Atrial Isomerism

Atrial Isomerism

In atrial isomerism, both atria have similar internal, external, and appendage configuration. They are considered either morphologically bilaterally right atria or bilaterally left atria. Validity of the concept of atrial isomerism, at least from the perspective of the purist, has been questioned.

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 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. Rarely, atria and their appendages 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. Because of this variability, Anderson prefers the term heterotaxy to denote presence of any of the numerous possible lateralization abnormalities, and then recommends describing the morphologic details for each patient.

Among 58 consecutive newborn cases of heterotaxy within a single institution, 25 had asplenia and 20 had polysplenia. In surgical series, left atrial isomerism is more common than right, in one surgical experience with 41 patients, 23 had left and 18 had right 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.

Conduction System

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

Anomalies of systemic venous connection are common. The inferior vena cava often does not connect directly to the atrium from below, but instead passes superiorly along the right-sided paravertebral gutter (azygos extension of inferior vena cava) or left-sided gutter (hemiazygos extension of inferior vena cava), emptying into a right-sided or left-sided superior vena cava ( Table 58-2 ). Azygos extension of the inferior vena cava occurs exclusively in patients with left atrial isomerism, in whom it occurs in about 75% of cases (see Table 58-2 ).

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 58-3 ). 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 58-2 ). Hepatic veins 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 58-4 ).

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 atrial isomerism.

Anomalies of Pulmonary Venous Connection

Extracardiac total anomalous pulmonary venous connection (TAPVC) is usually seen in patients with right atrial isomerism ( Table 58-5 ). 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.

Atrioventricular Connections

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 58-6 ).

Atrioventricular Septal and Other Atrial Septal Defects

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 34 ) 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 58-6 ). Most patients with atrial isomerism and AV septal defects have a common AV orifice (see “Complete Atrioventricular Septal Defect” under Morphology in Chapter 34 ) rather than two AV valve orifices. Rarely the atrial septum is well formed and intact or has only a probe-patent foramen ovale.

Ventricular Morphology and Ventricular Septal Defects

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 VSD have an AV septal defect, and in the remainder with intact AV septal structures, various types of VSD are present.

Pulmonary Outflow

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 58-6 ).

Ventriculoarterial Connections

In surgical series, ventriculoarterial connections are most commonly discordant, and an unusually high proportion (33%) of patients have double outlet right ventricle. In autopsy series, about 75% to 90% of specimens with right atrial isomerism have discordant ventriculoarterial connection (transposition) or double outlet right ventricle; in left atrial isomerism, this is true in about 20% to 65% of specimens (see Table 58-6 ). In some cases, such as double outlet from an indeterminate ventricle, the ventriculoarterial connection cannot be easily characterized.

Other Coexisting Cardiac Anomalies

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.

Summary

In the surgically more common left atrial isomerism, anomalies of systemic venous connection are common, as are abortive forms of cor triatriatum, but extracardiac TAPVCs are not. Common atrium and other types of AV septal defects occur in about half the cases. Univentricular AV connections are uncommon, as are solitary ventricular chambers, but double outlet right ventricle is common. Pulmonary stenosis is present in about half of cases.

In surgical patients with right atrial isomerism, anomalies of systemic venous connection are less common, as are abortive forms of cor triatriatum, but extracardiac forms of TAPVC occur more frequently. Common atrium and other forms of AV septal defect occur in more than 90% of patients, and solitary ventricular chamber occurs in nearly half. The fact that cardiac anomalies are more complex and numerous in hearts with right rather than left atrial isomerism probably explains the higher prevalence of left atrial isomerism in surgical series than in autopsy series.