| So much has happened to change our appreciation of cardiac structure since I first became involved with the study of the heart. Structures are now seen by the paediatric cardiologist with just as much resolution as the anatomist holding the heart in his or her hands, the clinician also being able to view the structures in motion, and to recognize changes in structure with time, commencing now with examination during fetal life. It is impossible in the time at my disposal to describe all the exciting events that have taken place during my period of 40 years involved with the study of cardiac anatomy, so I will concentrate on one specific lesion, namely an atrioventricular septal defect in the setting of common atrioventricular junction, showing that it remains a truism that we see further if we stand on the shoulders of the giants of yesteryear. In this respect, it is salutary to note that, at the latter part of the first half of the 19th century, Thomas Bevill Peacock had given an exquisite account of the lesion under discussion, emphasizing that the septal defect was at the base of the normally formed atrial septum, and pointing to the trifoliate arrangement of the left atrioventricular valve. We could have avoided much subsequent controversy had we abided by these excellent descriptions. It was, perhaps, other giants who deflected attention from the main findings, since Rokitansky, and subsequently Abbott, both described the left atrioventricular valve as a “cleft mitral valve”. Not until the late 1970's did Carpentier redress the balance, pointing to the unequivocal trifoliate arrangement of the left valve, but still describing it in terms of the Episcopal mitre. It was at this stage that we first became involved in study of this fascinating group of lesions. Carpentier had suggested that the structure of the atrioventricular junction showed variations from the “partial” to the “complete” variants of the entity, then usually described as “atrioventricular canal malformations”, or even as “endocardial cushion defects”. We were surprised to find, however, that when we stripped the valvar leaflets from the ventricular mass, it was no longer possible to determine whether individual hearts had started with two valvar orifices within the atrioventricular junction, or a common valvar orifice. This led to our detailed study of the hearts in the Pittsburgh collection, which showed that the basic phenotype was lack of atrioventricular septation in the setting of a common atrioventricular junction. Further studies showed us that the so-called “muscular atrioventricular septum” of the normal heart was no more than the area of overlap of the atrial and ventricular musculatures at the crux of the heart, and that the only true atrioventricular septum was the component of the membranous septum on the atrial aspect of the hinge of the septal leaflet of the tricuspid valve. This, in turn, revealed to us that shunting through the atrioventricular septal defect depended on the relationships between the bridging leaflets of the essentially common valve, and that the septal defect itself could be closed, producing an atrioventricular septal defect with no shunting! The real key to understanding, therefore, as emphasized by Peacock, is the trifoliate arrangement of the left atrioventricular valve. Even in those cases with no intracardiac shunting, the left valve retains its trifoliate configuration, and bears scant resemblance to the Episcopal mitre. Our experience, therefore, has reinforced the value of observation, but has also shown that description is greatly enhanced when words are used in their vernacular meaning. |
