Abdomen

Introduction

The abdomen constitutes that part of the body between the thorax and the pelvis. The abdominal cavity is bounded by the diaphragm above but is contiguous with the pelvis; the boundary is defined by the bony landmarks of the pelvic bones and lumbar spine. Anteriorly and laterally, the abdominal cavity is bounded by the soft muscular and fascial tissues of the anterior abdominal wall; posteriorly, the wall is more rigid, being formed by the parietal peritoneum that lies over the vertebral bodies with their muscular attachments.

From a functional perspective, the abdominal cavity essentially acts as a repository for a number of organ systems responsible for metabolic processing. This includes the hollow tubular structure of the bowel, which enters cranially at the gastro-oesophageal sphincter and develops into the remaining parts of the digestive system, carrying and processing nutrients and waste before, at the caudal end, passing these products back to the external environment. Organ systems such as the liver and kidneys are developed through a number of embryologic stages bringing a variety of different cell lines together for functional effect. Other structures that pass through the diaphragm and run into the pelvis include the great vessels, lymphatics and peripheral nerves. Although prenatal assessment of the abdomen may not inspire clinicians as much as some other structures, this is the powerhouse of metabolic well-being and includes and is bounded by many complex structures that need to be coordinated with surrounding tissues. Abnormalities of these systems can be lethal or cause significant morbidity in a neonate, and there is significant value in prenatal diagnosis that allows timely and appropriate intervention after birth.

Data from the European Congenital Anomalies Surveillance (Eurocat) Registry show that abdominal wall defects and gastrointestinal (GI) anomalies are the fifth most prevalent type of congenital anomaly, affecting approximately 1 in 400 pregnancies ( Table 32.1 ). About 75% of affected fetuses were liveborn, 3.2% of affected infants died in utero and 22% of women chose to interrupt the pregnancy. The range of GI pathologies is shown in Table 32.2 . Abdominal wall defects are commonest, with similar numbers of gastroschisis and exomphalos being identified although a significantly higher proportion of pregnancies affected by exomphalos were terminated. As a consequence, gastroschisis is the commonest abdominal surgical complication affecting liveborn infants. Herniation through the diaphragm is also common and is dealt with elsewhere. The remaining pathologies predominantly result from developmental errors leading to atresia of the variety of tubular structures seen through the alimentary canal.

Embryologic Development

The commonest abdominal anomalies seen prenatally, including gastroschisis and bladder extrophy, relate to failures in embryologic development of the abdominal wall. Formation of the abdominal wall involves a combination of lateral plate mesoderm and overlying ectoderm cell lines. The vertebrae and ribs and hypaxial flank muscles develop in the midline by 5 weeks’ gestation and then expand ventrolaterally and caudally. The rectus muscles reach the level of the umbilicus by 8 weeks’ gestation. Further rapid differentiation enables the development of the infraumbilical body wall. Between 4 and 10 weeks (when the extruded bowel returns to the intraabdominal cavity), there is a 25-fold increase in volume of the abdominal cavity. Differential rates of cell proliferation account for changes in shape, with a fivefold increase in abdominal circumference compared with length through this period. Processes of cell migration, reorganisation and cell-to-cell adhesion can all be disrupted, giving rise to the anomalies seen prenatally. Although exomphalos is also identified as an abdominal wall defect, the aetiology differs as the defect results from failure of gut loops to return to the body cavity after normal physiological herniation into the base of the umbilical cord.

The cloaca is the endodermal lined cavity that forms the boundary between the allantois (ventrally) and primordial hindgut (dorsally). This is divided, at 4 to 6 weeks, into anterior and posterior compartments by the urogenital sinus. Failure of development of this sinus will lead to a condition described as persistence of the cloaca in which the bowel, vagina and urethra remain confluent and drain into a common opening. The ventral part of the urogenital sinus develops into the bladder and urethra, and as the bladder ‘descends’ into the pelvis, the remaining portion of the allantoic duct involutes. Failure of involution will leave a patent urachal remnant. The ventral wall is normally reinforced through medial migration of mesoderm to form the lower part of the anterior abdominal wall. If this does not occur, the cloacal membrane can rupture, resulting in cloacal extrophy, bladder extrophy or epispadias depending on the timing of this event.

The gut and major intraabdominal viscera are formed from a tubular structure running through the craniocaudal axis of the early embryo. This tube is lined by endodermal tissue originating from the yolk sac. This is surrounded by a layer of mesoderm contributing to the gut tube wall as well as splanchnic (visceral) and somatic (parietal) mesoderms that continue to differentiate to form the supporting mesenteries. The vascular bundle that runs within the mesentery includes neural crest tissue that differentiates into the nerves and neurons found throughout the gut and associated viscera. The gut is traditionally divided into three parts (fore, mid and hind gut) that have different vascular supplies. Abnormalities of the bowel and other intraabdominal viscera can result from a range of failures in normal embryologic development, including anomalous differentiation of local cell populations, failure in tubal canalisation, failure to pull the gut into the abdominal cavity (or of closure of the ventral wall), failure in bowel rotation and anomalous vascular or neuronal connection. A number of resulting anomalies can potentially be detected in the prenatal period.

