Absent nasal bone

In a fetal sonographic assessment, an absent nasal bone is a feature which can sometimes be used as a surrogate marker for fetal aneuploidy.

Radiographic assessment

Antenatal ultrasound

It is assessed on a midline sagittal view. In this section, the nasal bone is often seen as a bright echogenic line. It is best visualized at around 11^th to 14^th weeks of gestation (1^st trimester). A magnified image may assist in visualization.

When the mid sagittal view is difficult to assess, some authors suggest a coronal view of the fetal face to look for paired echogenic structures located at the upper tip of the retronasal triangle ⁶. 

Significance

When the nasal bone is absent at 11 to 12 weeks, while the other ultrasound markers and serum biochemistry are normal; a follow up scan after a week is suggested.

The incidence of an absent nasal bone is related to nuchal translucency (NT), crown rump length (CRL), and ethnic origin, as well as aneuploidy. It is more common with increased NT, smaller CRL measurements and in fetuses of Afro-Caribbean parents. 

The reported prevalence range of an absent nasal bone on ultrasound for euploid as well as the following aneuploidic states are

• euploid: nasal bone absent in 0.5-3% ¹
• trisomy 21: nasal bone absent in 60-73% ^1-3
• trisomy 18: nasal bone absent in 53-57% ^1,3
• trisomy 13: nasal bone absent in 32-45% ^1,3
• Turner syndrome: nasal bone absent in 9% ³

See also

• hypoplastic nasal bone