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NOVEMBER
2002 Malalignment
of the outlet muscular ventricular septum
Ed.note: This issue of 'Current Opinion' is a two-part presentation on
a topic that has been of continuing interest to pediatric cardiologists, surgeons,
morphologists and scientists studying the developing heart. In the first part
of 'Current opinion', Professor Robert Anderson and Andrew Cook present their
concept of malalignment of the outlet septum, embellished with beautiful specimens
that illustrate their points. In the second part of the presentation, Girish Shirali
summarizes the echocardiographic approach to malalignment of the outlet septum.
Echocardiography brings 'to life' what we see in the specimens so clearly. Loops
of echocardiograms are included to emphasize key points.
Malalignment
of the Muscular Outlet Septum Robert
H. Anderson and Andrew C. Cook Cardiac
Unit, Institute of Child Health
University College, London, United Kingdom
In
our previous presentation, we discussed the embryologic formation of the muscular
outlet septum. We showed how the proximal parts of the fused cushions divide the
outflow tract of the developing heart, as illustrated in Slide
1, taken from a human embryo at Carnegie stage 18. Normally, the cushions
fuse with the crest of the primary muscular interventricular septum. With continuing
development, the cushions muscularise, becoming converted into the sleeve of free-standing
infundibulum that forms the greater part of the normal supraventricular crest.
When examining congenitally malformed hearts, the similarity is striking between
this mass of tissue seen in the developing heart (Slide
1), and the primary obstructive lesion observed in anomalies such as tetralogy
of Fallot. Thus, Slide
2 is a cross-section through the obstructed subpulmonary outflow tract from
a patient with tetralogy of Fallot. The parietal part of the outflow tract has
been surgically resected, and a patch has been inserted across the ventriculo-pulmonary
junction. On the septal side, however, the musculature remains intact. Note the
offsetting between the leaflets of the aortic and pulmonary valves. Note also
the hypertrophied outlet septum, along with the sleeve of subpulmonary infundibulum
it supports. The free-standing infundibular sleeve is separated by a plane of
extracardiac tissue from the aortic valvar sinus. The entire muscular mass is
clearly derived from the cushions of the outflow tract, but has retained its position
within the right ventricle, being malaligned with the remainder of the muscular
interventricular septum. In fact, when malaligned in this fashion, the muscularised
cushions no longer form an interventricular septal structure. In the specimen
shown, they have formed an outlet septum within the cavity of the right ventricle,
and one that is obstructing the subpulmonary outflow tract. This is an integral
part of the pathognomonic features of tetralogy of Fallot.
But,
in tetralogy of Fallot, the malaligned outlet septum has a particular relationship
to the other building blocks of the outflow tracts, as shown in Slide
3. The heart has been sectioned to replicate the subcostal oblique cut through
the right ventricular outflow tract. Note that the outlet septum is attached to
the remainder of the muscular interventricular septum antero-cephalad relative
to the septomarginal trabeculation, the structure also called the septal band.
The ventricular septal defect is cradled within the limbs of the septomarginal
trabeculation. The subpulmonary infundibulum is narrowed by a "squeeze"
between the malaligned outlet septum and a series of hypertrophied septoparietal
trabeculations. It is this combination of lesions, namely the deviated outlet
septum co-existing with hypertrophied septoparietal trabeculations, which is the
hallmark of tetralogy. Note also that, in the plane of the section, the ventriculo-infundibular
fold, or inner heart curvature, interposes between the leaflets of the aortic
and tricuspid valves. When traced medially in this heart, this musculature fades
away, leaving continuity between the leaflets of the aortic and tricuspid valves,
as shown in a different specimen in Slide
4. This section, again replicating the subcostal echocardiographic cut through
the right ventricular outflow tract, is also from a heart with tetralogy of Fallot.
