The nasal bone in fetuses with trisomy 21: sonographic versus pathomorphological findings.

Objective: To compare the sonographic findings of the nasal bone in fetuses with trisomy 21 with pathomorphological findings to determine whether the bone is truly absent.

Methods: Seventeen first-trimester fetuses with trisomy 21 were identified; the median gestational age was 12 weeks (range, 11-14) and the median maternal age was 38 (range, 27-47) years. Transabdominal ultrasound examination, preceding transabdominal chorionic villus sampling (TA-CVS) for karyotyping, included assessment of the fetal nose. The nasal bone was determined to be 'hypoplastic' or 'absent' and its length was measured. All pregnancies underwent termination after diagnosis. Serial sagittal sectioning with hematoxylin and eosin-staining of formalin fixed tissue was performed.

Results: Of the 17 cases, the nasal bone was sonographically evident, but with severe hypoplasia in 10 cases, absent in six, and in the remaining case it was not able to be assessed due to fetal position. Histomorphologically, in 16 cases a nasal bone was present, detectable by the evidence of an ossification center, and in one case the ossification structure was not clearly visualized. Retrospective review of ultrasound images could identify nasal bones in five of the six cases in which they were initially reported as being absent on ultrasound examination. These were visible, but less distinct and had decreased echogenicity, hence misinterpretation led to the false finding of an absent nasal bone when it was in fact present but hypoplastic.

Conclusion: Sonographic assessment of the fetal nasal bone should not distinguish between 'present' and 'absent', but instead between 'normal' and 'hypoplastic'. For reproducible results it is necessary to standardize the sonographic examination. The sonographic landmarks of the fetal nose are: the nasal bone, the skin above and the cartilaginous tip of the nose.