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Chapter Postnatal Growth of the Temporal Bone
ОглавлениеSome parts of the temporal bone are fully formed at birth and undergo no postnatal change, while other parts undergo changes in size and shape that accompany postnatal growth. This forms the basis for the clinical understanding that since, for example, the cochlea, vestibule, and other components of the inner ear and auditory ossicles are fully formed at birth, the length and thickness of an electrode for a cochlear implant will be the same for an infant as for an adult, or that bone malformations of the inner ear and auditory ossicles undergo no postnatal changes. On the other hand, the external auditory canal, mastoid air cells, internal auditory canal, vestibular aqueduct, and other components exhibit changes in shape that accompany postnatal growth. Also, because postnatal development of the mastoid air cells is impaired by recurrent otitis media in infancy and childhood, their form may vary widely due not only to age, but also to diseases of the middle ear. A proper understanding of the temporal bone’s normal structure with respect to age is important for accurate temporal bone imaging diagnosis and appropriate surgical intervention. In this chapter, we will examine CT images of each section of the temporal bone in children at different ages and explain how the various structures develop and change with age.
In this chapter, images taken from children at four months, one year, three years one month, and sixteen years ten months are used as representative examples, however these images are taken from different individuals and are not the product of an ongoing examination of a single child over time. Although the subjects all received CT exams for hearing loss or other reasons, samples were chosen from those who displayed no clear abnormalities in their CT images.
At birth, the bony framework of the external auditory canal consists of the annular tympanic bone alone, with lateral structures composed almost entirely of cartilage. As shown in the image taken at four months after birth, only the superior half of the bony external auditory canal exists, namely the floor (1b: ) of the tegmental air cells () and the medial portion of the posterior wall of the mandibular fossa (1a: ); the inferior half is still almost completely unformed (1b: ). In both the axial and the coronal sections, the bony external auditory canal is not so much a bony canal as a trumpet-shaped hollow in the base of the skull that spreads out laterally from the tympanic cavity (1a: , 1b: ). On the other hand, the lumen of the external auditory canal is cylindrical, with a thick layer of soft tissue between the auditory canal’s skin and bone surfaces. The diameter of the external auditory canal’s lumen in an infant of this age is approximately half that of an older child, and in clinical settings the external auditory canals of newborns and infants are so narrow that it is frequently difficult to observe the entire tympanic membrane, even with a magnifying otoscope.
Examination of the image taken at one year shows that the external auditory canal is both larger in diameter and longer than that of the 4-month-old infant. As can be seen in the axial section image, both the anterior wall (2a: ) and posterior wall (2a: ) of the bony external auditory canal have grown laterally. In the coronal section image, the lateral margin of the external auditory canal’s superior wall (2b: ) has moved laterally and slightly inferiorly, with the inferior portion of the bony wall growing laterally (2b: ), causing the external auditory canal to form a cylindrical shape overall. However, the external auditory canal is still trumpet-shaped (2a: ). Infants in this age range frequently require cochlear implantations, but direct observation of the round window niche while performing mastoidectomy and posterior tympanotomy is often impaired by the lateral portion of the bony external auditory canal wall. In order to obtain satisfactory operating field of vision of the round window niche and to further perform cochlear fenestration procedures, it is necessary to either remove a portion of the bony part of the external auditory canal or thin out the bony wall and temporarily fracture it to permit viewing from a more anterior position.
Axial Section Images
1a: 4 months old
2a: 1 year old
3a: 3 years, 1 month old
4a: 16 years, 10 months old
Coronal Section Images
1b: 4 months old
2b: 1 year old
3b: 3 years, 1 month old
4b: 16 years, 10 months old
In the image taken at three years old, we observe that the bony external auditory canal has expanded and elongated. Accompanying this growth, the lateral margin of the external auditory canal’s posterior wall has moved anteriorly (3a: ) and the lateral edge of the superior wall inferiorly (3b: ), with the entire structure gradually forming into a tunnel shape of constant diameter. The bone of the anterior and inferior walls has thickened and extended laterally (3a: , 3b: ).
