Case of the Week #606

LaShell Jones, RDMS RDCS-FE
Maternal Fetal Associates in Reston, Virginia

Posting Dates: May 31, 2024 - Jun 14, 2024

40-year-old multipara with type II diabetes presents at 16 weeks for genetic scan because she missed her 1st trimester screening exam. Ultrasound revealed the following findings. There were other numerous abnormalities in the rest of the scan apart from the ones shown here.

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Answer

We present a case of Trisomy 21. Non-invasive prenatal testing was concerning for Down's syndrome and amniocentesis confirmed the diagnosis.

Our images demonstrated the following:

Image 1: Absent Nasal Bone
Image 2: liver calcifications
Image 3-4: Echogenic bowel
Image 5: "Keyhole" Dandy Walker Variant
Image 6: Lagging Cerebellum measurement
Image 7: VSD
Image 8: Absent A - wave Ductus Venosus
Image 9: Pericardial Effusion
Image 10: Thick Nuchal Fold

Discussion

Down Syndrome, also known as trisomy 21, is the most prevalent chromosomal abnormality observed in newborns. Approximately 6,000 infants are born with Down Syndrome annually in the United States, equating to a frequency of about 1 in every 700 births. Various prenatal genetic screening methods and diagnostic tests are utilized to accurately identify Down Syndrome and other aneuploidies during pregnancy.

Ultrasound plays a crucial role in the screening for aneuploidies. It enables the detection of both major structural abnormalities and minor soft markers in fetuses affected by aneuploidies. Down Syndrome can manifest with anomalies in various systems such as cardiovascular, central nervous, craniofacial, musculoskeletal, gastrointestinal, and urinary tract. Notably, major structural anomalies like duodenal atresia and cardiac defects such as septal defects, tetralogy of Fallot, and atrioventricular canal defects may not always be identified through prenatal ultrasound screening.

Nuchal translucency (NT) measurement in the first trimester is indicative of the subcutaneous fluid-filled space at the back of the fetal neck. An increased NT measurement is associated with a higher risk of aneuploidies, including Down Syndrome, with a detection rate ranging from 64% to 70%. Additionally, the presence of a nuchal cystic hygroma or a hypoplastic/absent nasal bone in the first trimester can also indicate a risk of aneuploidies, including Down Syndrome.

During the second trimester, soft markers such as echogenic intracardiac foci, pyelectasis, short femur length, choroid plexus cysts, echogenic bowel, thickened nuchal skin fold, and ventriculomegaly are commonly assessed. Some of these markers, like echogenic bowel, thickened nuchal fold, and ventriculomegaly, have high likelihood ratios, prompting further genetic counseling and additional screening or diagnostic tests. Conversely, markers with lower predictive values may also warrant aneuploidy screening if not previously conducted.

While ultrasound is a valuable tool in identifying potential markers for Down Syndrome, it should not be solely relied upon for diagnosis. Combining ultrasound findings with maternal serum screening tests, such as first and second trimester screening, as well as integrated and sequential screening, enhances the sensitivity for detecting Down Syndrome. Cell-free DNA testing boasts a high detection rate of 99% for Down Syndrome. Diagnostic procedures like amniocentesis or chorionic villus sampling should be considered when screening results indicate a positive likelihood of aneuploidies.

References

American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins—Obstetrics; Committee on Genetics; Society for Maternal-Fetal Medicine. Screening for Fetal Chromosomal Abnormalities: ACOG Practice Bulletin, Number 226. Obstet Gynecol. 2020 Oct;136(4):e48-e69.