Effects of Long-Wavelength Lighting on Refractive Development in Infant Rhesus Monkeys

Mechanisms of emmetropization and what might go wrong in myopia

Studies in animal models and humans have shown that refractive state is optimized during postnatal development by a closed-loop negative feedback system that uses retinal image defocus as an error signal, a mechanism called emmetropization. The sensor to detect defocus and its sign resides in the retina itself. The retina and/or the retinal pigment epithelium (RPE) presumably releases biochemical messengers to change choroidal thickness and modulate the growth rates of the underlying sclera. A central question arises: if emmetropization operates as a closed-loop system, why does it not stop myopia development? Recent experiments in young human subjects have shown that (1) the emmetropic retina can perfectly distinguish between real positive defocus and simulated defocus, and trigger transient axial eye shortening or elongation, respectively. (2) Strikingly, the myopic retina has reduced ability to inhibit eye growth when positive defocus is imposed. (3) The bi-directional response of the emmetropic retina is elicited with low spatial frequency information below 8 cyc/deg, which makes it unlikely that optical higher-order aberrations play a role. (4) The retinal mechanism for the detection of the sign of defocus involves a comparison of defocus blur in the blue (S-cone) and red end of the spectrum (L + M-cones) but, again, the myopic retina is not responsive, at least not in short-term experiments. This suggests that it cannot fully trigger the inhibitory arm of the emmetropization feedback loop. As a result, with an open feedback loop, myopia development becomes "open-loop".

Myopia Control: Are We Ready for an Evidence Based Approach?

INTRODUCTION

Myopia and its vision-threatening complications present a significant public health problem. This review aims to provide an updated overview of the multitude of known and emerging interventions to control myopia, including their potential effect, safety, and costs.

METHODS

A systematic literature search of three databases was conducted. Interventions were grouped into four categories: environmental/behavioral (outdoor time, near work), pharmacological (e.g., atropine), optical interventions (spectacles and contact lenses), and novel approaches such as red-light (RLRL) therapies. Review articles and original articles on randomized controlled trials (RCT) were selected.

RESULTS

From the initial 3224 retrieved records, 18 reviews and 41 original articles reporting results from RCTs were included. While there is more evidence supporting the efficacy of low-dose atropine and certain myopia-controlling contact lenses in slowing myopia progression, the evidence about the efficacy of the newer interventions, such as spectacle lenses (e.g., defocus incorporated multiple segments and highly aspheric lenslets) is more limited. Behavioral interventions, i.e., increased outdoor time, seem effective for preventing the onset of myopia if implemented successfully in schools and homes. While environmental interventions and spectacles are regarded as generally safe, pharmacological interventions, contact lenses, and RLRL may be associated with adverse effects. All interventions, except for behavioral change, are tied to moderate to high expenditures.

CONCLUSION

Our review suggests that myopia control interventions are recommended and prescribed on the basis of accessibility and clinical practice patterns, which vary widely around the world. Clinical trials indicate short- to medium-term efficacy in reducing myopia progression for various interventions, but none have demonstrated long-term effectiveness in preventing high myopia and potential complications in adulthood. There is an unmet need for a unified consensus for strategies that balance risk and effectiveness for these methods for personalized myopia management.

Advances in myopia control strategies for children

Myopia has long been a global threat to public health. Timely interventions are likely to reduce the risk of vision-threatening complications. There are both established and rapidly evolving therapeutic approaches to slow myopia progression and/or delay its onset. The effective methods for slowing myopia progression include atropine eye-drops, defocus incorporated multiple segments (DIMS) spectacle lenses, spectacle lenses with highly aspherical lenslets target (HALT), diffusion optics technology (DOT) spectacle lenses, red light therapy (RLT), multifocal soft contact lenses and orthokeratology. Among these, 0.05% atropine, HALT lenses, RLT and +3.00 peripheral addition soft contact lenses yield over 60% reduction in myopia progression, whereas DIMS, DOT and MiSight contact lenses demonstrate at least 50% myopia control efficacy. 0.05% atropine demonstrates a more optimal balance of efficacy and safety than 0.01%. The efficacy of 0.01% atropine has not been consistent and requires further validation across diverse ethnicities. Combining atropine 0.01% with orthokeratology or DIMS spectacles yields better outcomes than using these interventions as monotherapies. Increased outdoor time is an effective public health strategy for myopia prevention while recent studies suggest that 0.05% low-concentration atropine and RLT therapy have promising potential as clinical myopia prevention interventions for high-risk groups. Myopia control spectacle lenses, being the least invasive, are safe for long-term use. However, when considering other approaches, it is essential to ensure proper instruction and regular follow-ups to maintain safety and monitor any potential complications. Ultimately, significant advances have been made in myopia control strategies, many of which have shown meaningful clinical outcomes. However, regular use and adequate safety monitoring over extended durations are imperative to foster confidence that can only come from extensive clinical experience.

The knowledge structure and research trends between light and myopia: A bibliometric analysis from 1981 to 2024

BACKGROUND

This bibliometric analysis explored the knowledge structure of and research trends in the relationship between light and myopia.

METHODS

Relevant literature published from 1981 to 2024 was collected from the Web of Science Core Collection database. Visual maps were generated using CiteSpace and VOSviewer. We analyzed the included studies in terms of the annual publication count, countries, institutional affiliations, prolific authors, source journals, top 10 most cited articles, keyword co-occurrence, and cocitations.

RESULTS

A total of 525 papers examining the relationship between light and myopia published between 1981 and 2024 were collected. The United States ranked first in terms of the number of publications and actively engaged in international cooperation with other countries. The New England College of Optometry, which is located in the United States, was the most active institution and ranked first in terms of the number of publications. Schaeffel Frank was the most prolific author. The most active journal in the field was Investigative Ophthalmology & Visual Science. The most frequently cited paper in the included studies was written by Saw, SM and was published in 2002. The most common keywords in basic research included "refractive error," "longitudinal chromatic aberration," and "compensation." The most common keywords in clinical research mainly included "light exposure," "school," and "outdoor activity." The current research hotspots in this field are "progression," "refractive development," and "light exposure." The cocitation analysis generated 17 clusters.

CONCLUSION

This study is the first to use bibliometric methods to analyze existing research on the relationship between light and myopia. In recent years, the intensity and wavelength of light have become research hotspots in the field. Further research on light of different intensities and wavelengths may provide new perspectives in the future for designing more effective treatments and interventions to reduce the incidence of myopia.

The Parameters Governing the Anti-Myopia Efficacy of Chromatically Simulated Myopic Defocus in Tree Shrews

Purpose

We previously showed that exposing tree shrews (Tupaia belangeri, small diurnal mammals closely related to primates) to chromatically simulated myopic defocus (CSMD) counteracted small-cage myopia and instead induced hyperopia (approximately +4 diopters [D]). Here, we explored the parameters of this effect.

Methods

Tree shrews were exposed to the following interventions for 11 days: (1) rearing in closed (n = 7) or open (n = 6) small cages; (2) exposed to a video display of Maltese cross images with CSMD combined with overhead lighting (n = 4); (3) exposed to a video display of Maltese cross images with zero blue contrast ("flat blue," n = 8); and (4) exposed to a video display of black and white grayscale tree images with different spatial filtering (blue pixels lowpass <1 and <2 cycles per degree [CPD]) for the CSMD.

Results

(1) Tree shrews kept in closed cages, but not open cages, developed myopia. (2) Overhead illumination reduced the hyperopia induced by CSMD. (3) Zero-blue contrast produced hyperopia but slightly less than the CSMD. (4) Both of the CSMD tree images counteracted small cage myopia, but the one low pass filtering blue <1 CPD was more effective at inducing hyperopia.

Conclusions

Any pattern with reduced blue contrast at and below approximately 1 CPD counteracts myopia/promotes hyperopia, but maximal effectiveness may require that the video display be the brightest object in the environment.

Translational Relevance

Chromatically simulated myopic blur might be a powerful anti-myopia therapy in children, but the parameter selection could be critical. Issues for translation to humans are discussed.

Effects of short-term exposure to red or near-infrared light on axial length in young human subjects

PURPOSE

To determine whether visible light is needed to elicit axial eye shortening by exposure to long wavelength light.

METHODS

Incoherent narrow-band red (620 ± 10 nm) or near-infrared (NIR, 875 ± 30 nm) light was generated by an array of light-emitting diodes (LEDs) and projected monocularly in 17 myopic and 13 non-myopic subjects for 10 min. The fellow eye was occluded. Light sources were positioned 50 cm from the eye in a dark room. Axial length (AL) was measured before and after the exposure using low-coherence interferometry.

