Myocarditis is an inflammatory disorder of the myocardium with necrosis of the myocytes and associated inflammatory infiltrate. It is usually caused by infection with a virus, particularly adenovirus or enterovirus (e.g., Coxsackievirus). These viruses typically cause other less serious diseases, such as gastrointestinal problems and the common cold. Although the utility of myocardial biopsy is debated, suspected myocarditis can be classified based on pathologic findings as defined in the Dallas or Marburg Criteria.
Investigations of the role of viruses in the development of myocarditis, DCM and ARVD/C are ongoing in the Phoebe Willingham Muzzy Pediatric Molecular Cardiology Laboratory, while viral diagnosis is provided by the John Welsh Cardiovascular Diagnostic Laboratory.
Dallas Classification (1987)
- Myocarditis: Myocardial necrosis, degeneration, or both, in the absence of significant coronary artery disease with adjacent inflammatory infiltrate with or without fibrosis
- Borderline myocarditis: Inflammatory infiltrate too sparse or myocyte damage not apparent
- No myocarditis
- Ongoing (persistent) myocarditis with or without fibrosis
- Resolving (healing) myocarditis with or without fibrosis
- Resolved (healed) myocarditis with or without fibrosis
WHO Marburg Criteria (1996)
Acute (active) myocarditis: A clear-cut infiltrate (diffuse, focal or confluent) of >14 leukocytes/mm² (preferably activated T-cells). The amount of the infiltrate should be quantitated by immunohistochemistry. Necrosis or degeneration are compulsory, fibrosis may be absent or present and should be graded.
Chronic myocarditis: An infiltrate of >14 leukocytes/mm² (diffuse, focal or confluent, preferably activated T-cells). Quantification should be made by immunohistochemistry. Necrosis or degeneration are usually not evident, fibrosis may be absent or present and should be graded.
No myocarditis: No infiltrating cells or <14 leukocytes/mm².
- Ongoing (persistent) myocarditis: Criteria as in 1 or 2 (features of an acute or chronic myocarditis).
- Resolving (healing) myocarditis: Criteria as in 1 or 2 but the immunological process is sparser than in the first biopsy.
- Resolved (healed) myocarditis: Corresponds to the Dallas classification.
Causes of Myocarditis
Most cases of myocarditis are believed to be caused by virus infection, either directly, due to virus replication, or indirectly, due to autoimmune disease. Occasionally, myocarditis may be due to drug hypersensitivity or toxicity.
The body's own immune system is responsible for defending it against all foreign invaders for example viruses and bacteria. However, sometimes, this system malfunctions, and starts to attack the body's own tissues - this results in a so-called auto-immune disease. It is thought that this process occurs in some cases of myocarditis.
Myocarditis also can be caused by alcohol, radiation, chemicals (hydrocarbons and arsenic), and drugs.