External validation of the 2025 ESC myocarditis risk stratification: incremental prognostic value of quantitative CMR markers

Background The 2025 European Society of Cardiology (ESC) guideline proposes a three-tier clinical risk stratification for acute myocarditis. External prognostic validation, particularly for separating low and intermediate risk, remains limited. Purpose To externally validate the 2025 ESC risk stratification and assess whether quantitative cardiovascular magnetic resonance (CMR) tissue markers improve prognostic discrimination. Methods We retrospectively included 319 patients with CMR-diagnosed acute myocarditis. Major adverse cardiac events were assessed during follow-up (median 3.0 years). We compared the original 2025 ESC model with three modified strategies: Model 1 replaced late gadolinium enhancement (LGE) segment count with quantitative LGE extent (optimal threshold ≥9.5%); Model 2 further incorporated extracellular volume (ECV) (threshold ≥37.5%); Model 3 tested a CMR-only three-tier scheme using ESC-defined left ventricular ejection fraction categories together with dichotomised LGE extent and ECV. Prognostic performance was evaluated using Kaplan–Meier analysis and Cox regression. Results Fifty-seven events occurred (17.9%). In the original ESC model, the high-risk group had higher risk than the low-risk group (hazard ratio 7.13; p=0.004), while the intermediate-risk group was not significantly different from low risk (hazard ratio 1.68; p=0.455). Model 1 improved separation, with higher risk in both intermediate (hazard ratio 4.54; p=0.045) and high-risk groups (hazard ratio 16.15; p<0.001) compared with low risk. Adding ECV (Model 2) enhanced discrimination, with intermediate- and high-risk groups showing markedly higher risk than low risk (HR 6.55 and 20.83; both p≤0.013). The CMR-only strategy (Model 3) provided clear three-tier separation, showing a graded increase in risk from low to intermediate (HR 3.92) and high risk (HR 14.11; both p≤0.002). Conclusion In this external cohort of acute myocarditis, the 2025 ESC scheme reliably identified high-risk patients but showed limited separation between low and intermediate risk. Quantitative LGE extent and ECV improved prognostic stratification and may support a practical CMR-based approach.