The objective of this longitudinal, observational study was to investigate change over time in ability to perform activities of daily living in myotonic dystrophy type 1 (DM1).

Adults with genetically confirmed DM1 were recruited as part of the PhenoDM1 study in the UK. Data on activities of daily living were recorded through the DM1-Activ^C at baseline and a follow-up visit after 12 (± 3) months. A subset of patients had advanced genetic testing to determine the size of the progenitor allele.

Our sample comprised 150 patients with DM1 (mean age: 45 years; 52% female). Mean follow-up was 383 days. Mean DM1-Activ^C total score at baseline was 71.24 (95% confidence interval 67.77–74.71) and at the follow-up visit 69.04 (65.54–72.54). Approximately 43% of patients had a lower score at the follow-up visit (indicating a decreased ability to perform activities of daily living), 24% a higher score (indicating an increased ability), and 33% the same score at baseline and follow-up. The mean annual change in the DM1-Activ^C total score, estimated at − 2.06 (− 3.54 to − 0.59), was significantly related to patients’ baseline score, but not sex, disease duration, timed test results, or cytosine-thymine-guanine repeat length.

Change over time in ability to perform activities of daily living as recorded through the DM1-Activ^C varies substantially between patients with DM1. Our data contribute to the understanding of the natural evolution of the disease, and should be helpful to inform the design of future trials based on the DM1-Activ^C.