Importance: Acute myocarditis, defined as a sudden inflammatory injury to the myocardium, affects approximately 4 to 14 people per 100 000 each year globally and is associated with a mortality rate of approximately 1% to 7%.
Observations: The most common causes of myocarditis are viruses, such as influenza and coronavirus; systemic autoimmune disorders, such as systemic lupus erythematosus; drugs, such as immune checkpoint inhibitors; and vaccines, including smallpox and mRNA COVID-19 vaccines. Approximately 82% to 95% of adult patients with acute myocarditis present with chest pain, while 19% to 49% present with dyspnea, and 5% to 7% with syncope. The diagnosis of myocarditis can be suggested by presenting symptoms, elevated biomarkers such as troponins, electrocardiographic changes of ST segments, and echocardiographic wall motion abnormalities or wall thickening. Cardiac magnetic resonance imaging or endomyocardial biopsy are required for definitive diagnosis. Treatment depends on acuity, severity, clinical presentation, and etiology. Approximately 75% of patients admitted with myocarditis have an uncomplicated course, with a mortality rate of approximately 0%. In contrast, acute myocarditis that is complicated by acute heart failure or ventricular arrhythmias is associated with a 12% rate of either in-hospital mortality or need for heart transplant. Approximately 2% to 9% of patients have hemodynamic instability, characterized by inability to maintain adequate end-organ perfusion, and require inotropic agents, or mechanical circulatory devices, such as extracorporeal life support, to facilitate functional recovery. These patients have an approximately 28% rate of mortality or heart transplant at 60 days. Immunosuppression (eg, corticosteroids) is appropriate for patients who have myocarditis characterized by eosinophilic or giant cell myocardial infiltrations or due to systemic autoimmune disorders. However, the specific immune cells that should be targeted to improve outcomes in patients with myocarditis remain unclear.
Conclusions and relevance: Acute myocarditis affects approximately 4 to 14 per 100 000 people per year. First-line therapy depends on acuity, severity, clinical presentation, and etiology and includes supportive care. While corticosteroids are often used for specific forms of myocarditis (eg, eosinophilic or giant cell infiltrations), this practice is based on anecdotal evidence, and randomized clinical trials of optimal therapeutic interventions for acute myocarditis are needed.