concerned and the atrial appendages, the right and left sides are mirror images of each other (enantiomeric) This is the essence of bodily isomerism The question marks refer to the fact that organ arrangement in the belly has no real pattern with either left or right isomerism Venoatrial Connections Anomalous venoatrial connections are the rule in patients with isomeric atrial appendages As already emphasized, some patients with isomeric atrial appendages can have an overall pattern of venous drainage that can be considered usual (Figs 26.8 and 26.9), or mirror imaged (Figs 26.10 and 26.11) Even in these patients, however, the venoatrial connections can never be anatomically normal when the appendages are isomeric In each individual therefore any pattern must be anticipated Certain features are sufficiently common, nonetheless, usually to permit the differentiation of right and left isomerism It is the connection of the pulmonary veins that is most reliable in permitting this distinction In right isomerism, because the pectinate muscles extend to the crux on both sides, the pulmonary venous connections are always anatomically abnormal Thus, when all the pulmonary veins connect to one of the morphologically right atrial chambers, be the chamber right- or left-sided, the anatomy is always abnormal when compared to the normal connections of the pulmonary veins to the morphologically left atrium This is because the morphologically left atrium never exhibits pectinate muscles extending to the crux, as is the case when both appendages are morphologically right In the minority of cases in which the pulmonary veins connect directly to an atrium with a morphologically right appendage, they join the atrial chamber posteriorly (Fig 26.12) Most usually, when the pulmonary veins do return directly to the heart in the setting of right isomerism, they make their connection via a fibrous confluence, typically opening centrally Therefore when there are morphologically right appendages bilaterally, the connections of the pulmonary veins will always be anomalous anatomically, even if the atrium receiving the veins is itself left-sided In about half the patients having isomeric right appendages, such semantic pitfalls do not arise This is because the pulmonary veins drain exclusively to an extracardiac source The site of anomalous connection is as varied as when totally anomalous pulmonary venous connection is seen with usual atrial arrangement (see Chapter 29) The problems of obstruction within the anomalous pulmonary venous pathway are the same Indeed, clinical experience suggests that an obstructed supracardiac pathway is more frequent in the setting of right isomerism than in usual arrangement The other universally constant feature of the venoatrial connections in the presence of isomeric right atrial appendages is absence of the coronary sinus (Fig 26.13), this channel being a component of the morphologically left atrioventricular junction When the pectinate muscles extend bilaterally to the crux, there is no room to enclose the sinus within the junctions Some patients with absence of the spleen may possess a coronary sinus, but this is because not all patients with “asplenia” have isomeric right atrial appendages FIG 26.8 Heart with isomerism of the left atrial appendages demonstrating quasi-usual venous relationships (A) The base of the heart shows bilateral tubular appendages and their narrow junctions (red arrows) with the atrial vestibule There is a right-sided superior caval vein with a large azygos vein indicative of azygos continuation of the inferior caval vein The pulmonary veins drain to the left-sided atrium (white box and arrows) (B) The posteroinferior view of the same heart The superior caval vein is marked with a black star and the right and left pulmonary veins with red stars (C) Short-axis view of the same heart demonstrating so-called noncompaction of both ventricles and the interventricular septum There was also transposition FIG 26.9 Heart with isomerism of the right atrial appendages and tricuspid atresia demonstrating quasi-usual venous relationships Pectinate muscles extend to the crux on both the right (A) and left sides (B) In A, the superior and inferior caval veins drain to the right-sided atrium; in B, the pulmonary veins drain to the left-sided atrium Note the large atrial septal defect AV, Atrioventricular