The Intracardiac Anatomy of Left Isomerism
It is again the extent of the pectinate muscles relative to the
atrioventricular junctions that serves as the distinguishing feature of
the morphologically left appendage. In left isomerism, there is always a
smooth vestibule between the margin of the appendage and the cardiac
crux. Since both atriums will have a morphologically left appendage, the
pulmonary venous connections are morphologically normal, albeit that
symmetrical drainage, with two pulmonary veins connecting to an atrium
on each side, is frequent. When all the pulmonary veins are connected to
one of other of the atrial cavities, it is frequent to find quasi-usual
or quasi-mirror-imaged venous drainage. It is the rule to find anomalies
in the systemic venoatrial connections, most usually interruption of the
inferior caval vein, with continuation through the azygos venous system
(Figures 6 and 7). The connection can then be to either a right-sided or
a left-sided superior caval vein. Such interruption can also be found
with usual atrial arrangement, even when there are multiple spleens and
left bronchial isomerism.16 The intracardiac anatomy
is then usually normal.
Bilateral superior caval veins are frequent, but in the form of a
coronary sinus, usually with one or other draining through a coronary
sinus. When there is a coronary sinus, it receives the coronary venous
drainage. When the coronary sinus is absent, the coronary veins connect
directly to the atrial chambers.
The atrial septum is much better formed in left as opposed to right
isomerism. Biventricular and mixed atrioventricular connections are also
more frequent than in right isomerism, usually with a common
atrioventricular junction, as are concordant ventriculo-arterial
connections. It is much more frequent with left isomerism, therefore, to
find patterns producing relatively normal circulations. If obstruction
is found within the ventricular outflow tracts, it is usually the left
ventricle that is involved. Because of the isomeric left atrial
appendages, the sinus node is always abnormal, typically being
hypoplastic and displaced towards the atrioventricular
junctions.37