Longitudinal reference centiles for the Gross Motor Function Measure-66 in children and adolescents with cerebral palsy.

Aim: To establish novel longitudinal reference values for the Gross Motor Function Measure-66 (GMFM-66) in children and adolescents with cerebral palsy aged 3 to 18 years, to enable more accurate assessments of changes in motor function.

Method: This was a single-centre retrospective analysis of patients who participated in a rehabilitation programme between January 2006 and March 2022. The GMFM-66 was used to measure gross motor function. Paired GMFM-66 measurements from the follow-up phase of the rehabilitation programme were used to establish a reference centile for the change in GMFM-66 over a 6-month period using the lambda-mu-sigma method.

Results: Reference centiles for GMFM-66 changes (over a 6-month period; ±1 month) were created using 1190 longitudinal data pairs of GMFM-66 measurements (mean age 8 years 4 months [standard deviation 7 years 11 months] at start of follow-up), Gross Motor Function Classification System levels I to V. The z-scores for GMFM-66 change of a validation dataset by the new tool and the previously described method to quantify a change in GMFM-66 by individual effect size were highly correlated (Pearson's rank correlation coefficient 0.981 [95% confidence interval 0.979-0.984], p < 0.001) INTERPRETATION: The new reference values showed a high correlation with the previously published reference values, which were limited to an age range of 3 to 12 years. The new reference values can be applied from an age of 3 to 18 years. This facilitates the evaluation of medical treatment after a 6-month period also for children with cerebral palsy who are older than 12 years.