Congenital Malformation of the External Auditory Canal and Middle Ear

Introduction

CAA is an intrauterine developmental anomaly characterized by the complete absence of the external auditory canal (EAC) or, less commonly, by various degrees of EAC stenosis. CAA is often associated with absence (anotia) or deformity (microtia) of the auricle and maldevelopment of the middle ear and ossicles. Occasionally, CAA may be associated with inner ear abnormalities, but, more commonly, hearing loss is conductive in nature with preservation of the cochlear function. CAA occurs in 1 in 10,000 to 20,000 live births, is three times more likely to be unilateral than bilateral, occurs more often in males, and affects the right side more often than it does the left. Aural atresia refers to the closure of the ear canal (or more precisely, the failure of the ear canal to develop) and to malformations of the middle ear space and ossicles. Rarely, CAA can be seen in patients with a normal pinna (generally associated with mutations in chromosome 18q), but in general, a more severe external deformity implies a more severe middle ear abnormality.

Lascaratos and Assimakopoulos noted that the Byzantine physician Paul of Aegina performed surgical treatment of CAA. However, Kiesselbach, in 1883, is often credited with the first surgical attempt to correct this malformation. In 1914, Page reported hearing improvement after surgery in five of eight patients. This report was followed in 1917 by that of Dean and Gittens, who reported an excellent hearing result in a single patient and reviewed the previously reported techniques. Despite these and other occasional reports of successful operations, the prevailing attitude toward surgical correction in these cases remained generally pessimistic. In 1947, Ombredanne in France and Pattee in the United States each reported a series of patients who experienced improved hearing after a successful operation. Pattee’s technique included removal of the incus to “mobilize” the stapes, whereas Ombredanne added fenestration of the lateral semicircular canal.

With the advent of modern tympanoplasty techniques in the 1950s, interest in atresiaplasty increased as the teachings of Wullstein and Zollner carried over into surgery of the congenital ear. Larger series with higher 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 skin graft placement. ^{, , ,} Ombredanne went on to report more than 600 cases by 1971 and 1600 cases with major and minor malformations by 1976. ^, Gill, Jahrsdoerfer, and many others have reported large surgical series since that time, paving the way for continued interest in refining techniques for atresiaplasty and improving hearing outcomes while minimizing morbidity.

Although the techniques of canalplasty, meatoplasty, tympanoplasty, and ossiculoplasty have improved considerably, surgical correction of CAA remains one of the most challenging operations performed by otologists. This is a complex surgical problem requiring the application of all tympanoplasty techniques and a thorough knowledge of the surgical anatomy of the facial nerve, oval window, and middle and inner ear, and their congenital variants. ^{, , , , , , , ,}

CAA is a first branchial arch anomaly, with the temporomandibular joint (TMJ) displaced posteriorly by the lack of development of the EAC. This posterior displacement narrows the distance between the glenoid fossa and the anterior wall of the mastoid tip. ^, CAA is often accompanied by some underdevelopment of the mandible or even hemifacial microsomia. Patients with hemifacial microsomia generally have a more hypoplastic middle ear space and ossicles and are candidates for atresia surgery only approximately 50% of the time because of the underdeveloped middle ear. Fusion of the incus and malleus is universal, but because of its origin from the second branchial arch and otocyst, the stapes bone is usually normal. ^, The mastoid and middle ear are smaller in volume, whereas the facial nerve generally courses more anteriorly and laterally from the second genu into the mastoid segment.

Embryology

A review of the normal embryologic development of the ear is critical to understanding the possible malformations the surgeon may encounter during surgery for CAA. The inner, middle, and external ears develop independently; an abnormality of one does not equate to a deformity of another. ^, Most frequently, abnormalities of the outer and middle ear are encountered in combination with a normal inner ear. ^,

Microtia (“small ear”) results from first and second branchial arch anomalies that occur during the first 6 weeks of gestation. The growth of mesenchymal tissue from the first and second branchial arches forms six hillocks around the primitive meatus that fuse to form the auricle. By the end of the third month of gestation, the primitive auricle has developed. The external auditory meatus derives from the first branchial groove (cleft). During the second month of gestation, 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 canalize and take shape in the sixth month and is fully canalized in the seventh month, causing the developing mastoid to separate from the mandible. Its subsequent posterior and inferior development carries the middle ear and facial nerves to their normal positions. ^{, ,} Although the middle and outer ears develop independently, the severity of microtia may indicate the status of middle ear development in aural atresia; minor degrees of microtia are associated with milder forms of middle ear maldevelopment.