RESULTS

Non-myopic subjects responded to red light with significant eye shortening, while NIR light induced minor axial elongation (-13.3 ± 17.3 μm vs. +6.5 ± 11.6 μm, respectively, p = 0.005). Only 41% of the myopic subjects responded to red light exposure with a decrease in AL and changes were therefore, on average, not significantly different from those observed with NIR light (+0.2 ± 12.1 μm vs. +1.1 ± 11.2 μm, respectively, p = 0.83). Interestingly, there was a significant correlation between refractive error and induced changes in AL after exposure to NIR light in myopic eyes (r(15) = -0.52, p = 0.03) and induced changes in AL after exposure to red light in non-myopic eyes (r(11) = 0.62, p = 0.02), with more induced axial elongation with increasing refractive error.

CONCLUSIONS

Incoherent narrow-band red light at 620 nm induced axial shortening in 77% of non-myopic and 41% of myopic eyes. NIR light did not induce any significant changes in AL in either refractive group, suggesting that the beneficial effect of red laser light therapy on myopia progression requires visible stimulation and not simply thermal energy.

Age and light damage influence Fzd5 regulation of ocular growth-related genes

Genetic and environmental factors can independently or coordinatively drive ocular axis growth. Mutations in FRIZZLED5 (FZD5) have been associated with microphthalmia, coloboma, and, more recently, high myopia. The molecular mechanism of how Fzd5 participates in ocular growth remains unknown. In this study, we compiled a list of human genes associated with ocular growth abnormalities based on public databases and a literature search. We identified a set of ocular growth-related genes from the list that was altered in the Fzd5 mutant mice by RNAseq analysis at different time points. The Fzd5 regulation of this set of genes appeared to be impacted by age and light damage. Further bioinformatical analysis indicated that these genes are extracellular matrix (ECM)-related; and meanwhile an altered Wnt signaling was detected. Altogether, the data suggest that Fzd5 may regulate ocular growth through regulating ECM remodeling, hinting at a genetic-environmental interaction in gene regulation of ocular axis control.

The influence of the environment and lifestyle on myopia

BACKGROUND

Myopia, commonly known as near-sightedness, has emerged as a global epidemic, impacting almost one in three individuals across the world. The increasing prevalence of myopia during early childhood has heightened the risk of developing high myopia and related sight-threatening eye conditions in adulthood. This surge in myopia rates, occurring within a relatively stable genetic framework, underscores the profound influence of environmental and lifestyle factors on this condition. In this comprehensive narrative review, we shed light on both established and potential environmental and lifestyle contributors that affect the development and progression of myopia.

MAIN BODY

Epidemiological and interventional research has consistently revealed a compelling connection between increased outdoor time and a decreased risk of myopia in children. This protective effect may primarily be attributed to exposure to the characteristics of natural light (i.e., sunlight) and the release of retinal dopamine. Conversely, irrespective of outdoor time, excessive engagement in near work can further worsen the onset of myopia. While the exact mechanisms behind this exacerbation are not fully comprehended, it appears to involve shifts in relative peripheral refraction, the overstimulation of accommodation, or a complex interplay of these factors, leading to issues like retinal image defocus, blur, and chromatic aberration. Other potential factors like the spatial frequency of the visual environment, circadian rhythm, sleep, nutrition, smoking, socio-economic status, and education have debatable independent influences on myopia development.

CONCLUSION

The environment exerts a significant influence on the development and progression of myopia. Improving the modifiable key environmental predictors like time spent outdoors and engagement in near work can prevent or slow the progression of myopia. The intricate connections between lifestyle and environmental factors often obscure research findings, making it challenging to disentangle their individual effects. This complexity underscores the necessity for prospective studies that employ objective assessments, such as quantifying light exposure and near work, among others. These studies are crucial for gaining a more comprehensive understanding of how various environmental factors can be modified to prevent or slow the progression of myopia.

Light exposure therapy for myopia control: a systematic review and Bayesian network meta-analysis

AIMS

To compare and rank the myopia control effects of different light wavelengths in children using a systematic review and Bayesian network meta-analysis (Bayesian NMA).

METHODS

The review protocol was registered with PROSPERO. We searched PubMed, EMBASE and MEDLINE for relevant clinical and animal studies published as of 2 February 2023. We included studies comparing red, violet or full-spectrum light with controls. Data extracted included descriptive statistics and study outcomes (axial length (AL) elongation and progression of spherical equivalent (SE) refraction). After quality assessment, estimates of treatment effect outcomes (mean differences (MDs) and 95% CIs) were first pooled for the animal and clinical studies in a traditional meta-analysis. To compare and rank the different light wavelengths, the Bayesian NMA was then conducted for all the included clinical studies (12 studies) and separately for only randomised controlled trials (8 studies). MDs, 95% credible intervals (CrIs) and ranks of the various light wavelengths were estimated in the Bayesian NMA.

RESULTS

When all clinical studies were included in the Bayesian NMA (12 studies), only red-light significantly slowed AL elongation, MD (95% CrI), -0.38 mm (-0.59 mm to -0.16 mm)/year and SE refraction progression, 0.72D (0.35D to 1.10D)/year compared with controls. It remained the only significant intervention when effect sizes from only RCTs (eight studies) were separately combined, (-0.28 mm (-0.40 mm to -0.15 mm)/year and 0.57D (0.22D to 0.92D)/year, for AL and SE refraction, respectively).

CONCLUSION

Myopia control efficacy varied among different wavelengths of light, with red light ranked as the most effective.

PROSPERO REGISTRATION NUMBER

Clinical studies: CRD42022368998; animal studies: CRD42022368671.

Repeated Low-level Red-light Therapy: The Next Wave in Myopia Management?

SIGNIFICANCE

Exposure to long-wavelength light has been proposed as a potential intervention to slow myopia progression in children. This article provides an evidence-based review of the safety and myopia control efficacy of red light and discusses the potential mechanisms by which red light may work to slow childhood myopia progression.The spectral composition of the ambient light in the visual environment has powerful effects on eye growth and refractive development. Studies in mammalian and primate animal models (macaque monkeys and tree shrews) have shown that daily exposure to long-wavelength (red or amber) light promotes slower eye growth and hyperopia development and inhibits myopia induced by form deprivation or minus lens wear. Consistent with these results, several recent randomized controlled clinical trials in Chinese children have demonstrated that exposure to red light for 3 minutes twice a day significantly reduces myopia progression and axial elongation. These findings have collectively provided strong evidence for the potential of using red light as a myopia control intervention in clinical practice. However, several questions remain unanswered. In this article, we review the current evidence on the safety and efficacy of red light as a myopia control intervention, describe potential mechanisms, and discuss some key unresolved issues that require consideration before red light can be broadly translated into myopia control in children.

Effect of red-light therapy on retinal and choroidal blood perfusion in myopic children

OBJECTIVE

To investigate the effect of repeated low-level red-light therapy (RLRLT) on retinal and choroidal blood perfusion in myopic children.

METHODS

Forty-seven myopic children (mean spherical equivalent refractive error [SE]: -2.31 ± 1.26 D; age range: 8.0-11.0 years) were enrolled and received RLRLT (power 2 mW, wavelength 650 nm) for 3 min twice a day, while 20 myopic children (SE: -2.75 ± 0.84 D; age range: 7.0-10.0 years) were included as a control group. All participants wore single-vision distance glasses. Refractive error, axial length (AL) and other biometric parameters were measured at baseline and during follow-up visits in the first, second and fourth weeks after initiation of treatment. Retinal thickness, subfoveal choroidal thickness (SFCT), total choroidal area (TCA), luminal area (LA), stromal area (SA) and choroidal vascularity index (CVI) were obtained using optical coherence tomography (OCT). The percentage retinal vascular density (VD%) and choriocapillaris flow voids (FV%) were measured using en-face OCT angiography.

RESULTS

After 4 weeks of treatment, a significant increase in SFCT was observed in the RLRLT group, with an average increase of 14.5 μm (95% confidence interval [CI]: 9.6-19.5 μm), compared with a decrease of -1.7 μm (95% CI: -9.1 to 5.7 μm) in the control group (p < 0.0001). However, no significant changes in retinal thickness or VD% were observed in either group (all p > 0.05). In the OCT images from the RLRLT group, no abnormal retinal morphology related to photodamage was observed. The horizontal scans revealed an increase in TCA, LA and CVI over time (all p < 0.05), while SA and FV% remained unchanged (both p > 0.05).

CONCLUSIONS

These findings indicate that RLRLT can enhance choroidal blood perfusion in myopic children, demonstrating a cumulative effect over time.

The effects of intensity, spectral purity and duty cycle on red light-induced hyperopia in tree shrews

INTRODUCTION

There have recently been several clinical studies suggesting that brief periods of exposure to red light (repeated low-level red light, 'RLRL') may produce a dramatic anti-myopia effect, calling for further investigations into its therapeutic parameters. Unfortunately, many experimental species used in refractive studies develop myopia in response to this wavelength. Tree shrews are the only animal model other than rhesus monkeys that consistently exhibit hyperopic responses to ambient red light. Here, tree shrews were used to study the influence of the spectral purity, duty cycle and intensity of red light on its anti-myopic effect.

