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Heterotaxy and Isomerism of the Atrial Appendages


Definitions

The term heterotaxy comes from the Greek heteros, meaning “other” and taxis, meaning “arrangement.” The nomenclature and definition of heterotaxy have been a matter of debate for years and remain controversial. The complex combinations of cardiac malformations found in this syndrome make its description challenging. Heterotaxy can be defined as an abnormal arrangement of the internal thoracoabdominal organs across the left-right axis of the body. Heterotaxy syndrome does not include situs inversus , in which patients have complete mirror-imaged arrangement of their internal organs along the left-right axis.

Isomerism (from the Greek isos , meaning “equal,” and meros , meaning “part”) can be defined as “a situation where some paired structures on opposite sides of the left-right axis of the body are, in morphologic terms, symmetrical mirror images of each other.” The concept of “atrial isomerism” is not universally accepted because atrial chambers as a whole are not entirely isomeric. However, true isomerism of the atrial appendages (considered in isolation) has been demonstrated previously ; the concepts of isomerism of the right atrial appendage (IRAA) and isomerism of the left atrial appendage (ILAA) are therefore more accurate. These two entities represent, in fact, two different subgroups of the heterotaxy syndrome itself. IRAA is most often associated with asplenia (asplenia syndrome), and ILAA is usually found with polysplenia (polysplenia syndrome). As described later ( Tables 57.1 and 57.2 ), IRAA and ILAA are typically, but not always, associated with a variety of cardiac and other thoracoabdominal abnormalities. The spleen is most commonly affected. Although certain associations of cardiac and noncardiac abnormalities are frequent, it is important to keep in mind that almost any combination can be found.

TABLE 57.1
Sequential Segmental Approach, Echocardiographic Views, and Cardiac Findings in Heterotaxy Syndrome
Steps Echocardiographic Views Cardiac Findings in Heterotaxy Syndrome
Isomerism of RAA Isomerism of LAA
  • 1.

    Localization of the heart in the chest + orientation of apex

Subcostal and apical The heart is right sided in up to half of all patients and in the middle in up to one-tenth
  • 2.

    Assessment of situs

Subcostal short axis Aorta and IVC on the same side of the spine, with IVC anterior to aorta Azygos vein posterior and to the left of aorta (azygos continuation of IVC)
  • 3.

    Atrial morphology

RAA: high parasternal long axis (angulation toward RVOT), subcostal short axis, right subclavicular
LAA: parasternal short axis at the level of the aortic valve		 Bilateral RAABilateral LAAFrequent anomalies of the interatrial septum
  • 4.

    Atrioventricular junction

Modified subcostal, apical four-chamber view A common AV junction is frequent —
  • 5.

    Ventricular morphology

All views, including modified subcostal, apical four-chamber Left ventricular hypoplasia in up to 40% —
  • 6.

    Ventriculoarterial connections

Parasternal long and short axis, apical four-chamber with anterior angulations Frequent DORV or TGA, and RVOT obstruction RVOT obstruction in ∼ 40%, aortic obstruction in one-fourth, occasional DORV
  • 7.

    Venoatrial connections

— — —
  • a.

    IVC and azygos vein

Abdominal and subcostal (long and short axis) Usually intact Interrupted with azygos continuation
  • b.

    SVC

Right SVC: Subcostal (long and short axis), suprasternal, and right subclavicular
Left SVC: Parasternal and suprasternal short axis with transducer directed leftward; most easily seen in the so-called ductal view		 Bilateral SVC in up to 70% —
  • c.

    Coronary sinus

Parasternal (long and short axis), suprasternal short axis Usually absent Can be absent up to 70%
  • d.

    Hepatic drainage

Subcostal (long and short axis) and apical Hepatic veins usually drain to IVC Anomalous hepatic venous drainage directly to the atrium in up to 40%
  • e.

    Pulmonary veins

Apical four-chamber, suprasternal short axis, subcostal four-chamber TAPVC is the rule, frequently to an extracardiac site Bilateral return (right pulmonary veins draining into the right-sided atrium and left veins to the left-sided atrium) common (approximately 60%)
AV , Atrioventricular; DORV , double outlet right ventricle; IVC , inferior vena cava; LAA , left atrial appendage; RAA , right atrial appendage; RVOT , right ventricular outflow tract; SVC , superior vena cava; TAPVC , total anomalous pulmonary venous connection; TGA , transposition of great arteries.

TABLE 57.2
Most Common Extracardiac Anomalies in Heterotaxy Syndrome
Data from Uemura H, Ho SY, Devine WA, Kilpatrick LL, Anderson RH. Atrial appendages and venoatrial connections in hearts from patients with visceral heterotaxy. Ann Thorac Surg . 1995;60:561-569; Lim JS, McCrindle BW, Smallhorn JF, et al. Clinical features, management, and outcome of children with fetal and postnatal diagnoses of isomerism syndromes. Circulation . 2005;112:2454-2461; Gilljam T, McCrindle BW, Smallhorn JF, Williams WG, Freedom RM. Outcomes of left atrial isomerism over a 28-year period at a single institution. J Am Coll Cardiol . 2000;36:908-916; Ticho BS, Goldstein AM, Van Praagh R. Extracardiac anomalies in the heterotaxy syndromes with focus on anomalies of midline-associated structures. Am J Cardiol . 2000;85:729-734.
Extracardiac Anomalies ∗ Isomerism of Right Atrial Appendage Isomerism of Left Atrial Appendage
Pulmonary morphology Bilaterally short and eparterial bronchi
Three lobes found bilaterally (73%)		 Bilaterally long and hyparterial bronchi
Two lobes on both sides (74%)
Spleen anomalies Functional asplenia with immunodeficiency Polysplenia
Gastrointestinal anomalies Malrotation of the gut
Dextroposition of the stomach		 Biliary atresia
Malrotation of the gut
Dextroposition of the stomach

∗ Genitourinary, musculoskeletal, craniofacial abnormalities and anomalies of the central nervous system are also found in less than or equal to 15% of patients with heterotaxy syndrome (IRAA or ILAA).

