Long-term follow-up of motor function and muscle strength in the congenital and childhood forms of myotonic dystrophy type 1

•Muscular symptoms are progressive in the congenital and childhood forms of DM1.•Deterioration of motor function often starts at the beginning of the second decade.•Age but not disease severity or sex predicts for changes of motor function.•Prognosis for the individual patient is difficult to predict due to great variation.

The aims of this study were to explore how motor function and muscle strength change over time in the congenital and childhood forms of myotonic dystrophy type 1, further to investigate whether sex, age, disease severity or size of the mutation could explain these changes. Motor function and isometric muscle strength were evaluated at three occasions during 1999-2013 in 57 patients aged 0.7-28.9 years. Median time between first and last assessment was 11.5 years ranging from 9.6 to 13.3 years. The study shows that motor function improves during the first decade, is most pronounced during the first six years, reaches a plateau during adolescence and starts to deteriorate in the beginning of the second decade. The most predictive variables for change are age (p < 0.0001) and number of CTG-repeat expansions (p = 0.0018). Sex or disease severity grade do not predict changes in motor function. Deterioration of muscle strength is most pronounced in ankle dorsiflexors. Knowledge of development and deterioration of motor function is important for clinical decision making and for planning of interventions. This knowledge can also be of interest for patient recruitment in drug trials, since treatment effect might be easier to evaluate in the stable phases of this progressive disorder.