As illustrated in Table 51.1, the evaluation begins with definition of visceral situs. Visceral (abdominal) situs is determined by the positions of the liver and stomach (see Fig. 51.1). The pancreas and spleen are generally located on the same side of the vertebral column as the stomach. Atrial and visceral situs often are considered together because they are usually concordant (the atrial and visceral situs are the same). However, exceptions occur, and venous connections and visceral situs may not necessarily agree with the apparent atrial situs. These features can be particularly variable in situs ambiguus. Situs ambiguus may be best defined as an uncertain or indeterminate situs (organ positions do not fit into any standard category). However, in many cases, the right and left components appear very similar to each other, and have been described as mirror images. Such cases also have been referred to as “isomeric” or as having bilateral right or left sidedness.

In visceral situs ambiguus with right isomerism (bilateral right sidedness), the spleen is usually absent (asplenia) and the liver is centrally located, symmetrically straddling the midline (see Fig. 51.1). In this situation, two mirror-image morphologic right hepatic lobes are present. In contrast to the normal arrangement, the inferior vena cava (IVC) and abdominal aorta will be found on the same side of the vertebral column. In addition, these patients almost always will have malrotations of the bowel.

Situs ambiguus with polysplenia has been described as bilateral left sidedness or left isomerism (5,6,7,8,9,10,11,12). However, the degree of right/left symmetry is less pronounced in these patients, compared to that seen in asplenia and bilateral right sidedness. In fact, the most common arrangement of the abdominal viscera in polysplenic patients is situs inversus. As a teaching tool, the concept of isomerism (mirror-image sidedness) is an attempt to simplify the typical features of complex anomalies in which multiple abnormalities tend to occur together. Thus, the asplenia syndrome (Ivemark syndrome) appears as a pathologic grouping of features emphasizing right sidedness, such as bilateral right bronchi and bilateral right (trilobed) lungs, bilateral right atria, and a symmetrical liver. Ivemark described the implications of asplenia on the pathogenesis of conotruncal anomalies in 1955. Van Mierop et al. (12) pointed out that the asplenia syndrome is associated with pulmonary stenosis or atresia, whereas in polysplenia syndrome, presence of single ventricle physiology and obstruction to pulmonary arterial blood flow are less common. In contrast, interruption of the IVC with azygos continuation of the inferior venous system and anomalous, bilateral
pulmonary venous connection of each lung to the ipsilateral atrium are common in polysplenia, but rare in asplenia. Tables 51.3 and 51.4 outline the frequency of both extracardiac and cardiac malformations in cases of heterotaxia. The frequencies represent a compilation of cases reported by multiple investigators (13,14,15,16).

Although splenic anomalies are often associated with complex congenital heart disease and with abnormalities of visceral situs, they are not the cause. Embryologically, the spleen, in contrast to the other thoracoabdominal viscera, does not initially develop as a midline structure. Rather, the splenic anlage is left sided from its inception. As a result, in the case of bilateral right-sided symmetry, the spleen would not be expected to develop—hence the association of right isomerism with asplenia. In contrast, left isomerism is frequently associated with polysplenia, although multiple spleens are usually found on only one side of the vertebral column (along the dorsal aspect of the stomach).

Clinically, the identification of the morphologic right atrium is important for establishing atrial situs. This is due to the fact that much of the anatomic left atrium is derived from the embryonic common pulmonary vein. As a result, its walls are relatively smooth and lack distinctive features for identification by clinical imaging modalities. In contrast, the anatomic right atrium is derived from the primitive atrium and has several muscular structures that make it more recognizable. Its most reliable intrinsic anatomic hallmark is the relatively thick superior limbus of the fossa ovalis, which can be observed by direct inspection at operation, at autopsy, or by tomographic imaging techniques (echocardiographic, CT, or cardiac MR scanning). In cases in which the atrial septum is absent, distorted, or cannot be visualized, atrial morphology may be inferred by inspection of the free atrial wall. In contrast to a left atrium, a morphologic right atrium will have a large pyramidal appendage, a crista terminalis, and pectinate muscles (14). In situs ambiguus and other complex cases, the atrial septum can essentially be absent, resulting in a common atrium. In these cases, assignment of atrial “identity” can be challenging and often must rely on secondary, associated features. The presence of pectinate muscles and the crista is helpful in anatomic specimens. However, these are not always clearly defined by cardiac imaging. In these situations, the connections of the coronary sinus and/or the suprahepatic segment of the IVC provide the most reliable and readily recognizable markers identifying the position of the morphologic right atrium (16,17). Anatomic determinants of the other cardiac segments also have been described and are summarized in Table 51.2.

