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).

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).

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”).

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).

Normal mandibular growth may depend on the presence of mandibular movement during intrauterine development. In one study, Sherer et al. (1995) examined the correlation between lack of mandibular movement, manifested by the absence of fetal swallowing and the development of subsequent micrognathia. For more than a 4-year period, these investigators studied 28 fetuses with polyhydramnios, defined as an amniotic fluid volume of >20 cm³. The study group consisted of 14 fetuses studied sonographically with absent mandibular movement and a nonvisualized stomach. These findings were interpreted as being consistent with absent fetal swallowing. The control group consisted of 14 fetuses with polyhydramnios but who had sonographic evidence of swallowing. In the study group, 2 fetuses were stillborn. Of the 12 liveborn infants, 11 died in the neonatal period between 1 hour and 7 days of life. All the fetuses in the study group had micrognathia confirmed at birth, whereas none of the fetuses in the control group had micrognathia. These authors concluded that lack of fetal swallowing movements were likely to be important in the development of micrognathia. In this study, four major groups of underlying anomalies were identified in cases of absent fetal swallowing:

1. 
   

   Complete absence of any fetal movements (fetal hypokinesia/akinesia sequence).

   
   
2. 
   

   Major central nervous system abnormalities with neurologic impairment of the swallowing mechanism.

   
   
3. 
   

   Abnormal karyotype.

   
   
4. 
   

   Isolated absent swallowing due to Moebius sequence (sixth and seventh cranial nerve palsy) (Sherer et al., 1995).

The most severe form of micrognathia is agnathia—total or virtual absence of the mandible. This extremely rare and lethal anomaly also known as otocephaly, is the result of a developmental field defect thought to be secondary to an insult to developing neural crest cell derivatives (Persutte et al., 1990). Prenatal sonographic diagnosis has been reported for this condition (Persutte et al., 1990) (Figures 28-2A and 28-2B).

The incidence of mild-to-moderate micrognathia in the general population is unknown. In a high-risk referral center, 56 cases of micrognathia were identified in 2086 fetuses (2.6%) studied in a tertiary sonographic unit for more than 8 years (Nicolaides et al., 1993). More recently, the incidence of micrognathia in a single center was estimated to be 1 in 1600 fetuses (Vettraino et al., 2003).

The first prenatal diagnosis of micrognathia was made in a patient deemed to be at risk for Pierre Robin syndrome because the parents already had an affected child. At 23 weeks of gestation, the facial structures were studied and considered to be within normal limits. The same patient returned at 35 weeks of gestation, when polyhydramnios and a striking micrognathia were demonstrated. This report indicated that significant fetal mandibular growth occurs during the third trimester (Pilu et al., 1986). In a later study of 2086 high-risk fetuses, severe micrognathia was diagnosed by the presence of a prominent upper lip and a small chin in 56 cases (Nicolaides et al., 1993). Significantly, all cases with micrognathia in this study had additional malformations or evidence of intrauterine growth restriction. A review of 20 prenatally diagnosed cases of Pierre Robin sequence noted a high incidence of associated polyhydramnios (60% of cases) and cleft palate (45% of cases) (Hsich et al., 1999). Micrognathia has been diagnosed in the first trimester (Teoh and Meagher, 2003).

All reports of sonographic studies performed on fetuses with micrognathia stress the extremely high incidence of associated anomalies and the grim prognosis for this finding. Bromley and Benacerraf (1994) described their experience with 20 fetuses diagnosed with micrognathia between 15 weeks of gestation and full term. Thirteen of these fetuses had polyhydramnios (a 70% incidence). These authors hypothesized that the polyhydramnios was due to difficulty in fetal swallowing. These authors recommended a full sonographic evaluation of the fetal face, which optimally includes a midsagittal profile, coronal sections of the lower face, andaxial evaluation of the orbital area. In this report, micrognathia was subjectively defined as an unusually small mandible with a receding chin demonstrated on the midsagittal view of the fetal face (Figure 28-3). The fetal mandible was not measured.

Several reports have attempted to establish sonographic nomograms of fetal mandibular length in relationship to gestational age (Otto and Platt, 1991; Paladini et al., 1999). To provide objective evidence of micrognathia, Chitty et al. (1993) created a growth chart for mandibular length in 184 normal fetuses between 12 and 27weeks of gestation. The fetal mandible was measured from the proximal end of the ramus at its insertion site into the temporomandibular joint to its distal end where it meets the cartilaginous symphysis mentis. The measurement was made in a plane that visualized one ramus of the jaw. The ultrasound beam was placed at a right angle to the plane of the mandible. A single measurement was recorded. Normal values obtained in this study are given in Table 28-1. At more than 28 weeks of gestation, it became difficult to identify and define fetal facial landmarks. This was due to change in fetal positioning and adjacent bony structure shadowing. These authors, as well as others, stated that accurate and reliable measurements of the mandible were not possible in the third trimester. In another study, fetal mandibular measurements were obtained in a scan plane parallel to the mandible that included the fetal hypopharynx. In this report of 204 women with uncomplicated pregnancies and reliable gestational dating, the anterior, posterior, and transverse fetal jaw measurements were shown to increase with gestational age (Watson and Katz, 1993).