BOUNDARIES

The nasopharynx lies above the soft palate and behind the posterior nares, which allow free respiratory passage between the nasal cavities and the nasopharynx (Figs 33.1, 33.2). The nasal septum separates the two posterior nares, each of which measures approximately 25 mm vertically and 12 mm transversely. Just within these openings lie the posterior ends of the inferior and middle nasal conchae. The nasopharynx has a roof, a posterior wall, two lateral walls and a floor. These are rigid (except for the floor, which can be raised or lowered by the soft palate) and the cavity of the nasopharynx is therefore never obliterated by muscle action, unlike the cavities of the oro- and laryngopharynx. The nasal and oral parts of the pharynx communicate through the pharyngeal isthmus which lies between the posterior border of the soft palate and the posterior pharyngeal wall. Elevation of the soft palate and constriction of the palatopharyngeal sphincter close the isthmus during swallowing.

The roof and posterior wall form a continuous concave slope that leads down from the nasal septum to the oropharynx. It is bounded above by mucosa overlying the posterior part of the body of the sphenoid, and further back by the basilar part of the occipital bone as far as the pharyngeal tubercle. Further down, the mucosa overlies the pharyngobasilar fascia and the upper fibres of the superior constrictor, and behind these, the anterior arch of the atlas. A lymphoid mass, the adenoid, lies in the mucosa of the upper part of the roof and posterior wall in the midline.

The lateral walls of the nasopharynx display a number of important surface features. On either side each receives the opening of the pharyngotympanic tube (also termed the auditory or Eustachian tube), situated 10–12 mm behind and a little below the level of the posterior end of the inferior nasal concha (Fig. 33.2). The tubal aperture is approximately triangular in shape, and is bounded above and behind by the tubal elevation which consists of mucosa overlying the protruding pharyngeal end of the cartilage of the pharyngotympanic tube. A vertical mucosal fold, the salpingopharyngeal fold, descends from the tubal elevation behind the aperture (Fig. 33.2) and covers salpingopharyngeus in the wall of the pharynx (see Fig. 33.4); a smaller salpingopalatine fold extends from the anterosuperior angle of the tubal elevation to the soft palate in front of the aperture. As levator veli palatini enters the soft palate it produces an elevation of the mucosa immediately around the tubal opening (see Fig. 33.4). A small variable mass of lymphoid tissue, the tubal tonsil, lies in the mucosa immediately behind the opening of the pharyngotympanic tube, and further behind the tubal elevation there is a variable depression in the lateral wall, the lateral pharyngeal recess (fossa of Rosenmüller). The floor of the nasopharynx is formed by the nasal, upper surface of the soft palate.

Fig. 33.4 A, Interior of the pharynx, exposed by removal of the mucous membrane, sagittal section. The bodies of the cervical vertebrae have been removed, the cut posterior wall of the pharynx retracted dorsolaterally and palatopharyngeus reflected dorsally to show the cranial fibres of the inferior constrictor. The dorsum of the tongue has been pulled ventrally to display a part of styloglossus in the angular interval between the mandibular and the lingual fibres of origin of the superior constrictor. B, Muscles of the left half of the soft palate and adjoining part of the pharyngeal wall, sagittal section.

MICROSTRUCTURE OF NON-TONSILLAR NASOPHARYNX

The nasopharyngeal epithelium anteriorly is ciliated, pseudostratified respiratory in type with goblet cells (see Fig. 2.2). The ducts of mucosal and submucosal seromucous glands open onto its surface. Posteriorly, the respiratory epithelium changes to non-keratinized stratified squamous epithelium which continues into the oropharynx and laryngopharynx. The transitional zone between the two types of epithelium consists of columnar epithelium with short microvilli instead of cilia. Superiorly, this zone meets the nasal septum; laterally, it crosses the orifice of the pharyngotympanic tube; and it passes posteriorly at the union of the soft palate and the lateral wall. Numerous mucous glands surround the tubal orifices.

