Diffuse Fibrosis in Hypertrophic Cardiomyopathy

BACKGROUND: Left ventricular (LV) myocardial extracellular volume (ECV), from cardiac magnetic resonance (CMR) T1 mapping, primarily reflects diffuse fibrosis and is a marker of adverse myocardial remodeling in hypertrophic cardiomyopathy (HCM). OBJECTIVES: The authors sought to evaluate the prognostic value of ECV beyond established clinical and imaging variables in a large contemporary HCM cohort. METHODS: We evaluated 1,050 consecutive patients with HCM and preserved LV ejection fraction (≥50%) who underwent CMR, including quantification of ECV (%) and late gadolinium enhancement (LGE) burden (% LV mass, >6 SDs above remote myocardium), between 2012 and 2021. The primary endpoint was a composite of mortality, appropriate implantable cardioverter-defibrillator (ICD) therapy, or heart transplantation. Statistical methods included multivariable Cox proportional hazards regression, restricted cubic spline analysis, assessment of incremental discrimination using changes in the C-statistic, and net reclassification improvement analysis. RESULTS: Median age was 60 years (IQR: 49-68 years) with 44% women. Median follow-up was 5.6 years (IQR: 2.5-7.8 years). The primary endpoint occurred in 124 patients (11.8%), including 111 deaths (10.6%), 12 appropriate ICD discharges (1.1%), and 1 heart transplantation (0.1%). In multivariable Cox regression, older age (HR: 1.05, 95% CI: 1.03-1.06; P < 0.001), NYHA functional class ≥II (HR: 1.96, 95% CI: 1.23-3.11; P = 0.004), diabetes mellitus (HR: 1.89, 95% CI: 1.27-2.81; P = 0.002), and higher ECV (HR: 1.05 per 1% increase, 95% CI: 1.02-1.09; P = 0.001) were independently associated with the primary outcome. Patients with ECV ≥32% had lower event-free survival than those with ECV <32% (adjusted HR: 1.81, 95% CI: 1.24-2.64; P < 0.001). CONCLUSIONS: Myocardial ECV was independently associated with long-term adverse outcomes. Multiparametric CMR assessment may improve risk stratification in HCM.