CHAPTER 3

ANATOMY

The Eustachian tube (ET) is part of a system of contiguous organs, including the nose, pharynx, palate, middle ear, and mastoid gas cells (Figure 3–1). In reality, the ET is not just a tube but an organ consisting of a lumen with its mucosa, cartilage, surrounding soft tissue, paratubal muscles (tensor veli palatini, tensor tympani, levator veli palatini, and salpingopharyngeus), and bony support (sphenoid sulcus and medial pterygoid plate); the term middle-ear cleft is often used to describe the ET and middle ear and mastoid gas cells (Table 3–1). The larynx, another organ in the airway, has many similarities to the ET in that both have comparable

       •    Anatomy (such as a lumen, which is covered by mucosa, cartilage support, and a muscular opening mechanism)

       •    Physiologic functions (pressure regulation/ventilation, protection, and clearance)

       •    Pathophysiology (the lumen is too open or too closed, or there is failure of the opening/closing mechanism)

The anatomy of the ET, within its system, is intimately related to its physiology (see Chapter 4, “Physiology”). Therefore, measurements made from fixed specimens (e.g., cadaver dissections and temporal bone histopathologic sections) must be viewed with the physical properties of the tissues in mind, especially the compliance of the walls of the lumen of the ET.

TABLE 3–1.  Composition of the Eustachian Tube as an Organ

Eustachian Tube (ET)

The ET can be divided into three portions: cartilaginous, junctional, and osseous. The cartilaginous portion is proximal and opens into the nasopharynx, and the osseous portion is distal and opens into the anterior middle ear. The junctional portion is that part of the tube at which the cartilaginous and osseous portions connect, which was previously thought to be the narrowest part of the tubal lumen, the isthmus. The cartilage is firmly attached to the skull base by the lateral and medial suspensory ligaments, separated by the Ostmann fat pad.¹ A three-dimensional study of nine human temporal bone histopathologic specimens by Sudo and colleagues demonstrated the isthmus portion of the lumen to be near the distal end of the cartilaginous portion and not at the junction of the cartilaginous and osseous portions (see Prenatal Growth).² Respiratory mucosa lines the entire tubal lumen. Figure 3–2 shows the anatomy of the tube and its muscles. It is important to describe the growth and development of the tube to understand why infants and young children have more middle-ear infections than older children and adults.

Developmental Anatomy

The structure of the ET in the adult is the culmination of 18 years of development and growth; thus, its structure and function can best be appreciated in the context of these processes. Further, identification of abnormalities and their consequences depends on knowledge of normal anatomy.

Prenatal Growth

In the embryo, the ET lumen develops from the persistence of the first pharyngeal pouch (Figure 3–3A). The entodermal lining of the first pharyngeal pouch, which extends laterally, makes contact with the ectoderm of the bottom of the first gill furrow on either side of the gill plate. The distal pouch becomes elongated and expanded to form the tubotympanic recess, the primordium of the middle-ear cavity (Figure 3–3B). The proximal portion then becomes narrowed to form the tube (Figure 3–3C). The lumen at this stage has a smooth margin with an unciliated low columnar epithelium.³ The structures associated with this lumen develop from the surrounding mesenchyme in a predictable sequence. From our laboratories, Swarts and colleagues studied tubal development in 20 human fetuses between 7 and 38 weeks postconception.⁴ Their results confirm and extend those of Wolff and Tos.^5,6 Figure 3–4 illustrates the differentiation and development of the tubal structures. Before 10 weeks postconception, only the epithelial lining of the lumen has differentiated. Between 10 and 12 weeks postconception, the levator veli palatini and tensor veli palatini muscles develop and become delineated from the surrounding mesenchyme (see Figure 3–4A). The first evidence of the third muscle, the tensor tympani muscle, is apparent approximately 2 weeks later. At about the same time (14 weeks postconception), the initial differentiation of the cartilage begins (see Figure 3–4B). This is indicated by a more darkly staining region just medial to the tubal lumen. Also at this time, the lumen begins to show folding of the epithelium into the rugae characteristic of the adult tube. Concomitant with these changes, glandular tissue appears in the pharyngeal wall, medial to the cartilage and between it and the more lateral lumen. Recent new data (2015) showed the cartilage of the tube is of dual embryonic origin.⁷ By 20 weeks postconception, the initial center of chondrification has increased in size, and a perichondrium is clearly differentiated in the anteromedial portions of the tube (see Figure 3–4C). An anteromedial to posterolateral gradient of development is apparent in the differentiation of the tubal structures. By parturition, these processes yield an ET very similar to that observed in the adult. The cartilage is clearly delimited by a perichondrium throughout its length and shows the classic J-shaped form. The muscles are well circumscribed and have positions, relative to the cartilage, reminiscent of those of the adult (compare figures 3–4D and 3–2). Glandular tissue has proliferated and now occupies the regions between the cartilage and nasopharynx, between the cartilage and the lumen, and between the lumen and the tensor veli palatini muscle.

