Atrial Isomerism


Definition

Atrial isomerism is a condition in which the right-sided and left-sided atria, normally morphologically different, are morphologically similar. 1 Thus, left atrial isomerism and right atrial isomerism can occur. Atrial isomerism is a specific phenotypic feature highly associated with generalized somatic laterality disorders characterized by abnormal arrangement of thoracic and abdominal viscera, including important structural cardiovascular anomalies. The terms situs ambiguus and heterotaxy are used to describe these laterality disorders. Attempts to classify specific constellations of the many clinical and phenotypic features that can occur with heterotaxy have resulted in descriptive terms such as asplenia syndrome and polysplenia syndrome . Experience shows that although there is some tendency for certain constellations to occur, exceptions are frequent or even the rule. About 3% of all congenital heart anomalies occur in the context of heterotaxy. The various cardiac anomalies found in heterotaxy are shown in Table 58-1 .

Table 58-1
Prevalence of Major Anatomic Cardiac Variables in 81 Patients with Prenatal and Postnatal Diagnosis of Heterotaxy Syndrome
Modified from Cohen and colleagues.
Anatomic Variables Prenatal Diagnosis ( n = 43)
N (%)
Postnatal Diagnosis ( n = 38)
N (%)
Isomerism of left atrial appendages 17 (39.5) 13 (34.2)
Isomerism of right atrial appendages 26 (60.5) 25 (65.8)
Right-sided heart 14 (32.5) 14 (36.8)
Interrupted inferior vena cava 15 (34.9) 8 (21.0)
Totally anomalous pulmonary venous return (extracardiac) 14 (43.8) 8 (21.0)
Common atrioventricular junction/common atrioventricular canal 31 (72.1) 30 (78.9)
Hypoplastic left heart syndrome 9 (20.9) 3 (7.9)
Double outlet right ventricle 13 (30.2) 17 (44.7)
Double outlet right ventricle with pulmonary atresia 13 (30.2) 13 (34.2)
Pulmonary outflow obstruction 33 (76.7) 28 (73.7)
Systemic outflow obstruction 9 (20.9) 6 (15.8)
Complete heart block 8 (18.6) 3 (7.9)

1 The adjectives left and right used to modify atrium or ventricle mean morphologically left or right. Position of a chamber or valve is referred to as right-sided or left-sided .

In this chapter, individual cardiovascular anomalies and commonly recognized constellations of cardiovascular anomalies that occur with heterotaxy are discussed. These complex associations are analyzed from a perspective that uses left atrial isomerism and right atrial isomerism as reference points or starting points for analysis.

Historical Note

Anomalies of right and left sidedness related to asymmetry of the body were recognized at least by the 15th century with Leonardo da Vinci's drawing of situs inversus. In the early 17th century, Marco Aurelio Severino described this anatomic variant, but it was Matthew Baille, a student of John Hunter, who is credited with scientifically describing anomalies of sidedness and their associated lesions in the latter part of the 18th century. In 1933 Kartagener drew attention to the association of situs inversus with sinusitis (Kartagener syndrome), providing an important clue to the possible morphogenesis of all anomalies of sidedness, although this had been suggested by Siewert in 1904. Biorn Ivemark in 1955 identified the syndrome of right atrial isomerism, asplenia, symmetry of thoracic organs, and conotruncal anomalies during his studies at Children's Hospital Boston.

Subsequently, general interest in the asymmetry of many bilateral animals, from snails to humans (the science related to chiral [asymmetric] bodies), has led to the hypothesis that left-right patterning is related to genetic factor processes that become reflected in midline left-right ciliary structure and function during embryogenesis. In the 1986 first edition of this book, Kirklin presented the UAB surgical experience with heterotaxy patients, consisting of 28 complete repairs and 28 palliative operations (including two Fontan operations).

Morphology and Morphogenesis

Morphology

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 ).

Table 58-2
Patterns of Inferior Vena Cava Drainage ( n = 183)
Data from Uemura and colleagues.
Atrial Connection Present Interrupted
Atrial Appendage Isomerism To Right-Sided Atrium (%) To Left-Sided Atrium (%) Via Right-Sided Azygos Vein (%) Via Left-Sided Azygos Vein (%) Other Patterns
Right 48 52 0 0 0
Left 12 12 34 40 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.

Table 58-3
Patterns of Superior Vena Cava Drainage ( n = 183)
Data from Uemura and colleagues.
Atrial Appendage Isomerism Unilaterally Present Bilaterally Present
To Right-Sided Atrial Roof (%) To Left-Sided Atrial Roof (%) Other Patterns (%) Both to Atrial Roof (%) One via Coronary Sinus (%)
Right 29 19 1 51 0
Left 22 14 2 38 24

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 ).