The liver develops from an embryologic structure found at the boundary of the embryonic pole and yolk sac known as the septum transversum. This brings ectodermal, mesenchymal and endodermal cell lines together. Internally, this aligns with the boundary between the foregut and midgut. In the early embryo, the liver buds out from the ventral surface. The cell lines undergo significant differentiation between 5 and 8 weeks of gestation to develop the complex architecture found between the portal field and central vein, including development of the biliary tree. The liver’s main embryonic and fetal functions are cardiovascular and haemopoietic, providing a vascular connection between the umbilical vein and the right side of the heart and producing blood stem cells before bone marrow development.

From an ultrasound perspective, embryonic development of the intraabdominal viscera can be followed from approximately 7 weeks of gestation. The anterior abdominal wall is already formed at this gestation, but the cord insertion can be visualised, and there is evidence of physiological herniation of the bowel into the cord at this stage. At 8 to 9 weeks, the abdominal cavity is almost completely filled by the liver and the stomach. The urethra becomes patent (through rupture of the cloacal membrane) at 9 weeks’ gestation, and the diaphragm develops at approximately 10 weeks. The bowel rotates and returns to the abdominal cavity by 11 weeks.

Sonographic Features at 12 Weeks’ Gestation

Although the routine second trimester (18- to 20-week) scan is still considered to be the ‘gold standard’ point for anatomical assessment, it is possible to detect major structural abnormalities, including some abdominal defects, at 11 to 13+6 weeks ( Fig. 32.1 ). In a series of 44,859 pregnancies that had a structured sequential anatomical survey completed at the 11- to 13+6-week scan, 488 (1.1%) fetuses had a structural abnormality, and 213 (43.6%) of these were successfully identified. This included all 104 cases of exomphalos, gastroschisis, megacystis and body stalk anomaly. In contrast, none of the three reported cases of bowel atresia were detected at 11 to 13+6 weeks.

The fetus is traditionally assessed in midsagittal section in the first trimester, which allows accurate assessment of crown rump length for confirmation of gestational age and measurement of nuchal translucency thickness, but this is not the most valuable plane for assessment of the abdomen and anterior abdominal wall. This is best achieved by rotating the probe so that the fetus is imaged in an axial section. The probe can then be manipulated to sweep through the abdomen, visualising structures of importance within a few seconds. Moving caudally from the thorax through the diaphragm, in the upper abdomen, the stomach should be visible to the left of the midline. The stomach is normally visible in all cases from 11 weeks’ gestation. Care should be taken to ensure that situs is appropriate. To the right of the midline at this anatomical level, the parenchyma of the liver can be seen, which is typically homogeneous with no echogenic foci. The hepatic portion of the umbilical vein can be used as a second landmark to define the correct plane for measurement of the abdominal circumference.

After the upper abdomen has been assessed, the probe can be swept down to the level of the umbilicus, and care should be taken to define the integrity of the insertion of the umbilical cord. In early pregnancy (i.e. <9 weeks’ gestation), space within the abdominal cavity is limited, and the developing small bowel extrudes through the umbilicus into the base of the cord. This should have returned to the abdominal cavity by 11 weeks’ gestation, and continued herniation of bowel or other intraabdominal viscus should be regarded as evidence of exomphalos. The features of this anomaly are discussed in more detail later. Colour Doppler can also be used to identify the umbilical arteries as they enter the abdomen, dividing around the bladder. A two-vessel cord can be detected if care is taken in this evaluation and has been reported to be associated with an increased risk for aneuploidy as well as with renal anomalies and is therefore a useful marker in the assessment of these disease processes. In addition to focusing on the umbilicus, it is important to check that there is no evidence of bowel herniation to the right of the midline; gastroschisis is also readily detected at 11 to 13+6 weeks when free loops of bowel, which are not contained within a membrane, can be seen floating freely in the amniotic fluid.

Sonographic Features at 20 Weeks’ Gestation

The abdomen and pelvis are traditionally assessed using three axial sections at 18 to 20 weeks ( Fig. 32.2 ). Moving sequentially from the thorax through the diaphragm, the upper abdomen is assessed in an axial section that demonstrates the echolucent stomach, the liver and the mid third of the umbilical vein at the level of the portal sinus. The upper poles of the kidneys should not be visible in this view. An additional cystic structure extending to the right may be visible, representing the fetal gallbladder. The abdominal circumference is measured at this level by placing callipers on the outer surface of the skin line and using either perpendicular linear measures or an ellipse to complete assessment. From an anatomical perspective, this view is used to check that the stomach is visible and to check situs and consistent echogenicity across the liver. In combination with coronal or longitudinal sections, this view is also used to demonstrate the integrity of the left and right hemidiaphragms.

Moving caudally, the fetal kidneys can be demonstrated lying either side of the spine. Anteriorly, the cord insertion is assessed to ensure integrity of the anterior abdominal wall. Turning into a longitudinal section, the distance between the insertion of the umbilicus and the genital tubercle can also be assessed. The lumen of the small and large bowel is not typically obvious at this gestation, and dilated loops can be detected if present. The small bowel may be echogenic, which has been associated with a range of pathologies described later. An axial section of the pelvis is used to demonstrate the presence of the bladder, and colour Doppler can be applied to show the bifurcation of the umbilical arteries and therefore the presence of a three-vessel cord. Moving caudally, the external genitalia can also be assessed.