Myocarditis in Children: Recognition, Diagnosis and Management

Myocarditis is inflammation of the heart muscle. In children it is notoriously variable — from a mild illness that resolves unnoticed to acute heart failure and dangerous arrhythmias within hours — and it remains one of the leading causes of sudden cardiac events in young people and of heart transplantation in children. The 2021 American Heart Association scientific statement sets out a modern diagnostic and management framework, in which cardiac MRI has replaced biopsy as the central non-invasive tool.

How it presents

Older children and adolescents may report chest pain, breathlessness, palpitations, unusual fatigue or fainting, often days after a viral illness with fever, sore throat, diarrhoea or myalgia. Infants and toddlers cannot describe symptoms and instead present with poor feeding, irritability, rapid breathing, sweating with feeds, pallor or failure to thrive — signs easily mistaken for bronchiolitis, sepsis or a gastrointestinal illness. A high index of suspicion is the single most important diagnostic factor: myocarditis should be considered in any child with unexplained respiratory distress or shock, especially with a recent viral prodrome.

Tests that establish the diagnosis

The work-up runs in layers. Troponin is a sensitive marker of heart muscle injury and is central to diagnosis, though a normal value does not exclude it. ECG is abnormal in most cases but is non-specific. Echocardiography assesses ventricular function, valve regurgitation, pericardial effusion and excludes congenital anomalies. Chest X-ray may show an enlarged heart or fluid. Cardiac MRI is now the key non-invasive test: using tissue-characterisation sequences, it detects oedema, hyperaemia and late gadolinium enhancement in a pattern characteristic of myocarditis, and the updated Lake Louise criteria formalise this. Endomyocardial biopsy — the historical gold standard — is now reserved for specific situations: fulminant presentations, suspected giant-cell or eosinophilic myocarditis, suspected cardiac sarcoidosis, or when the diagnosis remains uncertain and would change immunosuppressive treatment.

Causes

Viral infection is the commonest trigger, with parvovirus B19, enteroviruses, adenovirus and — in the COVID-19 era — SARS-CoV-2 among the recognised agents. Non-infectious causes matter too: immune-mediated myocarditis, hypersensitivity reactions to drugs, systemic autoimmune disease, and checkpoint-inhibitor cancer therapy. In many cases no cause is identified despite full investigation, which does not make the illness any less real.

Management

Care is largely supportive and escalates by severity. Hospitalised children are monitored continuously for arrhythmia, with echocardiography and, in severe cases, invasive haemodynamic monitoring. Heart failure is treated with diuretics, vasodilators afterload reduction, and inotropes where needed; mechanical circulatory support — ECMO or a ventricular assist device — bridges the sickest patients, and a substantial proportion of fulminant cases recover ventricular function with support. Arrhythmias and conduction block are managed according to standard algorithms, sometimes with temporary pacing. Intravenous immunoglobulin and corticosteroids are used in selected immune-mediated or biopsy-proven cases, but the evidence base is limited and their use should be individualised.

Recovery and follow-up

Exercise must be restricted during the acute phase and for a defined recovery period afterwards — typically several months — because intense exertion during active inflammation risks arrhythmia. Return to sport requires normalisation of function and freedom from significant arrhythmia, usually confirmed by exercise testing or ambulatory monitoring and sometimes repeat MRI. Long-term follow-up is essential: a proportion of children develop persistent ventricular dysfunction or dilated cardiomyopathy, and the first episode does not preclude recurrence.