Disclosure Congenital aural atresia (CAA) is a challenging problem encountered by the otologist. In CAA, the external ear canal fails to completely develop. Development of the ear canal may be halted anywhere in the process. Therefore, the clinician may encounter varying degrees of severity of this malformation. In the severe form of the disorder, no identifiable ear canal exists. If a semblance of an external auditory meatus is present, the ear canal may end in a shallow blind pouch. In less severe forms of the disorder, the ear canal may be stenotic with a pinpoint aperture leading into the medial ear canal. The condition is commonly accompanied by microtia, or incomplete development of the auricle, which is also a surgical reconstructive challenge. The ear canal and auricle assume separate developmental and embryologic origins. The current understanding and management of CAA is the result of 3 events. First, detailed anatomic studies of the temporal bone have improved our understanding of the condition. With embryologic studies, the sequence of development of the different regions of the ear (ie, external ear, middle ear, and inner ear) is better understood than it was before. Findings from these studies helped explain why some regions of the ear are more developed than others in patients with CAA and helped increase our appreciation for the anatomy found in CAA. Second, the advent of CT scanning furthered the ability to treat CAA. This single imaging modality allows the otologist to have a good picture of the anatomy and of what nature has left behind for the surgeon to work with. Before CT scanning, plain radiography and tomography were available, and surgeons could only surmise what the anatomy looked like inside and not predict a surgical outcome. Third, individuals such as Drs Jahrsdoerfer, De la Cruz, and Lambert, have obtained and shared their vast experience in managing CAA and have provided their insight into CAA with respect to surgical candidacy, hearing restoration, surgical correction, and management of complications. In CAA, the role of the otologist extends beyond diagnosis and treatment. The otologist must exercise sound decision making when determining the appropriate treatment and time course of management. This decision making usually occurs in the setting of anxious parents seeking an answer to their questions on how this deformity came about, what can be done to restore their child's hearing and appearance, and how soon treatment can be administered so that their child may return to their normal condition. Not all patients with CAA are candidates for surgical correction. Approximately 40% of patients are not surgical candidates because of the existing anatomy. Nevertheless, an evaluation of the hearing in both the affected and unaffected ears must be performed. Amplification must be provided as soon as possible when necessary. Providing early auditory stimulation is essential for speech and language development. For surgical candidates with favorable anatomy, the otologist must borrow the skills, techniques, and experience from middle ear and mastoid surgery to optimize the surgical result. Surgical success is based on restoration of useful hearing, long-term stability of hearing, and maintenance of a patent ear canal. Otologists agree that, when these goals are achieved, few accomplishments are as gratifying as successfully treating a patient with CAA. Problem: The main anatomic deformity in CAA is failure of the external ear canal to complete development. The severity of the deformity is variable. The anatomy of the ear depends on the point along the continuum where development is halted. In the most severe form of CAA (ie, when development of the ear is halted early), the ear canal cannot be identified. Bone fills the region the ear canal normally occupies, and no external portal is present to reach the middle ear and the sound-conducting mechanism to the inner ear. In mild cases, the ear canal is present, but it is stenotic and markedly narrowed. In these cases, the ear has developed more than in other cases. Frequency: Incidence of CAA is 1 case in 10,000-20,000 live births. A unilateral presentation of CAA is more commonly than a bilateral presentation, with an approximate ratio of 3:1. Etiology: Factors that cause the developmental sequence of the ear to cease in CAA are not known. Regions adjacent to the ear canal may be affected as well. In general, the inner ear is not affected because its development is complete by the time the ear canal begins to form. However, the middle ear, the ossicles, and the auricle may be affected because their development is concurrent with that of the ear canal. Pathophysiology: Intimate knowledge of the development of the ear is essential to understand the anatomic and clinical manifestations of CAA. The development of the ear consists of a complex series of events. The inner ear, middle ear cleft, ossicles, ear canal, and auricle each are derived from the 3 embryonic germ layers (ie, ectoderm, mesoderm, endoderm). These structures assume individual paths of growth and maturation. Embryology of the ear The formation of the ear canal begins with the invagination of the first branchial groove (ie, primary meatus). This area is located between the first branchial arch rostrally and the second branchial arch caudally. The branchial groove invaginates and advances