Endoderm from the first branchial pouch grows outward from the primitive foregut to form the eustachian tube and middle ear cleft. The plaque of tissue where the endoderm from the middle ear cleft meets the ectoderm of the EAC forms the tympanic membrane. While the first branchial pouch forms the eustachian tube, tympanic cavity, and mastoid air cells, Meckel’s cartilage (a first branchial arch component) forms the neck and head of the malleus and the body of the incus. Reichert’s cartilage (from the 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 branchial arch and the otic capsule. The ossicles attain their final shape by the fourth month of gestation. By the end of the seventh to eighth month, the expanding middle ear cleft surrounds the ossicles and covers them with mucous membrane. ^{, ,}

The facial nerve is a second branchial arch derivative, and at 4 to 5 weeks, this developing nerve divides the blastema, 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 intraosseous course of the nerve is dependent on this bony expansion. ^, The membranous portion of the inner ear develops during the third to sixth week from the otic placode on the lateral surface of the hindbrain. The surrounding mesenchyme transforms into the bony otic capsule.

CAA can range in severity from a thin membranous canal atresia to a solid core of tympanic bone, depending on the time of arrest during intrauterine development. ^{, ,} The inner ear is often normal in patients with EAC atresia (with or without microtia) because it derives from a completely separate embryologic anlage, the otic placode, and its development is completed before external and middle ear development. Nevertheless, one report found a 22% incidence of radiographic inner ear anomalies in patients with CAA, but interestingly, these were not generally associated with sensorineural hearing loss. Abnormalities in the course of the facial nerve, however, are quite common in the setting of CAA.

Classification Systems

Various classification systems for CAA have been proposed over the past 50 years to evaluate patient candidacy for surgery and to assist in surgical planning, patient counseling, and outcome comparison. ^, Bellucci proposed the classifying middle ear malformations into major and minor groups. The minor malformations include those with a patent EAC, an intact tympanic membrane, and some ossicular abnormality including the malleus bar, congenital stapes ankylosis, persistent stapedial artery, monopolar stapes, absent oval window, and malleus head fixation. The major malformations, the subject of this chapter, include those with an absent or severely stenotic ear canal accompanied by abnormalities of the middle ear and its structures.

Altmann, Chiossone, and Schuknecht also provided meaningful systems for examining patients with congenital atresia. ^{, ,} However, the most widely used anatomic system used to evaluate candidacy for surgery was proposed by Jahrsdoerfer et al., who developed a 10-point grading system to guide surgeons in preoperative assessment of the best candidates for hearing improvement ( Table 4.1 ). This system assigns a point value to each of the following: the radiographic appearance of the mastoid and middle ear and their degree of pneumatization, presence of the oval and round windows, facial nerve course, status of each of the ossicles (with the stapes accounting for two points), degree of middle ear aeration, and appearance of the auricle. The point allocation is qualitative but is based primarily on high-resolution computed tomography (CT) findings. Jahrsdoerfer and colleagues 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 that the patient is a fair candidate; a score of 6, a marginal candidate; and a score <5, a poor candidate. Other reports have validated the predictive ability of the Jahrsdoerfer grading system and have attempted to refine and provide more quantitative data, such as middle ear volume and incudostapedial joint angle for patient selection. ^{, ,}

The De la Cruz classification system considers mastoid pneumatization, inner ear anatomy, and the facial nerve and oval window-footplate relationship on high-resolution CT imaging. The malformations are divided into minor and major ( Table 4.2 ). The clinical importance of this classification is that surgery in patients with minor malformations has a good possibility of yielding serviceable hearing, whereas patients with major malformations are frequently inoperable but are treatable with an osseointegrated bone-conduction system such as BAHA (Cochlear Corporation) or Ponto (Oticon Medical).

