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