Situs Inversus~

Definition:

Situs inversus is a condition in which the organs of the chest and abdomen are arranged in a perfect mirror image reversal of the normal positioning.

Description:

Normal human development results in an asymmetrical arrangement of the organs within the chest and abdomen. Typically, the heart lies on the left side of the body (levocardia), the liver and spleen lie on the right, and the lung on the left has two lobes while the lung on the right has three lobes. This normal arrangement is known as situs solitus.

However, in about 1 in 8,500 people, the organs of the chest and abdomen are arranged in the exact opposite position: the heart is on the right (dextrocardia), as is the two-lobed lung, and the liver, spleen, and three-lobed lung are on the left. Yet because this arrangement, called situs inversus, is a perfect mirror image, the relationship between the organs is not changed, so functional problems rarely occur.

Causes and symptoms:

Early in the normal development of an embryo, the tube-like structure that becomes the heart forms a loop toward the left, identifying the left/right axis along which the other organs should be positioned. Although the mechanism that causes the heart loop to go left is not fully understood, at least one gene has been identified to have a role in this process. However, it is thought that many factors may be involved in causing situs inversus. Rarely, situs inversus can run in families, but most often it is an isolated and accidental event occurring in an individual for the first time in the family.

Most people with situs inversus have no medical symptoms or complications resulting from the condition. Although only 3-5% of people with situs inversus have any type of functional heart defect, this is higher than the rate of heart defects in the general population, which is less than 1%.

It is estimated that about 25% of people with situs inversus have an underlying condition called primary ciliary dyskinesia (PCD). PCD, also known as Kartagener's syndrome, is characterized as situs inversus, chronic sinus infections, increased mucous secretions from the lungs, and increased susceptibility to respiratory infections. PCD is caused by a defect in the cilia that impairs their normal movements.

Diagnosis:

Situs inversus should detected by a thorough physical examination. It is often picked up when a physician, using a stethoscope, hears otherwise normal heart sounds on the right side of the body instead of the left. To confirm the a suspected diagnosis of situs inversus, imaging studies such as MRI, CT, or ultrasound may be ordered, and a referral may be made to a cardiologist or internist for completeness. Imaging studies will also rule out the possibility of random arrangement of the organs, or heterotaxy, which has a much higher risk for serious medical complications.

Treatment:
There is no treatment for situs inversus. In the unlikely case that a heart defect is present, it should be treated accordingly by a cardiologist.

Individuals who have situs inversus should be sure to inform all physicians involved in their medical care. In addition to preventing unnecessary confusion, this will reduce the risk of missing a crucial diagnosis that presents with location-specific symptoms (such as appendicitis).

Alternative treatment:

Not applicable.

Prognosis:

The prognosis for an individual with situs inversus is good, and in the absence of a heart defect or other underlying diagnosis, life expectancy is normal.

Prevention:

There is no known method of preventing situs inversus.

Synopsis:

cardiovascular anomalies~

■dextrocardia with right aorta

■cardiac malformations

■vascular malformations


complete visceral heterotaxy (mirror image)~

■trilobar left lung

■bilobar right lung

■left liver

■inversed stomach

■right spleen


Associations:
renal malformations

■renal agenesis

■non-obstructive renal dysplasia

■bilateral renal dysplasia (12605452)

■multicytic kidneys (polycyctic kidneys)

■renal ectopia

■horseshoe kidney,

■renal hypoplasia (15877680)

renal cell carcinoma

situs inversus totalis with reno-pancreatic dysplasia (10678653, 9934983)

congenital pancreatic fibrosis (pancreatic dysplasia)

congenital hepatic fibrosis

short rib polydactyly syndromes (1481807, 1519646)

■short rib polydactyly syndrome type 3 (1481807)

NB: Children with situs inversus totalis have a much lower risk of cardiovascular malformations (3–9%) than children with situs ambiguus (over 80%). The risk of cardiovascular malformations in situs inversus totalis (3–9%) is higher than in the normal situation, situs solitus (0.6%).