Do optical treatments for the control of myopia progression produce proportional or absolute reductions in progression rates?

In recent decades there has been a marked increase in the prevalence of myopia in many countries, leading to concerns regarding both the cost of providing appropriate refractive corrections and the increased risk of ocular pathology associated with myopia. Thus, much effort is being expended to produce strategies which attempt to slow the rate at which myopia progresses in childhood and to reduce the final amount of adult myopia. Methods that may reduce the progression of myopia, at least partially, include pharmacological treatment such as low-dose atropine, various designs of spectacle and contact lenses and orthokeratology. In most related studies, refractive progression over a period of a few years of treated, initiallymyopic children is compared with that of a matched control group which receives normal single-vision correction. The comparison is typically presented as a percentage change in progression rate based on mean group values of the change in mean spherical equivalent (MSE) refraction. It is important that the treatment effects of myopia control strategies are measured and reported accurately. Brennan and colleagues have recently proposed that the assumption of a constant percentage treatment effect, in which treatment produces a greater absolute reduction in progression rate in those whose untreated progression is faster, may not be justified. They argue that “treatment tends more to an absolute effect rather than a relative effect across the progression range”. The difference is of importance, since a percentage effect would have the major benefit of reducing the potential for the high levels of myopia which carry the most risk of serious pathology. We wish to point out that the two possibilities either an absolute or a relative effect can be explored by considering the frequency distribution of the individual refractive changes occurring over a fixed period in untreated and treated myopic children. A treatment which reduced the progression rate for all myopic children by a constant absolute amount would not affect the shape of the frequency distribution, but would simply shift it along the progression axis towards less negative values. In contrast, a treatment producing a percentage reduction in all rates and hence a rescaling of the progression axis, would also preserve the shape of the frequency distribution but, since both negative and positive values of progression would be proportionately reduced, would compress it around zero progression. These effects are illustrated in Figure 1. This shows schematically an original (control) frequency distribution for progression, with a mean progression over 2 years of 1.15 D and a standard deviation of 0.71 D. If treatment results in all progressions being reduced by 50%, then the resultant narrower frequency distribution has the same basic shape as the control but its mean and standard deviation are reduced to 0.575 0.35 D. In contrast, a reduction of all progressions by a constant +0.575 D would give the same post-treatment mean of –0.575 D but the distribution would have the same form and standard deviation (0.71 D) as those for the control data. To clarify which of these treatment effects (if either) occurs in practice, we consider here the results of two relevant studies using optical methods (spectacles and contact lenses) to control myopia progression. These studies were selected on the basis that detailed information on progressions for individual children was available, rather than merely summary data in terms of means and standard deviations: unfortunately, we have not been able to locate similar published data for other forms of treatment, such as atropine or ortho-K. The first study involved the measurements of myopia progression rates in 183 Hong Kong Chinese schoolchildren (Lam et al.). Entry criteria included: ages 8–13 years;