Chapter 5 Congenital Malformation of the External Auditory Canal and Middle Ear

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Congenital aural atresia is characterized by aplasia or hypoplasia of the external auditory canal (EAC), often associated with absence or deformity of the auricle (microtia) and the middle ear, with occasional inner ear abnormalities. Aural atresia occurs in 1 in 10,000 to 20,000 live births ¹^–⁵; unilateral atresia is three times more common than bilateral atresia. This disorder occurs more commonly in males and on the right side.¹ EAC atresia is more often bony rather than membranous, and bony atresia is regularly accompanied by malformation of the middle ear cavity and structures of the middle ear.⁶^–⁹ More severe forms of congenital microtia are usually associated with EAC atresia; rarely, canal atresia may be seen in patients with a normal pinna.¹⁰ Generally, a more severe external deformity implies a more severe middle ear abnormality.^11,¹²

Kiesselbach, in 1883, is often credited with the first deep operation attempting to correct this malformation.⁸ Lascaratos and Assimakopoulos¹³ noted that the Byzantine physician Paul of Aegina performed a surgical treatment of congenital aural atresia.¹⁴ The procedure done by Kiesselbach resulted in facial paralysis. Because of the lack of middle ear microsurgery and the high complication rate, congenital aural atresia surgery was considered dangerous and to be avoided for the most part. In 1914, Page¹⁵ reported hearing improvement after surgery in five of eight patients. This report was followed in 1917 by Dean and Gittens,¹⁶ who reported an excellent hearing result in a patient and reviewed the various types of operations that had been tried by other surgeons. The prevailing attitude toward surgical correction in these cases remained generally pessimistic, however, despite these and other occasional reports of successful operations, until 1947. That year, Ombredanne¹⁷ in France and Pattee¹⁸ in the United States each reported a series of patients successfully operated on to improve hearing. Pattee’s technique included removal of the incus to “mobilize” the stapes¹⁸; Ombredanne¹⁷ added fenestration of the lateral semicircular canal.

With the advent of tympanoplasty techniques in the 1950s, interest in atresiaplasty increased as the teachings of Wullstein and Zollner carried over into surgery of the congenital ear.⁸ Larger series with greater success rates were reported as surgeons attempted to improve their results, using ossiculoplasty, mastoidectomy, differing degrees of bone removal, and different types and techniques of graft placement.^2,^4,^17,¹⁹^–²⁶ Ombredanne²⁷ went on to report on more than 600 aplasia cases by 1971 and 1600 cases with major and minor malformations by 1976.⁸ Gill’s report of 83 cases in 1969² and Jahrsdoerfer’s 1978 article⁸ are considered landmarks. Crabtree,²⁸ Jahrsdoerfer,^29,³⁰ Marquet,^31,³² and De la Cruz^6,³³ and their colleagues all reported on large surgical series, with modifications of classification and operative techniques.

Although techniques of canalplasty, meatoplasty, tympanoplasty, and ossiculoplasty have improved considerably, surgical correction of congenital aural atresia remains one of the most challenging operations performed by otologists. This is a complex surgical problem, requiring application of all tympanoplasty techniques and a thorough knowledge of the surgical anatomy of the facial nerve, oval window, and inner ear, and their congenital variants.^1,^6,^8,^17,^21,^26,^27,^29-^31,^33-⁴⁰ The temporomandibular joint is displaced posteriorly by the lack of development of the EAC, narrowing the distance between the glenoid fossa and the anterior wall of the mastoid tip.^19,⁴¹ Fusion of the incus and malleus is common, but because of its dual origin, the stapes footplate is usually normal.^4,⁴²

The surgical repair of aural atresia is recommended at age 6 years.⁶ The timing of repair must take into account any planned auricular reconstruction procedures. Criteria for patient selection must be stringent when attempting to achieve closure of the air-bone gap to within 20 to 30 dB. Preoperative counseling and several postoperative visits are essential for optimal results. In this chapter, we discuss these issues and provide guidelines for patient evaluation and selection, surgical techniques, and postoperative management.

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

First Branchial Arch	Second Branchial Arch
First hillock—tragus	Fourth hillock—antihelix
Second hillock—helical crus	Fifth hillock—antitragus
Third hillock—helix	Sixth hillock—lobule and lower helix

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:

Group 1 (mild): Some part of the EAC, although hypoplastic, is present. The tympanic bone is hypoplastic, and the ear drum is small. The tympanic cavity is either normal in size or hypoplastic.

Group 2 (moderate): The EAC is completely absent, the tympanic cavity is small, and its content is deformed, and the “atresia plate” is partially or completely osseous.

Group 3 (severe): The EAC is absent, and the tympanic cavity is markedly hypoplastic or missing.

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

Minor Malformations

1. Normal mastoid pneumatization

2. Normal oval window/footplate

3. Good facial nerve–footplate relationship

4. Normal inner ear

Major Malformations

1. Poor pneumatization

2. Abnormal or absent oval window/footplate

3. Abnormal course of facial nerve

4. Abnormalities of inner ear

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

Jahrsdoerfer and colleagues ⁵³ 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

Parameter	Points
Stapes present	2
Oval window open	1
Middle ear space	1
Facial nerve normal	1
Malleus-incus complex present	1
Mastoid well pneumatized	1
Incus-stapes connection	1
Round window normal	1
Appearance of external ear	1
Total available points	10
Rating	Type of Candidate
10	Excellent
9	Very good
8	Good
7	Fair
6	Marginal
≤5	Poor

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 associates ⁵⁴ 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’s ⁵⁵ 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’s ⁵⁶ 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.