Table 58-4
Patterns of Hepatic Vein Drainage ( n = 183)
Data from Uemura and colleagues.
Atrial Appendage Isomerism Confluence Present Via Independent Channels
Via IVC (%) Via Common Channel (Interrupted IVC) (%) Unilaterally to Atrium (%) Bilaterally to Atria (%) Other Patterns (%)
Right 76 0 6 18 0
Left 14 43 8 33 2
Key: IVC, Inferior vena cava.

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.

Table 58-5
Patterns of Drainage of Pulmonary Veins ( n = 183)
Data from Uemura and colleagues.
Direct Connections of All Pulmonary Veins to Atrial Chambers Via Sump Outside Heart (Confluence of All Pulmonary Veins Present) Others (Confluence of Pulmonary Veins Incomplete)
Atrial Appendage Isomerism To Left-Sided Atrium (%) To Right-Sided Atrium (%) Bilaterally to Chambers (%) Other Patterns (%) Via Superior Vena Cava (%) Via Portal Vein (%) Atresia of Alternative Channel (%) Some via Systemic Veins and Others Directly to Atrium (%) Via Multiple Channels Outside Heart (%)
Right 19 19 0 3 a 27 21 1 5 5
Left 26 14 60 0 0 0 0 0 0

a Via central confluence.

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 ).

Table 58-6
Summary of Anatomic Findings ( n = 93)
Data from Hirooka and colleagues.
Right Isomerism ( n = 61) Left Isomerism ( n = 32)
No. % No. %
UVH 39 64 9 28
Two ventricles 22 36 23 72
CAVV 56 92 18 56
Two AV valves 2 3 10 31
MA or TA 3 5 4 13
VA concordant connection (Ao from LV) 6 10 12 38
VA discordant connection (Ao from RV) 55 90 20 62
Pulmonary atresia 20 33 3 9
Pulmonary stenosis 36 59 17 53
Bilateral SVC 33 54 17 53
Right SVC 24 39 10 31
Left SVC 4 7 5 16
Right IVC 43 70 11 34
Left IVC 17 28 0
IVC absence 1 2 21 66
TAPVC 37 61 2 6
PAPVC 1 2 1 3
Key: Ao, Aorta; AV, atrioventricular; CAVV, common atrioventricular valve; IVC, inferior vena cava; LV, left ventricle; MA, mitral atresia; PAPVC, partial anomalous pulmonary venous connection; RV, right ventricle; SVC, superior vena cava; TA, tricuspid atresia; TAPVC, total anomalous pulmonary venous connection; UVH, univentricular heart; VA, ventriculoarterial.

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.

Morphogenesis

The genetic basis of heterotaxy is thought to be related to genetics of ciliary dysfunction on the embryologic midline node.

Clinical Features and Diagnostic Criteria

There are no clinical features absolutely specific to atrial isomerism, because there is no specific functional derangement uniformly associated with the atrial morphology. The clinical sign most intimately related to the atrial morphology itself is presence of abnormal P-wave morphology and slow atrial rhythm associated with left atrial isomerism. Asplenia, commonly associated with right atrial isomerism, is associated with an increased number of Howell-Jolly bodies in the routine blood smear in newborns, or persistent Howell-Jolly bodies in older infants. Clinical features depend, therefore, on the specific cardiac anomalies and the many possible noncardiac anomalies and disorders that may be present, including intestinal malrotation, absence of splenic function, primary ciliary dyskinesia, biliary atresia, central nervous system anomalies, craniofacial anomalies, intraabdominal vascular anomalies such as congenital extrahepatic portosystemic shunt, and musculoskeletal anomalies.

Atrial situs is best diagnosed preoperatively by determining thoracic situs, because atrial and thoracic situs are nearly always the same. Thoracic situs is best indicated by bronchial anatomy, which does not always correspond to lung lobulation. The length of each mainstem bronchus and its relationship to its respective pulmonary artery provide the most reliable clinical prediction of thoracic situs. The normal right mainstem bronchus is relatively short, and the right pulmonary artery is anterior and inferior to the bronchus; the normal left mainstem bronchus is relatively long, and the left pulmonary artery is posterior and superior to the bronchus.

Determining these relationships, and thus diagnosis of thoracic situs, is reliably accomplished from plain frontal and lateral chest radiographs, although meticulous attention must be paid to radiologic technique. If the ratio of the length of the shorter (normally right) bronchus divided by that of the longer (normally the left) is 2 or greater, there is thoracic lateralization; if the ratio is 1.5 or less, thoracic and usually atrial isomerism is present. Also, right isomerism is usually present when each pulmonary artery is anterior to its respective bronchus; left isomerism is usually present when each pulmonary artery is superior and posterior to its respective bronchus ( Fig. 58-1 ).

