Nasal bone measurement in Normal & Down's Foetus

 

Nasal bone hypoplasia in Downs—how difficult to image at 12-13.6 weeks scan !!!! Do you agree . In his original description of patients with trisomy 21, Langdon Down reported that the face was flat and the nose was small. Later, many workers  have observed absence of the nasal bone in 2/3 trisomy 21 fetuses examined at 19–22 weeks of gestation. The concern is that  at 11–14 weeks' gestation the nasal bone is not visible by ultrasonographic examination in about 70% of fetuses with trisomy 21 and in less than 1% of chromosomally normal fetuses. Furthermore, in trisomy 21 fetuses there was no significant difference in nuchal translucency thickness (NT) between those with and those without a visible nasal bone. Therefore, these two sonographic markers can be combined to provide a more effective method of early screening for trisomy 21, with an estimated detection rate of about 90% for a false‐positive rate of 5%,.

 

 

During the detailed ultrasound examination, which is routinely carried out before mid‐trimester amniocentesis for fetal karyotyping, the fetal profile and nasal bone are  also examined .The nasal bone was considered to be hypoplastic if it was absent or it appeared strikingly small, in which case it was measured and found to be always less than 2.5 mm.. In some study of 367 fetuses reporting on nasal bone length, measured by ultrasonography in normal fetuses, the length increased with gestation from a mean of 4.7 mm at 15 weeks to 8.2 mm at 22 weeks and the respective values for the 2.5th centile were 3.2 and 6.0 mm. In a recent study of 2050 normal fetuses at 15–22 weeks' gestation, the mean nasal bone length was 4.3 mm and the 2.5th centile was 2.8 mm at 15 weeks, and the respective values at 22 weeks were 7.5 and 5.6 mm.