First-trimester ultrasonographic screening for trisomy 21 has gained increasing popularity since the pioneer work of Nicolaides et al,1 which first demonstrated an association between increased nuchal translucency in the first trimester and chromosomal abnormalities. Subsequent studies from the same group demonstrated that the use of nuchal translucency thickness measurement in combination with maternal age is able to detect more than 70% of affected pregnancies for a false positive rate of 5%.2,3 To further improve the detection rate of trisomy 21 in the first trimester, other ultrasonographic markers have been extensively sought.3 Among them, absence of the nasal bone has emerged as one of the more promising first-trimester ultrasonographic markers of trisomy 21.4 According to the original series of 701 first-trimester high-risk pregnancies undergoing prenatal karyotyping by chorionic villus sampling, the nasal bone was ultrasonographically absent in 73% of fetuses with trisomy 21 and in only 0.5% of fetuses with a normal karyotype.4 Because this marker was found to be independent of other fetal and maternal variables, the authors concluded that nasal bone assessment can be incorporated as an additional marker in the algorithm for providing individual risks for trisomy 21 in the first trimester.4–7
Results from subsequent investigations by other groups, however, have been inconsistent. Although some authors have confirmed that nasal bone is indeed a useful ultrasonographic marker for the first-trimester screening of trisomy 21, both in high-risk8,9 and low-risk populations,10–12 others have found that the nasal bone is clearly identified in the first trimester in a large number of trisomy 21 fetuses, therefore suggesting that the nasal bone performs poorly as a screening test in the general population.13–15 These mixed findings draw attention to the complex issues surrounding nasal bone as a marker of trisomy 21, including the correct ultrasonographic technique for identifying the nasal bone in the first trimester,16 the significance of maternal ethnicity in nasal bone assessment,6,17 the optimal population (ie, high-risk, low-risk, or mixed) for nasal bone screening,14 and the gestational age at which nasal bone is best assessed, because the normal ossification process allows easier visualization in the late rather than in the early first trimester.6,18 The aim of this study was to report the experience from two fetal medicine specialists with first-trimester screening of trisomy 21 by assessing nuchal translucency thickness and nasal bone in a predominantly Latin American population.
PARTICIPANTS AND METHODS
The Fetal Medicine Center at Clinica Las Condes is a tertiary referral center that provides ultrasonographic screening for the local community as well as for patients seeking early prenatal diagnosis. Women presenting for first-trimester ultrasonographic examination with a single live fetus with a crown-rump length between 45 mm and 84 mm were prospectively recruited for this study, as approved by our institutional review board and after verbal informed consent was obtained from the patient. For the purposes of this study, only cases scanned by one of two fetal medicine specialists (W.S., V.D.), accredited and checked regularly by audits by the Fetal Medicine Foundation (London, England) for both nuchal translucency thickness measurement and nasal bone assessment, were included in this analysis.
In all cases, ultrasonographic examination was performed transabdominally using high-resolution ultrasound equipment (Accuvix XQ, Medison, Seoul, Korea; Voluson 730, GE Healthcare, Milwaukee, WI), following the guidelines established by the Fetal Medicine Foundation for nuchal translucency measurement and nasal bone assessment.2–4 If suboptimal views were obtained or either of these two markers was not properly visualized transabdominally, a transvaginal ultrasonogram or a repeat scan was performed according to patient’s preference. For screening purposes, an amplified view of the fetal head and upper thorax was obtained in a midsagittal plane to visualize the fetal facial profile and neck. Extensive effort was made to obtain the nuchal translucency and the nasal bone with an angle of insonation close to 90 degrees by rotating the transducer according to the position of the fetus. The nuchal translucency thickness was measured with electronic calipers placed in the “on-to-on” position.3 The presence of the nasal bone was assessed by focusing on the region of the nose, where under normal conditions three echogenic lines are identified.4 The two superficial lines represent the skin and, more caudally and anteriorly, the tip of the nose. Underneath the skin is a third echogenic line representing the nasal bone, which, with the overlying skin, forms an “equals” sign. Occasionally, because the nasal cartilage is not a single central structure, the transducer was tilted from side to side to ensure proper evaluation for presence or absence of the nasal bone. The nasal bone was considered to be present when an echogenic line, thicker and more echogenic than the overlying skin, was visualized. When this third echogenic line was not visualized, or was fainter, less echogenic, and thinner than the overlying skin, the nasal bone was considered to be absent. None of the women involved in this study underwent first-trimester maternal serum biochemistry for trisomy 21, because this test was not available at our institution during the time of the current study. Therefore, risk assessment for chromosomal abnormalities was based only on these two ultrasonographic markers, adjusted for maternal age and gestational age.
