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Echogenic Bowel
Introduction
Fetal echogenic bowel (FEB) is typically seen during second trimester prenatal ultrasound. FEB occurs when the fetal bowel appears with the same or greater echogenicity than do surrounding bony structures. When seen in the second trimester, FEB has been associated with increased risk for fetal cystic fibrosis, aneuploidy, gastrointestinal abnormalities, growth restriction, and viral infections. However, it is important to realize that the majority of patients with isolated FEB will have no associated adverse outcomes.
Disorder
Definition
While there is no standard definition for FEB, most practitioners use bowel that appears as bright as bone when minimal gain is used. Typically, the iliac wing is used as the bony reference point. The area may be singular or multifocal within the fetal abdomen, and should be homogeneously hyperechoic. Echogenic areas that create distal shadowing are suggestive of calcification. It is important to recognize that high frequency transducers can create the appearance of echogenicity, so consider using a lower frequency (e.g., 5 MHz) transducer to avoid overdiagnosis. Furthermore, newer machines with improved penetration often lead to an increase in the diagnosis of echogenic lesions. Similarly, women with lower body mass index (BMI) appear to be more often diagnosed with echogenic bowel. Therefore, care should be taken to adjust the gain to avoid overdiagnosis.
Prevalence and Epidemiology
The reported prevalence of FEB ranges from 0.2% to 1.4% during the second trimester.
Etiology and Pathophysiology
FEB is a nonspecific ultrasound finding that confers an increased risk for a range of pathologies. In many cases FEB is considered a normal variant, especially when isolated; however, one-third of cases may have a pathologic cause. Each pathologic entity has differing theories of the pathophysiology resulting in FEB, which will be addressed subsequently.
Bleeding
A bleeding event during pregnancy may result in blood in the amniotic cavity. The fetus then swallows and digests the blood, resulting in deposition of blood components into the gastrointestinal system. As blood is echogenic, the presence of blood in the fetal gastrointestinal system can increase its echogenicity. FEB can persist beyond 2 weeks after a bleeding event, therefore assessing the patient's recent history is important. Additional findings more consistent with a bleeding cause would be the presence of echogenic material in the fetal stomach or visualization of a subchorionic fluid collection.
Aneuploidy
FEB is considered a soft marker for fetal aneuploidy, most commonly trisomy 21. In some studies it increases the likelihood for trisomy 21 by 6.1-fold. FEB has been associated with other aneuploidies as well, with an overall aneuploidy risk of 6.7% in isolated FEB. In one retrospective cohort, the presence of FEB, when combined with other soft markers and major structural abnormalities, increased the chromosomal abnormality rate to 7.7% and 17.4%, respectively. The pathophysiology of FEB in aneuploidy is unclear. Fetuses with genetic abnormalities may have gastrointestinal complications, such as decreased intestinal motility or malformation. The likelihood ratio of 6.1 can be used to recalculate a patient's aneuploidy risk from their a priori age-related risk or from the results of analyte screening. It is unclear whether the results of cell-free DNA (cfDNA) screening should be adjusted based on the finding of FEB.
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