Likelihood ratios for fetal trisomy 21 based on nasal bone length in the second trimester: how best to define hypoplasia?

Authors


Abstract

Objective

To determine the best measure of fetal nasal bone hypoplasia for trisomy 21 risk assessment in the second trimester.

Methods

This was a prospective, observational study performed at a single institution between February 2003 and December 2005. Fetuses with nasal bone length recorded sonographically between 16 and 20.9 weeks and known karyotype were included. Definitions of nasal bone hypoplasia assessed included: non-visualized nasal bone, nasal bone < 10th percentile, nasal bone < 2.5th percentile, biparietal diameter/nasal bone ratio ≥ 10 and ≥ 11 and nasal bone multiples of the median (MoM) ≤ 0.5, ≤ 0.6, and ≤ 0.7.

Results

A total of 371 chromosomally normal and 11 trisomy 21 fetuses were included. Nasal bone hypoplasia based on MoM was superior to the other measures of hypoplasia for trisomy 21 risk assessment as reflected by a higher positive likelihood ratio. The risk for fetal trisomy 21 was higher with greater degrees of nasal bone hypoplasia.

Conclusion

Hypoplasia, as defined by MoM, is the superior approach when incorporating nasal bone evaluation into fetal trisomy 21 risk assessment in the second trimester. Copyright © 2007 ISUOG. Published by John Wiley & Sons, Ltd.

Introduction

Published studies have shown that absence or hypoplasia of the fetal nasal bone is associated with an increased risk for trisomy 211–12. In the first trimester, absence of the fetal nasal bone is highly efficacious in discriminating between normal fetuses and those affected by trisomy 211–5. Unfortunately, nasal bone assessment in the first trimester requires intensive training, likely limiting its widespread acceptance into clinical practice. With growth of the fetus and advancing gestation, the nasal bone is more easily imaged. While previously published data have shown that the nasal bone is more frequently visible in trisomy 21 fetuses in the second trimester of pregnancy5–8, it is frequently hypoplastic in fetuses affected by trisomy 21 when compared with normal fetuses5–7, 9, 10. Although several definitions for nasal bone hypoplasia in the second trimester have been proposed, including nasal bone length < 10th percentile5, < 2.5th percentile9, 11 and < 2.5 mm7, 10, and biparietal diameter to nasal bone ratio ≥ 10 and ≥ 116, 7, these definitions were arbitrary and were not based upon a prospective evaluation of their trisomy 21 screening performance.

This was a prospective study designed to evaluate the performance of nasal bone hypoplasia for fetal trisomy 21 risk assessment in the second trimester of pregnancy in fetuses of known karyotype. We sought to derive likelihood ratios for trisomy 21 based on alternative definitions of nasal bone hypoplasia in the second trimester, in an effort to determine how best to define fetal nasal bone hypoplasia for this purpose.

METHODS

This was an internal review board-approved, prospective observational trial performed at a single institution between February 2003 and December 2005. Patients at increased risk for fetal trisomy based on maternal age and/or serum screening referred between 16.0 and 20.9 weeks' gestation underwent a detailed ultrasound examination performed by trained sonographers and a maternal–fetal medicine specialist. Fetal karyotyping had not been performed prior to the ultrasound examination. In addition to the standard biometric and structural assessment, a detailed fetal aneuploidy survey was carried out. The following fetal markers of aneuploidy were assessed: short femur/humerus (for measured biparietal diameter), nuchal fold > 6 mm, choroid plexus cysts, nasal bone < 10th percentile, echogenic intracardiac foci, echogenic bowel, pyelectasis > 4 mm, iliac hip angle > 90° and two-vessel umbilical cord. Upon completion of the ultrasound examination, all patients were offered amniocentesis for fetal karyotyping. Only those fetuses with known karyotypes (obtained via amniocentesis or after delivery) and documented nasal bone length were included in this study.

All examinations were performed by sonologists trained in nasal bone assessment and the bone was assessed and measured as previously described5. Assessment of fetal nasal bone length in the second trimester can be achieved readily on standard images of the fetal profile, with little additional examination time required. Both nasal bones can be imaged by moving the sagittal imaging plane back and forth across the fetal profile. To account for potential discrepancies in the length between the paired nasal bones and in accordance with our previously published methodologies5, three measurements of the nasal bone length were obtained and the largest was recorded. The nasal bone was considered absent only if it was not visualized on all appropriate views. Using our previously published normative data, the median nasal bone length was plotted against the gestational age in weeks for the age range 16–20 weeks' gestation. Simple linear regression was performed on this plot to define the equation: median nasal bone length = − 0.214 + (0.04 × weeks' gestation) (R2 = 0.993). In a similar manner, an equation describing the 2.5th and 10th percentiles for the nasal bone length were determined: nasal bone length 2.5th percentile = − 0.098 + (0.025 × weeks' gestation) (R2 = 0.854) and nasal bone length 10th percentile = − 0.125 + (0.03 × weeks' gestation) (R2 = 0.922).

