Heterotaxy and Isomerism


Heterotaxy, aka situs ambiguous, isomerism or atrial isomerism is a complex, systemic congenital defect that impacts multiple organ systems, particularly the heart. This is an important defect that every echocardiographer should be aware of.


Heterotaxy is more accurately described as situs ambiguous. “Situs” refers to the positioning of the organs in the body. If the organs of the viscera and the thorax are normally positioned and related to each other, this is known as situs solitus. If the placement and positioning of the organs are a “mirror image” and the organs are “switched” or are “inverted”, for example, the spleen is on the right and the liver is on the left, this is known as situs inversus and is not included in the definition of situs ambiguous.


Situs ambiguous refers to organs that are not only abnormally positioned and mal-aligned, but they may also be isomeric, that is they have either two left sides or two right sides. This is particularly true of the heart where there may be two left atria or two right atria and appendages. This is also called atrial isomerism.


Patients with situs solitus and situs inversus do not experience organ dysfunction to the degree that situs ambiguous may cause. All patients with situs ambiguous lack organ symmetry and therefore experience significant systemic symptoms. Improper positioning of the intestines may cause blockage, pain, and inability to properly feed. The liver may be inflamed and normal biliary function may be absent or impaired which can lead to jaundice, vomiting and ascites (swelling of the abdomen). The spleen may be absent (asplenia) or there may be multiple spleens (polysplenia).


Of particular interest to the echocardiographer is what happens to the heart. The very first thing that the pediatric sonographer should do is test for situs. Perform a quick subxiphoid view and check the position of the spleen and liver. If the spleen is to the left and the liver to the right, and the apex of the heart is pointed to the left, then make a note on the worksheet that there is situs solitus.


The positioning of the heart will manifest in three ways. If the apex of the heart is pointed to the left and the structures of the heart are normal, then this is called levocardia. If the heart is positioned in the center of the chest cavity, then this is called mesocardia. If the apex of the heart is pointed to the right, then this is known as dextrocardia and is typically a part of the defect situs inversus and should not be confused with situs ambiguous. Patients with situs inversus often have few symptoms; the organs function properly and these patients can lead perfectly normal lives. In the case of situs ambiguous, there are multiple organ dysfunctions and there are severe, multiple heart defects in 75% of cases.


In the presence of situs ambiguous, the heart and the apex will be improperly positioned in the chest cavity. The atria are typically isomeric which means there will typically be two left atria or two right atria. Identifying the morphology of the atria is difficult with echocardiography alone and is much easier with TEE. Multiple other congenital heart defects are usually present.


Early in fetal development, the primitive veins (vena cavae and the pulmonary veins) will attempt to attach to their own morphologic atria. In the presence of two right atria (the most common type), the vena cavae will attach somewhat normally but the pulmonary veins will try to find their morphologic left atrium (which is not present). This leads to abnormal attachments of

the pulmonary veins and such defects as total anomalous pulmonary veins, right-to-left shunting, absent atrial septum, cyanosis, respiratory distress, dual sino-atrial nodes, conduction problems and poor systemic circulation.


If there is left atrial isomerism, or two left atria, then there will be abnormal attachment of the vena cavae. This leads to absent atrial septum, left superior vena cava, absent sino-atrial node, conduction problems, arrhythmias such as atrial fibrillation and atrial flutter and left-to-right shunting.


The best way to diagnose isomerism of the atria is to identify the atrial appendage with a TEE. The right atrial appendage has a large, triangular shaped appendage with pectinate muscles. The left atrium has a smaller, long, tubular appendage.


An ECG is performed to identify conduction defects, X-rays and CT scans of the thorax and viscera will identify organ malrotation, and ultrasounds of the spleen, liver, pancreas and kidneys further help identify gastro-intestinal malformations.


Ken Heiden RDCS