Micrognathia is seen in more than 50 conditions or syndromes, including bone dysplasias, fetal chromosomal abnormalities, fetal drug exposure (retinoic acid, aminopterin, trimethadione), and maternal phenylketonuria(23,30) (Table 11-10 ). These conditions or syndromes may be associated with other ultrasonographically recognizable fetal pathologic conditions, or micrognathia may be the only fetal abnormality that is ultrasonographically detectable. In a review of 20 fetuses with ultrasonographically identified micrognathia,(106) 5 of 20 fetuses (25%) had abnormal karyotypes, including 3 with trisomy 18 and 1 each with trisomy 13 and trisomy 9. Nicolaides and associates(31) detected chromosomal abnormalities in 37 of 56 (66%) fetuses with micrognathia. Other investigators detected micrognathia in 9 of 24 fetuses with facial anomalies; 38% of these had an abnormal karyotype, predominantly trisomy 18. These authors also noted a high incidence of skeletal dysplasias when micrognathia was discovered.
When micrognathia is detected on prenatal ultrasound, the prognosis is poor because of the frequently associated karyotypic and lethal anomalies.(25,31,106) Bromley and Benacerraf(106) noted polyhydramnios in 70% of cases. The etiology of polyhydramnios is uncertain but may be related to difficulty in swallowing associated with the micrognathia, or to other structural abnormalities.
当产前超声检查发现下颌过小时，预后通常较差，因为常伴有染色体和致命的畸形[25、31、106]。Bromley and Benacerraf报道70%的病例有羊水过多。羊水过多的病因还不确定，可能与下颌过小或其他的结构畸形引起的吞咽困难有关。
When micrognathia is detected prenatally, a diverse set of syndromes must be considered, and the following steps should be considered: (1) karyotyping; (2) search for other anomalies, including cardiac defects and skeletal dysplasias; (3) evaluation of the fetal ears to assist in diagnosing Treacher Collins or Goldenhar-Gorlin syndrome; (4) maternal history of drug exposure or familial syndromes; and (5) parental counseling regarding the poor prognosis, particularly when other ultrasonographic anomalies are identified. At birth, micrognathia may result in respiratory distress and difficult intubation.(106)
In the mid-sagittal view of the fetal face (facial profile), micrognathia presents as an unusually small mandible with a receding chin. Although percentile charts exist for the measurement of the mandible, the diagnosis of micrognathia is usually made subjectively when there is a visibly abnormal profile with a markedly receding chin(23,24) (Fig. 11-31 ). One must be careful when scanning in the parasagittal axis because hypoplasia of the nasal bridge and mandibular hypoplasia may be simulated (see Fig. 11-8 ). Unfortunately, hypoplasia of the mandible may not be obvious during the second trimester. Pilu and colleagues(107) reported a case with Pierre Robin syndrome in which a mid-sagittal facial view at 23 weeks' gestation was interpreted as normal; however, a similar view at 35 weeks demonstrated marked micrognathia. Unequivocal facial deformity of a fetus at risk for a specific syndrome allows confirmation of a diagnosis and permits accurate diagnosis. Normal ultrasonographic findings, however, do not absolutely exclude an affected fetus dysostosis (Treacher Collins syndrome).(108) Abnormalities seen on a facial profile view may assist in accurate diagnosis in multiple malformation syndromes, even in the absence of genetic or teratogen risk.(26) Of the 56 cases with sufficiently severe micrognathia to be diagnosed prenatally by ultrasonography by Nicolaides and colleagues,(31) additional malformations or growth retardation were demonstrated. These were associated with a poor perinatal outcome. Although micrognathia is present in more than 80% of fetuses with trisomy 18 or triploidy in pathologic series, Nicolaides and colleagues(31) detected micrognathia in only 21 of 83 fetuses (25% with trisomy 18) and in 9 of 83 fetuses (21% with triploidy). Therefore, the sensitivity of detection of micrognathia at ultrasound screening remains to be determined. The underestimation of cases may be related to unfavorable fetal positioning or oligohydramnios.(25,106) 超声表现 在胎儿面部的中矢状切面（胎儿侧面），下颌过小表现为下颚小且后缩。虽然有下颚测量的百分点图，但通常是发现了胎儿面部异常伴有明显的下颚后缩时主观上进行诊断（图11-31）[23、24]。必须认真扫查侧矢状面因为鼻梁发育不全和颚发育不全常被误认为下颌过小（图11-8）。不幸的是，中孕期常不能观察到下颚发育不全。Pilu和同事报道了一个 Pierre Robin综合征的病例，孕23周时中矢状面观察面部是正常的；但35周时相同的切面提示显著的下颌过小。特殊面部畸形可以确定诊断或作出精确诊断。然而，正常的超声表现并不能排除胎儿受影响因为超声发现综合征的可靠性依赖于表现的严重程度。例如：在常染色体显性下颌面部发育不全综合症中（Treacher Collins综合征），下颌过小和面裂的程度变化多样。面部发现畸形可能对于在多种异常综合症中进行精确诊断有帮助，甚至在缺少遗传和致畸剂危险因素的情况下。Nicolaides和同事报道的产前通过超声诊断为严重下颌过小的56例胎儿中，也可观察到其他的畸形和发育迟缓。并伴有围生期预后较差。虽然下颌过小在超过80%的18三体或三倍体胎儿中存在，Nicolaides和同事却只在83例胎儿的21例中（25%为18三体）和9例中（21%为三倍体）观察到下颌过小。因此，超声监测下颌过小的敏感性尚需要研究。病例的低估可能与胎儿位置或羊水过少有关[25、106]。