Omphalocele 

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.^{[1, 2, 3, 4]}

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.^[4]

Omphaloceles are associated with other anomalies in more than 70% of the cases; the severity of the associated anomalies determines the prognosis.^[5] The mortality rate is 80% when associated anomalies are present, and it increases to 100% when chromosomal and cardiovascular abnormalities are present.^{[6, 7, 8, 9]} Most associated anomalies are chromosomal.

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.^[10]

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.^[11] The finding of an omphalocele should prompt a targeted ultrasonographic examination to search for associated anomalies. Fetal echocardiography and karyotyping should also be performed.^[12]

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.

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.^{[13, 14, 15, 16, 17, 18]}

Limitations of techniques

A significant regional variation in the ultrasonographic detection of fetal abdominal wall defects has been described in Europe.^[19] This variation reflects differences in screening policies, equipment, and operator experience.^[20]

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.

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.

Prenatal MRI enhances the fetal anatomic evaluation; it can be a valuable adjunct to ultrasonography before surgical intervention for selected life-threatening fetal defects. MRI helps in corroborating and refining the ultrasonographic diagnosis of complex fetal defects.

The prognosis and mortality rate in omphalocele is determined more by the presence of associated anomalies, such as cardiovascular and chromosomal defects, than by the omphalocele itself. Prenatal MRI can be used to screen for anomalies such as complex cardiac defects and nervous system anomalies.

Degree of confidence

The diagnostic accuracy of MRI has been reported to be superior to that of sonography in selected cases of cerebral malformations because of the high resolution of the soft tissues and because of its more global depiction of complex fetal disorders.

Unlike ultrasonography, MRI is not operator dependent. Imaging, however, is dependent on magnet strength and the pulse sequences chosen. The field of view obtained with MRI is larger than that obtained with sonography; the large field of view allows good anatomic orientation. MRI is not limited by maternal obesity or oligohydramnios. Fetal movement can make MRI difficult.

MRI is considered safe for the developing fetus. At present, no clinical or experimental evidence suggests that MRI causes teratogenic or other adverse affects during pregnancy, although a few studies in laboratory animals have shown that prolonged, high-level exposure to electromagnetic radiation might result in teratogenicity.

A recommendation from the National Institute of Health Consensus Development Conference states, "MRI should be used during the first trimester pregnancy only when there are clear medical indications and when it offers clear advantage over other modalities."

False positives/negatives

The fine detail of internal structures of the abdomen and pelvis is not well demonstrated with fast T2-weighted sequences.

An omphalocele is diagnosed when a fetal anterior midline abdominal mass is demonstrated. The mass consists of abdominal contents that have herniated through a midline central defect at the base of umbilical cord insertion. The mean size of the defect is 2.5-5 cm. The image below depicts a mass with a smooth surface and contains abdominal viscera, usually the liver, bowel, and stomach.

The covering of the mass, which comprises the peritoneum and amnion, may rarely rupture. The membrane is not always visible. Wharton jelly may be detectable as a hypoechoic lining between the layers of the covering of the membrane.

The umbilical cord attaches to the apex of the herniated mass, where the umbilical vein can be seen within the mass. The cord may be widened where it joins the fetal skin.

Fetal ascites is common and is seen within the herniated sac. Polyhydramnios, and occasionally oligohydramnios, may be present. Other major anomalies may be apparent in as many as 70% of cases.

A number of sonographic features differentiate an omphalocele from a physiologic midgut herniation, shown in the images below.

A midgut herniation seldom exceeds 7 mm in diameter, whereas omphaloceles are much larger. Midgut herniation is invariably smaller in diameter than the abdomen, whereas the diameter of an omphalocele can be larger than that of the abdomen. The size of the omphalocele is best measured by using the ratio of the transverse diameter of the omphalocele to the transverse diameter of the abdomen. A midgut herniation seldom persists after 12 weeks of gestation or in a fetus with a crown-length measurement of more than 44 mm.