Initial Evaluation and Patient Selection

CAA is most commonly diagnosed in the newborn period because of its association with microtia. Nevertheless, children with very mild microtia or aural stenosis and conductive hearing loss (CHL) may escape the initial diagnosis during a physical examination. Generally, these children are discovered after failing a newborn hearing screening or a hearing screening in the pediatrician’s office or at school. After the degree of microtia and presence (or caliber) of the ear canal are assessed, more comprehensive evaluation of auditory function should be performed using air and bone-conducted auditory brainstem response (ABR) testing within the first few days to months of life.

As noted, most children with CAA have normal inner ear function. ABR testing of both ears is important to ensure that the nonatretic ear functions normally; total sensorineural hearing loss (SNHL) on the side of the normal-appearing ear has been reported.

In patients with bilateral CAA and normal bone-conduction thresholds, a bone-conduction hearing device should be applied as soon as possible, ideally within the first 3 to 6 months of life. In patients with unilateral CAA, the decision to place a bone conductor is an individual decision made by the family and the hearing healthcare team. As long as the hearing in the nonatretic ear is normal, and there are no other cognitive or developmental issues, the child with unilateral CAA should acquire normal receptive and expressive language.

The clinician evaluating the child with CAA should search for other associated abnormalities such as spine, kidney, or other branchial anomalies. The clinician should also recognize associated cephalic abnormalities (e.g., hemifacial microsomia or Goldenhar syndrome, Treacher Collins syndrome, Crouzon syndrome, or Pierre Robin sequence). In this subset of patients, surgical candidacy for atresia repair tends to be marginal because of poor aeration of both the middle ear and mastoid, more severe hypoplasia of the ossicular chain, and an anomalous course of the facial nerve. In patients with bilateral CAA who are poor candidates for atresia repair, a long-term bone-conduction hearing device or an osseointegrated bone-anchored hearing device are viable solutions (see Chapter 33 ).

Prompt and detailed counseling of the parents of a child with sporadic (nonsyndromic) CAA is necessary to alleviate concerns regarding the possible occurrence in subsequent children (which is no higher than that of the general population), to answer questions regarding future auricular reconstruction (i.e., microtia and atresia repair surgeries), and, most importantly, to ensure that proper hearing habilitation is instituted in a timely fashion. Parents should consider enrolling their child in speech therapy at an early age to optimize speech and language acquisition in preparation for “mainstreaming” at preschool age. Individual education programs (IEPs; 504[c] plans) are beneficial in the elementary school years. CAA surgery is not recommended until the child is 5 to 6 years old, so obtaining a CT scan at a younger age is unnecessary and a second CT scan may still be required later.

In the initial evaluation of the older patient (>5 years) with CAA, the most crucial elements remain the functional and anatomical integrity of the inner ear as documented by bone-conduction audiometry, and the middle and inner ear anatomy as viewed on high-resolution thin-section (≤1 mm) axial and coronal CT scans. The prognosis for hearing improvement with surgery is correlated with the degree of malformation and other anatomical structures, including the middle ear volume and incudostapedial joint angle. ^{, ,}

Timing of Surgical Repair

The timing of atresia surgery depends very much on the family’s decision for auricular reconstruction. Regardless of microtia repair, atresia repair is not recommended until the child is 5 to 6 years old. Before this age, there may be a tendency to scar and stenose the new canal, or form exostosis-like bony growths that may occlude the EAC. The mastoid and middle ear are better pneumatized in older children. In addition, younger children tend to be less cooperative in the office for the important postoperative care of packing removal and debridement. Failure to remove packing or clean the ear well could jeopardize an otherwise successful surgery. Finally, younger children tend to be more prone to the vagaries of the eustachian tube, and waiting until the child is 5 to 6 years old allows for the maturation of eustachian tube function with less risk of middle ear effusion.