INNERVATION

Much of the mucosa of the nasopharynx behind the pharyngotympanic tube is supplied by the pharyngeal branch of the pterygopalatine ganglion which traverses the palatovaginal canal with a pharyngeal branch of the maxillary artery.

ADENOID OR PHARYNGEAL TONSIL

The adenoid, or pharyngeal tonsil, is a median mass of mucosa-associated lymphoid tissue (MALT) situated in the roof and posterior wall of the nasopharynx (Fig. 33.3). At its maximal size (during the early years of life) it is shaped like a truncated pyramid, often with a vertically oriented median cleft, so that its apex points towards the nasal septum and its base to the junction of the roof and posterior wall of the nasopharynx.

Fig. 33.3 A, Nasopharyngeal tonsil following adenoidectomy by curettage. The rostral surface is to the left; surface folds radiate forwards from a median recess (arrow). In this example, the impression left by contact with the left Eustachian cushion is evident laterally (arrowhead). B, Transnasal endoscopic view of adenoid. 1. Adenoid (in posterior naris). 2. Inferior concha (posterior view). 3. Posterior end of nasal septum.

(A, Specimen provided by Professor MJ Gleeson, School of Medicine, King’s College London; B, by courtesy of Mr Simon A Hickey.)

The free surface of the pharyngeal tonsil is marked by folds that radiate forwards and laterally from a median blind recess, the pharyngeal bursa (bursa of Luschka), which extends backwards and up. The recess is present in the fetus and the young, but only occasionally present in the adult, and marks the rostral end of the embryological notochord. The number and position of the folds and of the deep fissures which separate them vary. A median fold may pass forwards from the pharyngeal bursa towards the nasal septum, or instead a fissure may extend forwards from the bursa, dividing the nasopharyngeal tonsil into two distinct halves which reflect its paired developmental origins (Fig. 33.3).

The prenatal origins of the nasopharyngeal tonsil are described on page 604. After birth it initially grows rapidly, but usually undergoes a degree of involution and atrophy from the age of 8–10 years (although hypoplasia may still occur in adults up to the seventh decade). Relative to the volume of the nasopharynx, the size of the tonsil is largest at 5 years, which may account for the frequency of nasal breathing problems in preschool children, and the incidence of adenoidectomy in this age group.

Vascular supply and lymphatic drainage

The arterial supply of the pharyngeal tonsil is derived from the ascending pharyngeal and ascending palatine arteries, the tonsillar branches of the facial artery, the pharyngeal branch of the maxillary artery and the artery of the pterygoid canal (see Figs 33.5, 33.6). In addition, a nutrient or emissary vessel to the neighbouring bone, the basisphenoid artery, which is a branch of the inferior hypophysial arteries, supplies the bed of the pharyngeal tonsil and is a possible cause of persistent postadenoidectomy haemorrhage in some patients.

Fig. 33.5 The tonsillar bed, sagittal section.

Fig. 33.6 Arterial supply to the palatine tonsil.

Numerous communicating veins drain the pharyngeal tonsil into the internal submucous and external pharyngeal venous plexuses. They emerge from the deep lateral surface of the tonsil and join the external palatine (paratonsillar) veins, and pierce the superior constrictor either to join the pharyngeal venous plexus, or to unite to form a single vessel that enters the facial or internal jugular vein; they may also connect with the pterygoid venous plexus.

PHARYNGOTYMPANIC TUBE

The pharyngotympanic tube (see Fig. 36.7) connects the tympanic cavity to the nasopharynx and allows the passage of air between these spaces in order to equalize the air pressure on both aspects of the tympanic membrane. It is about 36 mm long and descends anteromedially from the tympanic cavity to the nasopharynx at an angle of approximately 45° with the sagittal plane and 30° with the horizontal (these angles increase with age and elongation of the skull base). It is formed partly by cartilage and fibrous tissue and partly by bone.

