31 - Atrioventricular Septal Defects

Abstract

The focus of this chapter is the variant of atrioventricular septal defect in which the phenotypic feature is the commonality of the atrioventricular junction. The presence of the common junction reflects not only failure of formation of the normal atrioventricular membranous septum but also absence of the muscular sandwich that produces the normal offsetting between the leaflets of the tricuspid and mitral valves. The common atrioventricular junction itself can be guarded by a common atrioventricular valve or a common valve with dual orifices. The common valve has five leaflets, two confined to the right ventricle, one to the left ventricle, and the other two bridging the ventricular septum. The relationship between the left ventricular components of the bridging leaflets and the leaflet confined to the left ventricle produces a trifoliate left atrioventricular valve, with the third component, the mural leaflet, guarding less than one-third of the circumference of the left atrioventricular orifice. The space between the left ventricular components of the bridging leaflets, although previously described as a “cleft,” is in reality the zone of apposition between them. The relationship between the bridging leaflets and the septal structures underscores the potential for shunting across the atrioventricular septal defect. The relationship between the common junction and the atrial and ventricular chambers accounts for the concepts of dominant as opposed to balanced sharing of the junction. Multiple associated lesions can further complicate the morphologic arrangement. Having addressed the issues of morphogenesis, prevalence, and clinical features, we discuss and illustrate the techniques used to diagnose the anatomic variations. We conclude the chapter with considerations of the various approaches to surgical correction and a review of the impact of such complicating features as the presence of trisomy 21.