APLASIAS: HEMIFACIAL
MICROSOMIA, ROBIN
SEQUENCE AND TREACHER
COLLINS SYNDROME
20
20.1 Hemifacial Microsomia
Hemifacial microsomia is a condition that falls along the oculo-auriculo-vertebral spectrum. It comprises varying degrees of unilateral facial hypoplasia (although it may also present bilaterally), ranging from subtle asymmetries to severe facial aplasias.
20.1.1 Epidemiology
Hemifacial microsomia has been reported to occur in 1/3,500 to 1/5,600 live births. The male:female and right:left ratios occur1 with an incidence of approximately 3:2. It has been suggested that roughly two-thirds of cases are unilateral and up to one-third are asymmetrically bilateral, although a 10% bilateral incidence is more likely.2,3Most cases are sporadic; rarely, however, the condition is familial, with both autosomal dominant and recessive transmission being reported. Human studies have found the candidate gene at the 14q32 position. Other candidate genes have been identified in animal studies: Hfm, Far, Goosecoid.4
20.1.2 Etiology and pathogenesis
Numerous etiologic factors have been suggested for hemifacial micro-somia, including maternal vasoactive medication use, multiple gestation, primidone embryopathy, retinoic acid embryopathy, thalidomide embryopathy, maternal diabetes (preexisting or gestational), and even military service.4 Currently, however, vascular perturbation and neural crestopathy are thought to be the most likely mechanisms. Vascular perturbation, occurring in the distribution of the stapedial artery, has been associated with hemifacial microsomia-like features in the murine model. The stapedial artery arises as the second branch of the embryonic aorta and is transiently the main artery supplying the embryonic face.5 Neural crestopathy seems to involve both early loss and decreased migration of neural crest cells, which are known to be progenitors of the facial skeleton.6 The proposed final pathway for both these theories is a perturbation in chondrogenesis leading to either hypoplasia or aplasia of structures within the first and second branchial arches and the nasal placode.5,6
20.1.3 Diagnosis and evaluation
The diagnosis of hemifacial microsomia can be made solely on the basis of physical examination, although radiologic evaluations can help to further assess the degree of skeletal hypoplasia. It has been suggested that (1) concomitant ear and mandibular asymmetry (from a unilateral hypoplasia) or (2) isolated ear or mandibular asymmetry and the presence of two or more indirectly associated anomalies or a positive family history, are diagnostic.7,8 The oculo-auriculo-vertebral spectrum includes changes in all these structures, but the term “oculo-auriculo-vertebral spectrum” is typically reserved for more mildly affected children. Hemifacial microsomia tends to fall within the middle of the spectrum. Goldenhar’s syndrome, characterized by the presence of an epibulbar dermoid, falls further along the spectrum and should probably not be considered a variant.4 Most severe form of the oculo-auriculo-vertebral spectrum is craniofacial microsomia, which includes concomitant involvement of the orbit and skull. Examination of a child with hemifacial microsomia is best performed systematically, as follows:
20.1.3.1 Skull and forehead
Diminished height of the forehead and associated frontal recession may be markers for craniofacial microsomia. It is important to note that hemifacial microsomia may also occur, very rarely, in combination with single-suture craniosynostosis.
20.1.3.2 Orbit
Subtle aplasia or inferior dystopia of the orbit may be associated with craniofacial microsomia. In rare instances, there may be complete aplasia of the orbit with associated anophthalmia. The presence of an epibulbar dermoid is a marker for Goldenhar’s syndrome, and larger dermoids may impair vision by obstruction or infiltration of the cornea.9 Ocular closure may be incomplete due to facial nerve weakness or eyelid soft-tissue deficiency.
20.1.3.3 Ear
Preauricular skin tags and sinuses represent the mildest form of auricular deformity, with incompletely formed and complete anotia representing the other end of the spectrum. The ear may be positioned symmetrically with respect to the opposite side, but more commonly it is anteroinferiorly displaced. Conductive hearing loss is present in up to 90% of affected children, most frequently in the moderate-severe range (40–80 dB). Sensorineural hearing loss is present in 10%.10,11 Hearing deficits occur most commonly as the result of atresia (60%) or stenosis (36%) of the external acoustic canal; hypoplasia or atresia of the middle ear (30%); fused, malformed, or absent ossicles (95%); or poorly developed or absent mastoid air cells (65%). The severity of external ear and mandibular deformities is proportional to middle and inner ear abnormalities.
20.1.3.4 Zygoma
Absence or hypoplasia of the zygomatic arch is usually readily apparent by palpation lateral to the maxilla.
