Binocular lens treatment in tree shrews: Effect of age and comparison of plus lens wear with recovery from minus lens-induced 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.

Early Alterations in Inner-Retina Neural and Glial Saturated Responses in Lens-Induced Myopia

Purpose

Myopic marmosets are known to exhibit significant inner retinal thinning compared to age-matched controls. The purpose of this study was to assess inner retinal activity in marmosets with lens-induced myopia compared to age-matched controls and evaluate its relationship with induced changes in refractive state and eye growth.

Methods

Cycloplegic refractive error (Rx), vitreous chamber depth (VCD), and photopic full-field electroretinogram were measured in 14 marmosets treated binocularly with negative contact lenses compared to 9 untreated controls at different stages throughout the experimental period (from 74 to 369 days of age). The implicit times of the a-, b-, d-, and photopic negative response (PhNR) waves, as well as the saturated amplitude (Vmax), semi-saturation constant (K), and slope (n) estimated from intensity-response functions fitted with Naka-Rushton equations were analyzed.

Results

Compared to controls, treated marmosets exhibited attenuated b-, d-, and PhNR waves Vmax amplitudes 7 to 14 days into treatment before compensatory changes in refraction and eye growth occurred. At later time points, when treated marmosets had developed axial myopia, the amplitudes and implicit times of the b-, d-, and PhNR waves were similar between groups. In controls, the PhNR wave saturated amplitude increased as the b + d-wave Vmax increased. This trend was absent in treated marmosets.

Conclusions

Marmosets induced with negative defocus exhibit early alterations in inner retinal saturated amplitudes compared to controls, prior to the development of compensatory myopia. These early ERG changes are independent of refraction and eye size and may reflect early changes in bipolar, ganglion, amacrine, or glial cell physiology prior to myopia development.

Translational Relevance

The early changes in retinal function identified in the negative lens-treated marmosets may serve as clinical biomarkers to help identify children at risk of developing myopia.

Heterogenous thinning of peripapillary tissues occurs early during high myopia development in juvenile tree shrews

Myopia is an independent risk factor for glaucoma, but the link between both conditions remains unknown. Both conditions induce connective tissue remodeling at the optic nerve head (ONH), including the peripapillary tissues. The purpose of this study was to investigate the thickness changes of the peripapillary tissues during experimental high myopia development in juvenile tree shrews. Six juvenile tree shrews experienced binocular normal vision, while nine received monocular -10D lens treatment starting at 24 days of visual experience (DVE) to induce high myopia in one eye and the other eye served as control. Daily refractive and biometric measurements and weekly optical coherence tomography scans of the ONH were obtained for five weeks. Peripapillary sclera (Scl), choroid-retinal pigment epithelium complex (Ch-RPE), retinal nerve fiber layer (RNFL), and remaining retinal layers (RRL) were auto-segmented using a deep learning algorithm after nonlinear distortion correction. Peripapillary thickness values were quantified from 3D reconstructed segmentations. All lens-treated eyes developed high myopia (-9.8 ± 1.5 D), significantly different (P < 0.001) from normal (0.69 ± 0.45 D) and control eyes (0.76 ± 1.44 D). Myopic eyes showed significant thinning of all peripapillary tissues compared to both, normal and control eyes (P < 0.001). At the experimental end point, the relative thinning from baseline was heterogeneous across tissues and significantly more pronounced in the Scl (-8.95 ± 3.1%) and Ch-RPE (-16.8 ± 5.8%) when compared to the RNFL (-5.5 ± 1.6%) and RRL (-6.7 ± 1.8%). Furthermore, while axial length increased significantly throughout the five weeks of lens wear, significant peripapillary tissue thinning occurred only during the first week of the experiment (until a refraction of -2.5 ± 1.9 D was reached) and ceased thereafter. A sectorial analysis revealed no clear pattern. In conclusion, our data show that in juvenile tree shrews, experimental high myopia induces significant and heterogeneous thinning of the peripapillary tissues, where the retina seems to be protected from profound thickness changes as seen in Ch-RPE and Scl. Peripapillary tissue thinning occurs early during high myopia development despite continued progression of axial elongation. The observed heterogeneous thinning may contribute to the increased risk for pathological optic nerve head remodeling and glaucoma later in life.

Integrative Transcriptome and Proteome Analyses Elucidate the Mechanism of Lens-Induced Myopia in Mice

Purpose

The purpose of this study was to investigate the underlying molecular mechanism of lens-induced myopia (LIM) through transcriptome and proteome analyses with a modified mouse myopia model.

Methods

Four-week-old C57BL/6J mice were treated with a homemade newly designed -25 diopter (D) lens mounting by a 3D printing pen before right eyes for 4 weeks. Refraction (RE) and axial dimensions were measured every 2 weeks. Retinas were analyzed by RNA-sequencing and data-independent acquisition liquid chromatography tandem mass spectrometry. Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation, and STRING databases were used to identify significantly affected pathways in transcriptomic and proteomic data sets. Western blot was used to detect the expression of specific proteins.

Results

The modified model was accessible and efficient. Mice displayed a significant myopic shift (approximately 8 D) following 4 weeks' of lens treatment. Through transcriptomics and proteomics analysis, we elucidated 175 differently expressed genes (DEGs) and 646 differentially expressed proteins (DEPs) between binoculus. The transcriptomic and proteomic data showed a low correlation. Going over the mRNA protein matches, insulin like growth factor 2 mRNA binding protein 1 (Igf2bp1) was found to be a convincing biomarker of LIM, which was confirmed by Western blot. RNA-seq and proteome profiling confirmed that these two "omics" data sets complemented one another in KEGG pathways annovation. Among these, metabolic and human diseases pathways were considered to be correlated with the LIM forming process.

Conclusions

The newly constructed LIM model provides a useful tool for future myopia research. Combining transcriptomic and proteomic analysis may potentially brighten the prospects of novel therapeutic targets for patients with 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.

Prevalence of anisometropia and influencing factors among school-age children in Nantong, China: a cross-sectional study

Objective

To investigate the prevalence of anisometropia and associated parameters among school-aged children in Nantong, China.

Methods

This school-based, cross-sectional study examined students from primary schools, junior high schools, and senior high schools in an urban area of Nantong, China. Univariate and multivariate logistic regression analyses were used to investigate the specific correlations between anisometropia and related parameters. Non-cycloplegic autorefraction was assessed for each student. Anisometropia was defined as the spherical equivalent refraction (SE) difference ≥ 1.0 D between eyes.

Results

A total of 9,501 participants were validated for analyses, of which 53.2% (n = 5,054) were male, and 46.8% (n = 4,447) were female. The mean of age was 13.32 ± 3.49 years, ranging from 7-19 years. The overall prevalence of anisometropia was 25.6%. Factors such as myopia, scoliosis screening positive, hyperopia, female sex, older age, and higher weight had a significantly higher risk of anisometropia (p < 0.05).

Conclusion

There was a high prevalence of anisometropia in school-age children. Some physical examination parameters are closely related to children's anisometropia, especially myopia and scoliosis. Preventing myopia and controlling its progression may be the most important ways to reduce the prevalence of anisometropia. Correcting scoliosis may be an important factor in controlling the prevalence of anisometropia, and maintaining good reading and writing posture may be helpful in controlling the prevalence of anisometropia.

Longitudinal Changes of Bruch's Membrane Opening, Anterior Scleral Canal Opening, and Border Tissue in Experimental Juvenile High Myopia

Purpose

To investigate the relative positional changes between the Bruch's membrane opening (BMO) and the anterior scleral canal opening (ASCO), and border tissue configuration changes during experimental high myopia development in juvenile tree shrews.

Methods

Juvenile tree shrews were assigned randomly to two groups: binocular normal vision (n = 9) and monocular -10 D lens treatment starting at 24 days of visual experience to induce high myopia in one eye while the other eye served as control (n = 12). Refractive and biometric measurements were obtained daily, and 48 radial optical coherence tomography B-scans through the center of the optic nerve head were obtained weekly for 6 weeks. ASCO and BMO were segmented manually after nonlinear distortion correction.

