Heterotaxy syndrome is a rare and complex disorder of the chest and abdominal organ arrangements, and presents a diagnostic challenge to the radiologist. This article describes the morphological characteristics of heterotaxy and situs abnormalities, in particular left and right atrial isomerism, and suggests an approach in evaluating the spectrum of abnormalities associated with heterotaxy syndromes, using appropriate imaging modalities.
Heterotaxy includes a spectrum of malformations involving abnormal right-left axis determination. Thoraco-abdominal visceral laterality is deranged, often accompanied by midline defects of the face and brain. The incidence of heterotaxy is 1 in 5000–7000 live births with congenital heart disease.
Various human genes associated with heterotaxy syndrome have been identified with varying phenotypical expression.
The term ‘situs’ refers to the position of the atria and viscera in relation to the midline. Situs solitus is the usual arrangement of organs and vessels within the body, inferred when the systemic atrium, trilobed lung, liver, gallbladder and inferior vena cava (IVC) are positioned on the right; and the pulmonary atrium, bilobed lung, aortic arch, cardiac apex and stomach bubble are positioned on the left. In situs inversus, the anatomical arrangement of these structures is reversed and is the mirror image of situs solitus. Any other presentation with visceral malposition/dysmorphism associated with indeterminate atrial arrangement and vascular anomalies is termed situs ambiguous or heterotaxy.
Heterotaxy is subdivided into right or left isomerism, according to the atrial appendage anatomy. The atrium with a broad-based appendage receiving blood from the IVC, is called the systemic or right atrium. The atrium with a smaller, narrower appendage receiving blood from the pulmonary veins, is the pulmonary or left atrium.
Common morphological characteristics of right and left isomerism.
Characteristics | Right isomerism | Left isomerism |
---|---|---|
Cardiovascular malformations | single atrium with bilateral right atrial appendages<xxxx>mesocardia/dextrocardia<xxxx>single right ventricle<xxxx>double-outlet right ventricle<xxxx>atrioventricular discordance<xxxx>right-sided aortic arch<xxxx>malposition of the great arteries<xxxx>pulmonary stenosis/atresia<xxxx>bilateral SVCstotal pulmonary venous drainage | bilateral left atrial appendages<xxxx>unbalanced ventricles<xxxx>partial anomalous pulmonary venous drainage<xxxx>persistent left IVC draining into the left atrium<xxxx>interrupted hepatic portion of IVC<xxxx>left SVC |
Other malformations/dysfunction | bilateral right–sided lungs and bronchi: short bronchus, trilobed lungs<xxxx>asplenia<xxxx>symmetrical liver<xxxx>right-sided stomach<xxxx>intestinal malrotation | bilateral left-sided lungs and bronchi: long bronchus, bilobed lungs<xxxx>polysplenia<xxxx>midline liver<xxxx>extra-hepatic biliary atresia/hypoplasia<xxxx>extra-hepatic portal vein atresia<xxxx>intestinal malrotation |
Thorough radiological examination of every organ system is needed to delineate all rotational abnormalities and their associated complications in order to plan optimal management.
Flow diagram illustrating the imaging approach to heterotaxy syndrome.
The presence or absence of normal situs solitus can be determined on chest radiography, as well as certain features that are commonly seen in heterotaxy, such as dextrocardia (
Frontal chest radiograph demonstrating dextrocardia with right-sided aortic arch and right-sided stomach bubble in right isomerism.
Frontal chest radiograph demonstrating mesocardia with right-sided stomach bubble with prominent pulmonary vasculature in right isomerism.
Frontal chest radiograph demonstrating cardiomegaly with left-sided stomach bubble in left isomerism.
Echocardiography defines the intracardiac and cardiovascular anatomy.
Illustration of four possible atrial appendages.
2D echocardiogram 4-chamber view demonstrating pyramidal-shaped bilateral right atrial appendages.
2D echocardiogram 4-chamber view demonstrating bilateral finger-like atrial appendages in left isomerism.
2D echocardiogram 4-chamber view demonstrating a large atrioventricular septal defect.
2D echocardiograph colour Doppler 4-chamber view demonstrating classic double-outlet right ventricle.
While echocardiography remains the first-line investigation, cardiovascular magnetic resonance (CMR) is now the gold standard imaging modality in this context and is useful for assessment of cardiac and vascular structures and function. Furthermore, CMR and computed tomography (CT) are both useful for the study of other organs within the chest and abdominal cavities.
If cardiac CT or CMR are not available or not conclusive, cardiac catheterisation can be done to further delineate the cardiac anatomy as well as the location of the aorta and IVC for surgical planning, as was done in our hospital. Right isomerism is characterised by bilateral morphological right pulmonary arteries, as well as bilateral superior vena cavas (SVCs) (see the following video: https://youtu.be/bQggzTXTkDQ)
Illustration showing the position of the aorta and IVC in right isomerism.
Illustration showing interrupted IVC continuing with azygous or hemizygous vein. The aorta is located on the opposite side of the midline.
Bronchopulmonary anatomy is consistent with atrial appendage structures and reinforces the diagnosis of heterotaxy. Right isomerism is associated with trilobed lungs with eparterial bronchi (
Illustration of trilobed lungs with eparterial bronchus in right isomerism.
Illustration of bilobed lungs with hyparterhal bronchus in left isomerism.
Abdominal ultrasound is performed to determine the presence or absence of the spleen, the position of the liver, the presence and position of the gallbladder, and the position of the mesenteric arteries.
Asplenia in right isomerism has the complication of increased risk of infection. Polysplenia in left isomerism is more common in female subjects.
Intestinal malrotation occurs in both right and left isomerism, with a slightly higher incidence in right isomerism. Fluoroscopic upper gastrointestinal examination (such as barium meal with follow-through) can show the duodenal and jejunal locations; there is often mal- or non-rotation with malposition of the duodenal-jejunal junction (DJJ) but the position of the small bowel beyond the DJJ is variable.
There is a direct association of bowel malrotation with abnormal morphology, malposition and inversion of the pancreas as both develop embryologically in the dorsal mesogastrium.
Heterotaxy syndrome is associated with serious congenital heart defects as well as abnormalities in many other organ systems. Accurate and timeous investigation with appropriate imaging can greatly improve patient outcomes. As cardiac surgical techniques advance with increasing patient survival, so too will the need to investigate and address the many extra-cardiac systems involved.
This article was adapted from an electronic poster entitled ‘This is the left, right?’ presented at the European Congress of Radiology in Vienna, Austria, in 2013.
The authors declare that they have no financial or personal relationships which may have inappropriately influenced them in writing this article.
L-L.H. (University of Pretoria) wrote literature review, prepared images. B.J.M. (University of Pretoria) clinical details and images. F.E.S. (University of Pretoria) edited the document, selected images, coordinated between authors and university regarding videos. S.A. (University of Pretoria) assisted with layout, gave idea of the manuscript. Z.L. (University of Pretoria) coordinated between various authors and contributed to final editing