EMBRYOLOGY

A review of the normal embryologic development of the ear aids in understanding the myriad of possible combinations of malformations encountered in congenital aural atresia. The inner ear, middle ear, and external ear develop independently and in such a way that deformity of one does not presuppose deformity of another.^9,43 Most frequently, abnormalities of the outer and middle ear are encountered in combination with a normal inner ear.^44,45

Microtia is a result of first and second branchial arch anomalies. Growth of mesenchymal tissue from the first and second branchial arches forms six hillocks around the primitive meatus that fuse to form the auricle (Table 5-1). By the end of the third month, the primitive auricle has been completed. The external auditory meatus develops from the first branchial groove. During the second month, a solid core of epithelium migrates inward from the rudimentary pinna toward the first branchial pouch. This core, the precursor of the EAC, starts to hollow out and take shape in the sixth month. It canalizes in the seventh month, causing the developing mastoid to become separated from the mandible. Its subsequent posterior and inferior development carries the middle ear and facial nerve to their normal positions.^1,43,46–48 Some of the literature supports the notion that microtia grade can indicate the status of middle ear development in aural atresia. The better developed the external ear is, the better developed is the middle ear.

TABLE 5-1 Development of the Auricle

The first branchial pouch grows outward to form the middle ear cleft. The plaque of tissue where this cleft meets the epithelium of the EAC forms the tympanic membrane. While the pouch is forming the eustachian tube, tympanic cavity, and mastoid air cells, Meckel’s cartilage (first branchial arch) is forming the neck and head of the malleus and incus body. Reichert’s cartilage (second branchial arch) forms the remainder of the first two ossicles’ long processes and the stapes superstructure. The footplate has a dual origin from the second arch and the otic capsule. The ossicles attain their final shape by the fourth month. By the end of the seventh to eighth month, the expanding middle ear cleft surrounds the ossicles and covers them with a mucous membrane.^1,45,49

The facial nerve is the nerve of the second branchial arch. At 4.5 weeks, this developing nerve divides the blastema, which is the condensation of the second arch mesenchymal cells, into the stapes, the interhyale (stapedius muscle precursor), and the laterohyale (precursor of the posterior wall of the middle ear). The nerve’s intraosseous course is dependent on this bony expansion.^45,47 The membranous portion of the inner ear develops during the third to sixth week from an auditory placode on the lateral surface of the hindbrain. The surrounding mesenchyme transforms into the bony otic capsule.⁵⁰

Congenital aural atresia can range in severity from a thin membranous canal atresia to complete lack of tympanic bone, depending on the time of arrest of intrauterine development.^1,51,52 The common finding of a normal inner ear is explained because the inner ear is formed by the time of external/middle ear development arrest. Facial nerve course abnormalities are often seen.

CLASSIFICATION SYSTEMS

Of historical significance is a classification in congenital aural atresia developed in 1955 by Altmann.⁵¹ In this system, atresias are categorized into three groups, as follows:

Altmann’s classification system is purely descriptive, and most surgical candidates fall into groups 2 and 3.

The De la Cruz classification includes surgical feasibility guidelines using only high-resolution computed tomography (CT), taking into consideration mastoid pneumatization, inner ear normality, and facial nerve and footplate relationship.⁶ The malformations are divided into minor and major malformations (Table 5-2). The clinical importance of this classification is that surgery in cases of minor malformations has a good possibility of yielding serviceable hearing, whereas cases of major malformations are frequently inoperable, but treatable with the bone-anchored hearing aid (BAHA) system.³

TABLE 5-2 De la Cruz Classification of Congenital Aural Atresia

From De la Cruz A, Linthicum FH Jr, Luxford WM: Congenital atresia of the external auditory canal. Laryngoscope 95:421-427, 1985.

Jahrsdoerfer and colleagues⁵³ developed a widely used point grading system to guide surgeons in preoperative assessment of the best candidates for hearing improvement (Table 5-3). This system takes into account the parameters of mastoid pneumatization, presence of the oval and round windows, facial nerve course, status of the ossicles, and external appearance. Point allocation is based primarily on the findings on high-resolution CT. Jahrsdoerfer and colleagues⁵³ proposed that when the preoperative evaluation of the patient is itemized into this grading system, the best results (>80% success) are achieved with a score of 8 or better. A score of 7 indicates the patient is a fair candidate, a score of 6 indicates the patient is a marginal candidate, and with a score less than 5 the patient becomes a poor candidate.

TABLE 5-3 Jahrsdoerfer’s Grading System of Candidacy for Surgery of Congenital Aural Atresia

From Jahrsdoerfer RA, Yeakley JW, Aguilar EA, et al: Grading system for the selection of patients with congenital aural atresia. Am J Otol 13:6-12, 1992.

Ishimoto and associates⁵⁴ evaluated the relationship between hearing level and temporal bone abnormalities in patients with microtia, using Jahrsdoerfer’s CT scoring system and high-resolution CT scans of the temporal bone. They found that the hearing level in microtic ears correlated with the formation of oval/round windows and ossicular development, but not with the degree of middle ear aeration, facial nerve aberration, or severity of microtia.

Schuknecht’s⁵⁵ system of classification of congenital aural atresia is based on a combination of clinical and surgical observations. Type A (meatal) atresia is limited to the fibrocartilaginous part of the EAC. Meatoplasty is the surgical procedure of choice, and when performed in a timely fashion prevents formation of canal cholesteatoma and conductive hearing loss. In type B (partial) atresia, there is narrowing of the fibrocartilaginous and bony EAC, but a patent dermal tract allows partial inspection of the tympanic membrane. The tympanic membrane is small and partly replaced by a bony septum. Minor ossicular malformations exist, and hearing loss may be mild to severe. Type C (total) atresia includes all cases with a totally atretic ear canal, but a well-pneumatized tympanic cavity. There is a partial or total bony atretic plate, the tympanic membrane is absent, the heads of the ossicles are fused, there may be no connection to a possibly malformed stapes, and the facial nerve is more likely to have an aberrant course over the oval window. Type D (hypopneumatic total) atresia is a total atresia with poor pneumatization, common in dysplasias such as Treacher Collins syndrome. There are abnormalities of the facial nerve canal and the bony labyrinth. These patients are poor candidates for hearing improvement surgery.

