Longitudinal Construct Validity of the GMFM-88 Total Score and Goal Total Score and the GMFM-66 Score in a 5-Year Follow-up Study

Background. The Gross Motor Function Measure (GMFM) is the instrument most commonly used to measure gross motor function in children with cerebral palsy (CP). Different scoring options have been developed, and their measurement properties have been assessed. Limited information is available regarding longitudinal construct validity. Objective. The objective of this research was to study the longitudinal construct validity of 3 scoring options: the 88-item GMFM (GMFM-88) total, the GMFM-88 goal total, and the 66-item GMFM (GMFM-66). Design. A clinical measurement design was used in this study. Methods. Forty-one children with CP diplegia who were undergoing selective dorsal rhizotomy (SDR) were monitored with the GMFM for 5 years. The mean age at SDR was 4.4 years (range=2.5-6.6). Two subgroups for gross motor function before surgery were created according to the Gross Motor Function Classification System (GMFCS): GMFCS levels I to III and GMFCS levels IV and V. This study included results obtained before SDR and at 6, 12, and 18 months and 3 and 5 years after SDR. The effect size (ES) and the standardized response mean (SRM) were calculated. Results. At 6 months postoperatively, ES and SRM values were small (>= 0.5) for all GMFM scoring options. The GMFM-88 total and goal total scores showed large changes in ES values (range=0.8-0.9) and SRM values (range=0.9-1.3) at 12 months postoperatively, whereas the GMFM-66 scores showed lower ES values (range=0.3-0.4) and SRM values (range=0.7-0.8) for both subgroups. Later postoperatively, larger values for longitudinal construct validity were found. The ES and SRM values generally were lower for the GMFM-66 scores than for the GMFM-88 total and goal total scores. Limitations. All children underwent an extensive intervention, and changes in gross motor function were expected. Conclusion. All 3 scoring options showed large longitudinal construct validity in the long-term follow-up. The GMFM-88 total and goal total scores revealed large changes in gross motor function earlier postoperatively than the GMFM-66 scores.