How robust is ACTIVLIM for the follow-up of activity limitations in patients with neuromuscular diseases?

- ACTIVLIM: a valid and accurate measure of activity in neuromuscular diseases.
- ACTIVLIM is consistent across age, gender, language and time in a nationwide case mix.
- ACTIVLIM allows clinically meaningful follow-up of children and adults.
- A slight decrease in activity over one year is shown in most neuromuscular diseases.
- Over 1 year, level of activity is most decreased for ALS and most increased for CIDP.

This study aims to investigate the clinimetric properties of ACTIVLIM, a measure of activity limitations, when it is used in daily practice in a large nationwide representative cohort of patients with neuromuscular diseases. A cohort of 2986 patients was assessed at least once over 2 years in 6 national neuromuscular diseases reference centers. Successive Rasch analyses were conducted in order to investigate the scale validity, reliability, consistency across demographic and clinical sub-groups and its sensitivity to change. ACTIVLIM confirmed excellent fit to a unidimensional scale, with stable but 3-times more accurate item calibrations compared to the original publication. It showed a good reliability (R = 0.95), an appropriate targeting for 87% of the sample and an excellent invariance across age, gender, language and time. Despite some variations in the item difficulty hierarchy across diagnoses, ACTIVLIM exhibited a good capability to quantify small but significant changes in activity for various diagnostic groups. Overall, ACTIVLIM demonstrated very good clinimetric properties, allowing accurate quantitative measurement of activity limitations in both children and adults with a variety of neuromuscular diseases.