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Correction of Congenital Malformations
Congenital aural atresia (CAA) represents aplasia or hypoplasia of the external auditory canal (EAC) resulting from failed or aborted development and is often associated with other malformations of the temporal bone, including external, middle, and inner ear deformities. CAA occurs in one in 10,000 to 20,000 live births, preferentially affects males and the right side, and is unilateral more often than bilateral (3:1).1Bony atresia occurs more frequently than membranous atresia. Middle ear deformities, including fusion of the malleus and incus, are most likely encountered in cases of bony atresia. The stapes footplate is usually normal due to its dual embryonic origin. External deformities like microtia (absence or deformity of the pinna) are common in patients with CAA, and generally the severity of the external deformity correlates with the degree of middle ear deformity.2 The incidence of inner ear abnormalities in patients with CAA is relatively low. The facial nerve usually follows its normal course but may be displaced anteriorly and laterally in the vertical segment.
Surgical correction of CAA aims to create a patent, skin-lined EAC and close the postoperative air–bone gap to within 20 to 30 dB. This requires the application of all modern tympanoplasty techniques, including meatoplasty, canaloplasty, tympanic membrane grafting, and ossiculoplasty. Surgical success depends on the appropriate selection of surgical candidates and on a thorough understanding of the anatomy of the facial nerve, oval window, and inner ear and their variants.
■ Embryology
The inner, middle, and external ear develop independently and deformity of one does not necessitate deformity of another. Most frequently, abnormalities of the outer and middle are associated with normal inner ear structure and function.3
The primitive auricle forms by the end of the third month from six hillocks derived from the first and second branchial arches.4, 5 The external auditory meatus develops from the first branchial groove. During the second month, a solid core of epithelium extends inward from the primary meatus to the primitive tympanic cavity, forming the meatal plate. During the 21st week, this core resorbs and canalizes forming the precursor to the EAC. Subsequent posterior and inferior development carries the middle ear and facial nerve to their normal locations.
The eustachian tube, tympanic cavity, and mastoid air cells derive from the first branchial pouch, and the tympanic membrane forms from the plaque of tissue where this pouch meets the epithelium of the EAC.4, 5 Pneumatization occurs late in fetal development, is usually present by birth, and continues to expand post-natally. The neck and head of the malleus and the body of the incus develop from Meckel’s cartilage (first branchial arch), whereas Reichert’s cartilage (second branchial arch) forms the long processes of the malleus and incus and the stapes superstructure. The stapes footplate develops from the second arch and from the otic capsule. The ossicles achieve their final shape by the fourth month and, by the end of the seventh to eighth months, are covered with a mucous membrane from the expanding tympanic cavity.
The facial nerve is the nerve of the second branchial arch. Its general course is established by the end of the embryonic period; however, its ultimate intraosseous course depends on later bony expansion of the tympanic ring and cavity.6 The membranous inner ear develops during the third to the sixth week from an auditory placode on the lateral surface of the hindbrain. The surrounding mesenchyme transforms into the bony otic capsule.
■ Classification Systems
Classification systems aid in deciding which patients are the best candidates for atresiaplasty and in comparing outcomes. Multiple schemes have been developed. Altmann developed a purely descriptive classification system that categorizes CAA into three groups: mild, moderate, and severe based on the status of the EAC and tympanic cavity.7 Most surgical candidates fall into the latter two groups.
The De la Cruz classification divides abnormalities into minor and major categories. Minor malformations consist of (1) normal mastoid pneumatization, (2) normal oval window footplate, (3) reasonable facial nerve/footplate relationship, and (4) normal inner ear. Major malformations are (1) poor pneumatization, (2) abnormal or absent oval window footplate, (3) abnormal course of the facial nerve, and (4) abnormalities of the inner ear.
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.
