Conduction tissue changes associated with enlarged membranous septum--a cause of sudden death?

We have studied four hearts in which enlargement of the ventricular membranous septum was associated with histological abnormalities of the atrioventricular conduction tissues. Two of the cases showed unusually formed penetrating and non-branching main atrioventricular bundles. These two cases plus one other also showed right-sided penetrating and main atrioventricular bundles. In addition these three cases all showed extensive fibre loss affecting the origin of the left bundle, which in one heart was further accompanied by fibre loss in the main bundle. The conduction tissue damage in the fourth case, though more extensive, was not as clearly related to the enlarged membranous septum since the heart in this case was also enlarged and dilated (congestive cardiomyopathy). Our findings, while obviously not conclusive, suggest that conduction changes in the presence of an enlarged membranous septum could be added to the list of causes of sudden cardiac death. They certainly indicate that when an enlarged membranous septum is found in cases of sudden death, histological study of the conduction system is desirable.