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Omphalocele Imaging

Overview

Omphalocele is an anterior abdominal wall defect at the base of the umbilical cord, with herniation of the abdominal contents. The herniated organs are covered by the parietal peritoneum. After 10 weeks’ gestation, the amnion and Wharton jelly also cover the herniated mass.

The etiology of omphalocele is not known. Various theories have been postulated; these include failure of the bowel to return into the abdomen by 10-12 weeks, failure of lateral mesodermal body folds to migrate centrally, and persistence of the body stalk beyond 12 weeks’ gestation.

Omphaloceles are associated with other anomalies in more than 70% of the cases; the severity of the associated anomalies determines the prognosis.
The mortality rate is 80% when associated anomalies are present, and it increases to 100% when chromosomal and cardiovascular abnormalities are present.
Most associated anomalies are chromosomal.

In an examination of prevalence, epidemiology, and 1-year survival associated with omphalocele from 1995 to 2005, as reported by the National Birth Defects Prevention Network in the United States, prevalence of omphalocele was 1.92 per 10,000 live births. Neonates with omphalocele were more likely to be male,and born to mothers 35 years of age or older and younger than 20 years. The highest proportion of neonates with omphalocele had congenital heart defects (32%). Infant mortality was 28.7%, with 75% of those occurring in the first 28 days. The best survival was for isolated cases, and the worst was for neonates with co-occurrence of chromosomal defects and low birth weight.

In an analysis of anomalies associated with gastroschisis and omphalocele in the Texas Birth Defects Registry, the overall prevalence of abdominal wall defects among 3,806,299 Texas births from 1999 to 2008 was 7.4 per 10,000, with 4.8 per 10,000 for gastroschisis and 2.1 for omphalocele. Musculoskeletal (limb contractures or defects), cardiovascular, gastrointestinal, urogenital, and central nervous system defects were the most common anomalies. Of 1831 cases with gastroschisis, 594 (32%) had associated anomalies, compared to 654 (80%) of 814 omphalocele cases.

The anomaly is usually detected during routine ultrasonographic surveillance, during an investigation of a disparity of uterine size with time from conception or other obstetric indications, or during an evaluation of an increased maternal serum alpha-fetoprotein (AFP) level. Omphaloceles and gastroschisis are two open, ventral wall defects that are detected by means of AFP measurement. Acetylcholinesterase levels may also be increased.

Results of ultrasonographic studies suggest that in differentiating between normal physiologic herniation and a concern for omphalocele, one should note that physiologic midgut herniation should not exceed 7 mm in diameter and that physiologic herniation should not be apparent in fetuses with a crown-rump length greater than 44 mm.

Diagnostic amniocentesis is indicated when an omphalocele is suspected on antenatal sonograms.
The finding of an omphalocele should prompt a targeted ultrasonographic examination to search for associated anomalies. Fetal echocardiography and karyotyping should also be performed.

Small omphaloceles occur with a rate of 1 case in 5000 live births. Large omphaloceles occur with a rate of 1 case in 10,000 live births.

Preferred examination

Ultrasonography is an inexpensive, safe, noninvasive real-time technique that is widely available. It remains the imaging modality of choice for the prenatal assessment of the fetus. In experienced hands, ultrasonography is highly accurate in the diagnosis of most complications associated with pregnancy. It is also used as a guide to intervention in pregnancy. A significant regional variation in the ultrasonographic detection of fetal abdominal wall defects has been described in Europe.
This variation reflects differences in screening policies, equipment, and operator experience.

The rate of selective termination still appears to be relatively high, even in cases of omphalocele, without other evident anomalies, which generally have a good prognosis. Although ultrasonography is an accurate and sensitive means for detecting fetal anomalies, it still has limitations, and its dependence on operator skill is a major disadvantage. The diagnostic problems with ultrasonography must be considered when couples are counseled about a possible fetal anomaly.

A study of prenatal ultrasound examination and autopsy findings in fetuses and infants with gastroschisis and omphalocele found that of 11 cases with gastroschisis, only 1 was not detected at the prenatal ultrasound examination, and the rest had full agreement. Of 70 fetuses with omphalocele, 2 were not diagnosed at the prenatal ultrasound examination. The study also found that from 1985 to 2009, the main diagnosis for omphalocele improved from 85% to 95%, and the number of cases with major and minor autopsy findings not detected by ultrasound examination was reduced from 48% to 21%.

At present, the use of MRI in pregnancy is limited. As more experience is gained, fetal MRI may play a greater role in anomaly analysis. Prenatal MRI is at times a useful adjunct to ultrasonography and may enhance fetal anatomic evaluation when complex anomalies are suspected. MRI facilitates perinatal management and parental counseling.
 Besides the limited availability of MRI, its safety issues have not been completely resolved. Prudence currently dictates that MRI be used in the first trimester only if a clear medical indication is present and only when it offers a definite advantage over ultrasonography.

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