Myocarditis

**Definition:** Myocarditis (inflammatory cardiomyopathy) is inflammation of the heart muscle generally in the presence of a dilated cardiomyopathy that results from exposure to either discrete infectious external antigens such as viruses, bacteria, fungal or parasites; non-infectious external antigens such as hypersensitivity to drugs; or internal non-infectious triggers such as autoimmune or hypersensitive activation against self-antigens.

**Long definition:** The classic Dallas criteria for the pathological diagnosis of myocarditis require the presence of inflammatory cells simultaneously with evidence of myocyte necrosis on the same microscopic section when examining a myocardial biopsy. Borderline myocarditis is characterised by inflammatory cell infiltrate without myocardial necrosis. A negative biopsy does not necessarily rule out myocarditis.