Results

Lens-treated eyes developed high degree of axial myopia (-9.76 ± 1.19 D), significantly different (P < 0.001) from normal (0.34 ± 0.97 D) and control eyes (0.39 ± 0.88 D). ASCO-BMO centroid offset gradually increased and became significantly larger in the experimental high myopia group compared with normal and control eyes (P < 0.0001) with an inferonasal directional preference. The border tissue showed a significantly higher tendency of change from internally to externally oblique configuration in the experimental high myopic eyes in four sectors: nasal, inferonasal, inferior, and inferotemporal (P < 0.005).

Conclusions

During experimental high myopia development, progressive relative deformations of ASCO and BMO occur simultaneously with changes in border tissue configuration from internally to externally oblique in sectors that are close to the posterior pole (nasal in tree shrews). These asymmetric changes may contribute to pathologic optic nerve head remodeling and an increased risk of glaucoma later in life.

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.

Do monocular myopia children need to wear glasses? Effects of monocular myopia on visual function and binocular balance

Objective

This study aims to compare the binocular visual functions and balance among monocular myopic adolescents and adults and binocular low myopic adolescents and explore whether monocular myopia requires glasses.

Methods

A total of 106 patients participated in this study. All patients were divided into three groups: the monocular myopia children group (Group 1 = 41 patients), the monocular myopia adult group (Group 2 = 26 patients) and the binocular low myopia children group (Group 3 = 39 patients). The refractive parameters, accommodation, stereopsis, and binocular balance were compared.

Results

The binocular refractive difference in Group 1, Group 2, and Group 3 was -1.37 ± 0.93, -1.94 ± 0.91, and -0.32 ± 0.27 D, respectively. Moreover, uncorrected visual acuity (UCVA), spherical equivalent (SE) and monocular accommodative amplitude (AA) between myopic and emmetropic eyes in Group 1 and Group 2 were significantly different (all P < 0.05). There was a significant difference in the accommodative facility (AF) between myopic and emmetropic eyes in Group 2 (t = 2.131, P = 0.043). Furthermore, significant differences were found in monocular AA (t = 6.879, P < 0.001), binocular AA (t = 5.043, P < 0.001) and binocular AF (t = -3.074, P = 0.003) between Group 1 and Group 2. The normal ratio of stereopsis according to the random dots test in Group 1 was higher than in Group 2 (χ2 = 14.596, P < 0.001). The normal ratio of dynamic stereopsis in Group 1 was lower than in Group 3 (χ2 = 13.281, P < 0.001). The normal signal-to-noise ratio of the binocular balance point in Group 1 was lower than Group 3 (χ2 = 4.755, P = 0.029).

Conclusion

First, monocular myopia could lead to accommodative dysfunction and unbalanced input of binocular visual signals, resulting in myopia progression. Second, monocular myopia may also be accompanied by stereopsis dysfunction, and long-term uncorrected monocular myopia may worsen stereopsis acuity in adulthood. In addition, patients with monocular myopia could exhibit stereopsis dysfunction at an early stage. Therefore, children with monocular myopia must wear glasses to restore binocular balance and visual functions, thereby delaying myopia progression.

Chromatically simulated myopic blur counteracts a myopiagenic environment

There is a world-wide epidemic of myopia (nearsightedness), produced largely by human-made environmental visual cues that disrupt the emmetropization feedback mechanism that normally uses defocus cues to produce and maintain eyes in good focus. Previous studies have shown that the wavelength of light affects this process and that myopic defocus can slow the progression of myopia in children. We first asked if continuous exposure to a small cage with restricted viewing distance would produce an environmentally-induced myopia in tree shrews, small diurnal mammals closely related to primates. A group (n = 7) spent 11 days in a small cage with restricted viewing distance; one wall was a video display covered with Maltese crosses that included low-to-high spatial frequencies in the range visible to tree shrews. This group developed myopia (-1.2 ± 0.4 [stderr] D) that was significant relative to a colony group of seven animals (+1.0 ± 0.2 D) raised in mesh cages allowing more distant viewing. We then asked if chromatically-simulated myopic defocus, produced by blurring just the blue channel of the video display, would counteract this environmentally-induced myopia in a group of eight tree shrews. This group instead became significantly hyperopic (+4.0 ± 0.4 D) due to slowed axial elongation. These results demonstrate the high potency of chromatic cues in refractive regulation and may provide the basis for an anti-myopia treatment in humans.

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.

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.

One-year myopia control efficacy of spectacle lenses with aspherical lenslets

Aims

To evaluate the 1-year efficacy of two new myopia control spectacle lenses with lenslets of different asphericity.

Methods

One hundred seventy schoolchildren aged 8–13 years with myopia of −0.75 D to −4.75 D were randomised to receive spectacle lenses with highly aspherical lenslets (HAL), spectacle lenses with slightly aspherical lenslets (SAL), or single-vision spectacle lenses (SVL). Cycloplegic autorefraction (spherical equivalent refraction (SER)), axial length (AL) and best-corrected visual acuity (BCVA) were measured at baseline and 6-month intervals. Adaptation and compliance questionnaires were administered during all visits.

Results

After 1 year, the mean changes in the SER (±SE) and AL (±SE) in the SVL group were −0.81±0.06 D and 0.36±0.02 mm. Compared with SVL, the myopia control efficacy measured using SER was 67% (difference of 0.53 D) for HAL and 41% (difference of 0.33 D) for SAL, and the efficacy measured using AL was 64% (difference of 0.23 mm) for HAL and 31% (difference of 0.11 mm) for SAL (all p<0.01). HAL resulted in significantly greater myopia control than SAL for SER (difference of 0.21 D, p<0.001) and AL (difference of 0.12 mm, p<0.001). The mean BCVA (−0.01±0.1 logMAR, p=0.22) and mean daily wearing time (13.2±2.6 hours, p=0.26) were similar among the three groups. All groups adapted to their lenses with no reported adverse events, complaints or discomfort.

Conclusions

Spectacle lenses with aspherical lenslets effectively slow myopia progression and axial elongation compared with SVL. Myopia control efficacy increased with lenslet asphericity.

Trial registration number

ChiCTR1800017683.

Tree shrews do not maintain emmetropia in initially-focused narrow-band cyan light

We asked if emmetropia, achieved in broadband colony lighting, is maintained in narrow-band cyan light that is well focused in the emmetropic eye, but does not allow for guidance from longitudinal chromatic aberrations (LCA) and offers minimal perceptual color cues. In addition, we examined the response to a -5 D lens in this lighting. Seven tree shrews from different litters were initially housed in broad-spectrum colony lighting. At 24 ± 1 days after eye opening (Days of Visual Experience, DVE) they were housed for 11 days in ambient narrow-band cyan light (peak wavelength 505 ± 17 nm) selected because it is in focus in an emmetropic eye. Perceptually, monochromatic light at 505 nm cannot be distinguished from white by tree shrews. While in cyan light, each animal wore a monocular -5 D lens (Cyan -5 D eyes). The fellow eye was the Cyan no-lens eye. Daily awake non-cycloplegic measures were taken with an autorefractor (refractive state) and with optical low-coherence optical interferometry (axial component dimensions). These measures were compared with the values of animals raised in standard colony fluorescent lighting: an untreated group (n = 7), groups with monocular form deprivation (n = 7) or monocular -5 D lens treatment (n = 5), or that experienced 10 days in total darkness (n = 5). Refractive state at the onset of cyan light treatment was low hyperopia, (mean ± SEM) 1.4 ± 0.4 diopters. During treatment, the Cyan no-lens eyes became myopic (-2.9 ± 0.3 D) whereas colony lighting animals remained slightly hyperopic (1.0 ± 0.2 D). Initially, refractions of the Cyan -5 D eyes paralleled the Cyan no-lens eyes. After six days, they gradually became more myopic than the Cyan no-lens eyes; at the end of treatment, the refractions were -5.4 ± 0.3 D, a difference of -2.5 D from the Cyan no-lens eyes. When returned to colony lighting at 35 ± 1 DVE, the no-lens eye refractions rapidly recovered towards emmetropia but, as expected, the refraction of the -5 D eyes remained near -5 D. Vitreous chamber depth in both eyes was consistent with the refractive changes. In narrow-band cyan lighting the emmetropization mechanism did not maintain emmetropia even though the light initially was well focused. We suggest that, as the eyes diverged from emmetropia, there were insufficient LCA cues for the emmetropization mechanism to utilize the developing myopic refractive error in order to guide the eyes back to emmetropia. However, the increased myopia in the Cyan -5 D eyes in the narrow-band light indicates that the emmetropization mechanism nonetheless detected the presence of the lens-induced refractive error and responded with increased axial elongation that partly compensated for the negative-power lens. These data support the conclusion that the emmetropization mechanism cannot maintain emmetropia in narrow-band lighting. The additional myopia produced in eyes with the -5 D lens shows that the emmetropization mechanism responds to multiple defocus-related cues, even under conditions where it is unable to use them to maintain emmetropia.