Fetal nasal bone hypoplasia as defined by non-visualized nasal bone, nasal bone length < 10th percentile, nasal bone < 2.5th percentile (2 SD below the mean), biparietal diameter/nasal bone (BPD/NB) ratio ≥ 10 and ≥ 11 and multiples of the median (MoM) ≤ 0.5, ≤ 0.6 and ≤ 0.7 were compared. Likelihood ratios were determined for each measure of hypoplasia studied by dividing the incidence of hypoplasia in trisomy 21 fetuses by its incidence in the euploid group. When the nasal bone was not visualized, a length of 0.00 mm and a BPD/NB ratio ≥ 10 was recorded. In an effort to better understand the fetal risks associated with a short versus absent nasal bone, we determined separate likelihood ratios for nasal bone length ≤ 0.5, ≤ 0.6 and ≤ 0.7 MoM, exclusive of fetuses with an absent nasal bone.

Results

Karyotyping was available in 476 fetuses from women undergoing amniocenteses during the study period, and postnatal karyotyping for suspected aneuploidy was available in four additional neonates. Aneuploidy was diagnosed in 22/476 amniocenteses and in all four of the infants karotyped postnatally, including 11 cases of trisomy 21, four of trisomy 18, one of monosomy X and 10 others. Nasal bone length was recorded in all fetal trisomy 21 cases and in 23/26 cases of aneuploidy overall. Of the 454 chromosomally normal fetuses, nasal bone length was recorded for 371 from 365 patients (six twin pairs). The mean ± SD maternal age for these patients was 36.5 ± 4.1 years; the median gravidity and parity were 2 (range, 12–1) and 1 (range, 5–0), respectively. Fetal anomalies were noted at ultrasound in 9/11 (81.8%) trisomy 21 fetuses (Table 1). In the euploid group, 3/371 (0.8%) fetuses had absent nasal bone (Figure 1). The length of the nasal bone in the euploid group averaged 0.92 MoM. Documentation inconsistencies amongst the sonographers were to blame for the lack of available nasal bone length in some fetuses.

Figure 1.

Mid-sagittal view through the facial profile of an 18-week fetus with non-visualized nasal bone.

Table 1. Summary of the cases of trisomy 21 identified (n = 11)
GA (weeks)Sonographic findingsNB length (cm (MoM))Normal nasal bone length for GA (cm)
Median< 10th centile< 2.5th centile
  1. GA, gestational age; NB, nasal bone.

16.4Multiple anomalies0.12 (0.27)0.440.370.31
17.0Ventricular septal defect0.39 (0.84)0.470.390.33
17.3Nasal bone hypoplasia0.32 (0.67)0.480.390.33
17.6Normal0.48 (0.98)0.490.400.34
17.6Short fibula0.20 (0.41)0.490.400.34
17.9Echogenic intracardiac foci0.26 (0.52)0.500.410.35
18.0Multiple anomalies0.37 (0.73)0.510.420.35
18.0Multiple anomalies0.29 (0.57)0.510.420.35
20.1Normal0.59 (1.0)0.590.480.40
20.6Absent nasal boneAbsent0.610.490.42
20.6Multiple anomalies0.42 (0.69)0.610.490.42

The nasal bone was visualized in all but one trisomy 21 fetus (91%). Although present, the nasal bone in trisomy 21 fetuses was typically hypoplastic (Figure 2), with an average length of 0.67 MoM (for trisomy 21 fetuses with visualized nasal bone). A comparison of the alternative definitions for nasal bone hypoplasia in the second trimester is presented in Table 2. The incidence for each definition of nasal bone hypoplasia in euploid and trisomy 21 fetuses is presented, along with the calculated likelihood ratio for each sonographic index. Nasal bone hypoplasia as defined by MoM was superior to all other measures of nasal bone hypoplasia, as reflected in the higher likelihood ratios. In the calculation of these likelihood ratios, fetuses with non-visualized nasal bone and those with present but hypoplastic nasal bone were considered together. To better understand the significance of a visualized but hypoplastic nasal bone, likelihood ratios for nasal bone hypoplasia of ≤ 0.5, ≤ 0.6 and ≤ 0.7 MoM were calculated, exclusive of fetuses with non-visualized nasal bone (Table 3).

Figure 2.

Mid-sagittal view through the facial profile of an 18-week trisomy 21 fetus showing severe nasal bone hypoplasia (arrow).

Table 2. Comparison of alternative definitions of nasal bone (NB) hypoplasia
DefinitionEuploid fetuses (n = 371) (n (%))Trisomy 21 fetuses (n = 11) (n (%))Trisomy 21 likelihood ratio
  1. BPD, biparietal diameter, MoM, multiples of the median.

