Definition and Epidemiology
Myocarditis is a heterogeneous group of inflammatory heart diseases characterized by myocardial inflammation in the absence of ischemic etiology, typically driven by immune-mediated mechanisms triggered by infectious, autoimmune, toxic, or idiopathic insults. Histopathologically, it meets the Dallas criteria: myocardial inflammation (lymphocytic, giant cell, eosinophilic, or other infiltrates) associated with cardiomyocyte injury or necrosis.
- Incidence: Estimated at 10–22 cases per 100,000 person-years; underdiagnosed due to variable presentation and limitations of non-invasive testing.
- Demographics: Bimodal peak—young adults (20–40 years) often with viral or post-viral etiology; older adults more commonly associated with autoimmune or sarcoidosis-related disease.
- Mortality: Ranges from <1% in mild cases to >50% in fulminant presentations without timely intervention.
Etiology and Pathogenesis
Myocarditis follows a well-defined three-stage pathophysiological model:
| Stage | Mechanism | Key Triggers |
|---|---|---|
| Acute Inflammatory Phase | Direct viral/cytotoxic injury + innate immune activation (e.g., TLR signaling, cytokine release) | Enteroviruses (coxsackievirus B), parvovirus B19, HHV-6, SARS-CoV-2, influenza |
| Autoimmune/Post-infectious Phase | Molecular mimicry → cross-reactive adaptive immunity targeting cardiac antigens (e.g., myosin, β-adrenergic receptors) | Post-viral (e.g., adenovirus, SARS-CoV-2), post-vaccine (rare mRNA vaccine-associated cases; incidence ~10–40/million doses, mainly in young males) |
| Chronic Dilated Phase | Persistent inflammation → fibrosis → maladaptive remodeling → DCMP | Giant cell myocarditis (GCM), eosinophilic myocarditis (EM), immune checkpoint inhibitor (ICI)-associated |
Emerging Insights:
- Viral persistence without active replication can sustain inflammation via “bystander activation” (e.g., HHV-6 latency in cardiomyocytes).
- Autoantibodies against cardiac myosin, β1-adrenergic receptor, and muscarinic M2 receptor are detected in ~30–50% of cases—potential therapeutic targets.
- SARS-CoV-2-associated myocarditis: Myocardial injury (troponin elevation) occurs in 10–30% of hospitalized patients; true inflammatory myocarditis confirmed by CMR/EBM in ~10–25% of those with elevated troponin. Mechanisms include direct viral invasion (ACE2-mediated), hyperinflammation (IL-6, TNF-α surge), and microvascular dysfunction.
Clinical Presentation & Red Flags
Symptoms are nonspecific but should raise suspicion when occurring in individuals <45 years without traditional CVD risk factors:
| Symptom | Frequency | Clinical Implication |
|---|---|---|
| Chest pain (often pleuritic, positional) | 60–75% | May mimic ACS; no obstructive CAD on angiography is a key clue |
| Dyspnea / orthopnea | 70–85% | Suggests acute heart failure or fulminant presentation |
| Palpitations / presyncope | 40–60% | Indicates arrhythmia (NSVT, VT, AF) or conduction block |
| Fatigue / malaise | >90% | Often preceded by flu-like illness (1–3 weeks prior) |
Red Flags for Severe Disease:
- New-onset heart failure with hypotension (SBP <90 mmHg), tachycardia disproportionate to fever, or elevated JVP
