Situs Inversus

Situs inversus (SI-tus in-VER-sus) is a congenital condition in which major organs in the body are in reversed or mirrored position.

What Is Situs Inversus?

The normal position of internal organs of the thorax (THOR-ax) and abdomen (AB-do-men) is known as situs solitus (SI-tus so-LEE-tus). Situs inversus is a congenital condition in which some or all of the major organs of the thorax and abdomen are in  reversed or mirrored positions compared to normal. The prevalence of situs inversus is less than one in 10,000 people. It is usually an autosomal recessive disorder, although cases of X-linked transmission have been described.

Problems with rotation or mirrored position of body organs have been known for several centuries. A variant of situs inversus dextrocardia (the heart is positioned on the right side of the thorax) was first described in the 17th century. Full or complete situs inversus (totalis) was first described by the Scottish physician Matthew Baillic at the end of the 18th century.

In situs inversus totalis, all major organs are positioned as the exact mirror opposite to the usual location. The heart is located on the right side of the thorax, the liver is on the left side of the abdomen, whereas the stomach and spleen are on the right side. In situs inversus the right lung has two lobes, and the left lung has three lobes (the exact opposite of normal). In addition, all of the blood vessels, intestines, nerves, and lymph vessels are transposed. If all these organs are transposed as described, and in the absence of any congenital heart defects, no functional problems should be present in affected individuals. Individuals with situs inversus can live normal lives.

However, when there is dextrocardia without situs inversus, or situs inversus with levocardia (mirror image rotation of the heart in which the direction of rotation is opposite that of normal cardiac development), there is a much higher rate of significant congenital cardiac abnormalities. Many people with complete situs inversus are not aware of their unusual anatomy. The condition is often discovered during an unrelated medical examination.

Situs inversus is often discovered accidentally by a radiologist evaluating an x-ray, CT scan, MRI image or an image produced by ultrasound. It is also discovered by a physician making a diagnosis in which the bodily position of the problem does not match with the doctor's expectation. Marlowe had appendicitis. Normally, the appendix is in the right lower

portion of a person's abdomen. The normal pattern of pain associated with appendicitis is to become localized in the appendix. Marlowe's history was consistent with appendicitis. The explanation for the incorrect position is that Marlowe also has situs inversus.

On an x-ray of the large intestine, the cecum (initial portion of the large intestine) is on the left side instead of the right. The ascending colon is also on the left, the descending colon is on the right, and the sigmoid colon (the final portion of the large intestine) curves to the rightas it connects with the rectum.

Correct labeling of x-ray images is important. Situs inversus may be overlooked if labels are incorrectly applied by an x-ray technician. Such errors occur but they are uncommon.

Is Situs Inversus Serious?

Generally, situs inversus is not serious. As noted, situs inversus with levocardia is associated with a much higher rate of significant congenital cardiac abnormalities. People with known situs inversus should inform physicians and other medical personnel about their condition, as this will expedite medical care and prevent unnecessary confusion.

What Causes Situs Inversus?

Situs inversus results when events of twisting and folding embryonic development occur in a direction that is opposite to normal. Although the affected organs are mirror images of usual, they function in a normal manner.

Living with Situs Inversus

People with situs inversus have a normal life expectancy. When situs inversus is accompanied by anomalies involving the heart, life expectancy is reduced. The degree of reduced life expectancy depends on the severity of the defect. Not recognizing situs inversus may lead to problems during surgical procedures.

Males and females experience situs inversus in about the same proportion. It occurs with the same frequency in people of all races throughout the world.

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