Study Shows a Decline in Myopia with 2 Hours of Outdoor Time

Myopia is the most common ocular disorder worldwide, with worldwide prevalence expected to reach over 50% in the next 30 years,¹ as seen in Figure 1.² As such, myopia is becoming a major public health concern in many countries, especially in East and South-East Asia, where prevalence of myopia has rapidly increased over the past decades. While several well-researched interventions have been identified to correct or slow the progression of myopia, there remains relative paucity in research regarding preventing the onset of myopia.

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^{Figure 1}

Myopia Etiology

It is critical to emphasize myopia is not a benign or stationary condition. Once established, the condition progresses with increasing refractive error. In myopia, the spherical equivalent objective refractive error is less than or equal to –0.50 diopters in either eye. High myopia, representing advanced pathology and worse prognosis, is defined as a spherical equivalent objective refractive error of less than or equal to –5.00 diopters in either eye.

When posterior segment pathology is identified secondary to axial elongation, myopia can be considered pathologic. Vision loss from pathologic myopia and myopic degeneration can be attributed to multiple sequelae including macular choroidal neovascularization and retinal detachment.

Myopia is a complex disease with a multi-factorial etiology. Recent research surrounding the etiology of myopia continues to emphasize the major role of environmental factors in disease pathogenesis. Several human population studies have revealed that environmental factors, such as near work, digital screen time, and reading, play an important and independent role in the development of myopia.^3-7

Additionally, several epidemiological studies demonstrate myopia is more common in urban areas, among professionals, educated patients, computer users, and is associated with increased education.^8-10

It is also well documented that myopia shows evidence of familial inheritance. The Consortium for Refractive Error and Myopia (CREAM), the largest international genome-wide study of myopia and refractive error ever conducted, identified 24 genomic variations associated with a 10-fold increase in myopia prevalence suggesting a multifactorial pathophysiologic basis for myopia.¹¹

The genetic loci identified through this study also suggest that myopia appears to be a rather heterogeneous disease as genetic variants associated with myopia in different families and ethnic groups are often distinct. There is also evidence of the gene–environment interaction in myopia development, with studies demonstrating that the genetic background of an individual determines the impact of environmental factors on refractive eye development.¹²

Prevention of Myopia Through Outdoor Play

The first reported association between outdoor time and lower likelihood of myopic refractive error was by Mutti et al., where children who were encouraged to go outside during recess showed lower onset of myopia than those who continued their normal recess routine.¹³ However, there was no significant difference in myopia progression between the two groups.

Since then, this effect has been reported in several subsequent studies, with the effect of time spent outdoors on myopia shown to be independent of the associated reduced time spent indoors on near work.

However, more research needs to be done as the exact mechanism of controlling myopia by spending more time outdoors still needs to be investigated. Suggested mechanisms of the protective action of outdoor play include extended depth of focus through pupil constriction, exposure to ultraviolet light, and increase in dopamine release causing reduced axial elongation.

With the increase in the prevalence of myopia reaching alarming epidemic levels in some Asian countries, efforts have concentrated on trying to control both the progression and development of myopia. Recently, a promising population-based study illustrated the effect of outdoor play and the associated decline of myopia prevalence on a country-wide level after policy intervention promoted outdoor activities in kindergartens.¹⁴

Yang et al. conducted a cross-sectional study based on the Yilan Myopia Prevention and Vision Improvement Program (YMVIP) implemented since August 2014. This program effectuated myopia prevention strategies, such as increasing outdoor exercises (2 hours per weekday) in all kindergarten children in Yilan County. In total, 18,621 kindergarteners aged 5-6 years in six school-year cohorts from 2014 through 2019 were included in the study. Results show the prevalence of myopia among preschoolers declined continuously from 15.4% in 2014 to 8.4% in 2016.

Additionally, a significant dose-response association between the duration of exposure and the prevalence of myopia was evident (odds ratio 0.55, 95% CI, 0.49-0.61; after controlling for myopiogenic factors. However, further research is required to determine the optimal duration and strategy to implement outdoor time as an intervention to control or delay the onset of myopia.

Conclusions

Currently, studies such as by Yang et al. show promising results on the protective effect of minimizing myopiogenic risk factors on a country-wide level. Although research surrounding the treatment of the underlying pathogenesis of myopia and its complications is encouraging, emphasis on prevention may also prove to best serve patients in the future.

As the disease burden of myopia increases in the decades ahead, policy and thought must be given to prevent myopia on a policy-based level. As such, in addition to eye care professionals, policy makers also need to be aware of myopia risk factors and liaison with population-based researchers, optometrists, ocularists, and ophthalmologists to control the expected increased prevalence of the disease in the future.

References

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