Information on maternal demographics and first-trimester ultrasonographic findings was entered in a specially designed software provided by the Fetal Medicine Foundation (Sybase ASA, 11–13+6 weeks scan 1.0.0, Astraia Software, Munich, Germany), which generates individual risks for trisomies 21, 18, and 13. Information on fetal karyotype and pregnancy outcome was entered into the database as soon as it became available. Prenatal invasive procedures for fetal karyotyping were performed using chorionic villous sampling, amniocentesis, or cordocentesis, depending on gestational age and individual technical conditions. Chromosomal results were obtained by using conventional cytogenetic techniques, including short- and long-term cultures of trophoblast cells retrieved from chorionic villus sampling. Cases of chromosomal abnormality were identified from the cytogenetics laboratory logbook, which recorded all the cytogenetic studies performed prenatally, after a spontaneous abortion or fetal death, or in neonates with physical abnormalities. Information from the remaining cases was obtained from the delivery records and neonatal discharge summaries, which record the condition of the neonate at birth and the physical examination performed by a neonatologist. Infants not having chromosomal analysis performed were considered genotypically normal if no structural anomalies or physical dysmorphisms were detected during the neonatal physical examination.
Statistical analyses were performed using the Statistical Packages for Social Sciences for Windows (SPSS Inc., Chicago, IL). Differences between fetuses diagnosed with trisomy 21 and without trisomy 21 were calculated after excluding fetuses with other chromosomal abnormalities. Risks were expressed as odds ratios (ORs) with 95% confidence intervals (95% CIs). McNemar’s χ2 test was used to compare detection rates of trisomy 21 with nuchal translucency and nasal bone. P<.05 was considered statistically significant.
In a 3-year period from January 2003 to 2006, a total of 1,287 consecutive first-trimester singleton pregnancies fulfilling the entry criteria were recruited. The median maternal age was 33 years (range 14–47 years), with 456 (35.4%) women being 35 years or older at the time of the scan. The median gestational age at the time of ultrasound was 12 weeks (range, 11–14 weeks), with 245 pregnancies (19.0%) being scanned at 11 completed weeks, 522 (40.6%) at 12 completed weeks, and 520 (40.4%) at 13 completed weeks. Information on both the nuchal translucency thickness and presence or absence of the nasal bone was obtained from all pregnancies. In 110 cases (8.5%), the nuchal translucency thickness measurement was greater than the 95th percentile according to the specific gestational age,2,3 and 25 fetuses (1.9%) had absent nasal bone. In 12.1% of our population (n=156) there was an adjusted risk for trisomy 21 of 1 in 300 or higher based on maternal age, gestational age, nuchal translucency thickness measurement, and nasal bone assessment.
Karyotype results were available in 179 fetuses, of which 31 had trisomy 21 and 35 had other chromosomal disorders (Table 1). Among those 31 fetuses with trisomy 21, the nuchal translucency thickness was greater than the 95th percentile in 28 (OR 21.6, 95% CI 16.1–28.9), and the nasal bone was absent in 13 (OR 256, 95% CI 60.3–1086.4). An increased nuchal translucency thickness had a significantly higher detection rate of trisomy 21 than an absent nasal bone (90.3% compared with 41.9%, respectively; P<.01, McNemar’s χ2 test) An adjusted risk of 1 in 300 or greater was found in 29 (93.5%) pregnancies affected by trisomy 21. All but one (92.3%) of the 13 trisomy 21 fetuses with absent nasal bone had increased nuchal translucency thickness. In the group of chromosomally normal fetuses or phenotypically normal infants at birth (n=1,221), the nuchal translucency thickness was more than the 95th percentile in 64 cases (false positive rate of 5.2%), and the nasal bone was found to be absent in only two cases (false positive rate of 0.2%). The latter two cases were scanned before 12 weeks of gestation.
