Myocarditis

Dallas criteria[72]Aretz HT, Billingham ME, Edwards WD, et al. Myocarditis. A histopathologic definition and classification. Am J Cardiovasc Pathol. 1987 Jan;1(1):3-14. http://www.ncbi.nlm.nih.gov/pubmed/3455232?tool=bestpractice.com

• 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[73]Richardson P, McKenna W, Bristow M, et al. Report of the 1995 World Health Organization/International Society and Federation of Cardiology Task Force on the Definition and Classification of cardiomyopathies. Circulation. 1996 Mar 1;93(5):841-2. https://www.ahajournals.org/doi/10.1161/01.CIR.93.5.841 http://www.ncbi.nlm.nih.gov/pubmed/8598070?tool=bestpractice.com

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[74]The Criteria Committee of the New York Heart Association. Diseases of the heart and blood vessels: nomenclature and criteria for diagnosis, 6th ed. Boston, MA: Little Brown, 1964.

Class:

• Patients have cardiac disease but without resulting limitations of physical activity. Ordinary physical activity does not cause undue fatigue, palpitations, dyspnoea, or anginal pain

• 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

• 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

• 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 Heart Association (AHA) classification

The 2021 AHA statement on paediatric myocarditis lays out a new paradigm for myocarditis based on the following diagnostic strata:

• Possible myocarditis

• Clinically-suspected myocarditis

• Cardiac MRI-confirmed myocarditis

• Biopsy-proven myocarditis.