CHAPTER 193 Reconstruction of the Auditory Canal and Tympanum

• Atresiaplasty aims to create a patent, skin-lined external auditory canal and to achieve a postoperative air-bone gap within 20 to 30 dB.

• In bilateral or unilateral cases of congenital atresia, auricular reconstruction and atresiaplasty are recommended at 6 years of age. Reconstruction with porous high-density polyethylene (Medpor) can be done at an earlier age. In unilateral cases, atresiaplasty is indicated only in minor deformities.

• The microtia repair is performed first because of the need for an excellent blood supply to cover the autologous rib graft, and the hearing restoration surgery is performed 2 months after the last step of the microtia repair.

• 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.

• The following four anatomic parameters as seen radiographically are crucial for surgical planning: the status of the inner ear, the extent of pneumatization of the mastoid, the course of the facial nerve (both the relationship of the horizontal segment to the oval window and the location of the mastoid segment), and the presence of the oval window and stapes footplate.

• Ideal surgical candidates display a well-developed mastoid and a good oval window/footplate–facial nerve relationship.

• Ossicular reconstruction with the patient’s own ossicular chain is preferred to the use of a prosthesis.

• Complications of atresiaplasty include lateralization of the tympanic membrane, stenosis of the meatus, sensorineural hearing loss, and facial nerve palsy.

• Graft lateralization is the most common delayed cause of a poor hearing outcome, usually occurring in the first 12 months after surgery.

• Polymeric silicone sheets, hydroxylated polyvinyl acetate wicks, absorbable gelatin film disks, and tabs in the fascia graft have helped lower the incidence of tympanic membrane lateralization by keeping the tympanic membrane graft in position.

• The incidence of stenosis has been significantly reduced with the use of one-piece split-thickness skin grafts covering all exposed bone, and polymeric silicone sheets and hydroxylated polyvinyl acetate wicks in the external auditory canal.

• It is important that drilling on the ossicular chain be minimized when it is dissected away from the atretic bone to reduce the risk of noise-induced sensorineural hearing loss.

• Intraoperative facial nerve monitoring is essential in all cases because there are often no clearly identifiable landmarks in atretic ears.

• Bone-anchored hearing aids are an alternative to atresiaplasty for rehabilitation of hearing loss resulting from congenital aural atresia, particularly in patients who are not good candidates for surgery.

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 is unilateral more often than bilateral (occurring at a ratio of 3 : 1), preferentially affects males and the right side, and occurs in 1 in 10,000 to 1 in 20,000 live births.¹ Bony atresia occurs more frequently than membranous atresia. Fusion of the malleus and incus is the most common associated middle ear deformity, whereas the stapes footplate is usually normal.² Microtia is common in patients with CAA, and generally the severity of the external deformity correlates with the extent of middle ear deformity.³^–⁵ As opposed to middle ear malformations, the incidence of inner ear abnormalities in patients with CAA is relatively low, reflecting the earlier and independent embryonic development of the inner ear. The facial nerve typically lies in a normal position but may be displaced anteriorly and laterally in the vertical segment. Atresiaplasty aims to create a patent, skin-lined EAC and to achieve a postoperative air-bone gap within 20 to 30 dB. Surgical success depends on the appropriate selection of candidates for surgery and on proficient application of all modern tympanoplasty techniques, including meatoplasty, canalplasty, tympanic membrane grafting, and ossicular reconstruction.

Embryology

Knowledge of normal ear development aids understanding of the potential combinations of malformations possible in ears with CAA. This understanding helps physicians select appropriate surgical candidates and avoid complications during surgery. Because the inner ear, middle ear, and external ear develop independently, deformity of one does not necessitate deformity of another. Fortunately, inner ear structure and function are usually normal in ears with outer and middle ear abnormalities.⁶

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.^7,⁸ During the 21st week, this core resorbs and canalizes, forming the precursor to the EAC. Failure of resorption and canalization accounts for most cases of CAA. Subsequent posterior and inferior development carries the middle ear and facial nerve to their normal locations. By the end of the third gestational month, the primitive auricle has formed from six hillocks derived from the first and second branchial arches.

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.^7,⁸ Pneumatization of the middle ear and mastoid occurs late in fetal development, is usually present by birth, and continues to expand postnatally. Meckel’s cartilage (first branchial arch) gives rise to the neck and head of the malleus and the body of the incus, whereas Reichert’s cartilage (second branchial arch) forms the long processes of the malleus and incus as well as the stapes superstructure. The stapes footplate has a dual embryologic origin, developing from the second arch and from the otic capsule. The ossicles achieve their final shape by the fourth month and are covered with a mucous membrane from the expanding tympanic cavity by the end of the seventh to eighth month.

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.⁹ The auditory placode on the lateral surface of the hindbrain gives rise to the membranous inner ear during the third to the sixth week. The surrounding mesenchyme transforms into the bony otic capsule.

