DEFINITION

Myocarditis is a rare, potentially deadly, and often underdiagnosed cause of heart failure that primarily affects children and young adults. Historically, the diagnosis of myocarditis was confirmed by analysis of endomyocardial biopsy specimens. The Dallas criteria, proposed in 1986, define myocarditis as an inflammatory cellular infiltrate of the myocardium with or without myocyte necrosis and/or degeneration of adjacent myocytes. These criteria have been criticized due to inter-reader variability in interpretation, low sensitivity due to sampling error, discrepancy with other markers of viral infection and immune activation in the myocardium and lack of prognostic value. Immunohistochemical stains that detect cellular surface antigens such as anti-CD3, anti-CD4, and anti-CD28 (T lymphocytes), anti-CD8 (macrophages), and Class I and II anti-human leukocyte antigens may have greater sensitivity than the Dallas criteria and may have prognostic value.

In patients with previously unexplained heart failure, the presence of viral genomes may indicate active infectious lymphocytic myocarditis. The most common viruses screened in patients with suspected myocarditis are parvovirus B19 (PVB19), adenovirus, enterovirus, cytomegalovirus, Epstein–Barr virus, herpes simplex virus 1 and 2, human herpesvirus 6 (HHV-6), and hepatitis C virus.

Fulminant myocarditis is described as acute onset of severe heart failure due to viral myocarditis. Cardiac sarcoidosis is a rare form of inflammatory myocarditis distinguished histologically by non-necrotizing interstitial granulomas. Idiopathic giant cell myocarditis (GCM) is another rare form of inflammatory myocarditis that is characterized histologically by multinucleated giant cells, myocyte necrosis, and a lymphocytic inflammatory infiltrate.