Effects of low intensity ambient lighting on refractive development in infant rhesus monkeys (Macaca mulatta)

Optic Nerve Head Morphology and Macula Ganglion Cell Inner Plexiform Layer Thickness in Axially Anisometropic Rhesus Monkeys

Purpose

The purpose of this study was to determine the effects of axial elongation on optic nerve head morphology and macula inner retinal thickness in young rhesus monkeys.

Methods

Both eyes of 26 anisometropic, 1-year-old rhesus monkeys were imaged using optical coherence tomography (OCT). Before imaging, the animals were sedated, their eyes were dilated, and axial length was measured using an optical biometer. OCT imaging included a 20 degrees, 24-line radial scan centered on the optic nerve head (ONH) and two 20 degrees × 20 degrees raster scans, one centered on the ONH and the other on the macula. Radial scans were analyzed using programs written in MATLAB to quantify the Bruch's membrane opening (BMO) area and position, minimum rim width (MRW), anterior lamina cribrosa surface (ALCS) position, size of any scleral crescent, circumpapillary retinal nerve fiber layer (RNFL), and choroid thickness (pCh). Macula total retinal thickness (mTRT) and ganglion cell inner plexiform layer (GCIPL) thicknesses were quantified from macula scans. Linear least square regression was determined for OCT measures and axial length of the right eye, and for inter-eye differences.

Results

Animals were 341 ± 18 days old at the time of imaging. BMO area (R2 = 0.38), ALCS position (R2 = 0.45), scleral crescent area (R2 = 0.35), pCh thickness (R2 = 0.21), mTRT (R2 = 0.24), and GCIPL thickness (R2 = 0.16) were correlated with the axial length (all P < 0.05). For each of these parameters, the right-eye regression slope did not differ from the slope of the interocular difference (P > 0.57).

Conclusions

There are posterior segment morphological differences in anisometropic rhesus monkeys related to axial length. Whether these differences increase the risk of pathology remains to be investigated.

Inner Retinal Microvasculature With Refraction in Juvenile Rhesus Monkeys

Purpose

To characterize inner retinal microvasculature of rhesus monkeys with a range of refractive errors using optical coherence tomography angiography.

Method

Refractive error was induced in right eyes of 18 rhesus monkeys. At 327 to 347 days of age, axial length and spherical equivalent refraction (SER) were measured, and optical coherence tomography and optical coherence tomography angiography scans (Spectralis, Heidelberg) were collected. Magnification-corrected metrics included foveal avascular zone area and perfusion density, fractal dimension, and lacunarity of the superficial vascular complex (SVC) and deep vascular complex (DVC) in the central 1-mm diameter and 1.0- to 1.5-mm, 1.5- to 2.0-mm, and 2.0- to 2.5-mm annuli. Pearson correlations were used to explore relationships.

Results

The mean SER and axial length were 0.78 ± 4.02 D (-7.12 to +7.13 D) and 17.96 ± 1.08 mm (16.41 to 19.93 mm), respectively. The foveal avascular zone area and SVC perfusion density were correlated with retinal thickness for the central 1 mm (P < 0.05). SVC perfusion density of 2.0- to 2.5-mm annulus decreased with increasing axial length (P < 0.001). SVC and DVC fractal dimensions of 2.0- to 2.5-mm were correlated with axial length and SER, and DVC lacunarity of 1.5- to 2.0-mm annulus was correlated with axial length (P < 0.05).

Conclusions

Several inner retinal microvasculature parameters were associated with increasing axial length and SER in juvenile rhesus monkeys. These findings suggest that changes in retinal microvasculature could be indicators of refractive error development.

Translational Relevance

In juvenile rhesus monkeys, increasing myopic refraction and axial length are associated with alterations in the inner retinal microvasculature, which may have implications in myopia-related changes in humans.

Visual information and the development/control of myopia: Insights from nonhuman primate experiences

Over the past few decades, primarily by animal studies, correspondingly reinforced by epidemiological, clinical studies and controlled trials, researchers have identified that visual feedback regulates eye refractive developments, with visual image alterations being the most influential myopiagenic environmental factor. This article reviews studies using nonhuman primates to investigate visual risk factors for myopia development and evaluates and summarizes which visual factors contribute to the occurrence and progression of myopia. The possible underlying myopiagenic mechanisms and related myopia prevention/control strategies are also discussed.

