Diagnostic ultrasound ( PDFDrive )

CHAPTER
31
Chromosomal Abnormalities
Bryann Bromley and Beryl Benacerraf
SUMMARY OF KEY POINTS
• Screening for aneuploidy using the nuchal translucency (NT)
measurement should be performed by a credentialed
provider participating in an ongoing quality-monitoring
program.
• In chromosomally normal fetuses with an increased NT,
normal anatomy and no identiiable genetic syndromes,
there does not appear to be a higher risk of developmental
delay than in the general population.
• A “marker” identiied during a second-trimester sonogram
should prompt a detailed evaluation of the fetus and
correlation with a priori risk estimates for aneuploidy.
• The constellation of sonographic indings seen in the
second trimester may lead physicians to suspect a speciic
chromosomal abnormality.
• Cell-free DNA screening with reportable results has
superior test performance metrics compared with
traditional screening for targeted aneuploidies; however,
false-positive and false-negative results occur. Positive
results should be conirmed by a diagnostic test. Negative
results do not ensure a normal outcome.
CHAPTER OUTLINE
FIRST-TRIMESTER SCREENING FOR
ANEUPLOIDY
Nuchal Translucency and Trisomy 21
Serum Biochemical Markers
Combined First-Trimester Screening
Integrated and Sequential Screening
Standardization of Nuchal Translucency
Measurement Technique
Nuchal Translucency and Other
Aneuploidies
Cystic Hygroma
Nasal Bone
Other Markers for Aneuploidy
Reversed Flow in Ductus Venosus
Tricuspid Regurgitation
Thickened Nuchal Translucency With
Normal Karyotype
SECOND-TRIMESTER SCREENING
FOR TRISOMY 21
Nuchal Fold
Facial Proile (Nasal Bone)
Femur Length
Humerus Length
Urinary Tract Dilation
Echogenic Bowel
Echogenic Intracardiac Focus
Structural Anomalies
Adjunct Features of Trisomy 21
Combined Markers
TRISOMY 18 (EDWARDS
SYNDROME)
Choroid Plexus Cysts
TRISOMY 13 (PATAU SYNDROME)
TRIPLOIDY
MONOSOMY X (TURNER
SYNDROME)
PRENATAL SCREENING FOR
ANEUPLOIDY WITH CELL-FREE
DNA
DIAGNOSTIC TESTING
CONCLUSION
The current standard of obstetric care in the United States is
to ofer prenatal screening for aneuploidy to all women who
present for care before 20 weeks’ gestation. 1 If a woman chooses
to have a prenatal risk assessment for aneuploidy, multiple
sonographic markers and biochemical parameters are available
in both the irst and the second trimester. Recently, the paradigm
for prenatal screening has included the evaluation of cell-free
DNA in the maternal plasma and diagnostic testing has expanded
beyond the basic karyotype to include microarray analysis that
can detect small deletions and duplications in the genome, known
as “copy number variants” (CNVs). 2,3 he choices available depend
on the gestational age of the fetus at presentation for obstetric
care and the availability of resources within the local demographic
area. Screening algorithms are becoming progressively more
diverse with varying trade-ofs. 4 Counseling by an experienced
provider is recommended. 5
Background risk for aneuploidy (deviation from exact multiple
of haploid number of chromosomes) depends on maternal age,
fetal age, family history, and previously afected pregnancy.
Whereas trisomies 21, 18, and 13 increase in frequency as maternal
age increases, monosomy X and triploidy remain at a constant
rate 6 (Fig. 31.1).
Trisomy 21 (Down syndrome) is the most common aneuploidy
to result in a live birth and is the most frequently identiiable
genetic cause of mental retardation. he estimated prevalence
has increased over the past 20 years because of trends in advancing
maternal age and is estimated to be 1 in 504 in the second trimester.
7 Trisomies 18 and 13 are rarer, with a prevalence of 1 in
1088