To characterize lower extremity muscle fat fraction (MFF) in patients with myotonic dystrophy type 1 (DM1), myotonic dystrophy type 2 (DM2), and controls. Additionally, we examine the associations between MFF and disease-specific endpoints.

DM1 and DM2 are dominantly inherited muscular dystrophy primarily characterized by progressive muscle weakness, myotonia, and multisystemic features. A potential biomarker of disease severity includes MFF (%) derived from magnetic resonance imaging (MRI). However, how lower extremity MFF compares between DM1 and DM2 has not been previously examined against controls.

Mean age was 50±13 years for DM1, 60±13 for DM2, and 56±8 for controls. Disease duration was 19±14 for DM1 and 18±14 years for DM2. Mean MFF of the thigh was 35.7% (range: 15.3%-79.9%) for DM1, 34.4% (17.8%-58.0%) for DM2, and 13.2% (10.5%-16.6%) for controls. Mean MFF of the calf was 32.9% (range: 17.9%-70.1%) for DM1, 27.3% (21.3%-34.6%) for DM2, and 12.6% (10.9%-14.9%) for controls. Higher MFF were strongly correlated with shorter distance of 6MWT (ρ=-0.74 for thigh and ρ=-0.64 for calf, p-value<0.01), weaker grip strength (ρ=-0.77 for thigh and calf, p-value<0.001), and less steps completed (ρ=-0.78 for thigh and ρ=-0.77 for calf, p-value<0.001). MFF were also moderately correlated with disease duration (p-value>0.5).