Atrioventricular block secondary to a congenital aneurysm of the membranous septum. Histological examination of conduction system

Atrioventricular block which caused death in a 6-month-old baby is reported. Necropsy examination revealed the only abnormality in the heart to be an aneurysm of the membranous septum. Histological examination of the conduction system showed fibrosis and attenuation of the bundle of His at the lower border of the aneurysm.