Myocarditis — inflammatory myocardial injury — has a broad differential in young adults (18–40 yr). Accurate etiologic diagnosis guides prognosis and therapy.
| Category | Key Causes | Clinical Clues |
|---|
| Viral (most common) | Coxsackie B (enterovirus), Adenovirus, Parvovirus B19, HHV-6, EBV, CMV, Influenza, SARS-CoV-2, HIV, Hepatitis C | Preceding viral prodrome (fever, myalgia, URI, GI); often self-limited but can progress |
| Post-infectious / Immune-mediated | Post-viral autoimmune, MIS-A (multisystem inflammatory syndrome in adults), molecular mimicry | Weeks after infection; autoantibodies may be present |
| Drug-induced / Hypersensitivity | Checkpoint inhibitors (immune checkpoint myocarditis — medical emergency), clozapine, anthracyclines, penicillin, sulfonamides, lamotrigine, recreational drugs (cocaine, amphetamines) | Temporal relationship to drug initiation; eosinophilia (hypersensitivity) |
| Autoimmune / Systemic | SLE, Sjögren's, vasculitis (GPA, MPA, EGPA), sarcoidosis, giant cell myocarditis (GCM) | Systemic features; GCM: rapid progression, AV block, VT/VF, poor prognosis |
| Infectious (non-viral) | Lyme (Borrelia), Chagas (T. cruzi), Toxoplasma, Trypanosoma, bacterial (S. aureus, Streptococcus), fungal, parasitic | Travel/exposure history; specific serologies |
| Idiopathic | No identifiable cause after comprehensive workup | Diagnosis of exclusion |
Diagnostic Algorithm
- Clinical suspicion: Chest pain, dyspnea, palpitations, syncope, fatigue + elevated troponin/BNP.
- ECG: Diffuse ST/T changes, PR depression, low voltage, conduction delays, arrhythmias.
- Echo: Global/regional hypokinesis, LVEF, pericardial effusion, RV involvement.
- CMR (Gold Standard Non-Invasive): Lake Louise Criteria (2018): T2 edema + T1 elevation (LGE) + supportive (pericardial enhancement). Pattern suggests etiology (subepicardial LGE = viral/immune; mid-wall = sarcoid/GCM; subendocardial = ischemic).
- Endomyocardial Biopsy (EMB): Indicated for: fulminant presentation, high-grade AV block, VT/VF, suspected GCM/sarcoid/amyloid, failure of empiric therapy. Dallas criteria + immunohistochemistry + PCR/viral genome.
- Targeted labs: Autoimmune panel, viral PCR/serologies, Lyme/Chagas, eosinophils, drug levels, genetic testing (LMNA, DES, FLNC, etc.).
Management Principles
- Supportive: GDMT for HFrEF (avoid β-blockers acutely if hemodynamic compromise), diuretics, anticoagulation if EF <35% or AFib, temporary MCS (Impella, ECMO) for cardiogenic shock.
- Etiology-specific: Immunosuppression (GCM, sarcoid, autoimmune, checkpoint inhibitor — high-dose steroids ± infliximab/abatacept), antivirals (rarely indicated), antimicrobials (Lyme, Chagas), drug withdrawal.
- Activity restriction: 3–6 months; competitive sports restricted until normalization of EF, biomarkers, CMR, Holter.
- Follow-up: Serial echo/CMR, Holter, biomarkers. 50% recover EF; 20–25% progress to DCM.
Multidisciplinary care (heart failure, imaging, electrophysiology, rheumatology, infectious disease) optimizes outcomes.
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