Otology Anatomy and Embryology with Radiology Correlates

1

Which structures comprise the outer ear?

The external ear is composed of the auricle and the external ear canal, terminating at the tympanic membrane ( Fig. 32.1 ). The lateral one-third of the canal is cartilaginous and has hair follicles, along with ceruminous and sebaceous glands. The medial two-thirds of the canal is osseous and free of hair and adnexal structures. The length of the external canal, approximately 2.5 centimeters in adults, gives it a resonance frequency of 3 to 4 kHz.

2

What are the hillocks of His? What structure do they ultimately form?

The hillocks are six small buds of mesenchyme surrounding the dorsal end of the first branchial cleft. Hillocks 1, 2, and 3 arise from the mandibular (or first) branchial arch, while 4, 5, and 6 develop from the hyoid (or second) arch. These mesenchymal structures ultimately rearrange to form the auricle. Although the exact embryology is controversial, it is classically taught that the first hillock forms the tragus, the second and third form the helix, the fourth and fifth develop into the antihelix, and the antitragus is formed from the sixth ( Fig. 32.2 ).

3

What is the function of the auricle? How does its unique structure contribute to auditory function?

The cone-shaped auricle collects and directs sound down the ear canal toward the tympanic membrane. The shape of the auricle also creates small, unique, high-pitched frequency resonances that contribute to the ability to localize sound in the vertical space.

4

From which branchial structure does the external auditory canal develop?

The external auditory canal develops from the first (or mandibular) branchial groove.

5

What are preauricular pits and tags? What is their clinical significance?

Preauricular pits and tags are benign preauricular soft tissue malformations. Pits are depressions in the skin located anterior to the ear canal. Epithelial mounds or pedunculated skin are known as preauricular tags. Structural abnormalities, including preauricular pits and tags, malformed pinnae, and stenotic or atretic ear canals, may indicate hearing loss and can be associated with congenital syndromes. Presence of these findings suggests the need for a thorough clinical examination for other congenital anomalies, audiometric evaluation, and possibly genetic testing.

6

Which congenital syndromes are associated with external ear abnormalities?

• Treacher-Collins syndrome (mandibulofacial dysostosis): rare autosomal dominant condition with complete penetrance and variable expression consisting of downward-slanting palpebral fissures, auricular malformations with or without tags and preauricular blind fistulas, stenosis or atresia of the external ear canals, ossicular abnormalities, malar hypoplasia, flat nasal bridge, mandibular hypoplasia, cleft palate, and dental abnormalities.

• Goldenhar syndrome (oculo-auriculo-vertebral syndrome): rare disorder of variable inheritance patterns characterized by anomalous development of the first and second branchial arches, which often results in unilateral craniofacial malformations, including hemifacial microsomia, eye anomalies, strabismus, anotia, preauricular skin tags, and stenotic or atretic ear canals. It is also associated with severe scoliosis.

• Branchio-oto-renal syndrome: rare autosomal dominant disorder characterized by hypoplastic or absent kidneys, preauricular pits or tags, middle ear malformation or absence, and branchial cleft cysts or fistulae.

• CHARGE syndrome: rare syndrome with a cluster of associated malformations, including coloboma of the eye, heart defects, atresia of the choanae, retardation of growth or development, genital defects (hypogonadism), and ear anomalies (asymmetric pinnae with low-set, lop ears).

• DiGeorge sequence: 22q11 chromosomal deletion resulting in absence or hypoplasia of the thymus and/or parathyroid glands with cardiovascular and craniofacial anomalies, including low-set ears, micrognathia, hypertelorism, short philtrum, cleft palate, and choanal atresia.

• Crouzon syndrome: rare autosomal dominant syndrome characterized by premature skull bone fusion (craniosynostosis). Other physical features include exophthalmos, hypotelorism, strabismus, beak-shaped nose, hypoplastic maxilla, low-set ears, and ear canal stenosis or atresia.

7

Describe the middle ear. What structures can be found within the middle ear?

The middle ear is a 1 to 2 cm ³ air-filled cavity that houses the ossicles, the stapedius and tensor tympani muscles, and the chorda tympani nerve (containing taste fibers from the anterior two-thirds of the tongue and parasympathetic fibers to the submandibular and sublingual glands). The middle ear is bounded laterally by the tympanic membrane and medially by the lateral wall of the inner ear (otic capsule). It is continuous with the mastoid air cells via the antrum and nasopharynx via the eustachian tube (see Fig. 32.1 ).

8

What are the ossicles? What is their embryologic origin? What is their function?

The ossicular chain is composed of the malleus, incus, and stapes. The malleus attaches laterally to the tympanic membrane, the stapes couples medially to the inner ear via the oval window, and the incus bridges these two bones. The first branchial arch gives rise to the head and neck of the malleus and the body of the incus. The second branchial arch gives rise to the long process of the malleus, the long process of the incus, and the stapes suprastructure. The stapes footplate derives from both the second branchial arch and the otic capsule. The ossicles transform acoustic energy to overcome the impedance mismatch between the aerated external ear canal and the fluid-filled cochlea.

9

What is unique about the embryologic derivatives of the tympanic membrane?

The tympanic membrane consists of three layers, each of which is derived from a different germ layer. The outer epithelial layer derives from ectoderm, the middle fibrous layer derives from mesoderm, and the inner epithelial layer derives from endoderm. Neural crest–derived mesenchyme around the lateral margin of the membrane forms the tympanic annulus, which begins to ossify in the third month of gestation.

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