The cartilaginous part, which is approximately 24 mm long, is formed by a triangular plate of cartilage, the greater part of which is in the posteromedial wall of the tube. Its apex is attached by fibrous tissue to the circumference of the jagged rim of the bony part of the tube, and its base is directly under the mucosa of the lateral nasopharyngeal wall, forming a tubal elevation (torus tubarius; Eustachian cushion) behind the pharyngeal orifice of the tube (Fig. 33.2; see Fig. 30.1). The upper part of the cartilage is bent laterally and downwards, producing a broad medial lamina and narrow lateral lamina. In transverse section it is hook-like and incomplete below and laterally, where the canal is composed of fibrous tissue. The cartilage is fixed to the cranial base in the groove between the petrous part of the temporal bone and the greater wing of the sphenoid, and ends near the root of the medial pterygoid plate. The cartilaginous and bony parts of the tube are not in the same plane, the former descending a little more steeply than the latter. The diameter of the tube is greatest at the pharyngeal orifice, least at the junction of the two parts (the isthmus), and increases again towards the tympanic cavity.

The bony part, approximately 12 mm long, is oblong in transverse section, with its greater dimension in the horizontal plane. It starts from the anterior tympanic wall and gradually narrows to end at the junction of the squamous and petrous parts of the temporal bone, where it has a jagged margin for the attachment of the cartilaginous part. The carotid canal lies medially.

The mucosa of the pharyngotympanic tube is continuous with the nasopharyngeal and tympanic mucosae. It is lined by a ciliated columnar epithelium and is thin in the osseous part but thickened by mucous glands in the cartilaginous part. Near the pharyngeal orifice there is a variable, but sometimes considerable, lymphoid mass called the tubal tonsil.

At birth the pharyngotympanic tube is about half its adult length, it is more horizontal and its bony part is relatively shorter but much wider. The pharyngeal orifice is a narrow slit, level with the palate and without a tubal elevation.

BOUNDARIES

The oropharynx extends from below the soft palate to the upper border of the epiglottis (Figs 33.1, 33.2). It opens into the mouth through the oropharyngeal isthmus, demarcated by the palatoglossal arch, and faces the pharyngeal aspect of the tongue. Its lateral wall consists of the palatopharyngeal arch and palatine tonsil (see Fig. 30.3). Posteriorly, it is level with the bodies of the second, and upper part of the third, cervical vertebrae (Fig. 33.2).

SOFT PALATE

The soft palate is a mobile flap suspended from the posterior border of the hard palate, sloping down and back between the oral and nasal parts of the pharynx (Figs 33.2, 33.4). The boundary between the hard and soft palate is readily palpable and may be distinguished by a change in colour, the soft palate being a darker red with a yellowish tint. The soft palate is a thick fold of mucosa enclosing an aponeurosis, muscular tissue, vessels, nerves, lymphoid tissue and mucous glands; almost half its thickness is represented by numerous mucous glands which lie between the muscles and the oral surface of the soft palate. The latter is covered by a stratified squamous epithelium, while the nasal surface is covered with a ciliated columnar epithelium. In most individuals, two small pits, the fovea palatini, may be seen, one on each side of the midline: they represent the orifices of ducts from some of the minor mucous glands of the palate. In its usual relaxed and pendant position, the anterior (oral) surface of the soft palate is concave, and has a median raphe. The posterior aspect is convex and continuous with the nasal floor, the anterosuperior border is attached to the posterior margin of the hard palate, and the sides blend with the pharyngeal wall. The inferior border is free, and hangs between the mouth and pharynx. A median conical process, the uvula, projects downwards from its posterior border (see Fig. 30.3). Taste buds are found on the oral aspect of the soft palate.

The anterior third of the soft palate contains little muscle and consists mainly of the palatine aponeurosis. This region is less mobile and more horizontal than the rest of the soft palate and is the chief area acted upon by tensor veli palatini.

