Thoughts on concepts of development of the heart in relation to the morphology of congenital malformations.

In the past, it has often been the case that congenital malformations have been categorized in terms of their presumed embryologic development. The knowledge of development, however, has itself often been derived from studies of the normal heart during its development coupled with inferences drawn from the morphology of the abnormal hearts. This can lead to circular thinking which, often, has little basis in fact. It is our belief that cardiac embryology is an important science which should stand in its own right, but that knowledge of abnormal development should be derived from observation rather than inference. The potential dangers of concepts derived by extrapolation are illustrated with reference to hearts having deficiencies of atrioventricular septation ('endocardial cushion defects') and those with double inlet left ventricle ('single ventricle'). It is shown that description of these hearts is greatly facilitated by eschewing those concepts derived from 'armchair embryology'. Once a clear description is established, the scene is set to understand the real mechanisms underscoring the maldevelopment of these lesions.