Muscle Strength and Outcome Measures for Upper Limb Function in Adults With Myotonic Dystrophy Type 1: A Cross‐Sectional Study

BACKGROUND: Myotonic dystrophy Type 1 (DM1) is a multisystem neuromuscular disorder with progressive distal and proximal weakness, including in the upper limbs. Limited research has been conducted on hand function and outcome measures. METHODS: Eighty nine adult DM1 participants were included in this cross-sectional study. The study describes upper limb strength and function and examines associations between motor impairments and clinical outcome measures, including comparing strength measurement methods. Disease severity was classified with the muscular impairment rating scale (MIRS) and the Brooke upper extremity scale was used as a classification scale of upper limb function. Muscle strength was measured using handheld dynamometers, and fine motor function with the Nine-Hole Peg Test and Nut and Bolt task. Upper limb function was rated with the Revised Upper Limb Module and global motor function with the Motor Function Measure 32. Patient-reported outcome measures included ABILHAND-NMD, ACTIVLIM, and the Myotonia behavior scale. RESULTS: Participants showed a wide spectrum of motor impairment from minimal distal to severe proximal weakness. Strength measures correlated significantly with functional tests, while PROMs reflected perceived limitations but showed ceiling effects. Disease duration correlated with muscle strength and upper limb function, while age and subtype were less important. Myotonia severity showed only weak correlation with functional outcomes. CONCLUSION: No single measure fully captures upper limb function in DM1. A combination of strength testing, dexterity tasks, and PROMs provides a multidimensional assessment framework. Muscle strength assessment remains essential, and these findings support integrating multiple standardized measures for clinical trial endpoints and therapeutic evaluation in DM1.