The term micrognathia essentially means a small mandible.
A small mandible occurs secondary to abnormalities of the first branchial arch which in turn are caused by deficient or insufficient migration of neural crest cells and usually occur around the 4th week of gestation.
Micrognathia is associated with a vast array of other congenital anomalies which include:
- aneuploidic syndromic
non aneupliodic syndromic
- Fryns syndrome
- Goldenhar syndrome: hemifacial microsomia
- hydrolethalus syndrome
- lethal multiple pterygium syndrome
- Nager syndrome
- Neu Laxova syndrome
- Pena Shokeir syndrome
- Pierre Robin syndrome
- Seckel syndrome
- Smith Lemli Opitz syndrome
- Stickler syndrome
- TAR syndrome
- Treacher Collins syndrome: mandibulofacial dysostosis
- non syndromic non aneuploidic
Due to a large portion of normal mandibular growth occurring in the 3rd trimester, the condition is best diagnosed towards the latter half of pregnancy
- a true sagittal facial image would show a receding chin.
- the facial profile view is most useful in evaluating the mandibular size
Micrognathia is often a subjective finding best appreciated on a midline sagittal view.
Parameters used for objective measurement include:
- jaw index: (mandibular antero-posterior diameter/bi-parietal diameter) x 100
- frontal naso-mental angle 3
Ancillary sonographic features
If fetal swallowing is impaired there may be evidence of polyhydramnios.
Due to a high association rate with other anomalies, the detection of micrognathia warrants a careful search for other fetal abnormalities.
Severe micrognathia can potentially compromise neonatal respiration after birth.
The overall prognosis is highly variable dependent on the presence of other associated anomalies. Even when there is isolated fetal breathing (respiratory) difficulty at the time of birth, it is a concern 5.
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