- High-grade AV block (e.g., 2:1 AV block, complete heart block)
- Sustained VT/VF or bradyarrhythmias requiring pacing
- Marked elevation in troponin (>10× ULN) with minimal ST-T changes on ECG
Diagnostic Workup: Evidence-Based Algorithm
(Per 2023 ESC Guidelines for Cardiomyopathies and 2022 AHA Scientific Statement)
1. Initial Suspicion & Triage
- Clinical suspicion criteria (modified from ESC):
- Age <45 years + new cardiac symptoms
- Absence of CAD risk factors or angiographically confirmed non-obstructive CAD
- ≥1 of: elevated troponin, ECG abnormalities, LV dysfunction on imaging
2. Core Diagnostic Modalities
| Test | Indication | Evidence Strength | Key Interpretation |
|---|---|---|---|
| 12-lead ECG | All suspected cases (Class I) | Strong | • ST elevation (diffuse, not coronary territory) • PR depression (suggests concomitant pericarditis) • Low voltage + QRS widening: poor prognosis • Conduction abnormalities: AV block = higher mortality |
| Transthoracic Echo (TTE) | Suspected myocarditis (Class I) | Strong | • LV/LV dilation, hypokinesis (often apical sparing in acute phase) • LVEF <40% predicts adverse outcomes • Fulminant: severe LV dysfunction with preserved LV cavity size • Serial Echo: assess recovery over 1–3 months |
| Cardiac MRI (CMR) | Hemodynamically stable patients (Class I) | Strong | Lake Louise Criteria (2018 update): – T2-weighted: myocardial edema – STIR: high sensitivity for acute inflammation – LGE (late gadolinium enhancement): non-ischemic pattern (subepicardial/midwall, lateral wall) – ECE/T1 mapping: elevated native T1/ECV = active inflammation • Sensitivity 80–90% vs histology; false negatives in focal or subacute disease |
| Serum Biomarkers | Suspected myocarditis (Class I) | Strong | • High-sensitivity troponin (hs-cTn): >10× ULN favors myocarditis over unstable CAD • CK-MB: less specific; useful if hs-cTn unavailable • Serial measurements: decline correlates with resolution of inflammation |
3. Endomyocardial Biopsy (EMB)
Indications (Class I per ESC/AHA):
- Acute heart failure with LV ejection fraction <45% and hemodynamic compromise (shock, profound hypotension)
- New-onset AV block ≥ second degree or sustained VT
- Rapidly progressive cardiomyopathy with electrical instability
EMB Technical Considerations:
- Rastan technique: 3–5 right ventricular samples ( septal and free wall)
- Histology + immunohistochemistry + PCR: Required for diagnosis (Dallas criteria + molecular analysis for viral genomes, CD68/CD3 staining)
- Yield: Increases with higher inflammation grade; false negatives common in focal or subacute disease. Sensitivity ~50–70% in research centers.
Emerging Alternatives:
- Blood-based biomarkers: sST2, Galectin-3 for fibrosis; miRNA panels (e.g., miR-1, miR-133a) under investigation.
- PET/CT: ¹⁸F-FDG for active inflammation (especially in sarcoidosis or ICI myocarditis), but limited by high physiological myocardial FDG uptake.
Pathology-Guided Management
(Based on 2023 ESC Guidelines & International Study of Myocarditis [ISMC) Consensus)
A. General Supportive Care (All Patients)
- Hospitalization: Minimum 48 hours for all acute cases—even stable patients—due to risk of sudden arrhythmic death in first 7 days.