Peripheral refraction under different levels of illuminance

Peripheral refraction is believed to be involved in the development of myopia. The aim of this study was to compare the relative peripheral refraction (RPR) at four different levels of illuminance, ranging from photopic conditions to complete darkness, using an open-field autorefraction method. The RPR was calculated for each eccentricity by subtracting central from peripheral autorefraction measurements. The study included 114 myopic eyes from 114 subjects (mean age of 21.81 ± 1.91 years) and the mean difference in RPR between scotopic and photopic conditions (0 and 300 lux, respectively) was +0.32 D at 30° temporal and +0.37 D at 30° in the nasal visual field (NVF). Statistically significant differences were observed between 0 and 300 lux at 30° in the temporal visual field and at 30° and 20° in the NVF. Our results revealed a significant increase in relative peripheral hyperopia with increasing visual field eccentricity along the horizontal visual field in myopic eyes of young adults. Furthermore, this relative peripheral hyperopia increased as illumination decreased. These findings suggest that an increase in peripheral illuminance may protect against myopic eye growth.

Long-term blue light rearing does not affect in vivo retinal function in young rhesus monkeys

PURPOSE

Exposure to blue light is thought to be harmful to the retina. The purpose of this study was to determine the effects of long-term exposure to narrowband blue light on retinal function in rhesus monkeys.

METHODS

Young rhesus monkeys were reared under short-wavelength "blue" light (n = 7; 465 nm, 183 ± 28 lx) on a 12-h light/dark cycle starting at 26 ± 2 days of age. Age-matched control monkeys were reared under broadband "white" light (n = 8; 504 ± 168 lx). Light- and dark-adapted full-field flash electroretinograms (ERGs) were recorded at 330 ± 9 days of age. Photopic stimuli were brief red flashes (0.044-5.68 cd.s/m²) on a rod-saturating blue background and the International Society for Clinical Electrophysiology of Vision (ISCEV) standard 3.0 white flash on a 30 cd/m² white background. Monkeys were dark adapted for 20 min and scotopic stimuli were ISCEV standard white flashes of 0.01, 3.0, and 10 cd.s/m². A-wave, b-wave, and photopic negative response (PhNR) amplitudes were measured. Light-adapted ERGs in young monkeys were compared to ERGs in adult monkeys reared in white light (n = 10; 4.91 ± 0.88 years of age).

RESULTS

For red flashes on a blue background, there were no significant differences in a-wave (P = 0.46), b-wave (P = 0.75), and PhNR amplitudes (P = 0.94) between white light and blue light reared monkeys for all stimulus energies. ISCEV standard light- and dark-adapted a- and b-wave amplitudes were not significantly different between groups (P > 0.05 for all). There were no significant differences in a- and b-wave implicit times between groups for all ISCEV standard stimuli (P > 0.05 for all). PhNR amplitudes of young monkeys were significantly smaller compared to adult monkeys for all stimulus energies (P < 0.05 for all). There were no significant differences in a-wave (P = 0.19) and b-wave (P = 0.17) amplitudes between young and adult white light reared monkeys.

CONCLUSIONS

Long-term exposure to narrowband blue light did not affect photopic or scotopic ERG responses in young monkeys. Findings suggest that exposure to 12 h of daily blue light for approximately 10 months does not result in altered retinal function.

GABAB Receptor Activation Affects Eye Growth in Chickens with Visually Induced Refractive Errors

This study aims to explore the role of GABAB receptors in the development of deprivation myopia (DM), lens-induced myopia (LIM) and lens-induced hyperopia (LIH). Chicks were intravitreally injected with 25 µg baclofen (GABABR agonist) in one eye and saline into the fellow eye. Choroidal thickness (ChT) was measured via OCT before and 2, 4, 6, 8, 24 h after injection. ChT decreased strongly at 6 and 8 h after baclofen injection and returned back to baseline level after 24 h. Moreover, chicks were monocularly treated with translucent diffusers, −7D or +7D lenses and randomly assigned to baclofen or saline treatment. DM chicks were injected daily into both eyes, while LIM and LIH chicks were monocularly injected into the lens-wearing eyes, for 4 days. Refractive error, axial length and ChT were measured before and after treatment. Dopamine and its metabolites were analyzed via HPLC. Baclofen significantly reduced the myopic shift and eye growth in DM and LIM eyes. However, it did not change ChT compared to respective saline-injected eyes. On the other hand, baclofen inhibited the hyperopic shift and choroidal thickening in LIH eyes. All the baclofen-injected eyes showed significantly lower vitreal DOPAC content. Since GABA is an inhibitory ubiquitous neurotransmitter, interfering with its signaling affects spatial retinal processing and therefore refractive error development with both diffusers and lenses.

