Fetal anterior abdominal wall





Objectives


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 ).




FIGURE 62-1


Development and rotation of the midgut from the 6th to 11th week. The midgut is the primordium of the small intestines, cecum, appendix, ascending colon, and right one 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.


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:



  • 1.

    Is a limiting membrane present?


  • 2.

    What is the relation of the umbilical cord to the defect?


  • 3.

    Which organs are eviscerated?


  • 4.

    Is the bowel normal in appearance?


  • 5.

    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.



TABLE 62-1

Typical Features of Ventral Wall Defects and Associated Conditions
































































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
Pericardial defect
Cardiac 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

From Nyberg DA, McGahan JP, Pretorius DH, et al, editors: Diagnostic ultrasound of fetal anomalies: text and atlas, Philadelphia, 2003, Lippincott Williams & Wilkins.


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.



BOX 62-1

Differentiation of Omphalocele from 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.




Omphalocele


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 ).




FIGURE 62-2


Typical features of bowel-containing omphalocele (intracorporeal liver) shown on external examination (A) and on cross-sectional view (B).


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.




FIGURE 62-3


Longitudinal image of a 14-week fetus showing a small omphalocele with its covering membrane (arrows) projecting from the umbilical area.


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 ).




FIGURE 62-4


Typical features of liver-containing omphalocele shown on external examination (A) and on cross-sectional view (B).



FIGURE 62-5


A, Bowel omphalocele (B) in a fetus with trisomy 18. A, Abdomen. B, In the same fetus, an umbilical cord cyst (C) is noted coursing distally into the omphalocele (O). Other anomalies observed include a strawberry-shaped cranium, hypoplastic cerebellum, nuchal fold, and esophageal atresia. Additionally, a large atrial septal defect, hypoplastic left ventricle, single umbilical artery, and hydramnios were found.



FIGURE 62-6


A, Sagittal scan in an 18-week fetus in a spine-up position with a mass herniating from the anterior abdominal wall representing an omphalocele (arrows). A, Abdomen; C, cranium. B, In the same fetus, in a transverse direction, herniation of the liver into the omphalocele (o) is observed. Note the portal vessel within the liver. A, Abdomen; large arrows, first border of the omphalocele; s, spine.

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May 29, 2019 | Posted by in ULTRASONOGRAPHY | Comments Off on Fetal anterior abdominal wall
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