Lucan · Conditions

Myocarditis

Australian guidance: eTG Cardiovascular — Myocarditis

1. Definition and epidemiology

Myocarditis is inflammatory disease of the myocardium, characterised by inflammatory cell infiltration and cardiomyocyte necrosis in the absence of ischaemic injury. True population incidence is difficult to establish owing to subclinical presentations and variable diagnostic criteria, but autopsy studies suggest myocarditis accounts for 1 to 9% of all cardiac deaths and is a significant cause of sudden cardiac death in young adults under 35 years of age. In Australia, viral myocarditis is the most common form, with enteroviruses (particularly coxsackievirus B) and adenoviruses being the predominant agents identified in biopsy series.

2. Pathophysiology

The pathophysiology of myocarditis proceeds in three overlapping phases. In the acute phase (days 1 to 4), the inciting agent, most commonly a cardiotropic virus, directly infects cardiomyocytes via surface receptors such as the coxsackievirus and adenovirus receptor (CAR). Viral replication causes direct cytolysis and triggers innate immune activation, including natural killer cell recruitment and toll-like receptor signalling. In the subacute phase (days 4 to 14), adaptive immunity predominates: cytotoxic T lymphocytes and macrophages infiltrate the myocardium, generating the hallmark Dallas Criteria histological picture of an interstitial mononuclear infiltrate with adjacent myocyte necrosis. Molecular mimicry can redirect this adaptive response against native cardiac antigens including cardiac myosin, generating autoantibodies and perpetuating injury beyond viral clearance. In the chronic phase, persistent inflammation drives fibrosis and remodelling, leading to dilated cardiomyopathy in roughly 20% of cases. Giant cell myocarditis, an aggressive autoimmune variant, is characterised by multinucleated giant cells on biopsy and carries a poor prognosis without immunosuppression.

3. Clinical presentation and diagnostic criteria

Myocarditis has a broad clinical spectrum. The classic presentation is a young adult with a recent viral prodrome (fever, myalgia, upper respiratory tract symptoms 1 to 3 weeks prior) who develops chest pain, dyspnoea, palpitations, or fatigue. Chest pain is often pleuritic or positional, overlapping with pericarditis; when pericardial involvement coexists, the term myopericarditis is used.

Examination may reveal tachycardia disproportionate to fever, a soft S3 gallop, or a pericardial friction rub. In fulminant myocarditis, cardiogenic shock with haemodynamic compromise can occur rapidly. The Lake Louise Criteria (cardiac magnetic resonance imaging based) provide the primary non-invasive diagnostic framework: myocardial oedema on T2-weighted imaging plus non-ischaemic late gadolinium enhancement in a subepicardial or midwall pattern is highly specific. Endomyocardial biopsy using the Dallas Criteria remains the histological gold standard but has limited sensitivity due to sampling error and is reserved for cases with haemodynamic compromise, arrhythmia unresponsive to treatment, or diagnostic uncertainty.

4. Investigations and AU-specific cutoffs

Investigations should be ordered in a stepwise fashion:

1. Electrocardiogram (ECG): sinus tachycardia is the most common finding. Diffuse ST elevation (distinguishable from STEMI by saddle-shape morphology and PR depression) may occur. Ventricular arrhythmias or conduction block raise concern for giant cell myocarditis or cardiac sarcoidosis.
2. Cardiac troponin: elevated in the majority of symptomatic cases. In Australia, high-sensitivity troponin assays (99th percentile upper reference limit varies by assay) are standard; any elevation above the upper limit of normal is consistent with myocardial injury. The pattern differs from STEMI: troponin typically rises without a culprit lesion on angiography.
3. Full blood count, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR): markers of systemic inflammation support the diagnosis.
4. Transthoracic echocardiogram: assesses left ventricular (LV) systolic function, wall motion abnormalities, and pericardial effusion. LV ejection fraction (LVEF) below 40% defines heart failure with reduced ejection fraction (HFrEF) and guides management.
5. Cardiac magnetic resonance imaging (CMRI): first-line advanced imaging in haemodynamically stable patients; Lake Louise Criteria applied. T1 and T2 mapping improve diagnostic accuracy beyond visual assessment.
6. Coronary angiography or CT coronary angiography: performed when STEMI cannot be excluded clinically or on ECG.
7. Viral serology and polymerase chain reaction (PCR): low diagnostic yield and do not change acute management; not routinely recommended in clinical guidelines.
8. Endomyocardial biopsy: reserved for fulminant presentation, haemodynamic instability, or sustained ventricular arrhythmia unresponsive to therapy.

5. Management

Management is stratified by haemodynamic status and LV function. All patients with suspected myocarditis should be hospitalised for monitoring given the risk of rapid deterioration.