The images from a child at 16 years old are essentially the same as for an adult. Little change is expected to occur in the size of the temporal bone or the skull overall, limited to a slight thickening of cortical bone. The lateral margin of the external auditory canal has further extended laterally and has formed a complete tunnel shape (4a: ). The superior wall of the entrance to the external auditory canal is lower (4b: ) and the inferior wall has further thickened and grown laterally (4b: ). (Scale shown in images indicates 1 cm)
Mastoid Air Cell Development—Axial Section
The temporal bone is small at birth. The inner ear and auditory ossicles remain essentially unchanged, but the surrounding mastoid air cells expand with age, the external auditory canal grows, the internal auditory canal lengthens, and the petrous apex becomes extended. In newborns, the mastoid process is almost completely unformed and the temporal bone is positioned lower in the skull than in an adult, with its surface facing slightly inferior. After birth, the bony external auditory canal lengthens and the mastoid process becomes extended. The remarkable difference in temporal bone formation between an infant and an older child is mainly due to the different degrees of development of the mastoid air cells. Development of the mastoid air cells causes the lateral surface of the temporal bone to approach perpendicular, while the surface of the tympanic membrane gradually rises from its initial pronounced, downward-facing position [1].
Here, in order to examine the development of the temporal bone overall, and in particular the changes to the mastoid air cells, we present axial section images showing cross sections taken at the level of the inferior margin of the external auditory canal (1a-4a) and at the level of the malleus head and incus body in the middle area of the epitympanum (attic) (1b-4b).
First, we examine the development of the temporal bone in the axial section at the level of the inferior margin of the external auditory canal (1a-4a). In the image taken at four months, the temporal bone is not visible except for the inferior margin of the external auditory canal and the stylomastoid foramen (1a: ) in the vicinity of the tympanic annulus. In other words, very little development of the mastoid process, hypotympanum, and other components of the inferior temporal bone has taken place between birth and this age. The facial nerve also exits the temporal bone at this level. At one year, the air cells of the hypotympanum become visible, and a slight amount of bone formation can be observed at the medial margin of the external auditory canal and the mastoid process. The facial nerve is clearly contained in a bony canal (mastoid segment of facial nerve, 2a: ). At three years, development of the mastoid air cells is remarkable and extends to the sigmoid sinus posteriorly, and to the carotid canal anteriorly. However, the area around the facial canal (3a: ) has not yet undergone pneumatization. At sixteen years old, air cell development is nearly complete, with pneumatization extending to the petrous apex, hypotympanum, mastoid tip, and to the lateral region of the sigmoid sinus and posterior. The area surrounding the mastoid segment of the facial nerve is also pneumatized (4a: ).
Axial Section Images
1a: 4 months old
2a: 1 year old
3a: 3 years, 1 month old
4a: 16 years, 10 months old
Axial Section Images
1b: 4 months old
2b: 1 year old
3b: 3 years, 1 month old
4b: 16 years, 10 months old
Next we examine the image taken at the level of the attic (1b-4b). In the image, “a” marks the mastoid antrum and “s” marks the sigmoid sinus. At zero years, pneumatic cavities are limited to the attic (1b: ) and its lateral area, and to the mastoid antrum. The area between the sigmoid sinus and the mastoid antrum is still bone marrow, with no air cells. At one year, the formation of air cells is observed anterior to the attic in the supratubal recess (2b: ), with pneumatization of the area lateral to the attic and in the periphery of the mastoid antrum progressing nearly to the sigmoid sinus. Observation of the image at three years (3b) clearly indicates that, along with overall air cell development, the temporal bone itself has become larger. The air cells extend from the mastoid antrum, reaching the sigmoid sinus. In the image at sixteen years (4b), the temporal bone has grown further and pneumatization extends to its borders. Posteriorly, the mastoid air cells extend lateral to the sigmoid sinus.
