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Myocarditis is defined as inflammation of the myocardium on histologic examination. Cardiovascular complications may be significant and include myocardial dysfunction, arrhythmias, conduction abnormalities, and cardiac arrest.
- True incidence of acute myocarditis is difficult to estimate because of the wide range in clinical severity, various etiologies, and underdiagnosis.
- More than 50% of pediatric cases seen are in infants <1 year of age.
- Viral myocarditis has a seasonal distribution, which varies according to the viral species.
- Exposure to infectious agents, drugs, toxins, and systemic diseases
- Drug exposure
- Autoimmune disease
- Systemic disease
- Pathophysiology of myocarditis may vary based on cause (see “Etiology”).
- Viral myocarditis is best characterized and involves a complex interaction among the virus, host immune response, and environmental factors. Three stages include (1) viral injury and innate immune response, (2) acquired host immune response, and (3) recovery or chronic cardiomyopathy.
- Inflammatory response from innate and acquired immune response may result in significant damage to the myocardium and conduction system.
- Development of autoantibodies may also play a key role in acute and chronic myocardial damage.
- Virus may cause direct damage to the myocardium independent of inflammation, secondary to cleavage of structural proteins.
- Pathogenesis of nonviral myocarditis is poorly understood.
- Regardless of the cause, symptom severity increases with worsening ventricular function and/or with worsening arrhythmias.
- Fulminant myocarditis may be characterized by both severe systolic and diastolic dysfunction.
- Progressive left ventricular systolic dysfunction may lead to hypotension, acidosis, and end-organ dysfunction.
- Left ventricular diastolic dysfunction may result in elevated left ventricular end diastolic pressures, leading to pulmonary venous and arterial hypertension, with concomitant pulmonary edema and right-sided heart failure.
- Causes include infection, toxins, drugs, autoimmune disease, and systemic disease.
- Infectious causes include viral, bacterial, rickettsial, fungal, helminthic, spirochetal, and protozoal infections.
- Viral infection is the most common in developed countries including enteroviruses, erythroviruses, adenoviruses, and herpes viruses. Both RNA and DNA viruses have been implicated. Previously, the enteroviruses, specifically coxsackie B, were commonly seen. However, there has been a shift in the spectrum. Currently, parvovirus B19 is most commonly seen. There are growing reports of certain herpes viruses, specifically HHV6, becoming more prevalent.
- Nonviral infectious causes are far less common but must be considered especially in endemic areas, such as Central and South America where Chagas disease is prevalent.
- Nonviral myocarditis may be secondary to exposure to chemicals (arsenic and hydrocarbons), alcohol, radiation, drugs (chemotherapeutics), drug hypersensitivity, autoimmune disease such as systemic lupus erythematosus, or systemic disease such as Churg-Strauss or sarcoidosis.
- Giant cell myocarditis is a very rare form of myocarditis in children that is associated with autoimmune disease and drug hypersensitivity. These patients respond poorly to typical care and frequently require cardiac transplantation.