In atrial isomerism, both atria have similar internal, external, and appendage configurations. They are considered either morphologically bilaterally right atria or bilaterally left atria. ^{, ,} In some cases, even at autopsy, pure right or left atrial isomerism is not always present in the heterotaxy spectrum, and controversy exists about some details of isomerism. ^,

Atrial situs is most usefully determined by morphology of the atrial appendages, because all other studies provide indirect information. Right atrial appendage morphology is present when the appendage is blunt and has a broad junction with a smooth-walled atrium. This type of junction is accompanied by protrusion of the crista terminalis into the atrial cavity. Left atrial appendage morphology is present when the atrial appendage is long with windsock appearance and thin with constrictions along its length. Such appendages have a rather constricted junction with a smooth atrium, within which a crista terminalis is not identifiable. However, even with direct visualization, the shape of the atrial appendages and the nature of their junction with each atrium may be ambiguous, with appendages that have mixed right and left atrial morphology.

Atrial isomerism (right or left) commonly corresponds to thoracic isomerism; however, disharmony between atrial morphology and pulmonary and bronchial morphology occurs. ^{, , ,} Atrial and thoracic isomerism (i.e., bilateral atrial and thoracic right- or left-sidedness) usually corresponds to bilateral right-sidedness (asplenia) or left-sidedness (polysplenia) of the abdominal viscera, but there are exceptions to this correspondence. ^, Abdominal asplenia or polysplenia may occasionally exist without atrial isomerism, so splenic state does not always predict atrial morphology. In one autopsy study, right atrial isomerism was associated with true asplenia in 65 of 82 (79%) instances, a solitary spleen in 14 (17%), and multiple spleens in 3 (4%). Among 41 specimens with left atrial isomerism, polysplenia was present in 36 (88%), a solitary spleen in 3 (7%), and asplenia in 2 (5%).

In surgical series, either right or left atrial isomerism may predominate. ^, In an analysis of the Society of Thoracic Surgeons Congenital Cardiac Surgery Database, operations for right atrial isomerism were more frequent than for left atrial isomerism. Several heterotaxy studies performed in Asian populations show a strong predilection (80% of cases) for right atrial isomerism, suggesting there may be racial differences in the expression of left and right atrial isomerism. ^,

Right atrial isomerism is usually accompanied by bilateral sinus nodes, one in each atrium. ^{, ,} Two atrioventricular (AV) nodes may be present, with a sling of conduction tissue between them. In left atrial isomerism, the sinus node is absent in the majority of cases but when present is unusually positioned and often hypoplastic.

The AV node may be normally situated when ventricular architecture is right-handed (D-loop); when it is left-handed (L-loop), two AV nodes and a sling may be present. Other more severe conduction system abnormalities may also be present, because complete heart block occurs in some neonates with left atrial isomerism.

Supraventricular atrial tachycardias occur in right atrial isomerism in up to 25% of patients, whereas abnormal axis P waves with slow atrial or junctional rates are the rule in left atrial isomerism.

Anomalies of systemic venous connection are common. The inferior vena cava is often “interrupted” and does not connect directly to the atrium from below. Instead, it passes superiorly along the right-sided paravertebral gutter (azygos continuation of inferior vena cava) or left-sided gutter (hemiazygos continuation of inferior vena cava), emptying into a right-sided or left-sided superior vena cava ( Table 53.2 ). Azygos continuation of the inferior vena cava occurs nearly exclusively in patients with left atrial isomerism and is the most common systemic venous anomaly, occurring in about 74% of cases (see Table 53.2 ).

Bilateral superior venae cavae occur frequently: in half of patients with right atrial isomerism and in two-thirds of patients with left atrial isomerism ( Table 53.3 ). When present, each typically connects to the top corner of the corresponding atrium; however, in left atrial isomerism, one may connect to the coronary sinus.

When the inferior vena cava connects directly to the atria from below, it may connect to either the left- or right-sided atrium (see Table 53.2 ). Hepatic veins often connect directly to the atria from below, usually to one atrium but sometimes to both or to both sides of a common atrium. Such a direct hepatic vein connection is present in all patients with an azygos extension of the inferior vena cava, but it also occurs in patients whose inferior vena cava connects to the atria from below ( Table 53.4 ).

