Criteria
Dallas criteria[76]
Active myocarditis: the presence of an inflammatory infiltrate of the myocardium with necrosis and/or degeneration of adjacent myocytes not typical of the ischaemic damage associated with coronary artery disease.
Borderline myocarditis: the presence of an inflammatory infiltrate of the myocardium without necrosis or degeneration of adjacent myocytes.
World Health Organization Marburg Classification[77]
First biopsy:
Acute (active) myocarditis: a clear-cut infiltrate (diffuse, focal or confluent) of >14 leukocytes/mm² (preferably activated T cells). The amount of the infiltrate should be quantified by immunohistochemistry. Necrosis or degeneration is compulsory; fibrosis may be absent or present and should be graded.
Chronic myocarditis: an infiltrate of >14 leukocytes/mm² (diffuse, focal or confluent, preferably activated T cells). Quantification should be made by immunohistochemistry. Necrosis or degeneration is usually not evident; fibrosis may be absent or present and should be graded.
No myocarditis: No infiltrating cells or <14 leukocytes/mm².
Subsequent biopsies:
Ongoing (persistent) myocarditis. Criteria as in acute or chronic myocarditis.
Resolving (healing) myocarditis. Criteria as in acute or chronic myocarditis, but the immunological process is sparser than in the first biopsy.
Resolved (healed) myocarditis. Corresponds to the Dallas classification.
The amount and distribution of fibrosis should be described similarly as no (grade 0), mild (grade 1), moderate (grade 2) or severe (grade 3). Localisation or formation of fibrosis should be outlined as endocardial, replacement, or interstitial.
New York Heart Association functional classification[78]
Class I: Patients have cardiac disease but without resulting limitations of physical activity. Ordinary physical activity does not cause undue fatigue, palpitations, dyspnoea, or anginal pain.
Class II: Patients have cardiac disease resulting in slight limitation of physical activity. They are comfortable at rest. Ordinary physical activity results in fatigue, palpitation, dyspnoea, or anginal pain.
Class III: Patients have cardiac disease resulting in marked limitation of physical activity. They are comfortable at rest. Less than ordinary physical activity causes fatigue, palpitation, dyspnoea, or anginal pain.
Class IV: Patients have cardiac disease resulting in an inability to carry on any physical activity without discomfort. Symptoms of cardiac insufficiency or of the anginal syndrome may be present even at rest. If any physical activity is undertaken, discomfort is increased.
American College of Cardiology (ACC) classification[56]
The ACC describes the spectrum of myocarditis in four stages, mirroring the adult heart failure paradigm. Although this classification is based on adult studies, they suggest the approach can be adapted for paediatric populations.
Stage A: at-risk for myocarditis
Patients with risk factors for developing myocarditis but no current diagnosis, symptoms, or structural, biomarker, imaging, or histopathological markers of myocarditis.
Stage B: asymptomatic myocarditis
Patients with risk factors but no classic symptoms of acute myocarditis who have: 1) histopathology consistent with myocarditis, 2) cardiac MRI findings consistent with myocarditis, or 3) elevated high-sensitivity cardiac troponin levels with additional supportive evidence consistent with myocarditis.
Stage C: symptomatic myocarditis
Patients who have classic symptoms of acute myocarditis with: 1) histopathology consistent with myocarditis, 2) cardiac MRI findings consistent with myocarditis, or 3) elevated high-sensitivity cardiac troponin levels with additional supportive evidence consistent with myocarditis.
Stage D: advanced myocarditis
Patients with symptomatic myocarditis (meeting stage C criteria) who have haemodynamic or electrical instability requiring intervention.
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