Diagnostic ultrasound ( PDFDrive )

CHAPTER 31 Chromosomal Abnormalities 1101
femur length ratio of 0.91 or less identiies 40% of fetuses with
trisomy 21, with a 5% FPR. 125 Although all agree that fetuses
with trisomy 21 have shorter femurs than euploid fetuses, the
diference is quite small, and the clinical utility of this inding
remains controversial. 126 Additionally, femur length varies among
fetuses of diferent ethnicity. Asian fetuses tend to have shorter
femurs and black fetuses longer femurs compared with white
fetuses. 127 Such diferences may be suicient to question the
usefulness of the femur as a marker in many populations.
Humerus Length
he length of the humerus is a more sensitive and speciic marker
for trisomy 21 than the femoral length. 128 Benacerraf et al. 129
found that the measured humeral length was shorter than the
expected humeral length in fetuses with trisomy 21, with a
measured-to-expected humeral length ratio of less than 0.90 the
optimal criterion for detecting afected fetuses (expected humeral
length = −7.9404 + 0.8492 × BPD). he use of this ratio identiied
50% of the fetuses with trisomy 21, with a 6.2% FPR. 129
Urinary Tract Dilation
In the second trimester, mild urinary tract dilation is considered
present if the maximal anteroposterior diameter of the intrarenal
pelvis measures 4 mm or more 130 (Fig. 31.10A). Among fetuses
with trisomy 21, 17% to 25% have urinary tract dilation, compared
with 2% to 3% of euploid fetuses. 130,131
Echogenic Bowel
Echogenic bowel is seen in 0.2% to 0.8% of fetuses in the second
trimester. 132-134 To be considered echogenic, the bowel must appear
as a well-delineated homogeneous area that is as bright as the
adjacent bone when the operator is using a transducer with a
frequency of 5 MHz or less (Fig. 31.10B, Video 31.4). he
incidence of chromosomal abnormalities in the setting of
echogenic bowel ranges from 3% to 27%. 132-137 he cause of
echogenic bowel seen in association with aneuploidy may be
related to poor bowel motility and decreased water content of
meconium. 137 In addition to aneuploidy, echogenic bowel is
associated with cystic ibrosis, infectious causes such as cytomegalovirus,
primary bowel abnormalities, and severe growth
restriction. It has also been associated with fetal ingestion of
blood and impending fetal demise. 132-137
Echogenic Intracardiac Focus
An echogenic intracardiac focus (EIF) is a discrete, bright white
dot that appears as bright as bone in the region of the papillary
muscle of the heart (Fig. 31.10C). An EIF is usually located
within the let ventricle but can be seen in either or both cardiac
ventricles. Most are seen as a single focus, but they may occur
as multiple foci. his sonographic inding is caused by mineralization
of the papillary muscle and is seen in 16% of fetuses with
trisomy 21 as well as 5% of euploid fetuses. 138,139
An EIF is the most common marker to occur as an isolated
entity both in fetuses with trisomy 21 and in euploid fetuses. 90,91
An EIF carries a twofold to fourfold increased risk of trisomy
21. 90,91,94,139,140 his marker cannot be used reliably in patients of
Asian ancestry because of the high prevalence of the EIF in the
euploid Asian population. 141
he identiication of an EIF is confounded by technical considerations
such as cardiac position. 142 When the interventricular
septum is pointing directly toward or away from the transducer
beam in an apical or basal view, an EIF is detected more oten
than when the septum is imaged perpendicular to the transducer
beam in a lateral view. To be convinced that an EIF is present, it
must be as bright as bone and seen in several planes. Other
normal specular relectors in the fetal heart, such as the moderator
band in the right ventricle, can be mistaken for an EIF. Winn
et al. 143 evaluated 200 patients scanned between 18 and 22 weeks’
gestation. he rate of “true” echogenic intracardiac foci was 11 in
200 (5.5%). he rate of “false” echogenic foci was 34 in 200 (17%).
he most common locations for a false EIF were the moderator
band, endocardial cushion, and tricuspid valve annulus.
Recently, an aberrant right subclavian artery has been reported
as a useful prenatal ultrasound marker for the detection of trisomy
21. 144 A recent meta-analysis reported the prevalence of aberrant
right subclavian artery in fetuses with trisomy 21 is 23.6%
compared with 1.02% in euploid fetuses. he identiication of
an aberrant right subclavian artery should prompt genetic
sonography including fetal echocardiography and correlation
with a priori risk estimates. 145
Structural Anomalies
Approximately 25% of fetuses with trisomy 21 are identiied as
having a structural anomaly, including cardiac defects, duodenal
atresia, ventriculomegaly, and hydrops 90,91,132,146 (see Fig. 31.7).
Cardiac defects are seen in approximately 50% of newborns with
trisomy 21 and include atrioventricular septal defects and
ventriculoseptal defects. With meticulous technique, these can
be found prenatally in 80% to 90% of fetuses with trisomy 21. 147
Adjunct Features of Trisomy 21
Adjunctive features that may be seen on occasion in fetuses with
trisomy 21 but are not robust enough to adjust the risk of
aneuploidy include iliac angle measurements, ear length, clinodactyly,
hypoplasia of the middle phalanx of the ith digit, sandal
gap toe, and frontothalamic distance. hese features are diicult
to standardize, and there is substantial overlap between those
with and without trisomy 21. he identiication of an adjunctive
feature should prompt a genetic sonogram in which markers
with established likelihood ratios are evaluated and correlated
with a priori risk estimates. 148-154
Cases of trisomy 21 have been reported in fetuses with choroid
plexus cysts. 155 However, this is believed to result from the
population incidence of choroid plexus cysts rather than from
trisomy 21. 156
Combined Markers
he inding of individual sonographic markers can be used to
estimate a risk of aneuploidy. 156-163 In the early years of genetic
sonography, a scoring index was utilized that rated the markers
as “major” (score of 2 for thickened nuchal fold or major anomaly)
and “minor” (score of 1 for each inding, such as short femur,
short humerus, pyelectasis, echogenic bowel, and EIF). 157,158 A
cumulative score of 2 or more identiied 73% of fetuses with
trisomy 21, with an FPR of 4%. Nyberg et al. 90,162 suggested
integrating these markers into risk assessment by using Bayes