Objectives: Our goal was to compare histologically the mechanisms producing congenital complete heart block in normally structured hearts and in hearts with isomerism of the atrial appendages.
Background: It is known that several different histologic patterns can underscore the existence of congenital complete heart block in the normally structured heart, and that block is particularly frequent in the setting of isomerism of the atrial appendages. The histologic findings in the latter setting were compared and contrasted with those found in the normally structured heart.
Methods: Serial section techniques were used to study 14 hearts with isomerism of the atrial appendage (12 with left isomerism and 2 with right isomerism) and 7 normally structured hearts.
Results: Discontinuity between the atrioventricular (AV) node and the ventricular conduction tissues was found in 10 of the 12 hearts with left isomerism; the other 2 hearts had a normally formed conduction axis and heart block was not present in these cases. In both hearts with right isomerism, "slings" of ventricular conduction tissue connected dual AV nodes; congenital complete heart block was not present in either case. In six of the seven cases with a normally structured heart, anti-Ro antibodies had been found in the maternal serum. All six of these hearts had discontinuity between the atrial tissues and the ventricular conduction axis. Intraventricular discontinuity was found in the seventh case, in which anti-Ro antibodies were not found in the mother.
Conclusions: The pattern of congenital complete heart block in cases with left isomerism is discontinuity between the AV node and the conduction axis, in contrast to the pattern of atrial-axis discontinuity produced in the normally structured heart when anti-Ro antibodies are found in the maternal serum.