Objectives: We aimed to identify cardiac magnetic resonance (CMR) characteristics to discriminate biopsy-proven: (1) lymphocytic myocarditis from dilated cardiomyopathy (DCM), and (2) different stages of myocarditis (acute vs. chronic vs. healed myocarditis).

Method: 208 patients (age 51±15 years; 24% female) with endomyocardial biopsies (EMB) and CMR in the years 2008-2020 were evaluated and divided in groups of acute, chronic, healed lymphocytic myocarditis vs DCM. CMR protocol comprised functional, strain, T2 imaging, and late gadolinium enhancement imaging (LGE).

Results: Right ventricular EMB revealed: 1) acute myocarditis (n=7, 3%), 2) chronic myocarditis (n=117, 57%), 3) healed myocarditis (n=36, 17%), and 4) DCM (n=48, 23%). Left-ventricular ejection fraction (LV-EF) was higher, and LV end-diastolic volume index (LV-EDVI) was lower in myocarditis patients (acute: 41%, 83 ml/m²; chronic: 41%, 101 ml/m²; healed: 44%, 95 ml/m²) compared to DCM patients (29%, 131 ml/m²), p<0.0001.

Myocarditis patients demonstrated a higher myocardial edema prevalence (86% acute; 58% chronic; 25% healed) than DCM patients (17%), p<0.0001. Myocarditis patients also demonstrated a higher LGE prevalence (71% acute; 66% chronic; 72% healed) than DCM patients (40%), p=0.006. Strain analysis demonstrated lower longitudinal and radial strain rates in DCM patients vs. chronic myocarditis patients, p<0.003. Beside T2 images, myocarditis patients defined by their histological stage (acute/chronic/healed) showed no significant differences in any of the CMR characteristics.

Conclusions: Despite some significant differences in DCM versus myocarditis groups, CMR alone seems not to be sufficient to differentiate reliably between DCM vs. myocarditis patients. Moreover, different histological stages of myocarditis demonstrate significant overlap in CMR parameters, underlining the unmet need of further clinical information, and EMB in ambiguous cases to clarify definite diagnosis.