medially as an epithelial plate as early as the second month of fetal life. Its ingrowth temporarily meets the lateral growth of the first pharyngeal pouch. The first pharyngeal pouch is derived from endoderm and subsequently develops into the middle ear cleft. The union between the ingrowth of the first branchial groove and first pharyngeal pouch forms the meatal plate, which is the precursor to the tympanic membrane. At 6 months of fetal life, the epithelial plate begins to canalize from a medial to lateral direction to meet the primary meatus. At birth, the ear canal comprises the bony tympanic ring medially and a membranous cartilaginous portion laterally. Postnatally, the bony tympanic ring lengthens and transforms from a ring into a bony cylinder. Thus, the ear canal increases in length and reaches adult proportions by the age of 4-5 years. In CAA, development of the ear may be interrupted at any point. If the process is halted before canalization of the ear canal, total atresia occurs. If development is halted during canalization, the patency of the external ear canal varies depending on how far canalization has progressed. The inner ear begins development as early as the third week of fetal life with the formation of the otic placode, which is a local thickening of the ectoderm. The otic placode invaginates to form the otic pit. The epithelium of the otic pit fuses to become the otic vesicle, which forms the membranous labyrinth of the inner ear. A series of infoldings of the otic vesicle compartmentalizes the membranous labyrinth into the vestibule, cochlea, and endolymphatic regions. The inner ear completes its development by the 20th week of fetal life, which predates the formation of the ear canal and explains why patients with CAA generally have a cochlea that is functional and able to be stimulated. The middle ear cleft develops from lateral growth of the first pharyngeal pouch. The middle ear cleft eventually envelops the ossicles and incorporates them into the middle ear space. The ossicles are derived from both first and second branchial arches, specifically Meckel and Reichert cartilages, respectively. Arrest in the development of the ear canal affects the middle ear to the extent that the growth and maturation of the middle ear is incomplete and the middle ear space is contracted. The ossicular chain is deformed, typically featuring a fused malleus-incus complex, shortened incus and malleus, and hypoplastic stapes superstructure. The malleus neck may be fused to the bony atretic plate. The tympanic membrane may not be present. CAA commonly coexists with microtia. However, CAA may occur alone with a normal appearing auricle. The precursors of the auricle are the axonal hillocks, which fuse with each other around the ear canal. These axonal hillocks are derivatives of the first and second branchial arches. Each of the 6 hillocks is responsible for forming a distinctive part of the auricle. Hillock 1 forms the tragus; hillock 2 forms the crus helicis; hillock 3 forms the helix; hillock 4 forms the antihelix; hillock 5 forms the antitragus; and hillock 6 forms the ear lobule. The auricle assumes adult shape by the 20th week of fetal life. Microtia results when these hillocks do not fuse and continue to develop. In general, the middle ear is less developed when microtia is severe than when it is not. Clinical: The patient with CAA may present with a unilateral or bilateral problem. If the deformity is present in both ears, the severity of the deformity may vary between the ears. Colman graded the severity of CAA into 3 categories: minor, moderate, and severe. Patients with severe CAA have total canal atresia and usually present with unfavorable middle ear anatomy and temporal bone development for reconstruction. In comparison, patients with moderate CAA have more favorable anatomy (ie, usually an identifiable ear canal with deformed ossicles). Most patients with CAA present with this moderate form. In minor cases, the ear canal is present but narrowed, and the middle ear is better developed. CAA may coexist with syndromes that feature first and second branchial arch deformities (eg, Treacher-Collins syndrome, hemifacial microsomia, Goldenhar syndrome, other craniofacial abnormalities). The patient with CAA may present to the otologist in any of the scenarios discussed next. Detection at birth If the anatomic defect is detected at birth, the otologist meets the newborn and parents in consultation. The parents are usually anxious and eager to have their many questions answered. Reassure and educate the parents. Regardless of whether this condition is present in 1 or both ears, cochlear function must be assessed in both ears by means of specialized audiologic testing, such as auditory-evoked brainstem responses elicited from both air and bone conducted signals. If bilateral CAA and good cochlear function are present, the infant needs amplification with a bone-conducting hearing aid until a decision is made to pursue surgical correction just before the child starts school. Detection in the older child just before microtia repair The otologist may encounter the older child with CAA just before a plastic surgeon repairs microtia. In these patients, coordinate efforts and maintain open channels of communication between the surgeons and parents. Delineate a clear plan and schedule for the parents to prevent confusion and