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.

Transient Eye Shortening During Reading Text With Inverted Contrast: Effects of Refractive Error and Letter Size

Purpose

Myopes have a reduced ability to elicit transient axial eye shortening after imposed positive defocus, which may be due to changes in the biochemical signaling cascade controlling choroidal thickness. We have investigated whether reading with inverted text contrast can still elicit transient axial eye shortening in myopes, like it has been shown in emmetropes.

Methods

Changes in axial length before and after reading were measured with the Lenstar LS-900. Text with inverted contrast was read from a large screen at 2 m distance (angular subtense 35.9°, screen luminance matched in all conditions to 86 ± 7 cd/m²) for 30 minutes. Moreover, we investigated the effects of letter sizes. Two text sizes were tested: “small” text (letter height 13.75 arcmin) and “large” text (letter height 34.39 arcmin).

Results

Reading text with inverted contrast induced eye shortening (–10.2 ± 9.5 µm) in myopic eyes (n = 11; refraction –3.5 ± 1.9 diopters [D]), showing that an inhibitory signal was still generated by the retina as in emmetropes. In 15 subjects (refraction +1.7 to –4.2 D) we found that small text does not elicit significant differences in axial length (P = 0.09). However, with large text, changes in axial length were clearly different for the both contrast polarities (standard contrast, +1.7 ± 9.0 µm; inverted contrast, –9.7 ± 8.9 µm; P = 0.0017).

Conclusions

Although positive defocus may not be an effective intervention to inhibit further eye growth in myopes, other visual stimuli can still trigger choroidal thickening and possibly generate signals to decrease myopia progression.

Translational Relevance

Our results have shown the optimized text features, which may have a positive impact on myopia control.

"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.

Reappraisal of the historical myopia epidemic in native Arctic communities

PURPOSE

This study was developed to explain the extraordinary rise in myopia prevalence beginning after 1950 in Indigenous Arctic communities considering recent findings about the risk factors for school myopia development. Myopia prevalence changed drastically from a historical low of less than 3% to more than 50% in new generations of young adults following the Second World War. At that time, this increase was attributed to concurrent alterations in the environment and way of life which occurred in an aggressive programme of de-culturalization and re-acculturation through residential school programmes that introduced mental, emotional and physical stressors. However, the predominant idea that myopia was genetic in nature won the discussion of the day, and research in the area of environmental changes was dismissed. There may have also been an association between myopia progression and the introduction of extreme mental, emotional and physical stressors at the time.

RECENT FINDINGS

Since 1978, animal models of myopia have demonstrated that myopiagenesis has a strong environmental component. Furthermore, multiple studies in human populations have shown since 2005 how myopia could be produced by a combination of limited exposure to the outdoors and heavy emphasis on academic subjects associated with intense reading habits. This new knowledge was applied in the present study to unravel the causes of the historical myopia epidemics in Inuit communities.

SUMMARY

After reviewing the available published data on myopia prevalence in circumpolar Inuit populations in the 20th century, the most likely causes for the Inuit myopia epidemic were the combination of increased near work (from almost none to daily reading) and the move from a mostly outdoor to a much more indoor way of life, exacerbated by fewer hours of sunshine during waking hours, the lower illuminance in the Arctic and the extreme psychophysical stress due to the conditions in the Residential Schools.

Wide-field choroidal thickness and vascularity index in myopes and emmetropes

PURPOSE

To study regional variations in choroidal thickness (CT), luminal thickness and stromal thickness of the choroid, and choroidal vascularity index (CVI) in low myopic and emmetropic eyes using wide-field optical coherence tomography (OCT).

