Diagnostic ultrasound ( PDFDrive )

CHAPTER 31 Chromosomal Abnormalities 1093
approximately an 85% detection rate for Down syndrome at a
5% FPR. 22,25,33
Nuchal Translucency and
Other Aneuploidies
A thickened NT has been used to detect fetuses with trisomy
18. hese fetuses have an abnormal biochemical pattern with
very low β-hCG and very low PAPP-A. he BUN trial of 8514
screened pregnancies, using a risk cutof of 1 in 150, identiied
91% of fetuses with trisomy 18 with a 2% FPR. Additionally,
four in ive fetuses with trisomy 13 were identiied. 23 hese
trisomies result in fetuses that have multiple congenital malformations
and rarely survive beyond the irst year of life. he FASTER
trial reported a detection rate of 78% for all nontrisomy 21
aneuploidies at FPR of 6% when irst-trimester screening included
those cystic hygroma or combined screening. 45 Spencer and
Nicolaides 46 developed an algorithm for the detection of trisomy
18 and 13 that included maternal age, NT, free β-hCG, and
PAPP-A. Using a risk cutof of 1 in 150, they predicted a 95%
detection rate for these chromosomal defects with FPR of 0.3%.
In a population-based study of 452,901 women with singleton
gestations, the detection rates of aneuploidy based on irsttrimester
and sequential screening was 81.6% at a FPR of 4.5%.
he detection rates of trisomy 21, 18, and 13 were 92.9%, 93.2%,
and 80.4% respectively. he detection of monosomy X and
triploidy were 80.1% and 91.0%. 47 In a recent prospective validation
study the irst-trimester combined test detected 90%, 97%,
and 92% of trisomies 21, 18, and 13, respectively, as well as >90%
of cases of monosomy X, >85% of triploidies, and >30% of other
chromosomal abnormalities, at FPR of 4%. 48
Cystic Hygroma
he distinction between a irst-trimester cystic hygroma and a
thick NT is controversial. Historically, a cystic hygroma has been
diagnosed when the hypoechoic space at the back of the neck
extends down the fetal back and contains septations (Fig. 31.3,
Video 31.2). Malone et al. 49 reported on 134 cases of cystic
hygroma identiied in the FASTER trial from 38,167 screened
pregnancies. Chromosomal abnormalities were present in 51%
of these patients and major structural anomalies were identiied
in 34% of those without karyotypic anomaly. Survival with normal
outcome was conirmed in 17% of cases. hese investigators
reported an increased risk of aneuploidy, cardiac malformations,
and fetal death in fetuses with cystic hygroma compared with
those who had a simple, thickened NT. 49 Other investigators
have reported that septations are seen in all thickened NTs if
examined in the transverse suboccipitobregmatic plane and that
the incidence of adverse outcome is related to NT thickness
rather than appearance. 50,51
Nasal Bone
Dr. Down, in his original essay describing the physical features
of a category of developmentally delayed individuals, identiied
the nose as being small. 12 Not surprisingly, many fetuses with
trisomy 21 have a small or absent nasal bone. he fetal nasal
bone can be seen by ultrasound starting at approximately 11
weeks’ gestation. he irst-trimester nasal bone evaluation is
technically challenging, and competency in assessing nasal bone
reportedly takes an average of 80 scans. 52 he criteria for a nasal
bone evaluation are shown in Fig. 31.4 and Video 31.3. Initial
irst-trimester screening studies report that the nasal bone is
absent in 73% of fetuses with trisomy 21 and 0.5% of euploid
fetuses. 53 Cicero et al. 54 studied 5918 fetuses between 11 and 14
weeks and obtained a fetal proile in 99%. An absent fetal nasal
bone in euploid fetuses varied by ethnicity. A nasal bone was
not identiied in 2.2% of Caucasian fetuses, 9% of African Caribbean
fetuses, and 5% of Asian fetuses. Younger fetuses also have
a higher incidence of nonvisualization of the nasal bone. Absence
of the nasal bone was seen in 4.7%, 3.4%, 1.4%, and 1% of euploid
fetuses with CRL of 45 to 54 mm, 55 to 64 mm, 65 to 74 mm,
and 75 to 84 mm, respectively.
A
B
FIG. 31.3 Cystic Hygroma. (A) First-trimester fetus with skin thickening that extends around the entire body, consistent with cystic hygroma,
and bilateral distended jugular lymphatic sacs. Note the bilateral pleural effusions, indicating hydrops. (B) Axial scan through the head of the
same fetus shows septations within the nuchal thickening. See also Video 31.2.