As ontogeny proceeds, morphometric changes occur among the ET’s structures and with respect to the rest of the head. The most pronounced change is the increase in tubal length from 1 mm at 10 weeks to 13 mm at birth. Most of this increase occurs in the cartilaginous portion of the tube. If the increase in tubal length is standardized to changes in body size, such as crown-rump length or nasion–sella length, allometric growth is apparent; that is, the cartilaginous portion of the ET increases in length faster, between 16 and 28 weeks postconception, than either of these two measures of body size. The osseous portion of the ET displays isometric growth with respect to these measures of body size until 28 weeks, when its growth outpaces theirs. Changes also occur in lumen height. At about 10 weeks postconception, the lumen is an anteriorly opening flask, with a very short neck. As development proceeds, the neck of the flask (the ET) elongates, but throughout gestation, the diameter (height) grows isometrically with respect to body size; thus, the cylindrical configuration of the cartilaginous lumen persists until birth and for an undetermined period of time afterward. Finally, the angle between the tensor veli palatini muscle and the cartilage becomes more acute throughout ontogeny. This change follows the same gradient established in the differentiation of the tubal structures.

Because the fetal cranial base is relatively flat, the tube deviates from the horizontal plane only about 10 degrees, a condition that persists into early childhood. The cranial base angle increases during postnatal development, as do the vertical dimensions of the skull. The hard palate drops away from the skull base. As this occurs, the angle between the cartilaginous tube and the skull base increases.

Postnatal Growth

There have been at least eight major anatomic differences identified in the tube in the infant compared with the older child and adult; these are summarized in Table 3–2.^8–25 These developmental changes—described in detail subsequently along with others—are most likely related to the prevalence of otitis media being highest in infants and young children.

In studies from the anthropology department at the University of Pittsburgh, Sadler-Kimes and colleagues used a three-dimensional computer graphic technique to analyze the size, shape, and positional association of the ET cartilage, lumen, and paratubal muscles (Figure 3–5).⁸ They compared temporal bone histopathologic specimens from children below 7 years of age with those 7 years and older. In infants, the ET is about half as long as that in the adult; it averages about 18 mm. The tube lengthens rapidly during early childhood, essentially reaching a size by 7 years of age that is approximately that of the adult.^25–27 Ishijima and colleagues confirmed these findings following assessments of temporal bones, in which they reported the average path length of the tubal lumen in the 3-month-old infant to be 21.1 mm compared with the average adult length of 37.00 mm.⁹ They also found that the ratio of the length of the cartilaginous portion (with the junctional portion) to the osseous portion was 8:1 in the infant but 4:1 in the adult. The difference was attributed to developmental growth in the bony portion. They also confirmed the developmental difference in the angle of the tube: in children, the cartilaginous and bony portions aligned with the line that connects the pharyngeal and middle ear, whereas in adults, the cartilaginous tube is angled inferiorly and laterally from the osseous portion, which most likely corresponds to the craniofacial growth and development with advancing age. This change in tubal angle may be related to more efficient muscle opening of the tube with age, and the shorter tube in the infant and young child is most likely related to impairment of protective function (reflux, aspiration, or insufflation of nasopharyngeal secretions into the middle ear).

TABLE 3–2.  Developmental Differences Between Anatomy of the ET–Middle Ear in Infants Compared With Adults

TVP = tensor veli palatini muscle.