Activity restriction is central to acute management. Strenuous physical activity should be avoided for at least 3 to 6 months from symptom onset, regardless of symptom resolution, as exercise promotes viral replication and extends inflammatory injury. Return to competitive sport requires documented normalisation of LVEF, biomarkers, and absence of arrhythmia on ambulatory monitoring.

For patients with myopericarditis (myocarditis with prominent pericardial involvement and preserved LV function), anti-inflammatory therapy with colchicine combined with a nonsteroidal anti-inflammatory drug (NSAID) is used following the pericarditis protocol. Aspirin 750 to 1000 mg orally 8-hourly for 1 to 2 weeks (tapering by 250 to 500 mg every 1 to 2 weeks) plus colchicine 500 micrograms orally twice daily for 3 months (or once daily if weight is less than 70 kg) is the standard regimen.

When myocarditis causes LV dysfunction with LVEF at or below 40%, standard heart failure with reduced ejection fraction therapy is initiated following Australian guideline principles: angiotensin converting enzyme inhibitor (ACEI) or angiotensin receptor blocker (ARB), beta blocker, mineralocorticoid receptor antagonist (MRA), and sodium-glucose co-transporter 2 (SGLT2) inhibitor as the four-pillar regimen.

Immunosuppressive therapy with corticosteroids is not indicated for viral myocarditis, as it may worsen outcomes during active viraemia. Immunosuppression is reserved for specific histological diagnoses: giant cell myocarditis (prednisone plus ciclosporin or azathioprine), cardiac sarcoidosis, and eosinophilic myocarditis. Intravenous immunoglobulin has been used in paediatric myocarditis but evidence for adult use is limited.

Fulminant myocarditis with cardiogenic shock requires intensive care, inotropic support, and consideration of mechanical circulatory support (intra-aortic balloon pump or extracorporeal membrane oxygenation) as a bridge to recovery or transplant.

6. Pharmacology and AU PBS notes

Anti-inflammatory (myopericarditis phenotype): - Colchicine 500 micrograms orally twice daily (greater than or equal to 70 kg) or once daily (less than 70 kg) for 3 months. PBS listed (Schedule 4); reduce dose in renal impairment (eGFR below 30 mL/min/1.73 m2). AU brand: Colgout, Lengout. - Aspirin 750 to 1000 mg orally 8-hourly with taper; aspirin 100 mg (antiplatelet dose) is NOT equivalent. - Ibuprofen 600 mg orally 8-hourly as an alternative NSAID for 1 to 2 weeks, tapered. Use with caution in LV dysfunction.

Heart failure with reduced ejection fraction (when LVEF at or below 40%): - ACEI: ramipril 2.5 to 10 mg orally daily. PBS streamlined code for heart failure: PBS item 8519T. - Beta blocker: carvedilol 3.125 to 25 mg orally twice daily or metoprolol succinate 12.5 to 200 mg orally daily. - MRA: spironolactone 25 to 50 mg orally daily. Monitor potassium and renal function. - SGLT2 inhibitor: dapagliflozin 10 mg orally daily or empagliflozin 10 mg orally daily. PBS listed for HFrEF regardless of diabetes status.

Immunosuppression (giant cell myocarditis or cardiac sarcoidosis): - Prednisone 1 mg/kg/day (up to 60 mg/day) orally, tapered over months. - Ciclosporin or azathioprine added for giant cell myocarditis under specialist guidance.

7. Complications and monitoring

Key complications include: - Dilated cardiomyopathy: occurs in approximately 20% of patients; most common long-term sequela. Monitor with serial echocardiography at 1, 3, and 6 months post-diagnosis. - Ventricular arrhythmias and sudden cardiac death: highest risk in the acute phase and in giant cell or sarcoid myocarditis. Ambulatory ECG monitoring (Holter) for 24 to 48 hours at baseline; implantable cardioverter defibrillator (ICD) considered if sustained ventricular tachycardia persists after recovery. - Conduction system disease: varying degrees of atrioventricular block; temporary pacing may be required in fulminant disease. - Pericardial effusion and cardiac tamponade: monitor with serial echocardiography when effusion is present. - Recurrence: most common in myopericarditis; colchicine reduces recurrence. - Heart failure hospitalisation: patients with persistent LV dysfunction enrol in a multidisciplinary heart failure management program with weekly weight monitoring, fluid restriction, and regular biochemistry review.

Surveillance schedule: echocardiogram at 1, 3, and 6 months, then annually if LVEF has not fully recovered. Cardiology outpatient review at 1 month. Return to sport evaluation requires normal LVEF, normal 24-hour Holter, normal exercise stress test, and normalised serum troponin.

Written for Australian practice and grounded in the guidance named above. Study material only — not medical advice. Always check against current guidelines.