Omphalocele

Introduction

An omphalocele (Fig. 1) is an abdominal wall defect in which a variable amount of the abdominal contents protrude into the base of the umbilical cord. The parietal peritoneum covers the extruded abdominal wall contents.

Figure 1. Liver containing omphalocele.

The prevalence of omphalocele is approximately 1/2,280 to 1/10,000 births1. The CDC estimates that 775 babies in the United States are born with an omphalocele each year2.

Different etiologies have been proposed for bowel, in contrast to liver, containing omphaloceles. A defect in the formation of the lateral folds results in a liver-containing omphalocele (Fig. 2)3. 50% of omphaloceles contain liver.

The presence of only bowel (Fig. 3) in an omphalocele suggests a later process in development, i.e. failure of the abdominal contents to return to the abdominal cavity by 12 weeks’ gestation4.

Figure 2. Liver containing omphalocele - 30 wks.
Figure 3. Bowel containing omphalocele (arrow) 20 wks.

The incidence of associated anomalies with an omphalocele ranges from 45% to 77%3,5,6,7. Congenital heart defects, specifically Tetralogy of Fallot and atrial septal defects, are the most common abnormalities associated with omphalocele (Table I)8,9.

Table I. Associated anomalies with omphalocele.

The recurrence risk for an isolated omphalocele in a subsequent pregnancy is < 1%10.

Approximately 50% of fetuses with an omphalocele have a karyotypic abnormality with trisomy 18 considered the most common11,12,13. The risk of a chromosomal abnormality is increased in bowel containing omphaloceles12,14,15.

An omphalocele is, therefore, a reliable marker for both additional structure malformations and karyotypic anomalies16.

Maternal risk factors for having a child with an omphalocele include: smoking more than a pack of cigarettes a day17, obesity18, and using selective serotonin uptake inhibitors (SSRI) for depression19.

Stillbirth occurs in 16-30% of omphaloceles. The rate of reported intrauterine growth restriction is as high as 35%20,21. One-third of fetuses with an isolated omphalocele deliver preterm22.

Omphaloceles have been classified as small, giant, or ruptured23. A giant omphalocele in a neonate has been defined as a defect > 5 cm24 or > 6 cm25 that contains liver. Approximately 10-18% of omphaloceles rupture before delivery8.

Fetal lung development requires appropriate intra-abdominal pressures, chest and abdominal muscle development, and diaphragmatic movements. Because of the effect of a giant omphalocele on all of these developmental requirements, ventilatory insufficiency is an acknowledged complication with a giant omphalocele26. In addition to pulmonary hypoplasia, repair of a giant omphalocele results in increased intra-abdominal pressure and elevation of the diaphragm27,28. Immediate neonatal ventilatory support at delivery29 has resulted in survival of neonates with a giant omphalocele, who subsequently require long-term ventilatory support. An intra-operative intra-gastric pressure > 20-21 mm of mercury is associated with increased central venous pressure,anuria and bowel ischemia30. In these cases, a staged closure over 5-7 days is required9.

Conservative (not surgical) treatment of giant omphaloceles is associated with decreased complications and mortality31.

Syndromes and associations with an omphalocele are outlined in Table II31.

Table II. Syndromes and associations with an omphalocele57.

Beckwith-Wiedemann Syndrome

In the 1960’s Dr. Wiedemann in Germany and Dr. Beckwith in the United States independently described what came to be known as the Beckwith-Wiedemann syndrome (BWS)32.

The five common features of BWS include: macroglossia, macrosomia, a midline abdominal wall defect, ear creases/pits and neonatal hypoglycemia. At least two of these five common features is required to a make a diagnosis of BWS.

The incidence of BWS is 1/37,000 livebirths; 300 children are born with BWS in the United States each year33. There is up to a nine times higher chance of developing BWS after in-vitro fertilization34.

The inheritance of BWS is complex. Approximately 15% of cases are autosomal dominant with incomplete penetrance and the remainder occur sporadically.

An increased nuchal translucency and an omphalocele have been associated with BWS35.

In the 3rd trimester macrosomia, polyhydramnios and preterm birth are characteristic of Beckwith-Wiedemann syndrome. Hemihyperplasia may affect segmental regions of the body or selected organs. The overgrowth associated with BWS is due to increased IGF-2 action. Placentomegaly and a long umbilical cord are also frequent findings.