Near Infrared (NIR) Light Therapy of Eye Diseases: A Review

Near infrared (NIR) light therapy, or photobiomodulation therapy (PBMT), has gained persistent worldwide attention in recent years as a new novel scientific approach for therapeutic applications in ophthalmology. This ongoing therapeutic adoption of NIR therapy is largely propelled by significant advances in the fields of photobiology and bioenergetics, such as the discovery of photoneuromodulation by cytochrome c oxidase and the elucidation of therapeutic biochemical processes. Upon transcranial delivery, NIR light has been shown to significantly increase cytochrome oxidase and superoxide dismutase activities which suggests its role in inducing metabolic and antioxidant beneficial effects. Furthermore, NIR light may also boost cerebral blood flow and cognitive functions in humans without adverse effects. In this review, we highlight the value of NIR therapy as a novel paradigm for treatment of visual and neurological conditions, and provide scientific evidence to support the use of NIR therapy with emphasis on molecular and cellular mechanisms in eye diseases.

MicroRNA Profiling in Paired Left and Right Eyes, Lungs, and Testes of Normal Mice

Physiological and pathophysiological differences widely exist in paired organ systems. However, the molecular basis for these differences remains largely unknown. We previously reported that there exist differentially expressed miRNAs (DEMs) in the left and right kidneys of normal mice. Here, we identified the DEMs in the left and right eyes, lungs, and testes of normal mice via RNA sequencing. As a result, we identified 26 DEMs in eyes, with 23 higher and 3 lower in the left eyes compared with right eyes; 21 DEMs in lungs, with 15 higher and 6 lower in the left lungs compared with right lungs; and 54 DEMs in testes, with 6 higher and 48 lower in the left testes compared with right testes. Ten microRNAs (miRNAs) were further examined by quantitative PCR assays, and seven of these were confirmed. In addition, correlation analysis was performed between paired organ miRNA expressions and diverse body fluid miRNA expressions. Finally, we explored the functions and networks of DEMs and performed biological process and pathway enrichment analysis of target genes for DEMs, providing insights into the physiological and pathophysiological differences between the two entities of paired organs.

Establishment of correctly focused eyes may not require visual input in arthropods

For proper function, vertebrate and invertebrate visual systems must be able to achieve and maintain emmetropia, a state where distant objects are in focus on the retina. In vertebrates, this is accomplished through a combination of genetic control during early development and homeostatic visual input that fine-tunes the optics of the eye. While emmetropization has long been researched in vertebrates, it is largely unknown how emmetropia is established in arthropods. We used a micro-ophthalmoscope to directly measure how the lens projects images onto the retina in the eyes of small, live arthropods, allowing us to compare the refractive states of light-reared and dark-reared arthropods. First, we measured the image-forming larval eyes of diving beetles (Thermonectus marmoratus), which are known to grow rapidly and dramatically between larval instars. Then, we measured the image-forming principal anterior-median eyes of jumping spiders (Phidippus audax) after emergence from their egg cases. Finally, we measured individual ommatidia in the compound eyes of flesh flies (Sarcophaga bullata) that had developed and emerged under either light or dark conditions. Surprisingly, and in sharp contrast to vertebrates, our data for this diverse set of arthropods suggest that visual input is inconsequential in regard to achieving well-focused eyes. Although it remains unclear whether visual input that is received after the initial development further improves focusing, these results suggest that at least the initial coordination between the lens refractive power and eye size in arthropods may be more strongly predetermined by developmental factors than is typically the case in vertebrates.

Peripheral refraction and spherical aberration profiles with single vision, bifocal and multifocal soft contact lenses

PURPOSE

To compare the peripheral refraction and spherical aberration profiles along three visual field meridians of 16 commercial single vision (SV), bifocal (BF) and multifocal (MF) test contact lenses with a single vision control.

METHOD

Forty-four participants [24.2±2.4 years, SE: -0.50 to -4.50D] were randomly fitted, contra-laterally, with 6 SV's [Air Optix Aqua (control), Acuvue Oasys, Biofinity, Clariti, Night & Day and Proclear], 3 BF's [Acuvue Bifocal low and high add, MiSight] and 8 MF's [Proclear D & N in 1.5 and 2.5D adds; AirOptix, PureVision low & high adds]. Peripheral refraction was performed across horizontal, oblique and vertical meridians, with lenses on eye using the BHVI-EyeMapper. The power vectors M, J₀, J₄₅ and the spherical aberration coefficient were analysed. The peripheral refraction and aberration profiles of the test lenses were compared with the profiles of the control lens using curvature and slope coefficients.

RESULTS

Compared to the control, a relative peripheral hyperopic shift (M), a less negative J₀ curvature coefficient along the horizontal meridian, a less positive J₀ curvature coefficient along the vertical meridian, a less negative J₄₅ curvature coefficient along the oblique meridian and a more positive spherical aberration curvature coefficient along most meridians was seen with the Acuvue Bifocal and all center-near multifocal lenses. For the center-distance multifocal lenses the direction of the curvature coefficients of the same refraction and aberration components was opposite to that of the center-near lenses. The greatest differences in the slope coefficients when compared to the control were found for the Acuvue Bifocal lenses and all multifocal contact lenses for the refractive component M and the spherical aberration coefficient along the horizontal visual field meridian, with the Acuvue Bifocal and the center-near multifocal lenses having more positive coefficients and the center-distance lenses having more negative coefficients.

CONCLUSION

When worn on eye, different commercially available lens types produce differences in the direction and magnitude of the peripheral refraction and spherical aberration profiles along different visual field meridians. This information may be relevant to refractive development and myopia control.

Juvenile Tree Shrews Do Not Maintain Emmetropia in Narrow-band Blue Light

SIGNIFICANCE

In spectrally broad-band light, an emmetropization mechanism in post-natal eyes uses visual cues to modulate the growth of the eye to achieve and maintain near emmetropia. When we restricted available wavelengths to narrow-band blue light, juvenile tree shrews (diurnal dichromatic mammals closely related to primates) developed substantial refractive errors, suggesting that feedback from defocus-related changes in the relative activation of long- and short-wavelength-sensitive cones is essential to maintain emmetropia.

PURPOSE

The purpose of this study was to examine the effects of narrow-band ambient blue light on refractive state in juvenile tree shrews that had completed initial emmetropization (decrease from hyperopia toward emmetropia).

METHODS

Animals were raised in fluorescent colony lighting until they began blue-light treatment at 24 days of visual experience, at which age they had achieved age-normal low hyperopia (mean ± SEM refractive error, 1.2 ± 0.5 diopters). Arrays of light-emitting diodes placed atop the cage produced wavelengths of 457 (five animals) or 464 nm (five animals), flickered in a pseudo-random pattern (temporally broad band). A third group of five animals was exposed to steady 464-nm blue light. Illuminance on the floor of the cage was 300 to 500 human lux. Noncycloplegic autorefractor measures were made daily for a minimum of 11 days and up to 32 days. Seven age-matched animals were raised in colony light.

RESULTS

The refractive state of all blue-treated animals moved outside the 95% confidence limits of the colony-light animals' refractions. Most refractions first moved toward hyperopia. Then the refractive state decreased monotonically and, in some animals, passed through emmetropia, becoming myopic.