METHODS

Juvenile tree shrews (Tupaia belangeri) were raised from 24 to 35 days after eye opening under ambient lighting that was: standard white colony fluorescent light; pure narrow band red light of either 600, 50-100 or 5 lux; red light that was diluted with 10% white light (by lux) or 50% white and 2 s of pure red light that alternated with 2 s of pure white light (50% duty cycle). Refractive measures were taken with a NIDEK ARK-700 autorefractor and axial dimensions with a LenStar LS-900 Axial Biometer.

RESULTS

The pro-hyperopia effect of ambient red light was greatly reduced by even small amounts of concurrent white light 'contamination', but remained robust if 2-s periods of pure white light alternated with 2 s of red. Finally, the hyperopic effect of red light was maintained at reduced luminance levels in the 50-100 lux range and only failed at 5 lux.

CONCLUSIONS

These results have implications for understanding the mechanisms by which ambient red light affects refractive development, and possibly also for clinical therapies using RLRL. Nevertheless, it remains to be determined if the mechanism of the current clinical RLRL therapy is the same as that operating on tree shrews in ambient red light.

Repeated Low-Level Red-Light Therapy for Controlling Onset and Progression of Myopia-a Review

Repeated low-level red-light (RLRL), characterized by increased energy supply and cellular metabolism, thus enhancing metabolic repair processes, has gained persistent worldwide attention in recent years as a new novel scientific approach for therapeutic application in myopia. This therapeutic revolution led by RLRL therapy is due to significant advances in bioenergetics and photobiology, for instance, enormous progresses in photobiomodulation regulated by cytochrome c oxidase, the primary photoreceptor of the light in the red to near infrared regions of the electromagnetic spectrum, as the primary mechanism of action in RLRL therapy. This oxidase is also a key mitochondrial enzyme for cellular bioenergetics, especially for the nerve cells in the retina and brain. In addition, dopamine (DA)-enhanced release of nitric oxide may also be involved in controlling myopia by activation of nitric oxide synthase, enhancing cGMP signaling. Recent evidence has also suggested that RLRL may inhibit myopia progression by inhibiting spherical equivalent refraction (SER) progression and axial elongation without adverse effects. In this review, we provide scientific evidence for RLRL therapy as a unique paradigm to control myopia and support the theory that targeting neuronal energy metabolism may constitute a major target for the neurotherapeutics of myopia, with emphasis on its molecular, cellular, and nervous tissue levels, and the potential benefits of RLRL therapy for myopia.

The effects of ambient narrowband long-wavelength light on lens-induced myopia and form-deprivation myopia in tree shrews

Here we examine the effects of ambient red light on lens-induced myopia and diffuser-induced myopia in tree shrews, small diurnal mammals closely related to primates. Starting at 24 days of visual experience (DVE), seventeen tree shrews were reared in red light (624 ± 10 or 634 ± 10 nm, 527-749 human lux) for 12-14 days wearing either a -5D lens (RL-5D, n = 5) or a diffuser (RLFD, n = 5) monocularly, or without visual restriction (RL-Control, n = 7). Refractive errors and ocular dimensions were compared to those obtained from tree shrews raised in broad-spectrum white light (WL-5D, n = 5; WLFD, n = 10; WL Control, n = 7). The RL-5D tree shrews developed less myopia in their lens-treated eyes than WL-5D tree shrews at the end of the experiment (-1.1 ± 0.9D vs. -3.8 ± 0.3D, p = 0.007). The diffuser-treated eyes of the RLFD tree shrews were near-emmetropic (-0.3 ± 0.6D, vs. -5.4 ± 0.7D in the WLFD group). Red light induced hyperopia in control animals (RL-vs. WL-Control, +3.0 ± 0.7 vs. +1.0 ± 0.2D, p = 0.02), the no-lens eyes of the RL-5D animals, and the no-diffuser eyes of the RLFD animals (+2.5 ± 0.5D and +2.3 ± 0.3D, respectively). The refractive alterations were consistent with the alterations in vitreous chamber depth. The lens-induced myopia developed in red light suggests that a non-chromatic cue could signal defocus to a less accurate extent, although it could also be a result of "form-deprivation" caused by defocus blur. As with previous studies in rhesus monkeys, the ability of red light to promote hyperopia appears to correlate with its ability to retard lens-induced myopia and form-deprivation myopia, the latter of which might be related to non-visual ocular mechanisms.

Photobiomodulation at 660 nm promotes collagen synthesis via downregulation of HIF-1α expression without photodamage in human scleral fibroblasts in vitro in a hypoxic environment

PURPOSE

The increasing prevalence of myopia is a global public health issue. Because of the complexity of myopia pathogenesis, current control methods for myopia have great limitations. The aim of this study was to explore the effect of photobiomodulation (PBM) on human sclera fibroblasts (HSFs) under hypoxia, in the hope of providing new ideas for myopia prevention and control.

METHODS

Hypoxic cell model was established at 0, 6, 12, and 24 h time points to simulate myopia microenvironment and explore the optimal time point. Control, hypoxia, hypoxia plus light, and normal plus light cell models were set up for the experiments, and cells were incubated for 24 or 48 h after PBM (660 nm, 5 J/cm2), followed by evaluation of hypoxia-inducible factor 1α (HIF-1α) and collagen I a1 (COL1A1) proteins using Western blotting and immunofluorescence, and photo damage was detected by CCK-8, scratch test, and flow cytometry assays. We also used transfection technology to further elucidate the regulatory mechanism.

RESULTS

The change of target proteins is most obvious when hypoxia lasts for 24 h (p < 0.01). PBM at 660 nm increased extracellular collagen content (p < 0.001) and downregulated expression of HIF-1α (p < 0.05). This treatment did not affect the migration and proliferation of cells (p > 0.05), and effectively inhibited apoptosis under hypoxia (p < 0.0001). After overexpression of HIF-1α, the effect of PBM was attenuated (p > 0.05).

CONCLUSIONS

Photobiomodulation at 660 nm promotes collagen synthesis via downregulation of HIF-1α expression without photodamage.

L-opsin expression in chickens is similarly reduced with diffusers and negative lenses

Previous studies have shown that the expression of L- and M-opsins was reduced in chicken retina when eyes were covered with diffusers. The purpose of the current study was to find out whether this is a result of altered spatial processing during development of deprivation myopia or merely a consequence of light attenuation by the diffusers. Therefore, retinal luminances were matched by neutral density filters in fellow eyes that served as controls for diffuser-treated eyes. Furthermore, the effects of negative lenses on opsins expression were studied. Chickens wore diffusers or -7D lenses for a period of 7 days and refractive state and ocular biometry were measured at the beginning and at the end of the experiment. Retinal tissue was extracted from both eyes to quantify L-, M- and S-opsins expression by qRT-PCR. It was found that L-opsin expression was significantly lower in eyes wearing diffusers, compared to fellow eyes covered with neutral density filters. Interestingly, L-opsin was also reduced in eyes wearing negative lenses. In summary, this study shows that L-opsin expression is reduced due to the loss of high spatial frequencies and general contrast reduction in the retinal image, rather than by a decline in retinal luminance. Moreover, the fact that L-opsin was similarly reduced in eyes treated with negative lenses and diffusers suggests the existence of a common pathway for emmetropization, but it could also be just a consequence of reduced high spatial frequencies and lower contrast.

IMI 2023 Digest

Myopia is a dynamic and rapidly moving field, with ongoing research providing a better understanding of the etiology leading to novel myopia control strategies. In 2019, the International Myopia Institute (IMI) assembled and published a series of white papers across relevant topics and updated the evidence with a digest in 2021. Here, we summarize findings across key topics from the previous 2 years. Studies in animal models have continued to explore how wavelength and intensity of light influence eye growth and have examined new pharmacologic agents and scleral cross-linking as potential strategies for slowing myopia. In children, the term premyopia is gaining interest with increased attention to early implementation of myopia control. Most studies use the IMI definitions of ≤-0.5 diopters (D) for myopia and ≤-6.0 D for high myopia, although categorization and definitions for structural consequences of high myopia remain an issue. Clinical trials have demonstrated that newer spectacle lens designs incorporating multiple segments, lenslets, or diffusion optics exhibit good efficacy. Clinical considerations and factors influencing efficacy for soft multifocal contact lenses and orthokeratology are discussed. Topical atropine remains the only widely accessible pharmacologic treatment. Rebound observed with higher concentration of atropine is not evident with lower concentrations or optical interventions. Overall, myopia control treatments show little adverse effect on visual function and appear generally safe, with longer wear times and combination therapies maximizing outcomes. An emerging category of light-based therapies for children requires comprehensive safety data to enable risk versus benefit analysis. Given the success of myopia control strategies, the ethics of including a control arm in clinical trials is heavily debated. IMI recommendations for clinical trial protocols are discussed.

