Situs inversus (SI-tus in-VER-sus) is a congenital * condition in which major organs in the body are in reversed or mirrored position.
The normal position of internal organs of the thorax * and abdomen * is known as situs solitus (SI-tus SOH-lih-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 Baillie at the end of the 18th century. In situs inversus totalis, all major organs are positioned exactly opposite to the usual location. The heart is located on the right side of the thorax, and 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 medical examination for an unrelated condition.
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 when the bodily position of the problem does not match 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 localized in the appendix. Marlowe's history was consistent with appendicitis. The explanation for the incorrect position is that Marlowe also had situs inversus.
Correct labeling of x-ray images is important. Situs inversus may be overlooked if labels are applied incorrectly by an x-ray technician. Such errors occur but they are uncommon.
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 information will expedite medical care and prevent unnecessary confusion.
Situs inversus results when events of twisting and folding during embryonic development occur in a direction that is opposite to normal. Although the affected organs are mirror images of the usual arrangement, they function in a normal manner.
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.
Jones, Kenneth Lyons, Marilyn Crandall Jones, and Miguel del Campo. Smith's Recognizable Patterns of Human Malformation: Expert Consult, 7th ed. Philadelphia: Saunders, 2013.
Genetic and Rare Diseases Information Center. “Situs Inversus.” https://rarediseases.info.nih.gov/gard/4883/situs-inversus/resources/1 (accessed July 10, 2016).
American Academy of Family Physicians. PO Box 11210, Shawnee Mission, KS 66207-1210. Toll-free: 800-274-2237. Website: http://www.aafp.org (accessed July 10, 2016).
American Academy of Pediatrics. 141 Northwest Point Blvd., Elk Grove Village, IL 60007-1098. Toll-free: 800-433-9016. Website: http://www.aap.org (accessed July 10, 2016).
American College of Physicians. 190 North Independence Mall West, Philadelphia, PA 19106-1572. Toll-free: 800-523-1546. Website: http://www.acponline.org (accessed July 10, 2016).
* congenital (kon-JEH-nih-tul) means present at birth.
* appendicitis (ah-pen-dih-SY-tis) is an inflammation of the appendix, the narrow, finger-shaped organ that branches off the part of the large intestine in the lower right side of the abdomen.
* thorax (thor-AKS) is the part of the body between the neck and the abdomen, including the cavity enclosed by the ribs, breastbone, and backbone.
* abdomen (AB-do-men) the portion of the body between the thorax (ΤHOR-aks) and the pelvis. Also called the belly.
* dextrocardia is mirror-image rotation that is confined to the heart.
* CT scans is the shortened name for computed tomography (to-MOG-ra-fee), which uses computers to view structures inside the body. Formerly called computerized axial tomography (ΟAΤ).
* MRI, which is short for magnetic resonance imaging, produces computerized images of internal body tissues based on the magnetic properties of atoms within the body.
* ultrasound, also called a sonogram, is a diagnostic test in which sound waves passing through the body create images on a computer screen.
* appendix (ah-PEN-diks) is the narrow, finger-shaped organ that branches off the part of the large intestine in the lower right side of the abdomen. Although the organ is not known to have any vital function, the tissue of the appendix is populated by cells of the immune system.