A small bony prominence, produced by the pterygoid hamulus, can be felt just behind and medial to each upper alveolar process, in the lateral part of the anterior region of the soft palate. The pterygomandibular raphe (a tendinous band between buccinator and the superior constrictor) passes downwards and outwards from the hamulus to the posterior end of the mylohyoid line. When the mouth is opened wide, this raphe raises a fold of mucosa that indicates the internal, posterior boundary of the cheek; it is an important landmark for an inferior alveolar nerve block.

PALATINE TONSIL

The right and left palatine tonsils form part of the circumpharyngeal lymphoid ring. Each tonsil is an ovoid mass of lymphoid tissue situated in the lateral wall of the oropharynx (Fig. 33.5; see Figs 30.3, 30.5). Size varies according to age, individuality and pathological status (tonsils may be hypertrophied and/or inflamed). It is therefore difficult to define the normal appearance of the palatine tonsil. For the first 5 or 6 years of life the tonsils increase rapidly in size. They usually reach a maximum at puberty, when they average 20–25 mm in vertical, and 10–15 mm in transverse, diameters, and they project conspicuously into the oropharynx. Tonsillar involution begins at puberty, when the reactive lymphoid tissue begins to atrophy, and by old age only a little tonsillar lymphoid tissue remains.

The long axis of the tonsil is directed from above, downwards and backwards. Its medial, free, surface usually presents a pitted appearance. The pits, 10–20 in number, lead into a system of blind-ending, often highly branching, crypts which extend through the whole thickness of the tonsil and almost reach the connective tissue hemicapsule. In a healthy tonsil the openings of the crypts are fissure-like and the walls of the crypt lumina are collapsed so that they are in contact with each other. The human tonsil is polycryptic. The branching crypt system reaches its maximum size and complexity during childhood. The mouth of a deep tonsillar cleft (intratonsillar cleft, recessus palatinus) opens in the upper part of the medial surface of the tonsil. It is often erroneously called the supratonsillar fossa, and yet it is not situated above the tonsil but within its substance. The mouth of the cleft is semilunar, curving parallel to the convex dorsum of the tongue in the sagittal plane. The upper wall of the recess contains lymphoid tissue which extends into the soft palate as the pars palatina of the palatine tonsil. After the age of 5 years this embedded part of the tonsil diminishes in size. There is a tendency for the whole tonsil to involute from the age of 14 years, and for the tonsillar bed to flatten out. During young adult life, a mucosal fold, the plica triangularis, stretches back from the palatoglossal arch down to the tongue. It is infiltrated by lymphoid tissue and frequently represents the most prominent (anteroinferior) portion of the tonsil. It rarely persists into middle age.

The lateral or deep surface of the tonsil spreads downwards, upwards and forwards. Inferiorly, it invades the dorsum of the tongue, superiorly, the soft palate, and, anteriorly, it may extend for some distance under the palatoglossal arch. This deep, lateral aspect is covered by a layer of fibrous tissue, the tonsillar hemicapsule. The latter is separable with ease for most of its extent from the underlying muscular wall of the pharynx, which is formed here by the superior constrictor, and sometimes by the anterior fibres of palatopharyngeus, with styloglossus on its lateral side (Fig. 33.5). Anteroinferiorly, the hemicapsule adheres to the side of the tongue and to palatoglossus and palatopharyngeus. In this region, the tonsillar artery, a branch of the facial artery, pierces the superior constrictor to enter the tonsil, accompanied by venae comitantes (Fig. 33.5). An important and sometimes large vein, the external palatine or paratonsillar vein, descends from the soft palate lateral to the tonsillar hemicapsule before piercing the pharyngeal wall. Haemorrhage from this vessel from the upper angle of the tonsillar fossa may complicate tonsillectomy. The muscular wall of the tonsillar fossa separates the tonsil from the ascending palatine artery, and, occasionally, from the tortuous facial artery itself, which may lie near the pharyngeal wall at the lower tonsillar level. The glossopharyngeal nerve lies immediately lateral to the muscular wall of the tonsillar fossa (Figs 33.4A, 33.5): it is at risk if the wall is pierced, and is commonly temporarily affected by oedema following tonsillectomy. The internal carotid artery lies approximately 25 mm behind and lateral to the tonsil. In some individuals the styloid process may be elongated and deviate towards the tonsillar bed.

