While myopia may be corrected with spectacles or contact lenses, to restore clear vision, the hardships associated with myopia, both economic - $12.8 billion (in 1990) to correct refractive errors in the United States alone (Javitt & Chiang, 1994) - and social - the concomitant blinding conditions such as retinal detachment and glaucoma continue to accumulate. The critical question of whether myopia progression can be limited rests with the questions of how and why eyes grow beyond emmetropia to become myopic, the answers to which remain unclear. Bifocal and progression spectacle lenses have a proven history as myopia control treatments, but do not slow progression for all myopes. Here, we outline promising anti-myopia treatments in three categories, and discuss the direction of future research.
Optical interventions to prevent myopia progression can be grouped into spectacle wear (including bifocal and progressive addition lenses), soft contact lenses (including bifocal and multifocal lenses), and corneal refractive therapy, involving rigid contact lenses. The rationale for such optical treatments comes from animal studies with chicken, tree shrew, monkey, and guinea pig, all of which have shown increased ocular growth with exposure to hyperopic defocus (Wildsoet, 1997; Wallman & Winawer, 2004). In humans, hyperopic defocus may arise from under-accommodation at near, that is, a lag of accommodation.
Allen and O’Leary (2006) have shown that accommodative lag is an independent predictor of myopia progression in young adults. Hyperopic defocus in the retinal periphery, presumably a product of eye shape, appears to be another risk factor for myopia (Hoogerheide et al., 1971, Mutti et al., 2010). Equally important are results from animal studies showing that imposed myopic defocus slows ocular growth, and that some multi focal lens designs are more effective than single vision lens designs in achieving this end (Liu et al., 2010) (Fig. 1).
Fig. 1: Multifocal soft contact lenses are among treatments recently shown to be effective in slowing myopia progression; although the underlying mechanism for such treatment effects are not fully understood, novel myopia control lens designs are beginning to appear on the market.
Recent myopia research has provided a greater appreciation for the coordinated actions of the accommodation and vergence systems in creating a focused retinal image. While it is generally accepted that the retina uses image defocus to guide eye growth to an optimal image plane, it is now feasible to quantify retinal image quality as well as both predict and measure accommodation with different lens treatments. In a study of accommodation with three different soft contact lens types - single vision near, single vision distance, and bifocal - the accommodative lags of myopes were replaced by leads with both single vision near and bifocal lenses (Tarrant et al., 2008). In another related study involving multifocal contact lenses, the authors demonstrated beneficial effects on both accommodation and retinal image quality of lensinduced changes in spherical aberration (Tarrant et al., 2010).
A clinical study of myopia progression in children wearing single vision soft contact lens (SCL) and spectacles found no statistically significant differences in refractive error, axial length, or corneal curvature changes between the two groups over a threeyear period (Walline et al. 2008). This may assuage an anecdotal belief among practitioners that soft contact lens wearers are subject to “myopic creep”. Nonetheless, lack of evidence of exacerbation of myopia progression does not represent a therapeutic effect; treatments must slow progression. In a 12-month study involving SCLs, bifocal lenses significantly reduced rates of ocular elongation and myopia progression compared with single vision lenses (Aller et al. 2006). The subjects in this study may have been more amenable to the bifocal treatments, which neutralized their near eso fixation disparities, a study selection criterion. In a second study on a single pair of twins, the same authors found that the child wearing bifocal SCL showed no progression in the first year, and only slight myopic progression in the second year of lens wear (Aller & Wildsoet, 2008). The twin who wore single vision lenses progressed by -1.19 D, axial length increasing by 0.64 mm, over the first year, but regressed slightly (+0.44 D) after switching to bifocal lenses for the second year of the study.
