Definition and Etiology:

• Myocarditis is defined as inflammation of the heart muscle and can be caused by viral infections, autoimmune and systemic diseases, adverse drug reactions, and other etiologic agents.
• The most common cause of myocarditis is viral infections, with no standardized criteria for diagnosis or specific therapy being widely agreed-upon.

Clinical Presentation and Diagnosis:

• The clinical picture of myocarditis varies widely, from asymptomatic patients to critically ill patients with heart failure and cardiogenic shock. There are no widely agreed-upon criteria for making the diagnosis of myocarditis.
• On pathologic examination, myocarditis is usually apparent as infiltration of myocardium with lymphocytes, macrophages, and fibroblasts, accompanied by myocyte necrosis.

Incidence and Common Etiologic Agents:

• The global incidence of myocarditis is estimated at 22 cases/100,000 annually, with viral infections being the most common cause. The identification of viral etiologies may vary by geography and over time.
• In North America and Europe, the most common etiologic agents include Coxsackie B enterovirus, human herpesvirus 6, cytomegalovirus, and parvovirus B19.

Other Causes and Associations:

• Other causes of myocarditis include infectious illnesses such as Lyme disease, acute rheumatic fever, and diphtheria, as well as systemic and autoimmune diseases. Myocarditis can also be associated with infiltrative cardiomyopathies and specific forms like hypersensitivity or eosinophilic myocarditis.
• Chagas disease is the most common cause of myocarditis in Central and South America, and myocarditis can also be associated with doxorubicin cardiomyopathy or peripartum cardiomyopathy.

Pathogenesis and Pathological Features:

• On pathologic examination, myocarditis is usually apparent as infiltration of myocardium with lymphocytes, macrophages, and fibroblasts, accompanied by myocyte necrosis.
• Lymphocytic myocarditis is the most common type of myocarditis, and other types involve giant cells, eosinophils, or granulomas and can be associated with specific clinical conditions.

Associated Diseases and Conditions:

• Myocarditis can be associated with autoimmune and systemic diseases such as systemic lupus erythematosus, polymyositis, scleroderma, and sarcoidosis. Other specific forms include hypersensitivity or eosinophilic myocarditis and giant cell myocarditis.
• Myocarditis can also be associated with doxorubicin cardiomyopathy or peripartum cardiomyopathy.

Management and Therapy:

• There is controversy regarding the most appropriate medical therapy for myocarditis, and general management includes heart failure management and immunosuppressive therapy.