American College of Cardiology

Atrioventricular septal malalignment

Robert H. Anderson, Andrew C. Cook and Girish S. Shirali

Cardiac Unit, Institute of Child Health
University College, London, United Kingdom and Medical University of South Carolina, Charleston, SC

IIn our previous presentations, we discussed the development of the ventricular outflow tracts, and then showed how the muscular outlet septum, formed by myocardialisation of the proximal ends of the endocardial cushions that septate the outflow tract, could be malaligned relative to the apical muscular ventricular septum. We illustrated how this malalignment could obstruct the outflow tract of either the morphologically right or the morphologically left ventricle, and how the effect on the outflow tracts themselves depended on the ventriculo-arterial connections, with malalignment into the right ventricle producing subpulmonary obstruction when the ventriculo-arterial connections were condordant, but subaortic obstruction in the setting of discordant ventriculo-arterial connections. Such abnormal positioning of the muscular outlet septum is probably the best recognised form of malalignment, but there is an equally significant variant, namely that involving malalignment between the apical muscular septum and the atrial septum. Such atrioventricular septal malalignment is the phenotypic feature of hearts with straddling and overriding of the morphologically tricuspid valve. This will be the topic of this review, in which we will discuss this important lesion relative to the contentious topic of "ventricular septal defects of the AV canal type".

As indicated above, the essence of straddling and overriding of the tricuspid valve is malalignment between the leading edge of the atrial septum and the crest of the muscular ventricular septum (Figure 1 and echo 1). When describing hearts having this important malformation, we distinguish between the features of straddling and overriding. For us, straddling accounts for the situation in which the tension apparatus of an atrioventricular valve is attached in both ventricles, as shown diagrammatically in Figure 2, where some of the cords of the right AV valve straddle the ventricular septal crest (arrows). An echocardiographic sweep from the apical view (echo 2) demonstrates that the tricuspid valve straddles in a plane that is posterior to the 'classic' apical plane that is usually utilised to demonstrate both atrioventricular valves. In this case, the septal leaflet of the tricuspid valve has chordal attachments to the free wall of the left ventricle, while the anterior leaflet of the tricuspid valve has chordal attachments to the free wall of the right ventricle. We use the term overriding, in contrast, to describe the arrangement in which the atrioventricular junction is shared between the ventricles, shown diagrammatically in Figure 3, with the valvar orifice of the right AV valve in this setting overriding the crest of the muscular ventricular septum (arrows). An overriding right atrioventricular valve in the absence of straddling is demonstrated in echo 3, a subcostal short axis view of the AV valves in a patient with double inlet left ventricle with a hypoplastic right ventricle. Both AV valves open primarily to the left ventricle. The plane of the right AV valve orifice overrides the plane of the ventricular septum but it does not straddle the septum: it has no chordal attachments to the cavity of the right ventricle.

Valves usually straddle and override at the same time (Figure 4). But either straddling (Figure 2) or overriding (Figure 3) can exist in isolation. When defined in this fashion, either atrioventricular valves or arterial valves can override the septal crest. Only atrioventricular valves, however, can straddle, since the arterial valves do not possess any tension apparatus. Both of these features can vary. The straddling tendinous cords can be attached to the crest of the ventricular septum or can be attached on the septum towards the apex of the opposite ventricle, or can have an attachment to the parietal wall of the opposite ventricle (Figure 5). These variations, which constitute a spectrum, have a significant effect on potential surgical repair of the straddling valve. Equally important, however, is the variation in the degree of override. If most of the overriding orifice is connected to its appropriate ventricle, then the heart exhibits biventricular atrioventricular connections, which can be concordant, discordant, or ambiguous, depending on the segmental connections. If, in contrast, the overriding orifice is connected in its greater part to the ventricle already receiving the other atrioventricular valve, then there will effectively be a double inlet atrioventricular connection, which can be to a morphologically right or morphologically left ventricle. When the morphologically tricuspid valve overrides so as to produce an effective double inlet connection, however, the dominant ventricle is always of left morphology (Figure 6). The rudimentary and incomplete right ventricle, however, can either be right-sided (Figure 6) or left-sided (Figure 7). In this setting, the degree of straddling is still judged from the stance of the incomplete ventricle, so that when overriding is extreme, the tension apparatus is almost always attached on the parietal wall of the dominant left ventricle (Figure 8).

As explained in our opening paragraph, the essence of the malformation is malalignment of the atrial septum with respect to the apical muscular ventricular septum. This, of necessity, means that the hearts have separate right and left atrioventricular junctions. This feature is then crucial to the debate as to whether the hearts with straddling tricuspid valve have an "atrioventricular canal malformation". There is no question but that, in the hearts with straddling atrioventricular valve, some of the leaflets of the deformed valve bridge the ventricular septum. And the ventricular septum itself, when viewed from the left ventricular aspect, has a "scooped-out" arrangement. But neither of these features are phenotypic for the "atrioventricular canal malformation". In our opinion, it is the presence of a common atrioventricular junction that is the phenotype of the atrioventricular canal (Figure 9 and echo 4). In hearts with this phenotypic feature, the superior and inferior leaflets of the common atrioventricular valve can be firmly attached to the underside of the atrial septum as they bridge between the ventricles (Figure 10). This attachment to the atrial septum then confines shunting at ventricular level (echo 5). It is hearts of this type, with common atrioventricular junction but with the bridging leaflets attached to the undersurface of the atrial septum, that are the true ventricular septal defects of "atrioventricular canal type". They have all the features of hearts with common atrioventricular junction, including a trifoliate left valve and unwedging of the left ventricular outflow tract.

Taking the common atrioventricular junction as the phenotype of the "atrioventricular canal malformation" then introduces another significant feature. That is the finding that there can be atrioventricular septal malalignment of the type seen with separate atrioventricular junctions in the setting of the common junction. This is important from the stance of the conduction tissues. The atrioventricular bundle is carried on the crest of the muscular ventricular septum. In hearts with separate atrioventricular junctions and concordant atrioventricular connections, this bundle takes its origin from the atrioventricular node situated at the apex of the triangle of Koch (Figure 11). When there is a common atrioventricular junction, however, the atrial septum no longer is in contact with the crest of the muscular ventricular septum at the apex of Koch's triangle. The bundle is then displaced postero-inferiorly, and takes its origin from a node at the apex of an inferiorly displaced nodal triangle, rather than from the triangle of Koch (Figure 12). The margins of the nodal triangle, shown in green, are the leading edge of the atrial septum and the hingepoint of the inferior bridging leaflet.

But, when there is septal malalignment, irrespective of whether there is a common atrioventricular junction or separate atrioventricular junctions, the node is formed neither in the triangle of Koch nor the nodal triangle in the atrial septum (Figs. 11,12). Rather, it is formed within the atrial myocardium at the point where the muscular ventricular septum meets the atrioventricular junction (Figure 13). This point will obviously vary depending on the precise degree of overriding of the tricuspid valvar orifice, but the node will be formed postero-inferiorly when the atrioventricular connections are effectively concordant (Figure 12), but increasingly superiorly and anteriorly as the connection moves towards double inlet. Should the straddling tricuspid valve be left-sided, with left hand ventricular topology, the bundle will take its origin from an antero-superior node in the right atrioventricular junction irrespective of whether the atrioventricular connections are discordant or double inlet with left-sided rudimentary right ventricle.

Thus, it is the nature of the atrioventricular junction that determines the phenotype of the atrioventricular canal malformation, but malalignment between the atrial and ventricular septal structures that is the essence of hearts with straddling tricuspid valve.