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Thursday 23 November 2006

Definition: Isomerism is a defect in asymmetry of paired organs that usually have distinct right and left forms, but in this condition, are mirror images. Often the cardiac atrial appendages, the bronchi and the lungs are all involved.

The absence of visceral asymmetry is sometimes referred to as bilateral left-sidedness and bilateral right-sidedness.

- In left isomerism, the heart has two long narrow atrial appendages and both lungs are bilobed with long hyparterial bronchi. Left isomerism is often associated with polysplenia.

- In right isomerism, both atrial appendages are pyramidal in shape and the lungs are trilobed with short eparterial bronchi. Right isomerism is often associated with asplenia.

NB: Ivemark recognized the absence of L–R asymmetry as a pathological criterium and asplenia with isomerism was called Ivemark syndrome. Later asplenia/polysplenia syndrome appeared as a descriptive term. However it has been shown that isomerism of the atrial appendages is a much more constant feature than splenic anomalies.

Ivemark and asplenia/polysplenia syndrome do not constitute distinct etiological entities and these terms should be avoided.


- right isomerism
- left isomerism


- cardiac isomerisms

  • left cardiac isomerism
  • right cardiac isomerism

- atrial isomerisms

- pulmonary isomerism

  • right pulmonary isomerism
  • left pulmonary isomerism

Case records

- Case #10300: Heterotaxy (isomerisms and situs ambigus) with total anomalous pulmonary veins connection

See also

- heterotaxy (situs anomalies, laterality disorders)


- Peeters H, Devriendt K. Human laterality disorders. Eur J Med Genet. 2006 Sep-Oct;49(5):349-62. Epub 2006 Jan 3. PMID: 16461029