Abstract
Pediatric cardiologists treating patients with severe congenital cardiac defects define “visceral heterotaxy” on the basis of isomerism of the atrial appendages. The isomeric features represent an obvious manifestation of disruption of left-right asymmetry during embryonic development. Thus, there are two subsets of individuals within the overall syndrome, with features of either right or left isomerism. Within the heart, it is only the atrial appendages that are truly isomeric. The remainder of the cardiac components shows variable morphology, as does the arrangement of the remaining body organs. Order is provided in this potentially chaotic arrangement simply by describing the specific features of each of the systems. These features as defined by clinicians, however, seem less well recognized by those investigating the developmental origins of the disruption of symmetry. Developmental biologists place much greater emphasis on ventricular looping. Although the direction of the loop can certainly be interpreted as representing an example of asymmetry, it is not comparable to the isomeric features that underscore the clinical syndromes. This is because, thus far, there is no evidence of ventricular isomerism, with the ventricles distinguished one from the other on the basis of their disparate anatomical features. In similar fashion, some consider transposition to represent abnormal lateralization, but again, clinical diagnosis depends on recognition of the lateralized features. In this review, therefore, we discuss the key questions that currently underscore the mismatch in the approaches to “lateralization” as taken by clinicians and developmental biologists.
Highlights
Many features of the congenitally malformed heart remain difficult to understand and interpret to those who are unfamiliar with the lexicon of pediatric cardiologists
It is the recognition that only the atrial appendages are truly isomeric in the setting of the syndromes described by pediatric cardiologists as “visceral heterotaxy”, as we will show, that provides the basis for logical description of the cardiac findings in the two known clinical subsets
The precise mechanism underpinning the production of the features of isomerism as opposed to lateralization of the visceral organs, despite the huge strides made in establishing the steps involved in left-right determination [8,9], has yet to be identified
Summary
Many features of the congenitally malformed heart remain difficult to understand and interpret to those who are unfamiliar with the lexicon of pediatric cardiologists. It is established that, from the stance of cardiac morphology, the essential differences between the patients within the groups is the presence of isomeric rather than lateralized atrial appendages [4,5] Such categorization on the basis of left rather than right isomerism of the atrial appendages provides the necessary specificity to appreciate the markedly variable features to be found within the heart, and to provide correlation with the arrangement of the other thoraco-abdominal organs [6,7]. The International Nomenclature Committee responsible for providing definitions of congenital cardiac lesions have restricted “heterotaxy” to the lesions characterized by the presence of isomerism of the atrial appendages [10] These differences in the approach to symmetry and lateralisation have resulted in a current disconnect in the searches for the answers to the problems producing the clinical syndromes of isomerism. By discussing the fundamental anatomical differences between the types of lateralization represented by ventricular looping [12,13] as opposed to isomerism of the atrial appendages
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