Axial Shortening in Myopic Children after Repeated Low-Level Red-Light Therapy: Post Hoc Analysis of a Randomized Trial

INTRODUCTION

Axial length (AL) elongation in myopia is considered irreversible. We aimed to systemically report unexpected AL shortening observed in a randomized clinical trial (RCT) after repeated low-level red-light (RLRL) therapy.

METHODS

This is a post hoc analysis of a multicenter, single-masked RCT. Two hundred sixty-four myopic children aged 8-13 years allocated to RLRL treatment (intervention group) or a single vision spectacle (SVS, control group) were included. AL was measured using an IOL-master 500 at baseline, 1-, 3-, 6-, and 12-month follow-up visits. AL shortening was defined as AL reduction from baseline to follow-up visits at three cutoffs: > 0.05 mm, > 0.10 mm, and > 0.20 mm. Frequency of AL shortening at different cutoffs was calculated. Analysis was done with intent to treat (ITT).

RESULTS

At 12-months follow up, frequency of AL shortening > 0.05 mm was 26/119 (21.85%) and 2/145 (1.38%) for the RLRL group versus the control group, respectively. The frequency was 18/119 (15.13%) versus 0/145 (0%) for AL shortening > 0.10 mm, and 7/119 (5.88%) versus 0/145 (0%), for AL shortening > 0.20 mm, respectively (p < 0.001). Mean AL shortening after 12 months (SD) was -0.156 (0.086) mm in the RLRL group and -0.06 mm in the control group. Age was significantly associated with AL shortening in the multivariable analysis. For the RLRL group that exhibited AL shortening (n = 56), choroidal thickness (ChT) thickening (0.056 mm) could only explain 28.3% of AL shortening (-0.20 mm).

CONCLUSION

Nearly a quarter of children had > 0.05 mm AL shortening following 12 months of RLRL therapy, whereas AL shortening rarely occurred among controls.

TRIAL REGISTRATION

ClinicalTrials.gov (NCT04073238).

Comment on: "Polymer Co-Coating of Gold Nanoparticles Enables Their Integration Into Contact Lenses for Stable, Selective Ocular Light Filters"

T. j. Gawne,* Z. She, T. T. Norton

Exposure to cyan or red light inhibits the axial growth of zebrafish eyes

Myopia, or nearsightedness, is the most common type of refractive error and is characterized by a mismatch between the optical power and ocular axial length. Light, and more specifically the spectral composition of light, has been known to influence myopic axial growth. In this pilot study, we exposed zebrafish to illuminations that vary in spectral composition and screened for changes in axial length. The illumination spectra included narrow band ultra-violet A (UVA) (peak wavelength 369 nm), violet (425 nm), cyan (483 nm), green/yellow (557 nm), and red (633 nm) light, as well as broad band white light (2700 K and 6500 K), dim white light and broad spectrum (day) light. We found that rearing zebrafish in cyan or red light leads to a reduction of the ocular axial length. The results of this pilot study may contribute to new perspectives on the role of light and lighting as an intervention strategy for myopia control.

Limited bandwidth short-wavelength light produces slowly-developing myopia in tree shrews similar to human juvenile-onset myopia

During postnatal development, an emmetropization feedback mechanism uses visual cues to modulate the axial growth of eyes so that, with maturation, images of distant objects are in focus on the retina. If the visual cues indicate that the eye has become too long, it generates STOP signals that slow eye elongation. Myopia is a failure of this process where the eye becomes too long. The existing animal models of myopia have been essential in understanding the mechanics of emmetropization but use visual cues that lead to rapidly progressing myopia and don't match the stimuli that lead to human myopia. Form deprivation removes esssentially all spatial contrast. Minus lens wear accurately guides axial elongation to restore sharp focus: technically it is not a model of myopia! In contrast, childhood myopia involves a slow drift into myopia, even with the presence of clear images. We hypothesize that, in the modern visual environment, STOP signals are present but often are not quite strong enough to prevent myopic progression. Using tree shrews, small diurnal mammals closely related to primates, we have developed an animal model that we propose better represents this situation. We used limited bandwidth light to provide limited chromatic cues for emmetropization that are not quite enough to produce fully effective STOP signaling, resulting in a slow drift into myopia as seen in children. We hypothesize that this animal model of myopia may prove useful in evaluating anti-myopia therapies where form deprivation and minus lens wear would be too powerful.

Ubiquitous light-emitting diodes: Potential threats to retinal circadian rhythms and refractive development

The use of light-emitting diodes (LEDs) has increased considerably in the 21st century with humans living in a modern photoperiod with brighter nights and dimmer days. Prolonged exposure to LEDs, especially at night, is considered a new source of pollution because it may affect the synthesis and secretion of retinal melatonin and dopamine, resulting in negative impacts on retinal circadian clocks and potentially disrupting retinal circadian rhythms. The control of ocular refraction is believed to be related to retinal circadian rhythms. Moreover, the global prevalence of myopia has increased at an alarming rate in recent decades. The widespread use of LEDs and the rapid increase in the prevalence of myopia overlap, which is unlikely to be a coincidence. The connection among LEDs, retinal circadian rhythms, and refractive development is both fascinating and confusing. In this review, we aim to develop a systematic framework that includes LEDs, retinal circadian rhythms and refractive development. This paper summarizes the possible mechanisms by which LEDs may disrupt retinal circadian rhythms. We propose that prolonged exposure to LEDs may induce myopia by disrupting retinal circadian rhythms. Finally, we suggest several possible countermeasures to prevent LED interference on retinal circadian rhythms, with the hope of reducing the onset and progression of myopia.

Emmetropization and nonmyopic eye growth

Most eyes start with a hypermetropic refractive error at birth, but the growth rates of the ocular components, guided by visual cues, will slow in such a way that this refractive error decreases during the first 2 years of life. Once reaching its target, the eye enters a period of stable refractive error as it continues to grow by balancing the loss in corneal and lens power with the axial elongation. Although these basic ideas were first proposed over a century ago by Straub, the exact details on the controlling mechanism and the growth process remained elusive. Thanks to the observations collected in the last 40 years in both animals and humans, we are now beginning to get an understanding how environmental and behavioral factors stabilize or disrupt ocular growth. We survey these efforts to present what is currently known regarding the regulation of ocular growth rates.

Effects of artificial light with different spectral composition on eye axial growth in juvenile guinea pigs

The purpose of the study was to investigate the effect of artificial light with different spectral composition and distribution on axial growth in guinea pigs. Three-week-old guinea pigs were randomly assigned to groups exposed to natural light, low color temperature light-emitting diode (LED) light, two full spectrum artificial lights (E light and Julia light) and blue light filtered light with the same intensity. Axial lengths of guinea pigs' eyes were measured by A-scan ultrasonography prior to the experiment and every 2 weeks during the experiment. After light exposure for 12 weeks, retinal dopamine (DA), dihydroxy-phenylacetic acid (DOPAC) levels and DOPAC/DA ratio were analyzed by high-pressure liquid chromatography electrochemical detection and retinal histological structure was observed. Retinal melanopsin expression was detected using Western blot and immunohistochemistry. After exposed to different kinds of light with different spectrum for 4 weeks, the axial lengths of guinea pigs' eyes in LED group and Julia light group were significantly longer than those of natural light group. After 6 weeks, the axial lengths in LED light group were significantly longer than those of E light group and blue light filtered group. The difference between axial lengths in E light group and Julia light group showed statistical significance after 8 weeks (p<0.05). After 12 weeks of light exposure, the comparison of retinal DOPAC/DA ratio and melanopsin expression in each group was consistent with that of axial length. In guinea pigs, continuous full spectrum artificial light with no peak or valley can inhibit axial elongation via retinal dopaminergic and melanopsin system.

Axial Length Shortening and Choroid Thickening in Myopic Adults Treated with Repeated Low-Level Red Light

This study aimed to explore the effect of repeated low-level red light (RLRL) on axial length (AL), choroid blood flow, and anterior segment components in myopic adults. Ninety-eight myopic adults were randomly divided into the RLRL group (n = 52) and the control group (n = 46). Subjects in the RLRL group completed a 4-week treatment composed of a 3-min RLRL treatment session twice daily, with an interval of at least 4 h. Visits were scheduled before and on 7, 14, 21, and 28 days after the treatment. AL, subfoveal choroidal thickness (SChT), choroidal vascularity index (CVI), and anterior segment parameters were measured at each visit. A linear mixed-effects model showed that the AL of the subjects in RLRL decreased from 24.63 ± 1.04 mm to 24.57 ± 1.04 mm, and the SChT thickened by 18.34 μm. CVI had a slight but significant increase in the 0-6 zone. However, all the anterior segment parameters did not change after RLRL treatment. Our study showed that the choroid's thickening is insufficient to explain the axial length shortening. The unchanged anterior segment and improved choroid blood flow suggest that the AL shortening in this study is mainly related to changes in the posterior segment.