This time, the obstructing muscular structures have been transected. The muscular
outlet septum is shown by the cross-hatching, and the hypertrophied septoparietal
trabeculations by the arrows. The bracket shows the narrowed entrance to the subpulmonary
infundibulum. The yellow line marks the location of the interventricular communication,
which is overridden by the leaflets of the aortic valve. Note that the postero-inferior
rim of the ventricular septal defect is formed by fibrous continuity between the
leaflets of the aortic and tricuspid valves. As in the specimen shown in Slide
3, this feature makes the defect itself perimembranous, albeit with malalignment
of the muscular outlet septum. Simple malalignment of the muscular outlet septum,
however, is not sufficient to produce tetralogy of Fallot. This fact is exemplified
by the heart illustrated in Slide
5. Again the specimen is prepared to replicate the subcostal echocardiographic
cut. The muscular outlet septum, as in Slide
3, is attached antero-cephalad relative to the septomarginal trabeculation.
In this heart, however, the septoparietal trabeculations, marked by the stars,
are not hypertrophied, and the subpulmonary outflow tract, shown by the bracket,
is unobstructed. This is an example of the so-called "Eisenmenger" type
of ventricular septal defect. Note that, as in the heart shown in Slide
4, there is fibrous continuity between the leaflets of the aortic and tricuspid
valves, again making the defect perimembranous. But not all malalignment defects
are perimembranous. This is illustrated by the heart shown in Slide 6, again with
tetralogy of Fallot. This time we are looking up the right ventricular outflow
tract towards the interventricular communication. The four-pointed stars mark
the hypertrophied septoparietal trabeculations, which together with the malaligned
outlet septum narrow the subpulmonary outlet. In this heart, however, the posterior
limb of the septomarginal trabeculation fuses with the ventriculo-infundibular
fold, producing the muscular bar shown by the five-pointed star, which forms the
muscular postero-inferior rim of the ventricular septal defect. Thus, this malalignment
outlet defect possesses a muscular postero-inferior rim, and the entire right
ventricular border of the entrance to the overriding subaortic outflow tract is
muscular.
All
the hearts illustrated thus far have the pulmonary trunk arising from the right
ventricle, along with overriding of the aortic valve. Exactly the same type of
malalignment defect can be found, however, when it is the aorta that arises exclusively
from the right ventricle, but with overriding of the pulmonary trunk, in other
words with effectively discordant ventriculo-arterial connections. This is the
arrangement illustrated in Slide
7. In this heart, the ventricular septal defect is perimembranous because
of the presence of fibrous continuity between the leaflets of the pulmonary and
tricuspid valve in the postero-inferior rim of the defect. As with tetralogy of
Fallot, nonetheless, the same type of malalignment defect can be seen with a muscular
postero-inferior rim when the ventriculo-infundibular fold fuses with the septomarginal
trabeculation. This arrangement is shown in Slide
8, also with effectively discordant ventriculo-arterial connections.
Finally,
malalignment defects can be seen when the muscular outlet septum is deviated into
the left ventricular rather than the right ventricular outflow tract. The exemplar
of this lesion is shown in Slide
9, with the deviated muscular outlet septum producing marked obstruction to
the subaortic outflow tract. This is usually associated either with severe aortic
coarctation or interruption of the aortic arch. The heart shown in Slide
9, however, has effectively concordant ventriculo-arterial connections. The
counterpart of this arrangement is shown in Slide
10. The malalignment of the outlet septum, with marked deviation into the
left ventricle, is comparable to the heart shown in Slide
9. But, in the specimen shown in Slide
10, the ventriculo-arterial connections are effectively discordant. The aortic
valve overrides the crest of the muscular ventricular septum, albeit with most
of the aorta still connected within the right ventricle. The deviated outlet septum
produces severe subpulmonary obstruction. This is the anatomy that sets the scene
for the Rastelli type of correction, or more recently the REV maneuver.
