Evaluation of BTX/A therapy in children with CP: GMFM-88 vs. GMFM-66

Introduction: The GMFM is an observational measure that was developed and validated to assess gross motor abilities of children with CP. As the GMFM has been used in a variety of research situations its limitations have become more apparent, such as a lack of responsiveness to change for certain patient groups and long administration time. In an effort to improve interpretability and usefulness of the GMFM, its Canadian developers applied the Rasch Model of item analysis to the GMFM-88, resulting in creation of the GMFM-66. Aim of the retrospective study was to investigate whether the two versions would show differences in their discriminative capability and responsiveness to change in a sample of children with CP treated with BTX/A.

Method: Retrospective identification of n=57 children with spastic CP (mean age: 7.4 y, range: 2–17, GMFCS I, II n=17, GMFCS II n=14, GMFCS III, IV n=26) who had been assessed with the GMFM-88 at baseline and 3 months after BTX/A injection. Indications: adductor spasticity (n=26), pes equinus (n=15), 16 children received multi-level injections. Using the GMFM-66 software (Gross Motor Ability Estimator) each child's GMFM-88 scores were converted into the corresponding GMFM-66 scores and correlated with age and GMFCS-levels.

Results: The baseline scores of both measures did not show any significant difference in regard of discriminative capability; the amount of change in both measurements was related to age and severity of motor disability. However, in children with mild motor impairment the GMFM-88 was less sensitive to change compared to the GMFM-66 (p<0.001). For children functioning at low ability levels, the GMFM-88 seemed to be more sensitive to change.

Discussion: Depending on severity of motor disability, we could show that there are differences in the responsiveness to change of measurements obtained with the 66 or the 88-item version of the GMFM. These findings might have implications for the planning of future therapy studies in respect of which GMFM version should be selected to evaluate therapy outcome.