Quantitative Muscle MRI Depicts Increased Muscle Mass after a Behavioral Change in Myotonic Dystrophy Type 1.

Background Patients with myotonic dystrophy type 1 (DM1) increased their physical activity and exercise capacity following a behavioral intervention. However, it is unknown what is altered in muscles of patients with DM1 as a result of this intervention. The increased exercise capacity suggests that decelerated fat infiltration or increased muscle cross-sectional area (CSA) could be involved. Purpose To assess the effect of this activity-stimulating behavioral intervention on the lower extremity muscles of patients with DM1 with longitudinal quantitative muscle MRI. Materials and Methods In this prospective trial, participants with DM1 were randomized to a behavioral intervention (n = 14) or continued regular care (standard care; n = 13); no age-matched pairing was performed. Participants underwent MRI of the lower extremities at baseline and 10-month follow-up (January 2015 to March 2016). Fat fraction (FF), muscle CSA, and muscle water T2 (T2water) as markers for fat infiltration, muscle mass, and alteration in tissue water distribution (edema), respectively, were assessed with a chemical shift-encoded Dixon sequence and multiecho spin-echo sequence. Longitudinal within-group and between-group changes were assessed with paired-samples t tests and multivariable regression models. Results A total of 27 patients with DM1 (15 men) were evaluated. Patient age was comparable between groups (intervention, 45 years ± 13 [standard deviation]; standard care, 5 years ± 12; P = .96). Muscle CSA increased 5.9 cm2 ± 7.8 in the intervention group during the 10-month follow-up (P = .03) and decreased 3.6 cm2 ± 7.2 in the standard care group (P = .13). After 10 months, the mean difference between the groups was 9.5 cm2 (P = .01). This effect was stronger in muscles with baseline FF below the mean ± standard deviation of unaffected volunteers (-0.4 cm2 ± 0.15; P < .001). FF increased 0.9% ± 1.0 in the intervention group (P = .02) and 1.2% ± 1.2 for standard care (P = .02), with no between-group difference (P = .56). T2water did not change significantly in either group (intervention, P = .08; standard care, P = .88). Conclusion A behavioral intervention targeting physical activity increased lower extremity muscle cross-sectional area in patients with myotonic dystrophy, preferentially in healthy-appearing muscle. © RSNA, 2020 Online supplemental material is available for this article.