Micrognathia may occur in > 700 genetic syndromes.
Pierre Robin sequence is a common manifestation of micrognathia characterized by a U-shaped cleft soft palate and upper airway obstruction caused by glossoptosis (a tongue that falls to the back of the throat); conductive hearing loss may also be present. Feeding can be difficult, and sometimes cyanosis develops because the tongue is posterior and may obstruct the pharynx. Prone positioning during feeding may help, but uncoordinated swallowing may require nasogastric gavage feedings or a gastrostomy tube. If cyanosis or respiratory problems persist, tracheostomy or surgery to affix the tongue in a forward position (eg, sewing it to the inner lower lip) may be required. Otologic evaluation is indicated.
About one third of patients with micrognathia have associated anomalies that suggest an underlying chromosomal defect or genetic syndrome. When other anomalies are present, a clinical geneticist can help guide the evaluation because identification of the underlying syndrome is important for prognosis and family counseling. Some of the diagnoses to be considered include Treacher Collins syndrome (associated with downward slant of the eyes, coloboma of the eyelid, malformed pinna [microtia], and hearing loss), Nager syndrome, Goldenhar (oculoauriculovertebral) syndrome, and cerebrocostomandibular syndrome.
Surgical extension of the mandible can improve appearance and function. In the typical procedure, called distraction osteogenesis, an osteotomy is done and a distraction (separator) device is attached to both pieces. Over time, the distance between the two pieces is widened, and new bone grows in between to enlarge the mandible.
Congenital absence of the condyloid process (and sometimes the coronoid process, the ramus, and parts of the mandibular body) is a severe malformation. The mandible deviates to the affected side, resulting in severe malocclusion; the unaffected side is elongated and flattened. Abnormalities of the external, middle, and inner ears, temporal bone, parotid gland, masticatory muscles, and facial nerve often coexist. Syndromes to be considered include agnathia-holoprosencephaly, otocephaly, a severe form of cerebrocostomandibular syndrome, and Ivemark syndrome.
X-rays or facial CT of the mandible and temporomandibular joint show the degree of underdevelopment and distinguish agenesis from other conditions that result in similar facial deformities but do not involve severe structural loss. Facial CT is usually done before surgery.
Treatment of agnathia consists of prompt reconstruction with autogenous bone grafting (costochondral graft) to limit progression of facial deformity. Often, mentoplasty, onlay grafts of bone and cartilage, and soft-tissue flaps and grafts further improve facial symmetry. Distraction osteogenesis, in which an osteotomy is done and a distraction (separator) device is attached to both pieces of the mandible, is being increasingly used. Orthodontic treatment in early adolescence helps correct malocclusion.