Myocarditis is the term used to describe acute infective, autoimmune or toxic inflammation of the heart muscle (also known as myocardium). Most cases of myocarditis with onset in otherwise healthy individuals usually arise as a result of the infection by common viruses, although precise pathogenesis is not always easily determined. Symptoms can range from mild dyspnea and chest pain to cardiogenic shock and death.
The prognosis and treatment of myocarditis depends on the etiology, while clinical and hemodynamic data can provide guidance to decide when the patient needs to be referred to a specialist for endomyocardial biopsy. Important prognostic variables include the presence of heart block, degree of left and right ventricular dysfunction, and specific histopathological forms of myocarditis.
Even though a broad spectrum of etiologies has been implicated as causes of myocarditis, viral myocarditis still represents the prototype for the study of this disease and its evolution. Enteroviruses (namely Coxsackie group B serotypes) have traditionally been perceived as the predominant viral cause, but the development of new molecular techniques resulted in recognition of parvovirus B19 and human herpesvirus 6 as main viral causes of myocarditis.
Hepatitis C virus has also been associated with myocarditis, as HCV antibodies and RNA have been found in sera and cardiac tissue of patients with biopsy-proven myocarditis. Furthermore, in patients with human immunodeficiency virus (HIV) infection myocarditis is considered the strongest predictor of death among a large cardiomyopathy population, and is observed in more than 50% of performed autopsies.
Myocarditis can also be triggered by different non-viral infections, which should certainly be considered in patients with acute or chronic cardiomyopathy. The condition can often arise from the infection with Borrelia burgdorferi (a cause of Lyme disease), and it must be noted that patients with myocarditis due to Lyme disease are sporadically co-infected with babesia or erlichia. In areas of rural Central and South America, Trypanosoma cruziinfections can manifest as acute myocarditis.
Drug-induced hypersensitivity and systemic hypereosinophilic syndromes may result in a specific myocarditis; drugs implicated in this condition are antibiotics (namely penicillins, cephalosporins and sulfonamides), anticonvulsants, antipsychotics and antidepressants. Smallpox vaccination has also recently been recognized as a causative factor of myopericarditis.
Cardiac sarcoidosis and giant cell myocarditis, which are two idiopathic and histologically similar disorders, represent rare causes of inflammatory myocardial disease. Autoimmune diseases that are associated with myocarditis include Whipple’s disease, celiac disease, systemic lupus erythematosus, mixed connective tissue disease, systemic sclerosis and certain hematological abnormalities (i.e. thrombocytopenic purpura).
Three essential pathways have been implicated in the pathogenesis of myocarditis. Direct myocardial invasion by cardiotropic virus or other causes of the disease swiftly progresses to a second phase of immunologic activation. The activation of CD4+ T helper cells induces clonal expansion of B cells, resulting in additional myocytolysis, further local inflammation, as well as the production of circulating anti-heart antibodies.
Acute myocarditis is often initially diagnosed as nonischemic dilated cardiomyopathy in a patient with symptoms present for a few weeks to several months. However, clinical manifestations range from asymptomatic ECG abnormalities to devastating illness with cardiogenic shock, with the possible occurrence of atrial or ventricular arrhythmias, heart block, or an acute myocardial infarction–like syndrome.
Transient ECG abnormalities are commonly observed during community viral epidemics, and most patients remain entirely without any symptoms. Affected individuals sometimes report a typical viral prodrome that consists of fever, respiratory symptoms, myalgias (muscle pain) and occasional gastroenteritis, but these symptoms are highly variable. On the other hand, myocarditis can manifest as fulminant heart failure with an abrupt onset of hemodynamic collapse.
The true incidence of myocarditis in the community remains unknown, and the disease a complex and challenging diagnosis in cardiology. No large population-based studies defining the presenting symptoms of acute myocarditis are available, in part due to the absence of sensitive, non-invasive diagnostic tests. As the relationship between clinical findings and the presence of acute myocarditis remains vague, a very sensitive and specific diagnostic procedure is warranted.