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KEY POINTS
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Key Points

  • Facial malformation characterized by mandibular hypoplasia and a small receding chin that fails to maintain the tongue in a forward position.

  • Incidence is 1 in 1600 fetuses.

  • Very high incidence of associated anomalies.

  • Differential diagnosis includes Pierre Robin, Stickler, Treacher Collins, and Nager syndromes.

  • Fetal karyotype, including FISH studies for DiGeorge syndrome, is indicated.

  • DNA studies may be indicated if a syndrome is suspected.

  • Delivery should occur in a tertiary center due to the potential for neonatal airway problems.

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CONDITION
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Micrognathia is a facial malformation characterized by mandibular hypoplasia and a small, receding chin that fails to maintain the tongue in a forward position (Figure 28-1).

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Figure 28-1

Three-dimensional image demonstrating micrognathia.

Graphic Jump Location
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Early in embryonic development, the mandible grows slowly. Between 4 and 8 weeks of gestation, the developing tongue remains in the nasal cavity, between the palatine shelves, and a physiologic micrognathia is present. Around the 8th week of gestation, the mandible grows rapidly, and the tongue is normally pulled downward and forward. This allows the palatine shelves to come together to form the secondary palate. At this point in gestation, the mandible extends beyond the maxilla, but continued growth of the maxilla once again produces a relative micrognathia in the fourth and fifth months of gestation. The mandible continues to grow during the third trimester. If the compensatory growth of the mandible is incomplete at birth, a relative micrognathia can exist (Hawkins and Simpson, 1974).

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Micrognathia may result from environmental or genetic factors. For example, sharp flexion of the fetal neck in utero results in continuous pressure of the chin against the sternum, which impedes mandibular growth (Hawkins and Simpson, 1974). Micrognathia is also one component of many chromosomal and genetic syndromes (see “Differential Diagnosis”).

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Micrognathia is commonly thought of as one component of the Pierre Robin syndrome. This clinical triad consists of micrognathia, upper airway obstruction, and a U-shaped cleft palate. The name derives from a 1923 report by Pierre Robin, a French stomatologist, who described the association between newborn micrognathia and upper airway obstruction caused by glossoptosis (posterior displacement of the tongue). The symptoms associated with this “syndrome” are primarily due to an underlying mandibular abnormality (Figure 28-1). During fetal life, when the mandible is hypoplastic, the tongue cannot descend normally into the oral cavity at 7 to 11 weeks of gestation. If the tongue does not descend, it will remain pressed against the base of the cranium. This will then interfere with fusion of the medially growing palatal shelf and ultimately result in a cleft palate (Shprintzen, 1988).

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Normal mandibular growth may depend on the presence of mandibular movement during intrauterine development. In one study, Sherer et al. (1995)...

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