Survival Isomerism does impact survival, with greater mortality in those with isomerism when compared to those with similar congenital heart defects but without isomerism Survival has improved with time in isomerism patients with functionally univentricular circulation and biventricular circulation Mortality seems to be greatest in the first 3 years of life when all isomerism patients are grouped together Survival at 10 years of age is 61% and survival at 25 years of age is 35% If only those in the current era born after 2000 are considered, there is 70% survival at 13 years of age When the subsets of isomerism are compared, those with left isomerism tend to have better survival until approximately 14 years of age Survival at 5 years of age is 94% in those with left isomerism and 76% in those with right isomerism, while survival at 10 years of age is 83% in those with left isomerism and 64% in those with right isomerism As might be expected, those with biventricular circulation have increased survival when compared to those with functionally univentricular circulation In those with biventricular circulation, survival at 5 and 10 years of age is 89% and 84%, respectively Obstructed pulmonary venous connection, congenital atrioventricular block, moderate or greater atrioventricular regurgitation, presence of a common atrioventricular junction, pulmonary atresia, and need for extracorporeal membrane oxygenation are all associated with increased mortality in those with isomerism.10–24 Cardiac Anatomy Atrial Anatomy Structural isomerism, strictly enantiomerism, is seen when entities are mirror images of each other (Fig 26.1) In this respect, the usual arrangement of the organs within the body, when compared to its mirror-imaged variant, is an example of biologic enantiomerism (Fig 26.2) It is now established beyond reasonable doubt that within the heart, when assessed according to the extent of the pectinate muscles relative to the atrioventricular junctions, it is the atrial appendages that show the enantiomeric features.4 It is, however, only the atrial appendages that are uniformly isomeric One of the features of isomerism is anomalous venoatrial connections, although the patterns of flow through the anatomically abnormally connected veins can be quasi-usual or quasi–mirror imaged Emphasis is placed on the appendages simply because the venoatrial connections, along with vestibular morphology and septal structure, are all variable For example, the pulmonary veins may be connected to the morphologically right atrium, or to an extracardiac site The atrioventricular junction may be absent, as in tricuspid or mitral atresia Although the atrial septum has typical morphologically right and morphologically left sides, they are not always there These variable features, following the precepts established by the so-called morphologic method, cannot be used as reliable indicators of morphologically rightness or leftness.2 Instead, the heart with right isomerism will be characterized by the presence of appendages each having the morphology of the normal right appendage The uniformly characteristic feature of such rightness is the extent of the pectinate muscles around the atrioventricular junctions, so that they meet at the crux (Fig 26.3) The external features are usually distinctive, but not always constant with bilateral, morphologically right atrial appendages represented by a blunt, triangular tip and a broad junction with the venous component (Fig 26.4) This junction is marked by an extensive groove and corresponds internally to a prominent terminal crest It is the extent of the pectinate muscles that are the universal criterion for morphologically rightness on both sides of the atriums (see Fig 26.3) In contrast, in the heart showing left isomerism, each of the appendages will have the characteristic morphology of the normal left appendage, with the pectinate muscles contained within the tubular appendages, the posterior vestibular areas being smooth on both sides, and directly confluent with the venous components (Fig 26.5) The tubular appendage of the morphologically left appendage has a narrow or constricted junction with the smooth-walled component of the atrium (Fig 26.6) The pectinate ridges may extend more laterally than the constricted junction of the appendage, but they are always significantly more limited in their extent than the morphologically right atrium (compare Figs 26.3 and 26.5) Although the major morphologic features of isomeric appendages are to be found internally, the external features are also usually distinctive The presence of bilateral triangular, as opposed to bilateral tube-like, appendages, however, is not universally present It is the extent of the pectinate muscles that is the universal criterion for morphologically isomerism of the appendages (Fig 26.7) The arrangement of the appendages is in most cases harmonious with the arrangement of the thoracic organs Discordance between pulmonary arrangement and that of the appendages, however, is not uncommon Such discordance can also be found in patients with normal hearts For example, in the syndrome of biliary atresia with multiple spleens, it is possible to find bronchial isomerism, but usual atrial arrangement.25,26 The morphology of the appendages is the starting point of ongoing sequential analysis, but taking care to note the arrangement of the other systems of organs if discrepancies are identified FIG 26.1 Objects that are mirror images of each other This is the essence of structural isomerism, properly described as enantiomerism