Abstract

BACKGROUND AND OBJECTIVES: Delayed grip relaxation is a common symptom of myotonic dystrophy type 1 (DM1), differing from other muscle diseases. Preclinical studies suggest myotonia may reverse quickly with targeted treatment and video hand opening time (vHOT) could be a straightforward method for assessing myotonia in multicenter trials, but few studies have evaluated vHOT in large DM1 cohorts. This study aimed to evaluate how vHOT performs and relates to other disease aspects in a large, well-characterized DM1 population. METHODS: The vHOT was conducted in the END-DM1 natural history study across 22 international sites, including adult DM1 patients with a genetic or research criteria diagnosis. The primary outcome involved video-recorded hand opening after a maximum 3-second grip, performed twice at each study visit with 5-minute rest between trials, and blinded scoring at a central site. Muscle strength and function were assessed by myometry, timed functional tests, and patient-reported outcomes. Procedures were repeated after 1 year for a subset of participants. Wilcoxon signed-rank was used to evaluate differences and Spearman correlation for associations. RESULTS: A total of 591 patients with DM1 (mean age 43.7 ± 12.9 years, 57% female) were included, showing a broad spectrum of vHOT severity (median 3.9 seconds, interquartile range 1.8-7.9 seconds). At a single study visit, there were no systematic difference and good agreement between trials (mean difference 0.1 ± 3.7 seconds [p = 0.05], intraclass correlation coefficient 0.84 [95% CI 0.81-0.86]). vHOT correlated relatively weak with self-reported myotonia (ρ = 0.39), and even lower for other measures of muscle impairments such as grip strength (ρ = -0.21) or 9-hole pegboard (ρ = 0.12). 270 patients completed the 1-year follow-up, with vHOT showing no progression (mean difference 0.4 ± 4.7 seconds, p = 0.34). DISCUSSION: The vHOT procedure was performed successfully in a large international study, with grip relaxation delay varying from minimal to highly prolonged in an unselected cohort. The weak correlation with grip weakness supports the notion that myotonia and weakness are mechanistically distinct. Study limitations include underrepresentation of congenital DM1 and lack of other myotonia measures (e.g., handgrip relaxation myometry). It seems that vHOT is not suitable to assess disease progression, but stability over 1 year may support its use to assess improvement. TRIAL REGISTRATION INFORMATION: The END-DM1 observational study is registered with number NCT03981575.