External ear

The ear is located in the temporal bone of the skull and is composed of the inner, middle, and external ear (Figure 12-1). The structures of the ear evolve in the mesoderm, and development of the external ear begins during the sixth week of gestation when the six hillocks of His develop from the first and second branchial arches. The individual portions of the auricle, or flap of the ear, begin to fuse and assume the classic adult shape by the twelfth week of gestation, and fusion is complete by the twentieth week (Figure 12-2). The normal auricle should be no greater than 10 degrees off vertical plane or slope, and the superior portion should be in line with the outer canthus of the eye (Figure 12-3). Minor malformations of the auricle may be normal variants such as preauricular skin tags, a preauricular sinus or Darwin tubercle, a slight thickening or nodule at the upper portion of the helix (Figure 12-4). Malformations can also be the result of intrauterine position or may be related to genetic syndromes or other alterations in development occurring during the same gestational period.

Inner ear

Although the external ear formation coincides with the gestational formation of the internal ear structures, they develop separately. The auditory placode and the acousticofacial ganglion are present the fourth week of gestation. Over the next month, the first of the three turns in the cochlea develops. Arrest in development during this phase results in a common bony abnormality of the inner ear associated with congenital sensorineural hearing loss known as Mondini deformity. The final 2.5 turns of the cochlea occur by the twelfth week of gestation. The organ of Corti develops from the epithelium of the cochlea and is responsible for transmission of sound impulses to the eighth cranial nerve, the acoustic nerve. Improper development of the membranous labyrinth of the organ of Corti results in a Scheibe deformity, the most common congenital abnormality of the cochlear duct, resulting in sensorineural hearing loss (see later section, Hearing Loss). The semicircular canals first appear in the sixth week of gestation with differentiation of canal structures being complete by the sixteenth week. The sensory cells needed for equilibrium actually attain adult size by the twenty-third week of gestation. Alterations in development during this period due to chromosomal abnormalities or other causes can lead to hearing loss of particular tones.

Middle ear

Simultaneous with the early development of the inner ear is the formation of the first pharyngeal pouch in the oropharynx. The proximal portion of the pharyngeal pouch develops into the eustachian tube, and the distal portion becomes the tympanic cavity and supporting structures. The eustachian tube is 17 to 18 mm long at birth and lies 10 degrees off the horizontal plane. The development of the tympanic cavity with ossicles is not complete until the eighth month of gestation, but the malleus and incus reach adult size and shape by the eighteenth week of gestation and the stapes by the twentieth week of gestation.¹ The manubrium, malleus, and stapes and their supporting ligaments are formed by the Meckel and Reichert cartilage. Failure of ligaments to form properly results in a conductive hearing loss² (see later section, Hearing Loss). A conductive hearing loss is also seen in children with osteogenesis imperfecta, which causes otosclerosis, an abnormal destruction and redeposition of the bony structures of the labyrinth.

External ear (pinna)

The external ear is divided into sections: the outer portion is called the helix, just medial and parallel to the helix is the antihelix, and the concha is the cavity leading to the opening of the external canal (Figure 12-5). A firm protuberance on the anterior portion of the ear just at the entrance to the auditory canal is the tragus, and across from the tragus on the border of the antihelix is the antitragus. Beneath the tragus is the soft fold of skin that forms the ear lobe. Although the shape of the auricle varies slightly from person to person, the ears should be comparable bilaterally in size, shape, and position and not significantly varied bilaterally.

The external auditory canal, measuring approximately 2.5 cm in length, connects the outer ear to the middle ear and funnels sound waves to the tympanic membrane. The auricular muscles are innervated by the seventh cranial nerve, or facial nerve. The medial portion of the canal is innervated by the fifth cranial nerve, or trigeminal nerve, and the posterior canal by the tenth cranial nerve, or vagus nerve. The exterior third of the ear canal contains hair follicles, ceruminous glands, and sebaceous glands. The ceruminous gland, a modified apocrine sweat gland, secretes a milky substance that forms cerumen when exposed to the secretions of the sebaceous glands and air. Cerumen lubricates the skin, acts as a barrier to foreign objects entering the interior canal, and has protective antibacterial properties to reduce the incidence of skin infection in the external canal. Natural lateral movement of skin in the external canal facilitates drainage of cerumen and other debris from the external ear canal.

