Situs Inversus Imaging.docx

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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 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 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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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

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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.

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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).

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Radiograph of the upper abdomen from a barium enemaexamination in a 40-year-old man with situs inversus and dextrocardia. This image shows the

liver (L) in the left upper quadrant of the abdomen. The positions of the splenic flexure (SF)

and hepatic flexure (HF) are reversed.

Radiograph of the lower abdomen from a barium enema

examination in a 40-year-old man with situs inversus and dextrocardia. This image shows the

sigmoid colon (SC) on the right and the cecum (C) on the left.

Degree of confidence

The degree of confidence of radiographs is high. CT scan findings can be used to resolve any

remaining questions.

False positives/negatives

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, thecorrect 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.

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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 themirror-image anatomy.

Computed Tomography

CT scanning demonstrates the mirror-image anatomy of the viscera in situs inversus (see the

following images). 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.

Chest computed tomography scan obtained at the level of the

origins of the great vessels in a 40-year-old man with situs inversus and dextrocardia. This

image demonstrates mirror-image branching of the great vessels (*) and a left-sided superior 

vena cava (+).

Chest computed tomography scan obtained at the level of theaortic outflow tract in a 40-year-old man with situs inversus and dextrocardia. This image

shows reversal of the normal cardiac anatomy. The left atrium (LA), right atrium (RA), left

ventricle (LV), and right ventricle (RV) are shown. The descending aorta (DA) is on the

right.

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Computed tomography scan of the upper abdomen in a 40-

year-old man with situs inversus and dextrocardia. This image shows reversal of the normal

anatomy. The spleen (SP), stomach (ST), and liver (L) are shown. The descending aorta (DA)

is on the right.

Degree of confidence

The degree of confidence with CT scanning is high.

False positives/negatives

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.

Magnetic Resonance Imaging

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.

Degree of confidence

The degree of confidence with MRI is high.

False positives/negatives

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.

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Ultrasonography

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.