Corneal refractive therapy (CRT) offers the prospect of sharp daytime vision through the overnight wear of rigid gas permeable contact lenses. In one recent 2-year study, children who wore reshaping lenses showed less axial length growth than children wearing soft contact lenses (Walline et al., 2009), confirming the conclusion that CRT slows myopic eye growth reached in the earlier Longitudinal Orthokeratology (ortho-k) Research in Children study, though small scale. In the latter study, axial length increases differed by approximately 50% between ortho-k and single vision spectacle treated children after two years (Cho et al., 2005). Individual variability in treatment effects may reflect, to some extent, the unpredictability of eye growth, myopia risk, and treatment susceptibility, which appears to be tied to a child’s baseline myopia level. These corneal shaping effects must be distinguished from the effect of daily wear gaspermeable lenses, which were reported to also slow myopia progression compared to soft contact lenses, mainly due to corneal flattening and limited to the first treatment year (Walline et al. 2004). Standard rigid gas permeable lenses alone are not a sufficient or consistent myopia retardant.
Gwiazda (2009) has argued for more “personalized” treatment of myopia, i.e. treatment that would take into account the patient’s myopia level, progression risk(s) and, especially, binocular oculomotor function, since all appear to affect treatment potential, as discussed above. This point is exemplified in the large-scale COMET study lead by Gwiazda; while the treatment effect of progressive addition spectacle lenses over single vision lenses was found to be clinically insignificant for the cohort as a whole, further analyses showed a greater treatment effect for a subsetthose with lags of accommodation and near esophoria (Gwiazda et al., 2004). The inter-subject variability reported in the CRT study of Cho et al. (2005) support this argument. Improved under-standing of the treatment effects of multifocal soft contact lenses and CRT may allow more directed therapy - although both have positive benefits on accommodation and peripheral retinal defocus.
Finally, new strategies to combat myopia may take advantage of lifestyle and visual environment changes. Recent research suggests that spending time outdoors has a protective effect on myopia progression. The mechanism for this effect remains unclear, but diurnal retinal dopamine fluctuations, increased depth of focus and reduced blur due to pupil constriction, and reduced accommodative demand have been posited as contributing factors (Fig. 2).
Fig. 2: Outdoor activities appear to have protective value against myopia in children. While they are more likely to be active outdoors (i.e. not reading), on-going studies with animal models are investigating the influence of bright (sun) light on the release of retinal neurotransmitters, including dopamine (DA), which has been linked to slowed ocular growth.
In the Sydney Myopia Study, Rose et al. (2008) found that more time spent outdoors (as reported in parental surveys) was associated with lower myopia prevalence and more hyperopic refractions in 12-yearolds, but not 6-year-olds. Interestingly, playing sports indoors was not related to myopia level, suggesting that physical activity alone is not predictive of refractive development. This result is also backed up by Dirani et al. (2009), who found that myopia was negatively associated with both time spent outdoors and with time spent playing sports, but had no association with time spent in indoor sports.
Teasing apart the contributions of activity, environment, and light levels represents a major, albeit important challenge. Studies with animal models offer some insights. In chicks fitted with diffusers to induce form deprivation myopia, 15 minutes of daily exposure to sunlight or high intensity indoor lighting lead to shorter axial lengths and less myopic refractions (Ashby et al., 2009); in another study, high artificial illumination again reduced deprivation myopia, and this effect could be prevented with ocular injection of the dopamine antagonist, spiperone (Ashby et al., 2010). While exposure to high intensity light appears to stave off myopia development, in a form deprivation model at least, its effect on lens-induced myopia is simply to slow the inevitable (Ashby et al. 2010). Administering dopamine agonists, however, does inhibit both form deprivation and lens-induced myopia (Schmid & Wildsoet, 2004). It will be important that researchers remain open to alternative interpretations of such data and also bear in mind the practical limitations of posited anti-myopia therapy, such as sunlight, in moving forward.
In conclusion, promising avenues in myopia control continue to be iscovered. Bifocal soft contact lenses have been successfully used to slow myopia progression, especially in those with accommodative lags and near esophoria. Corneal refractive therapy is an attractive option that combines corneal flattening with freedom from daytime spectacle and contact lens wear, and its effects maintain as long as the nightly lens wearing regimen. New anti-myopia contact and spectacle lens designs are also on the horizon. Emerging works suggest spending time outdoors may be crucial to controlling myopia, although the underlying mechanism remains to be elucidated. We are moving rapidly towards myopia treatment plans that weight a patient’s oculomotor function and the defocus of the retinal image with other risk factors.