The following are key points to remember from a 2024 American College of Cardiology (ACC) expert consensus decision pathway (ECDP) on strategies and criteria for the diagnosis and management of myocarditis:

1. Acute myocarditis can be asymptomatic or present with nonspecific symptoms. However, having a high index of suspicion is key when patients present with chest pain, arrhythmias, or heart failure (HF), especially in the setting of a recent viral infection, prior myocarditis, autoimmune disease, family history of cardiomyopathy or sudden death, or cardiotoxin exposure.
2. Electrocardiograms (ECG), echocardiograms, and cardiac troponin assays are common diagnostic tests for suspected myocarditis. They provide useful prognostic information and data to guide management. However, normal testing does not rule out myocarditis in patients where index of suspicion is high.
3. Cardiac magnetic resonance (CMR) imaging provides valuable information in diagnosing myocarditis, characterizing the presence of myocardial edema, hyperemia, necrosis, fibrosis, pericardial inflammation, and left ventricular (LV) dysfunction. Additional imaging with myocardial scintigraphy and inflammation-sensitive radiotracers (e.g., ¹⁸F-FDG PET) may be of diagnostic value in select cases.
4. Endomyocardial biopsy use is generally limited to identifying causes of myocarditis with etiology-directed therapies (e.g., giant cell myocarditis) or ruling out myocarditis mimics. Use in all suspected cases of myocarditis is not routinely performed due to risks of the procedure and factors limiting diagnostic accuracy.
5. This document proposes a new classification system for myocarditis, which parallels the stages for HF. This new classification includes Stage A (exposure to myocarditis risk factors without symptoms or disease), Stage B (evidence of myocarditis without symptoms), Stage C (symptomatic myocarditis), and Stage D (symptomatic myocarditis with hemodynamic or electrical instability).
6. Outcomes in patients with biopsy-proven myocarditis vary, with approximately 50% having resolution in 2-4 weeks, 25% developing persistent cardiac dysfunction, and 12-25% progressing to end-stage HF or death. Predictors of poor outcomes include biventricular dysfunction, Stage D presentation, late gadolinium enhancement (LGE) on CMR, and specific etiologies (e.g., giant cell myocarditis). Transfer to an advanced HF center is recommended for patients with increased risk.
7. Pharmacological treatment of myocarditis is based on presentation and etiology. Nonsteroidal anti-inflammatory drugs and colchicine are uses for pericardial-type chest pain but should be avoided in cases of symptomatic HF. Immunosuppression is generally used in cases of eosinophilic, giant cell, cardiac sarcoid, and immune checkpoint inhibitor-associated myocarditis.
8. In patients with Stage C or D myocarditis, it is recommended to avoid strenuous physical activity for 3-6 months. Follow-up with CMR or echocardiography, 24-hour ECG monitoring, and exercise stress testing at the end of this time frame is advisable prior to returning to strenuous exercise.
9. Many patients with myocarditis have a genetic predisposition. Genetic counseling and testing for all patients is recommended if possible.
10. Longitudinal follow-up with imaging is important to assess for disease resolution or progression. Patients with Stage C or D myocarditis should have a repeat echocardiogram at 2-4 weeks with an office visit. At 6 months, low-risk patients (normal LV ejection fraction, no LGE on CMR, no electrical/hemodynamic instability) should get a repeat echocardiogram and higher-risk patients should get a CMR. Biomarkers also may be used to assess for evidence of subclinical deterioration.

Clinical Topics: Arrhythmias and Clinical EP, Heart Failure and Cardiomyopathies, Noninvasive Imaging, Implantable Devices, SCD/Ventricular Arrhythmias, Atrial Fibrillation/Supraventricular Arrhythmias, Acute Heart Failure, Magnetic Resonance Imaging

Keywords: Arrhythmias, Cardiac, Heart Failure, Magnetic Resonance Imaging, Myocarditis

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