12 Evaluation and Treatment of Malformations of the Ear
Atresia of the ear canal occurs in approximately 1 in 10,000 to 20,000 births. It may occur in isolation or be associated with microtia or various craniofacial dysplasias. Genetic transmission is rare in cases of isolated atresia, but may occur with various craniofacial syndromes. In approximately one-third of cases, aural atresia is bilateral.1
The evaluation of aural atresia involves both hearing and radiographic assessments. The immediate evaluation should center on the need for amplification, followed by a long-term rehabilitation strategy. Determining surgical candidacy and the operative procedure itself present unique challenges to the otologic surgeon. This chapter reviews concepts and protocols necessary to meet these challenges successfully.
Embryology
Development of the first pharyngeal pouch, the first and second branchial arches, the first branchial cleft, and the otic capsule must be understood to fully appreciate the problem of congenital aural atresia.
External Auditory Canal
The external ear canal is derived from the first branchial groove (cleft), between the mandibular and hyoid arches. A solid core of epithelial cells initially exists from the exterior down to the area of the tympanic ring and first pharyngeal pouch. During the middle trimester of fetal life, this epithelial core begins to canalize, progressing in a medial to lateral direction. If this process is arrested prematurely, a rudimentary tympanic membrane and small bony canal with a very stenotic membranous canal may develop. This situation predisposes to canal cholesteatoma formation as the desquamated squamous epithelium from the canal becomes trapped.
The medial portion of the external canal is formed by the tympanic bone, which begins to ossify in the 3rd embryonic month. Disruption of this process produces atretic bone at the level of the tympanic membrane, and results in atresia of the ear canal.2
Mastoid and Middle Ear
The eustachian tube, middle ear, and mastoid air cells are derived from the first pharyngeal pouch. Most patients with aural atresia have a smaller than normal middle ear and mastoid air cells system. A well-pneumatized mastoid air-cell system usually indicates good middle ear development, including ossicular formation. The relationship between the degree of microtia and middle ear development is disputed.3
The first and second branchial arches are responsible for forming the ossicles except for the medial or vestibular portion of the stapes footplate, which is derived from the otic capsule. The external ear canal and tympanic membrane develop from the first branchial cleft. Branchial arch (ossicular) and branchial cleft (external ear canal) deformities usually occur in combination.2
The membranous labyrinth is derived from the ectodermal otocyst and in isolated cases of aural atresia, sensorineural and vestibular functions are normal. Mild sensorineural hearing loss can occur in cases of aural atresia associated with other craniofacial deformities such as CHARGE, branchio-oto-renal syndrome, and Treacher-Collins syndrome.3 Within the broader context of craniofacial abnormalities, inner ear abnormalities have been noted by computed tomography (CT) in approximately one-fifth of patients. Most of these involve some dysplasia of the horizontal semicircular canal and/or vestibule.4 Absence or fixation of the stapes footplate in aural atresia is uncommon. This may result in part because the vestibular portion of the footplate is derived from the normal otic capsule. The rare circumstance of an absent stapes footplate and oval window may be related to abnormal development of the facial nerve.5–7
Facial nerve abnormalities are common in cases of aural atresia.8,9 Bony dehiscence of the tympanic segment of the facial nerve and a more anteriolateraly located mastoid segment of the facial nerve are the most common abnormalities noted. The typical obtuse angle of the facial nerve second genu becomes more acute and frequently courses over the round window, obscuring that structure. The distal portion of the mastoid segment of the facial nerve may exit the skull more superficially than normal, and can be vulnerable to a stretch injury from retractors. An association between preoperative facial nerve dysfunction and inner ear abnormalities on CT has been noted.4 A correlation between the degree of microtia and the extent of facial nerve abnormalities has also been observed.3,10
Classification
Classification of auricular deformities is straightforward.11 Grade 1 microtia represents a slight malformation with the auricle smaller than normal, but all parts discernable; grade 2 microtia denotes a severely deformed auricle consisting of a curving or vertical ridge of tissue; grade 3 microtia is defined by a rudimentary soft tissue structure without resemblance to an auricle; and grade 4 microtia represents complete absence of the auricle.
Classification of the external canal/middle ear deformities has been more problematic. The clinical examination, radiographic findings, surgical observations, or even histological studies have been used as classification parameters. Simply dividing these deformities into major and minor malformations has been proposed.12 This classification scheme is attractive because of its simplicity and clinical utility. The following descriptions reflect this classification approach. It should be emphasized that the main difference between the major and minor malformations relate to the status of the ear canal.
Major Malformation
The classic major malformation is defined by an absent external ear canal and tympanic membrane. Cases of severe canal stenosis may also be included in this group. The middle ear space is usually reduced in size, and the malleus and incus are both deformed and fused. The rudimentary malleus handle is typically fixed to the atretic bone. In severe cases, the middle ear space is a very hypoplastic or absent, and the ossicles are rudimentary and poorly discernable by CT scan. Dehiscence and displacement of the facial nerve can be expected in most major malformations. Deformity of the pinna (grades II to IV) is expected.
Minor Malformation
The significant finding in the minor malformation group is a conductive hearing loss. The pinna may be normal or only slightly misshapen. The external ear canal often shows some degree of stenosis, but may be normal in caliber. Abnormalities of the stapes and facial nerve can be severe and less predictable than in the major malformation group.
