Myocarditis: aetiology and histopathological diagnosis

Although the aetiology of myocarditis in a given patient often remains unknown, a large variety of infectious agents, including viruses, bacteria, protozoa, and fungi, systemic and autoimmune diseases, drugs, and toxins can cause myocarditis. The diagnosis of myocarditis is complex and challenging. The diagnostic gold standard is endomyocardial biopsy in which myocarditis is defined using the histological Dallas criteria and immunohistochemistry. Current diagnostic criteria require more than 14 infiltrating lymphocytes/mm2 (up to 4 macrophages may be included with the presence of CD3-positive T lymphocytes ≥7 cells/mm2). Inflammatory infiltrates are subdivided into lymphocytic, eosinophilic, polymorphic, giant cell, and cardiac sarcoidosis as these diagnoses imply a specific treatment and prognosis. Molecular biological analyses of cardiac tissues should include real-time polymerase chain reaction (PCR) and reverse transcription PCR assays for the most common cardiotropic infectious viruses such as enteroviruses, adenoviruses, cytomegalovirus, influenza viruses, human herpes virus 6, Epstein–Barr virus, hepatitis C virus, and parvovirus B19.