Acute Myocarditis

Any inflammatory process affecting the heart muscle is referred to as myocarditis . In most cases myocarditis results from an infection, although there are other causes as well. Involvement of the myocardium occurs commonly in some acute infectious diseases, especially those caused by viruses; but owing to the benign nature of myocarditis, it routinely goes unrecognized.

Many viral childhood diseases—mumps, measles, poliomyelitis—may lead to myocarditis. As a rule, they do not produce any symptoms, and hence the myocarditis cannot be recognized on physical examination; its presence can only be detected by special tests. Since it usually has no significant sequelae, testing for myocarditis is not indicated. However, in rare cases myocarditis may

lead to heart failure and even death. It is believed that myocarditis is principally responsible for the rare deaths from influenza. In the past, when diphtheria was common, myocarditis due to a toxin of the diphtheria bacillus was a dreaded complication of that disease. Myocarditis may coexist with viral pericarditis, to be discussed later in this chapter. It can also be caused by some drugs and by radiation therapy. As a part of the hypersensitivity mechanism, myocarditis is often a manifestation of diseases related to allergic reaction, such as acute rheumatic fever. Myocardial involvement occurs occasionally in infectious mononucleosis, viral hepatitis, and Lyme disease.

Even in those rare cases where heart failure results from acute myocarditis, complete recovery is the commonest outcome. Occasionally viral myocarditis may initiate dilated cardiomyopathy (see below). Heart failure caused by myocarditis may persist beyond the acute stage, turning into dilated cardiomyopathy. More often, however, there is an apparent recovery from acute myocarditis, with cardiomyopathy developing gradually months or years later. The possibility that some cases of dilated cardiomyopathy of unknown cause may also be the consequence of past, unrecognized myocarditis has been suggested, though not proved.

Clinically significant acute myocarditis presents itself usually by the onset of dyspnea. Signs of heart failure appear without obvious cause but are often associated with a fever. Diagnostic studies may show electrocardiographic abnormalities, which are particularly significant if day-to-day changes are observed. Chest X ray may reveal enlargement of the heart shadow. Echocardiography and nuclide ventriculogram are the definitive tests for determining the degree to which ventricular function is impaired.

Because of the self-limiting nature of myocarditis, in most cases no treatment is necessary. If a contributive allergic factor is suspected, the use of immunosuppressive drugs, including corticosteroids, may be justified.