Situs ambiguus

Situs ambiguous or situs ambiguus (the latter spelling is more correct etymologically, but the former spelling is very common), also known as heterotaxy, is a rare congenital defect in which the major visceral organs are distributed abnormally within the chest and abdomen.

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The normal position of the organs is known as situs solitus; situs inversus is a condition in which the usual positions of the organs are reversed from left to right as a mirror image of the normal condition. If these are the two extreme positions on a continuum of asymmetric thoracic and abdominal organ formation, situs ambiguous covers everything in between.

Classically, it comprises:

• Cardiac looping malformations – commonly Fallot’s tetralogy, transposition of the great vessels, pulmonary valve stenosis, and ventricular and atrial septal defects.
• Deranged abdominal organ asymmetry - the stomach and spleen are particularly prone to isolated reversal, and the stomach, liver, and a single adrenal gland are occasionally found in the midline.
• Organ malformations - chiefly asplenia-polysplenia, and more rarely a failure of the head of the pancreas to form, and horseshoe adrenals and kidneys.
• Rotation errors, causing volvulus and/or faulty peritoneal attachments.

More rarely, vascular abnormalities are found, including interrupted inferior vena cava, bilateral superior or inferior venae cavae, intrahepatic interruption of the inferior vena cava with connection to the azygos or hemiazygos veins, and aberrant portal veins.

Causes

Although its etiology is poorly understood, it has been found to be linked to maternal diabetes mellitus^[1], ^[2], family history of malformations, and parental cocaine use^[3], suggesting both genetic and environmental^[4] factors play a role.

See also

• Situs inversus
• Situs solitus
• Chirality (mathematics)
• Asplenia
• Polysplenia