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Anatomy, clinical manifestations, and diagnosis of heterotaxy (isomerism of the atrial appendages)

Alexander Lowenthal, MD
Theresa Tacy, MD
Rajesh Punn, MD
Section Editors
David R Fulton, MD
Candice Silversides, MD, MS, FRCPC
Deputy Editor
Carrie Armsby, MD, MPH


Heterotaxy, also referred to as isomerism of the atrial appendages, is defined as an abnormal assembly of the thoracic and abdominal organs from the normal arrangement known as "situs solitus." It is caused by disruption of left-right axis orientation during early embryonic development. Cardiac malformations are a major component of heterotaxy syndrome, resulting in significant morbidity and mortality. Abnormal cardiac development typically leads to atrial appendage isomerism, resulting in either bilateral paired right atria (right atrial isomerism [RAI]) or paired left atria (left atrial isomerism [LAI]).

This topic will review the anatomical variation, clinical manifestations, and diagnosis of heterotaxy (isomerism). The management and outcome of patients with heterotaxy are discussed separately. (See "Management and outcome of heterotaxy (isomerism of the atrial appendages)".)


Heterotaxy — Heterotaxy, derived from Greek (hetero, meaning "different," and taxy, meaning "arrangement"), is also referred to as visceral heterotaxy or heterotaxy syndrome. It is defined as an abnormal arrangement of the internal thoracic-abdominal organs across the left-right axis of the body. Patients with heterotaxy have been historically stratified into either the subsets of asplenia syndrome or polysplenia syndrome [1-3]. However, this classification is not useful in describing the cardiac malformations associated with heterotaxy.

Isomerism — In patients with heterotaxy, the normal asymmetry of the thoracic and abdominal organs is lost, resulting in an unusual degree of symmetry of organs and veins. The term "isomerism," derived from Greek (iso, meaning "equal," and meros, meaning "part"), refers to this abnormal developmental symmetry in which morphologic structures that normally develop on one side are found on both sides of the body, and is the currently accepted term used to describe hearts with isomeric atria and atrial appendages [4]. So, in affected patients, instead of distinct left and right sides, individuals with isomerism will have either two right sides or two left sides resulting in either two right atria or two left atria (atrial isomerism) [5]. Atrial isomerism is a major component of heterotaxy and causes significant morbidity and mortality because of discordance among the heart, systemic and pulmonary vessels, and other organs, and also among components of the heart.

Right atrial appendage isomerism, also referred to as right atrial isomerism (RAI), results in two right sides with bilateral right atria and atrial appendages, and an absence of left-sided structures (eg, coronary sinus). These patients usually have pulmonary venous anomalies, such as anomalous pulmonary venous connections or small pulmonary veins. Single ventricle physiology is predominant in RAI. These patients also typically have asplenia, as the spleen is a left-side abdominal organ. (See 'Right atrial isomerism' below.)

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Literature review current through: Dec 2017. | This topic last updated: Mar 03, 2016.
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