Congenital anamolies of Pinna and external auditory canal
Contents
Introduction:
Anamolies of Pinna and external auditory canal can be termed as microtia and congenital aural atresia respectively.
Definition:
Microtia:
is defined as the abnormal development of the Pinna resulting in a malformed auricle. The deformities caused could range from mild distortion of the anatomic landmarks to the complete absence of auricle.
Congential aural atresia:
is defined as a failure of development of external auditory canal. Congenitial aural atresia is always associated with a certain degree of microtia.
Microtia and congenital aural atresia are rare deformities. These deformities are always associated with other malformations of auditory system.
Etiology:
1. Exposure to teratogens like vitamin A
2. Vascular insults and genetic aberrations
3. Isolated microtia can occur with branchial arch anomalies
4. Can occur as a part of a single gene deletion or embryopathic development anamolies, eg. Goldenhar syndrome
5. Certain auricular deformities can occur as a result of multifactorial insults to the developing foetus
Incidence:
Microtia and congential auditory atresia occur in approximately 1 in every 20,000 live births. These deformities commonly occur unilaterally, more so on the right side. Men are affected thrice as common as women. The degree of auricular deformity usually correlates with the degree of middle ear deformity. The incidence of inner ear deformities are very rare in patients with congenital auditory atresia. Microtia is associated with other anomalies of face 50% of the time.
Women with four or more pregnancies are at increased risk of bearing a child with mirotia. The incidence of microtia is higher in Japanese population.
Embryology:
During the sixty week of intrauterine life the external ear begins to develop around the dorsal end of the first branchial cleft. On either side of this cleft lie the first (mandibular) and second (hyoid) arches. The auricle develops from these arches as 6 small buds of mesenchyme known as the six hillocks of His. The first arch gives rise to the hillocks 1 to 3 and the second arch gives rise to hillocks 4 to 6. Traditional theory suggests that hillock 1 becomes the tragus, hillocks 2 and 3 from the helix, hillocks 4 and 5 form antihelix and hillock 6 forms the lobule of the ear. More recently it has been suggested that the second arch contributes approximately to 85% of the auricle. The lobule is the last component of the pinna to develop.
The auricle begins to develop in the anterior neck region, then it is postulated to migrate dorsally and cephalad as the mandible begins to develop during the second and third months of gestation. By the 5th month of gestation the pinna lies in its adult location.
The external auditory canal begins to develop from the first branchial cleft during the first two months of gestation. During the first month a solid epithelial cell rest forms in this area and is in contact with the endoderm of the first pharyngeal pouch. There is an intervening mesoderm preventing direct contact between the ectoderm and endoderm.
Applied anatomy of pinna:
Pinna reaches mature size between ages of 13 - 15. The superior edge of the pinna should be in line with the lateral edge of eyebrow or upper eyelid. It also shows a posterior inclination ranging from between 5 - 30 degrees. The angle of the ear is parallel to that of the dorsum of nose, usually within a range of 15 degrees.
Evaluation:
While examining a patient with microtia attention should be paid to the mandible, oral cavity, cervical spine and eyes. This is done to rule out other associated cervico facial congenital anomalies. The quality of skin over the malformed pinna should also be noted. The integrity of facial nerve should be tested and documented.
Patients with unilateral microtia and congenital aural atresia usually have normal hearing on the opposite ear. Hearing status on the affected side should be recorded.
The role of CT scan in these patients is to assess the middle ear anatomy and inner ear anatomy. The presence of congenital cholesteatoma is common in these patients and hence must be sought in the CT scan of the affected side.
In order to select ideal candidates for repair microtia has been classified by Marx as:
Grade I : The pinna is malformed and smaller than normal. Most of the characteristics of the pinna, such as the helix, triangular fossa, and scaphae, are present with relatively good definition
Grade II: The pinna is smaller and less developed than in grade I. The helix may not be fully developed. The triangular fossa, scaphae, and antihelix have much less definition
Grade III: The pinna is essentially absent, except for a vertical sausage-shaped skin remnant. The superior aspect of this sausage-shaped skin remnant consists of underlying unorganized cartilage, and the inferior aspect of this remnant consists of a relatively well-formed lobule
GradeIV: Is complete anotia.
