You are in: eMedicine Specialties > Radiology > PEDIATRICS Situs InversusArticle Last Updated: Dec 5, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 12
  Author: Annamaria Wilhelm, MD, Staff Physician, Department of Radiology, Mayo Clinic Annamaria Wilhelm is a member of the following medical societies: American College of Radiology Coauthor(s): J Michael Holbert, MD, Associate Professor, Department of Radiology, Scott and White Memorial Hospital and Clinic Editors: Henrique M Lederman, MD, PhD, Consulting Staff, Department of Radiology, The Children's Hospital of Philadelphia; Professor of Radiology and Pediatric Radiology, Chief, Division of Diagnostic Imaging in Pediatrics, Federal University of Sao Paulo, Brazil; Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; Marta Hernanz-Schulman, MD, FAAP, Professor, Radiology, Radiological Sciences, and Pediatrics, Director, Department of Pediatric Radiology, Radiologist-in-Chief, Director, Department of Diagnostic Imaging, Vanderbilt University Medical Center, Vanderbilt Children's Hospital; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; John Karani, MBBS, FRCR, Consulting Staff, Department of Radiology, King's College Hospital, London Author and Editor Disclosure Synonyms and related keywords: situs inversus totalis, situs inversus with dextrocardia, situs inversus with levocardia, mirror image of situs solitus, mirror-image organs, situs solitus, situs ambiguous/ambiguus, Kartagener syndrome, Kartagener's syndrome, primary ciliary dysplasia, PCD INTRODUCTIONSection 2 of 12
  BackgroundMarco 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. (See also the eMedicine articles Kartagener Syndrome [in the Pulmonology section] and Pulmonary Hypoplasia [in the Pediatrics section], as well as Interpretation of the Chest Radiograph and A Case of Absent Right and Persistent Left Superior Vena Cava on Medscape.) PathophysiologySitus describes the position of the cardiac atria and viscera.1, 2, 3, 4, 5, 6, 7, 8 Situs solitus is the normal position, and situs inversus is the mirror image of situs solitus (see Image 1). 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 or situs inversus with dextrocardia.1, 2, 3, 4, 5, 6, 7, 8 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. When situs cannot be determined, the patient has situs ambiguous or heterotaxy.1, 2, 3, 4, 5, 6, 7, 8 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. (See also the eMedicine articles Heterotaxy, Asplenia, Heterotaxy, Polysplenia, Asplenia [in the Pediatrics section], and Asplenia/Polysplenia [in the Radiology section].) The 2 primary subtypes of situs ambiguous include the following1, 2, 3, 4, 5, 6, 7: (1) right isomerism, or asplenia syndrome, and (2) left isomerism, or polysplenia syndrome. 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. 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. FrequencyUnited StatesSitus inversus is present in 0.01% of the population. Mortality/MorbiditySitus inversus totalis that is associated with primary ciliary dyskinesia is known as Kartagener syndrome.10, 11, 12, 13, 14 Patients with primary ciliary dyskinesia have repeated sinus and pulmonary infections.10, 15, 16 Frequent pulmonary infections often result in bronchiectasis, which predominantly affects the lower lungs. (See also the eMedicine articles Primary Ciliary Dyskinesia, Bronchiectasis, and Sinusitis [in the Pediatrics section]; Bronchiectasis [in the Pulmonology section]; Bronchiectasis and Sinusitis [in the Radiology section]; and Sinusitis, Acute and Sinusitis, Chronic [in the Infectious Diseases section].) RaceNo racial predilection exists for situs inversus. SexThe male-to-female incidence is 1:1. AgeSitus abnormalities are congenital. Clinical DetailsSitus inversus occurs more commonly with dextrocardia.18 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 rare19; this condition occurs in 1 case per 22,000, and it is almost always associated with congenital heart disease. (See also the eMedicine articles Transposition of the Great Arteries and Corrected Transposition of the Great Arteries: Surgical Perspective [in the Pediatric section], as well as Newborns With Congenital Heart Disease Have Widespread Brain Abnormalities on Medscape.) Kartagener syndrome affects 20% of patients with situs inversus. Kartagener syndrome is a subgroup of disorders of primary ciliary dyskinesia in which the cilia have abnormal or no movement, most commonly because of absent or irregular dynein arms. The inheritance pattern of Kartagener syndrome is autosomal recessive with no male or female predominance. 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 ExaminationSitus abnormalities may be recognized first by using radiography or ultrasonography.9, 20, 21, 22, 23 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.24, 25 Limitations of TechniquesMost 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. DIFFERENTIALSSection 3 of 12
Asplenia/Polysplenia Ventricular Septal Defect Other Problems to Be ConsideredKartagener Syndrome RADIOGRAPHSection 4 of 12
FindingsIn 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 Image 2). 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.23 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 of 5 patients with situs inversus has Kartagener syndrome, evaluate the chest radiographs carefully for evidence of bronchiectasis (see Images 3-4). 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 Images 5-6). Degree of ConfidenceThe degree of confidence of radiographs is high. CT scan findings can be used to resolve any remaining questions. False Positives/NegativesThe 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 then, the patient is 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 SCANSection 5 of 12
FindingsCT scanning demonstrates the mirror-image anatomy of the viscera in situs inversus (see Images 7-9):
Degree of ConfidenceThe degree of confidence with CT scanning is high. False Positives/NegativesIn 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. MRISection 6 of 12
FindingsMRI 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 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. Degree of ConfidenceThe degree of confidence with MRI is high. False Positives/NegativesAs 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. ULTRASOUNDSection 7 of 12
FindingsEchocardiography 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 congenital heart disease, which then warrants a careful cardiac evaluation. Degree of ConfidenceThe degree of confidence with ultrasonography is high. False Positives/NegativesAlthough 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. NUCLEAR MEDICINESection 8 of 12
FindingsAny 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. Degree of ConfidenceThe degree of confidence with most nuclear medicine studies is moderate because of the limited anatomic detail. False Positives/NegativesRecording 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. ANGIOGRAPHYSection 9 of 12
FindingsAngiography 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. Degree of ConfidenceThe degree of confidence with angiography is high. False Positives/NegativesThe false-positive and false-negative angiographic findings are similar to those of fluoroscopy. INTERVENTIONSection 10 of 12
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. Medical/Legal Pitfalls
Special Concerns
MULTIMEDIASection 11 of 12
REFERENCESSection 12 of 12
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