Mastoid Air Cell Development—Coronal Section
The coronal section images here show cross sections anteriorly in the malleoincudal joint region (1a-4a) and posteriorly in the central part of the mastoid antrum (1b-4b). First, we will examine the anterior images at the level of the malleoincudal joint region. The arrow mark () in the images indicates the lateral wall of the attic.
At zero years, pneumatic cavities are visible not only in the attic, but also lateral to it. At this age, the inferior mastoid region (1a, previous page) showed no air cell development, but at the level of the attic, air cell development is present lateral to the lateral wall, indicating that air cell development in the temporal bone begins in the superior region of the temporal bone.
At one year, slight air cell development is visible immediately superior to the attic, while the temporal bone displays expansion and pneumatization in the area lateral to the attic’s lateral wall (tegmental air cells). As seen on the previous page, development of the mastoid part begins with growth of the bone itself, followed by growth and expansion of air cells within, whereas at the attic level superior to the external auditory canal air cell growth can be observed from early infancy, with pneumatization and bone growth and expansion progressing simultaneously as the child grows older.
This process continues from three to around sixteen years old, with the temporal bone expanding laterally and thickening vertically. The tegmen portion of the attic has almost no air cells during infancy, with only a cavity continuing from the mastoid antrum, but later air cells develop in the tegmen portion, which is basically a cavity, and this area develops and enlarges. On the other hand, as in the case of adhesive otitis media and cholesteatoma samples shown in Chapter 4, “Inflammatory diseases of the middle ear,” cases of recurrent otitis media are marked by impaired development of air cells lateral to the attic and a low-hanging middle cranial fossa floor. In such middle ear surgeries of the temporal bone, the low-hanging cranial base makes it difficult to secure sufficient field of view when approached laterally, necessitating an approach from inferior, which results in less working space. Therefore, it is important to examine air cell development of the tegmental air cells when determining whether middle ear surgery can be performed safely and securely. In observing the overall age-related lateral development of the temporal bone with respect to the medial and lateral portions of the attic’s lateral wall, it is apparent that, whereas the width of the medial attic hardly changes (in other words, it is almost fully formed in infancy), the lateral portion continues to grow and widen.
Coronal Section Images
1a: 4 months old
2a: 1 year old
3a: 3 years, 1 month old
4a: 16 years, 10 months old
Coronal Section Images
1b: 4 months old
2b: 1 year old
3b: 3 years, 1 month old
4b: 16 years, 10 months old
Next, we examine the cross section at the level of the mastoid antrum and the stylomastoid foramen (1b-4b). In the image, “a” indicates the mastoid antrum and “” indicates the stylomastoid foramen. At zero years, the mastoid (1b) contains no pneumatic cavities other than the antrum, the rest being composed entirely of bone marrow. Later, pneumatization occurs centered on the mastoid antrum and spreading toward the periphery, until at 16 years, air cells have developed medially beyond the labyrinth. Note that the stylomastoid foramen, where the mastoid segment of the facial nerve exits (1b-4b: ), has descended relative to the labyrinth with age. Also, with regard to the medial-lateral orientation, at zero years the stylomastoid foramen is almost exposed at the inferior margin of the temporal bones lateral surface, whereas in older children it is located medial to the mastoid process, deeply medial to the temporal bones lateral surface. When performing surgery on the infra-auricular region or inferior mastoid part in younger children, particularly infants, particular caution must be exercised in order to avoid damaging the facial nerve trunk. (Scale shown in images indicates 1 cm)
References
1 Gulya AJ: Developmental anatomy of the ear. Glasscock ME III and Shambaugh GE Jr. eds Surgery of the ear. Saunders. Philadelphia, 1990:5-33.
