Longitudinal MRI quantification of muscle degeneration in Duchenne muscular dystrophy

Objective: The aim of this study was to evaluate the usefulness of magnetic resonance imaging (MRI) in detecting the progression of Duchenne muscular dystrophy (DMD) by quantification of fat infiltration (FI) and muscle volume index (MVI, a residual-to-total muscle volume ratio). Methods: Twenty-six patients (baseline age: 5-12 years) with genetically proven DMD were longitudinally analyzed with lower limb 3T MRI, force measurements, and functional tests (Gowers, 10-m time, North Star Ambulatory Assessment, 6-min walking test). Five age-matched controls were also examined, with a total of 85 MRI studies. Semiquantitative (scores) and quantitative MRI (qMRI) analyses (signal intensity ratio -SIR, lower limb MVI, and individual muscle MVI) were carried out. Permutation and regression analyses according to both age and functional test-outcomes were calculated. Age-related quantitative reference curves of SIRs and MVIs were generated. Results: FI was present on glutei and adductor magnus in all patients since the age of 5, with a proximal-to-distal progression and selective sparing of sartorius and gracilis. Patients' qMRI measures were significantly different from controls' and among age classes. qMRI were more sensitive than force measurements and functional tests in assessing disease progression, allowing quantification also after loss of ambulation. Age-related curves with percentile values were calculated for SIRs and MVIs, to provide a reference background for future experimental therapy trials. SIRs and MVIs significantly correlated with all clinical measures, and could reliably predict functional outcomes and loss of ambulation. Interpretations: qMRI-based indexes are sensitive measures that can track the progression of DMD and represent a valuable tool for follow-up and clinical studies.