In spite of some broad agreement between authors, there are differences between different studies' findings. Russell et al37 suggest that after the age of 5 years, changes in a child's motor abilities are more related to developing and refining motor functions in specific environments, rather than the development of basic gross motor skills. They argue that GMFM is, therefore, not able to detect these changes and suggest that the PEDI may be a better tool to assess children 5 years and older.37 This would explain the finding of several studies3 , 38 that children younger than 5 years demonstrated greater change in gross motor function than older children. In fact, Russell et al37 and Rosenbaum et al47 found that there was no improvement at all in GMFM scores in children older than 5 years. However, other authors33 , 36 found the GMFM-66 to be sensitive in children between 5 and 18 years.
There is some disagreement between authors as to the responsiveness of the GMFM-88 versus -66. Wang and Yang3 found both GMFM versions responsive to children with GMFCS levels IV and V over a period of only 3.5 months, and other authors identified the GFMF-66 as the most sensitive OM they tested.33 , 36 However, this conflicts with the findings of Lundkvist Josenby et al,34 who report that whereas the GMFM-66 was as responsive as the GMFM-88 for identifying change at 3 and 5 years postintervention, the GMFM-66 was less sensitive at 12 and 18 months postintervention. The reason for this discrepancy might be found in the specific context of the studies and in the fact that Lundkvist Josenby et al34 worked with the smallest sample (n = 41). Wang and Yang3 found both GMFM-88 and -66 capable of detecting clinically meaningful change. However, they reported that the specificity of the GMFM-66 was 21% greater than that of the GMFM-88, meaning that the GMFM-88 is more likely to detect false positives than the GMFM-66. The latter may have further contributed to the finding of Lundkvist Josenby et al34 that the GMFM-88 was more sensitive than the GMFM-66.
A disadvantage the GMFM-66 and -88 share is that, as unidimensional OMs, they only examine gross motor function. It has been argued that they need to be complemented by other OMs to provide a comprehensive picture of a child's functioning/activity and participation.34 , 37 Furthermore, as measures of capacity, they do not give insight into a child's actual motor behaviors in his or her normal environment. Instead, they measure a child's single “performance” in a clinically controlled test setting.
The PEDI (a parent-reported OM) examines motor and self-care function, as well as participation (in its social function domain). The PEDI, therefore, reflects much more closely the ICF domains of activity and participation. Its clinical relevance is further supported by evidence that motor skills are not necessarily representative of overall functional improvements following therapeutic interventions.48
The PEDI is an OM that assesses the effect of a condition on a child in terms of his or her function across different domains, that is, how it affects his or her everyday life. On the basis of the parent's knowledge of the child, it reflects a child's function across different dimensions; that is, it assesses performance.35 This is more functionally relevant to children with CP. As the PEDI is a multidimensional OM, its mobility scale is a lot less detailed than the GMFM scales. In spite of this, its responsiveness to motor changes is well-established.35 , 38
When using the GMFM-66, on the contrary, an estimate of a child's motor ability can be made even if there are missing items, as items can be rated as “not tested” rather than given a zero score, on the basis of the GMFM-66's interval-level scaling.37 , 50 The latter also improves interpretability, as an equal rise in score corresponds to an equal rise in ability. It also allows therapists to predict what the next item that might be suitable as a therapeutic goal may be as each item is arranged in ascending difficulty.37 A further advantage of the GMFM-66 is that it has 22 fewer items than the GMFM-88 allowing for faster completion of the test. The disadvantage of the GMFM-66 is that it has a floor effect in children with low motor ability and ceiling effects in children older than 5 years.37 , 47 This means that it is much less useful when scoring children of low-motor ability and those older than 5 years.
sThe PEDI was designed with its scales deliberately skewed toward the lower end of ability in terms of functional skills.35 This means that it is useful in conjunction with the GMFM as it assesses some of the emerging motor skills that cannot be examined using the GMFM. However, despite the PEDI's ability to capture function at the lower end of the scale, it has no ceiling effects.35 , 38 On the contrary, because of its ability to assess the effect of a child's motor skills on function in his or her own environment, the PEDI emerges as being highly relevant for the age range where the GMFMs show least change (≥5 years). This is supported by Russell et al,37 , 47 who, as mentioned earlier, suggest that after the age of 5 years, changes in a child's motor abilities are related to developing and refining motor functions in specific environments, rather than the development of basic gross motor skills, as assessed by the GMFM. They suggest, therefore, that the PEDI is likely to be a better tool to assess children aged 5 years and older.37
While a range of OMs has been used to assess function in children with CP, not all of these have been validated for this user group. In the 7 studies identified for inclusion in this review, 6 evaluative OMs were specifically examined. Of these, only 3 OMs emerge as potentially appropriate for testing function in children with CP, the GMFM-88, GMFM-66, and PEDI. However, the GMFM-88 has weaknesses, namely its long completion time and floor and ceiling effects, which caution against its use in clinical practice. A weakness the GMFM-88 and -66 share is their unidimensionality; they only test gross motor capacity, that is, the gross motor ability a child demonstrates in a controlled environment, and not performance in other functional domains, including a child's actual motor and general activity behavior in his or her own daily environment.
In spite of its wider focus, the PEDI demonstrates excellent responsiveness to change across domains, including motor function. It is a measure of performance, that is, a child's actual activity in his or her everyday environment, and is, therefore, much more closely aligned with the ICF's activity and participation domains.6 A new version of the PEDI, the PEDI-CAT, is currently being developed to bring it up-to-date with recent trends in outcome measurement and to extend its range up to the age of 15 years. The latter is to be particularly welcomed with the current bias of validated measures of function for individuals with CP toward the younger age range.
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