Prevalence and Etiology Early reports of the prevalence of atrioventricular septal defects with a common atrioventricular junction have varied markedly.6,7 These differences almost certainly reflect the problems inherent in most epidemiologic studies due to the bias in selection The most accurate data are those provided by the study of a stable population backed up with verification at clinical study or autopsy Using such an approach, Samanek et al.8 calculated a prevalence of 0.19 per 1000 live births, which accounted for 2.9% of the patients examined with congenitally malformed hearts in Bohemia from 1952 to 1979 Of these cases, three-fifths had shunting confined at the atrial level, the so-called ostium primum defects, whereas the remainder had associated ventricular shunting Regional differences were also found, with most of the afflicted children being born in industrial areas and girls predominating in the overall numbers The lesion was even more frequent in stillborns, accounting for 6.2% of all congenitally malformed hearts.8 There is a strong association between deficient atrioventricular septation with Down syndrome (see Chapter 4) In Toronto, about one-third of patients with Down syndrome had an atrioventricular septal defect with a common valvar orifice, whereas only one-twentieth had the so-called ostium primum variant.9 In the Bohemian population, half of those with deficient atrioventricular septation had Down syndrome.8 This close association with trisomy 21 is cited as evidence against the usual multifactorial model put forward to explain the inheritance of congenital cardiac disease.10 Evidence has been found of autosomal dominant inheritance, not linked to chromosome 21, in large families involving many patients with atrioventricular septal defect,11 and there is appreciable evidence of familial recurrence.12,13 It is possible, therefore, to discern at least three different genetic patterns: one found in association with Down syndrome, a second emerging as an autosomal dominant trait, and the third being isolated There is a high rate of recurrence, particularly in females Major anatomic differences are present within the overall population of patients having atrioventricular septal defects with or without Down syndrome, such as the atrioventricular valve being common or divided into separate left and right valvar orifices It is intuitive to suggest that these differences must be a reflection of the different genetic mechanisms involved Anatomy Understanding Atrioventricular Septal Defects With a Common Atrioventricular Junction As shown in the following text, the abnormal structure and development of atrioventricular junctions are the phenotypic features of the group of lesions forming the focus of this chapter By atrioventricular junctions, we mean the areas of the heart where the atrial myocardium becomes contiguous with the ventricular myocardium To understand the abnormalities, it is necessary to emphasize the features of normality In the normal heart, the myocardial segments within these junctional areas are separated from one another save at the site of penetration of the bundle of His, which is part of the muscular axis responsible for atrioventricular conduction The separation within the junctions, providing the necessary electrical insulation, is produced largely by the fibrofatty tissues of the atrioventricular grooves These tissues form the greater part of the so-called valvar annuli, which also support the attachments of the leaflets of the atrioventricular valves In the normal heart, there are two atrioventricular junctions that surround the tricuspid and mitral valvar orifices.14 There is a central component present separating the junctions, which also abuts on the subaortic outflow tract (Fig 31.1, left) FIG 31.1 Cuts replicating the four-chamber echocardiographic planes taken in a normal heart (left) and a heart with atrioventricular (AV) septal defect and common atrioventricular junction (right) The essence of the normal heart is the presence of separate right and left atrioventricular junctions, with the atrioventricular component of the membranous septum separating the right atrium from the posterior extent of the left ventricular outflow tract The abnormal heart has a common atrioventricular junction, with an atrioventricular septal defect between the leading edge of the atrial septum and the crest of the muscular ventricular septum Relative to the plane of the atrioventricular junction, the septal defect has atrial and ventricular components In the normal heart, part of this separating component is made up of a true atrioventricular septum, this being the atrioventricular component of the membranous septum, which is relatively small (red arrow in Fig 31.1, left) It is the lack of this septal component, along with additional myocardial separating structures, that underscores the morphology of atrioventricular septal defects found in the setting of a common atrioventricular junction (Fig 31.1, right) In hearts with separate junctions, the septal leaflet of the tricuspid valve is usually attached at a considerably more apical level than is the corresponding leaflet of the mitral valve (see Fig 31.1, left) The posteroinferior part of the area between the hinges of the atrioventricular valvar leaflets, however, is not strictly septal This is because, in this area, the atrial myocardial wall overlaps the crest of the muscular ventricular mass, with an extension of the insulating inferoposterior fibrofatty atrioventricular groove separating the two muscular masses (Fig 31.2, left) In effect, it is a sandwich of muscular and fibrofatty tissues that is interposed between the cavities of the right atrium and the left ventricle The fibrous atrioventricular septum is found anterosuperior to this muscular area (see