The Phenotypic Features of Multiple Ventricular Septal Defects
When taking note of our current knowledge of cardiac development, we can now argue that all interventricular communications, depending on their borders, can be placed into one of three groups.11,15The significant feature of the first group is that the defects are within the substance of the ventricular septum. These are the so-called muscular defects (Figure 4A). As can also be inferred from the developmental evidence, they can be found anywhere within the muscular septum. Multiple muscular defects, therefore, can be found when opening through different parts of the septum. The most obvious multiple muscular defects, nonetheless, are found when the septum has not properly coalesced during development. Our analysis of the hearts contained within historical archives shows that this problem can manifest as two patterns, which we interpret as representing a spectrum of incomplete coalescence. At the milder end of the spectrum, the apical part of the septum is itself intact, but multiple discrete defects, of variable size, are found at the borders of the apical ventricular components with the ventricular inlets and outlets (Figure 5). In the heart shown in Figure 5, two of the defects are large, and are well seen from both the right and left side of the septum. The severe end of the spectrum is shown in Figure 6. In this heart, there is persisting excessive trabeculation at the apex of the left ventricle (Figure 6B). The entire apical part of the septum, furthermore, shows evidence of inappropriate coalescence of the muscular septum. As such, it is exceedingly difficult to recognise the multiple individual defects that percolate through the substance of the septum. This feature is even worse to recognise when assessed from the right ventricular aspect (Figure 6A). This arrangement is the so-called “Swiss cheese” variant. As is shown, it is impossible, on the basis of direct examination, to establish the precise number of fenestrations within such a septum. This is not the case when the spectrum of coalescence is less severe (Figure 5). As we will discuss in the our surgical review, this means that the “Swiss-cheese” variant can be difficult to repair, the more so since it is usually the most apical part of the septum that has failed to coalesce.
The group of defects reflect failure of closure of the tertiary interventricular communication.15 Its phenotypic feature is fibrous continuity between the leaflets of the mitral and tricuspid valves (Figure 4B).16 The defect incorporates within its borders the atrioventricular component of the membranous septum. It often additionally has a flap in its postero-inferior border formed by the interventricular part of the membranous septum. It is because the myocardial margins of the defect extend around these components of the membranous septum that the defect is designated as being perimembranous.17 The defect, which opens directly beneath the aortic root, can co-exist with muscular defects existing anywhere within the muscular part of the septum. The combination of particular importance is that which exists with a muscular inlet defect (Figure 7A). This is because, in this setting, the atrioventricular conduction axis extends through the myocardial bar which separates the two individual defects (Figure 7B). Should the heart be very small, as is the case in neonates and infants, the bar separating the defects may be of insufficient size to permit sutures to be placed so as to close each defect individually.18In this setting, therefore, it may be judicious to place a single patch covering the right ventricular exits of both defects. The alternative is to temporise until it is judged that the muscular bar is of sufficient size to permit sutures to be placed so as to close each defect without jeopardising the conduction axis.
The third group of defects is characterised by failure of formation of the muscular subpulmonary infundibulum (Figure 3C).15The phenotypic feature is fibrous continuity between the leaflets of the arterial valves (Figure 4C). This is the rarest type to be found in the setting of multiple defects, but must be anticipated to co-exist with muscular defects opening either to the apex or inlet of the right ventricle.
The final combination to be considered is not truly an example of multiple defects. This is when there is a large defect in the apical part of the muscular septum (Figure 8A). When viewed from the right ventricle, however, the defect is seen to be crossed by apical trabeculations, giving the impression of multiple defects (Figure 8B).4 The understanding of this defect has been obfuscated by suggestions that it extends into the infundibulum of the right ventricle.3 It represents a defect within the apical part of the muscular septum. As is now evident from development, formation of the infundibulum is a late event. It cannot be completed until the secondary interventricular communication is tunnelled into the aortic root. These processes, in themselves, show that an outlet defect could not open into the apical part of the right ventricle. It is also clear from the anatomical arrangement that the infundibular part of the right ventricle is found cranial to the limbs of the septomarginal trabeculation, or septal band.16