Above. During fetal life, the internal organs develop “sidedness”, that is, for example, the stomach is normally on the left side with the spleen and the liver is on the right side. Heterotaxy (atrial isomerism) is defined as a lack of differentiation between the left sided compared to the right sided organs. This lack of normal sidedness can include multiple organs including the heart and lungs. For example, the right lung has 3 lobes and the left lung has 2 lobes. Heterotaxy occurs, for example, when the 3 lobed right lung is present in both the right chest and the left chest.
Heterotaxy is categorized as either right-sided (right atrial isomerism) or left-sided (left atrial isomerism).
Right atrial isomerism may have the following features: absence of a spleen (asplenia). The morphologic right atrium will be on both the right side and left side of the heart, while the right lung anatomy (3 lobes) will be in both the left thorax and the right thorax. Severe cardiovascular abnormalities are often present, and there may be 2 sinoatrial nodes. Right atrial isomerism is frequently associated with transposition of the great arteries (TGA) with pulmonary stenosis or atresia, leading to cyanosis shortly after birth. About 3/4s of patients will have extra-cardiac total anomalous pulmonary venous return (TAPVR) (The normal cardiac veins entering the left atrial appendage (the right atrium) will not be seen.)
Left atrial isomerism is associated with polysplenia (evidence of multiple splenic tissues). The morphologic left atrium will be on both the right side and left side of the heart, while the morphologic left lung anatomy (2 lobes) will be in both the left thorax and the right thorax. Interruption of the inferior vena cava (IVC) with azygous continuation may be present. (Two vessels are seen behind the heart, the descending aorta (DA) and the azygous vein instead of one, the DA.) Two ventricles are usually present, and associated congenital heart disease is often mild.