Classification Systems

In atresiaplasty, surgery classification systems that help in the identification of best candidates and in comparison of outcome and multiple schemes have been developed. The De la Cruz classification divides abnormalities into “minor” and “major” categories (Box 193-1).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 best treated with the bone-anchored hearing aid (BAHA) system.

The point-grading system for preoperative assessment of the best candidates for hearing improvement developed by Jahrsdoerfer and colleagues ¹⁰ assigns points on the basis of preoperative temporal bone CT scan findings of mastoid pneumatization, presence of the oval and round windows, facial nerve course, status of the ossicles and mesotympanum, and presence of the stapes, as well as on the external appearance (Table 193-1). Each parameter receives 1 point except for the presence of the stapes, which receives 2 points. A score of 8 or higher predicts the best chance of surgical success (greater than 80% success). A score of 7 implies a fair result, a score of 6, a marginal result; and a score less than 6 indicates that the patient would have poor results from atresiaplasty.

Table 193-1 Jahrsdoerfer Grading System of Candidacy for Atresiaplasty

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

Total Score	Type of Candidate
10	Excellent
9	Very good
8	Good
7	Fair
6	Marginal
≤5	Poor

Modified from Jahrsdoerfer RA, Yeakley JW, Aguilar EA, Cole RR, Gray LC. Grading system for the selection of patients with congenital aural atresia. Am J Otol. 1992;13:6-12.

Ishimoto and colleagues ¹¹ evaluated the relationship between hearing level and temporal bone abnormalities in patients with microtia, using the Jahrsdoerfer scoring system and high-resolution CT (HRCT) scans of the temporal bone.¹¹ The hearing level in microtic ears best 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.

Other classifications systems have been developed by Altmann,¹² Chiossone,¹³ and Schuknecht.¹⁴ Congenital cholesteatoma occurs in 14% of patients with CAA and is not included in any of the grading systems because these systems are used only for predicting hearing results in patients undergoing elective procedures.

Initial Evaluation

Several issues should be addressed in infants presented with CAA. In addition to the malformations of the temporal bone, other congenital abnormalities may be present and should be excluded. Evaluation of auditory function with auditory brainstem response (ABR) audiometry should be performed during the first few days of life in patients with either unilateral or bilateral atresia, because ipsilateral and contralateral inner ear abnormalities may be associated with CAA. Occasionally, a patient with unilateral atresia has a total sensorineural hearing loss (SNHL) on the side of the normal-looking ear. To maximize speech and language development, proper hearing amplification must begin promptly. For the patient with bilateral CAA, a bone conduction hearing aid should be applied as soon as possible, ideally at 3 or 4 weeks of age. Hearing amplification is not necessary in the patient with unilateral atresia if hearing is normal in the contralateral ear.

Parents of a child with sporadic, nonsyndromic CAA are counseled that the possibility of CAA in their subsequent children is no greater than that for the general population. Options regarding future auricular reconstruction are also reviewed with the parents, and the need for proper hearing rehabilitation is stressed. Early enrollment in special education enhances speech and language development. Children with CAA in association with other syndromes (e.g., hemifacial microsomia, Treacher Collins or Crouzon’s syndrome, Pierre Robin sequence) are poor surgical candidates for whom long-term bone-conduction aids or BAHAs offer the best rehabilitation.

The two requirements for elective atresiaplasty are (1) audiometric evidence of cochlear function and (2) radiographic three-dimensional evaluation of the temporal bone. As noted, audiometric evaluation should occur within the first few days of life; however, radiologic evaluation with HRCT of the temporal bone is deferred until age 5 or 6 years. A patient with CAA may experience an infected or draining ear or acute facial palsy with or without a congenital cholesteatoma. In such a case, resolution of the infection and removal of the cholesteatoma, if present, take priority, and HRCT scanning may be indicated before age 5 or 6 years.

Timing of Ear Reconstructive Surgery

Auricular reconstruction precedes hearing restoration surgery by at least 2 months to optimize the blood supply for the autologous rib graft and complex flaps.¹⁵ For patients with binaural or unilateral atresia, microtia repair and atresiaplasty are recommended at 6 years of age. By this age, mastoid pneumatization is complete and the costal cartilage has developed sufficiently for auricular reconstruction. Auricular defects can be rehabilitated with an osseointegrated percutaneous mastoid implant prosthesis, with or without bone-conduction aids such as a BAHA.¹⁶ Alloplastic materials have been used for microtia repair in the past, but a high risk of extrusion is associated with their use. However, new materials, such as porous high-density polyethylene (Medpor), seem to be very well tolerated.^17,¹⁸ Medpor reconstruction can be done before or after atresiaplasty because intact blood supply is not indispensable to its success.

In unilateral cases, atresiaplasty surgery may be indicated in a patient who has “minor” unilateral atresia with normal middle ear, ossicles, and facial nerve and excellent pneumatization. In such a patient, atresiaplasty may be offered in childhood with the parents’ consent. An adult with unilateral atresia commonly requests atresiaplasty when presbycusis develops in the normal ear.