Anatomy and Physiology of the Eustachian Tube

Key Points

The eustachian tube is involved in pressure equalization, mucociliary clearance, and middle ear protection.
The principal dilator of the eustachian tube is the tensor veli palatini muscle.
Anatomic changes in the tube with growth and development include increased length and diameter, increasing tubal angle, change in cellular composition of tubal cartilage, and increased efficiency of tubal opening.

Since Eustachius first described the anatomy of his eponymous tube in 1563, understanding of its function in healthy and diseased ears has continued to evolve. An anatomic connection between the middle ear space and the nasopharynx was recognized, but the role played by the eustachian tube was unclear. Although this connection was initially thought to be an organ of respiration, later observers realized that it was vital to the health of the tympanic membrane and middle ear as a whole. Pioneering otologists, including Toynbee, Politzer, and Bezold, developed a paradigm that held that the eustachian tube regulates and modulates the pneumatic status of the middle ear and mastoid to maintain the appropriate milieu for optimal sound transmission by the tympanic membrane and ossicular chain.

Initial concepts regarding the physiology of the eustachian tube's function were found to be too simplistic. However, new information has led to a model of the eustachian tube as an “organ” because of its complex anatomy, function, and interdependent relationship with surrounding structures. A comparison of early otology texts with contemporary work reveals an evolution of the study of eustachian tube function, from limited therapeutics applied directly to the tube to an attempt to understand and modify intrinsic and extrinsic pathologic influences on its function.

Embryology and Postnatal Development

The eustachian tube is derived from the first pharyngeal pouch. The endoderm lining the tubotympanic sulcus invaginates between the first and second arch mesoderm to form the tubotympanic recess. The recess expands to form the tympanic cavity and the mastoid antrum. The osseous portion of the tube forms from the petrous and squamous parts of the temporal bone and the greater wing of the sphenoid bone. The cartilaginous tube has at least two discrete centers of chondrification. In patients with cleft palate, a deficiency in the development of the lateral lamina is often evident.

The muscles associated with the tube are the tensor veli palatini; dilator tubae, which is the medial bundle of the tensor veli palatini muscle ; tensor tympani; levator veli palatini; and salpingopharyngeus. These muscles appear early in development and form from the mesenchyme of the first (mandibular) arch together with the muscles of mastication. The tensor veli palatini, dilator tubae, and tensor tympani are innervated by the mandibular branch of the trigeminal nerve, whereas the levator veli palatini is innervated by the vagus nerve. The tensor veli palatini and dilator tubae change position during intrauterine development to form a more acute angle with the tube, particularly at the upper portion of the lateral lamina.

The eustachian tube is patent during embryologic development, which allows for free flow of amniotic fluid into the middle ear space. Early patency explains the occasional finding of keratin and lanugo hairs in the middle ear cleft in infants, and it may limit the ability to detect otoacoustic emissions during newborn hearing screening. It is conceivable that this route of entry of squamous epithelium may produce a congenital cholesteatoma, although the more likely mechanism seems to be epidermoid formation from the ectoderm of the first pharyngeal cleft.

The most rapid development of the petrous and squamous portions of the temporal bone occurs in the first 2 years of life, and eustachian tube development parallels this. Tubal length progresses from 17.5 mm in infants to reach an adult length of 37.5 mm. The length and area of the entire lumen increase more than twofold when comparing adults with young children, although the ratio of the cross-sectional diameter of the lumen to the tubal length remains constant.

The length of the osseous portion of the tube triples during development, whereas the cartilaginous tube increases 1.6-fold. The cartilaginous (nasopharyngeal) portion of the tube is three times as large in adults as in children, and the cross-sectional area, and presumably the efficiency of the dilatory muscles, increases with age.

More significantly, with an increase in cartilage size, important changes in cellular composition and shape occur with aging that result in a less “floppy” tube in adults compared with children. The cartilage cell density decreases in the midcartilaginous portion, and intercellular elastin, present in the hinge region between the medial and lateral laminae, is far more abundant in adults than in children. The medial lamina develops more than the lateral lamina, and when viewed in cross section, the lateral lamina and lateral wall of the tube develop more of an S shape; these changes allow for more efficient tubal recoil.

Other anatomic changes lead to an improvement in the efficiency of tubal function. During growth, there is a shift in the orientation of the cartilaginous portion relative to the osseous portion of the tube. When viewed from above, the long axis of the tube describes a greater curve with the increasing inferior and lateral movement of the cartilaginous tube, which augments middle ear protection. Additionally, the angle of insertion of the dilatory muscles (dilator tubae) becomes more perpendicular to the cartilaginous tube, and the tube becomes more dependent relative to the middle ear cleft.

The surface area of the tensor veli palatini, which is proportional to the force available to open the eustachian tube, is smaller in children, and so is the vector angle between the tensor veli palatini and membranocartilaginous portion of the eustachian tube. This is important for the tensor veli palatini's ability to efficiently displace the lateral wall of the eustachian tube in order to open the tubal lumen. All these elements may play a part in the decreased incidence of otitis media with age.

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