Anatomy of Hypopharynx 

The hypopharynx lies behind and adjacent to the larynx. It is also called the laryngopharynx.

Superiorly, it is bound by an imaginary line passing through the tip of the epiglottis. Inferiorly, it extends up to the lower border of the cricoid cartilage anteriorly and C6 vertebrae posteriorly. Initially, it is related to the larynx. Posteriorly related to C3-C6 vertebrae. It begins at the level of the hyoid bone, where it borders the oropharynx superiorly and funnels inferiorly to the cervical esophagus at the level of the inferior cricoid cartilage at the level of C6.

Subsites of Hypopharynx

The subsites of hypopharynx are:

• Pyriform sinuses laterally,
• Post cricoid area anteriorly.
• Pharyngeal wall posteriorly.

The two depressions in the hypopharynx are called the pyriform fossa. Another depression behind this is called the postcricoid region, which leads to the esophagus. The wall retracted down overlying this is called the posterior pharyngeal wall.

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UICc Classification Of The Anatomical Subsites (Hypopharynx)

The pharyngoesophageal junction, i.e., post-cricoid area, extends from the level of the arytenoid cartilage and connects folds to the inferior border of the cricoid cartilage, forming the anterior wall of the hypopharynx. The pyriform sinus extends from the pharyngeal epiglottic fold to the upper end of the esophagus. It is bound laterally by the thyroid cartilage and medially by the hypopharyngeal surface of the aryepiglottic fold ( C 13.1) arytenoids and cricoid cartilages.

The posterior pharyngeal wall extends from the superior level of the hyoid bone or floor of the vallecular to the level of the inferior border of the cricoid cartilage and from the apex of the one pyriform sinus to the other.

The paired pyriform sinuses are two in number and lie on the side of the aryepiglottic fold in the form of inverted pyramids. It begins at the pharyngeal or piloting folds superiorly and the

apex blending into the cervical esophagus at its inferior extent. Laterally, it is related to the thyroid cartilage and medially to the arytenoid and cricoid cartilage. They are lined with mucosa and bounded medially by quadrangular membranes of the larynx.

The lateral boundaries are the thyrohyoid membrane superiorly and the thyroid cartilage inferiorly. The foreign objects commonly lodged in the recesses and tumors can silently expand within the space asymptomatically until metastatic lymphadenopathy manifests.

The postcricoid area is a depression in the midline. It represents the mucosa underlying the posterior portion of the cricoid ring. It extends from the arytenoid cartilage to the inferior border of the cricoid cartilage and is laterally continuous with the pyriform fossa.

The posterior pharyngeal wall extends from the horizontal line drawn at the level of the superior margin of the hyoid bone or floor of the vallecular to the level of the inferior border of the cricoid cartilage and forms the apex of one piriform recess to the other.

The inferior constrictor is the main component of the wall behind the cricoid lamina and to the level of the upper margin of the cricoid cartilage. The inferior constrictor is composed of two separate muscles, i.e., the oblique fiber of thyropharyngeus and horizontal fiber of cricopharyngeus. The triangle area of dehiscence, or this fusion point of these muscles is Kilian's dehiscence, which is a site of potential weakness and the site of pharyngeal pouches. The outpouring of mucosa through Killian's dehiscence is called Zenker's diverticulum. It is a pseudo-difficult

because it does not contain all the layers of the pharyngeal wall but the herniation of mucosa.

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Blood Supply

The blood supply to the hypopharynx is via the external carotid system and includes branches of the superior thyroid artery and ascending pharyngeal and lingual arteries. The venous drainage mirrors the arterial system in addition to the prevertebral venous plexus.

Nerve Supply 

The Sensory Innervation

Through the branches of the glossopharyngeal and vagus nerves via the pharyngeal plexus. The internal branch of the superior laryngeal nerve.

The Motor Innervation

Through pharyngeal constrictors that come from the pharyngeal plexus. Posterior cricoarytenoid muscles are innervated by the recurrent laryngeal nerve.

Lymphatic Drainage 

The drainage can occur laterally to the jugulodigastric nodes. The second drainage pathway is posterior to the retropharyngeal nodes and can extend as high as the skull base in the nodes of Rouviere. Inferiorly, nodal metastases can occur in the paratracheal and paraesophageal nodes. Bilateral drainage is common, especially for lesions located in the medial pyriform and posterior pharyngeal wall.

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Pharyngeal Wall

Structure of pharyngeal wall

The pharyngeal wall consists of four layers from within to outwards.

