In the normal individual, the parietal components of the body are mirror-imaged and appropriately described as isomeric. The thoraco-abdominal organs, in contrast, are lateralized. However, in "visceral heterotaxy," the thoraco-abdominal organs also show some degree of isomerism, best seen in the arrangement of the bronchial tree. Whether isomerism can be found within the heart remains controversial. One of two recent publications in this journal emphasized the crucial features of bronchial isomerism; the other, in contrast, confused the situation of isomerism within the heart. In this review, we show how the topic of cardiac isomerism is clarified by concentrating on the anatomical features of the cardiac components and determining how best they can be described. Appropriate manipulation of developing mice produces unequivocal evidence of isomerism of the atrial appendages, but with no evidence of ventricular isomerism. In hearts from patients with so-called "heterotaxy," only the atrial appendages, distinguished on the basis of the pectinate muscles lining their walls, are uniformly isomeric, permitting the syndrome to be differentiated into the subsets of left as opposed to right atrial appendage isomerism. Thus, controversies are defused by simply describing the isomerism of the atrial appendages rather than "atrial isomerism," recognizing the frequency of abnormal venoatrial connections in these settings. Any suggestion of ambiguity is removed by the equally simple expedient of describing all the variable cardiac features, describing the arrangements of the thoracic and abdominal organs separately should there be discordances. Clin. Anat. 28:477-486, 2015.
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