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

Clinically Significant Axial Shortening in Myopic Children After Repeated Low-Level Red Light Therapy: A Retrospective Multicenter Analysis

INTRODUCTION

Myopia is recognized as a progressive eye disease. The aim of this study was to evaluate the frequency and associated factors of clinically significant axial length (AL) shortening among myopic children following repeated low-level red light (RLRL) therapy.

METHODS

The clinical data that were collected for the myopic children aged 3-17 years who received an RLRL therapy delivered by home-use desktop light device that emitted light at 650 nm for at least 1 year, were reviewed. The clinical data included AL, spherical equivalent refraction (SER), and visual acuity measured at baseline and follow-up. The primary outcomes were frequency of AL shortening of > 0.05 mm, > 0.10 mm, and > 0.20 mm per year, and associated factors of AL shortening per year.

RESULTS

A total of 434 myopic children with at least 12 months of follow-up data were included. The mean age of participants was 9.7 (2.6) years with SER of -3.74 (2.60) diopters. There were 115 (26.50%), 76 (17.51%), and 20 (4.61%) children with AL shortening based on cutoffs of 0.05 mm/year, 0.10 mm/year, and 0.20 mm/year, respectively. In the multivariable model, AL shortening was significantly associated with older baseline age, female gender, and longer baseline AL or greater spherical equivalent refraction (all P < 0.05). Among AL shortened eyes, the mean AL difference (standard deviation, SD) was -0.142 (0.094) mm/year. Greater AL shortening was observed among children who were younger and had longer baseline AL (all P < 0.05).

CONCLUSIONS

More than a quarter of children had AL shortening > 0.05 mm following RLRL therapy, and the overall mean AL change was -0.142 mm/year. Further studies should explore the mechanisms underlying AL shortening.

Prevalence and Characteristics of Myopia in Adult Rhesus Macaques in Southwest China

Purpose

To investigate the prevalence of myopia in a large cohort of adult rhesus macaques at Yunnan Province in southwest China and describe the characteristics of myopic rhesus macaque eyes.

Methods

A total of 219 rhesus macaques 14.07 ± 2.72 years old (range, 8-21) were randomly recruited for this study. We performed fundus photography and measurements of cycloplegic refractive error (RE) and axial length (AL) on macaques.

Results

A total of 429 eyes of 219 macaques were examined. The median RE was -1.25 diopters (D), and the median AL was 18.69 mm. The prevalence of myopia was 62.47%, and one-third of the myopic eyes were highly myopic. The presence of fundus tessellations was higher in myopic eyes than non-myopic eyes (42.54% vs. 6.21%). The cutoff value for the presence of tessellations was -3.52 D for RE and 19.38 mm for AL. In myopic eyes, there were significant differences between grade 1 and grade 3 fundus tessellations on RE (-5.57 ± 2.97 D vs. -8.13 ± 3.51 D) and AL (19.66 ± 0.55 mm vs. 20.60 ± 1.06 mm). Beta-peripapillary atrophy (β-PPA) was found in 48.10% of myopic eyes and 6.83% of non-myopic eyes. The presence of β-PPA is associated with the presence of fundus tessellations, AL, and RE. The presence of β-PPA was higher in grade 3 than grade 1 fundus tessellations (94.4% vs. 76%).

Conclusions

More than half of adult rhesus macaques in southwest China are myopic, and one-third of the myopic ones are highly myopic. Similar to humans, tessellated fundi and β-PPA are the characteristic signs of myopic rhesus macaques. Adult rhesus macaques are optimal animal models for research on the pathogenesis of myopia.

Translational Relevance

This study not only provides a reference for the refractive state and AL in myopic rhesus macaques but also indicates that adult rhesus macaques with spontaneous myopia are optimal animal models for research on the pathogenesis 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.

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.

Comparing low-coherence interferometry and A-scan ultrasonography in measuring ocular axial dimensions in young rhesus monkeys

We investigated a commercial low-coherence interferometer (LenStar LS 900 optical biometer) in measuring young rhesus monkey ocular dimensions. Ocular biometry data obtained using a LenStar and an A-scan ultrasound instrument (OPT-scan 1000) from 163 rhesus monkeys during 20-348 days of age were compared by means of coefficients of concordance and 95% limits of agreement. Linear regression was employed to examine and analyze the inter-instrument discrepancies. In young rhesus monkeys, the test-retest reliability of the LenStar was equal to or exceeded that of A-scan ultrasound (intraclass correlation = 0.86 to 0.93). The inter-instrument agreement was strong for vitreous chamber depth and axial length (coefficient of concordance = 0.95 and 0.86, respectively) and moderate for anterior chamber depth and lens thickness (coefficient of concordance = 0.74 and 0.63, respectively). The LenStar systematically underestimated ocular dimensions when compared to A-scan ultrasound (mean magnitude of difference = 0.11-0.57 mm). This difference could be minimized using linear calibration functions to equate LenStar data with ultrasound data. When this method was applied, the values between instruments were in excellent absolute agreement (mean magnitude of difference = 0.004-0.01 mm). In conclusion, the LenStar reliably measured ocular dimensions in young monkeys. When an appropriate calibration function is applied, the LenStar can be used as a substitute for A-scan ultrasonography.

"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.

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.