Malalignment
of the muscular outlet septum, therefore, can take various guises, depending on
whether the septum is deviated to be attached within the morphologically right
or left ventricle, and whether the ventriculo-arterial connections are effectively
concordant or discordant. Such malalignment of the muscular outlet septum, of
course, is also the essence of double outlet right ventricle, which we illustrate
in our next morphological presentation. Malalignment
Of The Muscular Outlet Septum: Echocardiographic Aspects Girish
S. Shirali, MD, FACC
Medical University of South Carolina
Charleston,
SC Echocardiographic
evaluation of the muscular outlet septum is best understood by utilizing sweeps,
loops and correlations with pathology specimens. As has been demonstrated beautifully
by Prof. Anderson et al, malalignment of the muscular outlet septum can result
in obstruction to either the pulmonary or the aortic outflow tract, depending
on the direction of malalignment (anterior vs. posterior), and the ventricluo-arterial
connection (concordant vs. discordant). A
combination of views and windows is effective in visualizing the ventricular outflow
tracts in their entirety. The subcostal short axis view and the subcostal long
axis view, together with parasternal long and short axis views, enable visualization
of the length of the entire length of the muscular outlet septum. From
the subcostal long axis view, loop
1 demonstrates tetralogy of Fallot with an over-riding aorta and a large anterior
malalignment type ventricular septal defect that extends to the membranous septum.
The latter feature is evidenced by the presence of continuity between the tricuspid
and aortic valves. The outlet septum is seen protruding into and raising the 'floor'
of the right ventricular outflow tract instead of its usual position between the
limbs of the septal band (septomarginal trabeculation). In its current 'malaligned'
position, it is in close proximity to the hypertrophied septoparietal trabeculations
of the right ventricular outflow tract, causing dynamic 'squeeze' of the subpulmonary
outflow tract. The stenosis and flow disturbance that results from this 'squeeze'
is shown in loop 2,
with turbulent, high velocity flow starting within the right ventricular outflow
tract. The
subcostal short axis view shown in loop
3 is from another patient with tetralogy of Fallot. This clearly demonstrates
how anterior malalignment of the outlet septum is integral to the malformation.
The resulting over-riding aorta and the ventricular septal defect (due to failure
of fusion of the outlet septum with the trabecular ventricular septum) are best
demonstrated from this view. This also demonstrates how the hypertrophied septoparietal
trabeculations of the right ventricular outflow tract and the malaligned outlet
septum together lead to dynamic subpulmonary stenosis. Alternatively, mild anterior
malalignment of the outlet septum may be enough to lead to a large ventricular
septal defect, but without the hypertrophied septoparietal trabeculations that
typify tetralogy of Fallot. The example shown in loop
4 is from a patient with the so-called 'Eisenmenger' type of ventricular septal
defect, with minimal obstruction to flow in the right ventricular outflow tract. Whether
or not the ventricular septal defect extends to the membranous septum has important
implications for the course of the conduction system. If the defect is indeed
(peri or para) membranous, the implication is that the bundle of His is located
in its postero-inferior margin. If, instead, the defect has a muscular postero-inferior
rim, then these concerns would be minimized. The distinction between these two
types of defect rims is readily evident by echocardiography. The fibrous continuity
between the tricuspid and aortic valves that Prof Anderson demonstrates on Slide
3 and Slide 4,
is evident in echo loop
1 and loop 2. Conversely, the muscular
postero-inferior rim of the defect demonstrated by Prof. Anderson on Slide
6, is demonstrated in a subcostal long axis view of the heart in loop
5. In this patient who has tetralogy of Fallot with pulmonary atresia, a bar
of muscle interposes between the aortic and tricuspid valves. This rim is immediately
anterior and inferior to the septal leaflet of the tricuspid valve, separating
the latter from the ventricular septal defect. Loop
6 demonstrates this feature from the parasternal short axis view. A prominent
rim of muscle is seen separating the ventricular septal defect from the septal
leaflet of the tricuspid valve. If
anterior malalignment of the outlet septum occurs in a patient with discordant
ventriculo-arterial connections (transposition: aorta arises from the right ventricle