Absent NB3 (0.8)1 (9)11.3
NB length < 10th percentile55 (14.8)8 (72.7)4.9
NB length < 2.5th percentile13 (3.5)5 (45.4)13.0
BPD/NB ≥ 1046 (12.4)8 (72.7)5.9
BPD/NB ≥ 1116 (4.3)7 (63.6)14.7
NB length ≤ 0.5 MoM4 (1.1)3 (27.2)24.7
NB length ≤ 0.6 MoM7 (1.9)5 (45.5)23.9
NB length ≤ 0.7 MoM16 (4.3)7 (63.6)14.8
NB length > 0.7 MoM355 (95.7)4 (36.4)0.38
Table 3. Trisomy 21 likelihood ratios (LR) based on nasal bone hypoplasia, excluding fetuses with absent nasal bone (n = 378)
Nasal bone lengthPositive LRNegative LR
≤ 0.5 MoM74.1
≤ 0.6 MoM36.4
≤ 0.7 MoM17.10.38

Discussion

To our knowledge, this is the first prospective study to evaluate the performance of nasal bone length in fetal trisomy 21 risk assessment in the second trimester of pregnancy in fetuses of known karyotype. Our primary focus was to determine how best to define fetal nasal bone hypoplasia for the purposes of second-trimester fetal Down syndrome risk assessment through the derivation of trisomy 21 likelihood ratios for alternative definitions of nasal bone hypoplasia/absence. We determined that nasal bone hypoplasia as defined by MoM was superior to all other measures of hypoplasia studied, as reflected in the higher positive likelihood ratios. Furthermore, we have shown that when the nasal bone is present, the risk for fetal trisomy 21 increases with greater degrees of nasal bone shortening; the positive likelihood ratio for fetal trisomy 21 increased from 17.1 with a nasal bone length of ≤ 0.7 MoM to 74.1 with a nasal bone length of ≤ 0.5 MoM (Table 3).

Previously reported rates of non-visualization of the nasal bone in trisomy 21 fetuses during the second trimester have ranged from 25 to 56%5–10. Our low rate of non-visualization of the nasal bone is likely related to several factors. Most importantly, our study was prospective, and the nasal bone was assessed in realtime by two examiners trained in nasal bone assessment. Reliance on a retrospective review of fetal profile images is unreliable for assessment of the presence/absence of nasal bone and likely accounts for the higher non-visualization rates in some studies6, 8. Our definition for absent nasal bone was unequivocal; even if it was extremely small, the nasal bone was considered present and was measured. Studies categorizing ‘exceedingly small’ nasal bones as absent would be expected to have a higher rate of nasal bone non-visualization7, 9, 10.

Strengths of our study include its prospective nature, our use of previously established normative data for nasal bone length, inclusion of only fetuses with known karyotypes, and the skill of the sonographers performing the nasal bone assessment. The derived likelihood ratios were meant to serve as a basis for comparison of the alternative definitions of nasal bone absence/hypoplasia studied. The actual likelihood ratios may differ with the inclusion of additional affected fetuses, amongst ultrasound centers with different levels of expertise, and amongst study populations with higher or lower fetal aneuploidy risk. Several limitations of the study are worth noting. The small number of trisomy 21 fetuses represents the major limitation. A larger percentage of normal fetuses had a nasal bone below the median (average, 0.92 MoM) and < 10th percentile (14.8%) than would be expected for a normally distributed population. The equation used to calculate the expected nasal bone length was determined in a separate, unscreened population and may have overestimated the expected length in the study population. This overestimation of the expected nasal bone length would have been applied equally to all fetuses and would therefore have been canceled out when calculating the likelihood ratios.

When performing fetal nasal bone evaluation for trisomy 21 risk determination, nasal bone absence is the most effective index in the first trimester whereas hypoplasia based on MoM, is the most effective in the second trimester. When gestational age is known, nasal bone hypoplasia defined by MoM is the best discriminator between trisomy 21 and normal fetuses, as evidenced by the high positive likelihood ratios. Few sonographic markers of aneuploidy possess likelihood ratios of similar magnitude (Table 4)13. Furthermore, defining nasal bone hypoplasia by MoM permits a more individualized risk assessment by allowing for risk adjustment based upon the degree of nasal bone hypoplasia. Conversely, the negative likelihood ratio associated with a normal nasal bone length, as defined by nasal bone length > 0.7 MoM, can provide significant fetal trisomy 21 risk reduction, with a negative likelihood ratio of 0.38. This risk reduction is far greater than that described in association with other second-trimester sonographic markers for trisomy 21 (Table 4)13. Assuming independence amongst nasal bone length and these other markers, the negative likelihood ratio associated with a normal nasal bone length has the potential to offset the marginal increase in fetal Down syndrome risk associated with these other, weaker, sonographic markers of aneuploidy.

Table 4. Trisomy 21 likelihood ratios (LR) of second-trimester sonographic markers13
MarkerPositive LRNegative LR
Nuchal fold53.050.67
Short humerus22.760.68
Short femur7.940.62
Pyelectasis6.770.85
Echogenic intracardiac foci6.410.75
Echogenic bowel21.170.87
Major defect32.960.79

Ancillary