unrealistic expectations. Several surgeries are required to repair microtia and CAA. The plastic surgeon is responsible for creating an auricle by means of a multistage procedure, and the otologist is responsible for creating a new ear canal in continuity with a tympanic membrane and mobile and contiguous ossicular chain. Clinical picture with canal stenosis A child with canal stenosis (mild atresia) may present to the clinician. Be aware that these patients with narrowed canals are at risk of canal cholesteatoma. Use a combination of diligent follow-up care and microscopic examination to ensure the patient does not develop a canal cholesteatoma. However, when the opening to the ear canal is too small to permit examination and when the patient's history (eg, otorrhea, otalgia) suggests a cholesteatoma, radiographic imaging with CT scanning is used to ensure that a canal cholesteatoma is absent. Children with canals 2 mm or smaller in diameter develop canal cholesteatoma more frequently than do other children. In addition, canal cholesteatomas are rarely, if ever, observed to occur in children younger than 3 years. Cole and Jahrsdoerfer reported these findings and noted that 50% of 54 stenotic ears with canals smaller than 4 mm developed canal cholesteatoma. Clinicians should readily incorporate CT in their diagnostic workup to ensure that no canal cholesteatoma, medial to the stenosis, is present. Clinical picture in the adult with CAA Adults with CAA who present to the otologist usually have a unilateral condition that was not corrected in childhood. In some patients, the condition was unnoticed, especially in those with stenotic ears. If it was noticed, the patients may have been told that their condition was irreparable or too risky to repair in light of possible facial nerve injury, which may have been the impression at the time. This is especially true if the contralateral ear was morphologically and functionally normal. In this situation, the patient may make an informed decision to proceed with surgery with the hopes of obtaining binaural hearing.
Before surgical correction is considered, the patient must fulfill 2 criteria. First, the patient with CAA must have normal cochlear function, as demonstrated on audiologic examination. Second, CT must demonstrate normal inner ear and internal auditory canal morphology. If patients meet these criteria, the next step is to identify patients with favorable anatomy who have a reasonable chance of success with surgical correction. Jahrsdoerfer devised a 10-point grading scheme, based on features from the CT scan and appearance of the external ear, that reflects the likelihood of success (see Imaging Studies). Patients who have a score of 7 or higher are good candidates for surgery.
Relevant Anatomy: Critical assessment of the anatomy is necessary in the evaluation and treatment of the patient with CAA. Even if surgical correction is not sought, the otologist must be able to examine and assess the CT scans, understand the anatomy, and explain the nature of the deformity to the parents or patient. Each case of CAA is different. The relevant anatomy includes the patency of the ear canal. In severe cases of CAA, an atretic plate is present, and no ear canal is identified. The atretic plate is the bony region just lateral to the middle ear space. This bony region forms the tympanic ring. In other cases, the tympanic bone is present, and its development should be evaluated. Assessment then should be made more medially to look at the middle ear space. The otologist should have an idea about the size of the middle ear cavity and the appearance of the ossicular chain. The shape, size, and continuity of the ossicles may be deduced from the CT scans. Evaluate the relationship between the temporomandibular joint and the middle ear space to determine if a new ear canal could be created and positioned appropriately. Evaluate the morphology of the membranous labyrinth to ensure no concurrent inner ear dysplasias exist. Check patency of the labyrinthine windows. Follow the course of the facial nerve through all of its segments. In cases of CAA, the facial nerve, if aberrant, typically courses more anterior and lateral in its proximal mastoid segment just after the second genu. In addition, the second genu takes a more acute bend than normal. Contraindications: An absolute contraindication to surgical correction of CAA is lack of cochlear function in the involved ear as evidenced by repeated audiologic examination. CT scan may reveal a dysplastic or aplastic inner ear. Surgical correction of CAA is pointless if the patient is unable to hear. However, if microtia is present, reconstruction of the auricle may be undertaken for cosmetic purposes. |
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Imaging Studies:
Medical therapy: Alternatives to surgical correction include amplification devices, such as a bone-conducting hearing aid or bone-anchored hearing apparatus/BAHA (Entific Medical Systems). Conventional hearing aids in patients with canal stenosis may be used. Experience using the BAHA device is considerable, especially in Europe. This device features a surgically-implanted, percutaneous titanium screw fixture that osseointegrates into the temporal bone. A sound transducer attaches to this titanium fixture and delivers the sound energy directly to the cochlea via bone conduction. Therefore, one does not need an ear canal or middle ear to hear. This type of technology is more efficient than the bone-conducting hearing aids because the sound energy is not attenuated by the skin and intervening soft tissues as it is with the traditional bone-conductors. SRTs less than 30 dB are obtainable with the use of these devices. Bilateral BAHA devices have even been used for patients with bilateral atresia. The situation may impart better sound localization ability. Furthermore, the use of the BAHA device does not preclude future reconstructive surgery. These alternatives may be considered in patients with unilateral or bilateral CAA who do not desire surgical correction or in whom the existing anatomy is not favorable (a score of 6 or less on the Jahrsdoerfer CT grading scale) but the cochlea is still functional and can be stimulated. Surgical therapy: Perform surgery if the patient is deemed a good candidate from radiologic (see Imaging Studies) and audiologic examinations. Surgery for correction of CAA is not necessary in patients with unilateral cases as long as normal hearing is present in the other ear. Surgery may be performed at any time. Surgery to correct CAA is usually performed when the child undergoes the multistage reconstructive surgery for microtia at the age of 6-7 years. This surgery is typically performed after the cartilaginous auricular framework is placed and the ear lobule is created and before the creation of the tragus and auricle elevation. Corrective surgery may be delayed until adulthood, when the patient can make an informed decision. Weigh the benefits of binaural hearing and improved sound localization against the potential complications of the surgery. Perform surgery in patients with bilateral cases with the goal to restore useful hearing, unaided or aided. Surgery should take place just before the child enters school, usually when aged 4-5 years. For surgery, choose the ear that appears to be morphologically and functionally best. Preoperative details: Perform a preoperative high-resolution temporal bone CT in all patients undergoing surgery. Prepare the appropriate area of the patient's body for obtaining a split-thickness skin graft to be used to line the neocanal. The skin on the inner surface of the upper arm is the usual donor site. Facial nerve monitoring is recommended. Consider monitoring of the facial nerve, particularly when revision is involved or when the preoperative CT shows an aberrant course of the facial nerve. In preoperative surgical counseling to the parents and patient, several points should be addressed. First, the estimated duration of surgery is 4-6 hours, and an overnight hospital stay is required. Second, most of the difficulty with surgical correction of the deformity lies in maintaining a patent ear canal. Therefore, a variety of ways to stent the ear canal may be needed in the postoperative period. Last, diligent follow-up in the office is needed after surgery. Intraoperative details: Begin surgery with a postauricular incision. Take care not to expose or disrupt the previously implanted cartilaginous auricular framework. Harvest temporalis fascia for tympanic membrane grafting. Expose the mastoid cortex by elevating the musculoperiosteum and by retracting it forward. Exposure of the mastoid cortex often reveals a depression in the bone that represents the tympanic bone remnant. This depression approximates the location of the meatus of the bony canal. Elevate the soft tissue anteriorly until the posterior aspect of the temporomandibular joint is encountered but not entered. Commence drilling by enlarging the tympanic bone remnant circumferentially and medially. The superior limit of the bone dissection is the middle fossa dura of the tegmen tympani. The anterior limit of bone dissection is the glenoid fossa of the temporomandibular joint. Take care not to enter the joint because it may prolapse into the ear canal and cause postoperative stenosis. The facial nerve may course and exit in the region of the glenoid fossa. After the anterior and superior limits of bone dissection are reached, proceed with removal of bone medially. Avoid aggressive bone removal posteriorly to prevent entry into the mastoid air cells with creation of a large mastoid cavity. A mastoid cavity incurs life-long maintenance and infections. As bone removal proceeds medially, sequentially identify landmarks. Identify the body of the incus in the epitympanum. Then identify the incus-malleus complex. The malleus neck often is fused with the atretic plate, which is the bony septum that separates the new ear canal from the middle ear space and delineates the level of the new tympanic membrane. The atretic plate is thinned. Take care especially when reaching the inferior-posterior aspect of the atretic plate, lateral to the middle ear, because the facial nerve often courses in this area. When the atretic plate is thinned sufficiently, pick away at the plate in piecemeal fashion to mobilize the ossicular mass. Strategies to reduce ossicular trauma that can transmit to the inner ear include leaving the dissection of the atretic plate off the ossicular mass until the very end and filling the middle ear space with gelatin sponge to prevent excessive mobility of the ossicular chain and to absorb any mechanical trauma. Perform inspection of the middle ear to locate the incudostapedial joint and stapes, then