Thanks for the great discussion! The study guide info on prognosis was based on the original motor development curves article (Rosenbaum PL, Walter SD, Hanna SE, et al. Prognosis for gross motor function in cerebral palsy: creation of motor development curves. JAMA. 2002;288(11):1357-1363. doi:10.1001/jama.288.11.1357), which includes a little more detail compared to the textbook chapter. Approximately 90% of gross motor skills were achieved by the age of 5 years in the study sample. This is an important take home message…that children functioning in GMFCS levels I and II are more stable in performance over time while children functioning in the other levels have higher risks for motor decline. Also, don’t be confused with the fact that all items of the GMFM are usually achieved by typically developing 5 yo children.
Table 2 in this Rosenbaum article provides the specific ages when 90% of skills are achieved, which range from 2.7 yrs (level V) to 4.8 yrs (level I). The authors of the original article stated…”Because the GMFM-66 assessments of children reported here were specifically made without the use of aids, such as walkers or crutches, these patterns of gross motor development probably represent the lower limit of what children in each level can, on average, accomplish in gross motor function….nor do the curves show how children apply their motor function in the context of activity or participation in daily life.” Also, the article acknowledges that we can’t predict how children might perform (such as peak and decline in motor function) with therapeutic innovations, such as SDR, botox, and intrathecal baclofen.
This original article was published over 20 years ago, however, follow up research has supported the results to varying extents. Hanna S, Rosenbaum P, et.al. (2009) Stability and decline in gross motor function among children and youth with cerebral palsy aged 2 to 12 years. Developmental Medicine and Child Neurology;.51:295-302 showed that GMFCS levels I and II didn’t have a clear peak and decline, however, levels III-V peaked between 6-8 years and had a corresponding decline represented by GMFM-66 points.
Keep in mind that all of these studies are using specific study samples and not population-based data. There will likely be some variability in ages and performance due to different study samples. For example, the original motor curves were based on children in Canada. If you dive into all of the follow up research, however, you’ll find similarities across different countries where standard of care for CP is similar.