ABSTRACT
Introduction Congenital heart disease (CHD) should be analyzed in a sequential segmental manner, especially when an abnormality of situs exists.1,2 The term “situs” is used to define the right-left orientation or arrangement of the body organs at three anatomical levels: abdominal, bronchopulmonary, and atrial.3-8 The term situs, however, does not define cardiac position. Usually there is an asymmetry of the abdominal and thoracic organs (Figure 17.1, Table 17.1). Abdominal asymmetry is characterized by a specific rightward-leftward orientation of the nonpaired solitary organs including the liver, stomach, and spleen, whereas thoracic asymmetry is due to asymmetry of the forms of the paired organs including the lungs, bronchi, pulmonary arteries, and atria. The organ arrangement at one level is usually consistent with other levels, but exceedingly rarely, abdominal, bronchopulmonary, and atrial arrangements may also be discordant.3,6,9-13 Three types of situs are possible: situs solitus, situs inversus, and heterotaxy (Figure 17.2, Table 17.2). Situs solitus indicates the normal arrangement of organs at the abdominal, atrial, and bronchopulmonary levels. Situs inversus refers to an inverted or mirror image arrangement of these organs. Heterotaxy (Greek, heteros [other or different] + taxis [arrangement]) indicates that the arrangement of the organs is different from the usual arrangement of situs solitus or situs inversus.4,12
How to determine visceral and atrial situs Laterality disorders (situs inversus and heterotaxy) affect about 1.1-1.2 per 10,000 live births.14 As these abnormalities of situs are harbingers of CHD and other anomalies, it is very important to routinely define the situs prior to the detailed fetal cardiac evaluation. The assessment should first begin with the determination of right and left sides of the fetus.15 Once the sonographer has clarified the “sidedness” of the fetus, the evaluation of situs at all three levels can be commenced. The abdominal situs is determined by observing the location of the liver, stomach, spleen, abdominal aorta, and inferior vena cava in the transverse view of the upper
abdomen (Figure 17.3).1 In situs solitus, the larger lobe of the liver is seen on the right, and the stomach and spleen are seen on the left (Figure 17.3a). The abdominal aorta is located posteriorly at the left anterior aspect of the spine, and the inferior vena cava is located more anteriorly on the right, almost always entering the right atrium. In situs inversus, this rightleft relationship is mirror imaged (Figure 17.3b). In heterotaxy with right isomerism or asplenia, the liver is characteristically symmetrical and the stomach tends to be located near the midline either on the right or on the left (Figures 17.3c,d). Occasionally, a part of the stomach is seen in the lower thorax.16 In most but not all cases with right isomerism, there is asplenia: by evaluating the posterior and lateral aspects of the stomach, the spleen can be identified if it is present. In most cases of heterotaxy and asplenia, a posteriorly located abdominal aorta and an anteriorly positioned inferior vena cava are seen on the same side of the spine as “juxtaposed” vascular structures.1,17
In heterotaxy with left isomerism or polysplenia, symmetry of the liver may appear less evident with its major portion often lying to one side of the abdomen (Figures 17.3d and 17.4).3,4 The arrangement of the liver and stomach often mimics that of either situs solitus or situs inversus. As the spleen develops in the dorsal mesogastrium, multiple spleens (polysplenia) aggregate in a single location along the greater curvature of the stomach18 (Figure 17.3d). Occasionally, multiple spleens may fuse together to form a single but multilobulated mass. This is in contrast to accessory spleens, in which a dominant spleen is present together with multiple small splenules that can be found anywhere in the peritoneal cavity. Polysplenia is commonly associated with interruption of the suprarenal infrahepatic segment of the inferior vena cava (interrupted IVC) and continuation of the lower-body venous drainage via the azygos or hemiazygos venous system (Figure 17.5). Although it can also occur with other types of body situs, interruption of the IVC is highly suggestive of heterotaxy with polysplenia. It can be diagnosed when the coronal or sagittal sonograms do not show the usual IVC connection to the atrial segment (Figure 17.5a).17,19-21 It can also be suspected when two equally sized vessels are seen in the posterior mediastinum in either the coronal or the transverse view, the posterior one being the dilated azygos or hemiazygos vein
and the other anterior structure being the descending aorta (Figures 17.5b-d).1,17,19-21 On color Doppler examination, the blood flow through these two parallel vessels is in opposite directions with the azygous vein seen draining into the superior vena cava (Figure 17.5d).
