Situs ambiguus

Situs ambiguus, also known as heterotaxy or heterotaxy syndrome, is a rare congenital condition characterized by an abnormal arrangement of the internal organs across the left-right axis of the body that is neither the normal arrangement (situs solitus) nor a complete mirror-image reversal (situs inversus). Instead, the organs are positioned in an indeterminate or discordant pattern. The condition primarily affects the thoracic and abdominal organs, including the heart, lungs, spleen, liver, stomach, and intestines. Situs ambiguus is broadly categorized into two subtypes: left isomerism (also called polysplenia syndrome), in which bilateral left-sidedness features predominate, and right isomerism (also called asplenia syndrome), in which bilateral right-sidedness features predominate. Congenital heart defects are the most clinically significant feature and are present in the vast majority of affected individuals. These may include atrioventricular septal defects, transposition of the great arteries, double outlet right ventricle, total anomalous pulmonary venous return, and abnormalities of the cardiac conduction system. Patients with right isomerism frequently have asplenia (absence of the spleen), which leads to increased susceptibility to life-threatening bacterial infections, particularly from encapsulated organisms. Patients with left isomerism may have polysplenia (multiple small spleens) with variable splenic function. Other associated features include intestinal malrotation, biliary atresia, and abnormalities of the inferior vena cava. Management of situs ambiguus is multidisciplinary and depends on the specific constellation of anomalies present. Treatment of complex congenital heart disease often requires staged surgical interventions, which may include palliative procedures or corrective open-heart surgery. Patients with asplenia require lifelong antibiotic prophylaxis and vaccination against encapsulated bacteria to prevent overwhelming sepsis. Intestinal malrotation may require surgical correction if it leads to volvulus. Prognosis varies widely depending on the severity of the cardiac defects and associated anomalies, with right isomerism generally carrying a more guarded prognosis than left isomerism. Genetic counseling is recommended, as the condition can be caused by mutations in several genes involved in left-right patterning, including ZIC3, NODAL, CFC1, ACVR2B, LEFTY2, GDF1, and others.

Common questions about Situs ambiguus