The nasal bone was assessed to be absent in 25 fetuses, 23 (92%) of which were chromosomally abnormal, including in 13 of the 31 (41.9%) fetuses with trisomy 21, three of the 12 (25%) with Turner syndrome, three of the 10 (30%) with trisomy 18, two of the three (66.7%) with trisomy 13, one of the two (50%) with triploidy, and one of the eight (12.5%) with other chromosomal abnormalities (Table 1). No evidence of trisomy 21 was found at the clinical examination of the single stillbirth in this series, which was due to placental abruption at 32 weeks, and neither of the two fetuses which spontaneously aborted were affected by trisomy 21 as determined by chromosomal analysis of products of conception.
This study demonstrates that, in our mixed high- and low-risk population, increased nuchal translucency thickness remains an extremely important first-trimester ultrasonographic marker of trisomy 21 when performed by adequately trained operators. Among the 31 cases of trisomy 21 identified in our study, 28 had a nuchal translucency thickness more than the 95th percentile, yielding a sensitivity of 90.3%. Increased nuchal translucency thickness, however, had a false-positive rate for detecting trisomy 21 of 5.2%. This is not surprising, because it is well known that increased nuchal translucency is also associated with many other conditions such as congenital heart defects, genetic syndromes, and congenital structural anomalies,19 in addition to other chromosomal abnormalities such as trisomy 18, trisomy 13, and Turner syndrome.2,3 The results of our study also suggest that, in comparison with nuchal translucency thickness, nasal bone assessment plays a considerably less prominent role in the prenatal detection of trisomy 21. In our population, only 13 of the 31 fetuses affected with trisomy 21 had an absent nasal bone in the first trimester, corresponding to a sensitivity of 41.9%, in stark contrast to the 73% detection rate originally reported in 20014 and the 62.1% detection rate recently reported by the same group.7 However, we were able to confirm the original observation that this ultrasonographic marker had an extremely low false-positive rate, because chromosomally normal fetuses rarely have an absent nasal bone on ultrasonography, suggesting that its incidence is not affected by other pathologic conditions that are associated with increased nuchal translucency thickness.19 Indeed, of the 25 first-trimester fetuses with absent nasal bone detected in our population, 23 (92%) were chromosomally abnormal, of which 13 had trisomy 21.
Beyond the potential explanation that nasal bone is inherently a poor marker for trisomy 21, there are several possible reasons why nasal bone was associated with a poor sensitivity compared with nuchal translucency thickness in our study. First, ultrasonographic examinations were performed by highly trained and motivated fetal medicine specialists familiar with first-trimester ultrasonographic screening. In contrast, the vast majority of ultrasonographic examinations in other large population studies was performed by ultrasonographers or physicians with a varying degree of training. Second, high-resolution, state-of-the-art ultrasound equipment was used in our study, which permitted excellent visualization and precise measurement of nuchal translucency thickness. However, high-resolution ultrasound equipment may also affect nasal bone assessment, as a hypoplastic nasal bone may be easily identified as present with high-quality equipment, but may be merely assessed as present or absent with a machine with lower resolution. According to a previous study,20 the length of the nasal bone is not clinically relevant; only its presence or absence is important for screening purposes. In our study, however, four of the 18 (22.2%) trisomy 21 fetuses had a clearly identifiable, echogenic but hypoplastic nasal bone, highlighting a potential role for nasal bone length in the first-trimester ultrasonographic screening of trisomy 21. Third, the more trained the operator in identifying the nasal bone, the higher the detection rate of fetuses with absent nasal bone is likely to be. Although it is difficult to demonstrate with certainty, it is reasonable to expect that if less experienced operators had evaluated our population, a lower incidence of absent nasal bone would have been found. Given that absent nasal bone has a low prevalence in the normal population, a ultrasonographer practicing in a low-risk population would probably have limited experience to detect this technically challenging ultrasonographic finding.
