A study recently published in the British Journal of Ophthalmology evaluated the effect of time outdoors on myopic shift in premyopic children.
Give me some background.
Premyopia refers to the early stages of myopia development prior to the clinical diagnosis of myopia —and highlights a critical window for intervention.
In fact: Several studies have demonstrated the potential benefits of time spent outdoors in reducing myopia incidence, with a relative reduction in myopia onset ranging from 11-52% in non-myopic children.
I’m sensing a but …
However: The study authors noted that while these studies included hyperopic and premyopic children, the effect of time outdoors among premyopic children was not specifically reported.
As such: There is a gap in research regarding how effective increased time outdoors is in preventing myopia among children who are already at high risk of developing myopia.
Now talk about the study.
In this post-hoc analysis of the Shanghai Time Outside to Reduce Myopia (STORM) study, investigators included 6- to 9-year old participants without myopia from the study who wore smartwatches to monitor time outdoors from 2017 to 2018.
Note: The cohort was organized into four groups based on the amount of time spent outside as follows:
- ≤60 minutes (min)/day
- 61 to 90 min/day
- 91 to 120 min/day
- >120 min/day
All participants were examined with cycloplegic refraction and daily time outdoors was objectively monitored using a wearable smartwatch.
Premyopia was defined as cycloplegic spherical equivalent (SE) from -0.50 to +0.75 diopters (D) and myopic shift was SE change from baseline to 1 year follow-up.
Findings?
Among 3,194 participants—42.9% premyopic; mean age: 8.2±0.6 years; 49.5% boys—there were no statistical differences between premyopic and hyperopic children in time outdoors (p=0.303).
- Hyperopes actually experienced reduced myopic shift with increasing outdoor time (plateau at ~120 minutes (min)/day).
Though: Premyopes exhibited a J-shaped nonlinear relationship between time outdoors and myopic shift, wherein the protective effect in premyopic children was limited and only reached statistical significance at the highest exposure levels.
Tell me more…
In comparison to the subgroup with <60 minutes of daily time outdoors, the difference in SE change in the other subgroups was not statistically significant:
- 61-90 min/day: -0.03 (95% confidence interval [CI]: -0.10 to 0.05)
- 91-120 min/day: -0.03 (95% CI: -0.11 to 0.05)
The reduced myopic shift was only observed with >120 min/day of time outdoors, although it was not statistically significant 0.04 (95% CI:-0.05 to 0.14).
And how many of these patients developed myopia?
Within 1 year, 31% of participants with baseline premyopia progressed to myopia, 0.3% of hyperopes became myopic, and 33.6% moved into the premyopic range.
Expert opinion?
The study authors highlighted the necessity of increasing time outdoors for premyopic children when they experience moderate light intensity (e.g., 3,000-5,000 lux) to attain a protective effect
They also noted several potential mechanisms that may explain the differences in effect between premyopic and hyperopic children, such as:
- Incident myopes start to experience greater lens power reduction and more extended axial elongation 2 years before the onset of myopia
- Premyopic individuals are at high risk of myopia and might also be experiencing a similar trend of lens power reduction and axial elongation, wherein the lens power fails to offset the axial elongation
- Premyopic retinas may not fully trigger the inhibitory arm of the emmetropization feedback loop and have the limited ability to inhibit eye growth as the myopic retina
However: The link between loss of ability to respond to positive defocus and loss of sensitivity to time outdoors and dopamine release is still not clear.
Limitations?
These included:
- The premyopia category includes children at different stages of refractive development; this heterogeneity could dilute observed effects and reduce the sensitivity to identify associations
- The light intensity captured by the smartwatch may differ from the light intensity received by the human eye
- Near work was measured by a questionnaire, resulting in potential report bias
- Possible confounding factors such as socioeconomic status, dietary patterns, and genetic risk beyond parental myopia were not fully captured in the dataset and could influence the results
- The study duration was only 1 year, which may not be enough to capture the long-term effects of outdoor time on myopia prevention in premyopic children.
Take home.
These findings demonstrate that increased time outdoors had a protective effect for hyperopic children—however, there was a more limited protective effect seen in premyopic children, and it was only observed with longer durations of time outdoors (>120 min/day).
As a result: Longer duration of time outdoors or additional interventions to prevent or delay myopia onset may be required in premyopic children.