GMFCS is a categorical scale (Palisano et al., 1997, 2008). Children and adolescents are allocated to one group or another. There’s absolutely no evidence, of course, that there is anything in the condition (or group of conditions) we call cerebral palsy to suggest that children’s gross motor abilities are distributed in such neat little packages. The spectrum of cerebral palsy is almost certainly a continuum and the gross motor abilities are almost certainly distributed along a continuum as well. The categories of the GMFCS do not represent actual discrete groups of children with gross motor abilities that are qualitatively different from those in the other groups. Rather, they are an administrative convenience. Medicine, and life in general, is littered with examples of continuously distributed parameters divided into essentially arbitrary categories simply because this is the easiest thing to do.
Remembering this is important when we engage in discussion about how clearly children can be allocated to the different categories and also how stable that categorisation is over time. If the classification is actually a convenient division of a continuous spectrum then there will be a number of children who fall very close to the border line between these groups. Some of them will lie sufficiently close to the boundary that they can’t be reliably categorised. One day they will illustrate the characteristics of one group and another day the characteristics of another. Alternatively one assessor will make a subjective decision to put the marginal patient in one group whereas another assessor will put them in the other group. Neither is wrong – it is just a consequence of taking people on a continuum and trying to put them in boxes. Just how many children inhabit this marginal space is unclear but in assessing the reliability of the classification system we should be anticipating at least some borderline children for whom it is not possible to allocate a definitive GMFCS level. I may not have been reading carefully enough but I’ve never seen any discussion of this in the relevant literature.
This also impacts on studies of stability of the GMFCS over time. We should expect that a fairly modest improvement in gross motor function should take a child who has been graded at the top end of one category at one time to lead them to be graded at the lower end of the next category up on a later occasion. Equally we should expect some children at the lower range of ability for any given range to drop a level if they deteriorate quite mildy. Some transition between neighbouring groups is thus an inevitable consequence of how the groups are defined and should be expected.
Palisano, R., Rosenbaum, P., Walter, S., Russell, D., Wood, E., & Galuppi, B. (1997). Development and reliability of a system to classify gross motor function in children with cerebral palsy. Dev Med Child Neurol, 39(4), 214-223.
Palisano, R. J., Rosenbaum, P., Bartlett, D., & Livingston, M. H. (2008). Content validity of the expanded and revised Gross Motor Function Classification System. Dev Med Child Neurol, 50(10), 744-750.