Myocarditis

Myocarditis could be caused by varieties of different disorders, many of which are infectious antigens. In Western countries, viral infections are by far the most common cause of myocarditis. Bacterial myocarditis is less common but should always be considered in patients with sepsis and ventricular dysfunction. The prevalence of bacterial myocarditis is difficult to estimate. The few published studies describe a post-mortem prevalence ranging from 0.2% to 1.5%. However, several of these studies included patients with bacterial myocarditis secondary to endocarditis.

Here I will be more focus on bacteria since we learned much more in bacteria. 

Bacterial myocarditis can develop by direct bacterial invasion or by toxins elaborated by the pathogen. The latter is a prominent feature of diphtheric and clostridial infections. Causes of ventricular dysfunction include: (1) myocardial inflammation and necrosis; (2) toxin-induced ventricular dysfunction; (3) sepsis-induced myocardial dysfunction; and (4) ventricular remodeling. Myocardial dysfunction is commonly associated with severe sepsis. Approximately 50%of patients with severe sepsis and septic shock can present with left ventricular systolic dysfunction.Right ventricular dysfunction is also common and usually follows the pattern of left ventricular dysfunction. The mechanisms involved in myocardial depression are most probably mediated by circulating cytokines such as tumor necrosis factor, interleukin-1 and interleukin-6. The subset of patients with sepsis-induced myocardial dysfunction that might have undiagnosed bacterial myocarditis is not known.

The definitive diagnosis of BM requires endomyocardial biopsy (EMB) or morphologically proven active myocarditis with evidence of bacterial invasion or positive tissue cultures. According to the Dallas criteria, active myocarditis is defined as”an inflammatory infiltrate of the myocardium with necrosis and/or degeneration of adjacent myocytes not typical of the ischemic damage associated with coronary heart disease.” EMB is highly specific for the diagnosis of myocarditis, but its sensitivity is low. Factors such as disease distribution (focal vs diffuse), stage of the disease process and sampling error influence the sensitivity of EMB. A clinical diagnosis of bacterial myocarditis may be reasonable when blood cultures are positive and there is proven active myocarditis on biopsy as long as other diagnoses have been excluded.