• Mucous membrane.
• Pharyngobasilar fascia
• A muscular coat
• Buccopharyngeal fascia

Mucosal Layer Of Pharyngeal Wall 

The mucosa of the nasopharynx is typically respiratory mucosa and is made up of ciliated pseudostratified columnar epithelium. The rest of the pharynx is lined by nonkeratinized stratified squamous epithelium. This change occurs at the transitional zone, i.e. at the lower end of the nasopharynx.

Submucosal Layer Of Pharyngeal Wall 

Submucosa is the layer beneath the mucosa. It is also termed lamina propria, which is made of connective tissue containing elastic tissue. It contains aggregates of gut-associated lymphoid tissue that forms Waldeyer's ring. The submucosa is thick and blends in with the fibrous

pharyngobasilar layer, which also forms the tonsillar capsule.

Pharyngobasilar Fascia Layer of Pharyngeal Wall 

The pharyngobasilar fascia is the innermost layer. It is firmly adherent to the skull base in the region of the occipital bone and petrous temporal bone. Superiorly, the fascia is thicker and fuses with the buccopharyngeal fascia to form a dense fibrous structure that suspends the pharynx from the base of the skull. The muscular wall does not fully reach the skull base and this fascial layer is important in maintaining the integrity of the pharynx.

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Muscular Layer of Pharyngeal Wall 

The muscular layer is made up of three pharyngeal constrictor muscles, i.e., superior, middle, and inferior, that interlock like three cones or flower pots, one inside the other, with each layer overlapping the next layer. The constrictor muscle sweeps backward to fuse on the posterior aspect of the pharynx at a midline raphe, the pharyngeal raphe.

The final layer, or outermost layer, of the pharynx is a thin areolar layer covering the pharyngeal constrictors that are also continuous with the covering of the buccinator muscle. This layer contains the pharyngeal plexus of veins and nerves.

Muscular Layer of Pharyngeal Wall 

The pharyngeal muscular wall is relatively thin. It comprises three circular constrictor muscles, all of which act to enable the reflux of swallowing to ensure prompt propulsion of the food bolus by a coordinator peristaltic wave into the esophagus. They are assisted by three longitudinal muscles, which act as elevators and dilators of the pharynx.

Gaps Between Muscles Of Pharyngeal Walls

The gap between the base of the skull and the superior constrictor is called the sinus of Morgagni. The levator veli palatini, auditory tube, tensor veli palatini, and ascending palatine artery pass through the sinus of the morgagni. The glossopharyngeal nerve and stylopharyngeus muscle pass through the gap between the superior and middle constrictors. The superior laryngeal vessel and internal laryngeal nerve pass through the gap between the middle constrictor and the inferior constrictor. The inferior laryngeal vessels and recurrent laryngeal nerves pass through the space below the inferior constrictor.

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Killian's Dehiscence

There are oblique fibers of the thyropharyngeus and horizontal fibers of the cricopharyngeus in the inferior constrictor. There is a triangular area of history called Killian's dehiscence. Another dehiscence present laterally between the upper longitudinal fibers of the esophagus and the cricopharyngeus is called Killian's Jamison's dehiscence. The dehiscence in the midline between the cricopharyngeus and the longitudinal fibers of the esophagus is called Laimer dehiscence.

Blood Supply of Pharynx 

The blood supply of the pharynx is through ascending pharyngeal branch or the external carotid, ascending palatine branch of facial or branch of external carotid, and the greater palatine

branch of maxillary. The venous drainage is through the plexus, which drains to the pterygoid plexus in the infratemporal fossa superiorly and the facial and internal jugular veins inferiorly.

Blood Supply of Soft Palate 

The blood supply of the soft palate mainly originates from the lesser palatine artery of the maxillary artery and the ascending pharyngeal artery and its palatine branches in particular. These arteries form a complex anastomosis with the ascending palatine artery, a branch of the facial artery.

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Nerve Supply of Pharynx 

The nerve supply of the pharynx is via a meshwork of interconnecting nerves on the posterior lateral surface of the pharynx. The pharyngeal plexus of nerves are formed by

• branch of the vagus, i.e. X nerve, which is a motor supply
• branches of the glossopharyngeal, i.e. IX nerve, which is a sensory supply
• sympathetic plexus

The sensory supply of the palate originates from the branches of the pterygopalatine ganglion, namely the greater and lesser palatine nerves. The site of their entry into the palate is a useful landmark for performing local anesthetic blocks, as it enters 1 cm medial to the region of the second molar.

Lymphatic Drainage 

The nasopharyngeal tonsils and some drainage from the tubal tonsils are to retropharyngeal nodes. The oropharynx drains into the upper jugular chain, particularly the jugulodigastric node.

The soft palate, lateral and posterior pharyngeal walls, and base of tongues drain into retropharyngeal and parapharyngeal nodes.

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