Causes: The causes of myocarditis are numerous and can be roughly divided into infectious, toxic, and immunologic etiologies, with viral etiologies most common in North America.
* Amongst the infectious causes, viral acute myocarditis is by far the most common.
o Identification of the coxsackie-adenovirus receptor protein explains the prevalence of these viruses as the causative agents in more than one half of cases. The receptor is the common target of the coxsackievirus B of the enterovirus family and serotypes 2 and 5 of the adenovirus family.
o Other viruses implicated in myocarditis include influenza virus, echovirus, herpes simplex virus, varicella-zoster virus, hepatitis, Epstein-Barr virus, and cytomegalovirus. Hepatitis C, in particular, is becoming a major focus of research.
o Human immunodeficiency virus (HIV) deserves special mention because it seems to function differently than other viruses. Although some evidence indicates that HIV directly invades myocytes, HIV genomes can be amplified from patients without histologic signs of inflammation. In addition, in patients who are infected with HIV, T-cell–mediated immune suppression increases the risk of contracting myocarditis due to other infectious causes, and this has been shown in myocardial biopsy samples.
* Nonviral infectious causes are numerous and varied. Worldwide the most common bacterial cause is diphtheria, and, in South America, the protozoal Chagas disease is a common entity. Streptococcal and staphylococcal species and Bartonella, Brucella, Leptospira, and Salmonella species can spread to the myocardium as a consequence of severe cases of endocarditis. Borrelia burgdorferi, the spirochete agent in Lyme disease, is also a known cause of myocarditis. Parasitic myocarditis from trypanosomiasis; trichinosis; and, in the immunocompromised host, toxoplasmosis have been identified.
* Toxic myocarditis has a number of etiologies including both medical agents and environmental agents.
o Among the most common drugs that cause hypersensitivity reactions are penicillin, ampicillin, hydrochlorothiazide, methyldopa, and sulfonamide drugs. This syndrome is associated with peripheral eosinophilia, fever, and rash in patients who have biopsy findings of an eosinophilic infiltrate of the myocardium.
o Numerous medications (eg, lithium, doxorubicin, cocaine, numerous catecholamines, acetaminophen) may exert a direct cytotoxic effect on the heart. Zidovudine (AZT) has been associated with myocarditis.
o Environmental toxins include lead, arsenic, and carbon monoxide.
o Wasp, scorpion, and spider stings
o Radiation therapy may cause a myocarditis with the development of a dilated cardiomyopathy.
* Immunologic etiologies of myocarditis encompass a number of clinical syndromes and include the following:
o Connective tissue disorders such as systemic lupus erythematosus (SLE), rheumatoid arthritis, and dermatomyositis that can often result in a dismal prognosis
o Idiopathic inflammatory and infiltrative disorders such as Kawasaki disease, sarcoidosis, and giant cell arteritis
* Rejection of the posttransplant heart may present as inflammatory myocarditis.