The clinical manifestations of acute myocarditis may range from none to acute fulminating congestive heart failure to sudden death.28 The cases of interest to the forensic pathologist are those involving individuals who are asymptomatic or have had only minor complaints and then suddenly collapse and die.

Myocarditis can be caused by infectious agents (bacterial, rickettsial, viral, protozoal, fungal), connective tissue diseases (e.g., rheumatic fever, rheumatoid arthritis), physical agents (chemical poisons, or drugs) or can be idiopathic. With infectious myocarditis, injury to the myocardium may be directly due to invasion by the organism or by toxin produced by the organism. Microscopically, there are patchy or diffuse areas of necrosis with interstitial inflammation. The inflammation may be focal and minor or extensive and severe. Degeneration and necrosis of muscle fibers is usually present. The infiltrate may vary from mostly neutrophils to lymphocytes, plasma cells, and esosinophils. Grossly, the appearance of the heart may be normal or pale and flabby with dilated chambers.

Most cases of infectious myocarditis are probably viral in origin. Initially, there is infiltration by neutrophils and lymphocytes accompanied by necrosis of muscle fibers. Subsequently, lymphocytes and macrophages predominate. There may or may not be subsequent interstitial fibrosis on healing. If fibrosis develops, it may be either minor or extensive and may or may not be associated with subsequent arrhythmias. In a case seen by one of the authors, a 17-year-old boy at age 13 had a documented episode of viral myocarditis. Following that, he developed arrhythmias with occasional episodes of ventricular extrasystoles and tachycardia. He was told never to engage in any strenuous activities. At age 17, while participating in a basketball game, he collapsed and died. For the 4 years following his episode of myocarditis, he had been followed by a cardiologist, who had documented his numerous episodes of arrhythmias. It was the expectation of the cardiologist that the heart would show extensive interstitial scarring. At autopsy, the heart appeared grossly normal. Multiple microscopic sections of the heart taken from all areas, including the conduction system, were completely unremarkable. This shows that a viral inflammation of the heart can cause injury to the conduction system of the heart that cannot be detected or evaluated microscopically. This condition is probably the cause of some instances of sudden death in which the autopsy findings are completely negative.

In occasional cases of fatal myocarditis, the myocarditis is not widespread, but consists of a single strategically located lesion. Thus, the case of a 32-year-old housewife found dead on the kitchen floor, whose autopsy and toxicological examination were completely negative. Microscopic sections of the heart showed a single focus of acute myocarditis in the conduction system. Death in this case was due to the unfortunate location of a relatively small lesion. This points out the importance of taking mul tiple sections of the heart, especially through the conduction system, in any complete examination of the heart for heart disease. It should be realized that a single small collection of mononuclear cells in the myocardium does not necessarily indicate that the individual is suffering from myocarditis. This is a "normal" finding in many hearts if one takes sufficient microscopic sections. In cases where myocardial disease is suspected, the authors recommend taking a minimum of six microscopic sections of myocardium for proper evaluation of the myocardium. At least one of these should involve the conduction system.

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