Normal Corneal Thickness and Endothelial Cell Density in Rhesus Macaques (Macaca mulatta)

Purpose

To define the normal range of central corneal thickness (CCT) and corneal endothelial cell density (ECD) in rhesus macaques (Macaca mulatta) and the effects of age, body weight, sex, and intraocular pressure (IOP) on these parameters.

Methods

Ophthalmic examinations were performed on 144 rhesus macaques without anterior segment pathology. The CCT was measured via ultrasound pachymetry (USP) and specular microscopy, and the ECD was semiautomatically and manually counted using specular microscopy. Rebound tonometry was used to measure IOP. Linear regression and mixed-effects linear regression models were used to evaluate the effects of age, body weight, sex, and IOP on CCT and ECD.

Results

We included 98 females and 46 males with an age range of 0.2 to 29.4 years. The mean CCT by USP and specular microscopy were 483 ± 39 and 463 ± 33 µm, respectively, and were statistically different (P < 0.001). The ECDs were 2717 ± 423 and 2747 ± 438 cells/mm2 by semiautomated and manual analysis, respectively. Corneal endothelial degeneration was identified in one aged rhesus macaque.

Conclusions

The mean USP and specular microscopy CCT values differed significantly, whereas the semiautomatic and manual ECD did not. The CCT was associated with the IOP and sex, whereas the ECD was associated with body weight and age (P < 0.05). As in humans, corneal disease in rhesus macaques is uncommon.

Translational Relevance

Establishing reference values is fundamental to use rhesus macaques as a model for corneal disease or to identify toxicity in studies of ocular drugs or devices.

Red light irradiation as an intervention for myopia

Myopia is one of the main causes of visual impairment worldwide. Preventing myopia and providing myopia-related interventions are of paramount importance. Based on a thorough review of the available literature, we conclude that red light irradiation can produce hyperopia, resulting in myopia prevention and control. Further, we suggest that red light irradiation may be a powerful tool for myopia prevention and control in the future. At the same time, red light has a protective effect on the cornea and retina at the cellular level, suggesting that red light irradiation may be a safe and effective modality for delaying myopia. Therefore, this form of irradiation is expected to play an important role in the prevention and control of myopia. However, more studies are needed to enhance the current state of knowledge and inform medical guidelines more comprehensively.

Retinal Transcriptomics Analysis Reveals the Underlying Mechanism of Disturbed Emmetropization Induced by Wavelength Defocus

PURPOSE

Wavelength signals play a vital role in refractive development. This study aimed to explore the retinal transcriptome signature in these processes.

METHODS

Guinea pigs were randomly divided into three groups exposed to white, blue, or green environmental light for eight weeks. Refraction and axial length were evaluated every 4 weeks, and the retinal transcriptome was profiled at 8 weeks.

RESULTS

Compared with the white group, ocular refraction significantly decreased and ocular axial length significantly extended in the green group whereas these parameters showed opposite trends in the blue group. RNA-sequencing showed that, compared with the white group, 184 and 171 differentially expressed genes (DEGs) were found in the blue and green groups, respectively. Among these DEGs, only 31 overlapped. These two sets of DEGs were enriched in distinct biological processes and pathways. There were 268 DEGs between the blue and green groups, which were primarily enriched in the extracellular matrix, and metabolism, receptor activity, and ion binding processes. In addition, nine human genes, including ECEL1, CHRND, SHBG, PRSS56, OVOL1, RDH5, WNT7B, PEBP4, CA12, were identified to be related to myopia development and wavelength response, indicating the potential role of these genes in human wavelength-induced myopia.

CONCLUSIONS

In this study, we identified retinal targets and pathways involved in the response to wavelength signals in emmetropization.

Candidate pathways for retina to scleral signaling in refractive eye growth

The global prevalence of myopia, or nearsightedness, has increased at an alarming rate over the last few decades. An eye is myopic if incoming light focuses prior to reaching the retinal photoreceptors, which indicates a mismatch in its shape and optical power. This mismatch commonly results from excessive axial elongation. Important drivers of the myopia epidemic include environmental factors, genetic factors, and their interactions, e.g., genetic factors influencing the effects of environmental factors. One factor often hypothesized to be a driver of the myopia epidemic is environmental light, which has changed drastically and rapidly on a global scale. In support of this, it is well established that eye size is regulated by a homeostatic process that incorporates visual cues (emmetropization). This process allows the eye to detect and minimize refractive errors quite accurately and locally over time by modulating the rate of elongation of the eye via remodeling its outermost coat, the sclera. Critically, emmetropization is not dependent on post-retinal processing. Thus, visual cues appear to influence axial elongation through a retina-to-sclera, or retinoscleral, signaling cascade, capable of transmitting information from the innermost layer of the eye to the outermost layer. Despite significant global research interest, the specifics of retinoscleral signaling pathways remain elusive. While a few pharmacological treatments have proven to be effective in slowing axial elongation (most notably topical atropine), the mechanisms behind these treatments are still not fully understood. Additionally, several retinal neuromodulators, neurotransmitters, and other small molecules have been found to influence axial length and/or refractive error or be influenced by myopigenic cues, yet little progress has been made explaining how the signal that originates in the retina crosses the highly vascular choroid to affect the sclera. Here, we compile and synthesize the evidence surrounding three of the major candidate pathways receiving significant research attention - dopamine, retinoic acid, and adenosine. All three candidates have both correlational and causal evidence backing their involvement in axial elongation and have been implicated by multiple independent research groups across diverse species. Two hypothesized mechanisms are presented for how a retina-originating signal crosses the choroid - via 1) all-trans retinoic acid or 2) choroidal blood flow influencing scleral oxygenation. Evidence of crosstalk between the pathways is discussed in the context of these two mechanisms.

Light Signaling and Myopia Development: A Review

INTRODUCTION

The aim of this article was to comprehensively review the relationship between light exposure and myopia with a focus on the effects of the light wavelength, illuminance, and contrast on the occurrence and progression of myopia.

METHODS

This review was performed by searching PubMed data sets including research articles and reviews utilizing the terms "light", "myopia", "refractive error", and "illuminance", and the review was concluded in November 2021. Myopia onset and progression were closely linked with emmetropization and hyperopia. To better elucidate the mechanism of myopia, some of the articles that focused on this topic were included. This article is based on previously conducted studies and does not contain any new studies with human participants or animals performed by any of the authors.

RESULTS

The pathogenesis and prevention of myopia are not completely clear. Studies have provided evidence supporting the idea that light could affect eye growth in three ways. Changing the corresponding conditions will cause changes in the growth rate and mode of the eyes, and preliminary results have shown that FR/NIR (far red/near-infrared) light is effective for myopia in juveniles.

CONCLUSION

This review discusses the results of studies on the effects of light exposure on myopia with the aims of providing clues and a theoretical basis for the use of light to control the development of myopia and offering new ideas for subsequent studies.

Light Color and the Commercial Broiler: Effect on Ocular Health and Visual Acuity

Light is a critical management factor for broiler production, and the wavelength spectrum, one of its components, can affect bird physiology, behavior and production. Among all the senses, sight is important to birds, and their visual system possess several adaptations that allow them to perceive light differently from humans. Therefore, it is critical to consider whether the exposure to monochromatic light colors influences broiler visual ability, which could affect behavioral expression. The present study examined the effects of various light colors on the visual systems of broiler chickens. Ross 708 males were raised from 0 to 35 days under three wavelength programs [blue (dominant wavelengths near 455 nm), green (dominant wavelengths near 510 nm) or white]. Broilers were given a complete ophthalmic examination, including chromatic pupillary light reflex testing, rebound tonometry, anterior segment biomicroscopy and indirect ophthalmoscopy (n = 36, day 21). To assess ocular anatomy, broilers were euthanized, eyes were weighed, and dimensions were taken (n = 108, day 16 and day 24). An autorefractor was used to assess the refractive index and the corneal curvature (n = 18, day 26). To evaluate spatial vision, broilers underwent a grating acuity test at one of three distances–50, 75, or 100 cm (n = 24, day 29). Data were analyzed as a one-way ANOVA using the MIXED procedure or Proc Par1way for non-normally distributed data. Significant differences were observed for refractive index and spatial vision. Birds raised under blue light were slightly more hyperopic, or far-sighted, than birds raised under white light (P = 0.01). As for spatial vision, birds raised under blue light took less time to approach the stimulus at distances of 50 cm (P = 0.03) and 75 cm (P = 0.0006) and had a higher success rate (choosing the right feeder, P = 0.03) at 100 cm than birds raised under white light. Improvements in spatial vision for birds exposed to blue light can partially explain the behavioral differences resulting from rearing broilers under different wavelengths.

