Analysis of the functional capacity outcome measures for myotonic dystrophy.

Objectives: Defining clinically relevant outcome measures for myotonic dystrophy type 1 (DM1) that can be valid and feasible for different phenotypes has proven problematic. The Outcome Measures for Myotonic Dystrophy (OMMYD) group proposed a battery of functional outcomes: 6-minute walk test, 30 seconds sit and stand test, timed 10 m walk test, timed 10 m walk/run test, and nine-hole peg test. This, however, required a large-scale investigation, METHODS: A cohort of 213 patients enrolled in the natural history study, PhenoDM1, was analyzed in cross-sectional analysis and subsequently 98 patients were followed for longitudinal analysis. We aimed to assess: (1) feasibility and best practice; (2) intra-session reliability; (3) validity; and (4) behavior over time, of these tests.

Results: OMMYD outcomes proved feasible as 96% of the participants completed at least one trial for all tests and more than half (n = 113) performed all three trials of each test. Body mass index and disease severity associate with functional capacity. There was a significant difference between the first and second trials of each test. There was a moderate to strong correlation between these functional outcomes and muscle strength, disease severity and patient-reported outcomes. All outcomes after 1 year detected a change in functional capacity except the nine-hole peg test.

Conclusions: These tests can be used as a battery of outcomes or independently based on the shown overlapping psychometric features and strong cross-correlations. Due to the large and heterogeneous sample of this study, these results can serve as reference values for future studies.