Diagnosis of Myocarditis According to ESC Guidelines
The diagnosis of myocarditis requires endomyocardial biopsy as the gold standard, showing inflammatory cells with necrotic myocytes on histological, immunological, and immunohistochemical criteria, though cardiac magnetic resonance (CMR) has become a routine, sensitive non-invasive alternative for confirmation before biopsy. 1
Essential First-Line Diagnostic Tests
All patients with suspected myocarditis must undergo the following mandatory investigations 1:
12-lead ECG - Look for QRS width >120 ms (predicts higher risk of death or transplantation), PR-segment depression and diffuse ST-elevation (suggests associated pericarditis), or low voltage with thickened LV walls (suggests myocardial edema) 1
Transthoracic echocardiography - Assess for wall motion abnormalities, wall thickening, and ventricular function 1
Cardiac biomarkers including troponins (though only elevated in 34% of histologically confirmed cases), creatine kinase, erythrocyte sedimentation rate, and C-reactive protein 1
Diagnostic Criteria Framework
The ESC references the Caforio criteria, which require clinically suspected myocarditis if ≥1 clinical presentation AND ≥1 diagnostic criterion from different categories are present, excluding angiographically detectable coronary disease (≥50% stenosis) or other known cardiovascular causes 1:
Clinical Presentations:
- Acute chest pain syndrome (82-95% of adults) 2
- Dyspnea (19-49%) 2
- Syncope (5-7%) 2
- Heart failure syndrome
- Arrhythmias or heart block 1
Diagnostic Criteria Categories:
- ECG/Holter/stress test abnormalities
- Elevated myocardiocytolysis markers (troponins, CK)
- Functional/structural abnormalities on imaging
- Tissue characterization by CMR 1
Important caveat: If the patient is asymptomatic, ≥2 diagnostic criteria must be met 1
Cardiac Magnetic Resonance Imaging
CMR is now routine and highly sensitive for confirming acute myocarditis even before endomyocardial biopsy 1:
- Characteristic pattern: Epicardial or mid-wall delayed gadolinium enhancement (DGE), NOT endocardial in coronary distribution 1
- T2-weighted sequences: Detect myocardial edema 1
- Early post-gadolinium T1-weighted sequences: Combined with DGE increases diagnostic accuracy 1
- T1 and T2 mapping: Increase sensitivity and reduce artifacts 1
- Prognostic value: Presence of DGE predicts subsequent ventricular arrhythmias and cardiovascular death 1
Endomyocardial Biopsy Indications
Endomyocardial biopsy should be performed especially in patients with life-threatening disease course 1:
- Acute fulminant myocarditis with refractory ventricular tachyarrhythmias in severe acute heart failure 1
- Hemodynamically compromising sustained ventricular arrhythmias or aborted cardiac arrest 1
- Giant cell myocarditis or sarcoidosis suspected (sensitivity 80-85% for giant cell) 1
- When immunosuppressive therapy is being considered 2
Critical technical point: Sensitivity increases with immunoperoxidase stains (anti-CD3, anti-CD68, anti-HLA antigens) or left ventricular biopsy for isolated LV disease 1
Clinical Phenotyping for Risk Stratification
The ESC guidelines distinguish two critical clinical settings 1:
Acute fulminant myocarditis: Refractory malignant ventricular tachyarrhythmias with severe acute heart failure, adverse short-term prognosis with early death from multisystem failure 1
Chronic inflammatory cardiomyopathy: Long-term evolution with LV dysfunction, high risk of sudden cardiac death similar to dilated cardiomyopathy 1
Warning Signs Requiring Hospital Admission
Patients with ventricular arrhythmias or heart block in acute myocarditis require prolonged ECG monitoring and mandatory hospital admission 1. Progressive wall motion abnormalities with deteriorating LV function, persistent or fluctuating troponin concentrations, widening QRS complex, and frequent non-sustained ventricular arrhythmias may precede sustained life-threatening arrhythmia 1.
Common Diagnostic Pitfall
Do not confuse myocarditis with pericarditis: While pericarditis requires ≥2 of 4 criteria (pericarditic chest pain, pericardial rubs, widespread ST-elevation/PR depression, pericardial effusion), patients with concomitant myocarditis will show elevation of myocardial injury markers (CK, troponin) in addition to inflammatory markers 1. The ECG changes in pericarditis reflect epicardial inflammation since the parietal pericardium is electrically inert 1.