BOUNDARIES

The laryngopharynx is situated behind the entire length of the larynx (known clinically as the hypopharynx) and extends from the superior border of the epiglottis, where it is delineated from the oropharynx by the lateral glossoepiglottic folds, to the inferior border of the cricoid cartilage, where it becomes continuous with the oesophagus (Figs 33.1, 33.2). The laryngeal inlet lies in the upper part of its incomplete anterior wall, and the posterior surfaces of the arytenoid and cricoid cartilages lie below this opening.

SPREAD OF INFECTION

Infection that spreads into the parapharyngeal space will produce pain and trismus. There may be swelling in the oropharynx which extends up to the uvula, displacing it to the contralateral side, and dysphagia. Posterior spread from the parapharyngeal space into the retropharyngeal space will produce bulging of the posterior pharyngeal wall, dyspnoea and nuchal rigidity. Involvement of the carotid sheath may produce symptoms caused by thrombosis of the internal jugular vein and cranial nerve symptoms involving the glossopharyngeal, vagus, cranial accessory and hypoglossal nerves. If the infection continues to spread unchecked, mediastinitis will ensue. A virulent infection in the retropharyngeal space may spread through the prevertebral fascia into the underlying prevertebral space: infection in this tissue space may descend into the thorax and even below the diaphragm, and results in chest pain, severe dyspnoea and retrosternal discomfort.

Pharyngeal infection from mucosa-associated lymph tissues such as the palatine tonsil, or as a result of a penetrating injury (e.g. from an ingested foreign body), may result in the spread of infection into the tissue spaces of the neck adjacent to the pharynx. This is an extremely dangerous situation because there is potential for rapid spread throughout the neck and, more dangerously, to the superior mediastinum, to cause overwhelming life-threatening infection.

PARAPHARYNGEAL SPACE TUMOURS

Tumours that develop in the parapharyngeal tissue space may remain asymptomatic for some time. When they do present, it may be with a diffuse pattern of symptoms, reflecting the effects of compression on the lower cranial nerves, e.g. dysarthria, resulting from impairment of tongue movements secondary to hypoglossal nerve damage; dysphagia, with overspill and aspiration of ingested material into the airway, resulting from loss of sensory information from the territory of the pharyngeal plexus nerves; motor dysfunction of the pharynx and larynx, resulting from loss of motor innervation via the pharyngeal plexus and the recurrent laryngeal branch of the vagus to the intrinsic muscles of the larynx.

ORAL PREPARATORY PHASE

In the oral preparatory phase, the essential action is chewing: the mandible is moved by the action of the jaw elevators and depressors (see Ch. 31), and the food is reduced by the grinding action of the teeth and simultaneously mixed with saliva. The lips are maintained as a tight labial seal by the contraction of orbicularis oris: buccinator performs a similar function for the cheeks. In this way, the sulci are closed, the vestibule normally remains empty, and any food that enters the vestibule is returned to the oral cavity proper. Buccinator also keeps the cheeks taut, ensuring that they are kept clear of the occlusal surfaces of the teeth during chewing: loss of the nerve supply to buccinator as a result of damage to the facial nerve results in painful and repeated lacerations of the cheeks.

