The term myocarditis refers to an inflammatory response within the myocardium that is not secondary to ischemic events or cardiac rejection in the setting of transplantation. The presence of myocyte necrosis is required for certain types of myocarditis — specifically, lymphocytic myocarditis that is triggered by viruses and augmented by autoimmunity — and the myocyte damage is believed to be mediated both by direct invasion of the myocardium and by immune insult.
Histologically, some overlap exists among categories of myocarditis, and no finding is specific for a single etiology (see Microscopic Findings). In general, the histologic patterns can be divided into the following categories:
Lymphocytic (including viral and autoimmune forms)
Eosinophilic (of which hypersensitivity myocarditis is the most common type, followed by hypereosinophilic syndrome)
Granulomatous (cardiac sarcoidosis [CS] and giant-cell myocarditis [GCM])
Neutrophilic (bacterial, fungal, and early forms of viral myocarditis)
Reperfusion type/contraction band necrosis (present in catecholamine-induced injury and reperfusion injury)
Pathologic classification is complicated, however, because tissue biopsies often are not obtained, and a presumptive clinical diagnosis must sometimes be confirmed by means of indirect methods such as serology.