ETIOLOGY

The majority of cases of idiopathic lymphocytic myocarditis (ILM) in the United States and Western Europe are thought to be viral in origin (Table 38.1) but there is considerable etiologic variation depending on the geographic location, season, and age of the host. The most commonly associated viruses are enteroviruses, particularly coxsackie B viruses. In research studies, enteroviral genomic sequences have been detected in approximately 25% of endomyocardial biopsies (EMB) from patients with either active myocarditis or dilated cardiomyopathy. Adenovirus and parvovirus are also common, particularly in children. Hepatitis C virus (HCV), echovirus, influenza virus, human immunodeficiency virus (HIV), and herpesviruses such as cytomegalovirus (CMV), Epstein–Barr virus (EBV), herpes simplex virus (HSV), and varicella zoster virus (VZV) also are among the more frequent viral etiologies. The myocardium sometimes demonstrates more than one virus by polymerase chain reaction (PCR) exam, but concurrent or past viral infection unrelated to the current myocarditis may be the cause of such findings. In the 2003 U.S. government program in which smallpox (vaccinia) vaccine was administered to approximately 38 000 potential first responders during a bioterrorism event, the incidence of myo/pericarditis was 5.5 per 10 000 vaccinees. Nonviral infectious causes such as Corynebacterium diphtheriae (diphtheria), Borrelia burgdorferi (Lyme disease), and American trypanosomiasis (Chagas disease) are found in the appropriate epidemiologic setting.