Clinical trials have demonstrated conclusively that correcting lenses can alter the rate of myopia progression in children. summarizes the changes in progression rate observed in some recent studies. The results have been segregated into groups based on the stated intent of the treatment strategy and whether the treatment lenses would have a substantial effect on peripheral refractive errors.
Figure 10 Relative efficacy of optical treatment strategies for slowing myopia progression. For most of the studies, refractive-error data were used to calculate the percentage difference in progression rate between the treated and control groups. For the orthokeratology (more ...)
An aim of the studies that employed traditional bifocals or progressive addition lenses was to decrease the level of accommodation during near work and/or to improve the quality of foveal vision, for example, by eliminating the degree of central hyperopic defocus associated with a lag of accommodation.27, 29, 30, 104
Although it is unclear how successful these treatment lenses were in accomplishing these goals,105
all of the studies found a statistically significant reduction in myopia progression relative to traditional spectacles. Although from a scientific perspective the results demonstrated that spectacle lenses could alter the course of myopia progression, the overall treatment effects were not clinically meaningful, possibly because the treatment zones of the lenses influenced a relatively small proportion of the visual field.
Recent studies employing distance under-correction strategies were based on observations in laboratory animals that showed that myopic defocus can slow axial elongation and produce hyperopic shifts in refraction. However, in children, under-correction strategies have failed to slow myopia progression26
and, in some cases, have been found to actually increase myopia progression relative to traditional single-vision spectacle prescription strategies.25
At first glance, these results seem to directly contradict the findings from laboratory animals. However, it is important to recognize that the overall optical effects of under-correcting a myopic human eye are likely to be very different from those imposed by a positive lens on the normal eyes of young animals. For example, with respect to distant targets, under-correction regimens will impose a relatively small amount of myopic defocus in the central retina (typically 0.5 to 0.75 D). Given the magnitude of relative peripheral hyperopia exhibited by most myopic humans, these small degrees of distance under-correction are very unlikely to eliminate the large amounts of hyperopic defocus in the periphery. In contrast, in animal studies, the powers of positive treatment lenses are generally much higher than the degree of under-correction. Moreover, the patterns of peripheral refractions in laboratory animals are likely to be very different from those of myopic children. For example, the majority of infant monkeys exhibit moderate amounts of central hyperopia and small amounts of relative peripheral myopia.106
As a consequence, positive treatment lenses employed with animals will typically impose relative myopic defocus on distant objects in both the central and peripheral retina, i.e., conditions that normally reduce the rate of axial elongation.
Correcting lenses that specifically target the pattern of peripheral refractions can reduce the rate of myopia progression. Recently, the Vision Cooperative Research Centre (Sydney, Australia) completed the initial clinical trials of new contact lenses and spectacle lenses that were designed to increase the curvature of the peripheral image shell while fully correcting central refractive errors, in essence, reducing the degree of relative peripheral hyperopia while maintaining clear central vision. Twelve-month data obtained for the new spectacle lens designs indicate that, relative to traditional spectacle lenses, 2 of 3 new designs (which differed in the amount and spatial distribution of the peripheral power zones) had no effect on progression rates. The third design, which had the smallest unrestricted central zone, reduced progression by an insignificant 17% in children 6–16 years of age. However, in a subgroup of younger children (6–12 years of age) with at least one myopic parent (i.e., children with relatively fast progression rates), this new lens significantly reduced myopia progression by 30%.107
The experimental contact lens designs were more successful. Specifically, in 7- to 14-year-old children, contact lenses that reduced relative peripheral hyperopia decreased the average rate of myopia progression by 36% over a 12-month period.108
It is likely that the contact lens designs were more effective than the best spectacle lens design because the optical treatment zones in the contact lenses were positioned closer to the line of sight and because, with contact lenses, the treatment zone remains centered during eye movements. Although these preliminary results provide proof that peripheral optical manipulations can slow myopia progression, much longer treatment periods will be necessary to prove clinical efficacy and much work is needed to optimize these lens designs.
Over-night orthokeratology optically corrects myopia by flattening the central cornea. For moderate degrees of central myopia, the primary changes in corneal topography are usually restricted to the central ±20° of the visual field. As a consequence, the normal pattern of peripheral refractions is changed dramatically. Specifically, beyond the area of central flattening, the peripheral refractions typically change from the relative peripheral hyperopia found at baseline to relative peripheral myopia,109–111
in essence establishing a peripheral refractive state that normally reduces axial growth in laboratory animals. In this respect, the initial small-scale clinical trials indicated that orthokeratology slows axial elongation in myopic children by about 50% over a two-year period.112, 113
These promising early results have recently been confirmed in a larger, prospective study that found a 36% reduction in axial elongation over a two-year period.114
Interestingly, in this study, the larger reductions in progression rate were observed in individuals with higher degrees of central myopia, possibly reflecting the fact that orthokeratology strategies would be expected to produce the highest degrees of relative peripheral myopia in highly myopic children.