METHODS

Sixty-nine healthy young adults between 20 and 38 years of age participated in this study, including 40 low myopes (mean ± SD spherical equivalent (MSE) refractive error: -3.00 ± 1.39 D, range: -6.00 to -0.62 D) and 29 emmetropes (MSE: -0.05 ± 0.09 D, range: -0.25 to +0.12 D). Wide-field CT, luminal thickness, stromal thickness and CVI were measured across five eccentricities (fovea, parafovea, perifovea; near-periphery and periphery) and four quadrants (nasal, temporal, inferior and superior), in vertical and horizontal meridians, while controlling for a range of extraneous factors potentially influencing the CT. Custom-written software was used to segment and binarize the OCT images.

RESULTS

Wide-field CT, luminal thickness and stromal thickness, averaged across all participants, exhibited significant topographical variation, with the foveal (379 ± 8 µm, 200 ± 4 µm, 179 ± 4 µm, respectively) and peripheral (275 ± 8 µm, 161 ± 4 µm, 114 ± 4 µm, respectively) regions presenting the thickest and thinnest regions (all p < 0.001). Wide-field CVI showed a progressively higher percentage (greater vascularity) with increasing eccentricity from the fovea towards the periphery (p < 0.001). Macular CT and stromal choroidal thickness were significantly thinner in myopes compared to emmetropes (p < 0.05). Myopes (55.7 ± 0.3%) showed a slightly higher CVI compared with emmetropes (54.4 ± 0.4%) (p < 0.05).

CONCLUSIONS

Low myopia in young adults was associated with significant choroidal thinning across the macular, but not extramacular regions, with this decrease in choroidal thickness mostly attributed to thinning in the stromal component of the choroid, rather than the luminal (vascular) component.

Long-Term Narrowband Lighting Influences Activity but Not Intrinsically Photosensitive Retinal Ganglion Cell-Driven Pupil Responses

Purpose: Light affects a variety of non-image forming processes, such as circadian rhythm entrainment and the pupillary light reflex, which are mediated by intrinsically photosensitive retinal ganglion cells (ipRGCs). The purpose of this study was to assess the effects of long- and short-wavelength ambient lighting on activity patterns and pupil responses in rhesus monkeys. Methods: Infant rhesus monkeys were reared under either broadband "white" light (n = 14), long-wavelength "red" light (n = 20; 630 nm), or short-wavelength "blue" light (n = 21; 465 nm) on a 12-h light/dark cycle starting at 24.1 ± 2.6 days of age. Activity was measured for the first 4 months of the experimental period using a Fitbit activity tracking device and quantified as average step counts during the daytime (lights-on) and nighttime (lights-off) periods. Pupil responses to 1 s red (651 nm) and blue (456 nm) stimuli were measured after approximately 8 months. Pupil metrics included maximum constriction and the 6 s post-illumination pupil response (PIPR). Results: Activity during the lights-on period increased with age during the first 10 weeks (p < 0.001 for all) and was not significantly different for monkeys reared in white, red, or blue light (p = 0.07). Activity during the 12-h lights-off period was significantly greater for monkeys reared in blue light compared to those in white light (p = 0.02), but not compared to those in red light (p = 0.08). However, blue light reared monkeys exhibited significantly lower activity compared to both white and red light reared monkeys during the first hour of the lights-off period (p = 0.01 for both) and greater activity during the final hour of the lights-off period (p < 0.001 for both). Maximum pupil constriction and the 6 s PIPR to 1 s red and blue stimuli were not significantly different between groups (p > 0.05 for all). Conclusion: Findings suggest that long-term exposure to 12-h narrowband blue light results in greater disruption in nighttime behavioral patterns compared to narrowband red light. Normal pupil responses measured later in the rearing period suggest that ipRGCs adapt after long-term exposure to narrowband lighting.

