Overview of the head and neck

Face and scalp

The scalp consists of five main layers: skin, subcutaneous tissues, epicranial aponeurosis, loose areolar connective tissue and peri­cranium. Posteriorly, it is continuous with the superficial fascia on the back of the neck. The scalp has a rich vascular, lymphatic and neural supply, and bleeds readily following trauma or surgical incision. Laterally, the scalp blends into the temporal region, where it has a looser texture.

On the face, three fascial layers (a subcutaneous fibroadipose tissue, a superficial musculo-aponeurotic system (SMAS) and the parotid–masseteric fascia) lie superficial to the plane of the facial nerve and its branches. The SMAS layer is important for raising parotid skin flaps and for aesthetic procedures. On the lateral side of the head, above the zygomatic arch, the temporoparietal fascia lies in the same plane as the SMAS but does not blend with it. It is superficial to the temporal fascia and blends superiorly with the epicranial aponeurosis. The parotid gland itself is surrounded by a fibrous capsule derived from the deep cervical fascia. The face has a good blood supply and there are also numerous vascular anastomoses that cross the midline; for example, the labial arteries supplying the lips bleed and pulse from both sides when transected.

Neck

The superficial cervical fascia contains a variable amount of adipose tissue, superficial lymph nodes and platysma. The aponeurotic fibres of platysma gradually fan out in this layer, either fading away within the posterior triangle, or becoming skin ligaments, or continuing into the fascia that covers pectoralis major and deltoid. Platysma is important surgically because most neck incisions entail raising subplatysmal flaps in order to gain access to the underlying structures, and to help preserve the blood supply to the skin, particularly in the previously irradiated neck.

Descriptions of the organization of the deep cervical fascia are largely based upon the classic work of Grodinsky and Holyoke. The deep cervical fascia is conventionally subdivided into three sheets (superficial or investing, middle and deep layers) and the carotid sheath, a condensation of the deep fascia that envelops the common and internal carotid arteries, internal jugular vein, vagus nerve and ansa cervicalis.

The superficial layer of the deep cervical fascia (SLDCF) encircles the neck, ensheathing trapezius and sternocleidomastoid. A simplified ‘rule of twos’ is often used to describe the SLDCF: it encloses two glands (submandibular and parotid), two muscles (sternocleidomastoid and trapezius) and two ‘spaces’ (suprasternal space and the ‘subvaginal’ space of the posterior triangle). Above the hyoid bone, and on both sides of the neck, the SLDCF splits to enclose the submandibular salivary gland and fuses with the periosteum along the superior nuchal line of the occipital bone, over the mastoid process, and along the entire base of the mandible. Inferiorly, it fuses with the periosteum covering the acromion, clavicle and manubrium of the sternum. Just above the manubrium, the SLDCF splits to enclose the suprasternal space, which contains a small amount of areolar tissue, the lower parts of the anterior jugular veins and the jugular venous arch, the sternal heads of the sternocleidomastoid muscles and lymph nodes.

The middle layer of the deep cervical fascia is subdivided into a muscular layer (surrounding the infrahyoid strap muscles) and a visceral layer (including the pretracheal and buccopharyngeal fasciae). The visceral layer extends from the base of the skull posteriorly, and from the hyoid bone and thyroid cartilage anteriorly and laterally. It invests the thyroid gland, larynx, trachea, pharynx and oesophagus, and continues into the superior mediastinum along the great vessels, ultimately fusing with the fibrous pericardium. Laterally, it merges with both the SLDCF and the carotid sheath.

The deep layer of the deep cervical fascia is subdivided into alar fascia anteriorly and prevertebral fascia posteriorly. The alar fascia is separated posteriorly from the prevertebral fascia by loose connective tissue that fills the so-called danger space ( Ch. 16 ) and is separated anteriorly from the pharynx/oesophagus, and the visceral layer of the middle deep cervical fascia, by loose connective tissue that fills the retropharyngeal space. The prevertebral fascia forms a fascial floor for the posterior triangle of the neck. The phrenic nerve (motor supply to the diaphragm) and the trunks of the brachial plexus in the root of the neck all lie deep to the prevertebral fascia, and so it is important to be careful not to breach this fascia when operating in this area. As the subclavian artery and the brachial plexus emerge from behind scal­enus anterior they carry the prevertebral fascia downwards and laterally behind the clavicle as the axillary sheath.

The fascial layers of the head and neck define a number of potential tissue ‘spaces’ or compartments above and below the hyoid bone ( Tables 3.1 and 3.2 ). These spaces are important because they allow the potential spread of infection, enabling rapid progression from the skull base to the mediastinum or to other spaces, with significant clinical sequelae. Use of the three layers of the deep cervical fascia ‘to delineate multiple compartments of the suprahyoid and infrahyoid neck is the current paradigm and allows segmentation of pathology into well-defined fascially enclosed spaces’. For example, airway compromise can occur quickly when the submandibular and sublingual spaces are involved bilaterally with infection (Ludwig's angina), or severe trismus may occur with involvement of the masseteric or pterygoid spaces. In health, the tissues within these spaces are norm­ally either closely applied to each other or filled with relatively loose connective tissue.