Atrial Function by Long Axis Shortening Enhances Risk Stratification Beyond Traditional CMR Markers in Possible Myocarditis.

Background Left ventricular ejection fraction (LVEF) and cardiac magnetic resonance (CMR) late gadolinium enhancement (LGE) are established risk markers for patients with myocarditis. Whether atrial functional impairment may occur in this clinical setting, and whether this finding offers incremental prognostic value, remains unknown.Objectives The authors sought to evaluate the association of left atrial (LA) and right atrial (RA) long axis shortening (LAS) assessed by CMR with major cardiovascular adverse events (MACE) in patients with possible myocarditis.Methods Patients meeting 2025 European Society of Cardiology criteria for "possible myocarditis" undergoing CMR at 2 tertiary centers, without coronary artery disease or alternative diagnosis, were included. LAS was associated with a composite of first MACE (heart failure hospitalization, sustained ventricular tachycardia, recurrent myocarditis, and all-cause mortality).Results A total of 787 patients (age 48 ± 16 years, 36% female) were included. LA-LAS was 21.5% ± 10.1%, and RA-LAS 29.5% ± 12.1%. After a median follow-up of 3.5 years (Q1-Q3: 1.8-6.5 years), 17.5% experienced a MACE. In the univariate analysis, both LA-LAS (per 5% decrease) (HR: 1.37; 95% CI: 1.23-1.52; P < 0.001) and RA-LAS (HR: 1.27; 95% CI: 1.19-1.37; P < 0.001), were associated with MACE. In multivariable models, LA-LAS below the median (22%) (HR: 2.17; 95% CI: 1.33-3.57; P = 0.002) and RA-LAS below the median (29%) (HR: 1.75; 95% CI: 1.12-2.70; P = 0.012) both independently predicted MACE. Adding LA-LAS and RA-LAS to the baseline model (clinical variables, left ventricular end-dastolic volume, LVEF, and LGE) resulted in improved risk stratification for MACE (chi-square increase by 11.3; P < 0.001; C-index from 0.72 to 0.75).Conclusions Atrial dysfunction can be present in possible myocarditis and atrial LAS was independently and incrementally associated with outcomes beyond traditional CMR markers and may refine future risk stratification in this clinical setting. (CMR Features in Patients With Suspected Myocarditis [CMRMyo]; NCT03470571; Inflammatory Cardiomyopathy Bern Registry [FlamBER]; NCT04774549).