Abstract
Exposure to sunlight has recently been postulated as responsible for the effect that more time spent outdoors protects children from myopia, while early life exposure to natural light was reported to be possibly related to onset of myopia during childhood. In this study, we had two aims: to determine whether increasing natural light exposure has a protective effect on hyperopic defocus-induced myopia, and to observe whether early postnatal exposure to natural light causes increased risk of refractive error in adolescence. Eight rhesus monkeys (aged 20-30 days) were treated monocularly with hyperopic-defocus (-3.0D lens) and divided randomly into two groups: AL group (n=4), reared under Artificial (indoor) Lighting (08:00-20:00); and NL group (n=4), exposed to Natural (outdoor) Light for 3 hours per day (11:00-14:00), and to indoor lighting for the rest of the light phase. After being reared with lenses for ca. 190 days, all monkeys were returned to unrestricted vision until the age of 3 years. Another eight age-matched monkeys, reared with unrestricted vision under artificial lighting since birth, were employed as controls. The ocular refraction, corneal curvature and axial dimensions were measured before lens-wearing (at 23±3 days of age), monthly during the light phase, and at the age of puberty (at 1185+3 days of age). During the lens-wearing treatment, infant monkeys in the NL group were more hyperopic than those in the AL group (F=5.726, P=0.032). Furthermore, the two eyes of most NL monkeys remained isometropic, whereas 3 of 4 AL monkeys developed myopic anisometropia more than -2.0D. At adolescence, eyes of AL monkeys showed significant myopic anisometropia compared with eyes of NL monkeys (AL vs NL: -1.66±0.87D vs -0.22±0.44D; P=0.002) and controls (AL vs Control: -1.66±0.87D vs -0.05±0.85D; P<0.0001). All differences in refraction were associated with parallel changes in axial dimensions. Our results suggest that exposure to natural outdoor light might have an effect to reduced hyperopic defocus-induced myopia. Also, the data imply that early life exposure to sunlight may help to maintain normal development of emmetropization later in life, and thus lower the risk of myopic anisometropia in adolescent monkey.
Highlights
Myopia is a common eye disorder, which is often considered to arise from complex interactions between genetic makeup and environment [1, 2]
A number of recent epidemiological studies have suggested that greater time spent outdoors has protective effect on the incidence and progression of school myopia [18, 31,32,33,34]
This positive effect is not associated with physical activity and is independent of near work, parent myopia, and ethnicity, which have been proposed as myopiagenic factors
Summary
Myopia is a common eye disorder, which is often considered to arise from complex interactions between genetic makeup and environment [1, 2]. When a minus lens (producing relative hyperopic defocus) is placed constantly over the eye of an infant animal, the eye elongates to compensate for the imposed hyperopic defocus until it is emmetropic with the lens in place, while the opposite is observed in the plus-lens condition. This vision-dependent mechanism has been found to be still active well beyond infancy[14]- into childhood and adolescence when the onset of “school” myopia typically occurs. Results from myopia studies based on defocus-induced myopia in animal models might be relevant to the etiology of myopia in children
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