Discussion
The evidence is now strongly in favour of discriminating between the recognized sub-sets of so-called “visceral heterotaxy” on the basis of isomerism of the atrial appendages.4-13 With the advances now made possible through the increased resolution of multi-detector computed tomography, and the ability to reconstruct the three-dimensional datasets, it is possible not only to recognize the features of the atrial appendages, but also to correlate the cardiac findings with the arrangements of thoracic and abdominal organs. The clinical findings endorse the conclusions long since made based on autopsy studies. Thus, right isomerism is usually, but not universally, accompanied by absence of the spleen. It is this feature that is believed to render such individuals susceptible to infection. The isomeric right appendages are the harbingers of the most severe cardiac and complex congenital malformations.2-13 Left isomerism is typically associated with polysplenia, but even the presence of multiple spleens does not protect against splenic incompetence.15-23,54
The markedly different features of right as opposed to left isomerism obviously pose different surgical challenges. Detailed knowledge of the likely co-existing cardiac anomalies, therefore, is advantageous prior to surgical intervention. Recognition of the presence of bilateral morphologically right atrial appendages, for example, enables the surgeon to anticipate the presence of bilateral sinus nodes. Recognition of isomeric left appendages, in contrast, alerts to an abnormal location of the atrial pacemaker.37,60 Appropriate precautions, therefore, should be taken to avoid injury not only to the sinus node, or nodes, but also their arteries.42
In those with left isomerism, the surgeon should anticipate interruption of the inferior caval vein. If performing a superior cavopulmonary connection, and prior to ligating the azygos venous channel, an enlarged azygos vein associated with an intact inferior caval vein should always be differentiated from azygos continuation of an interrupted vein. When the inferior caval vein connects directly to the atrial chambers, it may connect to either the left or right-sided atrium. In left isomerism, this should be distinguished from bilateral connection of hepatic veins. In those with right isomerism, in contrast, a hepatic vein often co-exists with an inferior caval vein (Figures 8 and 9).2-5,9-11
Although the arrangement of the inferior caval vein tends to distinguish those with left from right isomerism, this is not the case for the superior caval vein. At least half of patients with right isomerism, and up to two-thirds of those with left isomerism, have bilateral superior caval veins.2-13,15,19 When bilateral superior caval veins are present with right isomerism, each connects to the top corner of its atrium. This can make it difficult to divert the vein to the right-sided atrium, should this be required, or because of its short length even to the right superior caval vein.50 In left isomerism, in contrast, both superior caval veins are connected to atrial chambers in the fashion of drainage through a coronary sinus, although often unroofed.2,7,10,11 The hepatic venous connections are of particular concern in this setting. The veins usually connect to one atrium, but sometimes to both atriums, or to both sides of a common atrium (Figures 7 and 8).5,7,9-13 Such direct hepatic venous connections are not only found when the inferior caval vein is interrupted, but also when it connects directly to one or other atrial chamber. When performing the Kawashima procedure, search should be made for any interconnections between the inferior caval vein and the hepatic veins below their entry to the heart.5,7,9-13,43-47 Preoperative demonstration of such venous arrangements can facilitate construction of fenestrated Fontan pathways (Figure 9).44-46
Although not always recognised, totally anomalous pulmonary venous connection is universal in the setting of right isomerism, since both atrial appendages are morphologically right. Even should all pulmonary veins connect directly to one of the atrial chambers, such connections will be anatomically anomalous.2,3 The cardiac venous return is often obstructed. 2-5,14-22 The presence of pectinate muscles all around the muscular atrioventricular vestibules creates additional problems in producing an unobstructed pulmonary venous pathway.2-15,19-22 Pulmonary venous obstruction is the more frequent when the pulmonary arteries are hypoplastic, atretic, or discontinuous.2-15,19-22 Severe obstruction to pulmonary venous flow may mask the clinical importance of the pulmonary venous anomaly.2-22 On our institute, we continue to perform adjustable ligation of the vertical vein, routine left atrial augmentation, and interatrial septal fenestration when we recognise obstructive totally anomalous pulmonary venous connection, or when we encounter suprasystemic pulmonary arterial hypertension subsequent to weaning from bypass.48,49 It is almost certainly the intrinsic anatomic and histopathologic differences in the pulmonary veins of individuals with right isomerism that accounts for the higher incidence of reoperation when compared to individuals with lateralized atrial appendages.14,15,20,21,48,49,61-63
The pulmonary venous connections will always be anatomically normal when there is left isomerism, but such normally connected veins can be bilaterally symmetrically, with two connecting to each atrium with a morphologically left appendage.2-5,11 This produces a significant distance between the right and left-sided pulmonary veins,5,9-13 making for elongated and complex suture lines for any potential intraatrial baffle.14-22 The complexity of the required baffles can explain in part the reported incidence of postoperative arrhythmias, which has ranged between less than one-tenth to one-half. Also contributing to these problems are the absence, hypoplasia, or abnormal location of sinus node, along with abnormalities in the pathways for atrioventricular conduction.14-21,59,60
Another well recognized problem in the setting of left isomerism is the development of pulmonary arteriovenous malformations following the Kawashima operation, seen in up to three-fifths of cases (Figure 10).45,46,64 This is attributed to isolation of the lungs from exposure to hypothetical hepatic factors. Redirection of the hepatic venous return to the systemic venous circuit during completion of the Fontan circulation has been shown to result in subsequent resolution.45,46,64
Within the overall group, regurgitation of the common atrioventricular valve is a known risk factor for postoperative mortality, particularly after functionally univentricular repair. A systemic-to-pulmonary arterial shunt is known to cause volume overload, aggravating the regurgitation, and increasing mortality following completion of the Fontan circulation. 15,21,43-47 This has led to suggestions that the common valve be repaired or replaced before completion of the operations.41,45-47 Cardiac transplantation also represents a significant technical challenge. This may reflect previous palliative procedures, but is further exacerbated by the complex systemic and pulmonary venous anomalies.24,25 Even though the technical issues have largely been resolved, transplantation in the setting of isomerism remains associated with increased postoperative complications. Early and late survival is poor compared to other forms of congenital cardiac diseases.24,25,51-53