Use of correct nomenclature is important in all aspects of medicine. Many of the controversies that have bedeviled paediatric cardiology have devolved from the inappropriate use of words to describe the lesions to be found when the heart is congenitally abnormal. A continuing area of disagreement is the situation currently described by many as representing “heterotaxy”. When used literally, this word means any departure from the normal. Thus, all congenitally malformed hearts represent examples of heterotaxy. By convention, nonetheless, the term is used to describe the arrangement in which the bodily organs, including parts of the heart, are not in their usual or in their mirror-imaged patterns. The arrangements, therefore, represent the presence of the organs on the right and left sides of the body being mirror imaged, in other words isomeric; however, not all the organs are uniformly isomeric. In this review, we show that, when assessed on the basis of the morphology of the atrial appendages, specifically the extent of the pectinate muscles relative to the atrioventricular junctions, isomerism is an unequivocal finding within the heart. Only the atrial appendages, however, are truly isomeric. The potential problem of disharmony between the various systems of organs is resolved simply by accounting specifically for each of the systems. On these bases, we suggest that the isomeric arrangements can now readily be diagnosed in the clinical setting, and differentiated into their right and left isomeric variants. We propose that such distinctions will provide the key for establishing the genetic cues responsible for the formation of the isomeric as opposed to the lateralised arrangements.
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