Elbow Flexors Muscle Fat Fraction Is a Sensitive and Relevant Outcome Measure in Nonambulant Patients With DMD.

Duchenne muscular dystrophy (DMD) is an X-linked neuromuscular disease characterized by progressive muscle wasting. Objective and relevant outcome measures are needed for longitudinal follow-up and clinical trials. Quantitative MR muscle parameters in the legs are sensitive to change and related to function. However, data are limited for the upper limb, which is crucial for nonambulant DMD patients. This study investigated which MR parameters were sensitive to change over time and associated with loss of upper limb function. Muscle fat fraction (mFF) and contractile volume (CV) for elbow flexor and extensor muscles were obtained using chemical-shift-based fat-water separation scans at 3T (Philips, Ingenia) at baseline, 12, and 18 months. Standardized response means (SRM) were determined for different analysis volumes: center slice (CS), 5 (5S), 7 (7S), and all slices (AS). Parameters with SRM > 0.8 were associated with Performance of the Upper Limb, PUL2.0, using linear mixed models. Twenty nonambulant DMD patients participated. Baseline mFF ranged between 43.1% ± 12.6% for elbow extensors AS and 55.5% ± 18.9% for elbow flexors CS. SRM ranged from 0.37 for elbow flexors CV CS to 1.71 for elbow flexors AS and was consistently higher for increasing analysis volumes, longer follow-up time, and for mFF in comparison with CV. Associations with function were significant for all parameters and ranged from -0.25 to -0.34 (p < 0.001) for mFF and 0.12-0.49 (p < 0.001) for CV, except for elbow extensor CS CV (p = 0.87). Our study demonstrates that assessment of a larger part of the muscle increases the sensitivity to change over time in nonambulant DMD patients. Elbow flexor and extensor mFF 5S, 7S, and AS were sensitive to change over 12 months and related to loss of upper limb function in this population.