Jahrsdoerfer et al developed a point-grading system for preoperative assessment of the best candidates for hearing improvement.8 This system assigns points based on mastoid pneumatization, presence of the oval and round windows, facial nerve course, status of the ossicles and mesotympanum, presence of the stapes, and external appearance. Each parameter accounts for one point with the presence of the stapes receiving two points. A score of 8 yields the best chance of surgical success (> 80% success). A score of 7 implies a fair chance, 6 is marginal, and below this the patient is a poor atresiaplasty candidate. Other classification systems include those by Schuknecht and Chiossone.9, 10
■ Initial Evaluation and Patient Selection
Within the first few days of life auditory brainstem response (ABR) audiometry is performed in patients with either unilateral or bilateral atresia because ipsilateral and contralateral inner ear abnormalities may be associated with CAA. Occasionally, patients with unilateral atresia have a total sensorineural hearing loss (SNHL) on the side of the normal-appearing ear. Congenital abnormalities, in addition to those involving the ear, may be present and should be excluded.
Parents of a child with sporadic (nonsyndromal) CAA are counseled regarding possible occurrence in their subsequent children (no more than the general population), options regarding future auricular reconstruction, and most importantly, the necessity that proper hearing amplification begins promptly. In bilateral cases, a bone conduction hearing aid should be applied as soon as possible, ideally in the third or fourth week of life. A hearing aid is not necessary in unilateral cases with normal hearing in the contralateral ear. Early enrollment in special education enhances speech and language development. Syndromic children with CAA (e.g., hemifacial microsomia, Treacher Collins syndrome, Crouzon’s disease, or Pierre Robin syndrome) represent poor surgical candidates, and a long-term bone-conduction aid or BAHA offers the best chances for rehabilitation.
The studies required for elective atresiaplasty are (1) radiographic evidence of an inner ear and (2) audio-metric evidence of cochlear function. Whereas audio-metric evaluation should occur early, radiological evaluation with high-resolution computed tomography (HRCT) of the temporal bone in coronal and axial views is deferred until 5 or 6 years of age. Occasionally, a patient with CAA may present with an infected or draining ear or acute facial palsy; 14% have congenital cholesteatoma. In these cases, removal of the cholesteatoma and resolution of the infection become the first priority and HRCT scanning may be indicated at an earlier age.
■ Timing of Auricular Reconstruction and Atresiaplasty
Auricular reconstruction and atresiaplasty are recommended at 6 years of age because by this age the costal cartilage has developed sufficiently for auricular reconstruction, and mastoid pneumatization is complete. Microtia repair occurs first to optimize the blood supply for the complex flaps and autologous rib graft. Hearing restoration surgery follows microtia repair by at least 2 months.
In children with minor unilateral atresia with excellent pneumatization and normal middle ear, ossicles, and facial nerve, surgery can be performed with the parents’ consent. Often, older adults with unilateral atresia request surgery when their normal ear develops high-frequency hearing loss (presbycusis).
■ Management of the Atretic Ear
When faced with an individual with an atretic ear, the preoperative evaluation, counseling, and management can be complex. Table 5–1 summarizes some difficulties that may arise and possible solutions.
■ Preoperative Evaluation and Patient Counseling
An estimate of the size of the mastoid on physical examination can be determined by palpation of the mastoid tip, suprameatal spine of Henle (if present), condyle, and zygomatic arch. This is a useful measure because the new ear canal will be constructed at the expense of the mastoid air-cell system.
At this time, HRCT scanning in both axial and coronal planes is the only acceptable preoperative imaging study. The surgeon, in addition to the radiologist, should review the CT scan. The four imaging elements most critical for surgical planning in CAA are (1) the degree of temporal bone pneumatization; (2) the course of the facial nerve, both the relationship of the horizontal portion to the footplate and the location of the mastoid segment; (3) the existence of the oval window and stapes footplate; and (4) the status of the inner ear. Ideally, surgical candidates will have a well-developed mastoid and a good oval window footplate – facial nerve relationship. HRCT also provides information on thickness and form of the bony atretic plate, size and status of the middle ear cavity, presence of congenital cholesteatoma, and soft tissue contribution to the atresia, but these are less critical for the repair.
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