FIG 26.14 Heart and lungs from a patient with left isomerism, photographed from behind The venous return from the abdomen, apart from that from the liver, reaches the heart through the azygos vein, which runs together with the right-sided aorta The presence of separate connection of the hepatic veins, when combined with the relationship of the abdominal great vessels relative to the spine, had been considered a reliable means of distinguishing noninvasively the presence of right and left isomerism.19 We now know this not to be strictly accurate It is difficult, if not impossible, to distinguish with certainty cases as having isomerism simply by studying the relationships of the abdominal great vessels to the spine This is not to detract from the value of this feature when used as the initial step in the ultrasonographic assessment of sequential segmental anatomy Knowledge of the overall connections of the pulmonary veins, the inferior caval vein, and the drainage of the hepatic veins to the atriums, along with the arrangement of the coronary sinus, nonetheless, is a more accurate means of distinguishing between the right and left forms of isomerism Bilateral connections of superior caval veins to the roofs of the right- and left-sided atriums, however, are frequent in either setting In those with isomeric left appendages, these connections are anomalous on each side In those with isomeric right atrial appendages, in contrast, they are anatomically normal, with each caval vein appropriately related to a terminal crest (Fig 26.15), and with sinus nodes present subepicardially in the bilateral terminal grooves The drainage of the veins from the heart itself is also abnormal in both right and left isomerism This is no more than to be expected in right isomerism since, in the universal absence of the coronary sinus, there is no transverse channel within the atrioventricular groove to collect the venous return from the heart The variability in termination of the individual cardiac veins is surprising The veins can terminate directly, take a crooked course for a short distance along the atrioventricular groove, or traverse the atrial wall for some distance before draining into the atrium well away from the atrioventricular groove, often adjacent to the opening of a pulmonary or systemic vein (Fig 26.16) Such direct, crooked, or distant venous terminations are also to be found in hearts with isomeric left appendages, but a coronary sinus receiving all the coronary venous return is more frequent in cases with left isomerism.27 FIG 26.15 Internal aspect of the right-sided (A) and left-sided (B) atrial chambers from a patient with isomeric right atrial appendages The presence of terminal crests bilaterally is obvious FIG 26.16 Inferior surface of a heart with isomeric atrial appendages illustrating the concept of direct, crooked, and distant return of coronary venous drainage when the coronary sinus is absent Atrial Septum Although the extent of atrial septal deficiency fails positively to discriminate between patients having isomeric right or left atrial appendages, the septum tends to be better formed in those with left isomerism In those with right atrial appendages bilaterally, most frequently there is simply a strand of atrial tissue that spans a common atrial cavity It is rare to find the atrial septum completely lacking, but in most cases there is, effectively, a common atrium In about onefourth of cases, the septum is well formed superiorly in association with an atrioventricular septal defect, while very rarely the septum can be intact or the oval foramen be probe patent An effectively common atrium is to be expected in about half of cases with left isomerism An atrioventricular septal defect is also present in nearly half, while the septum can be virtually intact in nearly onefifth