The external ear canal is derived from the first branchial groove (cleft), between the mandibular and hyoid arches. It is initially represented by a solid core of epithelial cells that extends down to the area of the tympanic ring and first pharyngeal pouch. This core of cells remains in place until the middle trimester of fetal life, a time when most structures of the inner, middle, and outer ear are well differentiated. At this point, absorption of the epithelial cells begins, progressing in a medial to lateral direction. If this canalization process is arrested prematurely, it is possible to have a more normally developed tympanic membrane and bony external ear canal associated with an atretic or very stenotic membranous canal, a situation that predisposes to canal cholesteatoma formation as the trapped squamous epithelium continues to desquamate.

The medial portion of the external ear canal is formed by the tympanic bone. This structure begins to ossify in the third embryonic month, eventually forming the tympanic ring and osseous ear canal; the latter structure continues its lateral growth during the first and second postnatal years. Malformation of the tympanic bone produces atretic bone at the level of the tympanic membrane and results in atresia of the ear canal (2). The mandibular condyle articulates with this rudimentary tympanic bone.

The eustachian tube, middle ear, and mastoid air cells are derived from the first pharyngeal pouch. Although the middle ear cavity and mastoid air cells are smaller than normal in patients with aural atresia, no anatomic or clinical studies show impaired eustachian tube function in these ears. Pneumatization of the mastoid is a late embryologic event, starting at the seventh or eighth month and continuing into postnatal life. A well-pneumatized mastoid usually
indicates good middle ear development, including size of the tympanum and formation of the ossicles. The relationship between middle ear development and degree of differentiation of the pinna is disputed (3).

The ossicles, except for the vestibular portion of the stapes footplate, are formed from the first and second branchial arches. The external ear canal and tympanic membrane are derived from the first branchial cleft. Isolated branchial arch (ossicular) or branchial cleft (external ear canal) deformities are possible, but usually these malformations occur in combination (2). Other branchial arch defects may occur, especially mandibular hypoplasia. The stapes footplate is formed in part from the otic capsule, and in most cases it is normally developed in ears with congenital atresia. It is uncommon to encounter a fixed stapes footplate in the usual major congenital ear malformation, although the superstructure is frequently deformed. This information is important, because the stapes is often partially obscured by the lateral ossicular mass or the facial nerve in atretic ears, and its normal mobility may be difficult to determine with certainty.

Absence of the stapes footplate and oval window can occur, but this is usually encountered in a patient with a patent external ear canal and normal-appearing tympanic membrane rather than in a patient with aural atresia (4). It has been suggested that this condition is caused by abnormal development of the facial nerve (5, 6). By the fifth to sixth week of gestation, the horizontal and vertical segments of the facial nerve are evident (6). If anterior displacement of the facial nerve occurred at this time, the nerve could become interposed between the otic capsule and the stapes blastema, which is beginning to grow toward the otic capsule. This would interfere with further stapes development, resulting in a rudimentary ossicle attached to the incus. Continued growth of the stapes toward the otic capsule could result in the rudimentary crura becoming embedded in the displaced facial nerve (4). Because the stapes never contacts the otic capsule, an oval window does not form. With further anterior displacement of the facial nerve, it is possible for that structure to course across the promontory, inferior to the region of the oval window (4). It has been hypothesized that displacement of the facial nerve occurs because of underdevelopment of the first branchial arch (5). This results in a compensatory overshifting of the second branchial arch, and its nerve follows this shift, assuming a more anterior position.

The membranous labyrinth is derived from the ectodermal otocyst. In most cases of isolated aural atresia, sensorineural and vestibular functions are normal. A comprehensive review of inner ear abnormalities in aural atresia was recently reported by Vrabec and Lin (7). Their study included aural atresia within a broad spectrum of conditions termed “craniofacial microsomia” (e.g., CHARGE, branchiootorenal syndrome, Treacher Collins syndrome). They noted an inner ear anomaly by computed tomography (CT) in 22% of 105 patients, with an abnormality of the horizontal semicircular canal and/or vestibule accounting for over half (Fig. 148.1). A sensorineural hearing loss, usually mild and defined as a bone-conduction threshold greater than 20 dB involving two or more frequencies, was found in 15% of patients; the sensorineural hearing loss was usually mild.

Preoperative facial nerve weakness is uncommon in isolated aural atresia, but can be seen in 10% to 15% of patients with aural atresia in association with other craniofacial anomalies (7). Facial nerve abnormalities are common in cases of aural atresia (8, 9). Bony dehiscence of the fallopian canal frequently occurs, and the facial nerve may also take an anomalous course. Typically, the facial nerve makes an acute angle at the second genu, crossing the middle ear in a more anterior and lateral direction to exit into the glenoid fossa. This abnormal position of the mastoid segment of the facial nerve places it at jeopardy when drilling the posterior inferior portion of the new external ear canal. As the facial nerve exits the skull, it may lie just deep to the area of the tragus. Inadvertent injury to the nerve can occur if undermining of the auricle is necessary to better align the meatus and newly created external ear canal. A correlation between the degree of microtia and the extent of facial nerve abnormality has been observed (3, 10). An association between preoperative facial nerve dysfunction and inner ear abnormality by CT has also been noted (7).

