eustachian tube parts

eustachian tube parts

Cartilage: The contribution of the cartilage to the efficient functioning of the eustachian tube depends on its structure, composition, and attachment to the cranial base and paratubal muscles. The cartilage of the eustachian tube is shaped like an inverted J in cross section . Extrusion of this form yields its gross structure . The cartilage has been described as being composed of a short lateral lamina and an elongated medial lamina . It is misleading to speak of two laminae: the cartilage is actually a dome-shaped structure, with arms of different lengths. The lateral arm has a constant height. The medial arm, however, starts as a short structure, which increases rapidly in height to 13 mm just posterior and lateral to the attachment of the cartilage to the medial pterygoid plate. This attachment is visible as a slight protuberance of the posterior edge of this plate. Posteriorly, the cartilage height decreases to 9 mm, a height it retains until it enters the petrous temporal bone. In the midsection of the cartilaginous tube, a lateroinferior extension of the medial arm interposes itself between the lumen and the levator veli palatini muscle in most of the specimens examined (Swarts and Rood et al,  2005). The cartilage persists into the petrous temporal bone as a dome-shaped structure itself between the lumen and the superiorly placed tensor tympani muscle. Thus the suggestion in the literature that the cartilaginous tube ends where the osseous tube begins is incorrect..

A factor influencing cartilage function is its attachment to the cranial base. Two viewpoints exist with respect to this characteristic: first, that the cartilage is tightly bound to the cranial base in the sphenoid sulcus (sulcus tubae auditivae) throughout its length. Posteriorly, this attachment to the cranial base is reduced to a small insertion superior and slightly lateral to Rosenmüller’s fossa  The connective tissue of this attachment arises from the medial pharyngeal surface of the cartilage’s medial arm before it coalesces into a ligamentous structure that inserts on the cranial base. This insertion is seen on the skull base as a bony ridge forming the medial margin of the sulcus tubarius. In this region the superior surface of the cartilage is about 2 mm from the cranial base . Just before its entrance into the petrous temporal bone the medial arm of the cartilage is again broadly anchored to the cranial base. The tubal cartilage is similar in composition and elasticity to those found in the pinna and the nose. Several authors have reported dispersed elastic fibers within the tubal cartilage. Reiner  found elastic fibers that ran longitudinally and that spiraled laterally in the dog. The highest density of these fibers was just deep to the perichondrium of the inferior surface of the cartilage’s hook in an “almost cap-like arrangement” Guild (1955) found much the same distribution in human eustachian tubes. The radial organization of elastic fibers around the dome of the cartilage suggests that motion of the lateral arm relative to the medial arm of the cartilage is possible.

Lumen The structure of the adult eustachian tube lumen resembles two truncated cones attached at their narrow ends, their broadest ends representing the nasopharyngeal and tympanic orifices. The nasal orifice is 8.5 mm in height. This dimension decreases steadily to a minimum, 3.5 mm, after the eustachian tube enters the petrous temporal bone. A 20- degree angle exists between the roof of the lumen and its floor. The sum of this angle with that formed by the cranial base and roof of the lumen accounts for the 45-degree descent the eustachian tube makes in its course from the middle ear to the nasopharynx. The narrowed intratemporal region is known as the isthmus of the eustachian tube. Because of its reduced caliber it is often implicated as the critical component in the development of otitis media. However, Sadé et al  found no difference in the calibers of the isthmus in a comparison of children with otitis media and those without. The eustachian tube is lined with pseudostratified, columnar epithelium of the ciliated type, which sweeps material from the middle ear to the nasopharynx. The mucosa is continuous with the lining of the tympanic cavity at its upper end, as it is with the nasopharynx at its lower end. Associated with these ciliated epithelial cells are goblet cells that comprise about 20% of the cell population. This density of goblet cells reflects a reduction from their abundance in children. In adults, the highest densities of these cells occur near the nasopharyngeal orifice. The submucosa of the anterior one half of the cartilaginous eustachian tube also contains numerous mucoserous glands (Bunne M, et al , 2000). As with the goblet cell populations, the proportion of mucusproducing cells in the submucosa decreases with increasing age. Thus Tomoda and his colleagues postulated that the viscosity of the secretions from these cells also decreases with age. The products of these glands include mucopolysaccharides, lysozyme, secretory immunoglobulins, and surface active compounds Clearly, the activity of the eustachian tube is affected by the concentrations and distributions of these compounds. (Franz B et al,2002)

Lateral membranous wall Closely associated with the lumen of the eustachian tube is the lateral membranous wall. This structure, although not clearly delineated, is invoked in many descriptions of eustachian tube function. It is most clearly defined in the middle portion of the cartilaginous eustachian tube. Its medial boundary is the submucosa of the lumen. Laterally, a robust connective tissue layer serves as the insertion of the tensor veli palatini muscle. This fibrous lateral membrane is anchored superiorly to inferior curvature of the lateral arm of the eustachian tube cartilage. The region between these two boundaries is occupied by glandular tissue anteriorly and adipose tissue posteriorly. There is little evidence of connective tissue bridges spanning the space between these two components. Thus, it would appear that forces developed by the tensor veli palatini muscle would be passed to the lateralarm of the cartilage rather than to the lateral submucosa of the lumen.