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Myocarditis is a term used to describe an inflammatory infiltrative process within the muscular walls of the heart leading to degeneration and necrosis of cardiac myocytes. It is now understood that a spectrum of disease with overlapping clinical and histological characteristics exists, beginning with myocarditis and progressing to dilated cardiomyopathy. The clinical presentation ranges from the asymptomatic patient with mild ventricular dysfunction, ECG changes, and a self-resolving process to the patient with fulminant heart failure leading to dilated cardiomyopathy. Pericarditis, inflammation of the pericardium, may occur in isolation or with myocarditis and often presents in a similar fashion.

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The most common causes of myocarditis and pericarditis are infectious diseases, specifically viral etiologies, though specific diagnosis is achieved in less than half of cases.1 Therefore, myocarditis is most often deemed idiopathic. While the majority of cases are sporadic, there are reports of epidemics. Other causes of myocardial or pericardial inflammation include immune-mediated conditions, toxins, and medication side effects. This chapter focuses on the infectious etiologies of myocarditis and pericarditis, diagnosis, and disease management.

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Pathogenesis

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Current understanding of disease pathogenesis in myocarditis has come in large part from murine models. Three overlapping stages of disease have been described: (1) direct myocardial invasion by a cardiotropic triggering agent (usually thought to be viral); (2) immunologic activation; and (3) ongoing inflammation, circulation of antiheart antibodies and abnormal ventricular remodeling.2

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In acute viral myocarditis, data suggest that cardiotropic viral RNA (ribonucleic acid) enters the myocytes through endocytosis and produces viral protein that activates an immune cascade in the host. Inflammatory cellular infiltration with macrophages and natural killer cells enhances expression of inflammatory cytokines, specifically interleukin IL-1, IL-2, tumor necrosis factor (TNF), and interferon-γ,3,4 resulting in further inflammatory cell recruitment. Cytokines activate inducible nitric oxide synthase in cardiac myocytes.5 Nitric oxide has been shown to play an important role both in inhibiting viral replication and in producing intense myocardial inflammation.6,7 In addition, circulating autoantibodies directed against cardiac contractile, structural, and mitochondrial proteins have been detected in cardiac biopsy specimens in both humans and mice with myocarditis.8 Removal of autoantibodies by immunoabsorption techniques seems to improve cardiac function and decrease inflammation.911

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It is therefore deduced that a normal host immune response facilitates clearance of infectious agents. However, with immunologic imbalance, infectious agents may persist in the myocardium leading to ongoing immune-mediated myocyte destruction and myocardial injury. Detection of viral RNA in autopsy specimens of patients with dilated cardiomyopathy has supported the theory that persistence of viruses in the myocardium is capable of inducing ongoing myocardial injury resulting in acute or chronic dilated cardiomyopathy.

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Clinical Presentation

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History and Physical Examination

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With differences in age and overall immune status, pediatric myocarditis may have variable clinical presentations. Many cases of myocarditis are suspected to be subclinical without ...

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