The soft palate is depressed during this phase and premature spillage of food is common. Spillage occurs because the soft palate is not in continuous contact with the posterior part of the tongue, as was once thought (Hiiemae & Palmer 1999). Bolus formation appears to involve several cycles of food being transported from the anterior to the posterior part of the tongue through the palatoglossal and palatopharyngeal arches until a bolus accumulates on the oropharyngeal surface of the tongue (retrolingual loading). Throughout this phase, the lateral and rotatory tongue movements that deliver the food to the teeth for grinding and reduction are crucial for normal bolus formation. If effective tongue movements do not occur, chewing will be compromised. The end of this phase of swallowing is marked by the tongue holding the bolus of food that has been formed against the hard palate in readiness for transport to the posterior part of the oral cavity.

ORAL TRANSIT/TRANSFER PHASE

In the oral transit/transfer phase, the bolus formed in the oral preparatory stage is finally transported through the palatoglossal and palatopharyngeal arches into the oropharynx.

Genioglossus raises both the tongue tip and the part of the tongue immediately behind the tip. The soft palate is then fully lowered by contraction of palatoglossus and palatopharyngeus, and the posterior part of the tongue is simultaneously elevated: the apposed soft palate and tongue form a tight seal that helps to prevent premature entry of the bolus into the pharynx. Orbicularis oris and buccinator remain contracted, so keeping the lips and cheeks taut and the bolus of food central in the oral cavity. The bolus is accommodated in a shallow midline gutter that forms along the dorsum of the tongue, probably as a result of the co-contraction of the styloglossi and the genioglossi, aided by the superior longitudinal and transverse fibres of the intrinsic muscles.

The mandible is elevated and the mouth is closed. The floor of the mouth and the anterior and middle portions of the tongue are elevated, by co-contraction of the suprahyoid group of muscles (mylohyoid, digastric, geniohyoid and stylohyoid): the effectiveness of the suprahyoid muscles is increased as they contract against a fixed mandible (the mouth does not have to be closed to swallow, but it is much harder to swallow if it is open). Contraction of stylohyoid elevates the more posterior parts of the tongue and empties the longitudinal gutter. At the same time, the tongue flattens, probably as a result of the contraction of hyoglossus and some of the intrinsic lingual muscles, especially the vertical fibres. The elevated, flattened tongue pushes the bolus against the hard palate, and the sides of the tongue seal against the maxillary alveolar processes, helping to move the bolus further posteriorly. Contraction of styloglossus and mylohyoid completes the elevation of the posterior part of the tongue. At the same time, the posterior oral seal relaxes and the posterior tongue moves forward: the overall effect is of a cam-like action of the tongue, sweeping or squeezing the bolus towards the pillars of the fauces, finally delivering it to the oropharynx where the pharyngeal aperture is initially increased and then closed.

PHARYNGEAL PHASE

The delivery of the bolus to the oropharynx triggers the pharyngeal phase of the swallow. This phase, which is involuntary and the most critical stage of swallowing, involves the pharynx changing from being an air channel (between the posterior nares and laryngeal inlet) to a food channel (from the fauces to the upper end of the oesophagus). The airway is protected from aspiration during swallowing by hyolaryngeal elevation, and by resetting respiratory rhythm so that airflow ceases briefly as the bolus passes through the hypopharynx: the total time that elapses from the bolus triggering the pharyngeal phase to the re-establishment of the airway is barely 1 second.

The nasopharynx is sealed off from the oropharynx by activation of the superior pharyngeal constrictor and contraction of a subset of palatopharyngeal fibres to form a variable, ridge-like structure (Passavant’s ridge) against which the soft palate is elevated. From an evolutionary perspective, this ridge represents the remnant of a sphincter which encircled a more highly placed larynx: a high laryngeal position is the norm in other mammals and in the human infant (see below), but not in the human adult. Interestingly, the pharyngeal ridge becomes hypertrophic in an infant with a cleft palate, presumably in an attempt to produce a seal to the nasal airway. Ineffective velopharyngeal closure may result in nasal regurgitation of food.

