Named after Bartolomeo Eustachi, a 16-century Italian anatomist, the Eustachian tube is a canal that joins the middle ear to the nasopharynx. The Eustachian tube also is known as the auditory tube or pharyngotympanic tube.
The Eustachian tube plays a role in equalization, oxygenation, and drainage of the tympanic cavity in the middle ear. More specifically, the Eustachian tube permits equalization of pressure in the middle ear with respect to ambient pressure. In doing so, the Eustachian tube allows for regulation of the pressure across the tympanic membrane. It thus influences the tension in this structure and the attached ossicles, and in this way indirectly affects the effectiveness of sound wave transmission. It is believed that the Eustachian tube also may be involved in sound transformation through reverberation phenomena. Patency of the tube allows for air exchange in the tympanic cavity to replenish oxygen to the middle ear, in addition to providing an outlet for mucus and other fluid from the middle ear.
Partly a hollow tube in bone and partly a potential space in fibroelastic cartilage, the Eustachian tube is normally closed, as its proximal walls are collapsed. These can be actively pulled apart to open the tube with the help of accessory muscles or passively pushed apart by air exiting or entering the middle ear under pressure. The active opening of the Eustachian tube to relieve positive or negative pressure in the middle ear commonly is called “clearing the ear.”
The Eustachian tube originates from the tubotympanic recess, an embryonic structure derived from the first pharyngeal and second endodermal pouches. The proximal portion of the tubotympanic recess forms the tympanic cavity, and its distal portion becomes the Eustachian tube.
Blood is supplied to the Eustachian tube by several arteries of the external carotid, including the ascending pharyngeal branch and two branches of the maxillary artery, the middle meningeal artery and artery of the pterygoid canal. Venous returns drain into the pterygoid venous plexus, and lymphatics drain into the retropharyngeal lymph nodes.
Motor innervation to the muscle attachments of the Eustachian tube is provided by the pharyngeal plexus of the vagus nerve (cranial nerve X) and the mandibular branch of the trigeminal nerve (cranial nerve V). Specifically, the levator veli palatini and salpingopharyngeus muscles are innervated by cranial nerve X, and the tensor tympani and tensor veli palatini muscles are supplied by the mandibular branch of cranial nerve V. Sensory innervation to the Eustachian tube, middle ear, and pharynx is governed by the trigeminal nerve.
The Eustachian tube has four muscle attachments: the levator veli palatini, salpingopharyngeus, tensor tympani, and the tensor veli palatini. The Eustachian tubes can be actively opened in the actions of swallowing, chewing, and yawning through contraction of the soft palate, specifically through the action of the levator and tensor veli palatini. The tensor veli palatini partially originates from the auditory canal; hence, it makes a larger contribution to the opening of the Eustachian tube. The tensor tympani muscle originates in part from the Eustachian tube’s cartilaginous portion and works to dampen loud sounds such as chewing and speaking but has little direct effect on tube opening.
Anatomic and physiologic variations within the Eustachian tube may be causative factors in cases of Eustachian tube dysfunction. Manifestations include difficulty in opening the tube and problems with patency. These variations include narrower tube diameter; flaccidity of the cartilaginous portion of the tube or surrounding tissues; and mucosal hypertrophy of the tube lining, which leads to excess mucus production and clogging.
The structure of the Eustachian tube is made up of two anatomically distinct parts covered by the mucous membrane: a bony component and a cartilaginous component. The bony component courses through the petrous part of the temporal bone and represents one third (12 millimeters) of the tube's length. The Eustachian tube follows a straight course and slopes down postero-anteriorly and lateromedially at an angle of approximately 35 degrees in adults. The tube begins with an aperture on the anterior, or carotid, wall of the middle ear at the junction of the petrous and squamous constituents of the temporal bone and opens into the tympanic cavity, the walls of which are continuous with the middle ear. Its other end opens into the lateral wall of the nasopharynx, roughly at the level of the inferior turbinate. The Eustachian tube has a diameter of approximately 3 millimeters and is widest at its two ends and narrowest at its isthmus, which typically is found in the cartilaginous portion. It normally is closed due to pressure from adjacent Ostmann fat pads. The Eustachian tube is lined with ciliated epithelium, which sweeps mucus away from the middle ear in the direction of the nasopharynx.
In children, the Eustachian tube’s course is more horizontal than in adults (10 degrees concerning the horizontal versus 35 degrees in adults). This is believed to be a contributing factor in the development of acute otitis media in children due to impaired middle ear drainage and even reflux of nasopharyngeal contents into the middle ear through the Eustachian tube. Mucus cannot drain as readily with the smaller slope.
As a pathway connecting the middle ear to the pharynx, the Eustachian tube serves a dual purpose as an entrance and an exit to the middle ear. Gas is not the only substance that can be exchanged via this channel. Pharyngeal contents, most notably phlegm from the nasal passages and nasopharyngeal secretions, constitute liquid media which may be drawn or pushed inside the middle ear through the Eustachian tube and constitute a source of infectious organisms. Conversely, the Eustachian tube is also the principal outlet for middle ear effusions. In cases of obstruction, pressure may build up within the middle ear, leading to rupture of the tympanic membrane. This may occur in barotrauma or perforated otitis media.
Prostheses exist to facilitate Eustachian tube patency. Usually, these only are useful in the short-term, as mucus rapidly plugs them.
Eustachian tube dysfunction is not limited to obstruction. It also can involve intermittent opening or patulousness of the tube. In this case, patients may complain of autophony, which is characterized by hearing their chewing and speaking unusually loudly.
Some individuals can master active opening of their Eustachian tubes while others struggle. Difficulty in opening the Eustachian tube, and thus equalizing air pressure in the middle ear, may predispose the patient to ear pain with abrupt altitude changes. In general, air more readily escapes the middle ear than enters it under pressure. For this reason, those who fly in an airplane tend to experience more severe symptoms when going from low to high ambient pressure (landing) than from high to low ambient pressure (taking off). The Valsalva maneuver can help in opening the Eustachian tube by insufflating air inside of it. Divers must master this technique of opening their Eustachian tube to equalize while under water. Similarly to flying, divers must more frequently perform the Valsalva maneuver when descending into greater depth (low to high ambient pressure) than ascending to the surface (high to low ambient pressure).