Another limitation with nasal bone assessment is that this method is technically more difficult to perform than nuchal translucency thickness measurements, requiring appropriate training and supervision for a minimum of 120 examinations.16 Evaluation can only be obtained in the midsagittal plane, in which the fetus is facing the transducer, because the fetal skull impairs evaluation of the nasal bone if the fetus is facing away from the transducer. If the fetus is not facing the maternal abdominal wall at the time of the scan, the uterus and position of the transducer must be maneuvered to obtain correct positioning of the fetus relative to the transducer. On occasion, however, a proper evaluation will not be possible due to persistent position of the fetus, maternal habitus, or in pregnancies with a retroverted uterus, requiring examination with a transvaginal probe or a repeat scan. Because an absent nasal bone has been regarded as one of the most clinically significant markers for trisomy 21 in the first trimester,3 every effort was made to determine its presence or absence accurately in the current study. Nevertheless, it is still feasible that even with an adequate training, certain limitations, including those on time, interest, and resources, will hinder the incorporation of nasal bone assessment in the screening protocol of the unselected population.
Our findings also confirm that although absent nasal bone is indeed associated with trisomy 21, its relatively low sensitivity compared with nuchal translucency thickness and first-trimester biochemistry and the complex technical aspects involved in its accurate assessment currently limit its use in the general population. In addition, there are many concerns that, even with a solid technique and training, limit the incorporation of the nasal bone assessment in the first-trimester screening protocol in the unselected population.13–15 The most important confounding factor is the gestational age at the time of evaluation. Because ossification of the nasal bone is a continuous process, there is an increasing detection rate of the nasal bone as pregnancy progresses. In addition, ethnicity also plays an important role.6,17 It has been demonstrated that the visualization rate of the nasal bone is highest in the white population, intermediate in the Asian population, and lowest in the Afro-Caribbean population,6 although the role of paternal ethnicity has yet not been elucidated. In our study, this discrepancy was most likely not relevant, because this study was performed in a highly homogeneous population with less than 1% of the population representing Asian and Afro-Caribbean women. To our knowledge, based on a MEDLINE search of the English literature until June 2006 using the terms “nasal bone”, “first-trimester screening”, and “trisomy 21”, this is the first study reporting on screening in a predominantly Latin American population.
It has been recently suggested that nasal bone be incorporated into the first-trimester trisomy 21 risk assessment in only those patients with an intermediate risk after nuchal translucency and first-trimester biochemical screening, the so-called “contingency” screening.7 In this protocol, the first step is to determine the risk based on the maternal age, nuchal translucency thickness measurement, and maternal serum biochemistry. Women with risk for trisomy 21 greater that 1 in 100 should be offered fetal karyotyping, and those with risk lower than 1 in 1,000 should be reassured. However, in women with an intermediate risk from 1 in 100 to 1 in 999, a second ultrasonographic screening should then be carried out, at which time assessment of the nasal bone might play a clinically relevant role in determining which patients should be offered prenatal karyotyping. Evidence from the present study demonstrated that there is a limited role for nasal bone evaluation even in the contingency protocol. If the nasal bone is present upon initial examination, there is no reason for reevaluation and confirmation, because this will not affect significantly the adjusted risk, because more than one half of the fetuses with trisomy 21 will have a ultrasonographically identifiable nasal bone. However, a confident determination that the nasal bone is absent is clinically important, because the probability of trisomy 21 increases dramatically with absent nasal bone.
In conclusion, our study confirms that increased nuchal translucency thickness is a strong ultrasonographic marker for trisomy 21 in the first trimester. On the other hand, assessment of the nasal bone in the first trimester seems to have a less prominent role in identifying the fetus at risk for trisomy 21, because the presence of the nasal bone does not add very much to the current screening protocol using maternal age and nuchal translucency thickness. However, confident identification of an absent nasal bone has a high positive predictive value for trisomy 21 in the first trimester.
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