and pulmonary artery arises from the left ventricle), the ventricular septal defect
is in identical position, between the limbs of the septal band (septomarginal
trabeculation). However, now it is the 'floor' of the subaortic outflow that is
raised, resulting in subaortic crowding and the potential for hypoplastic structures
downstream (typically, coarctation of the aorta or interrupted aortic arch). These
features are shown in the next patient, who had transposition of the great arteries
and a large anterior malalignment type ventricular septal defect. This was associated
with severe hypoplasia of the aortic arch. Loop
7 is a subcostal short axis sweep that starts at the base of the heart and
proceeds apically. The first vessel that is seen is the aorta, which is committed
exclusively to the right ventricle. The aorta is identified based on its branching
pattern and its giving rise to both coronary arteries. The aortic valve and ascending
aorta are hypoplastic. The outlet septum (somewhat more mobile than usual) is
seen protruding into the subaortic outflow tract in a dynamic fashion (exaggerated
during systole). As the sweep proceeds towards the apex, the pulmonary artery
is seen arising (leftwards to the aortic valve) from the left ventricle, in continuity
with the mitral and tricuspid valves. In this instance, the pulmonary valve and
main pulmonary artery are dilated, with impressive size discrepancy between the
two arterial valves. Loop
8 is a parasternal long axis view in the same patient, demonstrating anterior
malalignment of the outlet septum into the subaortic region. Note the parallel
course of the great arteries. Both views show the dynamic 'squeeze' of the subaortic
outflow tract between the anteriorly malaligned outlet septum and the septoparietal
trabeculations of the right ventricle. This neonate had an excellent result from
a single stage operation consisting of an arterial switch, closure of the ventricular
septal defect to the native pulmonary (neo-aortic) valve, reconstruction of the
aortic arch and augmentation of the right ventricular outflow tract. The latter
part of the procedure, retaining the integrity of the native aortic (neo-pulmonary)
valve, ensured competent and unobstructed outflow from the right ventricle to
the pulmonary arteries. Posterior
malalignment of the outlet septum can result in a ventricular septal defect in
the same location (between the limbs of the septal band), but the outlet septum
is typically a thin sliver protruding into (and lowering the 'roof' of) the left
ventricular outflow tract. The next patient had a posteriorly malaligned outlet
septum causing subaortic stenosis, and interrupted aortic arch type B. This is
shown in loop 9. Note
the thin, hypoplastic but yet obstructive outlet septum and the hypoplastic aortic
annulus. If
posterior malalignment of the outlet septum into the left ventricular outflow
tract occurs in the setting of transposition, then, as shown by Prof. Anderson
in the final portion of his discussion, the result is subpulmonary obstruction.
The parasternal long axis view in loop
10 shows a large ventricular septal defect due to posterior malalignment of
the outlet septum. The latter structure is seen 'lowering the roof' of the left
ventricular outflow tract. The great artery arising from the left ventricle is
recognizable as the main pulmonary artery based on its course (diving posteriorly)
and its branching pattern. Severe subvalvar pulmonary stenosis is the result.
Loop 11 is obtained
from the same parasternal long axis window in the same patient, by tilting the
transducer rightwards. This demonstrates the relationship of the ventricular septal
defect to the large aorta seen arising anteriorly, exclusively from the right
ventricle. This patient underwent the Rastelli procedure, wherein the ventricular
septal defect was closed from the left ventricle to the aortic valve. A conduit
was placed to establish prograde flow from the right ventricle to the branch pulmonary
arteries. Since this procedure commits the pulmonary outflow tract to the left
ventricle, it necessitates that the pulmonary valve be oversewn. In
summary, malalignment of the outlet septum is seen in a wide range of structural
heart defects. Depending on the direction of malalignment and the nature of the
ventriculo-arterial connection, it can cause subpulmonary or subaortic obstruction.
This feature is uniquely amenable to diagnosis using echocardiography. The surgical
implications of malalignment are of great importance for tailoring the operation
to the individual patient. Identifying and understanding the central role of malalignment
is an important step towards establishing a complete diagnosis in patients who
exhibit this feature. |
