Gross Motor Function Measure-66 Item Sets for use with infants and toddlers at high risk for cerebral palsy: Construct validity and responsiveness.

Aim: To evaluate the construct validity and responsiveness of the Gross Motor Function Measure-66 Item Set (GMFM-66-IS), a standardized criterion-referenced observational measure, for use with children younger than 24 months with or at high risk for cerebral palsy (CP).

Method: Non-experimental integrative data analysis was performed on secondary data from three clinical trials involving children with or at high risk for CP (n = 79, 42 males, mean corrected age = 11.3 months [SD = 4.9]), and one observational study of typically developing children (n = 32, 14 males, mean age = 5.7 months [SD = 0.8]). The GMFM-66-IS and comparator instrument (gross motor subtest from the Bayley Scales of Infant and Toddler Development, Third Edition [Bayley-III] or Bayley Scales of Infant and Toddler Development, Fourth Edition [Bayley-4], depending on the study) were administered at baseline and 3 months later. Comparator groups were based on neurological impairment, clinical rating of gross motor change, and CP status. Correlations (r) and regression-adjusted standardized mean differences (Hedges' g) were computed.

Results: GMFM-66-IS and Bayley scores were correlated at baseline (r = 0.83), 3 months later (r = 0.88), and across time (r = 0.83). Children with mild impairment had higher mean GMFM-66-IS scores at baseline (g = 0.87) and 3 months later (g = 0.95). Children rated as demonstrating greater than expected gross motor change had larger mean GMFM-66-IS change scores than children demonstrating less than expected change (g = 0.62). Typically developing children had larger mean GMFM-66-IS change scores (g = 1.00).

Interpretation: GMFM-66-IS scores were supported by evidence of strong construct validity and moderate responsiveness.