20.1.3.5 Maxilla
Various degrees of maxillary hypoplasia may be present. A unilateral raised oral commissure suggests hemifacial microsomia. The degree of hypoplasia can be determined by having the patient bite on a tongue blade. It is important to remember that this hypoplasia is 3-dimensional. In addition to vertical shortening, the maxilla is narrower and retruded. This can be appreciated by observing the amount of rotation of the maxillary dental midline off the facial midline.
20.1.3.6 Mandible
The degree of mandibular hypoplasia mirrors the maxillary hypoplasia. Deviation of the chin point off the facial midline is usually the most obvious sign of hemifacial microsomia. However, a reduction in lateral facial fullness may also be noted because of the anomalous medial displacement of the hypoplastic ascending ramus. A mandible that can be rocked posteriorly on one side is indicative of aplasia of the temporomandibular joint (TMJ).
20.1.3.7 Intraoral examination
The amount of displacement of the maxillary and mandibular mid-lines off the facial midline should be noted. Dental crossbites are almost never seen; however, as the severity progresses, a lateral open bite may become present. Dental eruption may be delayed, and with more severe manifestations, tooth formation may fail. Examination of the palate may reveal asymmetric motion of the posterior palate, suggesting partial or incomplete unilateral palatal paresis. Velopharyngeal incompetence may result due to pharyngeal muscle hypoplasia or an overt or submucous orofacial cleft. Articulatory errors may be caused by malocclusions.
20.1.3.8 Nervous system
Examination of facial nerve function is necessary to document the degree, if any, of facial paralysis. The incidence of facial nerve weakness is between 22 and 45%.12–14 Trigeminal nerve deficits may also be present.
20.1.3.9 Extracranial abnormalities
Associated extracranial abnormalities may be found in more than one-half of patients in the following organ systems15–18: skeletal (40%), vertebral (24%, fusions and duplications15); cardiac (5–58%), atrial and ventricular septal defects (32%), conotruncal defects (39%); targeted growth defects (14%); and defects of the central nervous system (5–15%), genitourinary system (5–10%), gastrointestinal system (10%), and respiratory system (10%).
20.1.4 Classification
Although several classification systems have been described, two are most commonly used today. The OMENS-Plus system is the most comprehensive and is helpful for research analysis9–11 (Table 20.1). However, because of its completeness, it is more complex to use. Perhaps as a result, the most commonly referred-to clinical classification is the Pruzansky system, which focuses solely on the mandible20 (Table 20.2). Panoramic, plain mandibular films and computed tomography (CT) allow qualitative and quantitative assessments of mandibulo-maxillary deformity and may assist with surgical planning.21
Table 20.1. OMENS-Plus Classification: Presence of Extracranial Anomalies.19
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Orbit O0: Normal orbital size and position O1: Abnormal orbital size O2: Abnormal orbital position — superior or inferior O3: Abnormal orbital size and position Mandible M0: Normal mandible M1: Mandible and glenoid fossa are small with a short ramus M2: Mandibular ramus is short and abnormally shaped M2A: Glenoid fossa is in an anatomically acceptable position with reference to opposite TMJ M2B: TMJ is inferiorly, medially, and anteriorly displaced with severely hypoplastic condyle M3: Complete absence of ramus, glenoid fossa, and TMJ Ear E0: Normal ear E1: Mild hypoplasia and cupping with all structures present E2: Absence of external auditory canal with variable hypoplasia of the concha E3: Malpositioned lobule with absent auricle; lobular remnant usually inferiorly and anteriorly displaced Facial nerve N0: No facial nerve involvement N1: Upper (temporal and zygomatic) facial nerve involvement N2: Lower (buccal, mandibular, and cervical) facial nerve involvement N3: All branches of the facial nerve affected Soft tissue S0: No obvious subcutaneous tissue or muscle deficiency S1: Minimal deficiency S2: Moderate deficiency S3: Severe soft tissue deficiency |
TMJ = Temporomandibular joint
Table 20.2. Mulliken Modification of Pruzansky Mandibular Hypoplasia Grading System.
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Type I: The mandible and glenoid fossa are small with a short ramus Type II: The mandibular ramus is short and abnormally shaped Type IIA: The glenoid fossa is in an acceptable functional position in reference to the opposite TMJ Type IIB: The TMJ is inferiorly, medially, and anteriorly displaced Type III: There is complete absence of ramus, glenoid fossa, and TMJ |
TMJ = Temporomandibular joint
The most important initial test for any child presenting with hemifacial microsomia is polysomnography. Sleep-disordered breathing and obstructive sleep apnea are significantly more likely to occur when there is a significant degree of mandibular or TMJ hypoplasia, and any suggestion of obstructive sleep apnea needs to be further evaluated.22 Feeding may be impaired due to hypoplastic or absent masticatory muscles, nonfunctional or absent TMJs, occlusal imbalances, or orofacial clefting (10%). Therefore, it is also important to assess oral intake and weight gain in affected infants. Additionally, the value of otology and speech therapy should be initially assessed.
