On completion of this chapter, you should be able to:
Describe the embryology of the abdominal wall
Define and describe the anterior abdominal wall abnormalities discussed in this chapter
List the sonographic findings for an omphalocele and for a gastroschisis
Identify the fetal anomalies in pentalogy of Cantrell
Explain the sonographic findings in a fetus with amniotic band syndrome
Sonography has proven to be very effective for detecting anterior abdominal wall defects in utero. These defects occur during the first trimester as the midgut elongates and migrates into the umbilical cord. The midgut usually returns into the abdominal cavity by the 11th week of gestation. When this fails to occur, an abdominal wall defect is formed. The two most common defects are omphalocele and gastroschisis. Less common defects are ectopia cordis, limb–body wall complex, and cloacal exstrophy.
Embryology of the abdominal wall
By the end of the fifth week of development, an embryo is a flat disk consisting of three layers: ectoderm, mesoderm, and endoderm. In the sixth week, a process called folding helps the embryo transform itself into a cylindrical shape. This transformation is a critical part of the process of closing the abdominal wall.
As the embryo folds at the cranial end, the base of the yolk sac is partially incorporated as the foregut, which later develops as the pharynx, lower respiratory system, esophagus, stomach, duodenum (proximal to the opening of the bile duct), liver, pancreas, gallbladder, and biliary duct system.
The growth of the neural tube causes the embryo to fold at the caudal end, incorporating part of the yolk sac as the hindgut, which turns into the cloaca. It also causes the connecting stalk (located at the tail) to move to the ventral surface of the embryo, incorporating the allantois into the umbilical cord. The derivatives of the hindgut are the distal part of the transverse colon, the descending colon, the sigmoid colon, the rectum, the superior portion of the anal canal, the epithelium of the urinary bladder, and most of the urethra ( Figure 62-1 ).
The sides of the embryo fold, leading to the formation of the lateral and anterior abdominal wall. The midgut is the primordium of the small intestines (including most of the duodenum), cecum, vermiform appendix, ascending colon, and the right half to two thirds of the transverse colon. The connection of the yolk sac and body stalk will form the umbilical cord at the ventral region of the embryo. Expansion of the amniotic cavity will cover the umbilical cord by the amnion. Fusion of the midline begins during the seventh week of development and is completed by the eighth week.
The umbilical veins drain the placenta, body stalk, and the evolving abdominal wall. During the seventh week, the hepatic bud enlarges, and the right umbilical vein atrophies. The proximal portion of the left umbilical vein between the subhepatic portion and the common cardinal vein also atrophies. Branches of the aorta now replace the nutritive function of the umbilical veins with respect to the developing abdominal wall. The superior mesenteric artery is formed from the right omphalomesenteric artery.
Umbilication hernia of the bowel occurs during the eighth week of development as the midgut extends to the extraembryonic coelom in the proximal portion of the umbilical cord. The midgut grows faster than the abdominal cavity at this stage because of the increased size of the liver and kidneys. Thus, the herniation develops. The intestines return to the abdominal cavity by the 12th week of gestation.
Sonographic evaluation of the fetal abdominal wall
It should be possible to detect abdominal wall defects in utero, as the defects form early in embryologic development. It is very important to image the cord insertion site and the fetal anterior abdominal wall to evaluate for the presence or absence of such defects. If the urinary bladder and pelvis are evaluated closely, the diagnosis of bladder and cloacal exstrophy may also be made with sonography. The most common types of abdominal wall defects are gastroschisis, omphalocele, and umbilical hernia. Other types of abdominal ventral wall defects include ectopia cordia, bladder and cloacal exstrophy, amniotic band syndrome, and the limb–body wall complex. The following questions should be routinely answered:
Is a limiting membrane present?
What is the relation of the umbilical cord to the defect?
Which organs are eviscerated?
Is the bowel normal in appearance?
Are other fetal malformations evident?
