Subaortic stenosis and sudden death.

An obese white boy, 16 years old, collapsed and suddenly died while walking with friends. Since early childhood he had been known to have mild subaortic stenosis. At age 6 he exhibited first-degree heart block during an electrocardiographic exercise test. Neither then nor thereafter was he known to have any unusual symptoms until his death. Postmortem examination revealed marked cardiac enlargement without asymmetry or much dilatation. There was mild subaortic stenosis, but the heart was otherwise grossly normal. Special studies of his cardiac conduction system demonstrated fibrotic obliteration of the His bundle and proximal portions of both bundle branches. The same region had a much thickened central fibrous body from which the subaortic stenosis ridge protruded. Small arteries in the vicinity of the atrioventricular node were markedly narrowed. Because those vessels provide some of the blood supply to the His bundle region, their narrowing may have contributed to the fibrotic abnormalities observed. The extent of destruction of the His bundle makes a lethal failure of atrioventricular conduction the most likely terminal event.