Situs Inversus Imaging Overview Marco Severino first recognized dextrocardia in 1643. More than a centur y 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. Anatomy 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 of situs solitus (A) and the mirror ima ge of situs inversus (B). The right lung (RL), left lung (LL), right atrium (RA), and left atrium (LA) are shown. Types of situs inversus Situs inversus can be classified further into situs inversus with levocardia or situs inversus with dextrocardia. The classification of situs is independent of the cardiac apical position. The terms levocardia and dextrocardia indicate only the direct ion 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 sit us 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 orabsent. The 2 primary subtypes of situs ambiguous include (1) right isomerism, orasplenia syndrome, and (2) left isomerism, orpolysplenia 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
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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 insitus inversus. Situs inversus is present in 0.01% of the population.
Anatomy
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 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.
Types of situs inversus
Situs inversus can be classified further into situs inversus with levocardia or 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 situsinversus 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. 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.
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 3lobes. 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.
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 venacava, 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.
Kartagener syndrome is typified by bronchiectasis, sinusitis, and situs inversus and affects
20% of patients with situs inversus; however, only 50% of patients with Kartagener
syndrome have situs inversus.
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.
Limitations of techniques
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.
Differentials and other problems to be considered
The differential diagnosis includes appendicitis, asplenia/polysplenia, congenital coronary
abnormalities, sinusitis, and ventricular septal defect. Other conditions to be considered are
Kartagener syndrome, heterotaxy (see Heterotaxy Syndrome and Primary CiliaryDyskinesia), left isomerism (ie, Ivemark syndrome) (see Asplenia/Polysplenia), right
isomerism (ie, asplenia syndrome) (See Asplenia), situs solitus, and transposition of the great
arteries.
Radiologic intervention
If radiologic intervention is to be performed in a patient with situs inversus, the conditionshould be known from earlier diagnostic imaging. A question of improper image labeling
must be resolved before any procedure is initiated.
Special concerns
Failure to recognize situs inversus before performing a radiologic procedure may result in
intervention on the incorrect side in 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.
Radiography
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).
Posteroanterior chest radiograph in a 40-year-old man with situs
inversus and dextrocardia. This image shows that the cardiac apex (*) points to the right. A
right-sided aortic arch (A) is associated with slight deviation of the trachea (T) to the left. The
stomach (S) bubble is visible in the right upper quadrant.
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 of 5 patients with situs inversus has Kartagener syndrome, evaluate the chest
radiographs carefully for evidence of bronchiectasis (see the images below).
Posteroanterior chest radiograph in a 55-year-old woman withKartagener syndrome and situs inversus. This image shows a right-sided aortic arch (A) with
slight leftward deviation of the trachea (T), dextrocardia (*), and a stomach bubble (S) in the
right upper quadrant of the abdomen. Subtle bronchiectasis is also present in the lung bases
(see the next image).
Magnified view of the left lower lobe in a 55-year-old
woman with Kartagener syndrome and situs inversus (same patient as in previous image).
This image shows bronchiectasis (arrows).
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 following images).
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 congenital heart disease, which then warrants a careful cardiac
evaluation.
Degree of confidence
The degree of confidence with ultrasonography is high.
False positives/negatives
Although it is possible to switch the left and right sides of the ultrasonographic displays byholding 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 Imaging
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.
Degree of confidence
The degree of confidence with most nuclear medicine studies is moderate because of the
limited anatomic detail.
False positives/negatives
Recording the anterior and posterior projections incorrectly reverses the left and rightlabeling. As with other digital images, the nuclear medicine image can be reversed
electronically.
Angiography
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.