Significant macroglossia can lead to respiratory, feeding and speech difficulties.

The developmental delay associated with BWS is attributed to uncontrolled hypoglycemia during infancy, rather than to an inherent central nervous system abnormality.

Wilms tumor develops in 5-7% of children with BWS, with the vast majority occurring by the age of 836. Hepatoblastoma, neuroblastoma and rhabdomyosarcoma are also common in children with BWS.

Hemihyperplasia, omphalocele and Wilms tumor are due to specific phenotype-genotype correlations37.

Pentalogy of Cantrell

Pentalogy of Cantrell has a prevalence of 1/65,000 live births. Embryologically, it is due to a defect of the midline anterior body wall. The five characteristic abnormalities in this syndrome are: omphalocele; a defect of the lower sternum; agenesis of the anterior diaphragm; absence of the diaphragmatic portion of the pericardium; and ectopia cordis with cardiac anomalies. Because of the severity of the defects with Pentalogy of Cantrell, there are few reported survivors38,39. If there is a midline abdominal wall defect detected, even in the 1st trimester, with ectopia cordis, Penatology of Cantrell would be the most likely diagnosis40.

Omphalocele-Exstrophy-Imperfornaus-Spinal Defects (OEIS)

OEIS has an incidence of 1/250,000 livebirths. However, the incidence is probably higher as many cases are probably misdiagnosed as simply an omphalocele41. OEIS is associated with monozygotic twins, sporadic familial occurrence, trisomy 1842 and teratogens. The recurrence risk in subsequent pregnancies is < 1%43.

The cloaca is an embryonic structure where the genital, urinary and digestive organs join. A defect in its subsequent division results in the abnormalities of the sydrome44.

Sonographically, non-visualization of the fetal bladder, a midline abdominal wall defect and a lumbo-sacral meningomyelocele are identified.

Familial Occurrence of Omphalocele

The occurrence of omphalocele in siblings is rare. However, there are reported cases of X-linked recessive inheritance. As a result, the males would be affected and the females would be carriers45.

Fryns Syndrome

Fryns syndrome was initially characterized by a diaphragmatic hernia, hypoplasia of the distal phalanges and facial abnormalities. An omphalocele, rather than a diaphragmatic hernia, has been reported in a few cases46.

False Positive Diagnosis

Pressure from the transabdominal transducer can give an impression of an omphalocele47. A sonographic evaluation of a suspected omphalocele from different angles and with 3-dimensional sonography, can help to corroborate the diagnosis of a pseudo-omphalocele.

Sonographic Findings

The diagnosis of a bowel containing omphalocele cannot be made until after 12 weeks' gestation48,49. In the 1st trimester, the normal mid-gut herniation should not exceed 7 mm47. If the liver is out in an omphalocele cavity, the diagnosis has been made prior to 12 weeks’ gestation (Fig. 4)15.

Figure 4. Omphalocele (arrow) 12 wks.

The contents of an omphalocele are variable and may consist of, not only liver (Fig. 5), but also bowel, spleen, stomach, kidney and bladder.

Figure 5. Liver containing omphalocele.

In the 2nd and 3rd trimester, either a transverse or longitudinal image of the abdomen reveals a broad-based circular lesion at the level of the umbilical cord insertion. The surrounding sac may contain ascites50.

An allantoic cyst at the base of the umbilical cord (Fig. 6) with surrounding umbilical vessels is associated with omphaloceles51. The allantois forms from the yolk sac and normally regresses by 14 weeks’ gestation52.

Figure 6. Omphalocele containing liver; allantoic cyst (arrow)

There is a significant variation in the detection of omphalocele prenatally that is primarily determined by examination protocols and operator experience. Detection rates vary from 25-100% with a mean sensitivity of 75%53.

The diagnosis of omphalocele is originally made with 2-dimensional imaging. Three-dimensional sonography provides complimentary information on possible associated congenital anomalies (Fig. 7)16.

Figure 7. Omphalocele (arrow) (3D).

Prognosis

The prognosis of omphalocele is determined by the size of the defect and the associated anomalies54,55. If the defect is isolated, the neonatal survival is 75-95% 8,21,26,56. With associated anomalies, mortality reaches 80% and with a karyotypic abnormality or severe cardiac defect the mortality is near 100%22,50.

 

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References

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