Hemifacial microsomia, or craniofacial microsomia, is a congenital condition characterized by hypoplasia, or underdevelopment of the skeleton and soft tissue structures on one or both sides of the face. Hemifacial microsomia is the second most common congenital deformity involving the head and neck, with an incidence of 1 in 3,000 to 1 in 5,000 live births. 
Hemifacial microsomia has been known by many other names, including craniofacial microsomia and otomandibular dysostosis. Patients with hemifacial microsomia may have anomalies of the mandible (lower jaw), maxilla (upper jaw), ears, soft tissues and muscles of the face, and the facial nerve.
What Causes Hemifacial Microsomia?
The exact cause of hemifacial microsomia is unknown. During development, the mandible, maxilla, ears, facial soft tissues, muscles, and nerves develop from structures called the 1st and 2nd branchial arches. The most commonly accepted theory of the cause of hemifacial microsomia is a disruption of blood supply to these structures in the first 6 weeks of gestation.
Similarly, the genetics of hemifacial microsomia are poorly understood. There have been cases in which an autosomal dominant inheritance pattern is observed, meaning there is a 50% chance of passing the trait on to a child. There have also been cases in which an autosomal recessive inheritance pattern is observed, in which both parents must carry the gene, and there is a 25% chance of inheriting the trait. For the majority of patients, there is only a 2-3% chance of a child inheriting hemifacial microsomia.
What Is Affected In Patients With Hemifacial Microsomia?
There is a wide range in the presentation of patients with hemifacial microsomia. The majority of expression of the disease exists in the face, but as many as 55 % of patients may be affected in areas of the body outside of the cranial region. These may include the central nervous system (CNS), gastrointestinal (GI) tract, kidneys, lungs, and skeleton.
In the facial region, there may be varying malformation in the following areas:
Mandible (lower jaw)
The majority of patients with hemifacial microsomia have some degree of hypoplasia, or underdevelopment, of the mandible. This primarily affects the ramus, which is the vertical portion of the mandible containing the condyle, the mandible component of the temporomandibular joint (TMJ.) The involvement ranges from a mildly smaller mandibular ramus to a nearly abscent ramus and temporomandibular joint. This leads to the patient’s chin being deviated toward the affected side and an occlusal cant, which is an upward sloping of the plane of the teeth when viewed from the front.
Figure 1: Degrees of mandible deformity on hemifacial microsomia
Facial skeleton
In addition to the mandible, the maxilla (upper jaw) can be affected as well. This leads to shortening of the maxilla on the affected side. The zygoma, or cheekbone, may be deficient or absent. The orbit, or eye socket, may be decreased in size or malpositioned compared to the unaffected side.
Facial soft tissue
The soft tissues on the affected side are commonly deficient, or smaller than the other side. There may be preauricular skin tags, or small nodules of skin in front of the ear. It is common for there to be some degree of macrostomia. With macrostomia, meaning “large mouth”, the commissure (corner of the mouth) is positioned further toward the ear than on the unaffected side. Muscles of mastication, or the muscles needed to close the mouth and chew, are often smaller in patients with hemifacial microsomia. This may affect mouth opening and closing.
Ears
Most patients have some involvement of the auricle, or the external ear. This may range from a slightly smaller ear to a nearly absent external ear. Many patients have abnormalities in their ear canals, and problems with hearing on that side.
Nerves
The facial nerve is the nerve which controls the muscles of facial expression. Many patients with hemifacial microsomia have some weakness in facial expression due to hypoplasia of the facial nerve.
How Is Hemifacial Microsomia Diagnosed?
The diagnosis of hemifacial mircosomia is established by examining the patient. Upon diagnosis, other studies are often recommended, including hearing tests, xrays of the cervical spine (the part of the spine in the neck,) echocardiogram to assess the heart, and renal ultrasound to examine the kidneys.
How Is Hemifacial Microsomia Treated?
Hemifacial microsomia is a complex disorder, and different patients are affected differently. Evaluation and treatment by a multidisciplinary team is very important to ensure that all aspects of the disorder can be treated over time. This team typically includes craniofacial surgeons, otolaryngologists (ear, nose, and throat specialists), audiologists, dentists, orthodontists, and specialized nurses and pediatricians. Treatment of different aspects of hemifacial microsomia occurs throughout childhood and the teenage years. Timing of the treatment of each symptom depends on the severity of the deformity, overall growth, skeletal maturity (how far along in growth the facial skeleton is), dental eruption (growth of the teeth), and other important factors. Close coordination among members of the team is critical for successful treatment.
Because patients with hemifacial microsomia may be affected so differently, the specific surgical plan varies largely from patient to patient. For example, patients may have mild jaw deformities, requiring observation only, orthodontics, jaw surgery in the teenage years, or earlier intervention including bone grafting (taking bone from another part of the body), or distraction osteogenesis (please see distraction osteogenesis page), or free tissue transfer.
Below is a rough timeline of treatment for a patient with hemifacial microsomia. However, it is important to remember that each child’s treatment plan depends on their specific situation, what is affected, and how severely.
0-2 Years Of Age
During this time, the initial evaluation including hearing assessments, xrays, echocardiograms, and renal (kidney) ultrasounds are performed. Genetics and ophthalmology referrals may be made. The child’s feeding, weight gain, growth, and breathing are closely monitored. Surgical interventions during this time may include excision of the preauricular skin tags. Macrostomia can be repaired through a procedure called a commisuroplasty, in which the commissure, or corner of the mouth, is placed in a position symmetric with the other side. If the mandible is sufficiently underdeveloped that the patient has airway compromise, or difficulty breathing, coordinated procedures with the ENT and craniofacial surgeon may be necessary at this time.
2-6 Years Of Age
Hearing, nutrition, and development continue to be assessed during this time. In patients with severe jaw deformities in which the airway is compromised, procedures to increase the size of the mandible, such as distraction osteogenesis, may be done at this time. The patient will be followed by the ENT and ear tubes placed if necessary.
6-12 Years Of Age
This is the time in which ear reconstruction is typically performed. If appropriate, repair of aural atresia, or underdevelopment of the ear canal, can be done during this time period, as well. Certain patients benefit from early jaw surgery at this time. Procedures such as fat grafting to augment the facial soft tissues may be performed then also.
Teenage Years
Definitive jaw surgery is typically performed during the time of skeletal maturity, or when the facial skeleton stops growing. This leads to a more predictable result. In patients with severe jaw deformities, reconstruction may involve free tissue transfer. Augmentation of facial soft tissues may be done during this time period. Procedures for facial nerve palsy, such as facial reanimation surgery, may be performed at this point or sooner, depending on the needs for ear reconstruction and jaw surgery. Certain patients benefit from rhinoplasty, or nose surgery, if the abnormalities of the facial skeleton affect the nose.
Please contact the Craniofacial Team of Texas if you would like to schedule an appointment.
If you would like more information about this craniofacial anomaly, please contact the Craniofacial Team of Texas by calling 512-377-1142 or toll free 877-612-7069 to schedule an appointment or complete an online appointment request.
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