Myocarditis is inflammation of the heart muscle (myocardium). It can weaken the heart’s pumping function, disrupt its electrical system, and in some cases cause sudden cardiac arrest. It is an important and sometimes underdiagnosed cause of cardiac arrest, particularly in younger people who appear otherwise healthy.
**Causes**
The most common cause is viral infection, particularly enteroviruses (such as Coxsackievirus B), adenovirus, and parvovirus B19. Other causes include autoimmune conditions (such as systemic lupus erythematosus or sarcoidosis), toxic causes (certain medications, alcohol, recreational drugs), hypersensitivity reactions, and in some cases no cause is identified (idiopathic).
**How it presents**
Myocarditis can range from completely asymptomatic to rapidly fatal. Common presentations include chest pain (often sharp, worsening with lying flat), breathlessness, palpitations, fatigue, fever, and in some cases sudden cardiac arrest as the presenting event, particularly in young people.
**Diagnosis**
[Cardiac MRI](/glossary/cardiac-mri/) is the investigation of choice for suspected myocarditis, as it can detect inflammation and oedema in the heart muscle with high sensitivity. Blood tests (troponin, inflammatory markers) and ECG changes can also support the diagnosis.
**Treatment and recovery**
Most mild to moderate cases of viral myocarditis resolve with rest and avoidance of strenuous exercise, which is important during active inflammation. More severe cases may require intensive care support. Competitive sport and vigorous exercise are typically restricted for at least three to six months while inflammation resolves and cardiac function is reassessed. Some patients develop [dilated cardiomyopathy](/glossary/dilated-cardiomyopathy/) as a consequence of myocarditis.
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