Discussion
The evidence is now strongly in favour of discriminating between the
recognized sub-sets of so-called “visceral heterotaxy” on the basis of
isomerism of the atrial appendages.4-13 With the
advances now made possible through the increased resolution of
multi-detector computed tomography, and the ability to reconstruct the
three-dimensional datasets, it is possible not only to recognize the
features of the atrial appendages, but also to correlate the cardiac
findings with the arrangements of thoracic and abdominal organs. The
clinical findings endorse the conclusions long since made based on
autopsy studies. Thus, right isomerism is usually, but not universally,
accompanied by absence of the spleen. It is this feature that is
believed to render such individuals susceptible to infection. The
isomeric right appendages are the harbingers of the most severe cardiac
and complex congenital malformations.2-13 Left
isomerism is typically associated with polysplenia, but even the
presence of multiple spleens does not protect against splenic
incompetence.15-23,54
The markedly different features of right as opposed to left isomerism
obviously pose different surgical challenges. Detailed knowledge of the
likely co-existing cardiac anomalies, therefore, is advantageous prior
to surgical intervention. Recognition of the presence of bilateral
morphologically right atrial appendages, for example, enables the
surgeon to anticipate the presence of bilateral sinus nodes. Recognition
of isomeric left appendages, in contrast, alerts to an abnormal location
of the atrial pacemaker.37,60 Appropriate precautions,
therefore, should be taken to avoid injury not only to the sinus node,
or nodes, but also their arteries.42
In those with left isomerism, the surgeon should anticipate interruption
of the inferior caval vein. If performing a superior cavopulmonary
connection, and prior to ligating the azygos venous channel, an enlarged
azygos vein associated with an intact inferior caval vein should always
be differentiated from azygos continuation of an interrupted vein. When
the inferior caval vein connects directly to the atrial chambers, it may
connect to either the left or right-sided atrium. In left isomerism,
this should be distinguished from bilateral connection of hepatic veins.
In those with right isomerism, in contrast, a hepatic vein often
co-exists with an inferior caval vein (Figures 8 and
9).2-5,9-11
Although the arrangement of the inferior caval vein tends to distinguish
those with left from right isomerism, this is not the case for the
superior caval vein. At least half of patients with right isomerism, and
up to two-thirds of those with left isomerism, have bilateral superior
caval veins.2-13,15,19 When bilateral superior caval
veins are present with right isomerism, each connects to the top corner
of its atrium. This can make it difficult to divert the vein to the
right-sided atrium, should this be required, or because of its short
length even to the right superior caval vein.50 In
left isomerism, in contrast, both superior caval veins are connected to
atrial chambers in the fashion of drainage through a coronary sinus,
although often unroofed.2,7,10,11 The hepatic venous
connections are of particular concern in this setting. The veins usually
connect to one atrium, but sometimes to both atriums, or to both sides
of a common atrium (Figures 7 and 8).5,7,9-13 Such
direct hepatic venous connections are not only found when the inferior
caval vein is interrupted, but also when it connects directly to one or
other atrial chamber. When performing the Kawashima procedure, search
should be made for any interconnections between the inferior caval vein
and the hepatic veins below their entry to the
heart.5,7,9-13,43-47 Preoperative demonstration of
such venous arrangements can facilitate construction of fenestrated
Fontan pathways (Figure 9).44-46
Although not always recognised, totally anomalous pulmonary venous
connection is universal in the setting of right isomerism, since both
atrial appendages are morphologically right. Even should all pulmonary
veins connect directly to one of the atrial chambers, such connections
will be anatomically anomalous.2,3 The cardiac venous
return is often obstructed. 2-5,14-22 The presence of
pectinate muscles all around the muscular atrioventricular vestibules
creates additional problems in producing an unobstructed pulmonary
venous pathway.2-15,19-22 Pulmonary venous obstruction
is the more frequent when the pulmonary arteries are hypoplastic,
atretic, or discontinuous.2-15,19-22 Severe
obstruction to pulmonary venous flow may mask the clinical importance of
the pulmonary venous anomaly.2-22 On our institute, we
continue to perform adjustable ligation of the vertical vein, routine
left atrial augmentation, and interatrial septal fenestration when we
recognise obstructive totally anomalous pulmonary venous connection, or
when we encounter suprasystemic pulmonary arterial hypertension
subsequent to weaning from bypass.48,49 It is almost
certainly the intrinsic anatomic and histopathologic differences in the
pulmonary veins of individuals with right isomerism that accounts for
the higher incidence of reoperation when compared to individuals with
lateralized atrial appendages.14,15,20,21,48,49,61-63
The pulmonary venous connections will always be anatomically normal when
there is left isomerism, but such normally connected veins can be
bilaterally symmetrically, with two connecting to each atrium with a
morphologically left appendage.2-5,11 This produces a
significant distance between the right and left-sided pulmonary
veins,5,9-13 making for elongated and complex suture
lines for any potential intraatrial baffle.14-22 The
complexity of the required baffles can explain in part the reported
incidence of postoperative arrhythmias, which has ranged between less
than one-tenth to one-half. Also contributing to these problems are the
absence, hypoplasia, or abnormal location of sinus node, along with
abnormalities in the pathways for atrioventricular
conduction.14-21,59,60
Another well recognized problem in the setting of left isomerism is the
development of pulmonary arteriovenous malformations following the
Kawashima operation, seen in up to three-fifths of cases (Figure
10).45,46,64 This is attributed to isolation of the
lungs from exposure to hypothetical hepatic factors. Redirection of the
hepatic venous return to the systemic venous circuit during completion
of the Fontan circulation has been shown to result in subsequent
resolution.45,46,64
Within the overall group, regurgitation of the common atrioventricular
valve is a known risk factor for postoperative mortality, particularly
after functionally univentricular repair. A systemic-to-pulmonary
arterial shunt is known to cause volume overload, aggravating the
regurgitation, and increasing mortality following completion of the
Fontan circulation. 15,21,43-47 This has led to
suggestions that the common valve be repaired or replaced before
completion of the operations.41,45-47 Cardiac
transplantation also represents a significant technical challenge. This
may reflect previous palliative procedures, but is further exacerbated
by the complex systemic and pulmonary venous
anomalies.24,25 Even though the technical issues have
largely been resolved, transplantation in the setting of isomerism
remains associated with increased postoperative complications. Early and
late survival is poor compared to other forms of congenital cardiac
diseases.24,25,51-53