The term micrognathia describes a small mandible.

Epidemiology

Associations

Micrognathia is associated with a vast array of other congenital anomalies which include:

• aneuploidic syndromic

  • trisomy 9 ⁴

  • trisomy 13

  • trisomy 18 

• non-aneuploidic syndromic

  • arterial tortuosity syndrome

  • 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

  • congenital cardiac anomalies

  • certain skeletal dysplasias

    • achondrogenesis

Pathology

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 4^th week of gestation.

Radiographic features

Ultrasound

Due to a large portion of normal mandibular growth occurring in the 3^rd 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 anteroposterior diameter/biparietal diameter) x 100

• frontal nasomental angle ³

Ancillary sonographic features

If fetal swallowing is impaired there may be evidence of polyhydramnios.

Treatment and prognosis

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. In selected case a genioplasty may be an option in later life.

Complications

Severe micrognathia can potentially compromise neonatal respiration after birth.

Practical points

Due to a high association rate with other anomalies, the detection of micrognathia warrants a careful search for other fetal abnormalities.

See also

• microgenia