Situs solitus is a medical term indicating that all the abdominal and chest organs are in their expected positions, and are correctly oriented. The opposite is situs inversus, where the positions of the organs are flopped throughout the abdomen, creating a mirror image of what should be present. The position of the abdominal viscera can become important for activities like surgery, collecting biopsy samples, and assessing a patient with health problems that may be related to organ positioning.
In some cases, the positions of the organs are mixed up, and their positioning is described as situs ambiguus, referring to the fact that some organs are correctly positioned, while others are not. This can potentially pose a problem, as the organs fit together like the pieces of a puzzle in situs solitus, and switching their positions randomly could create problems. The patient might have difficulty with breathing, digestion, and other activities, for instance.
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In a very special form of situs ambiguus known as dextrocardia, all the organs are in the correct place except for the heart, which is moved to the right side of the chest. This may sometimes be described as dextrocardia with situs solitus, as a note that all the other organs are where they should be. Swapped organ positioning can be a cause for concern if it interferes with organ function.
When the organs are positioned in situs solitus, the heart, a bilobed lung, spleen, stomach, and aorta are located on the left side of the body. The right houses the trilobed lung, gallbladder, and liver. These organs can be seen on a medical imaging study, which can allow a doctor to take note of any that may have migrated out of position. Such studies may be ordered before surgery to learn more about the nature of an injury or issue, and to confirm that the organ is where the surgeon expects it to be. Otherwise, the surgeon might make an incision and encounter something unexpected.
The causes of abnormal organ positioning have to do with errors in fetal development, when budding stem cells fail to migrate into situs solitus. This is a complex and detailed process, and occasionally mistakes happen. These are not necessarily predictable or preventable. In some cases, they are associated with larger medical problems, including in some instances abnormalities that may be incompatible with life. Prenatal ultrasound can provide some information about fetal development and may help care providers prepare for issues that may need treatment shortly after birth.