchial eminence, which arises from the second, third, and fourth branchial arches.
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A Sistrunk procedure with removal of the midportion of the hyoid along with the thyroglossal duct (within the base of the tongue to the foramen cecum) and removal of the cyst is recommended to minimize the risk of recurrence of a thyroglossal duct cyst.
Surgeons use a number of nomenclature systems to organize the anatomy of the head and neck in an attempt to manage surgical approaches and anticipate deviations from normal anatomy. In most textbooks, the head and neck anatomy is divided into fascial layers and triangles or levels. The use of layers and levels is an organizational approach to manage the volume of anatomic detail in the neck by parceling it into reasonable functional units. For example, when a retropharyngeal abscess is diagnosed, surgeons can anticipate the spread of infections within the retropharyngeal space down to the mediastinum or into adjacent layers of the deep cervical fascia, and when an incision is made over the posterior triangle or level V, surgeons can reliably predict the structures they will encounter in precise order. Similarly, understanding the normal embryologic development of the face and neck is important to diagnose anomalies or masses of the head and neck and to define surgical approaches to address them. This chapter begins with brief descriptions of the fascial layers and the anatomic levels of the neck. The chapter continues with a discussion of the embryology of the face that includes the palate and tongue, the neck, the branchial or pharyngeal apparatus, and the contribution of this developmental anatomy to various abnormal and pathologic conditions of the head and neck.
Fascial Layers of the Neck
An understanding of the fascial relationships in the neck is important, not only because of the boundary relationships but also because fasciae form planes that provide routes of surgical access or pathways for hemorrhage and infection. For this reason, a brief discussion of the fascial planes is necessary before proceeding with the anatomy of the triangles or levels of the neck.
One of the earliest lessons in anatomy is that there are two types of fascia in the body, superficial and deep. In the region of the abdominal wall, superficial fascia consists of two layers—a fatty (Camper) layer and a deeper, membranous (Scarpa) layer. The deep fascia of the abdominal wall is not subdivided but is a layer that simply envelops the abdominal muscles. In the neck, the superficial layer of cervical fascia is a single layer of fascia that underlies the skin and contains the platysma muscle and cutaneous nerves and vessels. It is usually thin, except in the obese person, in whom it is thickened by adipose tissue. Its primary surgical significance is that it provides a fascial pad that protects underlying structures when a skin incision is made. In exceptionally lean people, however, the paucity of this layer may not protect underlying structures, such as the accessory nerve, so the surgeon should be wary when operating on such patients.
The deep cervical fascia is divided into three layers, illustrated best when the neck is viewed in cross section ( Fig. 2-1 ). These are the superficial (investing), middle (pretracheal or visceral), and deep (prevertebral) layers of the deep cervical fascia. The superficial layer of deep fascia underlies the platysma muscle and completely invests or encircles all of the superficial neck structures. For these reasons, the superficial layer is also known as the investing layer of deep fascia. In the region of the sternocleidomastoid and trapezius muscles, it splits and envelops the individual muscles. The superficial layer of the deep cervical fascia also invests the strap muscles and parotid and submandibular glands. The middle layer of the deep cervical fascia encloses the visceral structures of the neck: the trachea, esophagus, and thyroid gland. Hence, the synonym for the middle layer is the pretracheal or visceral fascia . The deep layer of the deep cervical fascia surrounds the deep muscles of the neck and cervical vertebrae (see Fig. 2-1 ). This layer is also known by its descriptive term, the prevertebral fascia . The muscles enclosed by the prevertebral fascia include the deep muscles of the neck: the levator scapulae; scalenus anterior, middle, and posterior; and longus colli and capitis, which lie on the anterior aspect of the cervical vertebrae. In addition, within the prevertebral fascia are the phrenic nerve and brachial plexus, located near the anterior and middle scalene muscles and the sympathetic chain lying anterior to the longus colli muscle. The superfical layer of deep fascia, along with the middle and deep layers, envelops the carotid and jugular vessels and vagus nerve to form the carotid sheath.
An effective means of visualizing the spatial relationships of these layers of fascia is to examine a cross section of the neck (see Fig. 2-1 ). This view is not only informative in defining the three layers of deep fascia but it also serves to relate them to the triangles of the neck (i.e., submandibular, posterior). Although the term triangle connotes a flat or planar form, the neck triangles are three-dimensional structures that should be visualized as pyramidal spaces that not only have three sides but also have a roof (top) and a floor (bottom). Most of the triangles are three-dimensional spaces bounded by bone and muscles with distinct fascial layers that form the roof and floor of the space. In general terms, each triangle contains blood vessels, nerves, lymphatic vessels, and lymph nodes. Using the fascial layers and triangles, the anatomy of the neck can be organized and simplified; for example, if you place a finger over the middle of the posterior triangle or level V, between the trapezius and sternocleidomastoid muscles (SCMs), you see that the roof (lateral wall) of the triangle is formed by the superficial layer of deep fascia. Palpation deeper into the triangle brings the tip of the finger into contact with the prevertebral fascia that forms the floor of the posterior triangle. If the superficial layer of the deep fascia is incised and you insert a finger into the space to explore anteriorly between the sternocleidomastoid and prevertebral muscles, you encounter the carotid sheath. This is a surgical approach to the retropharyngeal area or to the carotid vessels.
