Introduction
Background
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
Omphaloceles are associated with other anomalies in more than 70% of the cases. Most associated anomalies are chromosomal. The anomaly is detected during routine ultrasonographic examination of the fetus or during an investigation of an increased alpha-fetoprotein (AFP) level.
Pathophysiology
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
Associated anomalies are common (45-88%); the severity of the associated anomalies determines the prognosis.5 Chromosomal anomalies are common. It is thought that a chromosomal anomaly is more likely when oligohydramnios or polyhydramnios complicates the pregnancy. It has been reported that omphaloceles containing only bowel have a higher incidence of karyotypic abnormalities.6,7
The associated anomalies include the following:
- Chromosomal anomalies (40-60%): These include trisomies 18,13, and 21 and also Turner, Klinefelter, and triploidy syndromes.
- Cardiac defects (16-47%): These include ventricular and atrial septal defects, tetralogy of Fallot, pulmonary artery stenosis, pulmonary hypoplasia, a double-outlet right ventricle, bicuspid aortic valve syndrome, transposition of the great vessels, coarctation of the aorta, ectopia cordis, and absence of the inferior vena cava.8
- Respiratory insufficiency is occasionally seen in association with a giant omphalocele.
- Genitourinary anomalies (40%): These include bladder extrophy and omphalocele, bladder extrophy, imperforate anus and spinal anomalies (OEIS) complex; obstruction of the ureteropelvic junction; renal malpositioning (cephalic renal displacement)9 ; cloacal extrophy; and isolated imperforate anus.10
- Neural tube and head and neck anomalies: These include neural tube defects, holoprosencephaly, encephalocele, cerebellar hypoplasia, cleft lip, facial clefts, micrognathia, and cystic hygroma.11
- Gastrointestinal anomalies (40%): These include diaphragmatic hernia, malrotation, intestinal duplications, atresias, and ascites, absence of the gallbladder, abnormal fixation of the liver, tracheoesophageal fistula, and imperforate anus.12
- Musculoskeletal anomalies (10-30%): These include limb-body wall deficiency (LBWD), scoliosis, hemivertebra, camptomelic dwarfism, clubfeet, and syndactyly, as well as other digital anomalies.
- Maternal/fetal developmental abnormalities: These include oligohydramnios, polyhydramnios, intrauterine growth restriction (IUGR), single umbilical artery, allantoic cysts, placental chorioangioma, and fetal immaturity and prematurity.
- Beckwith-Wiedemann syndrome (5-10%): This encompasses a group of disorders involving an omphalocele, macroglossia, and visceromegaly.
One study has shown that periconceptional multivitamin use is associated with a 60% reduction in the risk of nonsyndromic omphalocele.13,14 These findings await replication.
Another study has shown that women younger than 20 years are at an increased risk for gastroschisis during pregnancy, whereas those aged 40 years or older are disproportionately more likely to have a pregnancy affected by an omphalocele.15,16,17,18,19
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Frequency
United States
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.
International
The frequency in the United Kingdom and in continental Europe is the same as that in the United States.20,21
Mortality/Morbidity
The prognosis depends on the associated anomalies. The mortality rate is 80% when associated anomalies are present, and it increases to 100% when chromosomal and cardiovascular abnormalities are present.22,23,24,25
- When an omphalocele is an isolated abnormality, the prognosis is good, and the mortality rate is around 10%.
- Excluding pregnancy termination, over half the fetuses with an omphalocele will die.
- Complications include infection, inanition, immaturity, hernia rupture, and intestinal obstruction. Infection and surgical complications are responsible for as many as 13% of deaths. Low birth weight causes more than 10% of deaths.
- An association with oligohydramnios and polyhydramnios portends a worse prognosis.
- The size of the omphalocele does not affect the prognosis.
- Whether the presence of fetal ascites is an indicator of a poorer prognosis is controversial. The incidence of associated anomalies is higher in a prenatal series as compared with a postnatal series. This observation is not surprising because many severely affected fetuses die in utero and are never seen by a surgeon.
Race
A study from Hawaii has shown that Pacific Islanders have a low risk of omphalocele, whereas Far East Asians have a low risk of gastroschisis.
Sex
The male-to-female ratio is 1:1.