Patient Evaluation
Most cases of aural atresia are evident at birth because of microtia or other craniofacial anomalies. A severe stenosis or blinding ending external ear canal in patients with a normal pinna, however, may escape diagnosis for years. In unilateral minor congenital ear malformations, the hearing loss may be discovered only on routine hearing screening in school, given the relatively normal pinna, external ear canal, and tympanic membrane.
The initial evaluation of the child with aural atresia must focus on both the functional and cosmetic aspects of this condition. Initially, overall hearing status and the need for amplification must be addressed. Consultation with other specialists, such as geneticists, microtia surgeons, and developmental pediatrics should also be undertaken.
Physical Examination
The initial examination focuses on overall craniofacial development and other areas of the body that may be associated with various syndromes. The mandible should be carefully inspected for hemifacial microsomia. Any other asymmetries of the face are noted. The degree of microtia and mastoid development are recorded. The caliber of the external ear canal is graded as normal, stenotic (mild or severe), blindly ending, or completely atretic. If observable, the mobility of the tympanic membrane is determined. Any cervical vertebral abnormalities are noted as are anomalies of the digits or limbs.
Achievement of neurologic milestones including speech and ambulation are assessed. A careful examination of facial function, including each division of the facial nerve, is conducted. An isolated lower face or lip paresis can occasionally be seen and noting this preoperatively is important. The most common anomaly of facial function is a congenital absence of the depressor anguli oris muscle.
Audiometric Evaluation
With unilateral atresia behavioral audiometry can be used in most patients. In very young infants or children, auditory brainstem response (ABR) testing may be necessary. Patients with bilateral atresia present more of a challenge because of the masking dilemma. As noted in the embryologic discussion, it is usual for sensorineural function to be normal or close to normal, although it is unsafe to assume this. Bone conduction ABR testing can provide objective data.13 With this testing, a bone conduction stimulus is provided and recording electrodes are placed around both ears. Wave I of the ABR is measured. Because there is minimal crossover of this small potential to the contralateral side, the surface electrodes are recording primarily the response from the underlying cochlea. Although the bone conduction signal is simultaneously stimulating both ears, the wave I responses being recorded are ear specific, thus providing a differential assessment of cochlear function.
In bilateral minor malformation cases with patent ear canals, electrocochleography will provide ear specific information. A deep canal recording electrode or transtympanic electrode can be used to record a robust wave I with the bone stimulus.
Computed Tomography
CT scans in both the axial and coronial projections should be performed at about the age of 4 or 5 years. Scanning infants does not change the immediate rehabilitative plans and is therefore not recommended. In addition, those studies may require anesthesia or be suboptimal in quality.
A CT scan is also recommended for patients with severe canal stenosis or minor ear malformations with significant conductive hearing loss. As noted previously, cholesteatoma can form in stenotic ear canals, but usually not before the age of 8 to 10 years.14 In the minor malformation ears concurrent abnormalities of the facial nerve often occur, and preoperative imaging is thus recommended.
Medical Rehabilitation
Bilateral Atresia
Amplification within the first 3 to 5 months of life should be the goal in infants with bilateral atresia. A traditional bone conduction hearing aid or soft band bone-anchored hearing aid (BAHA) is both appropriate. In patients with bilateral involvement the “best” (as determined by CT evaluation) ear should be selected for the initial surgical procedure.
Unilateral Atresia
Assuming normal sensorineural function by ABR and otoacoustic emission testing can be confirmed in the contralateral ear, no immediate intervention is required. These children will develop speech and language skills normally. Preferential seating in school and even an frequency modulated system should be considered. Most children are reluctant, however, to wear a hearing aid. Adults usually find the consequences of a unilateral hearing loss to be more disruptive in social and work situations, and are more receptive to the use of a hearing aid.
Surgical Management
Approximately 50% of carefully selected patients can be expected to have a hearing threshold of 25 dB or better after surgical intervention. As with any conductive hearing impairment, the option of a hearing aid (in this case BAHA) should be discussed with the patient. The probability that a reconstructed ear canal will require cleaning once or twice a year should also be discussed with the patient.
In general, there is more acceptance of surgery in bilateral cases of aural atresia. In properly selected patients with unilateral atresia, I and others contend that the benefits of binaural hearing and the possibility of achieving that with surgery provide a basis for offering corrective surgery.10,15
Selection Criteria
Normal or near normal sensorineural function in both ears should be present. Most patients after atresia surgery will have a residual conductive hearing loss of at least 10 dB. This loss superimposed on even a mild sensorineural deficit would make it difficult to eliminate the handicap of unilateral hearing loss or the need for hearing aids in bilateral cases.
Middle ear development as defined by the CT is critical for proper patient selection. Some degree of hypoplasia is expected, but the middle ear should at least be two-thirds of normal size, and all three ossicles, although deformed, should be readily identifiable (Fig. 12.1A to D). The position of the facial nerve should be examined closely. Although some anterior displacement of the mastoid segment is expected, a more severe anterior location will restrict access to the middle ear, diminishing the chance for a successful hearing outcome and increasing the risk of facial nerve injury.
A quantitative system for defining middle ear development has been proposed by Jahrsdoerfer (Table 12.1).16
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