Subendocardial Involvement as an Underrecognized Cardiac MRI Phenotype in Myocarditis.

Background Subendocardial late gadolinium enhancement (LGE) detected with cardiac MRI in myocarditis represents a diagnostic dilemma, since it may resemble myocardial ischemia. Purpose To explore and compare the histopathologic characteristics and clinical features and outcomes in patients with myocarditis with and without subendocardial involvement at cardiac MRI. Materials and Methods This retrospective study evaluated 39 patients with myocarditis pathologically proven by means of either endomyocardial biopsy or explant pathologic findings between 2015 and 2020. Patients were divided into two groups according to cardiac MRI phenotype: 18 with subendocardial involvement (mean age ± standard deviation, 40 years ± 17; 10 women) and 21 with no subendocardial involvement (mean age, 35 years ± 11; six women). The median follow-up period was 784 days (interquartile range [IQR], 90-1123 days). The Student t test, Mann-Whitney U test, and univariable Cox regression were used for statistical analyses. Results In the 18 patients with subendocardial involvement, 12 (67%) had lymphocytic myocarditis and six (33%) had giant cell myocarditis. Patients with subendocardial involvement compared with those without subendocardial involvement had lower left ventricular ejection fraction (mean ± standard deviation, 27% ± 11 vs 41% ± 19; P = .004), larger LGE extent (median, 13% [IQR, 10%-22%] vs 5% [IQR, 2%-17%]; P < .001), higher rates of cardiac death or transplant (eight of 18 patients [44%] vs one of 21 patients [4.8%]; P = .006), higher probability of giant cell myocarditis (six of 18 [33%] vs one of 21 [4.8%]; P = .02), and more major adverse cardiovascular events (MACE) (15 of 18 [83%] vs seven of 21 [33%]; P = .002). In a subgroup of patients with comparable LGE extent (median, 15% vs 16%; P = .40) and left ventricular ejection fraction (median, 27% vs 31%; P = .26), the prognostic difference in terms of MACE remained (15 of 17 patients [88%] vs five of 10 [50%]; P = .02). Conclusion Subendocardial involvement detected with cardiac MRI in myocarditis indicated more severe clinical features, including a higher frequency of severe lymphocytic myocarditis or giant cell myocarditis and worse prognosis. © RSNA, 2021 See also the editorial by de Roos in this issue.