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Omphalocele

Omphalocele (Liver evisceration)

Omphalocele (Liver + Ascites)
Portal veins in herniated liver

Findings:

A large soft-tissue mass is seen projecting from the anterior abdominal wall. This represents the fetal liver in a patient with a large omphalocele.

Discussion:

An omphalocele is a midline abdominal wall defect at the level of the umbilical cord insertion into the fetal abdomen. The pathogenesis is controversial. The most prevalent theory is that it is due to failure of the medial segments of the two lateral embryonic wall folds to fuse at approximately 3-4 weeks post conception. This defect at the umbilical ring allows the abdominal contents to herniate into a sac comprised of an outer layer of amnion and an inner layer of peritoneum and often Wharton's jelly. The most common eviscerated contents are liver and small intestine though virtually any organ can be seen within the omphalocele sac including colon, pancreas, spleen, omentum, stomach, heart or genitalia. As will be discussed below, absence of liver in the omphalocele sac is often associated with an abnormal karyotype. The umbilical cord insertion arises from the apex of the sac. The two other folds at the umbilical ring, the cephalic and caudal fold may not develop or fuse normally. Failure of the cephalic fold to fuse results in an upper midline omphalocele with multiple defects involving the heart, diaphragm, sternum and pericardium. When the caudal fold fails to develop normally, bladder or cloacal exstrophy may result. The two other theories of etiology of omphalocele include persistence of the body stalk or a failure of the midgut to return to an intrabdominal location during embryogenesis.

The incidence of omphalocele is 1 in 4000 to 1 in 5000 live births. If the omphalocele is the only abnormality, the recurrence risk is less than 1%. When associated with other abnormalities or a syndrome the recurrence risk may be as high as 25% dependent upon the specific syndrome. Omphalocele is frequently associated with other morphologic abnormalities, in many series as high as 70-80%. The most common associated abnormalities are cardiac, CNS, diaphragmatic,renal and skeletal. Approximately 40% of patients with an omphalocele will have an abnormal karyotype. As was mentioned above, liver is almost always identified within the omphalocele sac. When liver is absent and bowel is the only eviscerated structure within the sac, the incidence of karyotype abnormalities approaches 80-90%. A hypothesis that bowel only omphaloceles represent an arrest in normal development has been postulated.

Omphalocele with only bowel in sac (Trisomy 18)
Cord adjacent to bowel-only omphalocele

The perinatal mortality in fetuses with omphaloceles is between 25-75% and is invariably related to the associated malformations or karyotypic abnormalities. Infants with isolated omphaloceles have a relatively good prognosis, with a survival rate as high as 90%. While fetuses with omphaloceles are often delivered by cesarean section there are a number of studies demonstrating no significant difference between vaginal and cesarean delivery with regard to infant mortality, acute or long-term outcome or frequency of associated anomalies. The presence of liver in the omphalocele sac is not of prognostic importance and does not preclude a vaginal delivery.

Sonographically the diagnosis of an omphalocele is often made on the basis of several observations: 1) abnormal soft-tissue mass adjacent to the anterior adominal wall at the site of the umbilical cord insertion 2) small abdominal circumference 3) associated abnormalites. Despite the fact that the identification of the umbilical cord insertion site is considered part of the standard obstetrical ultrasound examination, studies demonstrating miss rates of 30% of anterior abdominal wall defects have been reported.

Allantoic cysts residing within the umbilical cord have been noted frequently in cases of omphaloceles. While they may occur in isolation, identification requires a search for a possible abdominal wall defect.

Omphalocele with herniation of bowel (arrowhead) and associated allantoic cyst (arrow)

Differential Diagnosis:

The major differential diagnosis is between an omphalocele and gastroschisis. The discrimination between the two can be made on the basis of a central defect with the umbilical cord inserting into it in the case of omphalocele and the presence of a surrounding membrane. Bowel loops are often seen to the right of a normally inserting umbilical cord and free floating in the case of a gastroschisis. The differentiation between these two entities can usually be readily made sonographically. The other condition that may simulate an omphalocele is when the fetal abdomen is compressed between the walls of the uterus or placenta due either to oligohydramnios or technical trasnducer compression. When an omphalocele is in an unusual location, the amniotic band syndrome or limb-body wall complex should be considered.

References:

van de Geijn EJ, Van Vugt JMG, Sollie JE, van Geijn HP. Ultrasonographic diagnosis and perinatal management of fetal abdominal wall defects. Fetal Diagn Ther 6:2-10, 1991

Moretti M, Khoury A, Rodriquez J, Lobe T, Shaver D, Sibai B. The effect of mode of delivery on the perinatal outcome in fetuses with abdominal wall defects. Am J Obstet Gynecol 163:833-838, 1990

Gray DL, Martin CM, Crane JP. Differential diagnosis of first trimester ventral wall defect. J Ultrasound Med 8:255-258, 1989

Benacerraf BR, Saltzman DH, Estroff JA, Frigoletto Jr. FD. Abnormal karyotype of fetuses with omphalocele: prediction based on omphalocele contents. Obstet Gynecol 75:317, 1990

Getachew MM, Goldstein RB, Edge V, Goldberg JD, Filly RA. Correlation between omphalocele contents and karyotypic abnormalities. Amer J Roentgenol. 158:133-136, 1991

Fink IJ, Filly RA. Omphalocele associated with umbilical cord allantoic cyst: sonographic evaluation in utero. Radiology 149:473, 1983

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Peter W. Callen, M.D.
Professor of Radiology, Obstetrics, Gynecology and Reproductive Science
University of California Medical Center, San Francisco, California