Association Between Color Vision Deficiency and Myopia in Chinese Children Over a Five-Year Period

Purpose

To explore the relationship of color vision deficiency with myopia progression and axial elongation in Chinese primary school children during a five-year cohort study.

Methods

A total of 2849 grade 1 students (aged 7.1 ± 0.4 years) from 11 primary schools were enrolled and followed up for five years. Cycloplegic autorefraction and axial length were measured annually. Color vision testing was performed using Ishihara's test and the City University color vision test.

Results

The prevalence of color vision deficiency was 1.68%, with 2.81% in boys and 0.16% in girls. Color-deficient cases consisted of 91.6% deutan and 8.3% protan. Over the five years, the cumulative incidence of myopia was 35.4% (17/48) in the color-vision deficiency group, which was lower than the 56.7% (1017/1794) in the color normal group (P = 0.004). Over the five-year study period, the change in spherical equivalent refraction in the color vision–deficiency group (−1.81 D) was also significantly lower than that in the color normal group (−2.41 D) (P = 0.002).

Conclusions

The lower incidence and slower progression of myopia in children with color-vision deficiency over the five-year follow-up period suggest that color-deficient individuals are less susceptible to myopia onset and development.

Recovery From Form-Deprivation Myopia in Chicks Is Dependent Upon the Fullness and Correlated Color Temperature of the Light Spectrum

Purpose

The purpose of this study was to evaluate the impact of full-spectrum light-emitting diodes mimicking sunlight (Sunlike LEDs) on ocular growth and refractive error development in a chicken model of myopia.

Methods

One-day old chicks (n = 39) were distributed into 3 groups and raised for 28 days in isoluminant (approximately 285 lux) fluorescent (n = 18, [FL-4000], correlated color temperature [CCT] = 4000 K) or Sunlike LED (n = 12, [SL-4000], CCT = 4000 K; n = 9, [SL-6500], CCT = 6500 K) white lighting environments. Form-deprivation myopia was induced monocularly from day 1 post-hatching (D1) until D14. On D14, form deprivation was halted and the recovery of form-deprived (FD) eyes was monitored until D28. Axial length (AL), refraction, choroidal thickness, and anterior chamber depth were measured in vivo on D1, D7, D14, D22, and D28. Differences in outcome measures between eyes and groups were compared using 2-way repeated-measures ANOVA.

Results

AL and myopic refraction of FD eyes increased similarly among groups during form-deprivation. FD eyes of animals raised under SL-4000 (D22: P < 0.001 and D28: P < 0.001) and SL-6500 (D22: P = 0.006 and D28: P < 0.001) recovered faster from axial elongation compared with animals raised under FL-4000. The refractive status of FD eyes reared under SL-6500, not under FL-4000 or SL-4000, was similar to control eyes on D28 (P > 0.05). However, SL-4000 and SL-6500 exhibited similar refraction on D28 than FL-4000 (P > 0.05). Choroidal thickness was significantly greater in FD eyes of chickens raised under SL-6500 than in animals raised under FL-4000 (P = 0.03).

Conclusions

Compared to fluorescent light, moderate intensities of full-spectrum Sunlike LEDs can accelerate recovery from form-deprivation myopia in chickens, potentially through a change in the choroid-mediated pathway.

Low-intensity, long-wavelength red light slows the progression of myopia in children: an Eastern China-based cohort

PURPOSE

To determine the effect of low-intensity, long-wavelength red light therapy (LLRT) on the inhibition of myopia progression in children.

METHODS

A retrospective study was conducted. One hundred and five myopic children (spherical equivalent refractive error [SER] -3.09 ± 1.74 dioptres [D]; mean age, 9.19 ± 2.40 years) who underwent LLRT treatment (power 0.4 mW, wavelength 635 nm) twice per day for 3 min each session, with at least a 4-h interval between sessions, and a control group of 56 myopic children (SER -3.04 ± 1.66 D; mean age, 8.62 ± 2.45 years) were evaluated. Both groups wore single-vision distance spectacles. Each child returned for a follow-up examination every 3 months after the initial measurements for a total of 9 months.

RESULTS

At 9 months, the mean SER in the LLRT group was -2.87 ± 1.89 D, significantly greater than that of the control group (-3.57 ± 1.49 D, p < 0.001). Axial length (AL) changes were -0.06 ± 0.19 mm and 0.26 ± 0.15 mm in the LLRT group and control group (p < 0.001), respectively. The subfoveal choroidal thickness changed by 45.32 ± 30.88 μm for children treated with LLRT at the 9-month examination (p < 0.001). Specifically, a substantial hyperopic shift (0.31 ± 0.24 D and 0.20 ± 0.14 D, respectively, p = 0.02) was found in the 8-14 year olds compared with 4-7 year old children. The decrease in AL in subjects with baseline AL >24 mm was -0.08 ± 0.19 mm, significantly greater than those with a baseline AL ≤24 mm (-0.04 ± 0.18 mm, p = 0.03).

CONCLUSIONS

Repetitive exposure to LLRT therapy was associated with slower myopia progression and reduced axial growth after short durations of treatment. These results require further validation in randomised controlled trials.

"Emmetropic, but not myopic human eyes distinguish positive defocus from calculated defocus in monochromatic red light"

Studies in animal models have provided evidence that broadband light and chromatic cues are necessary for successful emmetropization. We have studied this question in young human subjects by measuring short-term changes in axial length when they watched movies with calculated defocus (2.5D) or optically defocused movies (+2.5D) with red interference filters (620 ± 10 nm). Since filters cut luminance down by a factor of 10, a control experiment with neutral density filters (ND 1.0) was done. Ten myopes and 10 emmetropes were studied. Four experimental conditions were tested on two separate days. On the first day, movies with calculated defocus, and defocused by positive lenses were watched with ND filters. On the second day, movies with the same defocus patterns were watched with the red filters. Movies were presented on a large TV screen (LG OLED65C9, 65″) in a dark room at 2 m distance for 30 min. Changes in axial length before and after each stimulation were measured with the Lenstar (LS 900, with autopositioning system; Haag-Streit). Interestingly, the effects of calculated defocus or optical positive defocus on axial length were suppressed by 1.0 ND filters in myopes and emmetropes, with no clear trend. In contrast, narrow-band red light suppressed eye elongation with calculated defocus but not eye shortening with positive defocus in emmetropes. In myopes, as previously found in white light, there was a trend of axial eye elongation with positive lenses. In conclusion, the effect of positive lenses on eye growth did not require chromatic cues.

Short-Term Exposure to Blue Light Shows an Inhibitory Effect on Axial Elongation in Human Eyes Independent of Defocus

Purpose

Given the potential role of light and its wavelength on ocular growth, we investigated the effect of short-term exposure to the red, green, and blue light on ocular biometry in the presence and absence of lens-induced defocus in humans.

Methods

Twenty-five young adults were exposed to blue (460 nm), green (521 nm), red (623 nm), and white light conditions for 1-hour each on 4 separate experimental sessions conducted on 4 different days. In each light condition, hyperopic defocus (3D) was induced to the right eye with the fellow eye experiencing no defocus. Axial length and choroidal thickness were measured before and immediately after the light exposure with a non-contact biometer.

Results

Axial length increased from baseline after red light (mean difference ± standard error in the defocussed eye and non-defocussed eye = 11.2 ± 2 µm and 6.4 ± 2.3 µm, P < 0.001 and P < 0.01, respectively) and green light exposure (9.2 ± 3 µm and 7.0 ± 2.5 µm, P < 0.001 and P < 0.001) with a significant decrease in choroidal thickness (P < 0.05, both red and green light) after 1-hour of exposure. Blue light exposure resulted in a reduction in axial length in both the eyes (−8.0 ± 3 µm, P < 0.001 in the defocussed eye and −6.0 ± 3 µm, P = 0.11 in the non-defocused eye) with no significant changes in the choroidal thickness.

Conclusions

Exposure to red and green light resulted in axial elongation, and blue light resulted in inhibition of axial elongation in human eyes. Impact of such specific wavelength exposure on children and its application in myopia control need to be explored.

