Key Facts

Myocarditis is inflammation of the myocardium characterised by myocyte necrosis and inflammatory infiltrate. It has variable presentation from subclinical disease to fulminant heart failure and sudden death.

Epidemiology

• True incidence unknown: estimated 10-22 cases per 100,000 per year
• More common in young adults, especially males (2:1)
• Responsible for ~5-20% of sudden cardiac death in young adults
• COVID-19-associated myocarditis gained significant attention during the pandemic

Aetiology

Infectious:

• Viral (most common): Coxsackie B, parvovirus B19, adenovirus, HHV-6, EBV, CMV, influenza, SARS-CoV-2, HIV
• Bacterial: diphtheria, Lyme disease (Borrelia burgdorferi), TB
• Parasitic: Trypanosoma cruzi (Chagas disease — common in South America)
• Fungal: rare, immunocompromised

Non-infectious:

• Autoimmune: giant cell myocarditis, eosinophilic myocarditis, SLE, sarcoidosis
• Drug/toxin: cocaine, alcohol, anthracyclines (doxorubicin), immune checkpoint inhibitors, clozapine
• Hypersensitivity: drug reactions (penicillin, sulfonamides)

Pathophysiology

• Direct viral cytotoxicity → myocyte damage
• Immune-mediated injury: molecular mimicry, autoantibodies against cardiac proteins
• Inflammatory infiltrate and oedema → contractile dysfunction
• May progress to fibrosis and scar → dilated cardiomyopathy
• Arrhythmias from inflammation-induced electrical instability

Typical Presentation

• Preceding viral illness (1-4 weeks before) in many cases
• Chest pain: often sharp, may mimic ACS or pericarditis
• Heart failure symptoms: dyspnoea, fatigue, oedema
• Palpitations: arrhythmias (SVT, VT, heart block)
• Syncope or sudden death (rare presenting feature)

Classification

• Acute myocarditis: chest pain, troponin rise, ECG changes, new LV dysfunction
• Fulminant myocarditis: acute onset (<2 weeks) with severe haemodynamic compromise, cardiogenic shock
• Chronic/smouldering myocarditis: progressive heart failure, may present as DCM

Red Flags

• Cardiogenic shock (fulminant myocarditis)
• Sustained VT or high-degree heart block
• Rapidly declining LV function
• Young patient with sudden cardiac death risk
• Giant cell myocarditis: rapidly progressive, often fatal without treatment

Bedside

• ECG: ST changes (elevation or depression), T-wave inversions, low voltage, arrhythmias, heart block
• Observations: tachycardia, hypotension in severe cases

Bloods

• Troponin (hs-cTn): elevated (indicates myocyte necrosis)
• BNP/NT-proBNP: elevated (correlates with severity)
• CRP/ESR: elevated
• FBC: leucocytosis, eosinophilia (eosinophilic myocarditis)
• Viral serology: targeted based on clinical suspicion
• Autoimmune screen: ANA, anti-dsDNA, ANCA if indicated

Imaging

• Echocardiography: global or regional wall motion abnormalities, LV dysfunction, pericardial effusion
  • May be normal in mild cases
• Cardiac MRI (gold standard non-invasive): Lake Louise criteria
  • T2-weighted: myocardial oedema
  • T1 mapping/ECV: diffuse inflammation
  • Late gadolinium enhancement: mid-wall or subepicardial pattern (vs subendocardial in ischaemia)
• Coronary angiography: to exclude ACS (normal coronaries in myocarditis)

Special Tests

• Endomyocardial biopsy (EMB): gold standard for histological diagnosis
  • Dallas criteria: inflammatory infiltrate + myocyte necrosis
  • Indicated in: suspected giant cell myocarditis, haemodynamic compromise, new-onset HF unresponsive to treatment
  • Limited by sampling error (~30% sensitivity)
• PET-CT: may show active inflammation (useful in cardiac sarcoidosis)

Non-pharmacological

• Exercise restriction: avoid strenuous activity for ≥3-6 months (risk of arrhythmia and sudden death)
• Return to exercise only after: symptoms resolved, LV function normalised, no significant arrhythmias, inflammatory markers normalised, MRI shows resolution
• Bed rest in acute phase if symptomatic

Pharmacological

• Heart failure management: ACEi/ARB, beta-blocker, diuretics, MRA — as per standard heart failure guidelines
• Avoid NSAIDs: may worsen myocardial inflammation and impair healing (animal data)
• Anticoagulation: consider if severe LV dysfunction or intracardiac thrombus
• Antiarrhythmics: for symptomatic arrhythmias; avoid amiodarone long-term if possible
• Immunosuppression (corticosteroids ± azathioprine): for specific subtypes:
  • Giant cell myocarditis: aggressive immunosuppression (high-dose corticosteroids + ciclosporin/azathioprine)
  • Eosinophilic myocarditis: corticosteroids
  • Cardiac sarcoidosis: corticosteroids ± steroid-sparing agents
• NOT recommended for routine viral myocarditis (no proven benefit)

Surgical/Interventional

• Mechanical circulatory support: IABP, Impella, ECMO for fulminant myocarditis with cardiogenic shock
• Heart transplantation: for refractory end-stage heart failure
• ICD: if persistent severe LV dysfunction (EF ≤35%) after ≥3-6 months of optimal medical therapy
  • Wearable defibrillator (LifeVest): may be used as bridge during recovery period

Referral Criteria

• All suspected myocarditis: cardiology referral for cardiac MRI and management
• Haemodynamic compromise: CCU/ICU admission, MCS team involvement
• Suspected giant cell myocarditis: urgent EMB and immunosuppression

• Mild/moderate viral myocarditis: ~50-70% recover fully with normalised LV function
• ~20-30% develop chronic dilated cardiomyopathy
• Fulminant myocarditis: paradoxically, those who survive the acute phase (with intensive support) have better long-term outcomes (~90% survival) — vigorous immune response may clear virus effectively
• Giant cell myocarditis: rapidly fatal without immunosuppression; median survival ~6 months untreated; transplant-free survival ~50% at 5 years with treatment
• Sudden cardiac death: ~5-20% of unexplained SCD in young adults attributed to myocarditis
• COVID-19 myocarditis: generally mild; severe cases rare but occurred

Lake Louise Criteria for CMR Diagnosis of Myocarditis

Myocarditis vs MI on Cardiac MRI