The options for microtia repair, other than observation, include a prosthetic ear applied either with medical tape or osseointegrated titanium implants (e.g., Vistafix, Cochlear Corp.), Medpor porous polyethylene implant (Stryker Corp.), or autologous rib cartilage. Each method has its own risks, benefits, and cosmetic outcomes, which are beyond the scope of this chapter. For the purposes of atresia repair, however, the Medpor implant can be placed after atresia surgery (this method has been reported by one group), whereas the rib cartilage microtia reconstruction is placed before atresia surgery. Rib graft microtia repair is recommended at age 5 to 6 years, when the costal cartilage has developed sufficiently to allow for the reconstruction of the auricle.

If the family chooses the osseointegrated bone-conduction device for hearing habilitation (see Chapter 33 ), the otologic surgeon must communicate with the microtia surgeon regarding the timing and, more importantly, the placement of the device so that it does not interfere with future microtia reconstruction. The placement must allow for the possibility of transplanting costal cartilage or implanting the Medpor framework to an area with unscarred tissue. Many times the device must go a bit more posterior than if placed with a normal auricle. If positioned posteriorly, the implant can be placed at any time (after age 5, per Food and Drug Administration [FDA] regulations) regardless of the microtia repair. Most microtia surgeons, however, prefer to place the framework (cartilage or polyethylene) before implanting the bone-conduction device.

Rib graft auricular reconstruction must be performed before atresia surgery to avoid interfering with the critical blood supply to the surrounding soft tissue. However, Medpor reconstruction should be performed after atresia surgery because the atresia operation risks the exposure of the implant; an exposed Medpor implant will not heal because of the lack of blood supply to the implant and the attendant risk of infection or extrusion.

The child with a congenital ear canal cholesteatoma in the setting of congenital aural stenosis (CAS) demands more immediate surgical intervention. Such patients may present with an infected or draining ear or acute facial paresis. The estimated prevalence of cholesteatoma in CAS ranges from 19% to 48%, with females more likely than males to harbor cholesteatoma, and more likely in the left ear. ^, Children with hemifacial microsomia (Goldenhar syndrome) or Treacher Collins syndrome tend not to acquire these cholesteatomas. The diagnosis is suspected based on high-resolution CT imaging.

Infection, otorrhea, or otalgia in any child with CAS, regardless of age, should prompt urgent high-resolution CT imaging. In the child with ear canal cholesteatoma, imaging will reveal a rounded density lateral to the atretic plate with smooth, bony remodeling ( Fig. 4.1 ). The priority in these patients is the removal of the cholesteatoma and all epithelial elements and the resolution of the infection. With middle ear involvement, the operation should be staged for the purposes of hearing improvement.

Preoperative Evaluation and Patient Counseling ( Box 4.1 )

The preoperative evaluation of the patient desiring atresiaplasty rests on audiometric and radiographic data. Audiometrically, the patient must have normal bone-conduction thresholds in the atretic ear, indicating normal inner ear function. In patients with bilateral CAA, the masking dilemma can be overcome, and bone-conduction thresholds for each individual ear obtained through sensorineural acuity level (SAL) testing. Obtaining more reliable and accurate preoperative audiometric thresholds, especially in patients with bilateral CAA, is another reason to wait until the child is 5 to 6 years old to recommend atresia surgery.

Because of this age recommendation, CT imaging is also not recommended until the child is 5 to 6 years old. CT imaging in younger children delivers a higher total body radiation dose and increases the estimated lifetime attributable cancer mortality risk. CT imaging in younger children may also show middle ear fluid that the child will eventually outgrow. Opacification of the middle ear is a contraindication to atresia surgery, and the scan will have to be repeated to demonstrate a clear middle ear space before surgery. Finally, the middle ear and mastoid are still developing in younger children, and waiting until the child is aged 5 or 6 to image the ear will capture a more developed middle ear and mastoid.