Middle ear

The tympanic membrane is a thin layer of oval-shaped skin attached to the wall of the external canal and is approximately 9 to 10 mm in diameter (Figure 12-6). It is surrounded by a fibrous band called the annulus. The medial surface of the tympanic membrane is attached to the manubrium and lateral process of the malleus. The tympanic membrane has a resonance frequency of 800 to 1600 Hz, approximating the normal speech frequency of 500 to 2000 Hz found in humans. The tympanic membrane is divided into sections: (1) the pars flaccida is superior to the lateral process of the malleus, (2) the pars tensa comprises the majority of the tympanic membrane inferior to the lateral process of the malleus, and (3) Prussak’s space, which lies medial to the pars flaccida in the anterior superior quadrant of the tympanic membrane (see Figure 12-6). Prussak’s space is the most common location of retraction pockets and congenital or acquired cholesteatoma (Figure 12-7), an asymptomatic white mass in the middle ear that is thought to arise from the continued growth of the epidermoid layer over the tympanic membrane.¹ Tympanosclerosis, thickening and scarring of the tympanic membrane, is commonly seen after chronic infections of the middle ear (Figure 12-8).

The three ossicles of the inner ear, the malleus, incus, and stapes, the smallest bones in the body, transmit the movement of the tympanic membrane to the oval window and subsequently to the vestibular and cochlear branches of the eighth cranial nerve, the acoustic nerve (Figure 12-9). The head of the malleus articulates with the body of the incus at the incudomalleolar joint. The long crus or leglike structure of the incus, articulates with the head of the stapes at the incudostapedial joint. These joint areas are the most vascular regions of the ossicles, and therefore are the most susceptible to trauma or infection. The footplate of the stapes sits upon the oval window of the inner ear at the fibrous stapediovestibular joint. Because of the mechanical function of the three ossicles and the transmission of sound waves from the larger surface area of the tympanic membrane to the smaller surface area of the oval window, there is a net increase of 22 times the sound energy radiating from the tympanic membrane to the oval window.

The eustachian tube opens into the oropharynx just behind the nasal cavity and is the drainage and ventilatory structure for the middle ear. The eustachian tube connects the middle ear with the back of the throat. The middle ear is an air-filled space and when patent has the same air pressure as the outside air pressure. Normally, swallowing opens the eustachian tube and restores and equalizes the pressure between the middle ear and the outside air pressure. Changes in altitude or ambient pressure, such as occurs on an airplane, can change the middle ear pressure, resulting in pain or discomfort. Repeated swallowing or in infancy sucking on the bottle or breast can often equalize the eustachian tube pressure and reduce the associated pain. In the adult, the eustachian tube averages 35 mm in length. In infancy, it is half that length (around 18 mm), which allows bacteria and viruses to more easily migrate from the oropharynx to the middle ear (Figure 12-10). The musculature of the eustachian tube, which controls function, is innervated by the motor division of the fifth cranial nerve, or trigeminal nerve. In infants and children with cleft palate and other central craniofacial abnormalities such as Down syndrome, structural or functional abnormalities of the eustachian tube interfere with normal ventilation and clearance of the middle ear, increasing the incidence of otitis media in these children. Equalization of pressure in the middle ear is critical for normal sound wave vibration of the tympanic membrane.

Muscle maturation, elongation of the eustachian tube, and a more vertical position all contribute to the decreased incidence of otitis media and middle ear effusions in middle childhood, adolescence, and young adulthood.

Inner ear

The inner ear is the sensory end organ and is directly responsible for hearing and balance. The inner ear contains the vestibule, semicircular canals, and cochlea and is bathed in fluid that facilitates the transmission of sound waves to the auditory nerve and sensations of balance in the semicircular canals (see Figure 12-9). The sound waves pass over approximately 30,000 innervated hair cells in the cochlea, which are the primary receptors, transducers, and conveyers of sound energy to the brain.