Myocarditis
Myocarditis is inflammation of the heart muscle (myocardium). It can result from infections (especially viral), autoimmune conditions, certain medications and toxins, and other causes. Myocarditis can be mild and self-limited, or it can produce serious complications such as heart failure, arrhythmias, and, rarely, sudden cardiac death. Cardiac MRI is the most useful noninvasive imaging test for diagnosis and characterization, and management is directed at the underlying cause as well as the heart itself.
What is it?
Myocarditis is inflammation of the myocardium, the muscular tissue that makes up the bulk of the heart and contracts to pump blood throughout the body. Inflammation can weaken the heart muscle, disturb its electrical signaling, and—in severe cases—lead to heart failure or dangerous arrhythmias. Myocarditis ranges from very mild forms that resolve on their own to severe presentations requiring intensive care and advanced therapies.
There are many possible causes. Viral infections are the most common identified cause, including a wide range of viruses such as enteroviruses (notably Coxsackie), adenoviruses, parvovirus B19, human herpesviruses, influenza, SARS-CoV-2 (the virus that causes COVID-19), and others. Bacterial, fungal, and parasitic infections—such as Lyme disease, certain bacterial infections, and Chagas disease (in endemic regions)—can also cause myocarditis. Autoimmune and inflammatory conditions, including lupus, rheumatoid arthritis, vasculitis, sarcoidosis, and giant cell myocarditis, are important non-infectious causes. Certain medications and toxins—including some chemotherapy agents (such as anthracyclines), checkpoint inhibitor immunotherapy, certain antibiotics, antipsychotics, illicit substances, and excessive alcohol—can produce myocarditis as a drug-induced or toxic injury. Hypersensitivity reactions and, very rarely, vaccine-associated myocarditis (most often seen with mRNA COVID-19 vaccines in young males and typically mild) are also recognized.
Symptoms vary widely and depend on the cause, severity, and individual response. Many cases are mild and produce symptoms similar to a respiratory or “flu-like” illness, sometimes accompanied by chest pain, palpitations, or shortness of breath. More significant cases can present with persistent chest pain, marked shortness of breath, reduced exercise tolerance, fatigue, lightheadedness, fainting, leg swelling, or signs of heart failure. Severe myocarditis can cause cardiogenic shock, life-threatening arrhythmias, or sudden cardiac death. Some patients have no significant symptoms, and inflammation is identified only on imaging or laboratory testing.
Diagnosis combines clinical assessment, laboratory tests, and imaging. The ECG may show various abnormalities including changes that can mimic heart attack. Blood biomarkers—particularly troponin (a marker of heart muscle injury) and natriuretic peptides (such as BNP or NT-proBNP, markers of heart strain)—are commonly elevated. Echocardiography assesses heart function, chamber sizes, valve function, and pericardial effusion. Cardiac MRI with late gadolinium enhancement is the most useful noninvasive test for diagnosing myocarditis because it can detect myocardial inflammation, edema, and scar, often in patterns characteristic of the underlying cause. In selected patients—especially those with severe or rapidly progressive disease, or when specific diagnoses such as giant cell or eosinophilic myocarditis are suspected—endomyocardial biopsy may be performed. Additional testing for specific viral, autoimmune, or other underlying causes is guided by clinical suspicion.
Important to Know
Management of myocarditis is highly individualized and depends on the cause, severity, and clinical course. Most patients with mild myocarditis recover well with supportive care, activity restriction during the acute phase, and treatment of any underlying cause when identified. Symptoms often improve over weeks to months, and follow-up imaging may show resolution of inflammation, with or without residual scar.
For patients with heart failure related to myocarditis, standard guideline-directed medical therapy is used and may include beta-blockers, ACE inhibitors or ARBs (or ARNI agents), mineralocorticoid receptor antagonists, SGLT2 inhibitors, and diuretics. Arrhythmias are treated with appropriate medications, and patients with significant rhythm disturbances may require monitoring or, in selected cases, devices such as implantable defibrillators.
When myocarditis is caused by a specific autoimmune or inflammatory condition, treatment of that underlying disease is essential. Examples include corticosteroids and other immunosuppressive therapy for cardiac sarcoidosis, giant cell myocarditis, or eosinophilic myocarditis; specific antibiotic therapy for Lyme carditis; and discontinuation or modification of medications when drug-induced myocarditis is identified. Checkpoint inhibitor–associated myocarditis is a particularly important and potentially severe form that often requires high-dose corticosteroids and close coordination between cardiology and oncology.
Activity restriction during the acute phase is generally recommended because vigorous exercise during active inflammation may worsen the condition. Most expert guidelines suggest a period of reduced activity—often around 3 to 6 months for higher-risk cases—with gradual return to exercise based on follow-up imaging and other testing. Athletes and active individuals may require specialized clearance protocols.
Severe myocarditis may require intensive care, advanced heart failure therapy, mechanical circulatory support (such as ECMO or ventricular assist devices), and, in select cases, heart transplantation. Care is typically coordinated by a multidisciplinary team that may include cardiologists, heart failure specialists, electrophysiologists, infectious disease specialists, rheumatologists, oncologists, intensivists, and cardiac surgeons.
Most patients recover well, but some develop residual scar, persistent reduced heart function, or arrhythmias. Long-term follow-up with periodic imaging and clinical assessment is often recommended, particularly for patients with significant initial disease or higher-risk features.
Red flag symptoms include severe or worsening shortness of breath, severe chest pain, fainting, sustained rapid or irregular heart rate, severe leg swelling, signs of shock, or sudden collapse. These warrant urgent medical evaluation, as severe myocarditis can be life-threatening.