When the ratio of the diameter of the omphalocele to the transverse diameter of the abdomen is less than 60%, the lesion usually contains bowel and not liver.

The relative size of the omphalocele may decrease during pregnancy.

A measurement of the ratio of the transverse area of the lung to that of the thorax may be useful in predicting associated pulmonary hypoplasia in a giant omphalocele.^[21] However, antenatal detection requires prolonged ventilation, which may be difficult and requires further study.^{[22, 23, 24, 25]}

Degree of confidence

Ultrasonography is a sensitive technique, but it remains operator dependent. Its great advantage is that it can be quickly and frequently repeated as required. A definitive diagnosis of omphalocele is possible only beyond 12 weeks' gestation, when confusion with physiologic midgut herniation is no longer possible.

Investigators assessing the accuracy of routine ultrasonography in detecting fetal anterior abdominal-wall defects reported a detection rate of 60%, with a false-positive rate of 5.3%.^[26] In this study, sonographic examination was performed between weeks 16 and 22 in an unselected population. Fetuses with gastroschisis were incorrectly identified as having exomphalos in 14.7% of cases recognized before week 22. The diagnosis of associated, detectable anomalies was accurate in 71.6% of cases.

In a tertiary referral center, the detection of anomalous fetuses was shown to be significantly better with targeted ultrasonography than with routine ultrasonographic screening.^[27]

False positives/negatives

An anterior abdominal-wall mass has a wide differential diagnosis. It may be difficult to differentiate between a midline omphalocele and physiologic midgut herniation in early pregnancy; it may also be difficult to differentiate omphalocele from bowel herniation of gastroschisis.^[28]

Gastroschisis: Gastroschisis usually poses no problems of differentiation from an omphalocele. The only finding that the two share is that they both arise from the anterior abdominal wall. Gastroschisis has no membranous covering and usually presents with a ragged edge, which almost never contains liver. In addition, gastroschisis is typically right-sided, with the umbilical cord entering the abdomen to the left of the herniation (rather than on the herniation, as in an omphalocele). Spontaneous rupture of an omphalocele membrane may cause problems in differentiation, but this is so rare that it should seldom enter the differential diagnosis.

Physiologic bowel herniation: Physiologic herniation occurs at 10-13 weeks' gestation. The best method for differentiating this from an omphalocele is repeat sonography after 15 weeks' menstrual age. A large defect with liver exteriorized indicates an omphalocele at any gestational age.^[29]

Umbilical hernia: Ultrasonography reveals a prominent bulge of the anterior abdominal wall that contains omentum and/or bowel. This may protrude into the umbilical cord. Amniotic-fluid AFP levels may be elevated when the bowel herniates into the umbilical cord.^[30]

Extrophy of the urinary bladder: Sonographically, bladder extrophy may appear as an external, well-defined, solid or complex mass immediately superior to the fetal genitalia. Prolonged and repeated scans fail to reveal the fetal bladder. The renal collecting system and ureters need not be dilated, and unilateral or horseshoe kidneys may be found. Uterine and adnexal anomalies are relatively frequent. The pubis is abnormally wide, and the umbilical cord insertion may be abnormal.

Cloacal extrophy: Cloacal extrophy consists of a low omphalocele; bladder or cloacal extrophy; and frequently, other caudal anomalies, including meningomyelocele anal atresia and lower-limb anomalies. Most affected fetuses have a single umbilical artery. Ultrasonography usually shows a low, anterior abdominal mass below the umbilical cord; this is associated with absence of the urinary bladder.

Pseudo-omphalocele: During scanning of the fetal abdomen, pressure from the transducer may give an impression of an omphalocele, particularly during scanning in an oblique plane. The angle formed between such a pseudomass and the fetal abdominal wall is usually obtuse. A pseudo-omphalocele may also occur as a result of oligohydramnios or compression of the lateral thoracic wall from other causes. With medial compression of the lower thoracic wall, the abdomen may have an hourglass-like appearance when imaged in a transverse plane. Omphalocele associated with oligohydramnios is exceptionally rare.