Situs Inversus Imaging
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.
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.
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. [5, 6, 7, 8, 9, 10]
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.
Situs abnormalities may be recognized first by using radiography or ultrasonography. [2, 13, 14, 15, 16, 17] 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. [17, 18, 19, 20, 21, 22] 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.
In most patients with situs inversus, chest radiography shows dextrocardia, with the cardiac apex pointing to the right and the aortic arch and stomach bubble located on the right as well.
(See the image below.)
Confirming a mirror-image position of the atria allows confident diagnosis of situs inversus if the viscera are also reversed. The atrial morphology cannot be discerned on chest radiographs, but it can be determined indirectly by evaluating the bronchi.  In almost every patient, the side of the morphologic bronchus corresponds to the side of the morphologic atrium; therefore, situs inversus is confirmed if the bronchus intermedius is on the left, because the morphologic right atrium is also on the left. If a minor fissure can be identified, by inference, an eparterial bronchus and morphologic right atrium exist on that side.
In situs inversus, the longer hyparterial bronchus is on the right side and passes under the pulmonary artery; the shorter eparterial bronchus is on the left side and passes over the pulmonary artery. A left bronchus and right bronchus of equal length suggests isomerism. Because 1 in 5 patients with situs inversus have Kartagener syndrome, evaluate the chest radiographs carefully for evidence of bronchiectasis (see the images below).
Upper and lower gastrointestinal examinations are usually not performed for the diagnosis of situs inversus. However, situs inversus may be found incidentally during such examinations. In an upper gastrointestinal examination in a patient with situs inversus, the stomach is on the right, with the C loop of the duodenum curving to the left. The liver and spleen are also in mirror-image locations compared with their normal position. In a barium enema examination, the sigmoid colon curves to the right, leading to a right-sided descending colon and terminating in a left-sided cecum (see the images below).
The degree of confidence of radiographs is high. CT scan findings can be used to resolve any remaining questions. The most common cause of false-positive results is the technologist’s or radiologist’s inattention to proper labeling. This problem occasionally occurs when a technologist prepares for posteroanterior imaging of the chest and labels the image, but the patient is then seated and imaged in an anteroposterior projection (eg, because of patient debility); as a result, the correct labeling is reversed.
The most common cause of a false-negative diagnosis of situs inversus also results from inattention to labeling. The technologist may incorrectly revise a properly labeled radiograph in a patient with situs inversus, because the anatomy is reversed compared with the normal anatomy. A radiologist may incorrectly display an image so that it fits a mental template of what is normal without consciously noting the left or right marker. If a question of proper labeling exists, consult the technologist. If the projection of the image is known, the positioning of the name blocker can usually be used to reconstruct the correct labeling of the image. Alternatively, radiography may be repeated with supervision or special instructions to verify correct left-sided and right-sided labeling.
Most fluoroscopic machines have a button that electronically reverses the image. An experienced radiologist recognizes this reversal as soon as the table is moved to the left or right, because the expected direction of table travel is opposite to that observed on the image intensifier. An inexperienced operator can be confused by this apparent reversal of normal anatomy. Conceivably, a patient with situs inversus can be examined with a fluoroscopy machine, and the image can be reversed electronically in a misguided attempt to correct the mirror-image anatomy.
CT scanning demonstrates the mirror-image anatomy of the viscera in situs inversus (see the images below). The heart and great vessels are a mirror image of their normal anatomy; the left hemithorax contains a trilobed lung, whereas the right hemithorax contains a bilobed lung; and the liver and gallbladder are on the left side, whereas the spleen and stomach are on the right side. 
The degree of confidence with CT scanning is high. In preparing for CT scanning, the technologist records the patient’s position—prone or supine—and whether the patient is moved into the scanner head first or feet first. If the orientation is specified incorrectly, the left-right orientation is displayed incorrectly, and situs inversus is simulated.