CONCLUSIONS

From the tree shrew cone absorbance spectra, the narrow-band blue light stimulated both long-wavelength-sensitive and short-wavelength-sensitive cones, but the relative activation would not change with the refractive state. This removed feedback from longitudinal chromatic aberration that may be essential to maintain emmetropia.

Scleral structure and biomechanics

As the eye's main load-bearing connective tissue, the sclera is centrally important to vision. In addition to cooperatively maintaining refractive status with the cornea, the sclera must also provide stable mechanical support to vulnerable internal ocular structures such as the retina and optic nerve head. Moreover, it must achieve this under complex, dynamic loading conditions imposed by eye movements and fluid pressures. Recent years have seen significant advances in our knowledge of scleral biomechanics, its modulation with ageing and disease, and their relationship to the hierarchical structure of the collagen-rich scleral extracellular matrix (ECM) and its resident cells. This review focuses on notable recent structural and biomechanical studies, setting their findings in the context of the wider scleral literature. It reviews recent progress in the development of scattering and bioimaging methods to resolve scleral ECM structure at multiple scales. In vivo and ex vivo experimental methods to characterise scleral biomechanics are explored, along with computational techniques that combine structural and biomechanical data to simulate ocular behaviour and extract tissue material properties. Studies into alterations of scleral structure and biomechanics in myopia and glaucoma are presented, and their results reconciled with associated findings on changes in the ageing eye. Finally, new developments in scleral surgery and emerging minimally invasive therapies are highlighted that could offer new hope in the fight against escalating scleral-related vision disorder worldwide.

Short Interruptions of Imposed Hyperopic Defocus Earlier in Treatment are More Effective at Preventing Myopia Development

The purpose of this study was to evaluate the effect of interrupting negative lens wear for short periods early or late during the development of lens-induced myopia in marmosets. Sixteen marmosets were reared with a −5D contact lens on their right eye (plano on contralateral eye) for 8 weeks. Eight marmosets had lenses removed for 30 mins twice/day during the first four weeks (early interruption) and eight during the last four weeks (late interruption). Data were compared to treated controls that wore lenses continuously (N = 12) and untreated controls (N = 10). Interocular differences (IOD) in vitreous chamber (VC) depth and central and peripheral mean spherical refractive error (MSE) were measured at baseline and after four (T₄) and eight (T₈) weeks of treatment. Visual experience during the interruptions was monitored by measuring refraction while marmosets were seated at the center of a 1 m radius viewing cylinder. At T₄ the eyes that were interrupted early were not different from untreated controls (p = 0.10) and at T₈ had grown less and were less myopic than those interrupted later (IOD change from baseline, VC: +0.07 ± 0.04 mm vs +0.20 ± 0.03 mm, p < 0.05; MSE: −1.59 ± 0.26D vs −2.63 ± 0.60D, p = 0.13). Eyes interrupted later were not different from treated controls (MSE, p = 0.99; VC, p = 0.60) and grew at the same rate as during the first four weeks of uninterrupted lens wear (T₄ − T₀: 3.67 ± 1.1 µm/day, T₈ − T₄: 3.56 ± 1.3 µm/day p = 0.96). Peripheral refraction was a predictive factor for the amount of myopia developed only when the interruption was not effective. In summary, interrupting hyperopic defocus with short periods of myopic defocus before compensation occurs prevents axial myopia from developing. After myopia develops, interruption is less effective.

Gene expression signatures in tree shrew sclera during recovery from minus-lens wear and during plus-lens wear

Purpose

In juvenile tree shrews that have developed minus lens-induced myopia, if lens treatment is discontinued, refractive recovery (REC) occurs. However, in age-matched juvenile animals, plus-lens wear (PLW) produces little refractive change, although the visual stimulus (myopia) is similar (an "IGNORE" response). Because the sclera controls axial elongation and refractive error, we examined gene expression in the sclera produced by PLW and compared it with the gene expression signature produced by REC to learn whether these similar refractive conditions produce similar, or differing, scleral responses.

Methods

Eight groups of tree shrews (n = 7 per group) were examined. Four groups wore a monocular -5 D lens for 11 days until 35 days of visual experience (DVE). Lens wear was then discontinued, and the animals recovered for 0 h (REC-0), 2 h (REC-2h), 1 day (REC-1d), or 4 days (REC-4d). Starting at 35 DVE, three groups wore a monocular +5 D lens for 2 h (PLW-2h), 1 day (PLW-1d), or 4 days (PLW-4d). A normal group (PLW-0) was examined at 38 DVE to provide baseline measures. Using quantitative real-time PCR (qPCR), we examined scleral mRNA levels in recovering, plus-lens treated, and untreated control eyes for 55 candidate genes whose protein products included signaling molecules, metallopeptidases (MPs) and their inhibitors (tissue inhibitors of metallopeptidases [TIMPs]), and extracellular matrix proteins.

Results

No refractive recovery was measured in the REC-2h group. The scleral mRNA expression pattern for recovering versus untreated control eyes after 2 h of recovery was similar to that found for the group (REC-0) that had no recovery time. Many genes in both groups still had downregulated expression in the treated eyes versus the control eyes. The REC-1d group showed little refractive recovery (0.1 ± 0.1 D, mean ± standard error of the mean [SEM]), and the mRNA expression pattern was similar to that of the REC-2h group, but had fewer statistically significantly downregulated genes in the recovering eyes. The REC-4d group recovered refractively by 2.6 ± 0.4 D, and displayed a "STOP" gene expression signature of mostly upregulated mRNA expression in the recovering eyes compared with the untreated control eyes. The PLW-0 (normal) group and the PLW-2h group showed no statistically significant differential gene expression. The PLW-1d group showed a small hyperopic shift (0.1 ± 0.2 D). Two genes were differentially expressed: NPR3 was upregulated in the plus lens-wearing eyes, and IGF1 was downregulated. The PLW-4d group showed a similar hyperopic shift (0.3 ± 0.4 D), confirming that the plus lens-induced 5 D of myopia produced little refractive change. In the sclera, there was an IGNORE pattern of general differential upregulation of genes in the treated eyes (22 upregulated, one downregulated) that was distinct from the STOP signature found in recovery. Ten genes were upregulated in the REC-4d group and the PLW-4d group. However, ten other genes were differentially expressed in recovery, but not in plus-lens wear, while 12 genes were differentially expressed in plus-lens wear but not in recovery.

Conclusions

One day of recovery is not long enough for the emmetropization mechanism to produce significant gene expression changes in the sclera or refractive recovery. After 4 days, recovery and plus-lens wear produced altered scleral gene expression, but the patterns ("signatures") differed as to which genes showed altered expression, and whether the gene expression was up- or downregulated. Thus, myopia produced altered scleral mRNA expression in recovery and plus-lens wear, confirming that signals initiated by the retina reached the sclera, but the sclera in the elongated recovering eye responded differently from a normal sclera. This might have occurred because the recovering-eye sclera had remodeled during minus-lens compensation, making the sclera respond differently to the signals initiated by the retina. However, the myopia-produced retinal signals in plus lens-wearing animals also may have differed from those in the recovering eyes by the time the signals passed through the RPE and choroid to reach the sclera.

IMI - Report on Experimental Models of Emmetropization and Myopia

The results of many studies in a variety of species have significantly advanced our understanding of the role of visual experience and the mechanisms of postnatal eye growth, and the development of myopia. This paper surveys and reviews the major contributions that experimental studies using animal models have made to our thinking about emmetropization and development of myopia. These studies established important concepts informing our knowledge of the visual regulation of eye growth and refractive development and have transformed treatment strategies for myopia. Several major findings have come from studies of experimental animal models. These include the eye's ability to detect the sign of retinal defocus and undergo compensatory growth, the local retinal control of eye growth, regulatory changes in choroidal thickness, and the identification of components in the biochemistry of eye growth leading to the characterization of signal cascades regulating eye growth and refractive state. Several of these findings provided the proofs of concepts that form the scientific basis of new and effective clinical treatments for controlling myopia progression in humans. Experimental animal models continue to provide new insights into the cellular and molecular mechanisms of eye growth control, including the identification of potential new targets for drug development and future treatments needed to stem the increasing prevalence of myopia and the vision-threatening conditions associated with this disease.

