What is hemifacial microsomia?
Hemifacial microsomia is a subset of craniofacial microsomia. It is a spectrum of undergrowth conditions or malformations related to the craniofacial skeleton. They occur in 1 in 3,000 to 25,000 live births, second only to the incidence of cleft lip and palate. The lower jaw (or, mandibular) deformity is the most common manifestation of the disease process. In fact, the severity is classified on the basis of the lower jaw deformity (Pruzansky classification). The underlying problem is an underdevelopment of the craniofacial skeleton, including the external, middle, and inner ear structures as well as the jaw and overlying soft tissue structures (including the cranial nerves). As its name suggests, it usually occurs only on one side of the face (ie, hemifacial or unilateral). The occurrence of this condition is largely unknown.
Why does hemifacial microsomia occur?
It is still largely unknown why hemifacial microsomia occurs. Many syndromic conditions can include hemifacial microsomia, but the majority of isolated cases have no known cause. It occurs during development when 2 specific precursor tissues for facial development (ie, the first and second branchial arches) fail to form properly. As in the majority of cases of cleft lip and palate, the origin of hemifacial microsomia is felt to be mostly multifactorial in nature. In other words, numerous environmental exposures combined with certain unknown genetic factors come together to create the condition. Recent studies are looking into possible identifiable genetic sources that may be causing hemifacial microsomia.
What are the associated problems with hemifacial microsomia?
In hemifacial microsomia, the degree of visible deformity can be subtle or extreme depending on severity. The most common appearance is that of a small lower jaw with a chin point that is shifted to one side. This frequently contributes to bite irregularities or malocclusion (eg, lateral crossbite, cant, or open bite). Involvement of the external ear is common and can produce microtia or anotia (ie, underdeveloped or absent ear, respectively) with possible lack of the canal for hearing. Skin tags and sinus pits may also be present around the ear. The facial nerve (cranial nerve VII) can be affected, producing a one-sided facial weakness or paralysis. If the muscles used for biting are affected, then it may compound the problem of an asymmetric bite and simultaneously amplify the deficiency of facial volume. The cheekbone may also be deficient, and the outer corner of the eye may be displaced downward. Sometimes, the eye itself may be small or absent (ie, microphthalmia or anophthalmia, respectively), or have defects in the iris (or, colobomas). Eyelashes may be partially absent. Macrostomia (or, enlarged mouth opening) is the result of facial clefting.
In addition to the craniofacial deformities described, a number of other congenital deformities may be present. There can be problems with development of the spine, gastrointestinal, cardiac, renal, and pulmonary organs. Several named syndromes, such as Goldenhar and Treacher Collins, are known to have features of hemifacial microsomia.
Can hemifacial microsomia be inherited?
Although various genetic markers have been identified in hemifacial microsomia, the majority are of a single occurrence in a family and without a known cause. However, a multifactorial inheritance pattern (ie, some genetic tendency in combination with environmental factors) is suggested based on several observations. First, there does seem to be an increased recurrence risk in families with an affected relative. The risk of an affected individual having an affected sibling, child, or parent (ie, first-degree relative) is estimated at 2 to 3 percent or more. Second, maternal use of certain medications including aspirin, retinoic acid, pseudoephedrine, ibuprofen, and assistive reproductive medications are known to increase the chance of having a child with hemifacial microsomia. Lastly, the higher incidence in identical twins suggests a genetic link. A consultation with a genetics counselor is vital in accounting for all variables contributing to hemifacial microsomia and for providing a better foundation for family planning.
How is hemifacial microsomia treated?
Children with hemifacial microsomia may have a number of issues requiring intervention based on the severity of the condition and the presence of associated malformations. A multidisciplinary team of specialists is critical in managing issues related to breathing, speech, hearing, feeding, cardiac or kidney problems, orthopedic concerns, and psychological development. For correction of craniofacial issues, surgery is currently the only option. Ear tags can be removed very easily on an outpatient basis, whereas jaw deformities are much more complex and require a more aggressive surgical treatment.
