Microtia Reconstruction

Introduction

Microtia refers to a spectrum of congenital malformations marked by varying degrees of underdevelopment of the auricle. Anotia is the most severe degree of malformation, marked by an absence of any significant auricular structure. The reported global incidence of microtia ranges from 0.83 to 17.4 per 10,000 live births. ^{, ,} A 2005 review reported an overall prevalence of 1.69 per 10,000 live births, with significantly higher prevalence in the Hispanic (3.13 per 10,000) and Native American/Native Alaskan (4.67 per 10,000) populations. Overall, males are more frequently affected than females (2.5:1) with the right side more commonly involved than the left. Although microtia can occur bilaterally, the majority of affected individuals (77% to 93%) have unilateral involvement. Mendelian inheritance is more common in syndromic and familial cases, whereas a multifactorial or polygenic etiology is more probable in sporadic cases. Microtia may occur as an isolated condition, although it has been estimated that at least half of the cases occur as part of a sequence or syndrome.

The surgical management of microtia remains one of the greatest reconstructive challenges. Auricular reconstruction requires a three-dimensional (3D) understanding of ear anatomy, careful preoperative planning, surgical skill, artistry, and meticulous postoperative care. This chapter provides a review of microtia, a discussion of current surgical practices, and an exciting glimpse into future advances.

Embryology

The auricle begins to form in the fifth week of gestation as six mesenchymal buds, referred to as the hillocks of His. The prevailing theory in the literature posits that the hillocks of the second arch form the bulk of the pinna, those of the first arch form the tragus and at least part of the helical root, and the first branchial cleft persists as the external auditory meatus. Research published in 2005 supports a new model in which tissue marked by the hillock locations contributes to some specific parts of the auricle, whereas tissue caudal to the hillocks of the second arch gives rise to the free ear fold (from which the helix and scapha regions derive). Furthermore, this new research indicates that the first branchial cleft in mice does not give rise to the external auditory meatus. The auricular complex is initially positioned in the anterior neck region and then migrates dorsally and cephalad as the mandible develops during weeks 8 to 12 of gestation. ^,

The etiology of microtia is most likely multifactorial secondary to genetic and environmental effects, including teratogens and living at altitude. The underlying mechanisms are hypothesized to be neural crest cell disturbance and/or vascular disruption during development. Similar to a cleft palate, monogenic causes are more likely in syndromic or familial cases of microtia, whereas multifactorial causes are more probable in sporadic cases. Microtia is also a common feature of hemifacial microsomia, which is also known as craniofacial microsomia, Goldenhar syndrome, or oculo-auricular-vertebral spectrum disorder. Associated features of hemifacial microsomia include renal, cardiac, and vertebral anomalies, which may be detected prenatally. Microtia is also a feature of mandibulofacial dysostoses (e.g., Treacher Collins, Nager syndrome), and other craniofacial syndromes. The HOXA2 gene has been recently implicated in the development of microtia, along with several other genes. ^{, ,} In mouse models, exposure to retinoic acid between days 20 and 22 after fertilization will lead to disruptions of the second branchial arch, resulting in severe external and middle ear malformations. Rubella, other viral infections, alcohol, and thalidomide have also been suggested as possible contributors. Tissue from the lobule seems to be the least vulnerable, as there is almost always some portion of the lobule present, even in the most severe forms of microtia.

Anatomy

The auricle is an elaborate 3D structure that is remarkably constant in its general shape, despite individual variations. Given its undulating concavities and convexities and consistent soft contours, it is noteworthy that this complex structure is so frequently well formed. The skin on the anterior surface of the ear is firmly adherent, whereas the more robust subcutaneous layer on the posterior surface allows for greater mobility ( Fig. 18.1 ).

The height of the normal adult auricle ranges from 5.5 to 6.5 cm, and its width is normally 50% to 60% of the height. The auricle goes through rapid growth between 2 and 3 years of age and reaches 95% of its adult height by 4 to 6 years of age. The angle of protrusion of the ear from the posterior surface of the scalp is typically between 15 and 20 degrees. The distance to the scalp is 10 to 12 mm at the superior helix, 16 to 18 mm at the midhelix, and 20 to 22 mm at the lobule. The top of the helical rim parallels the level of the lateral aspect of the brow, and the vertical axis should be anteverted 15 to 20 degrees. Orienting the vertical axis of the auricle parallel to the line of the nasal dorsum is a useful landmark for surgical planning.

Microtia Classification

Underdevelopment of the auricle ranges in severity from preservation of the majority of the ear features with minor malformation to anotia, or complete absence of the auricular structure. In 1926 Marx described a three-grade classification system. In Grade I all structures are identifiable, but the ear is smaller than normal. Grade II represents deficiencies of the helix. Grade III is an auricle with no recognizable structures. Over the years, many other classification systems have been proposed. ^, We use descriptions that pair the grade with the phenotype: Grade 1 is an auricle of smaller dimension, though with preservation of all of the identifiable structures; Grade 2, conchal type, is an auricle with some structures absent but concha still identifiable; in Grade 3, lobular type, only the lobule is present; and Grade 4 is anotia ( Fig. 18.2 ).

Evaluation

Infants with microtia often present to the otolaryngologist early in infancy because of parental concerns related to the visible anomaly and the potential for hearing loss. A majority of patients with unilateral microtia have normal hearing in the unaffected contralateral ear. An age-appropriate hearing assessment should be performed in all microtia patients. Microtia is commonly associated with atresia of the external auditory canal (see Chapter 19).

The general health of the child should be assessed, with particular attention to the growth, body habitus, and lung function in the event that rib harvest is needed. Signs of hemifacial microsomia include mandibular asymmetry, maxillary asymmetry, epibulbar dermoids, and coloboma. Genetics consultation is helpful to guide further workup, such as renal ultrasound, cervical spine films to rule out vertebral anomalies, and panoramic dental films. Preoperative planning and discussion with anesthesiology is advised if a difficult airway is anticipated.

In patients with bilateral microtia and bilateral external auditory canal atresia, the option of bone conduction hearing aids should be discussed. Osseointegrated bone conduction devices can be considered once the calvarial bone has reached the required thickness of 3 to 4 mm, usually in children older than 5 years of age, which is the minimal age of implantation approved by the U.S. Food and Drug Administration. Delaying osseointegrated bone conduction device placement until after microtia reconstruction allows preservation of overlying soft tissue for auricular reconstruction and favorable positioning for the osseointegrated bone conduction device in relation to the newly constructed ear. Hearing rehabilitation of aural atresia is discussed in a separate chapter.