Clinical anatomy and physiology

The pharynx

The pharynx is a fibro muscular tube forming the upper part of the respiratory and digestive passages. It extends from the base of the skull to the level of the sixth cervical vertebra at the lower border of the cricoid cartilage where it becomes continuous with the oesophagus. It opens anteriorly into the nose, mouth and larynx from above downwards, and is therefore divided into three parts, the nasopharynx, oropharynx and hypo pharynx (Fig. 43.1).

Nasopharynx

The nasopharynx lies anterior to the first cervical vertebra and has the openings of the eustachian tubes in its lateral wall, behind which lie the pharyngeal recesses, the fossae of Rosen Muller. The adenoids are situated submucosally at the junction of the roof and posterior wall of the nasopharynx. The nasopharynx is closed off from the oropharynx during swallowing by the raising of the soft palate and contraction of the palatopharyngeal sphincter.

Oropharynx

This is bounded above by the soft palate, below by the upper surface of the epiglottis and anteriorly by the anterior faucial pillar which contains the palatoglossus muscle. The orophar­ynx therefore contains the palatine tonsils situated in the later­al wall between the anterior and posterior pillars of the fauces and the posterior third of the tongue. These palatine tonsils are part of the complete ring of lymphoid tissue (Waldeyer’s ring) which additionally comprises the adenoids and the lingual tonsils on the posterior third of the tongue. This ring of lym­phoid tissue occupying the entry of the air and food passages is constantly exposed to new antigenic stimuli. It is an important part of the mucosal-associated lymphoid tissue (MALT) which processes antigen and presents it to T-helper cells and B-cells. The tonsillar tissue produces immunoglobin G (IgG), IgA and a small amount of IgD. These immunoglublins are secreted into the pharynx and their output is increased in response to a wide variety of inflammatory processes (Fig. 43.2).

The tissue of Waldeyer’s ring undergoes physiological hypertrophy during early childhood as the child is exposed to increasing amounts of antigenic stimuli; there is often a similar hypertrophy of the cervical lymph nodes and, indeed, the abdominal lymph nodes (the aetiology of mesenteric adenitis). The tonsils contain tortuous crypts which can harbour pus and microorganisms. Clothing the lateral two-thirds of each tonsil is the capsule, a well-defined structure composed of fibrous and elastic tissue, and muscle fibres. The tonsil has an excep­tionally good blood supply. It is well to bear in mind that a tortuous facial artery may be closely related to the lower pole. On the lateral aspect of the tonsil is a varying number of paratonsillar veins which may be the source of serious venous bleeding following tonsillectomy. This may particularly occur when the bleeding end retracts into the upper part of the tonsillar fossa, and this must be found and ligated before the patient leaves the operating room.

Hypo pharynx

The hypopharynx is bounded above by the upper border of the epiglottis and anteriorly by the sloping laryngeal inlet. Its inferior border is the lower border of the cricoid cartilage where it continues into the oesophagus. The hypopharynx is commonly described anatomically of being composed of three areas: the piriform fossae, the posterior pharyngeal wall and the postcricoid area. The mucosa of these areas is in direct continuity with no distinct barriers, and disease processes, such as malignant neoplasms, can easily involve more than one area.

Swallowing

Swallowing is a complex neuromuscular act. Trauma and disease may result in dysphagia and at times aspiration of food and fluid into the airway. In considering the diseases which involve the pharynx and larynx, it is important to have a basic understanding of swallowing and the functions of the larynx. Swallowing consists of three stages: oral, pharyngeal and oesophageal (Fig. 43.3). Knowledge of the physiology of normal swallowing and the problems as a consequence of disease has been enhanced in the last two decades by use of video fluoroscopic techniques. This is the radio graphical evaluation of the passage of a bolus of radio-opaque liquid or solid from the point at which it enters the oral cavity down to its passage within the stomach. This investigation is considerably more accurate than the older-fashioned radiological examination of barium swallow, where the object was to con­centrate on the plain X-ray examination of the oesophagus, rather than the new technique which shows a complete video of all stages of swallowing. This can be reviewed on many occasions, if necessary, and gives far more detail than plain radiograph films.

