At the completion of this article, the reader should have a better understanding of the role of outdoor light exposure in myopia management, including:
- Will have reviewed the evidence on outdoor exposure as a protective factor against myopia
- Understand the critical role of light intensity, rather than the absence of near-work, in myopia
- Have reviewed RANZCO’s recommendation of two or more hours a day (with UV protection) for optimum daily outdoor light exposure to decrease myopia incidence
Dr Loreto (Loren) Rose
BSc (HONS) MBBS (HONS) FRANZCO PhD
Paediatric and general ophthalmic surgeon
Clinical Senior Lecturer, Macquarie Medical School
Founding member of Myopia Australia (www.myopiaaustralia.com.au)
Eyecare professionals increasingly find themselves in a bind. Time spent outdoors slows myopia progression, but too much UV light poses a danger to their eye health. Dr LOREN ROSE summarises the evidence on the role of increased lux visible light and the recommendations developed by RANZCO to navigate this set of circumstances.
What is the impact of myopia?
Myopia is an increasingly prevalent eye disorder, with its incidence expected to reach 52% by 2050 worldwide.1,2 It is predicted that one billion people worldwide will have high myopia with refractive errors ³ -6.0D and axial lengths of 26 mm. As this prediction becomes a reality, high myopia will be the leading cause of permanent blindness worldwide. The sight-
threatening complications of pathological myopia include retinal detachment, neovascular membrane secondary to myopic macular degeneration, glaucoma, and pre-senile cataract.3,4
Does family history play a part?
It has been established that genetics contribute to myopia development, and it is associated with increased risk with increased family history.5 Large genetic studies have identified hundreds of genetic loci which carry information from how large the eye is to how fast it will grow to even how susceptible it is to environmental influences affecting myopia progression. Genetic studies have implicated light-induced signalling as a driver for refractive error.6
Which environmental elements are protective? Which are risk factors for myopia development?
International studies show that outdoor exposure is a protective factor against myopia.
Environmental influences such as sunlight exposure and near work are implicated in the development and rate of myopia progression in many animal models and epidemiological studies.3,6-10 A meta-analysis and systemic review established that the incidence of myopia decreases with increased time spent outdoors in hours per week.7
Furthermore, increasing the time children play outdoors reduces myopia progression.11 It has been shown that the incidence of myopia can be reduced by increasing the time six-year-old children spend outdoors by 40 minutes a day.12
The combination of time spent outdoors, education levels, and near work that contributes to the prevalence of myopia can confound each other in studies.13-15 For example, children who spend more than two hours per day at ‘cram school’ (coaching centres after school) attendance have an increased incidence of myopia, which may be due to the reduced time spent outdoors or the increased time doing near activities or both.16
In 2007, the Orinda Longitudinal Study of Myopia reported the longitudinal evidence of a protective association between more time spent outdoors and a reduced risk of myopia onset in elementary school children in northern California.17 The risk of developing myopia in a high family history was reduced from 60% to 20% by spending at least 14 hours per week outdoors.
More recently, public health policy overseas has again highlighted the protective effect of natural light exposure on myopia prevalence. In 2020, the Tian Tian 120 project was published. In Taiwan, children were exposed to 120 minutes a day of outdoor time during school hours. The Taiwan trend of nine years of increasing prevalence of at least one eye of 20/25 or worse from 35% in 2001 to 50% in 2011 was reversed to 46% in 2015.18
Rose K et al19 analysed outdoor activity compared with near work and concluded that light intensity rather than the absence of near work was the critical factor. Sunlight contains the wavelengths of visible light and the shorter ultraviolet wavelengths known to cause eye and skin diseases.
What is the impact of UV light exposure versus bright light in protecting against myopia?
Although many studies are still ongoing, it is known from well-designed animal studies, including on rhesus monkeys,20 chicks21 and tree shrews22, that UV light is not critical for the regulation of ocular growth. UV-free lighting systems were used to inhibit scleral growth rates experimentally.