The complex anatomy of patients with heterotaxy syndrome should therefore be described by using a sequential segmental approach, which allows for a complete description of the cardiac structures’ relations and connections to each other (see Table 57.1 and Chapter 3, Chapter 6 ). However, in a few patients, the exact diagnosis will remain uncertain despite the most thorough assessment. The sequential segmental approach includes separate documentation of the following:

  • 1.

    The position of the heart in the chest and the orientation of the cardiac apex

    • Significant variations can occur in heterotaxy syndrome and can have a major impact on surgical interventions

  • 2.

    Cardiac segmental anatomy

    • Arrangement of the atrial appendages (atrial situs)

    • Ventricular topology (ventricular orientation, ventricular loop)

    • Morphology of the venoatrial, the atrioventricular (AV), and the ventriculoarterial junctions

    • Relationships of the arterial trunks in space

  • 3.

    Other malformations within the heart such as septal structures and coronary arterial distribution

  • 4.

    Description of the remaining thoracoabdominal organs including:

    • The spleen

    • The lungs and the bronchi

    • The intestines

    • The liver

This chapter presents a comprehensive and contemporary overview of heterotaxy syndrome, one of the most complex congenital heart diseases.

Morphology

Atrium

Morphologically right and morphologically left atria can be differentiated by studying the anatomy of their atrial appendages and the morphology of the atrial septum :

  • Anatomy of the atrial appendages ( Fig. 57.1 ): The morphologically right atrial appendage is a broad structure, and the pectinate muscles extend around the muscular AV vestibules.

    Figure 57.1, Heart specimens with isomerism of the left atrial appendage and isomerism of the right atrial appendage.

  • The morphologically left atrial appendage is a narrow finger-shaped structure to which the pectinate muscles are confined; there is continuity between the vestibule of the AV junction and the smooth-walled venous component of the atrium, uninterrupted by the presence of pectinate muscles.

  • Morphology of the atrial septum: The morphologically right side of the atrial septum contains the rim of the oval fossa, whereas its flap is on the left side.

Bronchopulmonary Anatomy

In heterotaxy syndrome, bronchopulmonary anatomy is often (but not always) consistent with the morphology of the atrial appendages.

  • In ILAA, patients typically have bilateral bilobed lungs, with two morphologically left-sided bronchi that follow a long course before the first branching, and branch inferior to the first lobar division of the pulmonary artery (hyparterial).

  • In IRAA, the following features are usually found: bilateral trilobed lungs with two morphologically right-sided bronchi that follow a short course before the first branching and a branch superior to the first lobar division of the pulmonary artery (eparterial).

Spleen

Most patients with heterotaxy have splenic abnormalities. As mentioned earlier, the spleen is absent in most patients with IRAA, whereas ILAA is associated with multiple spleens. However, the correlation between the morphology of the atrial appendages and the anatomy of the spleen is poorer than with the bronchopulmonary anatomy. The cardiac anatomy or subgroup of heterotaxy syndrome should therefore never be deduced from the splenic morphology; rather, each should be assessed and described separately.

Atrioventricular Junctions

In hearts of patients with univentricular physiology, three types of AV connections can be found: absent right, absent left, or double inlet. In patients with ILAA or IRAA who have biventricular arrangement, there is a concordant AV connection in half of the heart (eg, in ILAA the morphologically left atrium to the morphologically left ventricle) and a discordant connection in the other half (the morphologically left atrium to the morphologically right ventricle); therefore, the pattern of AV connections is mixed or nonconcordant/nondiscordant. A common AV junction with a common valve is frequent in both subgroups of heterotaxy syndrome, but particularly in IRAA.

Ventriculoarterial Junctions

In patients with heterotaxy syndrome, any type of ventriculoarterial connection may exist, regardless of the subgroup. However, discordant or double-outlet ventriculoarterial connections (often with pulmonary atresia) are more likely to occur in patients with IRAA, whereas concordant ventriculoarterial connections are more frequent in those with ILAA.

Venoatrial Connections

Venoatrial connection abnormalities are almost universal in patients with heterotaxy syndrome and, depending on their nature, have a profound impact on clinical presentation.

  • In IRAA, total anomalous pulmonary venous connection (TAPVC) of the extracardiac type is common. Even when the pulmonary veins are directly connected to the atrial cavity, the pattern of connection of the pulmonary veins to the atrial wall is usually abnormal. The coronary sinus is not formed according to the standard definition and is usually considered absent.

  • In ILAA, interruption of the inferior caval vein with azygos continuation is frequent, and the hepatic veins often connect directly to one or both atria.

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