Effect of Violet Light-Transmitting Eyeglasses on Axial Elongation in Myopic Children: A Randomized Controlled Trial

The fact that outdoor light environment is an important suppressive factor against myopia led us to invent violet light-transmitting eyeglasses (VL glasses) which can transmit violet light (VL), 360-400 nm in wavelength, for the suppression of myopia, and can meanwhile block harmful ultraviolet waves from sunlight. The current study is a double-blinded randomized clinical trial to investigate the myopia-suppressive effect of VL glasses compared to conventional eyeglasses (placebo glasses) that do not transmit VL. The subjects were children aged from 6 to 12 years old, the population in which myopia progression is generally accelerated, and the myopia suppressive effect was followed up for two years in a city in Japan. Periodical ophthalmic examinations, interviews, and measurements of reflection and axial length under mydriasis were performed at the initial visit (the baseline) and at 1, 6, 12, 18, and 24 months. The mean change in axial length in the VL glasses group was significantly smaller than in the placebo glasses group when time for near-work was less than 180 min and when the subjects were limited to those who had never used eyeglasses before this trial (p < 0.01); however, this change was not significant without subgrouping. The suppressive rate for axial elongation in the VL glasses group was 21.4% for two years.

[Relationship between Myopia and Light Exposure]

Epidemiological studies found that the incidence of myopia was increasing year by year and the age of onset of myopia was showing a trend of affecting increasingly younger children. Reducing the occurrence of myopia and controlling the increase of myopia diopter have always been the focus of research on the prevention and control of myopia. Large randomized controlled clinical trials have found that outdoor activities can effectively reduce the incidence of myopia and delay the progression of myopia. Basic experiments also revealed that there were certain connections between light exposure and myopia. We herein review the research progress, limitations and future directions of research on light exposure and myopia. From the perspective of light properties, increasing the intensity of light can slow the progression of myopia and reduce the occurrence of experimentally induced myopia. However, the actual mechanism of action is still unclear. The rhythmic changes of light exposure caused by the light/dark cycle may cause abnormalities in the secretion of melatonin and dopamine, and changes in the circadian rhythm of intraocular pressure and choroidal thickness, thus affecting myopia. The red light, with relatively longer wavelength and forming images behind the retina, tends to induce myopia more easily, while the blue light, with medium and short wavelength and forming images before the retina, tends to delay myopia progression. However, different species respond differently to lights of different wavelengths, and the relationship between light wavelength and myopia needs further investigation. Future research can be done to further explore the mechanism of action of how light exposure changes the progression of myopia, including the following aspects: how light changes dopamine levels, causing changes in downstream signal pathways, and thus controlling the growth of the axial length of the eye; how retinal photoreceptor cells receive light signals of different wavelengths in order to adjust the refractive power of the eyes; and how to design artificial lighting of reasonable intensity, composition and properties, and apply the design in myopia prevention and control.

The Role of Dopamine in Emmetropization Modulated by Wavelength and Temporal Frequency in Guinea Pigs

Purpose

Wavelength and temporal frequency have been found to influence refractive development. This study investigated whether retinal dopamine (DA) plays a role in these processes.

Methods

Guinea pigs were randomly divided into nine groups that received different lighting conditions for 4 weeks, as follows: white, green, or blue light at 0, 0.5, or 20.0 Hz. Refractions and axial lengths were measured using streak retinoscopy and A-scan ultrasound imaging. DA and its metabolites were measured by high-pressure liquid chromatography-electrochemical detection.

Results

At 0 Hz, green and blue light produced myopic and hyperopic shifts compared with that of white light. At 0.5 Hz, no significant changes were observed compared with those of green or blue light at 0 Hz, whereas white light at 0.5 Hz induced a myopic shift compared with white light at 0 or 20 Hz. At 20 Hz, green and blue light acted like white light. Among all levels of DA and its metabolites, only vitreous 3, 4-dihydroxyphenylacetic acid (DOPAC) levels and retinal DOPAC/DA ratios were dependent on wavelength, frequency, and their interaction. Specifically, retinal DOPAC/DA ratios were positively correlated with refractions in white and green light conditions. However, blue light (0, 0.5, and 20.0 Hz) produced hyperopic shifts but decreased vitreous DOPAC levels and retinal DOPAC/DA ratios.

Conclusions

The retinal DOPAC/DA ratio, indicating the metabolic efficiency of DA, is correlated with ocular growth. It may underlie myopic shifts from light exposure with a long wavelength and low temporal frequency. However, different biochemical pathways may contribute to the hyperopic shifts from short wavelength light.

Amber light treatment produces hyperopia in tree shrews

PURPOSE

Exposure to narrow-band red light, which stimulates only the long-wavelength sensitive (LWS) cones, slows axial eye growth and produces hyperopia in tree shrews and macaque monkeys. We asked whether exposure to amber light, which also stimulates only the LWS cones but with a greater effective illuminance than red light, has a similar hyperopia-inducing effect in tree shrews.

METHODS

Starting at 24 ± 1 days of visual experience, 15 tree shrews (dichromatic mammals closely related to primates) received light treatment through amber filters (BPI 500/550 dyed acrylic) either atop the cage (Filter group, n = 8, 300-400 human lux) or fitted into goggles in front of both eyes (Goggle group, n = 7). Non-cycloplegic refractive error and axial ocular dimensions were measured daily. Treatment groups were compared with age-matched animals (Colony group, n = 7) raised in standard colony fluorescent lighting (100-300 lux).

RESULTS

At the start of treatment, mean refractive errors were well-matched across the three groups (p = 0.35). During treatment, the Filter group became progressively more hyperopic with age (p < 0.001). By contrast, the Goggle and Colony groups showed continued normal emmetropization. When the treatment ended, the Filter group exhibited significantly greater hyperopia (mean [SE] = 3.5 [0.6] D) compared with the Goggle (0.2 [0.8] D, p = 0.01) and Colony groups (1.0 [0.2] D, p = 0.01). However, the refractive error in the Goggle group was not different from that in the Colony group (p = 0.35). Changes in the vitreous chamber were consistent with the refractive error changes.

CONCLUSIONS

Exposure to ambient amber light produced substantial hyperopia in the Filter group but had no effect on refractive error in the Goggle group. The lack of effect in the Goggle group could be due to the simultaneous activation of the short-wavelength sensitive (SWS) and LWS cones caused by the scattering of the broad-band light from the periphery of the goggles.

Long-Wavelength-Filtered Light Transiently Inhibits Negative Lens-Induced Axial Eye Growth in the Chick Myopia Model

Purpose

Eye growth and myopia development in chicks, and some other animal models, can be suppressed by rearing under near-monochromatic, short-wavelength blue light. We aimed to determine whether similar effects could be achieved using glass filters that transmit a broader range of short and middle wavelengths.

Methods

On day 6 or 7 post-hatch, 169 chicks were assigned to one of three monocular lens conditions (−10 D, +10 D, plano) and reared for 7 or 10 days under one of four 201-lux lighting conditions: (1) B410 long-wavelength–filtered light, (2) B460 long-wavelength–filtered light, (3) Y48 short-wavelength–filtered light, or (4) HA50 broadband light.

Results

At 7 days, B410 (but not B460) long-wavelength–filtered light had significantly inhibited negative lens induced axial growth relative to Y48 short-wavelength–filtered light (mean difference in experimental eye = −0.249 mm; P = 0.006) and HA50 broadband light (mean difference = −0.139 mm; P = 0.038). B410 filters also inhibited the negative lens-induced increase in vitreous chamber depth relative to all other filter conditions. Corresponding changes in refraction did not occur, and biometric measurements in a separate cohort of chicks suggested that the axial dimension changes were transient and not maintained at 10 days.

Conclusions

Chromatic effects on eye growth can be achieved using filters that transmit a broad range of wavelengths even in the presence of strong cues for myopia development.

Translational Relevance

Broad-wavelength filters that provide a more “naturalistic” visual experience relative to monochromatic light have potential to alter myopia development, although the effects shown here were modest and transient and require exploration in further species.

Effects of screen-based retinal light stimulation measured with a novel contrast sensitivity test

Myopia is increasing worldwide hence it exists a pressing demand to find effective myopia control strategies. Previous studies have shown that light, spectral composition, spatial frequencies, and contrasts play a critical role in refractive development. The effects of light on multiple retinal processes include growth regulation, but also visual performance and perception. Changes in subjective visual performance can be examined by contrast sensitivity (CS). This study was conducted to investigate whether retinal light stimulation of different wavelength ranges is able to elicit changes in CS and, therefore, may be used for myopia control purposes. In total, 30 right eyes were stimulated with the light of different wavelength ranges, including dominant wavelengths of ∼480 nm, ∼530 nm, ∼630 nm and polychromatic light via a commercial liquid crystal display (LCD) screen. Stimulation was performed screen full-field and on the optic nerve head only. CS was measured before any stimulation and after each stimulation condition using a novel and time-efficient CS test. Post-stimulation CS changes were analyzed by ANOVA regarding the influencing factors spatial frequency, stimulation wavelength and stimulation location. A priorly conducted verification study on a subset of five participants compared the newly developed CS test to a validated CS test. The novel CS test exhibited good reliability of 0.94 logCS and repeatability of 0.13 logCS with a duration of 92 sec ± 17 sec. No clinically critical change between pre- and post-stimulation CS was detected (all p>0.05). However, the results showed that post-stimulation CS differed significantly at 18 cpd after stimulation with polychromatic light from short-wavelength light (p<0.0001). Location of illumination (screen full-field vs. optic nerve head) or any interactions with other factors did not reveal significant influences (all p>0.05). To summarize, a novel CS test measures the relationship between retinal light stimulation and CS. However, using retinal illumination via LCD screens to increase CS is inconclusive.