Abnormalities of the anterior abdominal wall
Abdominal wall defects cause distortion of the normal contour of the ventral or anterior surface of the fetal abdomen. Table 62-1 summarizes the typical sonographic features and associated conditions of fetal abdominal wall defects.
|Type of Defect||Description||Sonographic Features||Other Anomalies|
|Gastroschisis||Paraumbilical defect||Typically, only bowel is eviscerated; occasionally other organs, but almost never the liver||Associated anomalies are uncommon; high rate of bowel-related complications|
|Omphalocele||Midline defect, contained by membrane||Variable||High risk of other anomalies and/or aneuploidy|
|Extracorporeal liver||Typically large||When isolated without other detectable anomalies, risk of aneuploidy is very low; however, high rate of cardiac anomalies|
|Intracorporeal liver||Typically small||>50% risk of aneuploidy when detected prenatally|
|Beckwith-Wiedemann syndrome||Syndromic condition||Macroglossia; omphalocele; visceromegaly||Omphalocele is typically intracorporeal liver type|
|Pentalogy of Cantrell||Omphalocele |
Anterior diaphragmatic hernia
Distal partial sternal defect
|May appear only as high omphalocele; pleural effusion, even transient, is highly suggestive of diaphragmatic hernia in this situation||None|
|Absent sternum||Absent sternum||Dynamic heart covered by skin||Rare; usually isolated|
|Ectopia cordis||Thoracic defect of sternum and skin||Dynamic heart not covered by skin||Rare; high rate of cardiac and other defects; may be associated with high omphalocele|
|Bladder exstrophy||Eviscerated urinary bladder||Nonvisualization of urinary bladder; soft tissue mass of anterior abdominal wall may be subtle||Increased risk of fetal aneuploidy|
|Cloacal exstrophy||Eviscerated cloaca with two hemibladders||Nonvisualization of urinary bladder; in one variation, ileal prolapse produces “elephant trunk” appearance||Severe, complex anomaly|
|Limb–body wall complex||Multiple anomaly condition||Bizarre, complex defect with ventral wall defect, close attachment to placenta, cranial defects, scoliosis||100% lethal, but no risk of aneuploidy|
The most common abdominal wall defects are the omphalocele, umbilical hernia (a form of omphalocele), and gastroschisis. The incidence of omphaloceles is roughly 1 in 4000 live births. Rarer abdominal wall defects include ectopia cordis, pentalogy of Cantrell, limb–body wall complex, amnion rupture sequence, and bladder and cloacal exstrophy. Overall, gastroschisis has an incidence of 12 per 10,000, although only rarely are affected infants born to older mothers. The role of the perinatal team is to distinguish among these lesions because clinical management, associated anomalies, delivery, and postnatal surgical survival vary, depending on the specific type of abdominal wall defect.
Box 62-1 outlines what the sonographer should investigate to distinguish between omphalocele and gastroschisis.
The sonographer should investigate the following to differentiate an omphalocele from a gastroschisis:
Look for the presence of a membrane; gastroschisis does not have one.
Look at the umbilical cord; the cord goes through the omphalocele, whereas gastroschisis is found to the right of the cord.
Determine which organs are eviscerated.
Determine whether the bowel is normal in texture.
Look for other anomalies, because omphaloceles often occur with chromosomal abnormalities.
During the 8th to 12th weeks of development, the fetal bowel normally migrates into the umbilical cord from the abdominal cavity. This normal embryologic herniation of the bowel permits the development of the intraabdominal organs and allows necessary bowel rotation. Because of the lack of space within the abdominal cavity and the large fetal liver and kidneys, the bowel is forced from the abdomen and into the extraembryonic coelom of the umbilical cord. This herniation permits the bowel to rotate around the superior mesenteric artery. These herniated loops of bowel normally return and rotate into position within the abdominal cavity by the 12th week of pregnancy. When bowel loops fail to return to the abdomen, a bowel-containing omphalocele occurs ( Figure 62-2 ).
An omphalocele develops when there is a midline defect of the abdominal muscles, fascia, and skin that results in herniation of intraabdominal structures into the base of the umbilical cord ( Figure 62-3 ). This herniation is covered by a membrane that consists of the amnion and peritoneum. The alpha-fetoprotein (AFP) level may be slightly elevated or within normal limits. Omphaloceles are characterized as two types: (1) those that contain the liver within the sac and (2) those that contain a variable amount of bowel without liver.
Fetuses with an omphalocele that contains only a bowel have a higher risk for chromosomal abnormalities and other anomalies ( Figures 62-4 and 62-5 ). Bowel within the omphalocele develops because the intestine fails to return to the abdomen (primitive body stalk remains). A liver omphalocele represents a developmental defect in abdominal wall closure. This type of omphalocele affects the abdominal wall muscles, fascia, and skin. Liver omphaloceles may contain a bowel and demonstrate a relatively large abdominal wall defect in comparison with the abdominal diameter ( Figures 62-6 and 62-7 ).