Myocarditis
Myocarditis is inflammation of the heart muscle (myocardium), most often caused by viral infection — classically coxsackievirus B. It can present with chest pain, arrhythmias, or heart failure, and may progress to dilated cardiomyopathy.
Myocarditis is inflammation of the myocardium, the muscular wall of the heart. It sits between two related conditions in the "-carditis" family: endocarditis (inflammation of the inner lining and valves) and pericarditis (inflammation of the surrounding sac). In myocarditis, inflammatory cells infiltrate the muscle itself, injuring cardiomyocytes and impairing contraction.
The most common cause in developed countries is viral infection, classically coxsackievirus B, along with adenovirus, parvovirus B19, HHV-6, and influenza. Non-viral causes include bacteria (including Borrelia burgdorferi in Lyme carditis, which characteristically produces AV conduction block), Trypanosoma cruzi in Chagas disease, autoimmune conditions like lupus and rheumatic fever, and drug hypersensitivity reactions.
Presentation ranges widely — from asymptomatic ECG changes to chest pain mimicking myocardial infarction, new arrhythmias, or acute heart failure. Young, otherwise healthy patients with heart failure symptoms after a recent viral illness are the classic vignette, and myocarditis is an important cause of sudden cardiac death in young adults. Laboratory findings include elevated troponin; endomyocardial biopsy shows lymphocytic infiltrate with myocyte necrosis. Many cases resolve, but some progress to dilated cardiomyopathy with chronic heart failure.
USMLE Step 1 tests myocarditis alongside endocarditis and pericarditis — expect vignettes distinguishing the three, identifying coxsackievirus B as the classic culprit, and connecting Lyme disease to carditis with conduction block.
Key takeaways
- Myocarditis is inflammation of the heart muscle, distinct from endocarditis (lining/valves) and pericarditis (sac).
- Coxsackievirus B is the classic cause; Lyme disease, Chagas disease, autoimmunity, and drug reactions also cause it.
- The classic vignette is a young patient with chest pain or heart failure after a viral illness, with elevated troponin.
- Myocarditis can progress to dilated cardiomyopathy and is a cause of sudden cardiac death in young adults.
- USMLE Step 1 tests distinguishing myocarditis from endocarditis and pericarditis in clinical vignettes.