The cartilaginous tube represents somewhat less than two-thirds of this distance, whereas the osseous portion is relatively longer and wider in diameter than it is in the adult. The cartilage, lumen, and levator veli palatini muscle all increase in cross-sectional area and volume after the age of 7 years compared with younger children. The cartilage and lumen of the tube change shape in older children, having more elongated structures. The distance from the levator veli palatini muscle to the other structures of the tube is significantly larger in older children. Also, the distances from the tensor veli palatini muscle to the lumen and to the levator palatini muscle is larger above age 6 years. Holborow demonstrated that in infants, the medial cartilaginous lamina is relatively shorter.²⁸ Cartilage mass also increases from birth to puberty (Figures 3–6 and 3–7).

The direction of the tube varies, from horizontal to an angle of about 10 degrees to the horizontal, and the tube is not angulated at the junctional portion but merely narrows; in the adult, the tube is approximately 45 degrees related to the horizontal plane.^29,30 This difference in angles between infants and adults has been thought to impair clearance of the ET–middle ear in the infant, but most likely the muscle vector of the tensor veli palatini is adversely affected in this age group (Figure 3–8). The angle that the tensor veli palatini muscle makes with the lumen is almost identical among children and adults; however, the angle between the tensor veli palatini muscle and the cartilage is different. In the child, the angle between the muscle and the cartilage is larger in the nasopharyngeal portion of the tube and decreases posteriorly toward the middle ear end of the tube, whereas in the adult, this angular relationship is stable throughout the length of the tube.¹¹ This angular difference between children and adults may be related to the known inefficient tubal function in children compared with adults and the increased incidence of otitis media in children. Some, or possibly all, of these developmental differences between the infant and the adult are most likely related to the relatively less efficient active tubal opening mechanism in the infant and young child,^31,32 which would make this age group susceptible to middle-ear disease.

From the laboratory of Professor Isamu Sando of the Department of Otolaryngology at the University of Pittsburgh, School of Medicine, aspects of ET anatomy have been studied and reported:

       1.  Yamaguchi and colleagues studied the cell density of the cartilage in the tube of human temporal bone histopathologic specimens from 26 weeks gestation to 85 years (24 were less than 3 years of age).¹⁷ They found that the density of the cartilage cells was statistically greater in specimens from children below the age of 7 years compared with specimens from older children and adults. This variation in cartilage cell density may be related to

             •    the observations that infants and young children have increased compliance of the ET (“floppy” tubes), which contributes to their inability to effectively open (dilate) the tubal lumen when the tensor veli palatini muscle contracts (functional obstruction) or

             •    the increased distensibility of the tube, which could promote insufflation of nasopharyngeal secretions into the middle ear during crying, closed-nose swallowing (the Toynbee phenomenon), and blowing the nose.^33,34

       2.  Matsune and colleagues assessed the amount of elastin in the hinge portion (the intermediate portion between the lateral and medial laminae) of the tube in temporal bone histopathologic specimens from infants and adults and found that the elastin was statistically less dense in the infant compared with the adult specimens.¹⁸ They postulated that the decreased amount of elastin in the cartilage of the child, compared with the adult, may be related to the hypothesis of functional obstruction (failure of the opening mechanism) in children. But, also, the relatively less dense elastin in the hinge portion in the infant could result in inadequate passive tubal closure and a more distensible lumen of the tube to nasopharyngeal positive pressures. Both of these possibilities would impair the protective function of the ET.

       3.  The postnatal development of the lumen of the tube was studied by Kitajiri and colleagues, who found that the area of the lumen increased almost fivefold from the newborn to age 20 years; the midcartilaginous portion increased most dramatically.¹³ The cross-sectional length of the lumen significantly increases during development, especially in the pharyngeal area of the tube. Suzuki and colleagues, after evaluations of temporal bone specimens, reported that the cross-sectional area width and height of the lumen in most of the cartilaginous portion of the tube was significantly smaller in children than in adults.¹⁴ They found that in children the lumen size was uniformly smaller throughout its length, compared with adults, in whom the cartilaginous lumen was larger at its pharyngeal end and then declined in lumen size distally. Figure 3–9 shows the remarkable growth and development of the tubal lumen, as well as the cartilage, glands, and muscle mass. The finding of a larger luminal area of the tube in the adult would promote a more effective pressure regulation of the tube compared with the infant. More recently, Ishijima and colleagues studied 11 temporal bone specimens from infants and adults and reported that the total volume of the tube was significantly larger in adults than in children, which was attributed to growth in the cartilaginous and junctional portions and not the osseous portion.³⁵