MRI is a valuable adjunct to echocardiography and angiography in demonstrating abnormalities of congenital heart disease and in aiding surgical planning. This imaging modality is particularly helpful in diagnosing atrial situs. The morphologic right atrium contains the ostium of the coronary sinus; a connection to the suprahepatic inferior vena cava; a large, wide-based, pyramidal atrial appendage; the crista terminalis; and the pectinate muscles. The morphologic left atrium has the ostia for the pulmonary veins and an atrial appendage with a narrow base and a tubular, hooked shape. [21, 24]
The degree of confidence with MRI is high. As with CT scanning, if the technologist incorrectly records whether the patient is moved head first or feet first into the bore or whether the patient is prone or supine, the image is reversed, and incorrect situs anatomy is simulated.
Echocardiography demonstrates the morphologic left and right atria. The morphologic right atrium has connections to the superior and inferior vena cava and a wide atrial appendage. The morphologic left atrium has a narrow left atrial appendage. Ultrasonography demonstrates the mirror-image anatomy of the abdominal viscera. Fetal ultrasonography can be used to detect situs inversus in utero; detection of this condition in utero alerts the physician to the possibility of PCD or congenital heart disease, which then warrants further evaluation. 
The degree of confidence with ultrasonography is high. Although it is possible to switch the left and right sides of the ultrasonographic displays by holding the transducer backwards or electronically reversing the image, this error is expected only with inexperienced users. False-positive or false-negative diagnoses with ultrasonography are unlikely.
Any nuclear medicine study that is used to evaluate the heart or viscera can be influenced by the presence of situs inversus. These studies include cardiac, pulmonary, hepatobiliary, splenic, and gastrointestinal imaging. For example, on a ventilation-perfusion pulmonary scan, the photopenic defect from the heart is reversed in cases of situs inversus with dextrocardia. The technologist must be able to recognize situs inversus anatomy, because nonstandard camera positioning is often necessary for optimal imaging.
The degree of confidence with most nuclear medicine studies is moderate because of the limited anatomic detail. Recording the anterior and posterior projections incorrectly reverses the left and right labeling. As with other digital images, the nuclear medicine image can be reversed electronically.
Angiography is unnecessary for the diagnosis of situs inversus. In fact, noninvasive methods are preferred. Although the atrial morphology can be analyzed to determine atrial situs, angiography is usually reserved for the evaluation of congenital heart disease. The degree of confidence with angiography is high. The false-positive and false-negative angiographic findings are similar to those of fluoroscopy.
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Annamaria Wilhelm, MD Staff Physician, Department of Radiology, Mayo Clinic
Annamaria Wilhelm, MD is a member of the following medical societies: American College of Radiology
Disclosure: Nothing to disclose.
John M Holbert, MD Professor of Radiology, Wake Forest University School of Medicine
Disclosure: Received royalty from Amirsys for independent contractor.
Bernard D Coombs, MB, ChB, PhD Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand
Disclosure: Nothing to disclose.
Marta Hernanz-Schulman, MD, FAAP, FACR Professor, Radiology and Radiological Sciences, Professor of Pediatrics, Department of Radiology, Vice-Chair in Pediatrics, Medical Director, Diagnostic Imaging, Vanderbilt Children’s Hospital
Disclosure: Nothing to disclose.
John Karani, MBBS, FRCR Clinical Director of Radiology and Consultant Radiologist, Department of Radiology, King’s College Hospital, UK
John Karani, MBBS, FRCR is a member of the following medical societies: British Institute of Radiology, Radiological Society of North America, Royal College of Radiologists, Cardiovascular and Interventional Radiological Society of Europe, European Society of Radiology, European Society of Gastrointestinal and Abdominal Radiology, British Society of Interventional Radiology
Disclosure: Nothing to disclose.
Henrique M Lederman, MD, PhD Professor of Radiology and Pediatric Radiology, Chief, Division of Diagnostic Imaging in Pediatrics, Federal University of Sao Paulo, Brazil
Henrique M Lederman, MD, PhD is a member of the following medical societies: Society for Pediatric Radiology
Disclosure: Nothing to disclose.
Situs Inversus Imaging
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