Uemura and colleagues report that the coronary sinus orifice is absent in about 40% of patients with left and in 100% of patients with right atrial isomerism. Other series, however, show substantial variation from these percentages. ^{, ,} Anomalies of systemic venous connection do not occur exclusively in patients with heterotaxy.

Extracardiac total anomalous pulmonary venous connection (TAPVC) is highly correlated with right atrial isomerism ( Table 53.5 ), present in 60% to 70% of cases of right atrial isomerism versus about 5% in left atrial isomerism. When pulmonary veins connect to an atrium, pattern of connection is variable. Importantly, there is usually the normal wide area of posterior atrial wall between the pulmonary veins when the heart is viewed from behind. Pulmonary venous obstruction may be present in up to 40% of patients with right atrial isomerism, especially when the connection is extracardiac. Pulmonary venous obstruction in left atrial isomerism is much less common. Atresia of the common pulmonary vein has been reported in right atrial isomerism.

About 75% of patients with left atrial isomerism have biventricular AV connections that are ambiguous. ^, However, there is a univentricular AV connection in about 50% to 75% of patients with right atrial isomerism, a considerably higher percentage than in any other type of atrial situs, and most of these patients have a solitary ventricular chamber ( Table 53.6 ). ^{, , ,}

The complexities of pulmonary and systemic venous connections, variability in the position and nature of AV valves through which the atria empty, and anomalous muscle bands that sometimes traverse the atria often make it difficult to apply conventional terms describing atrial septal defects (ASDs). However, a common atrium (see “ Common Atrium ” under Morphology in Chapter 32 ) is present in nearly half the cases. ^, AV septal defect is present in about 80% of patients, with a higher prevalence in right than in left atrial isomerism (see Table 53.6 ). Most patients with atrial isomerism and AV septal defects have a common AV orifice (see “ Complete Atrioventricular Septal Defect ” under Morphology in Chapter 32 ) rather than two atrioventricular valves (AVV) orifices. Rarely the atrial septum is well formed and intact or has only a probe-patent foramen ovale. ^,

Complexities of AV valves and connections and frequent occurrence of solitary ventricular chambers make it difficult to apply conventional terms. Only rarely is the ventricular septum intact; about 80% of patients with a ventricular septal defect (VSD) have an AV septal defect, and in the remainder with intact AV septal structures, various types of VSD are present.

Unobstructed pulmonary outflow is rare in right, but more frequent in left, atrial isomerism. In right atrial isomerism, pulmonary stenosis is present in slightly more than half of patients and pulmonary atresia in about one-third. In left atrial isomerism, pulmonary stenosis is present in about half and pulmonary atresia in less than one-tenth (see Table 53.6 ).

In surgical series, ventriculoarterial connections are most commonly discordant, and an unusually high proportion (33%) of patients have double outlet right ventricle (DORV). In autopsy series, about 75% to 90% of specimens with right atrial isomerism have discordant ventriculoarterial connection (transposition) or DORV; in left atrial isomerism, this is true in about 20% to 65% of specimens (see Table 53.6 ). ^{, ,} In some cases, such as double outlet from an indeterminate ventricle, the ventriculoarterial connection cannot be easily characterized.

Anomalies other than those inherent in atrial isomerism are infrequent in surgically treated patients. In autopsy series, obstructive lesions on the left side of the heart, excluding left ventricular hypoplasia and mitral stenosis, are common.

The salient morphologic features associated with right and left atrial isomerism are summarized in Boxes 53.1 and 53.2 . Right atrial isomerism is strongly associated with asplenia, and left atrial isomerism is strongly associated with polysplenia, but exceptions occur. Extracardiac TAPVC is strongly associated with right atrial isomerism, occurring with about four times the frequency compared to left atrial isomerism. Anomalies of systemic venous return, particularly interrupted inferior vena cana, are strongly associated with left atrial isomerism, occurring about four times the frequency in right atrial isomerism. Pulmonary valve stenosis or atresia is much more common in right atrial isomerism. Univentricular AV connections and AV septal defects occur in over half the cases. Common atrium and other forms of AV septal defect occur in more than 90% of patients with right atrial isomerism, and solitary ventricular chamber occurs in about half. Discordant ventricular arterial connection or DORV occurs in greater than 75% of patients with right atrial isomerism and in 30% to 50% of left atrial isomerism.