       4.  Aoki and colleagues measured the amount of the Ostmann’s fat pad from temporal bone specimens that ranged in age from neonates to adults and found an increase in volume with advancing age until adulthood, primarily in height, but little growth in width (Figure 3–10).¹⁹ Because the fat pad is positioned in the inferolateral aspect of the ET, it may prevent excessive dilation of the tubal lumen. On the other hand, the relatively greater mass of the fat pad in the infant could contribute to less effective opening of the lumen of the tube. Also, the differences of the amount and position of the fatty tissue between children and adults may be related to better protective function in the adult.²⁰ Moodi and colleagues (2010) used magnetic imaging to show changes in Ostmann fat pad with age, which decreased in size with advancing age.²¹

       5.  Sando and colleagues found that the inferior portion of the lumen of the ET contains numerous folds in the mucosa, which increase the surface area, whereas the superior portion of the tubal lumen has relatively no folds.³⁶ These folds progressively decrease until the age of 20 years (Figure 3–11).²³ The significance of this developmental change is uncertain at present but may be related to the growth of the tubal luminal area. From the same laboratory, Ozturk and colleagues (2011) described mucosal folds in the surface of the posterior wall of the tube, which they postulated as being microturbinates that might provide important protection and clearance functions and a role in the pathogenesis ET dysfunction.³⁷ The height of the pharyngeal orifice of the infant tube is about half of that of the adult, but the width is similar and is more exposed in the infant than it is in the adult because it lies lower in the shallower nasopharyngeal vault.

       6.  Orita and colleagues studied 32 temporal bones (age 1 day to 19 years) and reported that there was a greater proportion of mucous glands and insufficient serous glands to the age of 7 years compared with adults, which they postulated may be related to the pathogenesis of otitis media.²⁰

       7.  From studies in 12 temporal bones (3 months to 81 years of age), Suzuki and colleagues reported that the length of the tensor veli palatini muscle attachment and its ratio to the length of the tube, especially in the cartilaginous portion, increase with advancing age in infancy to adulthood but then decrease in later life.¹⁴

       8.  Orita and colleagues described differences in the connective tissue in the lateral portion of the tube between infants and older children and adults, in that there was more of it in the older specimens, which they postulated may be related to the poorer tubal function in youngsters.²⁴

Significance of Developmental Differences

The observed differences in the anatomy of the ET as an organ among the infant, young child, and adult cited earlier provide convincing evidence to explain some of the major functional differences also identified between these age groups; this, in turn, aids our understanding of the increased incidence of middle-ear disease in the pediatric population. Pressure regulation function is less efficient in the young, most likely owing to ineffective active opening of the tubal lumen by contraction of the tensor veli palatini muscle, which is probably due to either the difference in the muscle vector, the highly compliant tubal cartilage, or both. Inefficient pressure regulation function of the tube results in middle-ear underpressures—especially during periods of upper respiratory infections—which, if prolonged, progress to middle-ear effusion. Because infants and young children have a shorter and less stiff tube than the older child and adult, nasopharyngeal secretions can reflux or be insufflated more readily into the middle ear and result in middle-ear infection (see Chapter 4; Chapter 5, “Pathophysiology”; and Chapter 6, “Pathogenesis”).

