10th May 2025

Diagnosis

Patients typically present with a classical history of chest pain and on elucidating the history report recent or current infective symptoms, e.g. upper respiratory tract symptoms or recent fevers. The chest pain is often atypical, described as sharp or stabbing and may be positional if the pericardium is also involved (i.e. the pain is worse when lying flat). Some patients may be asymptomatic with only subclinical infection, or present late with symptoms of heart failure.

Troponin is frequently significantly and persistently raised and is often described as a “troponin leak”, although it can also be normal. Inflammatory markers are frequently raised, with CRP elevated in 80-90% of patients. PCR testing and IgG antibodies of common viral pathogens are recommended, although will frequently be negative.

ECG is frequently abnormal with non-specific ST- and T-wave changes, ventricular ectopics and AV conduction abnormalities. However diagnostic value is limited as the changes are neither specific nor sensitive enough alone for a definitive diagnosis.

Echocardiography is essential for ruling out any signs of heart failure due to myocarditis but do not make the diagnosis. Suggestive echocardiographic features of myocarditis are:

• New structural or functional abnormalities
• Regional wall motion abnormalities or global ventricular dysfunction without ventricular dilatation or with, generally mild, dilatation
• Increased wall thickness due to myocardial oedema
• Pericardial effusion
• Intracardiac thrombi, not explained by other conditions (e.g., coronary artery disease (CAD), acute coronary syndrome (ACS) or valvular heart disease).

Endomyocardial biopsy

This is indicated for those with progressive or persistent severe cardiac dysfunction and/ or life-threatening arrhythmias. The aim is to identify specific aetiology and treatment, e.g. giant cell myocarditis, eosinophilic myocarditis, sarcoidosis, and systemic inflammatory disorders. Histopathologically it is characterised by inflammatory cellular infiltrates with or without evidence of myocyte injury. Ideally, 5-7 samples are recommended – 3 for pathology, 2 for infections (PCR) and 2 for RNA viruses/ viral replications and therefore the type of infiltrate can be used to classify myocarditis:

• lymphocytic myocarditis: associated with pathogens, especially viral causes, as well as drugs, radiation and autoimmune disorders
• eosinophilic myocarditis: relatively uncommon, associated with parasitic infections, hypersensitivity reactions, eosinophilic inflammatory disorders and neoplastic processes
• Giant-cell myocarditis: frequently idiopathic, but also associated with autoimmune disorders such as thyroid disease or inflammatory bowel diseases
• granulomatous myocarditis: associated with sarcoidosis

However, biopsy provides many limitations, mostly due to its invasive nature and infrequent clinical use (often due to limited clinical centres that perform the procedure).

Cardiac MRI

Therefore more commonly cardiac magnetic resonance imaging (CMR) is used as a non-invasive alternative and the current European Society of Cardiology guidelines for acute and chronic heart failure consider CMR a useful method in patients with suspected myocarditis.

Cardiac MR findings suggestive of myocarditis:

• Oedema
• Inflammation and fibrosis detection
• Quantification and localisation through T1 and T2 mapping
• Extracellular volume assessment and late gadolinium enhancement (LGE)

ESC Working Up on Myocardial and Pericardial Diseases (2013) diagnostic criteria

As per ESC guidelines, myocarditis should be suspected if: 

•  ≥1 clinical presentation and ≥1 diagnostic criteria from different categories, in the absence of (1) angiographically detectable coronary artery disease (coronary stenosis ≥50%); 
• (2) known pre-existing cardiovascular disease or extra-cardiac causes that explain the symptoms (for instance, valve disease, congenital heart disease, hyperthyroidism, etc.)

These are summarised in the below table:

Table 2: Clinical Presentation and Diagnostic Criteria

Other investigations to consider:

• Coronary angiography or CT coronary angiography (CTCA)  to rule out coronary artery disease or ACS in those in which the clinical picture is unclear
• Cardiac PET, especially in those with suspected systemic autoimmune disease or cardiac sarcoidosis