Swallowing is mediated via efferent fibres passing to the medulla oblongata through the second division of the trigem­inal nerve (V), glossopharyngeal (IX) and vagus nerves (X). The afferent pathway is from the nucleus ambiguous, and is mediated via the glossopharyngeal (IX), vagus (X) and hypoglossal (XII) nerves.

Damage to these major cranial nerves at any point along their pathway, by trauma or disease, may cause dysphagia with or without aspiration.

In the oral voluntary phase of swallowing the lips, cheeks, tongue, floor of mouth, teeth and palate participate in pre­paring the food. The food is held in the oral cavity, lateralised for mastication and then formed into a bolus. The tongue propels the formed bolus posteriorly into the oropharynx in the second involuntary pharyngeal phase. The soft palate is elevated to prevent nasopharyngeal escape, and the lateral and posterior walls contract to propel the food downwards. The larynx moves upwards and forwards as the posterior tongue moves backwards and downwards.

The main function of the larynx is in fact not the production of voice but the protection of the tracheobroncheal airway and lungs. It is basically a three-tiered sphincter mechanism: the first tier consisting of the epiglottis, aryepiglottic folds and arytenoids; the second the false cords; and the third level the true vocal folds. The whole larynx not only moves upwards and forwards but the three-tier sphincter mechanism closes on every occasion that swallowing occurs.

As the food enters the piriform fossa there is an anticipatory relaxation of the upper oesophageal sphincter and the food enters the oesophagus. Oesophageal peristaltic waves then convey the food down into the stomach. Gravity aids both the involuntary pharyngeal and oesophageal phases of swallowing.

Relations of the pharynx (Fig. 43.4)

The parapharyngeal space — this potential space lies lateral to the pharynx and is of importance when understanding disease of the pharynx and neck. It is a potential space which extends from the base of the skull above to the superior mediastinum below, and it is occupied by the carotid vessels, internal jugular vein, deep cervical lymph nodes, the last four cranial nerves and the cervical sympathetic trunk. Infection and suppuration of the cervical lymph node in the parapharyngeal space most commonly occurs from infections of the tonsils or teeth (particularly the third lower molar tooth). It may then spread through­out the parapharyngeal space up to the skull base or down to the paraoesophageal region and superior mediastinum. Retropharyngeal space — this potential space lies posterior to the pharynx bounded anteriorly by the posterior pharyngeal wall and its covering buccopharyngeal fascia and posteriorly by the cervical vertebrae and their covering muscles and fascia. It contains the retropharyngeal lymph nodes, which are usually paired lateral nodes, but which are separated by a tough median partition which connects the prevertebral with the buccopharyngeal fascia. These nodes are more developed in infancy and young children, and it is at this age that they are most likely to he involved in inflammatory processes which, if severe, may affect swallowing and respiration as a consequence of gross swelling and suppuration of the retropharyngeal space.

Larynx

The larynx is the protective sphincter which closes off the airway during swallowing. In humans and some other mam­mals it is also responsible for the generation of sound, which in humans is modified by the pharynx, oral cavity, nasal passages, sinuses, tongue, lips and teeth to produce speech. The larynx has a mainly cartilaginous framework which may ossify in later life and which consists of the hyoid bone above, the thyroid and cricoid cartilages, and the intricate arytenoid cartilages posteriorly.

The cricoid cartilage is the only complete ring in the entire airway and bounds the sub glottis which is the narrowest point of the airway. This is the commonest site for damage from endotracheal tubes, occasionally causing laryngotracheal stenosis.

An anatomical description of the larynx divides it into the supraglottis, glottis and subglottis (Fig. 43.5). The true vocal folds (often incorrectly called the vocal cords) are normally white in contrast to the pink mucosa of the rest of the larynx and airway. The true vocal folds meet anteriorly at the mid-level of the thyroid cartilage, whereas posteriorly they are separate and attached to an arytenoid cartilage. This arrangement produces the ‘V’ shape of the glottis (Fig. 43.6).

