Eustachean tube is otherwise known as pharyngotympanic tube, middle ear ventilation tube. It is bony cartilagenous in nature. It connects the middle ear with the nasopharynx. In adults it lies at an angle of 45 degrees to the horizontal plane. In infants this inclination is about 10 degrees. In adults its length is 38mm. For descriptive purposes it can be divided into posterior 1/3 which is osseus in nature and anterior 2/3 which is cartilagenous in nature.
This eustachean tube is shorter straighter and wider in infants predisposing middle ear infections through this tube. The osseus portion of the eustachean tube also known as protympanum lies completely within the petrous portion of the temporal bone. The lumen of the osseus portion of the eustachean tube is triangular and is open always in contrast to the fibrocartilageous portion which is kept closed at rest, and opens during swallowing, or during a valsalva manuver. The osseous and cartilagenous portion of the eustachean tube meet at an irregular bony portion and form an angle of about 160 degrees with each other. The cartilagenous tube courses anteromedially and inferiorly, angled between 30 and 40 degrees. The cartilagenous portion of the tube is not completely surrounded by cartilage, but is deficient inferolaterally where it is covered by a membrane. The cartilage is crook shaped covering the medial, lateral and superior walls of the cartilagenous portion of the tube. The tubal lumen is shaped like two cones joined at their apices. The junction of the cones is the narrowest portion of the lumen and is known as the isthumus, and is usually situated at the junction of the cartilagenous and bony portion of the tube.
Figure showing various portions of Eustachean tube
The cartilagenous portion of the eustachean tube doesnot follow a straight course in the adult but extends along a curve from the junction of the osseous and cartilagenous portions to the medial pterygoid plate, approximating the skull base during most of its course. The eustachean tube crosses the superior border of the superior constrictor muscle to enter the nasopharynx. The medial cartilagenous portion of the tube presses against the pharyngeal wall to form a prominent fold, the torus tubaris. The torus is the site of origin of the salpingopalatine muscle and is the point of origin of the salpingopharyngeal muscle.
The mucosal lining of the eustachean tube is continuous with that of the nasopharynx and middle ear (ciliated columnar epithelium). Certain differences in the mucosal lining is evident, mucous glands predominate at the nasopharyngeal orifice, and this gradually changes into a mixture of goblet cells at the tympanum.
Muscles associated with eustachean tube: The muscles associated with the eustachean tube are 4 in number. They are tensor veli palatini, levator veli palatini, salpingopharyngeus, and tensor tympani.
Usually the eustachean tube is closed; it opens during such actions like swallowing, yawning thus equalising the middle ear pressure. Active dilatation of the tube is induced by the tensor veli palatini muscle. Closure of the tube has been attributed to passive reapproximation of tubal walls by extrinsic forces exerted by surrounding elastic fibres.
Blood supply: The eustachean tube is supplied by the ascending palatine artery, pharyngeal branch of internal maxillary artery, the artery of the pterygoid canal, ascending pharyngeal artery, and the middle meningeal artery. The venous drainage is via the pterygoid plexus.
Nerve supply: The pharyngeal orifice of the eustachean tube is supplied by a branch from the otic ganglion, the sphenopalatine nerve, and the pharyngeal plexus. The reminder of the tube receives its sensory supply from the tymapnic plexus and the pharyngeal plexus. The glossopharyngeal nerve has an important role in the innervation of the eustachean tube.
Functions of the eustachean tube:
Ventilation: It ventilates the middle ear cavity via the nasopharyngeal airway.
Figure showing ventilatory functions of middle ear
Protection: It protects the middle ear cavity from microbes of nasopharynx.
Figure showing protective function of eustachean tube
Drainage: Drains the secretions from the middle ear cavity into the nasopharynx.
Diagram showing drainage function of eustachean tube
Features of Infant eustachean tube: In infants the eustachean tube is about half as long as in the adults, averaging about 18 mm. The osseous portion is longer than the cartilagenous portion. It is shorter, straighter, and wider than that of adults. The tensor veli palatini muscle is less efficient in infants. The tube is also mostly horizontal in infants. Hence infants are more prone for middle ear infections arising from the eustachean tube.