Figure 58-1, Chest radiographs in atrial isomerism. A, Left isomerism, frontal chest film. Each bronchus has a similar length. B, Lateral view. Pulmonary arteries are superior and posterior to tracheobronchial tree. C, Right isomerism, frontal chest film. Each bronchus has a similar length. D, Lateral view. Pulmonary arteries are anterior and inferior to bronchi.

Prenatal diagnosis of heterotaxy can often be made by echocardiography, but prenatal diagnosis has no impact on survival. Echocardiography after birth can reliably identify the typical constellations of morphologic abnormalities associated with both left and right atrial isomerism and as a result can strongly suggest the diagnosis ( Figs. 58-2 and 58-3 ). Echocardiography, however, is limited in delineating all morphologic details related to atrial isomerism, particularly when complex pulmonary artery and pulmonary venous anomalies are present (e.g., pulmonary atresia with discontinuous branch pulmonary arteries, mixed TAPVC). Specific characteristics of the atrial appendages cannot usually be identified with certainty, and the relationship of bronchi to pulmonary arteries cannot be determined. Complex pulmonary artery and pulmonary vein anomalies, and the extracardiac thoracic and abdominal features of left and right atrial isomerism, can best be determined by computed tomography and cineangiography ( Figs. 58-4 and 58-5 ). Specific hemodynamic data can only be obtained by cardiac catheterization.

Figure 58-2, Atrial isomerism with asplenia (probably bilateral right-sidedness). Echocardiography can reliably identify most cardiovascular defects associated with the various atrial isomerism (heterotaxy) syndromes. Because the constellation of findings is variable and complex, multiple echocardiographic views are required. This subcostal coronal view demonstrates enlarged atrial chamber. Almost complete absence of atrial septum is evident, with only a central band present. There is a large ostium primum defect and a large ostium secundum defect. The single atrioventricular valve and ventricular mass are also seen. Key: AS, Atrial septum; ASD1, ostium primum defect; ASD2, secundum atrial septal defect; V, ventricle.

Figure 58-3, Atrial isomerism with polysplenia (probably bilateral left-sidedness). A, Four-chamber view demonstrating common atrium and lateral entry of left and right pulmonary veins into left-sided component of common atrium. This is a typical finding in this form of atrial isomerism. Also noted is the atrioventricular septal defect with two well-formed ventricles and a small inlet ventricular septal defect. B, View of right-sided atrium with hepatic veins draining directly into atrial chamber, in the setting of interrupted inferior vena cava. C, Suprasternal notch coronal view demonstrating enlarged superior vena cava and azygos vein entering atrial chamber, in the setting of interrupted inferior vena cava. Key: A, Atrium; AZ, azygos vein; HV, hepatic veins; LV, left ventricle; PV, pulmonary veins; RV, right ventricle; SVC, superior vena cava; VSD, ventricular septal defect.

Figure 58-4, Conventional angiogram (posteroanterior view) demonstrates injection of the right splenorenal shunt (arrows) via the right femoral vein to inferior vena cava catheter. The catheter has passed through the shunt, with the catheter tip ( * ) in the shunt near the splenic vein connection. The large tortuous shunt is seen extending from the midsplenic vein to the right renal vein at its junction with inferior vena cava–azygos vein junction. The more proximal splenic vein on the right is not filled. There is faint filling of the right renal vein. The portal vein (right branches fill better than left branches) and superior mesenteric vein are patent. Key: PV, Portal vein; RV, right renal vein; SMV, superior mesenteric vein; SV, midsplenic vein.

Figure 58-5, Two-year-old with polysplenia syndrome. Axial slice from a contrast-enhanced abdominal computed tomography venogram demonstrates right liver and left polysplenia. There is a splenorenal shunt connecting splenic vein with left renal vein (arrow) behind the more proximal splenic vein. This drains posteriorly to hemiazygos vein. The entire course of the shunt cannot be seen on a single image. There is partial visualization of a retroaortic right renal vein, also draining to hemiazygos vein. Key: Ao, Abdominal aorta; HZ, hemiazygos vein; L, liver; RV, left renal vein; SPL, polyspenia; SV, splenic vein.

Almost all children with right atrial isomerism are in sinus rhythm, and most have a normal P-wave axis. Complete AV block coexists in about 10% of patients with left atrial isomerism but is rare in right atrial isomerism.

At operation, the surgeon must make direct observations of the atrial appendages and atrial walls to confirm or deny the preoperative diagnosis of right or left atrial isomerism.

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