March 3, 2025
By Dwight Akerman, OD, MBA, FAAO, Dipl AAO, FBCLA, FIACLE
Is the percentage treatment effect the best and most accurate way to express the effectiveness of a myopia control therapy?
Many eye care practitioners believe that the most effective way to describe myopia management therapies is by comparing their percentage reductions in spherical equivalent refraction (SER) and axial length (AL) to those of a control group. Indeed, manufacturers of myopia therapies often highlight the percentage of their product’s myopia control effect in marketing materials.
For example:
- In a randomized controlled trial, myopia control intervention #1 reduced axial length elongation by 0.16 mm compared to the control group after one year. The manufacturer’s marketing literature asserted this as a 50% decrease in axial length progression relative to the control.
- In another randomized controlled trial, myopia control intervention #2 reduced axial length elongation by 0.17 mm compared to the control after one year. The manufacturer’s marketing literature asserted this as a 75% decrease in axial length progression compared to the control.
These percentages could misleadingly imply a clinically significant difference in efficacy when compared to the control: 75% vs. 50%. Is there a clinically significant difference between these two interventions after one year? In short, no.
The percentage of myopia control effectiveness offers advantages, such as providing a standardized and easily understood measure of treatment efficacy. However, its main drawback is that this metric cannot facilitate comparisons between different treatments assessed in separate clinical studies, as the efficacy percentage is unique to each specific study. Currently, determining the relative efficacy of various myopia control interventions is impossible because no randomized controlled trials have been published in peer-reviewed literature that directly compare two or more interventions.
Factors like study duration, participant ethnicity, and age can all affect the percentage. Shorter studies generally demonstrate a higher efficacy rate due to an initial surge in treatment effect during the first year.
Many experts prefer to describe efficacy using the Cumulative Absolute Reduction in Axial Elongation (C.A.R.E.) rather than a percentage of the myopia control effect. Brennan et al. introduced this metric in their paper on Efficacy in Myopia Control.
C.A.R.E. was designed to enable accurate, direct comparisons of treatment efficacy. It serves as an alternative to percentage efficacy, which encounters challenges when comparing across studies due to various factors, such as study duration and the characteristics of the control group, that influence percentage efficacy. Rather than expressing efficacy as a relative percentage based on progression measured in the control group, C.A.R.E. provides an absolute efficacy: the total reduction in axial growth of the treatment group compared to the control group. C.A.R.E. is time-dependent and must be referenced regarding the time scale. In the examples illustrated above, the C.A.R.E. value for intervention #1 above is 0.16 mm at 12 months.
Based on their extensive analysis, Brennan and co-researchers have reached the following key interpretations:
- Axial length is the preferred metric for monitoring the progression of myopia.
- The efficacy of myopia control treatments should be expressed as an absolute, not a percentage effect.
- The efficacy of treatments tends to be independent of age, progression rate, and initial refractive error but not time.
- Past progression is not a reliable indicator of future progression, and all myopic children aged 12 and under should be treated aggressively.
The main advantage of the C.A.R.E. metric is that it provides an empirically determined, evidence-based effect size that clinicians, myopia control candidates, and parents can expect from a specific treatment over a defined time frame. However, differences in study design and variability within control groups must be considered when analyzing and interpreting this metric. Analyses of studies investigating the same treatment across diverse ethnic populations have shown that this figure is more reliable than simple percentages, demonstrating greater consistency.
The takeaway message is that the percentage of myopia control efficacy cannot be used to compare different treatments studied in separate research because the percentage is specific to each study. The C.A.R.E. metric is a better way to assess treatment efficacy.
Best professional regards,
Dwight H. Akerman, OD, MBA, FAAO, Dipl AAO
Chief Medical Editor
dwight.akerman@gmail.com
