This is due to a dense interstitial infiltrate of inflammatory cells. The normal appearing red myocardium elsewhere could also show microscopic infiltrates, which are not dense enough to see grossly. Cardiotropic organisms, mostly viral, are blood borne. Inflammatory response is to the organisms and necrotic muscle. Postinfectious immunogenic cases may be due to complexing of antigenic products of organisms with tissues, or cross reacting determinants common to both organism and host tissues.
Epidemiology varies with cause.
Heart may only be dilated and flabby with no obvious myocardial visible changes. Dense interstitial inflammatory response may be seen as a gray diffuse or focal discoloration of the myocardium. Valves not affected. Mural thrombi may result. Cellular infiltrate is usually mononuclear with dominance of lymphocytes.