<scp>MRI</scp> ‐Determined Muscle Fat Fractions and Contractile Volumes in Myotonic Dystrophy Type 2

INTRODUCTION: Although magnetic resonance imaging (MRI) has been used to selectively characterize patterns of muscle involvement in myotonic dystrophy type 2 (DM2), the relationship between quantitative MRI-derived muscle composition and functional performance remains incompletely defined. This study aimed to characterize the imaging phenotype in DM2 using whole-body MRI scans and to evaluate the association between MRI-based muscle measurements and clinical endpoints. METHODS: Clinical endpoints collected included strength, physical function, and patient-reported outcomes. AI-based algorithms were utilized to segment 38 muscles bilaterally from Dixon MRI scans collected in a prospective, cross-sectional study. The segmentations were used to calculate muscle fat fraction (MFF) and contractile volume (CV) for each muscle. Associations between MRI measurements and clinical outcomes were examined using Spearman correlations. RESULTS: Adult patients (6 females, 6 males) with DM2 and no contraindications to MRI were enrolled at a single center. The mean age was 52.5 ± 15.4 years and mean disease duration was 12.5 ± 7.8 years. The gluteus maximus and erector spinae muscles had the highest average MFF. Mean MFF in fully captured muscles ranged from 7.4% to 36.5%. Higher MFFs were moderately associated with shorter 6-min walk distances (ρ = -0.62, p = 0.03) and slower 10-m walk/run times (ρ = 0.61, p = 0.04). Higher CVs were moderately associated with longer 6-min walk distance (ρ = 0.67, p = 0.02) and faster 10-m walk/run (ρ = -0.67, p = 0.02). DISCUSSION: These results identify a pattern of proximal muscle involvement and moderate associations between MFFs and CVs with physical function. Larger longitudinal studies are needed to validate the utility of imaging biomarkers in DM2.