The hyperopic effect of narrow-band long-wavelength light in tree shrews increases non-linearly with duration

During postnatal refractive development, an emmetropization mechanism uses refractive error to modulate the growth rate of the eye. Hyperopia (image focused behind the retina) produces what has been described as "GO" signaling that increases growth. Myopia (image focused in front of the retina) produces "STOP" signaling that slows growth. The interaction between GO and STOP conditions is non-linear; brief daily exposure to STOP counteracts long periods of GO. In young tree shrews, long-wavelength (red) light, presented 14 h per day, also appears to produce STOP signals. We asked if red light also shows temporal non-linearity; does brief exposure slow the normal decrease in hyperopia in infant animals? At 11 days after eye opening (DVE), infant tree shrews (n = 5/group) began 13 days of daily treatment (red LEDs, 624 ± 10 or 636 ± 10 nm half peak intensity bandwidth) at durations of 0 h (normal animals, n = 7) or 1, 2, 4, or 7 h. Following each daily red period, colony lighting resumed. A 14 h red group had no colony lights. Refractive state was measured daily; ocular component dimensions at the end of the 13-day red-light period. Even 1 h of red light exposure produced some hyperopia. The average hyperopic shift from normal rose exponentially with duration (time constant 2.5 h). Vitreous chamber depth decreased non-linearly with duration (time constant, 3.3 h). After red treatment was discontinued, refractions in colony lighting recovered toward normal; the initial rate was linearly related to the amount of hyperopia. The red light may produce STOP signaling similar to myopic refractive error.

Tree shrew (Tupaia belangeri) as a novel laboratory disease animal model

The tree shrew (Tupaia belangeri) is a promising laboratory animal that possesses a closer genetic relationship to primates than to rodents. In addition, advantages such as small size, easy breeding, and rapid reproduction make the tree shrew an ideal subject for the study of human disease. Numerous tree shrew disease models have been generated in biological and medical studies in recent years. Here we summarize current tree shrew disease models, including models of infectious diseases, cancers, depressive disorders, drug addiction, myopia, metabolic diseases, and immune-related diseases. With the success of tree shrew transgenic technology, this species will be increasingly used in biological and medical studies in the future.

Multi-Scale Modeling of Vision-Guided Remodeling and Age-Dependent Growth of the Tree Shrew Sclera During Eye Development and Lens-Induced Myopia

The sclera uses unknown mechanisms to match the eye's axial length to its optics during development, producing eyes with good focus (emmetropia). A myopic eye is too long for its own optics. We propose a multi-scale computational model to simulate eye development based on the assumption that scleral growth is controlled by genetic factors while scleral remodeling is driven by genetic factors and the eye's refractive error. We define growth as a mechanism that changes the tissue volume and mass while remodeling involves internal micro-deformations that are volume-preserving at the macroscale. The model was fitted against longitudinal refractive measurements in tree shrews of different ages and exposed to three different visual conditions: (i) normal development; (ii) negative lens wear to induce myopia; and (iii) recovery from myopia by removing the negative lens. The model was able to replicate the age- and vision-dependent response of the tree shrew experiments. Scleral growth ceased at younger age than scleral remodeling. The remodeling rate decreased as the eye emmetropized but increased at any age when a negative lens was put on. The predictive power of the model was investigated by calculating the susceptibility to scleral remodeling and the response to form deprivation myopia in tree shrews. Both predictions were in good agreement with experimental data that were not used to fit the model. We propose the first model that distinguishes scleral growth from remodeling. The good agreement of our results with experimental data supports the notion that scleral growth and scleral remodeling are two independently controlled mechanisms during eye development.

Long-wavelength (red) light produces hyperopia in juvenile and adolescent tree shrews

In infant tree shrews, exposure to narrow-band long-wavelength (red) light, that stimulates long-wavelength sensitive cones almost exclusively, slows axial elongation and produces hyperopia. We asked if red light produces hyperopia in juvenile and adolescent animals, ages when plus lenses are ineffective. Animals were raised in fluorescent colony lighting (100-300 lux) until they began 13days of red-light treatment at 11 (n=5, "infant"), 35 (n=5, "juvenile") or 95 (n=5, "adolescent") days of visual experience (DVE). LEDs provided 527-749 lux on the cage floor. To control for the higher red illuminance, a fluorescent control group (n=5) of juvenile (35 DVE) animals was exposed to ∼975 lux. Refractions were measured daily; ocular component dimensions at the start and end of treatment and end of recovery in colony lighting. These groups were compared with normals (n=7). In red light, the refractive state of both juvenile and adolescent animals became significantly (P<0.05) hyperopic: juvenile 3.9±1.0 diopters (D, mean±SEM) vs. normal 0.8±0.1D; adolescent 1.6±0.2D vs. normal 0.4±0.1D. The fluorescent control group refractions (0.6±0.3D) were normal. In red-treated juveniles the vitreous chamber was significantly smaller than normal (P<0.05): juvenile 2.67±0.03mmvs. normal 2.75±0.02mm. The choroid was also significantly thicker: juvenile 77±4μmvs. normal 57±3μm (P<0.05). Although plus lenses do not restrain eye growth in juvenile tree shrews, the red light-induced slowed growth and hyperopia in juvenile and adolescent tree shrews demonstrates that the emmetropization mechanism is still capable of restraining eye growth at these ages.

Progression of High Anisometropia in Children

PURPOSE

To investigate the onset and rate of progression of high anisometropia in myopic children younger than 13 years.

METHODS

A retrospective study was performed on children with anisometropia younger than 13 years with myopia of more than 4.00 diopters (D) in the more ametropic eye and a difference in spherical equivalent refraction of 4.00 D between both eyes. All children had a complete ophthalmologic examination, including measurement of visual acuity and cycloplegic refraction every 3 to 6 months for at least 5 years. Change in the spherical equivalent and the cylindrical error for both eyes and changes in the difference in spherical equivalent refraction between both eyes were calculated for each patient at each visit. Linear, polynomial, logarithmic, and exponential fitting models were tested for both eyes and for the anisometropic difference between both eyes. The regression line with the greatest R² value was considered best fit.

RESULTS

Sixty-three patients fulfilled the inclusion criteria. The more ametropic eye grew in a regular fashion during the first 2 years of life, followed by a rapid decrease in the rate of growth to become almost stable after 4 years of age. The increase in myopia best fit a third-degree polynomial (cubic) model (R² = 0.98). The less ametropic eye showed only a small increase in myopia during the follow-up period. The anisometropic difference between both eyes increased gradually during the first 2 years, then remained stable.

CONCLUSIONS

High anisometropic myopia progresses rapidly in the first few years of life before becoming stable. [J Pediatr Ophthalmol Strabismus. 2017;54(5):282-286.].

The wavelength composition and temporal modulation of ambient lighting strongly affect refractive development in young tree shrews