Chiossone’s⁵⁶ classification is based primarily on the location of the glenoid fossa. In type I, the fossa is in the normal position; in type II, it is moderately displaced; in type III, the fossa overlaps the middle ear; and in type IV, in addition to the fossa overlapping the middle ear, there is lack of mastoid pneumatization. Patients with types I and II are ideal surgical candidates. Type III cases have a tendency toward graft lateralization. Patients with type IV are not surgical candidates. In atresiaplasty surgery, a classification scheme is useful for surgical planning, patient counseling, and comparison of outcomes.

INITIAL EVALUATION AND PATIENT SELECTION

When aural atresia is noted in a newborn, several issues must be addressed. Where one congenital abnormality is found, others must be sought. A high-risk registry for hearing loss is helpful in this regard.⁵⁷ After the degree of aural deformity is assessed by physical examination, evaluation of auditory function in unilateral and bilateral atresia should be performed using auditory brainstem response audiometry within the first few days of life. An 11% to 47% incidence of inner ear abnormality is associated with congenital aural atresia.¹² Occasionally, in unilateral cases, there is a total sensorineural hearing loss (SNHL) on the side of the normal-appearing ear, which might otherwise be missed.^6,33

In bilateral cases, a bone-conduction hearing aid should be applied as soon as possible, ideally in the third or fourth week of life. In unilateral cases in which the opposite ear hears normally, a hearing aid is unnecessary. A child with aural atresia and associated cephalic abnormalities (e.g., hemifacial microsomia and Treacher Collins, Crouzon, or Pierre Robin syndrome) should be recognized.^57–61 In this subset, surgical correction has poor results,⁵⁵ and a long-term bone-conduction hearing aid or BAHA in these nonoperable bilateral congenital aural atresia situations is beneficial (see later).³

Prompt and careful counseling of the parents of a child with sporadic (nonsyndromal) congenital aural atresia is necessary to alleviate concerns regarding possible occurrence in subsequent children (no more than the general population), to answer questions regarding future auricular reconstruction, and, most important, to ensure that proper hearing amplification is instituted in a timely fashion. The child should be enrolled in special education at an early age to maximize speech and language acquisition, in preparation for “mainstreaming” at preschool age. Radiologic and surgical evaluations are deferred until the child is 6 years old (see later).

In the initial evaluation of an older individual with congenital aural atresia, the most crucial elements remain the functional and anatomic integrity of the inner ear. Audiometry and high-resolution thin-section (≤1 mm) CT in coronal and axial views are necessary. Prognosis for hearing improvement depends on the presence and degree of malformations. Auricular reconstruction must be done before hearing reconstruction to avoid interfering with the blood supply to the surrounding soft tissue, which is indispensable for microtia repair. Reconstruction with alloplastic materials, such as porous high-density polyethylene (Medpor), can be done before or after atresiaplasty because intact blood supply is not indispensable.

A patient with congenital aural atresia may present with an infected or draining ear or acute facial palsy; 14% have congenital cholesteatoma.⁶ The priority in these cases is removal of the cholesteatoma and resolution of the infection. Preoperative audiometry and high-resolution CT scanning may be necessary at an earlier age in children with repeated infections in the atretic ear.^6,55,62

There are two requirements for planning surgery in congenital aural atresia: radiographic three-dimensional evaluation of the temporal bone and audiometric evidence of cochlear function.^8,63 Other conditions mandating prompt surgical intervention are congenital cholesteatoma, a draining postoperative atretic ear, and acute facial palsy. The CT scan should always be reviewed for cholesteatoma, which necessitates surgery at any age.^55,62,64 It is not included in any of the grading systems because these are used only for predicting hearing results in elective atresiaplasty surgery.

TIMING OF AURICULAR RECONSTRUCTION AND ATRESIAPLASTY

In bilateral or unilateral atresia, auricular reconstruction and atresiaplasty are recommended at 6 years of age. Before this age, there may be a tendency to form exostosis-like bony growth that may occlude the EAC, and there is less patient cooperation. By age 6, the costal cartilage has developed sufficiently to allow for reconstruction of the auricle, and the mastoid has become as pneumatized as possible. The microtia repair should be done first because the complex flaps and use of autologous rib graft demand excellent blood supply.^65,66 Many surgeons do not perform cartilage microtia reconstruction on an ear with previous reconstruction attempts.⁶⁴ The hearing restoration surgery is performed 2 months after the last step of the microtia repair. Rehabilitation of auricular defects can be done using an osseointegrated percutaneous mastoid implant prosthesis, with and without bone-conduction aids, such as BAHA.^3,48,67–69 Alloplastic materials have been used for microtia repair in the past, but there is a high risk of extrusion associated with their use. Newer materials, such as Medpor, seem to be very well tolerated, however.^70–72 As noted earlier, Medpor reconstruction can be done before or after atresiaplasty because intact blood supply is not indispensable.

In unilateral cases, atresiaplasty surgery may be indicated in a patient with “minor” unilateral atresia with normal middle ear, ossicles, and facial nerve, and excellent pneumatization. In such patients, atresiaplasty may be offered in childhood with the parents’ consent.³³ We often see older adults with unilateral atresia who request surgery when their normal ear begins having high-frequency hearing loss (presbycusis).