Two other classification systems one by Jahrsdorfer and the other by De la cruz are commonly used these days. De la cruz classification divides the malformations into major and minor categories. Obviously ears with minor deformities are better surgical candidates for successful reconstruction, where as ears with major deformites should be managed with hearing aids.
Surgical therapy for unilateral microtia: This is performed as a staged procedure using autogenous rib cartilage. This technique was refined by Tanzer.
Tanzer surgery:
Is performed in four stages. There is a three month gap between these stages.
First stage: Rib cartilage is harvested and sculptured into the shape of pinna and is placed under skin pocket of the microtic ear.
Second stage: Formation of the lobule
Third stage: Elevation of the ear with insertion of a post auricular skin graft
Fourth stage: Formation of the tragus with a skin/cartilage composite graft from the contralateral ear and full-thickness skin graft for the conchal area from the contralateral ear
Nagata technique:
This is a two staged procedure developed by Nagata. This involves constructing the auricular framework form the sixth to ninth rib cartilages. The framework is created using stainless steel sutures. The framework is placed and the lobule remnant is transposed. Six months later a reconstruction is performed and the graft is released.
The aim of congenital aural atresia repair is to provide serviceable hearing to the patient. Children associated with congenital aural atresia with other syndromes like Treacher Collins syndrome should not be operated upon, and should be managed with implantable hearing devices. Congenital aural atresia should be performed two months after microtia repair to preserve blood supply to skin and subcutaneous blood supply.
Before proceeding with repair of congenital aural atresia a high resolution CT scan must be performed. The mastoid cavity and middle ear anatomy should be completely analysed. The presence of a mastoid cavity is a must for surgery because the canal is created at the expense of the mastoid cavity.
The factors that must be taken into consideration before performing the surgery include:
1. Status of inner ear
2. Temporal bone pneumatisation
3. Course of facial nerve
4. Presence of foot plate and round window
5. Presence of cholesteatoma
Three possible approaches can be followed for congenital aural atresia repair. They are:
1. Mastoid
2. Anterior
3. Modified anterior
Mastoid approach:
In this approach the external auditory canal is created at the expense of mastoid cavity. It involves drilling out the mastoid and identifying the sino dural angle. This is a risky procedure because of distorted anatomy of the facial nerve in these patients.
Anterior approach:
Is the most common approach used these days. In this approach a post auricular incision is made and the subcutaneous tissue and periosteum are raised anteriorly up to the level of glenoid fossa. If any remnant of tympanic bone is present drilling is started at the cribriform area, and if no tympanic bone is present the drilling begins at the temporal line just posterior to the glenoid fossa. Drilling is continued anteriorly and medially till epitympanum is entered. The most common anamoly encountered in the middle ear of these patients is a fused malleal - incudal joint. Stapes is usually normal in these patients. The atretic bone is carefully removed uncovering the ossicles. The facial nerve usually lie medial to the ossicular mass, and must be protected at all costs. It could commonly be injured in the posterior - inferior middle ear space. Drilling is continued till the canal is about 10mm in size. Ossicular chain reconstruction is performed and a neo tympanum is fashioned using temporalis fascia graft. Split thickness skin graft is used to line the external auditory canal. A wide meatoplasty is fashioned and a large wick is inserted to stent the canal.
Modified anterior apporach:
This approach is used in patients with a thick atretic plate because of poor orientation during dissection. This poor orientation may risk carotid artery, facial nerve, and lateral semicirular canal to injury. Orientation in these patients could be achieved by an initial posterior dissection up to the level of sinodural angle. This enables the surgeon to identify the level of lateral canal and ossicular mass. From here on the approach is similar to that described under anterior approach.
Complications of these surgical procedures:
1. Injury to facial nerve 2. Injury to lateral canal 3. Sensorineural hearing loss 4. Rejection of graft material 5. Restenosis 6. The morbidity associated with rib harvest is significant and includes scarring, deformity and risk of pneumothorax.
Prosthetic devices:
Tissue expanders and other prosthetic materials can be used in these surgical procedures. The use of tissue expanders before implantation of microtia framework can avoid the use of skin grafts and could also reduce the number of surgical procedures.
