Eustachian (auditory) tube
The auditory tube, more commonly known as the Eustachian tube, is a part bony, part fibrocartilaginous tube which connects the middle ear with the nasopharynx. It is also known as the pharyngotympanic tube.
It serves to equalize pressure within the tympanic cavity with ambient air pressure. It does this by opening during activities such as swallowing, yawning, or the Valsalva maneuver (i.e. an effort to breathe out forcibly with the mouth and nose firmly closed). Opening of the auditory tube may also occur during isolated changes in atmospheric pressure (e.g. 'popping' of the ears on ascent/descent inflight or crossing mountainous terrain etc.)
In this article, we will discuss the gross and functional anatomy of the auditory tube. We will also discuss the clinical relevance of the structure, including dysfunction of the auditory tube.
|Definition||A narrow osseo-fibrocartilaginous tube which connects the middle ear with the nasopharynx|
|Function||Serves to equalize pressure in the middle ear during changes in atmospheric pressure|
|Muscles||Tensor veli palatini muscle
Levator veli palatini muscle
Tensor tympani muscl
Bony part: tubal branches of middle meningeal artery/caroticotympanic branches of internal carotid artery
Cartilaginous part: deep auricular artery/pharyngeal branches of maxillary artery, ascending palatine artery (facial artery)
Bony part: tubal branch of tympanic nerve (CN IX)
Cartilaginous part: pharyngeal nerve (CN V3) (via pterygopalatine ganglion)
- Clinical points
Although commonly known as the Eustachian tube (after Bartolommeo Eustachio, an Italian anatomist, 1524-1574), the preferred name for this structure is the auditory (or pharyngotympanic) tube which better describes its anatomical relations.
The auditory tube connects the anterior wall of the middle ear (a.k.a tympanic cavity) to the lateral wall of the nasopharynx (at the level of the inferior nasal concha). It extends through the petrous portion of the temporal bone before passing into the fleshy lateral wall of the nasopharynx, supported by cartilages embedded in its wall; hence the auditory has both bony and cartilaginous parts. In adults, the auditory tube is approximately 43 mm long and has a variable diameter throughout; it is widest at the tympanic and pharyngeal openings and narrowest at the isthmus, where the bony and cartilaginous parts meet.
The bony part of the auditory tube comprises the posterolateral third of this structure. It arises is located in an anterior subcompartment of the tympanic cavity, known as the protympanum (the region anterior to the tympanic ring), which communicates with the musculotubal canal of the temporal bone; this canal is further divided into two semicanals by the cochleariform process: one for the auditory tube, the other for the tensor tympani muscle (see below). The former forms the tympanic opening of the auditory tube.
Within the temporal bone, the bony part of the auditory tube narrows and terminates in the region of the sphenopetrosal suture, a point known as the isthmus of the auditory tube.
Note: Recent studies involving endoscopy of the middle ear have suggested that the bony part of the auditory tube is better described as an anterior extension of the tympanic cavity rather than part of the auditory tube itself.
The cartilaginous part of the auditory tube is supported by a curled triangular plate of fibrocartilage located in a groove between the greater wing of the sphenoid bone and petrous portion of the temporal bone. The apex of this triangle is attached to the bony part of the auditory tube while the base projects towards the mucous membrane of the nasopharynx, posterior to the pharyngeal opening of the auditory tube; here it forms an elevation on the nasopharyngeal wall known as the torus tubarius. A collection of a collection of lymphoid nodules, known as tubal tonsils, are located near the pharyngeal opening of the auditory tube.
The lining, or mucosa, of the auditory tube is continuous with that of the tympanic cavity where it arises, as well as with that of the nasopharynx where it terminates. Therefore the mucosa of bony part of the auditory initially begins as a [thin layer of] simple cuboidal/columnar epithelium, which transitions to a [thicker layer of] ciliated pseudostratified epithelium in the cartilaginous part (similar to that of respiratory epithelium found in the nasopharynx). The latter contains mucous glands known as tubal glands.
The auditory tube is normally collapsed, i.e. closed, but opens upon positive pressure and swallowing. There are four muscles that are responsible for auditory tube functioning. The tensor and levator veli palatini muscles open the tube, by contracting the muscles of the soft palate.
Levator veli palatini
Levator veli palatini 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 X), via the pharyngeal plexus. It functions by elevating during swallowing and hence preventing the food entering the nasopharynx.
Tensor veli palatini
Tensor veli palatini 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 is an elongated triangular muscle that originates from the cartilaginous part of the auditory 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.
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 auditory 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
Not exactly a muscle but these fatty bodies, which are found inferomedial to the auditory tube, act to ensure closure of the auditory tube, thereby reducing the transference of pressure from the nasopharynx to the middle ear.
Test your knowledge on the structures of the middle ear with this quiz.
The bony part of the auditory tube is supplied by tubal branches of the middle meningeal artery (branch of the maxillary artery) and/or caroticotympanic arteries (branches of the internal carotid artery), while the cartilaginous part may be supplied by any of the following vessels: deep auricular artery (maxillary artery), pharyngeal branch of artery of pterygoid canal (maxillary artery), ascending palatine artery (facial artery). Venous drainage is carried to the pterygoid venous plexus.
The bony part of the auditory tube is innervated via a tubal branch of the tympanic nerve/plexus (derived from the glossopharyngeal nerve (CN IX). The cartilaginous part, on the other hand, is primarily innervated by branches of the pharyngeal nerve (branch of the maxillary nerve (CNV3) via the pterygopalatine ganglion).
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When there are changes in atmospheric pressure, the auditory tube serves to equalize pressure in the middle ear. For example, 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 auditory 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, similar to what occurs in the case of a descending plane.
The tube also drains mucus from the middle ear because of its ciliated epithelium and aerates the middle ear.
The auditory tube connects the middle ear to the nasopharynx. Therefore, it acts as a tract that can connect the two cavities and transmit infections. Otitis media is an infection of the middle ear and most commonly affects children. Their auditory tubes are not much smaller (38 mm vs 43 mm on average). However, the angle of the tube with the horizontal plane is probably more important (as low as 10 degrees in children vs 35 degrees in adults), leading to reflux of nasopharyngeal secretions into the middle ear. It also most frequently follows an upper respiratory tract infection.
Eustachian tube dysfunction (obstructive vs. patulous)
If the auditory tube is patulous, it remains intermittently open. This results in autophony i.e. the patient can hear themselves chewing, and their own heartbeat etc. Obstructive disorders result from conditions such as glue ear, enlarged adenoids, hay fever and persistent rhinitis.
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