Ear reconstruction using either rib cartilage or a porous polyethylene framework (ie, Medpor® implant) can successfully reproduce the shape of an ear. Recreation of an outer ear canal is possible but is largely dependent on the formation of middle and inner ear structures for successful restoration of hearing. Most children with complete occlusion of the ear canal are candidates for a bone-anchored hearing aid (or, BAHA device).
Facial soft tissue asymmetry can be improved with fat grafting from the abdomen to the area of volume loss in the face. In severe cases, a large area of tissue with its blood supply can be transplanted to the face and sculpted to give more effective filling. This is known as a fasciocutaneous free flap. Whereas fat grafting can be done on an outpatient basis, the free flap option requires a short period of hospitalization to ensure graft survival. In some instances, facial augmentation can be done using artificial implants for cheek bone and chin improvement, or to fill wasting in the temples.
For facial weakness or facial paralysis, various options exist for improving movement. For eye exposure issues due to failure of eyelid closure, an upper eyelid gold or platinum weight can be inserted to assist in closure. Alternatively, a piece of fascia taken from another part of the body can be inserted into the upper eyelid opening mechanism in order to lengthen it, thereby achieving good closure and avoiding the use of a foreign implant. In many instances, a lower eyelid procedure may also be required. Specifically, a fusion of the outer corner of the eyelids can help suspend the lower eyelid and prevent the chance of drying. This is called a tarsorrhaphy. Alternatively, a piece of tendon from the wrist area can be used to re-suspend the lower eyelid from the corners of the eyes. Newer muscle transplantation techniques have been tried with marginal success.
For mouth function, restoration of movement at the corner of the mouth is critical for many reasons. Being able to tighten the lips can help achieve closure and improve speech, drooling, and smiling. Several options exist for smile reanimation from cross-face nerve grafting to gracilis muscle free flaps to temporalis muscle transposition. Please refer to the section on Facial Paralysis for further details.
Macrostomia (large mouth opening due to a sideways cleft) repair is done as early as 6 months of age. This involves closing the layers of the cheek in a specific pattern in order to restore symmetry and maintain proper oral function. The surgery takes less than 2 hours and can be done on an outpatient basis.
Revisional surgery is required in many cases of hemifacial microsomia. These can include additional contouring procedures or scar revisions. As a child grows, changes in the craniofacial skeleton may cause shifting of tissues, uncovering new areas that will need to be addressed. The timing of intervention is very important in getting optimal results and should be custom-tailored for each patient.
Recovery from surgical correction of each component of hemifacial microsomia is highly variable. Whereas ear reconstruction can potentially be performed as an outpatient, other aggressive interventions such as jaw surgery and facial reanimation may require a brief hospitalization. Considerations must be made for airway management when operating on the jaws or inside the mouth.
Generally, avoidance of aggressive play and close supervision is required for the first 2 to 4 weeks following surgery. This restriction will need to be tailored to the procedure performed and is directly proportional to the aggressiveness of the intervention.
Jaw or oral procedures will usually have dietary restrictions (soft diet only) for 6 weeks following surgery. All procedures require some level of scar management. The plastic surgeon should guide patients at every step.
All procedures for correcting individual deformities related to hemifacial microsomia will carry a different set of risks and potential complications. Thankfully, all procedures are relatively safe when appropriate preoperative evaluation is performed. Airway complications are the most serious issue encountered. Anesthesia pre-evaluation is necessary to stratify risk and to make provisions at the time of surgery for successful intubation and postoperative management. Many of the risks encountered during surgery for hemifacial microsomia are directly proportional to the severity of the condition. For example, airway obstruction carries a higher likelihood of occurring when the lower jaw is extremely small (ie, Pruzansky III classification) and shifted than when it is only slightly small (ie, Pruzansky I classification).
Bleeding, infection, and anesthetic complications are risks of any surgical procedure. In addition, poor scarring or wound problems can occur rarely. More frequently, achieving perfect symmetry is an impossibility. Improvements can be substantial, but reversing the deformity to a normal state cannot be realized in the majority of patients. Overcorrection or undercorrection may occur. Injected fat may resorb and incompletely correct a contour deficit. Many times, revisional surgery will be required to improve scars, contour, or symmetry, as mentioned above.