The piriform sinus, also commonly spelled pyriform sinus, is a feature of the pharynx, or human throat. It is contained in the hypopharynx, which is the lower portion of the pharynx that is found between the hyoid bone on the front of the neck and the lower border of the cricoid cartilage where the larynx, or voice box, begins. The piriform sinus is a narrow fossa, or depression, situated to either side of the opening to the larynx. It is a common site of hypopharyngeal cancers, with nearly 70 percent of cancers afflicting the lower throat occurring here.
Beginning immediately behind the nasal and oral cavities, the pharynx or throat consists of three sections. The nasopharynx, also known as the upper or epipharynx, links the nasal cavity to back of the mouth. Situated behind the tongue is the middle section, the meso- or oropharynx, where the mouth opens to the throat. The lowest portion of the pharynx is the hypopharynx or laryngopharynx, which begins behind the hyoid bone, a U-shaped bone to which the muscles of speech and swallowing attach. Running downward toward the larynx, it ends where the flap of soft tissue known as the epiglottis closes off the larynx — the path to the trachea and lungs — from the esophagus during the act of swallowing.
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The hypopharynx can be subdivided into three sections: the posterior pharyngeal wall, the postcricoid area, and the piriform sinus. Above the esophagus in the back of the throat is the posterior pharyngeal wall, while the postcricoid area lies forward, within the space beyond the opening of the epiglottis to the larynx and behind the posterior wall of the cricoid cartilage of the larynx. Forming the lateral or side portion of the hypopharynx, visible when viewed from the front, is the piriform sinus. It forms a pair of teardrop-shaped recesses that drop down on either side of the epiglottis where it opens to the larynx. Not uncommonly, food that is not swallowed properly can get stuck in this space, increasing the risk that it will be aspirated, or enter the trachea instead of the esophagus.
Also of clinical significance is the frequency of pharyngeal cancer in the piriform sinus. While only a third of hypopharyngeal cancers are found in the postcricoid and posterior pharyngeal regions, nearly 70 percent are located in the piriform sinus. These cancers tend to be squamous cell carcinomas, and they tend to afflict people who have previously been frequent users of tobacco and/or alcohol.