20.1.5 Treatment
The treatment of hemifacial microsomia requires multiple specialists; therefore, it is best referred to craniofacial centers. The following is an overview of available treatment strategies.
20.1.5.1 Ear reconstruction
The initial surgical intervention in a child with hemifacial microsomia is ideally delayed until the child is around 8 years old, when external ear reconstruction may be performed. By age 6, the size of the ear is approximately 85% of the final adult size. However, delaying surgery until age 8 allows for growth of the costal cartilage to a size sufficient to permit creation of a sturdier, higher-profile framework. Of course, the optimal time for surgery must take into account associated psychosocial issues and is best decided by the patient and family.
Current treatment options include artificial ear prosthesis placement, alloplastic framework reconstruction, and costochondral ear reconstruction. Artificial ear prostheses require minimal surgery (for bony anchored prostheses), and can have excellent aesthetic results. However, these results are impaired by seasonal variations in color match that vary with sun exposure. They may require lifelong changes (resulting in higher costs) and are known to include long-term issues with bony anchoring. They are also subject to trauma.
Alloplastic ear implants are technically much easier to perform than autogenous reconstructions and offer reproducibly good aesthetic results. Their primary drawback is that placing artificial material under a thin skin covering has a high complication rate that likely increases with length of follow-up; therefore, they probably should be thought of only as “temporary ears.”23,24
Costochondral ear reconstruction is currently the most widely accepted method of microtia reconstruction. This technique has the potential to provide both optimal appearance and durability. However, these multistaged reconstructions are technically challenging, with results highly dependent on the experience and skill of the surgeon. For this reason, microtia reconstruction is best referred to those with a special interest in this procedure and sufficient experience to produce reliable results. Reconstruction of the external auditory canal, tympanic membrane, and ossicles may be possible depending on the degree of aplasia. It is our opinion that for most patients with microtia, the results of this procedure do not provide sufficient justification. Moreover, the creation of an external auditory canal not only detracts from the aesthetic appearance of the ear but also requires routine lifelong cleaning by an otolaryngologist to maintain canal patency.
20.1.5.2 Skeletal reconstruction
Skeletal reconstruction is partially dependent on the degree of hypoplasia. There are currently three basic approaches to treating the facial skeleton: distraction osteogenesis, standard orthognathic corrections, and reconstructions using rib grafts. The following is a brief overview of the advantages and disadvantages of these general categories of treatment.
20.1.5.2.1 Distraction osteogenesis. These procedures are generally limited to treating children with milder variations of hemifacial microsomia, and are not recommended for those with absent TMJs (Pruzansky Type III). One advantage of distraction is that it allows gradual concomitant lengthening of the soft-tissue envelope. However, the primary reason for the current popularity of this technique may be that it is technically straightforward and does not require any significant orthognathic training. Distraction is associated with a fairly high complication rate, including external facial scars, injury to permanent dentition, facial nerve injury, device failures, and TMJ issues.25–28 Aside from requiring multiple stages, the largest issue with distraction is that it does not treat the associated maxillary hypoplasia; therefore, isolated lengthening of one side of the mandible (which is already anomalously medialized) creates problematic crossbites.
20.1.5.2.2 Traditional orthognathic surgery. This surgical approach is also best reserved for children with intact TMJs. The advantage of this approach is the complete correction of both maxillary and mandibular hypoplasia while idealizing occlusion, in a single stage. The facial midline can be completely aligned, although the lateral hypoplasia will persist. The disadvantage of this technique is that it is technically more challenging. Experience with orthognathic surgery is necessary, as severe cases often require 3-dimensional repositioning of the maxilla, unaided by splint guidance.
20.1.5.2.3 Rib graft reconstruction. Agenesis of the TMJ requires rib bone and cartilage graft reconstruction. These procedures, similar to orthognathic two-jaw surgery, are technically demanding. Reconstruction of the absent TMJ requires reconstruction of both the ascending ramus and the condyle as well as the zygomatic arch and glenoid fossa. Intraoral exposures are associated with significant infection rates; therefore, extraoral approaches may be justified.
20.1.5.3 Summary of reconstructive options
The timing for surgical intervention may have come full circle. The traditional orthognathic correction, first proposed by Obwegeser,29 was performed only in skeletally mature individuals. Taking into consideration the psychosocial issues associated with more significantly affected children, Munro popularized a complete orthognathic correction, including rib grafting, in younger children.30
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