United Kingdom
sejjran@ucl.ac.uk
Orcid 0000-0001-5163-9021
It is now more than 40 years since, together with my good friend and
colleague Gaetano Thiene, we addressed the issue of the anatomical
substrate of mitral atresia.1 Shortly thereafter, with
access to the excellent archive of congenitally malformed hearts held at
Children’s Hospital of Pittsburgh, my colleagues and I explored the
anatomical heterogeneity that is encountered when such valvar atresia is
found in the setting of a patent aortic root.2 We
encountered exactly the same problems that are now described by the team
working at the All India Institute of Medical Sciences in New Delhi. The
investigators working in New Delhi encountered an additional problem.
They recognised that the problems existing in providing a simple
classification for these lesions are greatly increased in the setting of
isomeric, rather than lateralised, atrial
appendages.3,4 Hence, they have produced two excellent
manuscripts, rather than seeking to combine the overall findings. Their
reports now serve to show how analysis of computed tomographic datasets
is rapidly establishing itself as the new “gold standard” for the
description of the anatomy of the congenitally malformed heart. The
technique has several advantages over gross dissection. In the first
instance, it shows the anatomy of the heart within the chest. For too
many years cardiac morphologists have described the heart as if removed
from the chest and positioned on its apex, producing the so-called
“Valentine arrangement”. It is slowly becoming recognised that
attitudinally appropriate description enhances the ability to make
direct comparisons between the structures as now revealed by
three-dimensional imaging and the names used to describe
them.5 Traditionalists can sometimes find it difficult
to accept, for example, that the leaflet of the valve guarding the
diaphragmatic aspect of the right atrioventricular junction is
inferiorly located, and hence should not be described as being
“posterior”. It is the activities of groups such as those working in
New Delhi, whose investigations are increasingly seen published in the
pages of the Journal, which are demonstrating the advantages of
describing cardiac structures in their attitudinally appropriate
settings. A second great advantage of virtual dissection of computerised
tomographic datasets is that the technique shows the cardiac components
in their correct orientation to each other, There is no need to open and
stretch the heart, as is usually the case when performing gross cardiac
dissection. And sections can be taken again and again, and in different
planes, when assessing the three-dimensional dataset. A third advantage
of computerised tomographic investigation is that it reveals the details
not only of the cardiac structures, but also the arrangement of the
remaining thoracic and abdominal organs. In the investigations now
published in this issue of the journal, for example, the Indian
investigators were able to demonstrate disharmony between the
arrangement of the atrial, thoracic, and abdominal arrangements in three
of their patients with lateralised atrial chambers, and in two
individuals with isomeric atrial appendages.3,4Significantly, the disharmony in those with isomeric appendages was
between the lungs, bronchuses, and abdominal organs, but still with the
bronchial arrangement providing an accurate guide to the type of
isomerism present. Even more significantly, when using their tomographic
datasets, it proved possible in all instances to identify the presence
of isomeric atrial appendages on the basis of the extent of their
pectinated walls relative to the vestibules of the atrioventricular
junctions.6 This will now surely be the way forward to
identifying and segregating those patients currently described in
confusing fashion as having “heterotaxy”. As the authors show, it is
the abdominal organs that are “heterotaxic”, whereas it is only the
lungs, bronchuses, and atrial appendages which are able to show the
features of isomerism.7
The major topic of the two investigations, however, was “mitral
atresia” in the setting of a patent aortic root. As the investigators
emphasise, atresia of the mitral valve is also one of the features of
the hypoplastic left heart syndrome. In this setting, as in the hearts
with patent aortic root, the atresia itself can be the consequence of
presence of an imperforate leaflet, but more frequently is due to
absence of the left atrioventricular connection, when the myocardial
floor of the left atrial chamber is separated by a layer of fibroadipose
tissue from the underlying crest of the ventricular mass. When mitral
atresia is found in the setting of the hypoplastic left heart syndrome,
it is usually found with co-existing aortic atresia, and the left
ventricle is then often little more than a slit in the inferior wall of
the ventricular mass. As the authors from New Delhi rightly emphasise,
the main distinguishing feature between the hypoplastic left heart
syndrome and the variants seen with patent aortic root is the integrity
of the ventricular septum.8 The cases with patent
aortic root and ventricular septal defect, therefore, can be viewed as a
different set of phenotypes. It is the categorisation of these
phenotypes that then creates significant problems when seeking to
describe them all as “mitral atresia”. As the computerised tomographic
datasets reveal, when the mitral valve is atretic in the setting of
congenitally corrected transposition with usual atrial arrangement, then
it is right-sided, rather than being left-sided. The Indian
investigators, nonetheless, have rightly include some cases in their
series in which the left atrioventricular valve is atretic, and is
producing blockage of the direct outlet from the pulmonary venous
atrium. And, as they discuss, had the left-sided atrioventricular
connection been formed in this setting, it would have been guarded by a
morphologically tricuspid valve. It is for this reason that the
investigators correctly describe atresia of the left atrioventricular
valve, rather than the mitral valve. This description then also holds
good for the patients they identified with isomerism, since when the
appendages are isomeric, it is not always possible to identify pulmonary
and systemic venous atrial chambers. It does remain possible to
distinguish between the right and left atrioventricular junctions. There
is one problem remaining with their descriptions, which relates to the
nature of the atresia when the atrial chambers are themselves
mirror-imaged, as was the case in four of their patients with
lateralised atrial chambers. If these patients truly had atresia of the
left-sided atrioventricular valve, then this arrangement would be the
equivalent of tricuspid atresia as seen when there is usual atrial
arrangement. Since I was the referee of the manuscripts during the
process of peer review, I should have asked for clarification of this
point at an earlier stage. This small indiscretion on my part should not
detract from the superb account the authors provide of cardiac anatomy
in an unusual setting. Their figures and descriptions point to the
quality of diagnosis now being achieved in the All India Institute. The
nature of the material they describe also points to the numbers of
patients now coming forward for treatment, and the variability in the
combinations of lesions encountered. Above all, their account emphasises
the great value of simple description of the overall findings.