The Eustachian tube is also known as the pharyngotympanic tube. It connects the middle ear to the lateral wall of the nasopharynx. It equalizes pressure across the two cavities, and opens during activities such as swallowing, yawning or the Valsalva manoeuvre, as well as during changes in atmospheric pressure. In this article we will discuss the gross and functional anatomy of the Eustachian tube. We will also discuss the clinical relevance of the structure, and provide a summary of key points at the end of the article. We will finally conclude with some review questions to test the reader’s understanding of the article content.
The Eustachian tube connects the anterior wall of the middle ear to the lateral wall of the nasopharynx (at the level of the inferior nasal concha). The tube extends from the petrous portion of the temporal bone to the fleshy lateral wall of the nasopharynx, and hence has both bony and cartilaginous elements. In adults, the Eustachian tube is approximately 43 mm in length and has a variable diameter throughout, i.e. it is widest at the pharyngeal opening and tympanic cavity, and narrowest at the isthmus.
The bony section comprises a third of its total length. It starts at the carotid (anterior) wall of the tympanic cavity, narrows and ends at the connection of the squamous and petrous portions of the temporal bone. The uneven margin this creates enables the attachment of the cartilaginous section of the Eustachian tube.
The cartilaginous section of the Eustachian tube is formed from a triangular plate of fibrocartilage. The apex of this triangle is attached to the medial end of the bony section of the Eustachian tube, and the base of the triangle is directly beneath the mucous membrane of the nasopharynx. This triangular fibrocartilage lies in a groove between the greater wing of the sphenoid bone, and the petrous portion of the temporal bone. The cartilaginous section of the tube is also inclined further downwards, has more mucous glands, and significant adenoid tissue and the tubal tonsils surrounding the pharyngeal opening, i.e. the torus tubarius.
The Eustachian tube is normally collapsed, i.e. closed, but opens upon positive pressure and swallowing. There are four muscles that are responsible for Eustachian tube functioning. The tensor and levator veli palatini muscles open the tube, by contracting the muscles of the soft palate.
Levator veli palatini- This is a thick round muscle that arises from the petrous portion of the temporal bone and auditory tube cartilage. It descends in an oblique fashion, running superior to the superior pharyngeal constrictor, and blends with the contralateral muscle. It is innervated by the pharyngeal branch of the vagus nerve (cranial nerve 10), via the pharyngeal plexus. It functions by elevating during swallowing, and hence preventing the food entering the nasopharynx.
Tensor veli palatini- This is a broad ribbon like muscle that tenses the soft palate which during swallowing, prevents food entry to the nasopharynx. It lies lateral and anterior to the levator veli palatini. It arises from the pterygoid process as well as the spine of the sphenoid bone. As it also takes some of its origin from the auditory tube, it is the main muscle which opens it upon contraction. It descends between the medial and lateral pterygoid plates, and terminates as a tendon that winds around the pterygoid hamulus, and inserts onto the palatine aponeurosis that lies behind the hard palate, i.e. the palatine bone.
Salpingopharyngeus- This is an elongated triangular muscle that originates from the cartilaginous part of the Eustachian tube. It descends and merges with the palatopharyngeus muscle. It elevates the pharynx during swallowing. It is innervated by the vagus nerve via the pharyngeal plexus.
Tensor tympani- The tensor tympani is the larger of the two muscles of the auditory canal, the other being stapedius. The muscle functions to dampen down sounds such as chewing. It arises from the cartilaginous portion of the eustachian tube as well as the adjacent section of the greater wing of the sphenoid bone. It inserts onto the handle of the malleus. The innervation to the muscle arises from the nerve to tensor tympani, which arises from the mandibular (V3) division of the trigeminal nerve (cranial nerve 5).
Ostmann fat pad- These are fatty bodies which are found inferomedial to the eustachian tube. They act to ensure closure of the eustachian tube, thereby reducing the transference of pressure from the nasopharynx to the middle ear.
When a plane takes off, and reaches a higher altitude, the atmospheric pressure drops. This drop creates a force on the tympanic membrane, which expands the volume of the middle ear. Air can then be drawn through the Eustachian tube to equalize the pressure in the middle ear. When the plane descends, the atmospheric pressure increases, pushing the tympanic membrane medially, therefore compressing air in the middle ear. The same phenomena occur in a diver. As they descend deeper into the ocean, the hydrostatic pressure rises, similarly to what occurs in the case of a descending plane.
When there are changes in atmospheric pressure, the Eustachian tube is required to equalize pressure in the middle ear. The tube also drains mucus from the middle ear because of its ciliated epithelium and aerates the middle ear.