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Cardiac manifestations of Duchenne muscular dystrophy (DMD) include progressive cardiac dysfunction and an elevated resting heart rate (HR). We hypothesized this elevated HR reflects autonomic dysfunction that can be identified by heart rate variability (HRV) analyses which will be associated with myocardial fibrosis by cardiac magnetic resonance imaging (cMR). DMD patients ( N = 74) and controls ( N = 17) had time and frequency domain HRV analyses calculated via Holter monitoring. Cardiac magnetic resonance imaging was performed on DMD cases only. χ test, T test, ANOVA, and logistic regression were used to perform comparisons between groups. A p value of <0.05 was used for statistical significance. DMD cases had higher resting average HR than controls (99.4 ± 8.9, 85.4 + 6.2, p < 0.001). Among HRV variables, decreases were seen in the following: standard deviation of R to R intervals, the percent RR intervals differing by >50 ms from previous RR interval, the root-meansquare of successive differences of RR intervals, the standard deviation of the mean R to R segment (SDANN), low frequency, and high frequency domain, all p values 0.001. Maximum HR and SDANN most significantly associated with positive LGE on cMR ( p = 0.008, p = 0.016). DMD cases on beta blocker had an average HR lower than those not on beta blocker ( p = 0.009), but with no difference in HRV analysis. DMD patients have reduced HRV and therefore autonomic dysfunction prior to the onset of heart failure which is associated with myocardial fibrosis. [ABSTRACT FROM AUTHOR]