The airway is sealed at the laryngeal inlet by closure of the glottis (see Ch. 34), and the epiglottis is retroflexed over the laryngeal aditus as a result of passive pressure from the base of the tongue and active contraction of the aryepiglottic muscles. The conventional view that laryngeal closure during swallowing occurs from inferior to superior, i.e the vocal folds adduct first and the epiglottis covers the arytenoids and glottis last, has been challenged by studies using simultaneous electromyography and fibreoptic endoscopic evaluation of swallowing, FEES, which have reported that the aryepiglottic folds close before vocal fold adduction during a swallow. It is probably reasonable to assume that the sequence of events that close the glottis may alter according to the type of swallow and consistency of the bolus. What is not in dispute is that the hyoid bone and larynx are raised and pulled anteriorly by the suprahyoid muscles and the longitudinal muscles of the pharynx, so that the laryngeal inlet is brought forward under the bulge of the posterior tongue, i.e. out of the path of the bolus. This action helps expand the hypopharyngeal space and relax the upper oesophageal sphincter, which is also raised by several centimetres. The bolus passes over the reflected anterior surface of the epiglottis and is swept through the laryngopharynx to the upper oesophageal sphincter.

The sequential contraction of the three pharyngeal constrictor muscles is often assumed to be the driving force propelling the bolus towards the oesophagus. However, evidence that the head of the bolus moves faster than the wave of pharyngeal contraction suggests that, at least in some situations, the kinetic energy imparted to the bolus as it is expelled from the mouth into the oropharynx may be sufficient to carry it through the pharynx. This energy is generated by pressure gradients created within the pharynx by the tongue driving force, the hypopharyngeal suction pump, and the ‘stripping action’ of the pharyngeal constrictors.

The tongue driving force, or the tongue thrust pressure force, is a positive pressure that squeezes the bolus towards the laryngopharynx. It is generated by the upward movement of the tongue pressing the bolus against the contracting pharyngeal wall and requires a tight nasopharyngeal seal (created by elevation of the soft palate). There is a view that the tongue driving force is the most important factor responsible for moving the bolus down the pharynx. The hypopharyngeal suction pump is caused by the elevation and anterior movement of the hyoid and larynx, which creates a negative pressure in the laryngopharynx, drawing the bolus towards the oesophagus, aided by a more negative pressure inside the oesophagus. The pharyngeal constrictors generate a positive pressure wave behind the bolus. Their sequential contraction may facilitate clearance (‘stripping’) of the pharyngeal walls and piriform sinuses: if this is so, residues that remain in the valleculae must reflect inadequate tongue force generation at the end of the oral phase of swallowing.

OESOPHAGEAL PHASE

The third, or oesophageal, stage begins after the relaxation of the upper oesophageal sphincter has allowed the bolus to enter the oesophagus. Sequential waves of contractions of the oesophageal musculature subsequently propel the bolus down to the lower oesophageal sphincter, which opens momentarily to admit the bolus to the stomach. The oesophageal phase of swallowing is much more variable than the other phases, and lasts between 8 and 20 seconds.

SWALLOWING PATTERN GENERATOR

The patterning and timing of striated muscle contraction during swallowing are generated at a brainstem level in a network of neural circuits which collectively form a central swallowing pattern generator (SPG). Experimental neurophysiological and tracer studies in animals have shown that the SPG includes several brainstem motor nuclei and two groups of interneurones in the dorsal and ventral medulla, a dorsal swallowing group (DSG) and a ventral swallowing group (VSG). The DSG is located in the nucleus of the solitary tract and possibly in the adjacent reticular formation, and contains neurones that generate the swallowing pattern, probably triggered by convergent information from both cortical and peripheral inputs. The afferent feedback from the branches of the superior laryngeal nerve which innervate the valleculae, epiglottis and supraglottic part of the larynx, and which relay through the nucleus of the solitary tract, facilitates laryngeal closure during swallowing (Jafari et al 2003). The VSG is in the ventrolateral medulla above the nucleus ambiguus and contains neurones that act as ‘switches’, distributing the swallowing drive to motoneurone pools in the trigeminal, facial and hypoglossal nuclei and in the nucleus ambiguus.

