This chapter reviews inflammatory heart diseases; their infectious, autoimmune/immune-mediated, and toxic etiology; diagnostic algorithms; pathologic characteristics; clinical manifestations; and management (see Fuster and Hurst’s Central Illustration). The most common etiology of infectious myocarditis is viral; immune-mediated myocarditis is often related to underlying systemic autoimmune diseases, and the most common toxic causes include drugs. The term myocarditis is the key descriptor of myocardial inflammation; it does not specify the immune phenotype of inflammatory cells or the cause. The diagnostic process involves suspicion based on clinical presentation of recent onset of symptoms in previously healthy individuals with exposure to infectious or toxic agents, detection of suggestive imaging abnormalities, and demonstration of the histological evidence of myocardial inflammation with an effort to define etiologic basis. Treatment aims at resolution of the cause, when known, and at the management of cardiac manifestations, regardless of the cause, according to guidelines addressing the clinical phenotype. In most cases, acute myocarditis undergoes spontaneous resolution, but it can evolve toward chronic inflammatory disease with the most common phenotype mimicking dilated-hypokinetic cardiomyopathy. Recent reports have suggested that in some of the viral infections, the viral genome may be chromosomally incorporated and eventually transmitted. Finally, more eventful outcomes are observed when inflammation occurs in patients genetically predisposed to develop dilated cardiomyopathy.
The term myocarditis describes the presence of inflammatory cells in the myocardium. This descriptive diagnosis encompasses a wide spectrum of etiologically different diseases that share, as a unique criterion, the pathologically proven evidence of myocardial inflammation.1 The definition does not specify the inflammatory burden, the immunophenotype of inflammatory cells, and the cause. In clinics, the formulation of a diagnosis of myocarditis implies that the observed cardiac phenotype is attributable to the inflammation of the myocardium.
The Historical Definition
In the 1980s, the World Health Organization and the International Society and Federation of Cardiology defined myocarditis as an inflammatory disease of the myocardium wherein the diagnosis is established by histologic, immunologic, and immunohistochemical criteria. Idiopathic, autoimmune, and infectious forms of inflammatory cardiomyopathy were recognized.2 Whereas the Dallas criteria3 insisted on the demonstration of mononuclear cellular infiltration accompanying a demonstrable and ongoing myocyte damage, the definition of myocarditis recently updated by the European Society of Cardiology (ESC) working group on myocardial and pericardial diseases requires the presence of ≥14 lymphocytes/mm2 including CD3+ T lymphocytes ≥7 cells/mm2 but no more than 4 monocytes/mm.3,4 These descriptive definitions did not include the etiologic basis of myocardial involvement and clinical information.
The Heterogeneous Etiology
Myocarditis can occur in immunocompetent and immunodeficient hosts and be infectious, noninfectious (autoimmune, immune-mediated, or auto-inflammatory), and toxic (Table 46–1). The causes of infectious myocarditis include DNA and RNA viruses, bacteria, fungi, protozoa, and helminths. ...