to verify ossicular continuity and footplate mobility. If discontinuity is present, use a partial or total ossicular replacement prosthesis for reconstruction. Position the center of the neocanal around the ossicular mass. The new ear canal should be 1.5 cm in diameter, approximately 1.5- 2 times the caliber of the normal external ear canal. Graft the tympanic membrane with the temporalis fascia, ensuring that the graft sits on the ossicular mass with a slight tented effect. The edges of the graft may drape several millimeters onto the bony canal wall. Harvest the split-thickness skin graft, and use this skin to line the ear canal. Cover all exposed areas of the bony canal with skin. Stabilize the tympanic membrane graft, and prevent the graft from lateralizing by placing a silicone annular button over it. Fill the ear canal with packing to stabilize the skin grafts. Leave the lateral-most edge of the skin graft free to suture to the skin edge of the concha. Mobilize the auricle and position it over the neocanal. If the microtic auricle was reconstructed, create a meatal opening by removing both skin and cartilage. The meatus should lead directly into the bony canal. More cartilage present at the meatus lessens the chance of stenosis. Additionally, debulk any excessive soft tissue in this area to prevent stenosis. Suture the skin edges of the skin graft to the skin edges of the auricular meatus. Close the postauricular incision in layered fashion. If the auricle was mobilized extensively, perform undermining to reapproximate the postauricular skin edges. Postoperative details: Instruct the patient to return to the clinic after 1 week for removal of the canal packing and silicone button. At this time, the skin grafts may be evaluated for take. Local wound care may be given to areas of granulation or nonhealing. Schedule the second postoperative visit 1 month later to assess take of the skin and tympanic membrane grafts. Follow-up care: Base surgical success on restoration and stabilization of hearing and maintenance of a patent ear canal. Monitor these during the follow-up period with examination and audiometric examination.
Injury to the facial nerve is one of the most feared complications of surgery for correction of CAA. This complication has historically deterred surgeons from correcting this condition. However, in experienced hands and with improved imaging techniques, the complication rate has been below 1%. The facial nerve is estimated to be aberrant in 25-30% of patients. The displaced portion is typically at the second genu, where it takes an acute turn. As a result, the mastoid segment of the facial nerve courses more anterior and lateral than usual. It traverses the middle ear at the level of the round window and leaves the temporal bone at the glenoid fossa instead of the stylomastoid foramen. In surgery, it possesses an inferior and posterior relationship with the atretic plate. In a review of facial nerve injuries in more than 1000 patients with CAA, Jahrsdoerfer and Lambert reported injury in 10 patients. Patients with low-set ears, canal stenosis, and cholesteatoma had facial nerves vulnerable to injury during surgery. The newly created ear canal also may become stenotic. This complication is usually due to poor cartilaginous support in the meatal portion of the canal. If excessive soft tissue was not debulked in this area, it may contribute to narrowing in this region. Stenosis of the bony ear canal may occur over time because of osteoneogenesis. If the skin graft does not completely cover the bone of the canal, this also may contribute to canal narrowing. Hearing loss may ensue after surgery. If a high-frequency sensorineural hearing loss occurs, an iatrogenic injury from the transmission of mechanical or acoustic trauma to the inner ear during surgery may be present. This occurs with excessive manipulation of the ossicles or when the atretic plate, which is in continuity with the ossicles, is removed aggressively with the high-power drill. If the hearing loss is conductive, unrecognized ossicular fixation or discontinuity, repeat fixation of the ossicles, displacement of the ossicular prosthesis, or lateralization of the tympanic membrane graft may be present. Other potential complications include cholesteatoma and chronic external and middle ear infections.
The goal of CAA surgery is to restore hearing to an SRT of 15-25 dB without the need for any amplification. With careful surgical candidate selection, these results should be achieved. Appropriate candidates are patients who have a score of 7 or higher on the Jahrsdoerfer CT grading scale (see Imaging Studies). In general, approximately one half of patients have an SRT less than 25 dB after surgery. In 1998, Lambert assessed long-term stability of hearing results and noted that some degradation of hearing occurs after the first year, with approximately one third of patients requiring revision surgery for hearing loss. In these revisions, approximately one half of patients obtain an SRT of 25 dB or less, and approximately two thirds of patients obtain an SRT of 30 dB or less.
Improvement in the ability to treat CAA lies with ways to maintain patency of the surgically-reconstructed ear canal. This improvement may come in the form of genetically engineered tissue grafts or advanced stent materials.
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