Idiopathic acute myocarditis with complete atrioventricular block in a baby. Clinicopathological study of the atrioventricular conduction system.

A 17-month-old Japanese girl with an idiopathic acute myocarditis had symptoms of vomiting, slight fever, and liver enlargement, but no edema. Clinical diagnosis of acute myocarditis was not made until she had Stokes-Adams syndrome and electrocardiogram revealed complete atrioventricular block on the day of death. At autopsy, idiopathic acute myocarditis was detected diffusely in the right and left ventricles. Vomiting and liver enlargement were due to congestive heart failure. Serial sections of the atrioventricular conduction system revealed diffuse and severe acute inflammatory changes in the right bundle and the left bundle branches, especially in the terminal portions. Acute inflammation was focally noted in the atrioventricular node and the His bundle. The complete atrioventricular block probably followed the severe acute inflammation of the bundle branches. Our case suggest that idiopathic acute myocarditis may be underdiagnosed in babies, as there is no way to determine whether there is dyspnea and palpitation on exertion, and idiopathic fibrosis of conduction system with or without conduction disturbances in children and adults may be sequelae of healed myocarditis in babies.