The situation in which the bodily organs are “jumbled up”, frequently described in terms of visceral heterotaxy, was first brought to prominence by Ivemark, who emphasised the situation in terms of anatomy and development of the spleen. Putschar and Mannion then indicated that “between the normal situs, which is asymmetrical, and the situs inversus, which is the asymmetrical mirror-image of normality, a symmetrical situs sometimes exists, and this is the essence of bodily isomerism”.
In the setting of the congenitally malformed heart, however, the isomeric features are found uniformly only in the atrial appendages. To date, these such subtle features have largely been recognised at autopsy, but if specifically sought for, they should be identified by the echocardiographer, even when working in the prenatal setting.
The positive diagnosis of cardiac isomerism, therefore, depends on the recognition of isomeric atrial appendages. There is no evidence of isomerism at ventricular or arterial level. The relationship of the great vessels as they traverse the diaphragm, nonetheless, has been shown to be helpful in pointing to the need for more specific examination of the atrial appendages. When analysed on this basis, there can only be left or right isomerism, although the isomeric features are not always found uniformly throughout the bodily organs. Should there be disharmony between the systems, the specific findings should be described for each system, thus removing any suggestion of ambiguity.
The distinction between left and right isomerism is crucial for counselling, not only for immediate decisions regarding the progress of the pregnancy in question, but for future potential pregnancies. Distinguishing between pregnancies developing with right and left isomerism could also provide the key for determining the genes responsible for the production of laterality.