Shortly after birth, the eyes of most animals (including humans) are hyperopic because the short axial length places the retina in front of the focal plane. During postnatal development, an emmetropization mechanism uses cues related to refractive error to modulate the growth of the eye, moving the retina toward the focal plane. One possible cue may be longitudinal chromatic aberration (LCA), to signal if eyes are getting too long (long [red] wavelengths in better focus than short [blue]) or too short (short wavelengths in better focus). It could be difficult for the short-wavelength sensitive (SWS, "blue") cones, which are scarce and widely spaced across the retina, to detect and signal defocus of short wavelengths. We hypothesized that the SWS cone retinal pathway could instead utilize temporal (flicker) information. We thus tested if exposure solely to long-wavelength light would cause developing eyes to slow their axial growth and remain refractively hyperopic, and if flickering short-wavelength light would cause eyes to accelerate their axial growth and become myopic. Four groups of infant northern tree shrews (Tupaia glis belangeri, dichromatic mammals closely related to primates) began 13 days of wavelength treatment starting at 11 days of visual experience (DVE). Ambient lighting was provided by an array of either long-wavelength (red, 626 ± 10 nm) or short-wavelength (blue, 464 ± 10 nm) light-emitting diodes placed atop the cage. The lights were either steady, or flickering in a pseudo-random step pattern. The approximate mean illuminance (in human lux) on the cage floor was red (steady, 527 lux; flickering, 329 lux), and blue (steady, 601 lux; flickering, 252 lux). Refractive state and ocular component dimensions were measured and compared with a group of age-matched normal animals (n = 15 for refraction (first and last days); 7 for ocular components) raised in broad spectrum white fluorescent colony lighting (100-300 lux). During the 13 day period, the refraction of the normal animals decreased from (mean ± SEM) 5.8 ± 0.7 diopters (D) to 1.5 ± 0.2 D as their vitreous chamber depth increased from 2.77 ± 0.01 mm to 2.80 ± 0.03 mm. Animals exposed to red light (both steady and flickering) remained hyperopic throughout the treatment period so that the eyes at the end of wavelength treatment were significantly hyperopic (7.0 ± 0.7 D, steady; 4.7 ± 0.8 D, flickering) compared with the normal animals (p < 0.01). The vitreous chamber of the steady red group (2.65 ± 0.03 mm) was significantly shorter than normal (p < 0.01). On average, steady blue light had little effect; the refractions paralleled the normal refractive decrease. In contrast, animals housed in flickering blue light increased the rate of refractive decrease so that the eyes became significantly myopic (-2.9 ± 1.3 D) compared with the normal eyes and had longer vitreous chambers (2.93 ± 0.04 mm). Upon return to colony lighting, refractions in all groups gradually returned toward emmetropia. These data are consistent both with the hypothesis that LCA can be an important visual cue for postnatal refractive development, and that short-wavelength temporal flicker provides an important cue for assessing and signaling defocus.

Upregulated expression of N-methyl-D-aspartate receptor 1 and nitric oxide synthase during form-deprivation myopia in guinea pigs

This study aimed to investigate the expression of N-methyl-D-aspartate receptor 1 (NMDAR1) and neuronal constitutive nitric oxide synthase (ncNOS) during form-deprivation myopia (FDM). FDM models were established in guinea pigs with facemasks. NMDAR1 expression in the retina was detected by immunohistochemistry and Western blot analysis. ncNOS mRNA expression was examined by in situ hybridization. cGMP content was measured by radioimmunoassay. In control group, NMDAR1 and ncNOS were expressed in binocular retinas, and there was no significant difference in NMDAR1 and ncNOS expression and cGMP content between the two eyes. However, NMDAR1 and ncNOS expression and cGMP content in the retina of FDM eyes were significantly higher than that of contralateral untreated eyes. Furthermore, ncNOS mRNA level and cGMP content was highly correlated. In conclusion, FDM upregulates the expression of NMDAR1 and ncNOS and increases cGMP content in the retina. NMDAR1/NO-cGMP pathway may contribute to abnormal visual signals during myopic progression.

Rapid, accurate, and non-invasive measurement of zebrafish axial length and other eye dimensions using SD-OCT allows longitudinal analysis of myopia and emmetropization

Refractive errors in vision can be caused by aberrant axial length of the eye, irregular corneal shape, or lens abnormalities. Causes of eye length overgrowth include multiple genetic loci, and visual parameters. We evaluate zebrafish as a potential animal model for studies of the genetic, cellular, and signaling basis of emmetropization and myopia. Axial length and other eye dimensions of zebrafish were measured using spectral domain-optical coherence tomography (SD-OCT). We used ocular lens and body metrics to normalize and compare eye size and relative refractive error (difference between observed retinal radial length and controls) in wild-type and lrp2 zebrafish. Zebrafish were dark-reared to assess effects of visual deprivation on eye size. Two relative measurements, ocular axial length to body length and axial length to lens diameter, were found to accurately normalize comparisons of eye sizes between different sized fish (R2=0.9548, R2=0.9921). Ray-traced focal lengths of wild-type zebrafish lenses were equal to their retinal radii, while lrp2 eyes had longer retinal radii than focal lengths. Both genetic mutation (lrp2) and environmental manipulation (dark-rearing) caused elongated eye axes. lrp2 mutants had relative refractive errors of -0.327 compared to wild-types, and dark-reared wild-type fish had relative refractive errors of -0.132 compared to light-reared siblings. Therefore, zebrafish eye anatomy (axial length, lens radius, retinal radius) can be rapidly and accurately measured by SD-OCT, facilitating longitudinal studies of regulated eye growth and emmetropization. Specifically, genes homologous to human myopia candidates may be modified, inactivated or overexpressed in zebrafish, and myopia-sensitizing conditions used to probe gene-environment interactions. Our studies provide foundation for such investigations into genetic contributions that control eye size and impact refractive errors.

Limited change in anisometropia and aniso-axial length over 13 years in myopic children enrolled in the Correction of Myopia Evaluation Trial

PURPOSE

We investigated changes in anisometropia and aniso-axial length with myopia progression in the Correction of Myopia Evaluation Trial (COMET) cohort.

METHODS

Of 469 myopic children, 6 to <12 years old, enrolled in COMET, 358 were followed for 13 years. Cycloplegic autorefraction and axial length (AL) in each eye were measured annually. The COMET eligibility required anisometropia (interocular difference in spherical equivalent refraction) of ≤ 1.00 diopter (D). For each child, a linear regression line was fit to anisometropia data by visit, and the regression slope b was used as the rate of change. Logistic regression was applied to identify factors for significant changes in anisometropia (b ≥ 0.05 D/y, or a cumulative increase in anisometropia ≥ 0.50 D over 10 years). Similar analyses were applied to aniso-AL.

RESULTS

A total of 358/469 (76.3%) children had refractions at baseline and the 13-year visit. The mean (SD) amount of anisometropia increased from 0.24 D (0.22 D) at baseline to 0.49 D (0.46 D) at the 13-year visit. A total of 319/358 (89.1%) had slopes |b| < 0.05 D/y and 39 (10.9%) had slopes |b| ≥ 0.05 D/y, with only one negative slope. Similarly, 334/358 (93.3%) children had little change in aniso-AL over time. The correlation between changes in anisometropia and aniso-AL over 13 years was 0.39 (P < 0.001). The correlation between changes in anisometropia and myopia progression was significant (r = -0.36, P < 0.001). No correlation was found between baseline anisometropia and myopia progression (r = -0.02, P = 0.68).

CONCLUSIONS

Myopia and axial length progressed at a similar rate in both eyes for most children in COMET during the period of fast progression and eventual stabilization. These results may be more generalizable to school-aged myopic children with limited anisometropia at baseline. (ClinicalTrials.gov number, NCT00000113.).

Dynamics of active emmetropisation in young chicks--influence of sign and magnitude of imposed defocus

PURPOSE

Young eyes compensate for the defocus imposed by spectacle lenses by changing their rate of elongation and their choroidal thickness, bringing their refractive status back to the pre-lens condition. We asked whether the initial rate of change either in the ocular components or in refraction is a function of the power of the lenses worn, a result that would be consistent with the existence of a proportional controller mechanism.

METHODS

Two separate studies were conducted; both tracked changes in refractive errors and ocular dimensions. Study A: To study the effects of lens power and sign, young chicks were tracked for 4 days after they were fitted with positive (+5, +10 or +15 D) or negative (-5, -10, -15 D) lenses over one eye. In another experiment, biometric changes to plano, +1, +2 and +3 D lenses were tracked over a 24 h treatment period. Study B: Normal emmetropisation was tracked from hatching to 6 days of age and then a defocusing lens, either +6 D or -7 D, was fitted over one eye and additional biometric data collected after 48 h.

