To investigate the association of muscle fat fraction in the lower extremities with disease-specific endpoints in patients with DM2.

Myotonic dystrophy type 2 (DM2), a dominantly inherited muscular dystrophy, results from CCTG repeat expansion in the CNBP gene. DM2 is primarily characterized by progressive proximal muscle weakness, myotonia, and multisystemic features. A potential biomarker of disease severity includes increased intramuscular fat infiltration, which can be quantified by fat fraction (%) via magnetic resonance imaging (MRI). However, the relationship between this measure and clinical endpoints remains unstudied in DM2.

Lower extremity MRIs were obtained from seven patients with genetically-proven DM2 (86% female) using a two-point 3D Dixon volumetric interpolated breath-hold examination (VIBE) protocol with a 3T scanner. Muscle volumes (15mm thickness) were segmented from the proximal, middle, and distal levels of the thigh and calf in both legs. Muscle fat fraction (%) was calculated from the Dixon sequence by dividing the mean signal intensity of the fat volumes by the summed mean signal intensity of the fat and water volumes. Lower extremity fat fraction was correlated with quantitative muscle strength and functional outcomes, including a six-minute walk distance (meter) and 10-meter gait speed (second) using Spearman’s correlation (ρ).

Higher muscle fat infiltration of the lower extremities is associated with poorer physical performance, lower grip strength, and CCTG repeat length in patients with DM2. A larger study is required to evaluate fat fraction as a potential biomarker for disease progression and therapeutic response in DM2.