The size of the inner ear does not change after birth, but the internal auditory canal continues to grow from birth to around puberty. Here we present a summary of normal development; for further details including examples of anomalies, please refer to Chapter 3, “Congenital anomalies: IAC stenosis.” There are many reports showing measurements of the length and diameter of the internal auditory canal based on high-resolution CT findings, including cases in which hearing loss is both present and absent. According to the report by McClay [1], there is no significant difference in horizontal and vertical diameters of the internal auditory canal between groups with and without senso-rineural hearing loss. However, in cases with inner ear malformation, the incidence of sensorineural hearing loss was higher when the diameter of the internal auditory canal was 2 mm or less.
While there is no universal standard established for abnormal enlargement of the internal auditory canal, the above-mentioned report by McClay et al [1] defines enlargement as a horizontal or vertical diameter of 8 mm or greater. However, enlargement of the internal auditory canal is generally considered to be unrelated to hearing loss.
Concerning age-related changes to the length of the internal auditory canal, Lang [2] states that it grows from around 5-7 mm at birth to 10-15 mm in adulthood. Measurements using 3-dimensional computer reconstructions based on histopathological specimens of the temporal bone [3] also show that, while the internal auditory canal’s diameter increases very little with age, it does lengthen. In measurements taken according to McClays method in temporal bone CT images shown here (1a,b-4a,b), the length of the internal auditory canal was 5.8 mm at four months, 7.7 mm at one year, 10.6 mm at three years, and 12.9 mm at sixteen years, indicating that it lengthens with age. The diameter of the internal auditory canal varies considerably between individuals, but does not show the same clear trend toward enlargement as for length. Consequently, the growth pattern for the internal auditory canal is best understood as a steady lengthening with little change in diameter.
The development of the internal auditory canal includes changes in direction as well as length. In the coronal section images, we see that the medial end of the internal auditory canal is elevated and its path slightly angled in infants, with an approach toward horizontal accompanying growth (1b-4b). In the axial section images, the medial end appears to be facing slightly posteriorly in early years, whereas in older children it faces almost directly medially. These spacial changes to the direction of the internal auditory canal may reflect relative growth differences between the temporal bone and the cerebellum, brain stem, and 7th and 8th cranial nerves in the posterior cranial fossa.
(Scale shown in images indicates 1 cm)
Axial Section Images
1a: 4 months old
2a: 1 year old
3a: 3 years, 1 month old
4a: 16 years, 10 months old
Coronal Section Images
1b: 4 months old
2b: 1 year old
3b: 3 years, 1 month old
4b: 16 years, 10 months old
References
1 McClay JE, Tandy R, Grundfast K, et al: Major and minor temporal bone abnormalities in children with and without congenital sensorineural hearing loss. Arch Otolaryngol Head Neck Surg 2002;128:664–671.
2 Lang J: Neuroanatomie der Nn. Opticus, Trigeminus, Facialis, Glossopharyngeus, Vagus, Accessorius und Hypoglossus. Arch Otorhinolaryngol 1981;231:1–69.
3 Sakashita T and Sando I: Postnatal development of the internal auditory canal studied by computer-aided three-dimensional reconstruction and measurement. Ann Otol Rhinol Laryngol 1995;104:469–475.
The vestibular aqueduct and endolymphatic sac play an important role in the endolymphatic dynamics of the inner ear. There are reports that patients with Ménière’s disease have shorter, narrower vestibular aqueducts [1] and shorter distances between the posterior semicircular canal and the posterior cranial fossa surface of the temporal bone [2, 3 ] than in non-Méniér’s disease cases, making this an important point to focus on in CT images of the temporal bone for patients with vertigo. However, Ménière’s disease is an illness found mainly in adults, so attention has rarely been focused on the vestibular aqueduct in temporal bone images of children. But it has become clear that enlarged vestibular aqueduct syndrome is frequently a cause in childhood hearing loss, so observation of the vestibular aqueduct is now considered to be of major diagnostic significance in children as well.
