What are the Dallas criteria for diagnosing myocarditis?

Dallas Criteria for Myocarditis

The Dallas criteria, established in 1986, define myocarditis histologically as an inflammatory infiltrate of the myocardium with necrosis and/or degeneration of adjacent myocytes, not typical of ischemic damage associated with coronary artery disease. 1

Historical Definition and Components

The Dallas criteria were formally established to standardize the histopathologic diagnosis of lymphocytic myocarditis on endomyocardial biopsy (EMB) specimens. 1 The criteria specifically require:

• Inflammatory infiltrate present in the myocardium 2
• Myocyte necrosis and/or degeneration adjacent to the inflammatory cells 2, 3
• Pattern NOT typical of ischemic coronary artery disease 1, 2

The criteria classify findings into categories: active myocarditis, borderline myocarditis, or no myocarditis based on the presence and extent of inflammation and myocyte damage. 1

Critical Limitations Recognized by Guidelines

The American Heart Association, American College of Cardiology, and European Society of Cardiology joint scientific statement explicitly questions the Dallas criteria as the gold standard for myocarditis diagnosis due to three major flaws: 1

Sampling Error

• Myocardial inflammation is often patchy and focal, not diffuse 1, 3
• Standard EMB obtains only 3-4 small fragments (1-2 mm each), which may miss areas of active inflammation 2
• This leads to high false-negative rates even when myocarditis is present 1

Interobserver Variability

• Significant disagreement exists among pathologists interpreting the same biopsy specimens using Dallas criteria 1
• Studies show wide variation in myocarditis diagnosis rates (0% to 63%) when different pathologists apply these criteria 1
• The subjective nature of identifying "inflammatory infiltrate" and "myocyte damage" contributes to inconsistent interpretation 4

Lack of Correlation with Viral Genomes

• Dallas criteria myocarditis does not correlate well with demonstration of viral genomes in heart tissue by molecular techniques 1
• Patients can have positive viral PCR without meeting Dallas criteria, and vice versa 1

Clinical Impact and Prognostic Value

The information gained from Dallas criteria does not alter prognosis or therapy in most patients with acute dilated cardiomyopathy. 1

Key evidence demonstrating limited clinical utility:

• Myocarditis Treatment Trial: 111 patients with active or borderline myocarditis by Dallas criteria showed no benefit from 24-week immunosuppression (prednisone plus azathioprine or cyclosporine) compared to conventional therapy 1
• IMAC-1 Trial: Among patients with acute nonischemic dilated cardiomyopathy, only 16% had Dallas criteria myocarditis, and intravenous immunoglobulin showed no benefit 1
• Grogan et al study: Survival rates in patients with Dallas criteria myocarditis were identical to those without inflammation 1

Superior Alternative Diagnostic Approaches

Immunohistochemical (IHC) criteria have largely superseded Dallas criteria, with significantly higher diagnostic sensitivity. 5, 3

Modern Immunohistochemical Standards

• Threshold: >14 leukocytes/mm² with >7 T lymphocytes/mm² is the accepted diagnostic cutoff 2
• Diagnostic yield: IHC confirms myocarditis in 54% of suspected cases versus only 28% using Dallas criteria (P <0.01) 5
• Prognostic value: IHC-confirmed inflammation predicts worse outcomes—21% required heart transplant/LVAD or died versus 7% without inflammation 5

Molecular Biological Techniques

• PCR or nested-PCR can detect low-copy viral genomes from small EMB specimens 2
• Positive PCR results should be accompanied by parallel blood sample testing at the time of EMB 2
• Viral genome detection provides etiologic diagnosis beyond what Dallas criteria can offer 3, 6

Novel Immune Markers

• HLA-ABC and HLA-DR expression on cardiomyocytes may guide therapy in chronic dilated cardiomyopathy 1
• These markers are more sensitive for persistent immune activation than lymphocyte infiltration alone 1
• Not yet in routine clinical use but show promise for identifying patients who may benefit from immunomodulation 1

Current Guideline Recommendations

The AHA/ACC/ESC Writing Group does NOT recommend performing EMB using Dallas criteria for routine evaluation of new-onset heart failure of 2 weeks to 3 months duration that responds to usual care within 1-2 weeks. 1

This recommendation reflects the consensus that Dallas criteria-based diagnosis:

• Rarely changes clinical management 1
• Does not predict response to immunosuppressive therapy 1
• Has been superseded by more sensitive and specific techniques 2, 5

Common Pitfalls to Avoid

• Do not rely solely on Dallas criteria when EMB is clinically indicated—always request immunohistochemistry and viral PCR 2, 3
• Do not assume negative Dallas criteria rules out myocarditis—sampling error and low sensitivity mean significant inflammation may be missed 1, 5
• Do not use Dallas criteria results alone to guide immunosuppressive therapy decisions—clinical trials show no benefit even when Dallas criteria are positive 1