Pathophysiology
Clinical meaning
Myocarditis is inflammation of the myocardium (heart muscle) that can result from infectious, autoimmune, or toxic causes, with viral infection being the most common etiology. Coxsackievirus B (an enterovirus) is historically the most frequently identified causative agent, though adenovirus, parvovirus B19, human herpesvirus 6 (HHV-6), influenza, SARS-CoV-2, and Epstein-Barr virus are also important causes. The pathophysiology of viral myocarditis proceeds through three overlapping phases. In Phase 1 (viral invasion, days 1-3), the virus enters the body through the respiratory or gastrointestinal tract and reaches the heart via the bloodstream. Coxsackievirus B binds to the coxsackievirus-adenovirus receptor (CAR) on cardiomyocytes, gaining entry to the cardiac muscle cells. Once inside, the virus hijacks cellular machinery to replicate, causing direct cardiomyocyte necrosis and releasing intracellular contents (troponin, CK-MB) into the bloodstream. In Phase 2 (immune response, days 4-14), the innate and adaptive immune systems mount an inflammatory response. Natural killer cells, macrophages, and T lymphocytes infiltrate the myocardium to destroy virus-infected cardiomyocytes. While this immune response is necessary to clear the virus, it causes significant collateral damage to uninfected cardiomyocytes, creating patchy areas...