It is well established that spending time outdoors may prevent the development of myopia, although the underlying mechanism is unknown.6,17,18,29 The dominant theory is that exposure to brighter sunlight outside stimulates a release of dopamine in the retina, and dopamine can promote slower, normal growth of the eye, thereby leading to a lower risk of myopia.30
Recently, some novel findings have supported that the differences in the spectral composition of lighting in indoor and outdoor environments may contribute to the higher prevalence of myopia in children who spend less time outdoors.13 The chromaticity signals from LCA can promote the normal rate of eye growth and development of ocular refraction,31 which may help to explain why outdoor activity has a protective effect against myopia and to highlight the possible effects of artificial light in the increasing prevalence of childhood myopia. Therefore, the manipulation of the chromaticity of light to which the eyes are exposed may be valuable for the management or prevention of childhood myopia. To date, we know relatively little about how variations in the wavelength composition of ambient lighting affect refractive development in humans. Our goal was to determine whether environments dominated by long-wavelength red light inhibit the development of myopia. The study programme integrated red light with a wavelength of 650 nm in natural light, which is beneficial to the human body, to replace natural light, irradiate the retina with safe power and effective time, and stimulate the retina to produce and release more dopamine, thereby effectively controlling axial eye growth and preventing the occurrence and progression of myopia.
In the present study, children who received repeated 650-nm red light irradiation developed relative hyperopic refractions, and significant changes occurred over time (P = 0.019, Table 2). Corresponding to refractive development, the treated eyes of all children exhibited statistically significantly slower axial eye growth than before treatment. Our results showed that low-intensity, single-wavelength red light irradiation can effectively control AL elongation and slow myopia progression (Table 2, Fig. 2). Interestingly, the decreases in both the SER and AL in the older group (8–14 years) were greater than those in the younger group (4–7 years), which is consistent with previous studies. The decreases in the SER and AL in the longer AL group (≥ 24 mm) were greater than those in the shorter group (< 24 mm). Notably, no significant differences in corneal curvature were found pretreatment and posttreatment. These results suggest that the slowed myopia progression was mainly due to AL shortening and not corneal curvature flattening. Regardless of the exact mechanism that was responsible for the hyperopic shifts found in the present study, the results suggest that exposure to long-wavelength lighting may, at least under certain circumstances, be beneficial for reducing myopia progression. The results also support the emerging view that the eye can utilize chromatic cues associated with LCA to regulate ocular growth.13,31
Whether certain spectral ranges of light can more potently delay and inhibit myopia than others is a topic of ongoing research. The effects of the spectral composition of light on refractive development have been studied in a number of animal models. When infant rhesus monkeys wore spectacles with red filters or were raised in red light, they became consistently more hyperopic.13,14 Similarly, light from red LEDs also acted as an inhibitory stimulus for axial eye growth in tree shrews, even in adolescent animals.10,12 In contrast, upon their return to short-wavelength light, tree shrews tended to become more myopic. Nonetheless, previous studies on the effect of various spectral compositions of light on refractive development have yielded inconsistent findings. In contrast to monkeys and tree shews, chickens,24 guinea pigs23 and fish20 became more myopic under red light and more hyperopic under blue light, which was interpreted as an attempt by the eye to compensate for LCA. Why manipulations of the spectral composition have opposite effects in chicks, guinea pigs, and fish compared to tree shrews and rhesus monkeys is an important open question. The disparity of the results of previous studies highlights that our understanding of how chromatic cues influence eye growth is not complete. There are, however, similarities between the results of our study and previous studies in monkeys and tree shews, namely, that long-wavelength red light acted as an inhibitory stimulus for axial eye growth and induced hyperopic shift in children.
The choroid, a highly vascularized layer located between the retina and sclera, plays a crucial role in relaying signals derived from the retina to the sclera, producing mediators that regulate scleral metabolism during visual cue-modulated ocular development, further affecting extracellular matrix (ECM) remodelling in the sclera, and playing an active role in emmetropization or in the pathogenesis of myopia.32,33 It has been determined that there is a direct correspondence between changes in choroid thickness and choroidal blood flow in both animal models and humans. Decreases in choroidal blood flow may result in reduced levels of oxygen and nutrient supply to the neighbouring avascular sclera.33 Such modulation affects myopia development in experimental animal models and humans.34,35 In this study, 650-nm red light therapy increased the choroidal thickness, possibly in response to an improvement in scleral blood perfusion through the choroid and reduced scleral hypoxia.
Previous studies have demonstrated that narrow-band, long-wavelength red light can thicken the choroid and restore the elasticity of scleral fibres.33 When the choroid is exposed to 650-nm red light, the warm effect of the red light will open the neck-like stenosis at the opening of the small arteries of the choroidal lobules, increase the blood flow into the lobules, increase the microcirculation blood volume, increase the oxygen permeability of the choroidal blood vessels and the oxygen absorption capacity, and thicken the choroid. Additionally, the increase in choroid thickness can move the retina towards the focal plane of the eye (choroidal accommodation), change the AL.32
Various studies36,37 on the association between the AL and choroidal thickness and the association between choroidal thickness and choroidal components in adults have been described with the help of binarization techniques applied to EDI-OCT images.27,38,39 In myopic children, subfoveal choroidal thinning with longer ALs was found to be associated with a reduction in the LA.28 Changes in the LA may directly influence choroidal thickness, as blood vessels represent the main component of the choroid, which might be a helpful signal for inhibiting myopia development.28 Studies using chicks have shown that myopia resulted in smaller vessel diameters and lower blood vessel densities.40,41 After short treatment durations (9 months), the SFChT, TCA and LA all significantly increased; however, the stromal area slightly decreased, with no significant difference compared to that at baseline. Univariate linear regression analyses showed that the changes in the AL were significantly associated with age and baseline AL.
In conclusion, the results demonstrate that repeated 650-nm, low-intensity, single-wavelength red light effectively reduced myopia progression, inhibited axial elongation and increased the choroidal thickness, possibly in response to an improvement in scleral blood perfusion through the choroid and reduced scleral hypoxia. These findings require further exploration in a longitudinal study, as well as furth0er research in animal models.