One can encounter cases of truly bilateral right atria (or right atrial isomerism). In these patients the appearance of both atrial appendages and the atrial free walls will have the characteristics of a right atrium. The coronary sinus is usually absent in these cases and IVC connection can be ambiguous as well (bilateral or midline hepatic veins). Even more rarely, in polysplenia syndrome with left atrial isomerism, both atrial appendages and free walls may resemble a left atrial appendage. The left atrial appendage usually has a narrow base and a tubular, hooked shape (Fig. 51.3) (18). However, in most cases with asplenia or polysplenia, the atrial anatomy is poorly defined, with a large common atrium, a thin rudimentary remnant of atrial septum, anomalies of systemic and pulmonary venous connections, and abnormal, but similarly shaped bilateral appendages.

Van Praagh et al. (16) have argued that the concept of bilateral or two right atria and bilateral or two left atria is anatomically unrealistic. These researchers and also De la Cruz and Nadal-Ginard (17) emphasized the importance of the suprahepatic segment of the IVC as a reliable marker for the morphologic right atrium, because embryologically, the sinus venosus is incorporated into the right atrium, and IVC-to-right atrium concordance is diagnostically accurate, even when there is visceroatrial discordance. With incorporation of the sinus venosus, the coronary sinus also can be recognized as a reliable marker for the morphologic right atrium. The morphologic right atrium, then, can be determined by a normally formed coronary sinus, a larger and more anterior appendage, and connection of the suprahepatic portion of the IVC. This approach allows clinical assignment of atrial situs in most cases, although the most important clinical issues are often related to the position and spatial arrangement of the venoatrial connections rather than the morphologic identities of the atria themselves.

Pulmonary situs is determined by the sidedness of the morphologic right and left lungs. Clinically, this is defined by the relationship of the pulmonary arteries to their adjacent bronchi, and not by the number of lobes in each lung. In a morphologic right lung, the pulmonary artery travels anterior to the upper lobe and intermediate bronchi. In contrast, a morphologic left lung is characterized by a pulmonary artery that courses over the main bronchus and posterior to the upper lobe bronchus. The distance from the carina to the origin of the upper lobe bronchus is 1.5 to 2 times greater for the morphologic left lung than for the right lung (19). These relative distances may be used clinically to infer pulmonary situs. The simplest method is by examination of the air bronchograms on a plain chest x-ray. This ratio applies regardless of the sidedness of the aortic arch (i.e., regardless of the bronchus over which the aorta travels) (Fig. 51.4). In the setting of pulmonary isomerism (both lungs having the same morphology), however, the ratio approaches unity, because the lengths of the two main bronchi are similar. In these cases, bilateral trilobed (right) lungs should suggest the diagnosis of situs ambiguus with the asplenia syndrome (right isomerism), and bilateral bilobed (left) lungs the polysplenia syndrome (left isomerism). However, lung isomerism does not exist in all cases of cardiac isomerism and pulmonary isomerism can occur in patients without features of cardiac heterotaxia.

The cardiac and vascular segments must be evaluated not only according to morphology, but also in terms of spatial orientation. For example, atria and ventricles should be described both by their morphology and their right or left sidedness. In general, the position and orientation of the ventricular septum is used to describe the location of the ventricular chambers. In hearts with levocardia with normally positioned ventricles, the right ventricle (RV) is to the right and anterior, while the left ventricle (LV) is left and posterior to the septum. In contrast, in situs inversus totalis the usual ventricular arrangement is the LV–right posterior and the RV–left anterior. Mesocardia is characterized by a vertical midline septum with side-by-side ventricles. Rarely, the ventricular septum is horizontal rather than vertical and results in superoinferior (over-and-under or “upstairs–downstairs”) ventricles.

The position of the aorta is described relative to the pulmonary trunk. Normal hearts are characterized by a right posterior aorta. After the morphology and position of the cardiac segments are determined, the manner in which they connect to one another is evaluated. The three forms of connections to be investigated are venoatrial, AV, and VA.

The difficulties encountered in the accurate definition of atrial situs based on atrial anatomy alone emphasize the importance of accurate determination of systemic and pulmonary venous connections, as described in Tables 51.1 and 51.2. Regardless of the designation of a right or left atrium or the shape of the atrial appendages, the localization of the superior vena cava (SVC), IVC, hepatic veins, coronary sinus, and pulmonary venous connections should be determined individually and recognized as unilateral or bilateral. This determination is particularly important for surgical management and should be a routine part of any imaging study.