Contribution of M-opsin-based color vision to refractive development in mice

M-opsin, encoded by opn1mw gene, is involved in green-light perception of mice. The role of M-opsin in emmetropization of mice remains uncertain. To answer the above question, 4-week-old wild-type (WT) mice were exposed to white light or green light (460-600 nm, a peak at 510 nm) for 12 weeks. Refractive development was estimated biweekly. After treatment, retinal function was assessed using electroretinogram (ERG). Dopamine (DA) in the retina was evaluated by high-performance liquid chromatography, M-opsin and S-opsin protein levels by Western blot and ELISA, and mRNA expressions of opn1mw and opn1sw by RT-PCR. Effects of M-opsin were further verified in Opn1mw^-/- and WT mice raised in white light for 4 weeks. Refractive development was examined at 4, 6, and 8 weeks after birth. The retinal structure was estimated through hematoxylin and eosin staining (H&E) and transmission electron microscopy (TEM). Retinal wholemounts from WT and Opn1mw^-/- mice were co-immunolabeled with M-opsin and S-opsin, their distribution and quantity were then assayed by immunofluorescence staining (IF). Expression of S-opsin protein and opn1sw mRNA were determined by Western blot, ELISA, or RT-PCR. Retinal function and DA content were analyzed by ERG and liquid chromatography tandem-mass spectrometry (LC-MS/MS), respectively. Lastly, visual cliff test was used to evaluate the depth perception of the Opn1mw^-/- mice. We found that green light-treated WT mice were more myopic with increased M-opsin expression and decreased DA content than white light-treated WT mice after 12-week illumination. No electrophysiologic abnormalities were recorded in mice exposed to green light compared to those exposed to white light. A more hyperopic shift was further observed in 8-week-old Opn1mw^-/- mice in white light with lower DA level and weakened cone function than the WT mice under white light. Neither obvious structural disruption of the retina nor abnormal depth perception was found in Opn1mw^-/- mice. Together, these results suggested that the M-opsin-based color vision participated in the refractive development of mice. Overexposure to green light caused myopia, but less perception of the middle-wavelength components in white light promoted hyperopia in mice. Furthermore, possible dopaminergic signaling pathway was suggested in myopia induced by green light.

Short Wavelength (Blue) Light Is Protective for Lens-Induced Myopia in Guinea Pigs Potentially Through a Retinoic Acid-Related Mechanism

Purpose

To investigate the effect of short-wavelength light (SL) on guinea pigs with lens-induced myopia (LIM) and the possible retinoic acid (RA)–related mechanisms.

Methods

Two-week-old guinea pigs (n = 60) with monocular −5D lenses were reared under white light (WL, 580 lux) or SL (440 nm, 500 lux). The left eyes were uncovered as control. Refractive error (RE) and axial length (AL) were measured at baseline, one week, two weeks, and four weeks after intervention. Retinal RA was measured from four guinea pigs after two and four weeks of treatment with HPLC. Two-week-old guinea pigs (n = 52) with monocular −5D lens were fed with either RA or its synthesis inhibitor citral every third day in the morning, and half from each group were reared under WL or SL conditions. RE and AL were recorded at baseline and two and four weeks after intervention. Retinal RA was measured after four weeks of intervention.

Results

At the end of treatment, guinea pigs exposed to SL were less myopic than to WL (2.06 ± 1.69D vs. −1.00 ± 1.88D), accompanied with shorter AL (P = 0.01) and less retinal RA (P = 0.02). SL reduced retinal RA even after exogenous RA supplementation (P = 0.02) and decelerated LIM compared to WL (1.66 ± 1.03D vs. −3.53 ± 0.90D). Citral slowed ocular growth, leading to similar RE in W+CI and S+CI groups (3.39 ± 1.65D vs. 5.25 ± 0.80D).

Conclusions

Overall, SL reduced LIM in guinea pigs, even in those supplemented with oral RA, accompanied by reduced retinal RA levels. Oral RA accelerated eye elongation, but citral equally decelerated eye elongation under SL and WL with no significant retinal RA reduction.

Violet light suppresses lens-induced myopia via neuropsin (OPN5) in mice

The increasing prevalence of myopia is a significant public health concern. Unfortunately, the mechanisms driving myopia remain elusive, limiting effective treatment options. This report identifies a refractive development pathway that requires Opn5-expressing retinal ganglion cells (RGCs). Stimulation of Opn5 RGCs with short-wavelength violet light prevented experimental myopia in mice. Furthermore, this effect was dependent on the time of day, with evening exposure being sufficient to protect against experimental myopia. Thus, these studies suggest Opn5 RGCs may contribute to the mechanisms of emmetropization and identify the OPN5 pathway as a potential target for the treatment of myopia.

Myopia has become a major public health concern, particularly across much of Asia. It has been shown in multiple studies that outdoor activity has a protective effect on myopia. Recent reports have shown that short-wavelength visible violet light is the component of sunlight that appears to play an important role in preventing myopia progression in mice, chicks, and humans. The mechanism underlying this effect has not been understood. Here, we show that violet light prevents lens defocus–induced myopia in mice. This violet light effect was dependent on both time of day and retinal expression of the violet light sensitive atypical opsin, neuropsin (OPN5). These findings identify Opn5-expressing retinal ganglion cells as crucial for emmetropization in mice and suggest a strategy for myopia prevention in humans.

Quantitative proteomic analysis of scleras in guinea pig exposed to wavelength defocus

Myopia is the most common optical disorder in the world, and wavelength defocus induced ametropia and myopia have attracted great attention. The objective was to identify and quantify scleral proteins involved in the response to the wavelength defocus. Guinea pigs were randomly divided into 3 groups that received different lighting conditions for 8 weeks: white light, short wavelength light, and long wavelength light. Refraction and axial length were measured, Hematoxylin-Eosin staining and transmission electron microscope were adopted to observe the scleral structure, and scleral proteome was also detected to analyze protein abundance by employing TMT labeling method. After light stimulation, the long- and short -wavelength light induced myopic and hyperopic effect on the guinea pig's eye and induced distinct protein signature, respectively. 186 dyregulated proteins between the short- and long-wavelength group were identified, which were mainly located in extracellular region and involved in metabolic process. We also found that 5 proteins in the guinea pigs scleras in response to wavelength defocus were also human myopic candidate targets, suggesting functional overlap between dyregulated proteins in scleral upon exposure to wavelength defocus and genes causing myopia in humans. SIGNIFICANCE: Wavelength defocus induces refractive errors and leads to myopia or hyperopia. However, sclera proteomics respond to wavelength defocus is lacking, which is crucial to understanding how wavelength defocus influences refractive development and induces myopia. In this proteome analysis, we identified unique protein signatures response to wavelength defocus in sclera of guinea pigs, identified potential mechanisms contributing to myopia formation, and found that several human myopia-related genes may involve in response to wavelength defocus. The results of this study provide a foundation to understand the mechanisms of myopia and wavelength defocus induced ametropia.

IMI Risk Factors for Myopia

Risk factor analysis provides an important basis for developing interventions for any condition. In the case of myopia, evidence for a large number of risk factors has been presented, but they have not been systematically tested for confounding. To be useful for designing preventive interventions, risk factor analysis ideally needs to be carried through to demonstration of a causal connection, with a defined mechanism. Statistical analysis is often complicated by covariation of variables, and demonstration of a causal relationship between a factor and myopia using Mendelian randomization or in a randomized clinical trial should be aimed for. When strict analysis of this kind is applied, associations between various measures of educational pressure and myopia are consistently observed. However, associations between more nearwork and more myopia are generally weak and inconsistent, but have been supported by meta-analysis. Associations between time outdoors and less myopia are stronger and more consistently observed, including by meta-analysis. Measurement of nearwork and time outdoors has traditionally been performed with questionnaires, but is increasingly being pursued with wearable objective devices. A causal link between increased years of education and more myopia has been confirmed by Mendelian randomization, whereas the protective effect of increased time outdoors from the development of myopia has been confirmed in randomized clinical trials. Other proposed risk factors need to be tested to see if they modulate these variables. The evidence linking increased screen time to myopia is weak and inconsistent, although limitations on screen time are increasingly under consideration as interventions to control the epidemic of myopia.