Lateralisation abnormalities are characterised by one or more viscera located on the wrong side of the body. They are caused by a malrotation of the fetal cardiac and/or gastrointestinal tube. The normal anatomical positon of all organs is called situs solitus. Complete inversion of the normal anatomy, referred to as situs inversus totalis, occurs in 1:8000 cases.
Any kind of incomplete inversion is known as situs inversus partialis: such a condition is encountered in 1:7000 cases. Three common variations of situs inversus partialis are: situs inversus thoracalis (dextrocardia and inversion of mediastinal structures), situs inversus abdominalis (inversion of abdominal strutures), and isomerism of either the right or left half-body. The latter is characterised by a symmetric duplication of either the right or left lung, bronchial system and heart atrium and by a centrally positioned, symmetric liver. Polysplenia is observed in left isomerism, and asplenia in right isomerism.
Associated malformations are far more frequent and severe in sinus inversus partialis than in situs inversus totalis. 95% of the newborns with situs inversus partialis suffer from major cardiovascular abnormalities, such as septal defects, great vessel transposition, right or left ventricular double outlet, or pulmonary stenosis. Asplenia or polysplenia are found in 80% of cases of situs inversus partialis. Venous malformations are common.
In 20-25% of cases, situs inversus totalis is part of a Kartagener syndrome. This is characterised by the tetrad of sinus inversus totalis, chronic sinusitis, bronchiectasis, and male infertility, all of which result from immotile cilia syndrome. Minor cardiovascular malformations and asymptomatic gut abnormalities, such as incomplete bowel rotation and atresia, are common in patients with situs inversus totalis.