As is generally known, the inner ear is essentially completely formed at birth and does not undergo any postnatal development. The vestibular aqueduct is also part of the inner ear but, unlike other inner ear components, it continues to grow and develop postnatally. In this respect, the vestibular aqueduct is more similar to the internal auditory canal than the inner ear. According to Fujita and Sando’s measurements of histopathological specimens of the temporal bone, the length of the vestibular aqueduct from the vestibule to the posterior cranial fossa is approx. 5 mm in infants, gradually lengthening to approx. 6 mm at three years and approx. 8 mm at seventeen years [4]. Accompanying this growth process, the exit to the posterior cranial fossa moves laterally and inferiorly, until in adults it is located posterior to the posterior semicircular canal and inferior to the level of the lateral semicircular canal.
In the temporal bone CT images shown here (vestibular aqueduct indicated by arrow, posterior semicircular canal indicated by “p”), at zero years the vestibular aqueduct is at the same level as the lateral semicircular canal, with the opening considerably medial to the posterior semicircular canal (1b: ). Also, there is no temporal bone posterior to the posterior semicircular canal, which appears to be protruding into the posterior cranial fossa. In the finding for the one-year-old child, the vestibular aqueduct extends slightly inferior to the lateral semicircular canal, and the posterior semicircular canal is contained inside the temporal bone without protruding posterior to the posterior cranial fossa surface of the temporal bone. At three years, the vestibular aqueduct has further lengthened and the opening to the posterior cranial fossa (3a: ) is nearly posterior to the posterior semicircular canal. Also, the posterior cranial fossa surface of the temporal bone has developed further posteriorly, with the posterior semicircular canal positioned relatively more anterior. In the finding for the older, sixteen-year-old child, representation of the vestibular aqueduct itself is clearer than in the infants, and this may be thought to correspond to the vestibular aqueduct’s enlargement. The pathways within the temporal bone are also longer than in younger children and the vestibular aqueduct’s posterior cranial fossa outlet (4a: ), where the endolymphatic sac is located, shows a slight depression.
Axial Section Images
1a: 4 months old
2a: 1 year old
3a: 3 years, 1 month old
4a: 16 years, 10 months old
Axial Section Images
1b: 4 months old
2b: 1 year old
2c: 1 year old
3b: 3 years, 1 month old
3c: 3 years, 1 month old
4b: 16 years, 10 months old
4c: 16 years, 10 months old
Ménière’s disease is not necessarily limited to a single cause and is considered to be a product of compound factors. Although, as previously stated, Ménière’s disease patients have hypoplastic vestibular aqueducts and suppressed development of the temporal bone posterior to the posterior semicircular canal, mastoid air cell development in other areas is basically satisfactory and differs from the uniform inhibition of air cell development within the temporal bone seen in chronic otitis media. I have experienced cases of adult patients with typical bilateral Ménière’s disease in whom CT images indicate that their posterior semicircular canals protrude into the posterior cranial fossa, from which one may postulate that Ménière’s disease may be attributable to some form of congenital or genetic factor inhibiting temporal bone growth posterior to the posterior semicircular canal. (Scale shown in images indicates 1 cm)
References
1 Krombach GA, van den Boom M, Di Martino E, et al: Computed to-mography of the inner ear: size of anatomical structures in the normal temporal bone and in the temporal bone of patients with Menière’s disease. Eur Radiol 2005;15:1505–1513.
2 Yazawa Y, Kitahara M: Computed tomographic findings around the vestibular aqueduct in Menière’s disease. Acta Otolaryngol Suppl 1991;481:88–90.
3 Nidecker A, Pfaltz CR, Matefi L, Benz UF: Computed tomographic findings in Meniere’s disease. ORL J Otorhinolaryngol Relat Spec 1985;47:66–75.
4 Fujita S, Sando I: Three-dimensional course of the vestibular aqueduct. Eur Arch Otorhinolaryngol 1996;253:122–125.
