Longitudinal Psychometric Properties of the Myotonic Dystrophy Health Index in a Large Multicenter Cohort of People Living With Myotonic Dystrophy Type 1.

INTRODUCTION/AIM: The Myotonic Dystrophy Health Index (MDHI) is a patient-reported outcome measure assessing disease burden in DM1. In prior cross-sectional studies, the MDHI correlated with disability status and with several functional measures and activities of daily living. To aid its use and interpretation, we investigated the longitudinal performance of MDHI.

METHODS: Patients with genetically confirmed DM1 were enrolled as part of an observational longitudinal study within the Myotonic Dystrophy Clinical Research Network (DMCRN). Internal consistency and construct validity were evaluated to confirm previous findings. Both the Minimal Detectable Change (MDC) and the Minimal Clinically Important Difference (MCID) were calculated using distribution- and anchor-based methods, respectively. Sensitivity to change and responsiveness were also investigated, along with potential factors associated with the progression of overall burden.

RESULTS: 451 DM1 patients were assessed, with 147 completing 24-month follow-up. The MDHI showed excellent internal consistency (α = 0.95) and confirmed construct validity. MDC for the total score ranged from ±4.24 (effect size change-based approach) to ±9.72 (Standard Error Method-based approach) points, while MCID ranged from small changes (-0.12 improvement/+1.25 worsening) to large changes (-2.17 improvement/+4.71 worsening). Changes in MDHI total and subscale scores were statistically significant at individual- or subgroup-levels and consistently mirrored changes in corresponding clinical measures.

DISCUSSION: The MDHI demonstrates robust sensitivity and responsiveness to individual and subgroup-level changes in disease burden, offering valuable insights into patient-perceived progression. These findings support MDHI use over time and as a potential additional outcome measure at subgroup levels in clinical trials.