- Heart Failure Management:
- Standard GDMT for HFrEF: ACEi/ARB/ARNI, β-blockers (after hemodynamic stabilization), MRA, SGLT2i
- Avoid digoxin if LV dysfunction present (increased arrhythmia risk)
- Fulminant myocarditis: Early mechanical support may be life-saving:
- VA-ECMO for cardiogenic shock (bridge to recovery or transplant)
- Impella for temporary hemodynamic support
- Note: Mortality remains >50% in refractory cases without ECMO
B. Immunosuppression: Indication-Specific Recommendations
| Condition | Recommendation | Supporting Evidence |
|---|---|---|
| Acute Lymphocytic Myocarditis (viral or idiopathic) | Do NOT use routine immunosuppression (Class I) | ICON trial (2013):无 benefit in LV function; increased infections |
| Giant Cell Myocarditis | Early combination ISN: Cyclosporine + mycophenolate + prednisone ± anti-thymocyte globulin | Meta-analysis (JACC 2021): 1-yr survival 85% vs 30% with supportive care alone |
| Eosinophilic Myocarditis | 1. Discontinue offending agent (e.g., IL-5 inhibitors, DPP-4 inhibitors, antibiotics) 2. High-dose steroids (methylprednisolone 1g/day × 3–5 days → taper) for hemodynamic instability | Hypersensitivity myocarditis case series (Circulation 2020); Eosinophilic Granulomatosis with Polyangiitis (EGPA) protocols |
| Immune Checkpoint Inhibitor (ICI)-Associated Myocarditis | Prompt high-dose steroids (methylprednisolone 1–2 g/day × 3 days) ± infliximab if steroid-refractory | NEJM 2020: Mortality 46% vs 9% with early steroids; avoid infliximab if heart failure present (may worsen dysfunction) |
| Autoimmune/ICI Myocarditis + LV Dysfunction | Consider trial of immunosuppression (Class IIa) | Retrospective data show improved LVEF with cyclosporine/mycophenolate in non-viral lymphocytic myocarditis |
Immunosuppression Cautions:
- Screen for latent TB, hepatitis B/C before starting cyclosporine/anti-thymocyte globulin
- Monitor for PJP prophylaxis with multi-drug regimens (e.g., TMP-SMX)
- Avoid in active viral myocarditis (PCR+ for enterovirus/parvovirus) due to risk of uncontrolled replication
C. Arrhythmia Management
- Bradycardia: Temporary pacing for high-grade AV block—do not delay pacing. Permanent pacemaker if conduction disease persists >6–12 months.
- Tachyarrhythmias:
- NSVT: Often self-limiting; treat underlying inflammation
- Sustained VT/VF: ICD after recovery (Class IIb); avoid ICD during acute phase due to high arrhythmia recurrence from reversible inflammation
Prognosis & Follow-up
- Short-term: Mortality 5–10% in hospitalized patients; up to 50% in fulminant cases.
- Long-term:
- 60–70% recover full LV function within 3 months
- 20–30% develop persistent LV dysfunction → DCMP (annual risk of SCD: 5–10%)
- Risk stratification for ICD implantation:
- LVEF ≤35% despite optimal medical therapy for ≥3 months
- NSVT + LV dilation + late gadolinium enhancement on CMR
Follow-up Protocol:
- Echo & ECG at 1, 3, and 6 months
- CMR at 3–6 months: Assess resolution of inflammation (T2 signal normalization) vs fibrosis (LGE persistence)
- Hs-cTn and NT-proBNP monthly until stable
Future Directions & Unmet Needs
- Viral Eradication Strategies: Pleconaril (enterovirus capsid binder) in phase II trials.
- Targeted Biologics:
- Il-1β antagonists (anakinra): In refractory cases (IL-1BLOCK-MI trial ongoing)
- Anti-IL-6 (tocilizumab): For cytokine-driven myocarditis
- Precision Diagnostics: Next-generation sequencing of endomyocardial samples for viral/bacterial DNA/RNA; autoantibody profiling.
Summary: Key Clinical Pearls
- Suspect myocarditis in young patients with unexplained heart failure, arrhythmias, or conduction disease—even without classic viral prodrome.
- CMR is the cornerstone non-invasive test—do not delay for biopsy if stable.
- EMB is indicated only in hemodynamically unstable or electrically unstable patients—avoid routine use.
- Immunosuppression is NOT standard of care for acute lymphocytic myocarditis but is life-saving in GCM, ICI-myocarditis, and autoimmune variants.
- Hospitalize all acute cases ≥48 hours: Arrhythmic events often occur beyond initial ED triage.
References: 2023 ESC Guidelines for Cardiomyopathies; Circulation 2022;145:e76–e95 (AHA Scientific Statement); Eur Heart J 2021;42:3073–3083 (ISMC Consensus); NEJM 2020;383:1667–1678 (ICI Myocarditis); JACC 2021;77:1985–1999 (Immunosuppression Meta-analysis).