RESULTS

In study A, animals treated with positive lenses (+5, +10 or +15 D) showed statistical similar initial choroid responses, with a mean thickening 24 μm h(-1) over the first 5 h. Likewise, with the low power positive lenses, a statistically similar magnitude of choroidal thickening was observed across groups (+1 D: 46.0 ± 7.8 μm h(-1); +2 D: 53.5 ± 9.9 μm h(-1); +3 D 53.3 ± 24.1 μm h(-1)) in the first hour of lens wear compared to that of a plano control group. These similar rates of change in choroidal thickness indicate that the signalling response is binary in nature and not influenced by the magnitude of the myopic defocus. Treatments with -5, -10 and -15 D lenses induced statistically similar amounts of choroidal thinning, averaging -70 ± 15 μm after 5 h and -96 ± 45 μm after 24 h. Similar rates in inner axial length changes were also seen with these lens treatments until compensation was reached, once again indicating that the signalling response is not influenced by the magnitude of hyperopic defocus. In study B, after 48 h of +6 D lens treatment, the average refractive error and choroidal changes were found to be larger in magnitude than expected if perfect compensation had taken place, with a + 2.4 D overshoot in refractive compensation.

CONCLUSION

Taken together, our results with both weak and higher power positive lenses suggest that eye growth is guided more by the sign than by the magnitude of the defocus, and our results for higher power negative lenses support a similar conclusion. These behaviour patterns and the overshoot seen in Study B are more consistent with the behaviour of a bang-bang controller than a proportional controller.

Gene expression signatures in tree shrew sclera in response to three myopiagenic conditions

PURPOSE

We compared gene expression signatures in tree shrew sclera produced by three different visual conditions that all produce ocular elongation and myopia: minus-lens wear, form deprivation, and dark treatment.

METHODS

Six groups of tree shrews (n = 7 per group) were used. Starting 24 days after normal eye-opening (days of visual experience [DVE]), two minus-lens groups wore a monocular -5 diopter (D) lens for 2 days (ML-2) or 4 days (ML-4); two form-deprivation groups wore a monocular translucent diffuser for 2 days (FD-2) or 4 days (FD-4). A dark-treatment (DK) group was placed in continuous darkness for 11 days after experiencing a light/dark environment until 17 DVE. A normal colony-reared group was examined at 28 DVE. Quantitative PCR was used to measure the relative differences in mRNA levels for 55 candidate genes in the sclera that were selected, either because they showed differential expression changes in previous ML studies or because a whole-transcriptome analysis suggested they would change during myopia development.

RESULTS

The treated eyes in all groups responded with a significant myopic shift, indicating that the myopia was actively progressing. In the ML-2 group, 27 genes were significantly downregulated in the treated eyes, relative to control eyes. In the treated eyes of the FD-2 group, 16 of the same genes also were significantly downregulated and one was upregulated. The two gene expression patterns were significantly correlated (r(2) = 0.90, P < 0.001). After 4 days of treatment, 31 genes were significantly downregulated in the treated eyes of the ML-4 group and three were upregulated. Twenty-nine of the same genes (26 down- and 3 up-regulated) and six additional genes (all downregulated) were significantly affected in the FD-4 group. The response patterns were highly correlated (r(2) = 0.95, P < 0.001). When the DK group (mean of right and left eyes) was compared to the control eyes of the ML-4 group, the direction and magnitude of the gene expression patterns were similar to those of the ML-4 (r(2) = 0.82, P < 0.001, excluding PENK). Similar patterns also were found when the treated eyes of the ML-4, FD-4, and DK groups were compared to the age-matched normal eyes.

CONCLUSIONS

The very similar gene expression signatures produced in the sclera by the three different myopiagenic visual conditions at different time points suggests that there is a "scleral remodeling signature" in this mammal, closely related to primates. The scleral genes examined did not distinguish between the specific visual stimuli that initiate the signaling cascade that results in axial elongation and myopia.

Optical treatment strategies to slow myopia progression: effects of the visual extent of the optical treatment zone

In order to develop effective optical treatment strategies for myopia, it is important to understand how visual experience influences refractive development. Beginning with the discovery of the phenomenon of form deprivation myopia, research involving many animal species has demonstrated that refractive development is regulated by visual feedback. In particular, animal studies have shown that optically imposed myopic defocus slows axial elongation, that the effects of vision are dominated by local retinal mechanisms, and that peripheral vision can dominate central refractive development. In this review, the results obtained from clinical trials of traditional optical treatment strategies employed in efforts to slow myopia progression in children are interpreted in light of the results from animal studies and are compared to the emerging results from preliminary clinical studies of optical treatment strategies that manipulate the effective focus of the peripheral retina. Overall, the results suggest that imposed myopic defocus can slow myopia progression in children and that the effectiveness of an optical treatment strategy in reducing myopia progression is influenced by the extent of the visual field that is manipulated.

Mutations in LRPAP1 are associated with severe myopia in humans

Myopia is an extremely common eye disorder but the pathogenesis of its isolated form, which accounts for the overwhelming majority of cases, remains poorly understood. There is strong evidence for genetic predisposition to myopia, but determining myopia genetic risk factors has been difficult to achieve. We have identified Mendelian forms of myopia in four consanguineous families and implemented exome/autozygome analysis to identify homozygous truncating variants in LRPAP1 and CTSH as the likely causal mutations. LRPAP1 encodes a chaperone of LRP1, which is known to influence TGF-β activity. Interestingly, we observed marked deficiency of LRP1 and upregulation of TGF-β in cells from affected individuals, the latter being consistent with available data on the role of TGF-β in the remodeling of the sclera in myopia and the high frequency of myopia in individuals with Marfan syndrome who characteristically have upregulation of TGF-β signaling. CTSH, on the other hand, encodes a protease and we show that deficiency of the murine ortholog results in markedly abnormal globes consistent with the observed human phenotype. Our data highlight a role for LRPAP1 and CTSH in myopia genetics and demonstrate the power of Mendelian forms in illuminating new molecular mechanisms that may be relevant to common phenotypes.

Why do only some hyperopes become strabismic?

Children with hyperopia greater than +3.5 diopters (D) are at increased risk for developing refractive esotropia. However, only approximately 20% of these hyperopes develop strabismus. This review provides a systematic theoretical analysis of the accommodation and vergence oculomotor systems with a view to understanding factors that could either protect a hyperopic individual or precipitate a strabismus. The goal is to consider factors that may predict refractive esotropia in an individual and therefore help identify the subset of hyperopes who are at the highest risk for this strabismus, warranting the most consideration in a preventive effort.

Response to interrupted hyperopia after restraint of axial elongation in tree shrews

PURPOSE

To determine if early restraint of axial elongation in response to plus lenses increases the subsequent response to interrupted hyperopia in tree shrews.

METHODS

The normal interrupted hyperopia group (n = 5) had normal visual exposure until 24 days of visual experience (VE). Then, from 24 to 45 days of VE, the animals wore binocular -4-diopter (D) lenses, which shifted the refractive state of the eyes in the direction of hyperopia. Interrupted hyperopia was produced by removing the lenses for 2 hours per day. The early-restraint interrupted hyperopia group (n = 5) wore binocular +4-D lenses continuously from 11 to 24 days of VE, becoming emmetropic with the lenses in place and hyperopic when they were removed. Then, from 24 to 45 days of VE, the lenses were removed 22 hours per day and replaced for 2 hours per day. This created the same initial regimen of interrupted hyperopia as in the normal interrupted hyperopia group. A plus lens control group wore binocular +4-D lenses (n = 5) continuously from 11 to 45 days of VE to assess the stability of the refractive compensation.

RESULTS

In the normal interrupted hyperopia animals, 2 hours of relief from the imposed hyperopia was sufficient to prevent myopia development. In the early-restraint interrupted hyperopia animals, 2 hours of relief from the hyperopia did not prevent myopia development; the eyes became myopic while wearing the lens. The control animals compensated for the +4-D lenses and maintained a stable with-the-lens emmetropia through 45 days of VE, demonstrating that the myopic shift in the early-restraint group was caused by the interrupted hyperopia.

CONCLUSIONS

Compensation for plus lenses, involving slowed axial elongation, increases the response to subsequent interrupted hyperopia. Similar to previous reports of an eye size factor in elongated eyes, these data provide evidence for an eye size mechanism operating, in this case, in eyes that have restrained their axial length.

The effect of simultaneous negative and positive defocus on eye growth and development of refractive state in marmosets

PURPOSE

We evaluated the effect of imposing negative and positive defocus simultaneously on the eye growth and refractive state of the common marmoset, a New World primate that compensates for either negative and positive defocus when they are imposed individually.