Thin-section (≤1 mm), high-resolution CT imaging in the axial and coronal planes must demonstrate favorable anatomy ( Fig. 4.2 ). The Jahrsdoerfer classification system (see Table 4.1 ) can prognosticate hearing results 1 month after surgery: patients who score 7 or higher have an 80% to 90% chance of achieving normal or near-normal hearing (speech reception threshold [SRT] ≤30 dB HL); patients with scores below 7 have only a 40% to 50% chance of achieving normal or near-normal hearing. The middle ear volume was identified as the single best predictor of hearing outcome in this and other studies on middle ear anatomy and hearing outcomes. ^,

Alternatively, the otologic surgeon may inspect the four most important imaging elements helpful in planning reconstruction of a congenitally malformed ear: (1) the degree of pneumatization of the temporal bone; (2) the course of the facial nerve, including the relationship of the horizontal portion to the footplate and the location of the vertical segment; (3) the presence of the oval window and stapes footplate; and (4) the status of the inner ear. ^{, ,} CT also provides information on the thickness and form of the atretic bone, size and status of the middle ear cavity, distance between the atretic bone and ossicles, attachment of the malleus neck to the atretic plate, and soft tissue contribution to the atretic ear canal, all of which are important factors for surgical planning.

The lack of pneumatization of the middle ear space is a major cause of inoperability in CAA. ^, In addition, the obstruction of the oval window by an overhanging facial nerve will prevent ossiculoplasty and hearing improvement. If the oval window or footplate is absent, surgery is not recommended because the long-term hearing outcomes of the oval window drill-out procedure are marginal with significantly higher risk. In such patients, the osseointegrated bone-conduction system is an extremely useful alternative.

Patients and surgeons alike must understand the risk of facial nerve injury in CAA surgery. This risk stems from the more acute angle the nerve takes at the mastoid genu compared with its typical 90- to 120-degree takeoff angle. The nerve also often lies more lateral and anterior than usual ( Fig. 4.3 ). On high-resolution CT, it is important not to mistakenly “identify” the vertical course of the facial nerve in the marrow bone leading to the styloid process and the hypoplastic mastoid process ( Fig. 4.4 ). Even in atretic ears in which the facial nerve does not have an abnormal course, a significantly reduced distance is found between the facial canal and the TMJ, and the facial canal and the posterior wall of the new canal. ^, Nevertheless, in experienced hands, facial nerve injury in atresia surgery is low, especially with intraoperative monitoring, and has been estimated at less than 1%.

The goal of atresia surgery is the creation of a clean, dry, skin-lined ear canal and eardrum with normal to near-normal hearing. The patient and parents are counseled regarding the success of atresiaplasty repair and the chances of successful hearing improvement based on the anatomy noted previously. Patients are informed that a split-thickness skin graft (STSG) from the upper arm (or lateral thigh) is needed to line the new bony EAC. Initially, frequent postoperative visits are necessary for cleaning the desquamated epithelium from the skin graft.

The complications of facial nerve paresis or paralysis (<1%), SNHL (approximately 5% to 10%), postoperative canal stenosis (10% to 15%), loss of the skin graft with resultant mucosalization of the ear canal and moisture or drainage (10% to 15%), and loss of early hearing gains secondary to tympanic membrane graft lateralization or refixation of the ossicular chain (10% to 15%) are discussed with the family ( Box 4.2 ).

Surgical Technique ( Box 4.3 )

The keys to successful atresia repair are the careful selection of patients for surgery and meticulous technique and attention to detail at each step. The operation is made more difficult by fatigue and poor ergonomics, which may cause back pain or discomfort.

Early atresiaplasty operations failed because the surgeon opened the mastoid and used the antrum and horizontal semicircular canal as landmarks of the middle ear. This posterior technique often led to draining ears and wet cavities. Poor tympanoplasty and skin grafting techniques also contributed. Jahrsdoerfer pioneered the anterior approach—using the tegmen and bone overlying the TMJ to follow the most direct path into the epitympanum of the middle ear and opening as few mastoid air cells as possible—the preferred technique today. An important concept is that while the human ear canal is centered on the mesotympanum with the tympanic membrane attached to the umbo and manubrium of the malleus, the reconstructed neo-canal is centered on the epitympanum with the tympanic membrane draped over the fused head of the malleus-body of the incus complex.