The Hypopharynx

Boundaries

Three Subsites

1. Piriform sinuses (fossae) - the most common site of hypopharyngeal malignancies
   • Paired, inverted pyramid-shaped recesses flanking the laryngeal inlet
   • Formed by anterior, medial, and lateral walls
   • Base is at the pharyngoepiglottic fold; apex is just below the cricoid cartilage
   • The medial piriform mucosa forms the posterolateral wall of the paraglottic space, separated from the endolarynx only by the aryepiglottic folds and lateral cricoarytenoid muscles
2. Postcricoid region
   • Midline, inferiorly located subsite
   • Extends from just below the posterior arytenoid cartilages to the esophageal introitus
   • Lies medial to the tracheoesophageal groove, so tumors here can involve the recurrent laryngeal nerve, paratracheal nodes, and thyroid gland
3. Posterior pharyngeal wall
   • Extends from the level of the hyoid bone to the superior aspect of the cricopharyngeus muscle
   • Separated from vertebral/paravertebral structures by the retropharyngeal space
   • Tumors can invade prevertebral tissues early

Histological Layers (Wall Structure)

1. Mucosal lining - stratified squamous epithelium over loose stroma
2. Fibrous layer - pharyngeal aponeurosis
3. Muscular layer - posterior cricoarytenoid muscles (anterior) and middle/inferior constrictor muscles (posterior); the inferior constrictor condenses distally into the cricopharyngeus muscle
4. Fascial layer - buccopharyngeal fascia

• Killian's triangle - a potential defect at the superior junction of the inferior constrictor and cricopharyngeus, through which posterior pharyngeal wall tumors (and Zenker diverticula) can herniate
• Weakness at the thyrohyoid membrane (just inferior to the hyoid bone), where tumors may spread laterally along the vascular pedicle and superior laryngeal nerve

Blood Supply

• Arterial: Primarily the superior thyroid artery (branch of external carotid); collaterals from lingual and ascending pharyngeal arteries
• Venous: Mirrors the arterial supply

Innervation

• Sensory: Glossopharyngeal nerve (CN IX) and vagus nerve (CN X) - sensory signals pass to the nucleus solitarius in the brainstem, coordinating deglutition
• The internal branch of the superior laryngeal nerve passes through the superior lateral wall of the piriform sinus and through the thyrohyoid membrane
• This nerve synapses in the jugular ganglion alongside Arnold's nerve (from the external auditory canal) - this explains referred otalgia in piriform sinus tumors

Lymphatic Drainage

• Piriform sinus - lymphatics pass through the thyrohyoid membrane to the jugulodigastric lymph nodes
• Postcricoid region - paratracheal and inferior cervical nodes
• Posterior wall - retropharyngeal and upper jugular nodes

Clinical Relevance

• The hypopharynx is a common site for squamous cell carcinoma (SCC), accounting for the majority of hypopharyngeal neoplasms
• Tumors here often present at an advanced stage because the region is "occult" (hidden) and symptoms are non-specific (dysphagia, otalgia, neck mass)
• Risk factors: tobacco, alcohol
• Piriform sinus tumors can invade the larynx due to close anatomical relationships
• Imaging with PET-CT is standard for staging; barium studies are particularly useful for piriform sinus apex and postcricoid assessment
• Treatment (surgery, radiotherapy, chemotherapy) carries a poor prognosis at advanced stages due to the challenging anatomy and reconstructive demands

"bordered anteriorly by the larynx"

Quick visual clarification

FRONT (anterior)          BACK (posterior)
─────────────────────────────────────────────
   LARYNX    |   HYPOPHARYNX   |  VERTEBRAE
             ↑
        larynx is the anterior
        border of the hypopharynx

• The larynx sits in front of the hypopharynx
• The hypopharynx wraps around the back and sides of the larynx
• The piriform sinuses (part of the hypopharynx) are actually the two grooves running on either side of the larynx
• Behind the hypopharynx are the retropharyngeal space and vertebral column