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Situs Inversus Imaging

Practice Essentials

Marco Severino first recognized dextrocardia in 1643. More than a century later, Matthew Baillie described the complete mirror-image reversal of the thoracic and abdominal organs in situs inversus. Situs inversus is present in 0.01% of the population. Situs describes the position of the cardiac atria and viscera. Situs solitus is the normal position, and situs inversus is the mirror image of situs solitus (see the image below). Cardiac situs is determined by the atrial location. In situs inversus, the morphologic right atrium is on the left, and the morphologic left atrium is on the right. The normal pulmonary anatomy is also reversed, so that the left lung has 3 lobes and the right lung has 2 lobes. In addition, the liver and gallbladder are located on the left, whereas the spleen and stomach are located on the right. The remaining internal structures are also a mirror image of the normal.   

Schematic drawings illustrate the standard anatomy

Schematic drawings illustrate the standard anatomy of situs solitus (A) and the mirror image of situs inversus (B). The right lung (RL), left lung (LL), right atrium (RA), and left atrium (LA) are shown.

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Types of situs inversus

Situs inversus can be classified further into situs inversus with levocardia and situs inversus with dextrocardia. The classification of situs is independent of the cardiac apical position. The terms levocardia and dextrocardia indicate only the direction of the cardiac apex at birth; they do not imply the orientation of the cardiac chambers. In levocardia, the base-to-apex axis points to the left, and in dextrocardia, the axis is reversed. Isolated dextrocardia is also termed situs solitus with dextrocardia. The cardiac apex points to the right, but the viscera are otherwise in their usual positions. Situs inversus with dextrocardia is also termed situs inversus totalis because the cardiac position, as well as the atrial chambers and abdominal viscera, is a mirror image of the normal anatomy. Situs inversus totalis has an incidence of 1 in 8,000 births. Situs inversus with levocardia is less common, with an incidence of 1 in 22,000 births.

When situs cannot be determined, the patient has situs ambiguous or heterotaxy. In these patients, the liver may be midline, the spleen absent or multiple, the atrial morphology unclear, and the bowel malrotated. Often, normally unilateral structures are duplicated or absent. The 2 primary subtypes of situs ambiguous include (1) right isomerism, or asplenia syndrome, and (2) left isomerism, or polysplenia syndrome. Heterotaxy syndromes have an incidence of 1 in 10,000 newborn births but account for about 4% of all congenital heart disease (CHD).

In classic right isomerism, or asplenia, bilateral right-sidedness occurs. These patients have bilateral right atria, a centrally located liver, and an absent spleen, and both lungs have 3 lobes. The descending aorta and inferior vena cava are on the same side of the spine. Right isomerism has an incidence of between 1 in 10,000 and 1 in 20,000 births, male predominance, and  a nearly 100% incidence of CHD. It often presents in childhood with a cyanotic heart defect such as a common AV canal, univentricular heart, transposition of the great arteries, or total anomalous pulmonary venous return.

In left isomerism, or polysplenia, bilateral left-sidedness occurs. These patients have bilateral left atria and multiple spleens, and both lungs have 2 lobes. Interruption of the inferior vena cava with azygous or hemiazygous continuation is often present. Left isomerism has an incidence of between 1 in 10,000 and 1 in 20,000 births and a female predominance. Associated cardiac malformations include partial anomalous pulmonary venous return, atrial septal defect (ASD), and a common atrioventricular (AV) canal.

The features of situs ambiguous are inconsistent; therefore, situs ambiguous cases are challenging and require thorough evaluation of the viscera.
The location and relationships of the following should be reviewed carefully: abdominal viscera, hepatic veins, superior vena cava, inferior vena cava, coronary sinus, pulmonary veins, cardiac atria, atrioventricular connections and valves, cardiac ventricles, position of the cardiac apex, and aortic arch and great vessels.

Other features of situs inversus

Situs inversus occurs more commonly with dextrocardia.
A 3-5% incidence of congenital heart disease is observed in situs inversus with dextrocardia, usually with transposition of the great vessels. Of these patients, 80% have a right-sided aortic arch. Situs inversus with levocardia is rare,
and it is almost always associated with congenital heart disease.

The typical clinical phenotype of primary ciliary dyskinesia (PCD) includes any or all of the following: neonatal respiratory distress; chronic, persistent lower respiratory symptoms (early onset and persistent wet cough); chronic, persistent upper respiratory symptoms (nasal congestion and otitis media), and a laterality defect (situs inversus or ambiguous). The presence of any 2 of these features provides a strong clinical phenotype for PCD. At least 12% of PCD patients have situs ambiguous, and these patients have a 200-fold increased probability of having structural congenital heart disease as compared to the general population with heterotaxy.

The recognition of situs inversus is important for preventing surgical mishaps that result from the failure to recognize reversed anatomy or an atypical history. For example, in a patient with situs inversus, cholecystitis typically causes left upper quadrant pain, and appendicitis causes left lower quadrant pain. A trauma patient with evidence of external trauma over the ninth to eleventh ribs on the right side is at risk for splenic injury.
If surgery is planned on the basis of radiographic findings in a patient with situs inversus, the surgeon should pay careful attention to image labeling to avoid errors such as a right thoracotomy for a left lung nodule.

Preferred examination

Situs abnormalities may be recognized first by using radiography or ultrasonography.
However, computed tomography (CT) scanning is the preferred examination for the definitive diagnosis of situs inversus with dextrocardia. CT scanning provides good anatomic detail for confirming visceral organ position, cardiac apical position, and great vessel branching. Magnetic resonance imaging (MRI) is usually reserved for difficult cases or for patients with associated cardiac anomalies.
 Most patients with situs inversus with levocardia require additional imaging to evaluate the associated cardiac anomalies. When radiation exposure is a concern, MRI or ultrasonography may be preferred.

The differential diagnosis includes appendicitis, asplenia/polysplenia, congenital coronary abnormalities, sinusitis, and ventricular septal defect. Other conditions to be considered are PCD, heterotaxy (see Heterotaxy Syndrome and Primary Ciliary Dyskinesia), left isomerism (ie, Ivemark syndrome) (see Asplenia/Polysplenia), right isomerism (ie, asplenia syndrome) (see Asplenia), situs solitus, and transposition of the great arteries.

If radiologic intervention is to be performed in a patient with situs inversus, the condition should be known from earlier diagnostic imaging. A question of improper image labeling must be resolved before any procedure is initiated.
 Failure to recognize situs inversus before performing a radiologic procedure may result in intervention on the incorrect side of the patient. Attention to the left and right sides of the patient and the left and right labeling of images is helpful to prevent mistakes in diagnosis and/or surgical intervention.

Discordance between the direction of the cardiac apex and the abdominal situs suggests congenital heart disease. Situs ambiguous and situs inversus with levocardia have this discordance between the direction of the cardiac apex and the abdominal situs; thus, further imaging is usually needed.

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