What is situs inversus?

[Anonymous]

What is situs inversus?

[Genetic Counselor]

Situs inversus is a rare congenital condition in which the internal organs of the chest and abdomen are arranged in a mirror-image pattern compared with their usual positions. The word situs refers to the position or arrangement of organs, while inversus means reversed. This condition develops before birth, during the early stages of embryonic development, when the body establishes the left–right orientation of its organs.

Normally, the heart is positioned predominantly on the left side of the chest, the liver is mainly on the right side of the abdomen, and the stomach and spleen are located on the left. In situs inversus, these positions are reversed. The heart may be on the right, a condition called dextrocardia; the liver may be on the left; and the stomach and spleen may be on the right.

There are two commonly described forms of situs inversus:
– Situs inversus totalis: The major organs of both the chest and abdomen are arranged in a mirror image of their usual positions.
– Situs inversus with variations: The heart or other organs may show a reversed arrangement without every organ following the same pattern. Such cases require careful assessment to determine the exact anatomy.

Situs inversus is different from heterotaxy, in which organs do not follow the usual left–right arrangement but also do not form a complete mirror image. Heterotaxy can be associated with complex congenital heart defects, abnormal blood vessels, and abnormalities of the spleen.
The condition is often discovered incidentally during an ultrasound, chest X-ray, CT scan, MRI, or echocardiogram performed for an unrelated reason. Some people remain unaware of their condition until adulthood because they do not experience noticeable symptoms.

In many individuals, situs inversus totalis does not interfere with normal organ function. However, some people may have an associated condition called primary ciliary dyskinesia (PCD). This is a genetic disorder affecting microscopic structures called cilia, which help clear mucus from the respiratory tract. When cilia do not function properly, individuals may experience recurrent sinus infections, chronic cough, ear infections, or bronchiectasis.
The reversal of organ positions occurs because of differences in the biological processes that establish the body’s left–right axis during embryonic development. Several genes and cellular mechanisms contribute to this process, although the cause is not identified in every individual.

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