Karouta and Ashby noted that UV exposure does not underlie the ability of bright light to retard the development of deprivation-myopia or the ability of bright light to maintain normal untreated eyes in a hyperopic state23,24 Instead, their data indicate that the ability of light to retard the development of deprivation myopia is driven by intensity-dependent increases in retinal dopamine release.21
Furthermore, they noted that broadening the spectral output of the lighting system to include UV output was unlikely to induce an even greater protective effect against the development of myopia, as the development of deprivation myopia can be abolished in rhesus monkeys at 20,000 lux20 and chicks at 40,000 lux of bright light alone.25 Hammond et al26 compared UV-free light to illuminance-matched UV-containing light and found no difference in modifying the emmetropisation process in myopia animal models.
Indoors, light is typically between 50 lux in a home27 to 320-500 lux in an office.27,28 Outdoors, there is a huge increase from 20,000 lux on a cloudy day to 100,000 lux in direct sunlight. Hence, even if there were additional slowing in myopia from UV exposure, the increased risk of UV-related blinding eye diseases seems unnecessary.
Ho CL et al,11 in their meta-analysis of the dose-response relationship between outdoor exposure and myopia indicators, found that more than 120 minutes of daily outdoor light exposure decreased myopia incidence by 50%, spherical equivalent refraction by 32.9% and axial elongation by 24.9% for Asian children aged 4-14 years. Furthermore, less than 40 minutes outdoors daily is associated with more rapid axial length progression.29 Hence, at least two to three hours of outdoor activity should be encouraged with UV-protecting eyewear during childhood.
Who is the audience?
Public awareness of the increasing incidence and lifelong visual complications of myopia is currently limited. Parental understanding of the causes and health risks of myopia is poor, and parents/caregivers may be nonchalant regarding myopia in their child. Increasing public awareness is obviously important in improving myopia control.
What messaging is needed to highlight risks and benefits?
In recommending children increase outdoor time, a child’s subsequent risk of skin cancer and UV-related eye diseases, including periorbital skin cancers,30 ocular surface tumours including limbal squamous cell carcinomas,31 pterygium,32 cortical cataracts33-35 and increased risk of age-related macular degeneration34 must be balanced with their risk of myopia.36
Children have the least naturally-developed protection against UVR; 80% of UV eye exposure occurs before a child turns 18.37 It has been demonstrated that 30% of children 9-11 years show UV damage to their eyes using UV fluorescence photography. By 12-15 years of age, 80% of children had damage detected using UV fluorescence, but even more alarmingly, 30% had clinically evident pingueculae or pterygium.38
This result was again demonstrated by Kempen JH et al,39 who showed that areas with the highest prevalence of skin cancers had the lowest levels of myopia by using the data from the Beaver Dam, Rotterdam, the Blue Mountains Eye Studies and the Melbourne Vision Impairment Project. Therefore, the degree of UV damage to the eye correlates with the degree of high lux light.
The presence of pterygium has been associated with reduced axial length measurements. Only 1.8% of patients with axial lengths greater than 26 mm have pterygia compared to 51.4% of pterygia patients with axial lengths less than 23 mm in the same population group.40 Although UV light is not the protective component of sunlight for limiting myopia, it is simultaneously present with the bright lux of light from the sun when people are outdoors.
By increasing the exposure of the paediatric eye to an increased lux of visible light and limiting exposure to UV radiation, it is possible to limit both UV-related ocular diseases and myopia. With the protective benefit of high lux visible light to reduce myopia and by excluding the damaging impact of the invisible UVR on the eye with maximally protective sunglasses, both these silent epidemics can be combated simultaneously.
RANZCO’s policy is to optimise exposure to outdoor light while protecting from UV light to reduce skin malignancy (recognising that periorbital skin cancers account for 5-10% of all skin cancers that occur in the body).41,42 Importantly, based on the evidence, RANZCO recommends maintaining exposure to sufficient high-intensity sunlight to minimise myopia progression.
Therefore, as the standard setters in ophthalmology, and with a vision to stop preventable blindness, RANZCO recommends two or more hours a day with UV protection to the skin and ocular surface to limit the progress of myopia while protecting against the risk of cancer.
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