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

Abstract

IntroductionThere 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.MethodsJuvenile 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.ResultsThe 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.ConclusionsThese 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.

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

“…We note that we have previously shown that ambient narrow-band red light induces hyperopia in tree shrews, but even small amounts of added white (including blue) light greatly reduce this effect. 49 However, the chromatic simulations of myopic blur used here operate in broad-spectrum light. We hypothesize that, at least for this species, the red light by itself is interpreted as zero blue (short-wavelength) spatial contrast, similar to the flat blue pattern used here.…”

Real World Study of Cumulative Absolute Reduction in Axial Elongation after Photobiomodulation Therapy

“…Based on studies in tree shrews, the dose-response effect was not obvious if the lighting was within 50 ~ 600 lux 23 . Likewise, we recorded no signi cant difference of CARE between the group of 0.37mW and the group of 1.20mW at any follow-up timeline.…”

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

“…Alternatively, the myopia control effect of the ambient narrowband red light in animals and low-level red-light therapy in humans could be mediated by differences in treatment duration or other light characteristics between these two lighting environments, for example, illumination (diffuse ambient red in the tree shrew and monkey experiments vs. potentially highly localized retinal intensities in low-level red light) or phase (diffuse vs. coherent). 75 Another possibility is that the myopia control efficacy of low-level red-light therapy in children is occurring via a different mechanism. It has been proposed that the effect of intense red light could instead be due to direct stimulation of mitochondria, which have components with an absorbance peak around 650 nm.…”

“…Regardless, given the similarity in the peak sensitivity, bandwidth, and spectral purity of the light, 75 the myopia control effect observed with low-level red-light therapy could be mediated via the same mechanism responsible for the long-wavelength light-induced hyperopia in tree shrews and monkeys. Specifically, the emmetropization mechanism could be using the long-wavelength image contrast as a cue that the eye is too long and generating STOP signals that slow eye growth 47,76 .…”

“…It may be that the emmetropization mechanism in humans is generating much stronger STOP signals because of boosting by either a much greater effective light intensity in low-level red-light therapy or a greater sensitivity to red light in humans than in rhesus monkeys and tree shrews. Alternatively, the myopia control effect of the ambient narrowband red light in animals and low-level red-light therapy in humans could be mediated by differences in treatment duration or other light characteristics between these two lighting environments, for example, illumination (diffuse ambient red in the tree shrew and monkey experiments vs. potentially highly localized retinal intensities in low-level red light) or phase (diffuse vs. coherent) 75 …”