Adult Anatomy

In the adult, the length of the ET has been reported to be as short as 30 mm³⁸ and as long as 40 mm,³⁹ but the usual range of length reported in the literature is 31 to 38 mm.^30,40–44 The study by Sudo and colleagues, in which they constructed a wire frame model from their temporal bone histologic specimens, found the average length of the cartilaginous, junctional, and osseous portions from the temporal bone specimens to be 23.6, 3.0, and 6.4 mm, respectively, for a total length of 33 mm.² It forms a 42 ± 9–degree angle with a parasagittal plane through the medial pterygoid plate. The tube in the adult begins in the nasopharynx and passes posteriorly and laterally through the petrous temporal bone. The tube does not take a straight course from the nasopharynx to the middle ear but rather a slowly curving inverted-S course. Speilberg found that in adults the tube makes two curves from the tympanic cavity, arching downward and forward across the space between the anterior canal wall and the bony external auditory meatus in the condyle of the mandible.³⁸ Before the pharyngeal orifice, it makes another slight curve downward and forward. Additional observations⁴ support Speilberg’s observations, although the variability is great.

The nasopharyngeal end of the tube lies about 20 mm above the plane of the hard palate.²⁹ The cartilage protrudes into the nasopharynx; this protrusion is known as the torus tubarius (see Figure 3–2). A thick layer of epithelium continuous with the soft tissue lining of the nasopharynx covers it. An observer viewing the nasopharynx endoscopically cannot see the torus tubarius but can see the mucosa overlying the cartilage. Any inferences drawn from observing motion in this area must consider this point of the anatomy.

Lumen and Mucosa

In the adult, the osseous and cartilaginous portions of the ET lumen resemble two truncated cones attached at the junctional area, 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 tube enters the petrous portion of the 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 ascent that the tube makes in its course from the nasopharynx to the middle ear.

LUMEN AND ISTHMUS   It is now known that the isthmus, the narrowest segment of the tube, is not at the junctional portion where the cartilaginous and osseous portions meet but is within the cartilaginous part. The three-dimensional measurements of the tube in the temporal bone specimens by Sudo and colleagues revealed the isthmus to be 21 mm from the pharyngeal orifice and 3 mm from the pharyngeal margin (the most anterior margin of bone surrounding the tubal lumen) of the junctional portion.² The measurements of the cross-sectional area of the lumen at the isthmus in all specimens were remarkably consistent in length, whereas there was variation in the other portions of the tube: pharyngeal orifice, 7.37 ± 5.65 mm³; isthmus, 0.65 ± 0.19 mm³; pharyngeal margin, 3 mm³; tympanic margin (i.e., the most posterior margin of the cartilaginous portion), 3.35 ± 2.04 mm³ of the junctional portion; tympanic orifice, 18.63 ± 5.48 mm³. The reduced caliber of the lumen at the isthmus is a critical component of the physiologic protective function of the ET—the flask effect in preventing nasopharyngeal secretions from entering the middle ear (see Chapter 4). However, any increase in the lumen at the isthmus would result in impairment of protection of the middle ear, owing to reflux, aspiration, or insufflation of nasopharyngeal secretions into the middle ear. Sadé and colleagues found no difference in the calibers of the isthmus in a comparison of children with otitis media and those without.⁴⁵ This finding lends support to the hypothesis that functional (failure of the opening mechanism) obstruction of the tube, as opposed to anatomic (mechanical) obstruction, is a cause of otitis media.⁴⁶

MUCOUS MEMBRANE   The lumen is lined with pseudostratified, columnar epithelium of the ciliated type, which sweeps material from the middle ear to the nasopharynx (Figures 3–12 and 3–13). The mucosa is continuous with the lining of the tympanic cavity at its distal end, as it is with the nasopharynx at its proximal end. Associated with these ciliated epithelial cells are goblet cells that comprise about 20% of the cell population.⁴⁷ Tos and Bak-Pedersen studied temporal bones from premature and newborn infants, children, and adults who were free of signs of otitis media and made 30,000 to 60,000 counts of goblet cells in different portions of the ET, for example, pharyngeal to tympanic ends and the lateral, medial, floor, and roof of the mucosa of the lumen (Figure 3–14).⁴⁸ They found very low densities in all parts of the tube in premature infants, increasing in the pharyngeal portions gradually through childhood and attaining a very high density in the adult. A similar density was reported between the lateral and medial walls, but it was lower in the roof and higher in the floor of the tube, which is consistent with the findings reported by Sando and colleagues (Figure 3–15).³⁶

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