OP12.02: Absent nasal bone without other features of Trisomy 21 in pregnant Caucasian women: check the clavicles for cleidocranial dysplasia

Failed visualisation of the fetal nasal bone is strongly associated with aneuploidies (T21, 13, and 18). It may also rarely be observed in euploid fetuses, mostly of Afro-Caribbean and Asian ethnicities. Absent nasal bone can be the presenting sign for cleidocranial dysplasia. Case report: A 42-year-old pregnant woman was referred at 24 weeks for an absent nasal bone (Fig 1). In addition to this finding, the fontanelles were large with wide sutures (Fig 2), the brain parenchyma was unusually clearly visible, and the clavicles were hypoplastic (Fig 3). Fetal growth was normal. The child's father had two children from a previous relationship who had missing clavicles. The diagnosis of cleidocranial dysplasia was made and confirmed by amniocentesis with detection of RUNX2 gene mutation. Discussion: Cleidocranial dysplasia is an autosomal dominant skeletal dysplasia characterised by abnormal bone and dental development with normal cognitive function. 80% of cases are caused by a mutation in the RUNX2 gene. Prenatal genetic testing for this mutation is available. The prenatal sonographic diagnosis is primarily based on clavicular hypoplasia or aplasia and insufficient ossification of the cranium. In the few published cases, there are occasional references to the absence of the nasal bone, yet in most cases this is retrospective. Absent nasal bone may help to establish the prenatal diagnosis. Prenatal diagnosis is important, since the abnormal ossification of the skull and chest may expose the fetal brain and lungs to potential damage during natural birth. In conclusion, in absence of fetal nasal bone and low risk for fetal aneuploidies or if aneuploidies are excluded, consider cleidocranial dysplasia and check the clavicles.