Congenital Aneurysm of the Muscular Interventricular Septum Associated With Bifascicular Block.

Isolated congenital aneurysm of the muscular interventricular septum is rare. We present a patient with congenital aneurysm of the basal muscular ventricular septum, who also develops conduction abnormalities with first-degree heart block, right bundle branch block, and left posterior fascicular block. The case details the natural history of the aneurysm over a 10-year period follow-up during which the patient remained asymptomatic with evidence of regression of the aneurysm. Given the aneurysm's location close to the proximal right bundle and left posterior fascicle, we believe that the cause for the aneurysm also injured both fascicles resulting in bifascicular block. The diagnosis of bifascicular block was confirmed using the electrocardiogram-derived vectorcardiography loops. These conduction abnormalities have remained stable. The case illustrates the utility of vectorcardiography in diagnosing bundle branch conduction defects. The case also illustrates the importance of anatomical considerations when encountering congenital heart defects.