Diagnostic ultrasound ( PDFDrive )

1100 PART IV Obstetric and Fetal Sonography
A B C
FIG. 31.10 Second-Trimester Markers for Trisomy 21. (A) Urinary tract dilation. Transverse scan through the fetal abdomen at 18 weeks
shows urinary tract dilation. The maximal diameter of the intrarenal renal pelvis measures 4 mm (calipers). Measurements are optimally performed
with the fetal spine at 12 or 6 o’clock. (B) Echogenic bowel. Sagittal scan through the fetal abdomen in the second trimester reveals a focus of
echogenic bowel (arrow). Note that the bowel is as bright as bone. See also Video 31.4. (C) Echogenic intracardiac focus (EIF). Axial scan through
the fetal chest showing the four-chamber view of the heart with a “double” EIF in the left ventricle.
TABLE 31.3 Pooled Estimates of
Detection Rates (DRs), False-Positive Rate
(FPR), Positive and Negative Likelihood
Ratios (LR + and LR − )
Marker DR FPR LR + LR −
Nuchal fold 26.0 1.0 23.30 0.80
Short femur 27.7 6.4 3.72 0.80
Short humerus 30.3 4.6 4.81 0.74
Echogenic bowel 16.7 1.1 11.44 0.90
Echogenic intracardiac focus 24.4 3.9 5.83 0.80
Urinary tract dilation 13.9 1.7 7.63 0.92
Ventriculomegaly 7.5 0.2 27.52 0.94
Absent or hypoplastic nasal 59.8 2.8 23.27 0.46
bone
Aberrant right subclavian
artery
30.7 1.5 21.48 0.71
Modiied from Agathokleous M, Chaveeva P, Poon LC, et al.
Meta-analysis of second-trimester markers for trisomy 21. Ultrasound
Obstet Gynecol. 2013;41(3):247-261. 109
fetuses. A receiver-operator characteristic curve for the prediction
of trisomy 21 based on biparietal diameter (BPD)/nasal bone
length (NBL) reveals that a cutof of 11 or greater identiies 69%
of trisomy 21 fetuses with a 5% FPR. Other authors recommend
measurement of less than 5th percentile or an absolute measurement
of less than 2.5 mm as thresholds to predict aneuploidy. 113-116
Vintzileos et al. 117 retrospectively evaluated the signiicance
of the nasal bone ossiication in fetuses referred for genetic
sonogram; 29 fetuses with trisomy 21 were compared to 102
euploid fetuses. Absence of the nasal bone was seen in 41% of
fetuses with trisomy 21 and none of the euploid fetuses. he
absence of the fetal nasal bone is as powerful a marker for trisomy
21 as the thickened nuchal fold. 118
Odibo et al. 119 suggested that evaluating the NBL as a multiple
of the median is the optimal method of using this marker. hese
investigators examined 3634 women at increased risk for aneuploidy
and nasal bone assessment was possible in 3197 women
(88%), 23 of whom had fetuses with trisomy 21. An NBL of less
than 0.75 MoM provided the best deinition of nasal bone
hypoplasia and had a sensitivity and speciicity of 49% and 92%,
respectively, compared with 61% and 84% for BPD/NBL greater
than 11. hese investigators favor the incorporation of absent
nasal bone as a major marker for trisomy 21 in the secondtrimester
genetic sonographic screening because of its ease of
identiication and better speciicity compared with MoM of less
than 0.75.
here is variation in the prevalence of a hypoplastic or absent
nasal bone depending on ethnicity. Cicero et al. 115 reported that
8.8% of patients of African Caribbean ancestry had an absent
or hypoplastic nasal bone, compared with 0.5% of Caucasian
fetuses, thus limiting the utility of this marker in patients of
African Caribbean heritage.
hree-dimensional ultrasound has been used to evaluate the
presence or absence of the fetal nasal bone. In 20 fetuses with
trisomy 21, Benoit and Chaoui 120 found 9 had either an absent
or a hypoplastic nasal bone on two-dimensional ultrasound.
he three-dimensional evaluation showed bilateral nasal bone
absence in six fetuses and unilateral nasal bone absence in
three. Prenasal thickness (PT) has recently been reported as a
useful marker for the detection of trisomy 21 in the second and
third trimester. In a retrospective evaluation of 159 fetuses with
trisomy 21, Vos et al. reported on the NBL and PT as well as
ratios of NBL/PT and prefrontal space ratios to identify fetuses
with trisomy 21. 121 hese ratios were not gestational age related
and at least one of these four indings was identiied on 95.3%
of fetuses with trisomy 21. he NBL/PT and prefrontal space
ratios individually identiied 86% and 80% of fetuses with trisomy
21. In the setting of a normal karyotype and otherwise normal
detailed imaging, the outcome for an isolated absent nasal bone is
favorable. 122
Femur Length
A short femur was one of the earliest recognized features in the
sonographic detection of trisomy 21. 123 he expected femoral
length (FL = −9.645 + 0.9338 × BPD) accounts for 94% of variation
in normal length. 124 Based on BPD, a measured-to-expected