Nerve supply

The sensory nerve supply to the larynx above the vocal folds is from the superior laryngeal nerve and below the vocal

folds from the recurrent laryngeal nerve. Both of these nerves are branches of the vagus (X). The motor nerves supply to the larynx is from the recurrent laryngeal nerve, which is a branch of the vagus and which supplies all intrinsic muscles. Only one of these intrinsic muscles, the posterior cricoarytenoid, abducts the vocal folds during respiration. All other intrinsic muscles adduct the cords.

Lymphatics

The lymphatic drainage of the supraglottis above the vocal folds is to the upper deep cervical nodes, whilst in contrast that of the subglottis is to inferior deep cervical nodes and to the paratracheal and mediastinal nodes.

•The vocal folds themselves have a very sparse lymphatic drainage. The commonest form of malignant disease affecting the true vocal folds is squamous cell carcinoma, and tumours confined to the true vocal folds do not metastasise to lymph nodes unless they spread into supraglottic or subglottic tissue.

All of the intrinsic muscles of the larynx are supplied by the recurrent laryngeal nerve, and therefore damage to this nerve or to the vagus nerve will cause paralysis of the vocal fold on the side of the damage.

Phonation/speech

During expiration air from the lungs passes out through the larynx under pressure and the vocal folds channel this into a column of high-speed vibrating air, thus producing sound. This sound is converted into intelligible speech by the remainder of the vocal tract, i.e. the pharynx, tongue, lips, teeth and the resonating chambers of the nose and sinuses. The larynx functions by closing the vocal fold against the air being exhaled from the lungs but the rise in subglottic pressure forces the vocal folds apart slightly for an instant with accompanying vibration of the vocal fold epithelium. The opening and closing occurs in rapid sequence to produce a vibrating column of air which is the source of sound. The pitch of the sound is controlled by the frequency of the vocal fold epithelial vibration, which in turn is determined by the thickness, length and tension of the vocal folds controlled by the intrinsic musculature. The loudness or intensity of the sound is governed by the expiratory air pressure and the amplitude of the vocal fold vibrations.

Paralysis or overt disease of the vocal folds or closely associated laryngeal structures will give rise to disturbance of the sound and creation of the symptom of hoarseness (Table 43.1).

The neck

The neck is divided into anterior and posterior triangles by the sternocleidomastoid muscle. The anterior triangle extends from the inferior border of the mandible to the sternum below, and is bounded by the midline and the sternocleidomastoid muscle. The posterior triangle extends backwards to the anterior border of trapezius and inferiorly to the clavicle. The upper part of the anterior triangle is commonly subdivided into the submandibular triangle above the digastric muscle and the submental triangle below. The lymphatic drainage of the head and neck is of considerable clinical importance (Fig. 43.7). The most important chain of nodes is the deep cervical nodes which run adjacent to the internal jugular vein. The other main groups are the submental, submandibular, preauri­cular and postauricular, occipital and posterior triangle nodes.

The upper deep cervical nodes, which contain the large jugulo­digastric node, drain the oropharynx, including the tonsils, postero­lateral aspects of the oral cavity, superior aspects of the larynx and piriform fossae, and are the commonest site of enlargement due to disease in these areas.

The upper deep cervical nodes may be palpated along the anterior border of the sternocleidomastoid muscle but when the muscle is well developed or the neck is obese it may be difficult to evaluate their true size and under these circumstances clinical examination may be unreliable.

Metastatic spread of squamous cell carcinoma, which accounts for 80 per cent of malignant disease of the head and neck, most commonly occurs with tumours of the nasopharynx, tongue base, tonsil, piriform fossae and supraglottic larynx. When an enlarged neck node is detected and the possibility of malignant disease suspected, it is these five primary sites which must receive careful investigation.

Metastatic spread to regional lymph nodes in the neck is managed surgically in many cases by resection of the neck nodes, and a detailed knowledge of their anatomy is thus required. The parotid and submandibular salivary glands, and the thyroid and parathyroid glands, are additional important neck structures which will be covered separately in their respective chapters.