Age
The earliest that an omphalocele can be detected is at 12 weeks of menstrual age. Woman aged 40 years or older are disproportionately more likely to have a fetus with an omphalocele.
Anatomy
Physiologic umbilical herniation occurs during the eighth week of development when the fetal midgut extends into the extraembryonic celom, occupying the proximal segment of the umbilical cord. This herniation is associated with the development of normal bowel rotation in fetal life.26
The bowel undergoes a 270° counterclockwise rotation, the first 90° of which occurs by week 10, extracoelomically at the base of the umbilicus. The remaining 180° clockwise turn occurs after the bowel returns to the abdomen. Rotation occurs around the axis of the superior mesenteric artery. Theoretically, this may occur because of the large fetal liver and kidneys and because the rapid development of midgut outgrows the abdominal cavity at this stage of development. The midgut returns to the abdominal cavity at 12 weeks of menstrual age.
One of the theories regarding the cause of omphaloceles is failure of the bowel to return to the abdomen. Physiologic umbilical herniation should not persist beyond 12 weeks of gestation. After 12 weeks' gestation, one must consider omphalocele as the cause of mass in the base of the umbilical cord and beyond the confines of the abdominal wall.
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.
Presentation
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 AFP level. Omphaloceles and gastroschisis are 2 open, ventral wall defects that are detected by means of AFP measurement. Acetylcholinesterase levels may also be increased.27
A weak band of acetylcholinesterase can be detected on chromatography. This finding is thought to be related to the proximity of nerve terminations to the amniotic fluid.28
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.29,30,31,32,33,34
Limitations of Techniques
A significant regional variation in the ultrasonographic detection of fetal abdominal wall defects has been described in Europe.20 This variation reflects differences in screening policies, equipment, and operator experience.35
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.
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.
Differential Diagnoses
Other Problems to Be Considered
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.36
Umbilical hernia
An umbilical hernia is due to a defect in the linea alba, subcutaneous tissues, and skin covering the protruding bowel. Umbilical hernias are common in the first months in 20% of African-American infants and in 3% of white neonates. It is frequent in premature infants, occurring in more than 5% of those weighing less than 1500 g.
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.
Gastroschisis
Gastroschisis results from herniation of the bowel, and rarely other structures, through a small (2-5 cm) off-midline abdominal wall defect. This herniation protrudes into the amniotic cavity, usually in the right paraumbilical region. No covering membrane is present. Gastroschisis is thought to result from a vascular event to the omphalomesenteric artery.
Affected patients have malrotated bowel. Vascular compromise may occur from a volvulus, and it may result in bowel obstruction, ischemia, or atresia. Bowel-wall thickening may occur, perhaps in part due to the reaction of bowel (without a membrane separating it) with amniotic fluid. Besides malrotation, gastroschisis is not associated with other abnormalities. Affected patient karyotypes are normal. The prognosis is usually favorable. The presence of liver and spleen herniation has been reported to modify the prognosis and workup. The pancreas, stomach, bladder, uterus, ovaries, and fallopian tubes may also be herniated.
Amniotic band syndrome
Amniotic band syndrome (ABS) is a common cause of abdominal wall defects; in appearance, it may be similar to gastroschisis. An atypical location of the abdominal wall defect should suggest the diagnosis of ABS. Membranes contiguous with such a defect may be identified.
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.
Thoracoabdominal pentalogy of Cantrell
Ectopia cordis associated with a ventral wall defect should suggest the diagnosis of the pentalogy of Cantrell. This syndrome is often associated with other anomalies.37
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.
Limb-body wall complex
Limb-body wall complex (LBWC) is a lethal condition with severe anterior abdominal wall defect. The defect is lateral and involves the size of the umbilical cord insertion point. The abdominal contents lie outside within a sac of amnion and mesoderm. LBWC is frequently associated with congenital heart disease, cranial anomalies (encephalocele), limb abnormalities, and scoliosis. Chromosomal defects are usually absent.
Multiple cavernous hemangiomas
Multiple cavernous hemangiomas are most frequently found over the lower body. They are often associated with Klippel-Trenaunay-Weber syndrome, which is diagnosed in the presence of multiple surface masses that cause limb hypertrophy. Hydrops may occur.
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.
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Further Reading
Keywords
exomphalos, anterior abdominal wall defect
