From that paper I posted Symptoms: Myocarditis can manifest like a myocardial infarction with sudden-onset angina pectoris, arrhythmias, and/or heart failure developing within days. Most patients with myocarditis initially have such non-specific symptoms that these are often categorized in the context of the preceding infection and not as being of cardiac origin. Diagnosis: Electrocardiography results and laboratory chemical findings that are characteristic of acute myocarditis—such as the changes to the ST segment and raised cardiac enzymes that are typical for acute myocardial involvement. As the pathophysiological changes of infectious and non-infectious myocarditis occur at a cellular and subcellular level, confirmation of a specific pathogen or inflammation requires a direct examination of myocardial tissue, which can be obtained without problems by means of a biopsy.