Background Myocardial replacement fibrosis is one of the major histologic features of hypertrophic cardiomyopathy (HCM), but its characteristics have not been well delineated. Purpose To clarify the characteristics of replacement fibrosis in HCM and to evaluate the prognostic value of the regional extent of fibrosis. Materials and Methods This prospective study evaluated participants with HCM who underwent contrast-enhanced cardiac MRI from March 2011 to April 2019. For each participant, global and 16-segment extent of late gadolinium enhancement (LGE) in the left ventricle (LV) at cardiac MRI was analyzed. The primary end point was all-cause death. Results Among the 798 study participants enrolled (median age, 49 years [interquartile range {IQR}: 38-59 years]; 508 men), 588 (74%) underwent whole-exome sequencing. Thirty-five participants (4%) experienced death from any cause during a median follow-up of 2.9 years (IQR: 1.5-4.7 years). Spearman analysis showed weak correlations between the extent of LGE and wall thickness (LGE of global LV and maximal LV wall thickness, r = 0.35 [P < .001]; LGE and thickness of septum, r = 0.30 [P < .001]). In the 16-segment model, the distribution of LGE was visually inhomogeneous and higher in the basal anterior, basal septal, midanterior, and midseptal regions (P < .001). This similar distribution of LGE was observed in participants with asymmetric septal hypertrophy, those with apical HCM, participants positive for mutation and those negative for mutation, and participants with MYH7 and MYBPC3 mutations. Cox analysis indicated that both the global extent of LGE (adjusted hazard ratio = 1.68 per 10% increase in LGE; P < .001) and the regional extent of LGE (ie, basal, midventricular, and apical regions of LV when on the short-axis view; septum, anterior free wall, inferior free wall, and lateral free wall when on the long-axis view) were associated with adverse outcomes. Conclusion In hypertrophic cardiomyopathy, myocardial replacement fibrosis weakly correlated with hypertrophy, was inhomogeneous and asymmetric, and was predominantly distributed in the interventricular septal wall and anterior free wall at the basal and mid levels. Greater extent of fibrosis was associated with poor prognosis, regardless of its location in the left ventricle. © RSNA, 2021 See also the editorial by Hanneman in this issue.

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