Chronic myocarditis in patients with atrial fibrillation of unknown origin: diagnostic challenges and unresolved issues

Aim . To analyze the significance of contrast-enhanced cardiac magnetic resonance imaging (MRI) and endomyocardial biopsy (EMB) in the diagnosis of myocarditis in patients with atrial fibrillation (AF) of unknown origin. Material and methods . The study included 57 patients with AF of unknown origin aged 45,5 [38,8;53,3] years (men, 77,2%). Paroxysmal AF was observed in 56,1% of patients, while persistent AF — in 22,8%, and long-standing persistent AF — in 21,1%. Thirty-five patients underwent contrast-enhanced cardiac MRI and Lake-Louise criteria assessment. Simultaneously with AF ablation, all patients underwent EMB from the interventricular septum, outflow tract, and right ventricular apex with histological diagnosis of myocarditis based on the Dallas criteria modified by the World Heart Federation. Results . According to contrast-enhanced cardiac MRI, delayed contrast accumulation was diagnosed in 91,4%. In 17,1% of cases, signs of edema were detected on T2-weighted images, while the average left ventricular (LV) edema level was 1,6±0,3 (reference <2,0). In 25,7% of patients, signs of hyperemia were detected with average LV hyperemia index of 3,4 [2,2;4,0] (reference <4,0). Myocarditis was diagnosed in 12 patients (34,3%) with 3 criteria detected in 2 patients, 2 in 10 patients (6 with hyperemia + fibrosis, 4 with edema + fibrosis), and 1 (fibrosis) in 22 patients. Histological criteria of lymphocytic myocarditis were detected in 28 out of 57 patients (49,1%): focal in 19 (67,9%), diffuse in 6 (21,4%), and diffuse-focal in 3 (10,7%). Edema signs on MRI was the factor most strongly associated with the total number of detected T-lymphocytes in myocardial fragments, assessed using ROC analysis (AUC 0,782 (95% confidence interval 0,61-0,93) p=0,032). The combined use of MRI and EMB allowed to increase the accuracy of myocarditis diagnosis from 34,3 to 62,9%. Conclusion . Myocarditis was diagnosed in 34,3% of patients according to MRI and in 49,1% according to EMB. Edema according to MRI data was the only sign that had predictive value for the severity of inflammatory cell infiltration in myocardial fragments. Histological analysis in combination with MRI increased the accuracy of myocarditis diagnosis in patients with AF of unknown origin from 34,3 to 62,9%.