In the major malformation group, the external ear canal and tympanic membrane are usually absent, although cases of severe canal stenosis are also included. A small rudimentary tympanic membrane attached to a bony septum is occasionally seen in the canal stenosis patients, but typically the stenosis prevents visualization of the medial aspect of the ear canal. The size of the middle ear space is reduced, and the malleus and incus are deformed, fused, and fixed to the atretic bone. In severe cases, the middle ear space is very hypoplastic, and ossicles are rudimentary or absent. Dehiscence or displacement of the facial nerve can be expected in most major malformations. Grade II or III microtia is common, and inner ear function is usually normal.

The significant defect in the minor malformation group involves the middle ear. A conductive hearing loss exists because of absence or deformity of one or more ossicles or fixation of the ossicular chain. Abnormalities of the stapes may be more severe in the minor malformation group than in the major group. The middle ear space and tympanic membrane are normal or only slightly smaller. The external ear canal is patent but may be mildly stenotic. Dehiscence or displacement of the facial nerve can occur, and the pinna is normally developed or only slightly deformed.

No immediate medical intervention is necessary in the infant discovered to have unilateral atresia, assuming there is normal hearing in the contralateral ear by ABR and OAE testing. The parents can be reassured that speech, language, and intellectual development will proceed normally. Preferential seating in school and possibly an FM system are advised, but a hearing aid is not recommended because of poor acceptance by most children. Many adults, however, find the consequences of unilateral hearing loss from atresia to be a significant aggravation at work and in social settings, and they more readily accept hearing aids. In patients with atresia of the ear canal, a bone-conduction hearing aid must be used. If the canal is only stenotic, an air-conduction aid is preferred because of cosmesis, improved sound localization (i.e., stimulation of one cochlea only), broader frequency response, less sound distortion, and comfort.

Early amplification within the first 3 to 4 months of life is essential in infants with bilateral atresia. Initial medical and audiologic evaluations can be completed within the first few months of life and a bone-conduction hearing aid or soft band BAHA can be fitted soon thereafter.

Although most otologic surgeons would consider atresia repair in bilateral cases, many are reluctant to operate on unilateral atresias. The issue is not the unilateral aspect of the hearing loss because most otologic surgeons will
explore the middle ear of a child with a large unilateral conductive hearing loss due to other causes (e.g., trauma, infection). The concern is the degree and predictability of hearing improvement that can be achieved, potential lifetime care of a mastoid cavity, and the risk to the facial nerve in atresia surgery. These concerns have prompted many surgeons to recommend delaying surgery in unilateral cases until adulthood, when patients can make their own decision based on the risks and benefits.

An improvement in the hearing threshold to 25 dB or better eliminates the handicap of unilateral hearing loss. This degree of hearing improvement is not possible in all atresia patients, but it can be achieved in at least 50% of carefully selected patients. A mastoid cavity is not created if the “anterior” surgical approach is used, and risk to the facial nerve is minimized by understanding the abnormal development of this structure and by using intraoperative facial nerve monitoring. I and others contend that the benefits of binaural hearing and the possibility of achieving that goal are sufficiently great to offer corrective surgery to carefully selected children with unilateral atresia (10, 15).

Patients with bilateral atresia present less of a surgical dilemma. The goal in these cases is to restore sufficient hearing so that amplification is no longer needed. In contradistinction to ear selection for other otologic disorders, the “best” (as determined by CT evaluation) ear is selected for the initial surgical procedure. Most surgeons recommend operating as the child approaches school age and, depending on the hearing result, on the second ear within the next several years. As with many conductive hearing losses (e.g., otosclerosis), the possible use of amplification with a BAHA should be discussed. The probable need for yearly or semiannual cleaning of the reconstructed ear canal should also be acknowledged.

Most patients undergoing atresia repair have a residual conductive deficit of at least 10 dB. Sensorineural function should be normal to achieve binaural hearing in unilateral cases or to obviate the need for a hearing aid in bilateral cases. Normal or near-normal sensorineural function in the contralateral ears is also important to avoid operating on the better hearing ear.

Although audiometric criteria can be defined quantitatively, the real art of patient selection is centered on the CT evaluation of the middle ear. Hypoplasia of the middle ear space, ranging from mild to severe, occurs in most cases of congenital atresia, and ossicular development can be expected to correlate directly with middle ear size. The risk of surgical complications will be minimized and the chances for a successful hearing result are increased if the middle ear and mastoid size are at least two-thirds of the normal size and if all three ossicles, although deformed, can be identified (Figs. 148.1 and 148.2). CT demonstration of the oval and round windows and a near-normal course of the facial nerve further define the ideal surgical candidate. The relationship of the facial nerve to the oval window (i.e., normally positioned or overhanging) and the position of the vertical segment are noted. Severe anterior displacement of
the vertical segment of the nerve restricts access to the middle ear space, reducing the chance for a successful hearing result and increasing the chance of facial nerve injury.