What Makes The Great Outdoors Great? Peering More Into Select Environmental Risk Factors For Myopia

The worldwide increase in myopia prevalence among children is very concerning for eye care providers and public health experts. Current projections expect nearly 5 billion people globally to have myopia by the year 2050, with about 1 billion of these patients with greater than 6 diopters of myopia¹. Myopia is much more than the mere inconvenience of needing glasses. Retinal changes associated with myopia, such as maculopathy and retinal detachments, can have significant impacts on vision.² It is well-established that the risk for these changes escalates with the amount of myopia. The earlier age of myopia onset compounded with the rapid rate of progression justifies the idea that “there is no safe level of myopia”.³

With this at the forefront, an abundance of research has been published looking at the common denominators in myopic children. The conversation returns to the balance between nature and nurture. Though genetic factors play a role, there is increasing evidence that environmental factors may occupy the heavier part of the scale.⁴ Many experts have analyzed the profiles of children with different visual stimuli and lifestyles, searching for possible harbingers of myopia. Consider a recent finding in the recent epidemiological literature: Children of lower socioeconomic status living in rural Australia had lower rates of myopia than those living in urban areas.⁵
The environmental factor of rurality is implicated, but what is the underlying relationship to refractive error? When combing through possible contributors within the visual environment, there are some mechanisms worth noting. Light exposure as well as spatial properties in visual stimuli are two of many features that have surfaced with interesting links to myopia.

Light

There appears to be something special about natural light in relation to myopia prevalence: children who spent more time outdoors had a lower incidence of myopia of myopia and later age of onset.⁶ Though not evaluated in the study with Australian children, outdoor time may have a role in its findings. In addition to numerous studies focused on myopia, the public health literature affirms that children living in rural areas spend more time outdoors than urban counterparts.⁷ A closer look at the type and duration of outdoor light exposure should be taken to understand its mediating effect on myopia.

A point to explore regarding light exposure is the significant difference between indoor and outdoor light intensity. For reference, common values for the illuminance of a well-lit classroom can be 500 lux, a shaded outdoor area during the day as 5000 lux, and an outdoor field with direct sunlight upwards of 100,000 lux! When enumerated as such, the large discrepancies in light intensities flag itself as a potential predicament. Light stimulates the retina to produce and release dopamine, which is a signal for axial elongation at least in animals.⁸ In humans, dopamine has not been directly implicated but there is evidence to suggest that axial elongation is associated with less bright light exposure.⁹ Given the low light intensity in a traditional classroom, strong considerations should be made about how children spend their day at school and outside of school. Time outdoors during school¹⁰ and even classroom design change¹¹ can be promising modifications to increase light intensity within the traditional school model.

Children with myopia have been shown to spend less time in bright light conditions as compared to children with emmetropia.¹² The specifics of the dose-response curve between light exposure and myopia prevalence is still being investigated, but the inverse relationship has been established. More time spent in moderate intensity light, even if it is not direct sunlight, can be sufficient to benefit both children with and without myopia.¹³

Spatial Properties

In addition to light, the content of the visual environment seems to have impact on the eye’s development. This is an emerging thought in the understanding of the visual advantages for rural and natural environments over urban and indoor environments. Flitcroft and colleagues recently analyzed urban, indoor, and natural environments and found urban and indoor scenes lack fine details, technically called high spatial frequencies.¹⁴ Think how thick of a brush you would need to paint a picture of drab office building walls versus a lush forest with scattered fallen leaves in all directions. The natural environment is rich in details and has relevance to myopia, when modeled in vision research studies using animals. When artificially degraded, stimuli absent of high spatial frequencies can prompt myopic growth in animals.^15,16 The low details in man-made environments could presumably have a paralleled effect in the human eye, and could be an unrecognized contributor of myopia.=

In Summary

Numerous previous studies have shown a lower myopia prevalence in children living in rural areas in other parts of the world.^17-20 The strength of this association is not in the label of rurality itself but in underlying common denominators. Are there takeaways that can extend beyond the rural population? Surely, there are cases to be made about light and content of visual stimuli, to name a few. The outdoors and natural environment have components that seem to give proper feedback for eye growth. How time is spent at school and on weekends can be reasonable starting points for careful consideration. Parents, educators, and administrators should account for the evidence thus far to thoughtfully optimize children’s visual environments. The great outdoors can be great for children’s eyes, if they make the time.

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Dr. Tiffany Yanase Park graduated magna cum laude from Southern California College of Optometry and completed a pediatric optometry residency at Children’s Hospital Colorado. Dr. Yanase Park now works as part of a university-based pediatric ophthalmology department in Southern California.

References

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