Right atrial isomerism (RAI) is a severe complex congenital heart defect resulting from embryonic disruption of proper left-right axis determination. RAI is usually characterized by complete atrioventricular septal defect with a common atrium and univentricular AV connection, total anomalous pulmonary drainage, and transposition or malposition of the great arteries. Affected individuals present at birth with severe cardiac failure. Other associated abnormalities include bilateral trilobed lungs, midline liver, and asplenia, as well as situs inversus affecting other organs. Left atrial isomerism (LAI) is a related disorder with a somewhat better prognosis. LAI is characterized by bilateral superior vena cava, interruption of the intrahepatic portion of the inferior vena cava, partial anomalous pulmonary venous drainage, and ventricular septal defect. Patients with LAI may have polysplenia and bilateral bilobed lungs, as well as situs inversus affecting other organs. Both RAI and LAI malformation complexes have classically been referred to as Ivemark syndrome (summary by Eronen et al., 2004 and Kaasinen et al., 2010). [from
OMIM]