Micrognathia

Micrognathia can be due to a short mandible or a backward displacement of the mandible. It is seen in 1 in 1,500 live births.

Immediate morbidity and mortality depends largely on appropriate expertise at birth to facilitate airway management. Longer-term morbidity and mortality is largely determined by an underlying diagnosis and/or associated anomalies.

Polyhydramnios is a feature of micrognathia owing to obstruction of the gastrointestinal tract by the tongue.

There are many associated anomalies, and the more are present, the greater likelihood there is of an underlying genetic diagnosis.

Watch this brief ultrasound video from the Fetal Medicine Foundation to see how features of micrognathia can be identified during the 20-week scan.

• Around 30% of micrognathia cases will have an underlying chromosomal anomaly, particularly trisomy 18 and triploidy.
• Micrognathia can be associated with many genetic syndromes, including:
  • Pierre Robin sequence, which is associated with micrognathia, cleft palate and glossoptosis and may be associated with an underlying genetic diagnosis and/or other structural anomalies;
  • Treacher Collins syndrome, which is associated with hypoplasia of the maxilla and zygoma, micrognathia, cleft palate and malformed or absent ears (nearly all cases of Treacher Collins syndrome have heterozygous variants in TCOF1, POLR1D or POLR1B, or homozygous variants in POLR1C or POLR1D, while around 3% will not have an identified genetic cause); and
  • otocephaly (or Agnathia-Microstomia-Synotia syndrome), which is associated with severe micrognathia or agnathia with midline anomalies including holoprosencephaly, encephalocele, cyclopia, aglossia or midline ear positioning.

Recurrence depends on the underlying cause. Isolated micrognathia has no increased risk of recurrence. Where common aneuploidy is diagnosed, the recurrence risk is around 1%.

If a genetic syndrome or single-gene disorder is identified, the recurrence risk is 25%–50%, depending on the inheritance pattern.

Antenatal management

• Regular ultrasound monitoring every four weeks during the antenatal period.

Delivery

• Where significant micrognathia is expected, there can be difficulties with airway management at birth. Delivery should therefore be planned in a unit with appropriate expertise for difficult airway management in a neonate.
• Delivery should be timed to facilitate appropriate neonatal cover at the time of birth, in case of complications. This is usually at around 38 weeks.
• There is no contraindication to vaginal birth, though elective caesarean section is sometimes preferred for planning purposes.

Postnatal management

• Immediate availability of airway management team is required.
• Multidisciplinary management of neonatal and paediatric care is required.

For clinicians

• Fetal Medicine Foundation: Micrognathia
• NHS England: National Genomic Test Directory

References:

• Antonakopoulos N and Bhide A. ‘Focus on prenatal detection of micrognathia‘. Journal of Fetal Medicine 2019: volume 6, pages 10–112. DOI: 10.1007/s40556-019-00210-0
• Paladini D. ‘Fetal micrognathia: Almost always an ominous finding‘. Obstetrics & Gynaecology 2010: volume 35, issue 4, pages 377–384. DOI: 10.1002/uog.7639

For patients

• Cleft Lip and Palate Association: Pierre Robin sequence
• International Society of Ultrasound in Obstetrics and Gynaecology: Micrognathia