The effects of reduced ambient lighting on lens compensation in infant rhesus monkeys

Although reduced ambient lighting (~50 lx) does not increase the degree of form-deprivation myopia (FDM) in chickens or infant monkeys, it does reduce the probability that monkeys will recover from FDM and that the normal age-dependent reduction in hyperopia will occur in monkeys reared with unrestricted vision. These findings suggest that low ambient lighting levels affect the regulatory mechanism responsible for emmetropization. To study this issue, infant rhesus monkeys (age ~ 24 days) were reared under dim light (55 ± 9 lx) with monocular -3D (dim-light lens-induced myopia, DL-LIM, n = 8) or +3D spectacle lenses (dim-light lens-induced hyperopia, DL-LIH, n = 7) until approximately 150 days of age. Refractive errors, ocular parameters and sub-foveal choroidal thickness were measured periodically and compared with normal-light-reared, lens-control monkeys (NL-LIM, n = 16; NL-LIH, n = 7). Dim light rearing significantly attenuated the degree of compensatory anisometropias in both the DL-LIM (-0.63 ± 0.77D vs. -2.11 ± 1.10D in NL-LIM) and DL-LIH treatment groups (-0.18 ± 1.93D vs. +1.71 ± 0.39D in NL-LIH). These effects came about because the treated and fellow control eyes had a lower probability of responding appropriately to the eye's effective refractive state. Vision-induced interocular differences in choroidal thickness were only observed in monkeys that exhibited compensating refractive changes, suggesting that failures in detecting the relative magnitude of optical errors underlay the abnormal refractive responses. Our findings suggest that low ambient lighting levels reduce the efficacy of the vision-dependent mechanisms that regulate refractive development.

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.

The development of and recovery from form-deprivation myopia in infant rhesus monkeys reared under reduced ambient lighting

Although reduced ambient lighting ("dim" light) can cause myopia in emmetropizing chicks, it does not necessarily lead to myopic changes in emmetropizing rhesus monkeys. Because myopia is rarely spontaneous, a question remained whether dim light would hasten the progression of visually induced myopia. To determine the effects of dim light on the development of and recovery from form-deprivation myopia (FDM), seven 3-week-old infant rhesus monkeys were reared under dim light (mean ± SD = 55 ± 9 lx) with monocular diffuser spectacles until ~154 days of age, then maintained in dim light with unrestricted vision until ~337 days of age to allow for recovery. Refractive errors, corneal powers, ocular axial dimensions and sub-foveal choroidal thicknesses were measured longitudinally and compared to those obtained from form-deprived monkeys reared under typical laboratory lighting (504 ± 168 lx). Five of the seven subjects developed FDMs that were similar to those observed among their normal-light-reared counterparts. The average degree of form-deprivation-induced myopic anisometropia did not differ significantly between dim-light subjects (-3.88 ± 3.26D) and normal-light subjects (-4.45 ± 3.75D). However, three of the five dim-light subjects that developed obvious FDM failed to exhibit any signs of recovery and the two monkeys that were isometropic at the end of the treatment period manifest abnormal refractive errors during the recovery period. All refractive changes were associated with alterations in vitreous chamber elongation rates. It appears that dim light is not a strong myopiagenic stimulus by itself, but it can impair the optical regulation of refractive development in primates.

Light and myopia: from epidemiological studies to neurobiological mechanisms

Myopia is far beyond its inconvenience and represents a true, highly prevalent, sight-threatening ocular condition, especially in Asia. Without adequate interventions, the current epidemic of myopia is projected to affect 50% of the world population by 2050, becoming the leading cause of irreversible blindness. Although blurred vision, the predominant symptom of myopia, can be improved by contact lenses, glasses or refractive surgery, corrected myopia, particularly high myopia, still carries the risk of secondary blinding complications such as glaucoma, myopic maculopathy and retinal detachment, prompting the need for prevention. Epidemiological studies have reported an association between outdoor time and myopia prevention in children. The protective effect of time spent outdoors could be due to the unique characteristics (intensity, spectral distribution, temporal pattern, etc.) of sunlight that are lacking in artificial lighting. Concomitantly, studies in animal models have highlighted the efficacy of light and its components in delaying or even stopping the development of myopia and endeavoured to elucidate possible mechanisms involved in this process. In this narrative review, we (1) summarize the current knowledge concerning light modulation of ocular growth and refractive error development based on studies in human and animal models, (2) summarize potential neurobiological mechanisms involved in the effects of light on ocular growth and emmetropization and (3) highlight a potential pathway for the translational development of noninvasive light-therapy strategies for myopia prevention in children.