Establishing the anatomic hallmarks of congenitally malformed hearts.

A detailed knowledge of the precise morphology of the congenitally malformed heart has never been more important. In fields as diverse as clinical genetic counseling, epidemiology, surgery, and development biology, a proper description of morphology is essential to understand the cause and prevention of cardiac malformations. There has been a tendency in the past to categorize malformed hearts on the basis of putative developmental faults that are deduced from abnormal morphology, for example "endocardial cushion" or "looping" defects. We consider that this is potentially misleading. A better approach is exactly to determine cardiac structure so as to identify the "hallmark" (or "prototype") morphology of a given lesion. The hallmark will constitute those features, among all the changes in a malformed heart, that best define its fundamental character. As an example of the confusion produced by mechanistic extrapolation, consider the cleft in the anterior leaflet of an otherwise normal mitral valve, compared with the space between the leaflets bridging the ventricular septum in hearts with common atrioventricular junctions and deficient atrioventricular septation ("atrioventricular canal malformations"). It is probably correct to presume that both of these entities result from failure of fusion of the atrioventricular endocardial cushions. It is quite inappropriate, in contrast, to group these two lesions together morphologically, given the major anatomical differences seen in the formed hearts (Sigfússon et al. 1995). Therefore it is also important from the stance of developmental biology that the precise morphology of a genetically or chemically induced lesion be recognized if appropriate inferences are to be drawn for consideration of morphogenetic mechanisms and of comparable lesions in humans.