Promising results have also been obtained using treatment lenses that produce relative myopic defocus in the peripheral and central fields simultaneously. For example, Cheng et al.28
found that executive bifocal spectacles reduced myopia progression by 46% on average over a two-year period in 8–13 year-old children and Anstice and Phillips,115
using multifocal contact lenses that had optical profiles similar to traditional center-distant bifocal contact lenses, reported a 36% reduction in myopia progression in an 8-month trial. Although the primary intent in both studies was to manipulate central vision, in both cases the treatment lenses also produced relative myopic defocus over a very large part of the periphery. With the executive bifocal, when children were viewing through the distance portion of the lens, the near add produced relative myopic shifts over approximately half the visual field. In this respect, the larger treatment effects observed with executive bifocals versus progressive addition lenses may reflect the executive bifocal’s effectively larger treatment zone, i.e., relative myopic shifts were produced over a larger portion of the visual field. With the Anstice and Phillips’ multifocal contact lens, children postured their accommodation for the distance portions of the lenses at all distances and, thus, experienced superimposed relative myopic defocus essentially across the entire visual field.
Inspection of shows that optical treatment strategies that influence the eye’s effective refractive state over a substantial part of the peripheral field are likely to be more effective than traditional bifocals and progressive addition lenses and that peripheral treatment strategies can produce clinically meaningful reductions in the myopia progression. With respect to these peripheral treatment strategies, it is important to note that there is another commonality in all of these designs. In addition to treating a large part of the visual field, each of these strategies produces simultaneous myopic defocus, which animal studies show is a strong stimulus for reducing axial growth116–118
. For example, with the Anstice and Phillips’ contact lenses, the myopic defocus is spatially superimposed on the in-focus central retinal image. With the Vision CRC contact lenses and spectacle lenses and with orthokeratology, the myopic defocus is spatially restricted to the periphery, but it exists simultaneously with clear central vision.
As originally suggested by research on laboratory animals, it seems likely that any lens design that imposes myopic defocus across a substantial part of the eye will be effective in slowing the rate of myopia progression. The more consistently the imposed myopic defocus is maintained over time and across fixation distances, the more likely it is to be effective. Because central vision controls accommodation, peripheral treatment strategies, like that illustrated in , can consistently produce relative myopic shifts over time and they offer a number of other advantages. First, peripheral treatment strategies produce an anti-myopia treatment effect while maintaining optimal central vision, in contrast to strategies that impose simultaneous myopic defocus across the central retina. Second, peripheral treatment strategies, by reducing the degree of peripheral hyperopic defocus, can actually produce measurable improvements in peripheral vision, a potentially valuable side benefit.119
Third, peripheral treatment strategies can be implemented using all of our traditional optical treatment methods (i.e., spectacles, contact lenses, orthokeratology and corneal laser surgery). This is an advantage because it will be important to develop effective anti-myopia designs in all of these modalities to meet patient needs. At the least, we should design correcting lenses that do not induce peripheral optical conditions that may actually promote myopia progression (e.g., like many traditional negative spectacle lenses).
While I believe that we are on the verge of having a number of optical treatment options that do effectively slow myopia progression, it is important to note that many of the clinical results just outlined are preliminary in nature and these new lens designs must be investigated in larger and longer trials. Moreover, there are still many issues associated with these optical treatment strategies that must be resolved. For example, the optimal peripheral image shell manipulations required to slow myopia progression is not known. In this respect, it seems likely that as we develop a better understanding of the role of the periphery in regulating eye growth, we can increase the effectiveness of peripheral treatment strategies. For example, to date, all of the optical treatment strategies have employed a single power profile; no attempts have been made to optimize the peripheral optical manipulations to take into account the substantial inter-subject differences in the pattern of peripheral refractions or the significant intra-ocular variations in peripheral refraction. It will be important to improve the efficacy of these potential optical treatment strategies, because optical treatment strategies will not eliminate existing myopic errors (except possibly in very young children). Instead, they will probably only slow subsequent progression, which is a strong argument for employing these treatment options at as early an age as practical. But given the increasing prevalence of myopia, having an optical treatment strategy that produces a clinically meaningful reduction in myopia progression would have huge public health benefits.