Nasal bone evaluation with ultrasonography: a marker for fetal aneuploidy

It is clear that evaluation of the fetal nasal bone is becoming a powerful tool in prenatal screening for DS. It will be argued for some time to come as to what the findings equivalent to nasal bone absence or hypoplasia are in either radiological or histomorphological studies. During the 11-14-week scan, the most powerful predictor of DS is the virtual absence of the nasal bone (severe hypoplasia) as viewed on ultrasound. The idea of using NBLs in screening at this point in gestation needs to be investigated further. The optimal time for nasal bone evaluation during this time period appears to be when the fetal CRL is in the range 65-74 mm. The incidence of nasal bone absence in the euploid population appears to be the lowest (approximately 1.5%) and the incidence of nasal bone absence is still very high (approximately 73%) in fetuses with DS, with the resultant likelihood ratio being approximately 4818. To arrive at the appropriate risk assessment, several factors including the patient's race, the NT measurement and the CRL measurement must be taken into consideration. However, even in the Afro-Caribbean population where the incidence of nasal bone absence in euploid fetuses appears to be the highest, the likelihood ratio remains significant (approximately 7.2). The fact that the nasal bone presence decreases the risk of DS by approximately three-fold inevitably results in a decreased false-positive rate. Nasal bone evaluation in the second trimester makes use of both the phenomenon of nasal bone absence and the fact that the NBL in DS fetuses with an identifiable nasal bone is shorter than in euploid fetuses. The finding of an absent nasal bone on the 15-22-week scan increases the risk of DS 83 times6. In the case where the nasal bone is present, if the measurement is short, either with the absolute measurement being 2.5 mm or less or if the BPD/NBL ratio is ≥ 10, the sensitivity of the test approaches 81% with a false-positive rate of 11%. The importance of a standardized view of the nasal bone cannot be overstated. The fetus must be imaged facing the transducer. The fetal face is viewed longitudinally and strictly in the midline. In this view, a normal nasal bone is identified as a thin echogenic line within the bridge of the nose. The skin over the nasal bridge can be as echogenic as the nasal bone itself. This is especially true during the 11-14-week scan. Therefore, the nasal bone must be clearly identified as a second echogenic line beneath the skin. A second surface echogenic line in front of the nasal bridge is usually seen. This represents the skin over the tip of the nose. An echogenic line or lines can also often be seen within the tip of the nose. These lines probably represent the cartilage and/or parts of the vomeral bones. This view was proposed and described by The Fetal Medicine Foundation3. One source of some confusion is the angle of insonation with which the nasal bone should be imaged. For example, the method proposed by The Fetal Medicine Foundation uses an angle of insonation of 45° with respect to a reference line, which connects the fetal forehead and the chin. The angle of insonation described in the paper by Minderer et al. is also 45° but the reference line here connects the forehead and the tip of the fetal nose. In practice, the actual angle of insonation only roughly approximates to the 45° proposed in these two descriptions. A much simpler way to describe the angle of insonation is to use the longitudinal axis of the nasal bone as the reference line, i.e. the nose should be viewed with the longitudinal axis of the nasal bone being at 90° to the insonating beam2,19 (Figure 1). Absence of the nasal bone is best demonstrated in the same view with the angle of insonation being perpendicular to the echogenic skin over the nasal bridge. The nasal bone length is obtained by simply measuring the nasal bone from one end to the other. The optimal way to measure the length is with accuracy to within 0.1 mm. This is especially important during the 11-14-week scan where the nasal bone size is small4,5,11,17; therefore, each 0.1 mm represents a large percentage of the actual measurement. A 90° angle of insonation allows for the best visualization of the nasal bone. However, the scatter of the ultrasound beam at both ends of the nasal bone in this view can artificially increase the nasal bone length. One way to reduce this scatter is to change the angle of insonation so that it is closer to either 45° or 135° (Figure 1). In addition to the lateral scatter of the ultrasound beam, the nasal bone during the 11-14-week scan often has short 'extensions', which are less echogenic than the nasal bone itself.