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