Divided Left Atrium—or Triatrial Heart?

In the accompanying case report, Nair and his colleagues describe the association between the so-called ‘‘cor triatriatum’’ and unroofing of the course of a persistent left superior caval vein through the left atrioventricular groove. Cor triatriatum is rare, but it can be found in various unusual settings other than the ‘‘classical’’ form. One of these rarer variants is the association emphasized by Nair and his associates. In their discussion, these authors discuss concepts of development and suggest that these issues are yet to be resolved. Might I suggest that resolution of the debates is more likely to emerge if, as Thilenius and colleagues suggested, we consider the lesion under discussion in terms of division of the left atrium, rather than implying that the abnormal hearts possess three atrial chambers? It is surely no more than conventional wisdom that dictates the need to describe the entity as cor triatriatum? Nair and associates themselves described how they recognized distal and proximal components of the morphologically left atrium. The existence of a triatrial heart must devolve on their definition of atrial chambers, since logic dictates that only if we have criteria for uniform recognition of a third atrial chamber can we justifiably describe triatrial hearts. Concepts of cardiac embryology are helpful in arbitrating this problem. The account provided by Knauth and colleagues has now been confirmed and extended by ongoing studies in man and mouse. Taken together, the studies show that the initial atrial chamber is part of the so-called primary heart tube. This component, in the definitive heart, is incorporated largely as the body of the morphologically left atrium. Only a small part persists in the morphologically right atrium, being positioned eventually between the left venous valve and the septum. It is ballooning of the atrial appendages from the primary heart tube, which then produces the components permitting distinction of the morphologically right and left atrial chambers. These appendages are the most constant atrial components when hearts are congenitally malformed. They are the parts that satisfy the criterion enshrined by Van Praagh and colleagues as the ‘‘morphological method. ’’ This important principle states that structures within the heart should be defined on the basis of their intrinsic morphology and not on the basis of other features that are themselves susceptible to change. The other atrial components, namely, the vestibules, derived from the atrioventricular canal musculature, the venous components, and the septum are themselves all variable and hence are disqualified as serving as criteria for the recognition of atrial chambers. The developmental evidence, therefore, supports the notion that there can only be two atrial chambers. The morphological evidence then shows that these atrial chambers, be they morphologically right or left, can be divided to produce the entities described inappropriately as cor triatriatum. The morphologically right atrium can be divided when there is persistence of the valves of the systemic venous sinus. The morphologically left atrium, in contrast, is divided by the presence of the fibromuscular shelf described by the current authors. It has been suggested that the shelf is induced by presence of a persistent left superior caval vein. This seems unlikely since the authors conclusively show that the shelf can still exist when the walls normally separating the persisting left vein from the cavity of the left atrium have disappeared during fetal development. This explanation also founders when it is recognized that not all cases of divided left atrium are associated with persistent patency of the left superior caval vein. Another explanation has been based on entrapment of the pulmonary vein. But, in their excellent overview, Thilenius and colleagues showed that the shelf could also divide the left atrium even when there was totally anomalous connection of the pulmonary veins. Hence, we still do not know why the fibromuscular shelf is formed, although we do know that it divides the morphologically left atrium into proximal and distal components. The oval fossa can empty into either of these parts. The proximal part can exist even in the presence of totally anomalous pulmonary venous connection, implying that the shelf itself is derived in some way from the atrial component of the primary heart tube. Resolution of the morphogenesis of the lesion and understanding of its variations will be facilitated by recognition that the lesion