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SIR – In response to the concerns raised by David Scrutton in his letter1 regarding the use of the Gross Motor Function Classification System (GMFCS), we appreciate the opportunity to clarify the issues he discusses. We fully agree with his observation that ‘There are reasons other than motor disability dictating whether a child achieves or fails to achieve an ability…’. Note, however, that the GMFCS was developed as a classification system to categorize motor function of children with cerebral palsy (CP). It was never intended to explain why a child does or does not reach a particular level of ability. We also agree that the addition of more levels within the classification might provide finer detail, but two issues should then be considered. The first is whether there is clinical value in more fine-grained functional subgroups. Second, it must be recognized that there is always a trade-off between complexity and reliability of a classification system, and the reliability of the additional levels would need to be demonstrated.

Two points require clarification in discussing the potential uses of the Functional Mobility Scale (FMS). First, the FMS does not, as suggested by Scrutton, have 22 separate mobility grades. The three items (5, 50, and 500 metres) are considered as separate entities representing three importantly different environmental settings (home, school, and the wider community). In scoring the FMS there are 8 categories to select from for FMS 5m, and 7 categories for each of the 50 and 500m categories. Second, a common misunderstanding is that the FMS and GMFCS are interchangeable when in fact they have been developed for different purposes. The FMS is an outcome measure designed to assess mobility at one point in time and to explore change over time. The GMFCS is a classification system and has not been developed to be, nor should it be used as, an assessment tool. As stated, it is ‘simple yet reliable’ and its increasing use internationally has aided communication amongst clinicians. The FMS and GMFCS provide complementary information.

With regard to the potential misuse of the GMFCS in constraining people’s thinking about interventions, we would argue that in fact it can be a very helpful way of guiding treatment decisions, as has been developed by our colleagues in Germany.2 Rather than being prescriptive, the GMFCS can direct clinicians to ensure that children receive care appropriate to their level of function and their age. The motor growth curves3 illustrate the potential for gross motor function for children classified within each GMFCS level. From these data we can see that, for example, it would not be appropriate to perform extensive gait correction surgery on a child in level IV when their prognosis for motor function is for transfers and, at best, limited walking with primarily wheelchair use for mobility. Hip surveillance data of children with CP tell us that the rate of hip subluxation increases linearly from level I (0%) through to V (>90% risk in level V).4 The GMFCS level can thus provide a guide to help service providers monitor and treat hip problems in children with CP.

The categorical nature of the GMFCS means that a change in GMFCS level may occur after an intervention, especially when the individual was on the border between two levels to begin with. It must also be recognized that assignment of a GMFCS level is a matter of judgment – and human judgment (even within the same individual over time) can be fallible. The apparent transition of a child in GMFCS level III to level II after surgery for crouch gait does not undermine the concept of the relative stability of GMFCS nor does it support the use of the GMFCS as an outcome measure. Most of the better quality intervention studies report a change in GMFCS level in <10% of participants. Hence the few participants who change GMFCS level are ‘the exceptions which prove the rule’. The ‘rule’ being that the GMFCS is relatively stable in children and young people with CP over time.

We agree that there are limitations with both the FMS and the GMFCS; however, they complement each other well by providing simple yet useful information regarding the mobility activity of children with CP. They should be used in conjunction with a host of other valid and reliable assessment and classification tools to address a wide range of clinical and research questions.

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