ACC 2024 Expert Consensus Decision Pathway on Myocarditis

Comprehensive Summary: Imaging Recommendations with Focus on CMR

Citation: Drazner MH, et al. 2024 ACC Expert Consensus Decision Pathway on Strategies and Criteria for the Diagnosis and Management of Myocarditis. JACC 2025;85(4):391-431

Significance: First ACC Expert Consensus providing structured decision pathways for myocarditis diagnosis and management, including explicit follow-up imaging protocols stratified by risk.

1. Novel Stage-Based Classification System

The document introduces a staging system analogous to heart failure:

Stage	Definition	Imaging Role
A	Risk factors for myocarditis (e.g., recent viral illness + troponin elevation) but no confirmed diagnosis	Baseline echo recommended
B	Subclinical myocarditis: CMR or histological evidence without symptoms	CMR follow-up recommended
C	Symptomatic myocarditis	Risk-stratified imaging protocol
D	Advanced: cardiogenic shock, refractory VT, need for MCS	CMR when stabilised

Key implication: Stage B formally recognises patients with incidental CMR findings of myocarditis—a previously under-addressed group.

2. CMR: The Gold Standard for Noninvasive Diagnosis

Position Statement

CMR is widely endorsed for assessing myocarditis with a Class 1 indication in various guidelines. The document positions CMR as: - Gold-standard for noninvasive multiparametric tissue characterisation - Essential for assessing cardiac structure and function - Key tool for prognostication through serial imaging

Updated Lake Louise Criteria (2018) Application

The document applies the updated Lake Louise Criteria as established by Ferreira et al.¹

Diagnosis requires ≥1 T2-based + ≥1 T1-based criterion:

T2-based (oedema): - Global or regional ↑ myocardial T2 relaxation time - ↑ signal intensity on T2-weighted imaging

T1-based (injury/fibrosis): - ↑ native T1 - ↑ extracellular volume (ECV) - Late gadolinium enhancement (LGE) in non-ischaemic pattern

Important nuance: Having only 1 criterion (T1 OR T2 alone) may still support diagnosis in appropriate clinical context, but with lower specificity—“added caution needed.”

3. CMR Technical Considerations

Timing

Diagnostic sensitivity depends on timing relative to myocardial injury
“MRI-negative myocarditis” may occur when:
- Methods not sufficiently sensitive/high-resolution
- Technical limitations (tachyarrhythmia, motion, artefacts)
- Insufficient experience at imaging centre

Protocol Recommendations

Standard: Cine imaging, T1/T2 mapping, LGE
Additional: “Reasonable to include stress perfusion” to help exclude ischaemic aetiology
Advanced: CMR strain (feature-tracking or tissue tagging) for subtle dysfunction

LGE Pattern Recognition

Pattern	Association
Mid-wall/subepicardial, lateral/inferior	Classic myocarditis
Diffuse subendocardial + LV thrombus	Eosinophilic myocarditis
Ring-like mid-wall/subepicardial	Desmoplakin and other genetic cardiomyopathies
Midwall septal	Poor prognosis marker

4. Structured Follow-up Imaging Protocol (Table 3)

This is a major contribution—first explicit guidance on imaging timing by risk.

Low-Risk Stage C (all of: normal LVEF, no LGE, haemodynamically/electrically stable):

Timepoint	Test
2-4 weeks	Office visit + Echocardiogram (with strain)
6 months	Echocardiogram

Stage D or Medium/High-Risk Stage C (any of: ↓LVEF, NSVT/higher-grade ectopy, significant bradyarrhythmia, LGE on CMR, ↑18F-FDG on PET, haemodynamic instability/clinical HF):

Timepoint	Test
2-4 weeks*	Office visit + Biomarkers + ECG + Echocardiogram
Interval	Titrate GDMT for HFrEF
6 months	CMR

*Patients with symptomatic HFrEF should be seen within 1 week of discharge

Athletes: CMR may be repeated as early as 3 months (for return-to-sport decisions)

5. Prognostic Value of Serial CMR

Key Findings from Evidence Review

Follow-up CMR at 3-6 months: - More sensitive at detecting persistent inflammation than serial echo or biomarkers² - T1 and T2 may remain elevated after normalisation of T2 ratio, EGE ratio, and ECV

Prognosis by CMR findings (Aquaro et al., n=202 with follow-up CMR, 7-year follow-up)³:

Follow-up CMR Finding	n	Cardiac Events	Event Rate
LGE + oedema persist	22	19	86%
LGE without oedema	—	—	Intermediate
No LGE, no oedema	—	0	0%

Independent predictors of cardiac events (multivariable analysis)³: 1. Midwall septal LGE pattern on follow-up CMR (HR 2.8; 95% CI 1.1-7.2; p=0.028) 2. Persistence of LGE without oedema (HR 4.5; 95% CI 1.3-14.5; p=0.008)

6. Other Imaging Modalities

Echocardiography

First-line imaging: widely available, no contraindications
Features suggesting myocarditis: ↑ wall thickness, ↓ LVEF, WMA, pericardial effusion
Strain imaging: Can detect subclinical dysfunction; reduced GLS associated with VA and outcomes
Limitation: Not specific for myocarditis (cannot differentiate from other cardiomyopathies)

Nuclear/PET Imaging (Section 4.2.5)

FDG-PET: - Detects metabolically active inflammatory cells - Requires careful patient preparation (12-24h high-fat/low-carb diet, then 6-12h fasting) - Useful when CMR not feasible (arrhythmia, non-CMR-compatible device)

Hybrid PET/CMR: - May offer incremental value vs either modality alone - Improves sensitivity for detecting inflammation in areas difficult for CMR (e.g., near devices)

7. Role of CMR in Return-to-Sport Decisions

Figure 11 Algorithm: Athletes abstain from strenuous activity until: 1. Symptoms resolved 2. Biomarkers normalised 3. No significant arrhythmias on 24h monitoring 4. CMR shows resolution of inflammation (can be done at 3 months)

Abnormal follow-up CMR (any of: ↓LVEF/RVEF, evidence of non-ischaemic inflammation, LGE) = continued restriction from competitive sports.

8. Diagnostic Pathway Integration

CMR is positioned as a key decision node when: - Troponin elevated + symptoms suggest myocarditis - Echo shows new dysfunction without clear aetiology - Distinguishing myocarditis from ACS (include stress perfusion) - Incidental finding of LGE on CMR done for other indications (→ Stage B) - Assessment for recovery/persistence of inflammation

Complementary Roles: CMR vs EMB

CMR	EMB
Noninvasive	Invasive (0.6-5% complication rate)
Assesses whole myocardium	Sampling error
Cannot determine histotype	Gold standard for histotype (viral, GCM, etc.)
High sensitivity/specificity when criteria met	Needed when specific aetiology changes management

9. Summary: What Clinicians Should Do

Use stage-based approach to classify patients (A-D)
Order CMR for diagnosis when myocarditis suspected (include T1/T2 mapping, LGE)
Consider stress perfusion in CMR protocol if ischaemia not excluded
Risk-stratify patients based on LVEF, LGE presence, arrhythmias
Follow structured protocol for repeat imaging:
- Low-risk: Echo at 2-4 weeks and 6 months
- Medium/high-risk: Echo at 2-4 weeks, CMR at 6 months
Recognise prognostic patterns: Persistent LGE ± oedema on follow-up = higher risk
Athletes: CMR at 3 months acceptable for return-to-sport clearance
Consider FDG-PET when CMR inconclusive or contraindicated

Key CMR References

Ferreira VM, Schulz-Menger J, Holmvang G, et al. Cardiovascular magnetic resonance in nonischemic myocardial inflammation: expert recommendations. J Am Coll Cardiol 2018;72:3158-76. [Updated Lake Louise Criteria]
Berg J, Kottwitz J, Baltensperger N, et al. Cardiac magnetic resonance imaging in myocarditis reveals persistent disease activity despite normalization of cardiac enzymes and inflammatory parameters at 3-month follow-up. Circ Heart Fail 2017;10:e004262.
Aquaro GD, Ghebru Habtemicael Y, Camastra G, et al. Prognostic value of repeating cardiac magnetic resonance in patients with acute myocarditis. J Am Coll Cardiol 2019;74:2439-48. [Key prognostic serial CMR study]
Messroghli DR, Moon JC, Ferreira VM, et al. Clinical recommendations for cardiovascular magnetic resonance mapping of T1, T2, T2* and extracellular volume: a consensus statement by the Society for Cardiovascular Magnetic Resonance. J Cardiovasc Magn Reson 2017;19:75. [T1/T2 mapping technical standards]

Document prepared for www.cardiac-imaging.org Last updated: January 2026