situs ambiguus

Last reviewed 01/2018

Heterotaxy is synonymous with 'visceral heterotaxy' and 'heterotaxy syndrome'

• defined as an abnormality where the internal thoraco-abdominal organs demonstrate abnormal arrangement across the left-right axis of the body
• by convention, heterotaxy does not include patients with either the expected usual or normal arrangement of the internal organs along the left-right axis, also known as 'situs solitus', nor patients with complete mirror-imaged arrangement of the internal organs along the left-right axis also known as 'situs inversus' (1)
• situs ambiguus is defined as an abnormality in which there are components of situs solitus and situs inversus in the same person
  • situs ambiguus, therefore, can be considered to be present when the thoracic and abdominal organs are positioned in such a way with respect to each other as to be not clearly lateralised and thus have neither the usual, or normal, nor the mirror-imaged arrangements
  • most common anatomic defects associated with situs ambiguus include severe heart malformations, splenic abnormalities, and malposition of the abdominal viscera
  • mortality is high because of the overall complexity of malformations, particularly of the heart
  • situs ambiguus can be divided into 2 broad groups:
    • right-sided isomerism (asplenia) is characterized by itus ambiguus with bilateral right-sidedness
    • left-sided isomerism (polysplenia) is a second type of situs ambiguus characterized by bilateral left-sidedness
      • polysplenia may be regarded as bilateral left-sidedness and may be associated with left atrial isomerism
      • congenital cardiac anomalies are more common and are often more severe in asplenia than in polysplenia
        • include endocardial cushion defects, pulmonary atresia or pulmonary stenosis, transposition of the great vessels, total anomalous pulmonary venous return, and a double-outlet right ventricle
        • cyanotic heart diseases, including severe atrioventricular canal defects, tend to be more common in asplenia, whereas acyanotic defects, which usually occur with increased pulmonary blood flow, are more common in polysplenia
          
          

  Congenital defects associated with situs ambiguus comprise of:

• 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

Cause of this condition is poorly understood

• its occurence has been found to be linked to maternal diabetes mellitus, family history of malformations, and parental cocaine use
  • familial situs ambiguus has been reported with autosomal and X-linked transmission
  • data would suggest that both genetic and environmental factors play a role

Reference:

• 1. Jacobs JP et al. The nomenclature, definition and classification of cardiac structures in the setting of heterotaxy. Jacobs JP, Anderson RH, Weinberg PM, Walters HL 3rd, Tchervenkov CI, Del Duca D, Franklin RC, Aiello VD, Béland MJ, Colan SD, Gaynor JW, Krogmann ON, Kurosawa H, Maruszewski B, Stellin G, Elliott MJ. Cardiol Young. 2007 Sep;17 Suppl 2:1-28