Triangles and Levels of the Neck
Nomenclature of the cervical lymphatic tissues historically has been inconsistent with descriptive terms such as deep, superficial, anterior, and lateral . As the focus of oncologic surgery has shifted from radical extirpation to functional preservation, it has become more important to standardize the classification system for the cervical nodes. The American Academy of Otolaryngology–Head and Neck Surgery, along with the American Society of Head and Neck Surgery, has developed a standardized classification system for the cervical lymphatic system based on levels I through VI of the neck ( Fig. 2-2 ).
Lymph node levels I through VI can also be divided into subzones (see Fig. 2-2 ). Level I includes lymph nodes in the submental (Ia) and submandibular (Ib) triangles. Levels II through IV include the jugular nodes in fibroadipose tissue located between the lateral border of the sternohyoid muscle and the posterior border of the SCM. Level II encompasses the upper jugular lymph nodes and extends from the skull base superiorly to the carotid bifurcation or hyoid inferiorly. The spinal accessory nerve divides level II into anterior (IIa) and posterior (IIb) compartments. Level III includes the middle jugular nodes around the middle third of the jugular vein and extends from the carotid bifurcation superiorly to the omohyoid or cricothyroid notch inferiorly. Level IV includes the lower jugular group of lymph nodes that surround the jugular vein from the omohyoid to the clavicle. Level V is the posterior triangle group, which encompasses lymph nodes located between the posterior border of the SCM and the anterior border of the trapezius muscle. Level V includes the nodes that surround the lower half of the spinal accessory nerve (Va) and the transverse cervical artery (Vb or the supraclavicular nodes). Level VI includes the anterior compartment group that surrounds the visceral structures of the neck from the hyoid superiorly to the sternal notch inferiorly. The lateral boundary on either side of level VI is the medial aspect of the carotid sheath. This space crosses the midline and includes the perithyroidal, paratracheal, and precricoid nodes in addition to the nodes along the recurrent laryngeal nerve.
Development of the Head and Neck
The development of many structures in the head and neck is intimately related to the branchial or pharyngeal apparatus. These are transient embryonic structures that undergo substantial remodeling so that their original embryonic form is essentially unrecognizable by the time a child is born. The derivatives of these structures, nevertheless, are important to adult morphology; hence, aberrations in branchial apparatus development may produce significant malformations.
Embryology of Branchial Arches
At approximately 4 to 5 weeks of gestation, the area of the future face and neck of the embryo consists of five or six pairs of fingerlike masses of tissue named the branchial arches ( Fig. 2-3 ). Prominent in lateral profile, these masses are aligned transversely to the plane of the neck and are separated externally by indentations, termed the branchial clefts . The outer surfaces of the arches, as well as the clefts, are lined by ectoderm, whereas the substance of the branchial arches contains mesoderm and neural crest cells. Only the first branchial cleft contributes to a definitive structure of the embryo: the external auditory canal and the outer layer of the tympanic membrane. The remaining second, third, and fourth branchial clefts fuse with the epicardial ridge in the lower part of the neck.
The pharyngeal pouches are outpouchings from the foregut region ( Fig. 2-4 ) and are composed of endoderm. The derivatives of the arches, clefts, and pouches are distinct, because they are derived from different embryonic germ layers: mesoderm, ectoderm, and endoderm, respectively. To generalize, it can be stated that in the adult, the derivatives of the branchial arch will be structures composed of muscle, bone, or similar mesodermal derivatives, and the derivatives of the endodermal pharyngeal pouch will be glandular or associated with the digestive tract.
Derivatives of Branchial Arches
In the early phase of branchial arch development, the mesodermal mass of the arch forms a bar of cartilage, which remodels into bone, cartilage, or other connective tissue elements in the adult. Similarly, the adult musculature of the face and neck develops from the mesoderm of the branchial arches. Each arch also has an associated cranial nerve and artery. Because of the proximity of the developing branchial arches to the brainstem, each branchial arch receives motor or sensory innervation from an adjacent cranial nerve. A comparable parallel to this pattern occurs in the trunk, where a muscle is derived from the myotome region of a somite and receives its innervation from the adjacent segmental spinal nerve. In both cases, regardless of where the primordial muscle cell migrates, it retains its primary embryonic innervation. After each arch receives its cranial nerve innervation, the adult pattern is established, regardless of its future migration onto the back of the head or base of the neck. The development of each branchial arch is considered separately.
First Branchial Arch.
The trigeminal nerve supplies motor innervation to all the muscles derived from the first branchial arch. In addition, sensory innervation is provided not only over the region of the mandible via the third division of the trigeminal nerve but also over the maxillary process of the first arch and the frontal nasal process via the second maxillary and first ophthalmic division, respectively, of the trigeminal nerve. The artery of the first branchial arch is the internal maxillary artery.
For skeletal derivatives of the first arch, the proximal part of the Meckel cartilage is remodeled and contributes to the formation of the ramus of the mandible ( Fig. 2-5 ). The distal part of the cartilage withers, and the body of the mandible is formed from intramembranous bony growth. Other structures formed by the proximal part of the Meckel cartilage are the sphenomandibular ligament, anterior malleolar ligament, malleus (except the manubrium, which is from the second arch), and the incus (except for its long process, which is from the second arch). The maxillary process is derived from the dorsal portion of the first arch and contributes to the development of the premaxilla, maxilla, zygoma, and part of the temporal bone.
