Myocarditis
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Direct answer
Myocarditis is inflammation of the myocardium with myocyte injury in the absence of ischaemic cause, most often viral — Coxsackie B virus is the classical organism — with the heart dilated, flabby and infiltrated by inflammatory cells. Diagnosis on endomyocardial biopsy uses the Dallas criteria: an inflammatory infiltrate with myocyte necrosis or degeneration not explained by infarction. It presents from an asymptomatic picture through new heart failure in a young adult after a flu-like illness, arrhythmias and chest pain, to sudden death, and it may resolve or progress to dilated cardiomyopathy.
What you must remember
- Commonest causes: viral infections top the list (Coxsackie B and other enteroviruses, adenovirus, parvovirus B19, human herpesvirus 6), followed by drugs and toxins (anthracyclines, clozapine, immune checkpoint inhibitors) and immune-mediated disease.
- Specific infections: Trypanosoma cruzi (Chagas disease) with pseudocysts of amastigotes in myocytes; diphtheritic myocarditis from toxin; Lyme borreliosis; and HIV-associated disease.
- Dallas criteria: biopsy showing lymphocytic infiltrate with myocyte necrosis or degeneration defines active myocarditis; borderline myocarditis has inflammation without definite myocyte injury.
- Giant cell myocarditis: rapidly progressive, often fulminant disease of young adults with multinucleate giant cells on biopsy; poor prognosis, treated with intensive immunosuppression or transplantation.
- Morphology: soft, pale, dilated ventricles; interstitial lymphocytes and macrophages with focal myocyte necrosis; healing proceeds to interstitial fibrosis.
- Clinical clues: young patient, preceding viral illness, tachycardia out of proportion to fever, new arrhythmia or heart failure, raised troponin with normal coronaries; MRI with late gadolinium enhancement supports the diagnosis non-invasively.
- Outcomes: many cases recover; a minority progress to dilated cardiomyopathy, the reason viral myocarditis is a leading background diagnosis in explanted hearts.
Common confusion
Myocarditis is confused with myocardial infarction because both raise troponin and can cause chest pain with ECG changes. The discriminating features are age, viral prodrome and normal coronary arteries in myocarditis, versus atherothrombotic occlusion and coagulative necrosis in infarction. A second confusion is lymphocytic versus giant cell myocarditis — similar presentation, but giant cell disease is more fulminant and demands biopsy. Do not equate "myocarditis" with "pericarditis", though myopericarditis with pleuritic, positional pain often coexists.
Exam-focused takeaway
Stems describe a young adult with a recent febrile illness developing heart failure or ventricular arrhythmia, or a Latin American immigrant with cardiomyopathy and conduction disease (Chagas). Histology questions hinge on the Dallas criteria or pseudocysts; organism-drug associations (anthracycline, clozapine) appear as one-liners. Answer with the cause category — viral, toxic, immune, parasitic — and mention dilated cardiomyopathy as the long-term risk.
Frequently asked questions
Which virus most classically causes myocarditis?
Coxsackie B virus, an enterovirus, though parvovirus B19 and human herpesvirus 6 are frequently identified in biopsy series.
What are the Dallas criteria?
Endomyocardial biopsy criteria defining myocarditis as an inflammatory infiltrate with myocyte necrosis or degeneration not characteristic of infarction.
How does Chagas myocarditis appear histologically?
Pseudocysts containing Trypanosoma cruzi amastigotes within myocytes, with chronic interstitial inflammation and eventual dilated cardiomyopathy with apical aneurysm.
Why is giant cell myocarditis feared?
It runs a fulminant course in young adults with heart failure and refractory arrhythmias, requiring aggressive immunosuppression or transplantation.
Can myocarditis cause dilated cardiomyopathy?
Yes — unresolved inflammation and myocyte loss progress to fibrosis, chamber dilatation and systolic failure months to years later.