The patterns of activation in the smooth muscle of the lower part of the oesophagus are generated locally in intramural plexuses driven by vagal autonomics.

SWALLOWING IN THE NEONATE

In the adult, the tip of the epiglottis is significantly lower than the inferior edge of the soft palate. In the neonate, the larynx is high in the neck and the epiglottis may extend above the soft palate so that the laryngeal airway is in direct continuity with the posterior nares (Fig. 33.10): a potential space is therefore formed between the soft palate above, the epiglottis behind and the tongue anteroinferiorly. In other mammals with an oropharyngeal anatomy similar to that of the human infant, up to 14 cycles of tongue movement or oral phases cause the accumulation of food in this space. Subsequent emptying of the space is a single event followed by movement of the bolus down the oesophagus. The ratio of accumulation cycles to swallow events in the human neonate is approximately 1.5 : 1, which is lower than in other mammals, but still implies some temporary accumulation. In the case of a liquid bolus, accumulated material may be passed laterally to the epiglottis through the piriform fossae rather than over the flexed epiglottis, although it is not known whether this happens in the human infant.

Fig. 33.10 Sagittal section of the head of a neonate. Note the relatively high position of the larynx, the opening being at the level of the soft palate A, B, epiglottis.

(By permission from Berkovitz BKB, Holland GR, Moxham BJ 2002 Oral Anatomy, Embryology and Histology, 3rd edn. Edinburgh: Mosby.)

Swallow safety is critical at all ages. In the neonate, where coordination of suckling, swallowing and breathing is not fully developed, the potential risk of airway obstruction and/or aspiration of ingested milk or other material during swallowing is reduced because the intranarial larynx prevents the bolus from entering the larynx before and after a swallow (see Ch. 34). The change towards adult anatomy and coordination of the phases of swallowing starts a few months after birth. Differential growth in length of the human pharynx causes the larynx to take up its low adult position and the epiglottis to lose contact with the soft palate: the larynx reaches its final position around the time of puberty. The adult anatomy does not allow any significant accumulation of food to occur immediately anterior to the epiglottis, which means that the transport of food through the fauces has to bear a 1 : 1 relationship to pharyngeal and oesophageal transit. Moreover, the lowered larynx compromises the previously protected airway: the hyoid and larynx are therefore raised and pulled forwards during a swallow in order to minimize the risk of deglutitive aspiration.

PHARYNGOSTOMY AND EPIGLOTTOPEXY

Loss of control of the pharyngeal phase of swallowing, e.g. due to neurological disease or ablative head and neck surgery, may result in aspiration of food, especially fluids, leading to pneumonia. This problem may be addressed surgically by pharyngostomy or epiglottopexy. In pharyngostomy, a fine bore feeding tube is passed into the lower oesophagus or stomach via the nose or anterior abdominal wall. Alternatively, a tube may be passed through the cervical skin, fascia and platysma directly into the piriform fossa. This is achieved by passing a curved forcep into the piriform fossa and pushing it laterally, displacing the contents of the carotid sheath and tenting up the platysma and cervical skin from the inside. By cutting down on to the forcep, it is possible to grasp the feeding tube and pull it into the piriform fossa prior to feeding it on into the oesophagus. The tract formed by such a puncture epithelializes and may be used for long-term alimentation. In epiglottopexy, the neck and the pharynx are opened to expose the laryngeal inlet, the aryepiglottic folds are denuded of mucosa to encourage their adhesion, and the epiglottis is sutured down to the aryepiglottic folds to shield the laryngeal inlet. The resultant compromise of the airway can be offset by the creation of an alternative airway via a tracheostomy. The pharyngeal wall and cervical skin are reconstituted by suturing.