METHODS

Ten marmosets were reared with multizone contact lenses of alternating powers (-5 diopters [D]/+5 D), 50:50 ratio for average pupil of 2.80 mm over the right eye (experimental) and plano over the fellow eye (control) from 10 to 12 weeks. The effects on refraction (mean spherical equivalent [MSE]) and vitreous chamber depth (VC) were measured and compared to untreated, and -5 D and +5 D single vision contact lens-reared marmosets.

RESULTS

Over the course of the treatment, pupil diameters ranged from 2.26 to 2.76 mm, leading to 1.5 times greater exposure to negative than positive power zones. Despite this, at different intervals during treatment, treated eyes were on average relatively more hyperopic and smaller than controls (experimental-control [exp-con] mean MSE ± SE +1.44 ± 0.45 D, mean VC ± SE -0.05 ± 0.02 mm) and the effects were similar to those in marmosets raised on +5 D single vision contact lenses (exp-con mean MSE ± SE +1.62 ± 0.44 D. mean VC ± SE -0.06 ± 0.03 mm). Six weeks into treatment, the interocular growth rates in multizone animals were already lower than in -5 D-treated animals (multizone -1.0 ± 0.1 μm/day, -5 D +2.1 ± 0.9 μm/day) and did not change significantly throughout treatment.

CONCLUSIONS

Imposing hyperopic and myopic defocus simultaneously using concentric contact lenses resulted in relatively smaller and less myopic eyes, despite treated eyes being exposed to a greater percentage of negative defocus. Exposing the retina to combined dioptric powers with multifocal lenses that include positive defocus might be an effective treatment to control myopia development or progression.

Anisometropia in children from infancy to 15 years

PURPOSE

To investigate anisometropia in children from age 6 months to 15 years.

METHODS

Children with refractions at 6 months (n = 1120), 5 years (n = 395), and 12 to 15 years (n = 312) were included in this study. All children were refracted in the laboratory by noncycloplegic retinoscopy. Myopes had spherical equivalent refraction (SER) of the less ametropic eye of less than -0.50 D, hyperopes had SER of the less ametropic eye greater than or equal to 1.00 D, and emmetropes had SER of the less ametropic eye from -0.50 to +1.00 D.

RESULTS

The mean difference in refraction between the two eyes was similar at 6 months (0.11 D) and 5 years (0.15 D), increasing to 0.28 D at 12 to 15 years. Using a cutoff of 1.00 D SER for anisometropia, the prevalence was 1.96%, 1.27%, and 5.77% at 6 months, 5 years, and 12 to 15 years, respectively. At 12 to 15 years, the prevalence of anisometropia in the myopes was 9.64% and in the hyperopes was 13.64%, both significantly higher than that in the emmetropes (3.38%, P < 0.05). The degree of anisometropia at 12 to 15 years was significantly associated with the refractive error of the less ametropic eye at 12 to 15 years, with and without adjustment for relevant covariates (P < 0.05). Infants with significant astigmatism (cylinder power ≥ 1.00 D in one or both eyes) have an increased risk of anisometropia (P < 0.05).

CONCLUSIONS

The prevalence of anisometropia increases between 5 and 15 years, when some children's eyes grow longer and become myopic. However, anisometropia was found to accompany both myopia and hyperopia, suggesting that other mechanisms in addition to excessive eye growth may exist for anisometropia development, especially in hyperopia.

Visual guidance of recovery from lens-induced myopia in tree shrews (Tupaia glis belangeri)

PURPOSE

To examine, in tree shrews, the visual guidance of recovery from negative lens-induced myopia by measuring the effect of wearing low-power negative or positive lenses during recovery. To learn if removing a negative lens for 2 h per day, after compensation has occurred, is sufficient to produce recovery.

METHODS

Starting 16 days after natural eye opening (days of visual experience), juvenile tree shrews wore a monocular -5 D lens for 11 days to produce compensation (age-appropriate refraction while wearing the lens). Recovery in four groups was started by discontinuing -5 D lens wear, which caused the treated eyes to be refractively myopic, and substituting: no lens (n = 7), a plano lens (n = 8), a -2 D lens (n = 6) or a +2 D lens (n = 10). In a fifth group (n = 6), the -5 D lens was removed for 2 h each day but worn the remainder of the time. Non-cycloplegic refractive measurements were made daily for the first 10 days and then less frequently. After 31-35 days, the lens-guided recovery period was ended for most animals; periodic measures were continued to assess post-lens recovery changes.

RESULTS

All the eyes responded to the -5 D lens and were myopic (-4.8 ± 0.1 D, mean ± S.E.M.) compared to the untreated fellow control eye. In all groups except the -2 D lens group, some animals exhibited slow or incomplete recovery. During recovery, the treated eye of most animals recovered until its refraction, measured with the recovery-lens in place, was near to that of the control eye. Measured without the lens, the -2 D group was myopic and the +2 D group was hyperopic. With the lens in place, the plano-lens, -2 D lens, and +2 D lens groups remained slightly myopic (-1.0 ± 0.3 D, -0.6 ± 0.2 D and -1.3 ± 0.1 D, respectively). The rate of recovery during the first four days was unrelated to the amount of myopia initially experienced by the recovering eyes. Removal of the -5 D lens for 2 h each day produced recovery.

CONCLUSIONS

During recovery, the emmetropization mechanism uses the presence of myopia, but perhaps not the magnitude, to guide eyes toward a refractive state similar to the control eye, regardless of whether the optically-recovered eye is longer or shorter than the fellow control eye. Wearing a goggle frame containing a lens of any power limits the recovery. The recovery signal can be intermittent, present for only 2 h per day, and still mediate recovery in competition with increasing amounts of hyperopia as recovery progresses.

Perspective: how might emmetropization and genetic factors produce myopia in normal eyes?

Substantial evidence has emerged over the past decades for a role of genetics in the development of human refractive error. There is also an emmetropization mechanism that uses visual signals to match the axial length to the focal plane. There has been little discussion of how these two important factors might interact. We explore here ways in which genetic factors driving axial growth may interact with the emmetropization mechanism, mostly to produce emmetropic eyes but often to produce myopia. An important factor may be a normal, yet reduced ability of juvenile eyes to use myopia to restrain genetically driven axial elongation. Reduced ability to respond to myopia by slowing axial elongation may contribute to the development of myopia in cases where genetics alone would make the axial length longer than the focal plane.

Patterns of mRNA and protein expression during minus-lens compensation and recovery in tree shrew sclera

PURPOSE

To increase our understanding of the mechanisms that remodel the sclera during the development of lens-induced myopia, when the sclera responds to putative "go" signals of retinal origin, and during recovery from lens-induced myopia, when the sclera responds to retinally-derived "stop" signals.

METHODS

Seven groups of tree shrews were used to examine mRNA levels during minus lens compensation and recovery. Starting 24 days after eye opening (days of visual experience [VE]) lens compensation animals wore a monocular -5D lens for 1, 4, or 11 days. Recovery animals wore the -5D lens for 11 days, which was then removed for 1 or 4 days. Normal animals were examined at 24 and 38 days of VE. All groups contained 8 animals. Scleral mRNA levels were examined in the treated and contralateral control eyes with quantitative real-time polymerase chain reaction (qPCR) for 27 genes divided into four categories: 1) signaling molecules, 2) matricellular proteins, 3) metalloproteinases (MPs) and tissue inhibitors of metalloproteinases (TIMPs), and 4) cell adhesion and other proteins. Four groups (n=5 per group) were used to examine protein levels. One group wore a -5D lens for 4 days. A second group recovered for 4 days after 11 days of -5D lens treatment. Two groups were used to examine age-matched normal protein levels at 28 and 39 days of VE. The levels of six scleral proteins that showed differential mRNA expression were examined with quantitative western blots.

RESULTS

Nineteen of the genes showed differential (treated eye versus control eye) expression of mRNA levels in at least one group of animals. Which genes showed differential expression differed after 1 and 4 days of compensation and after 1 or 4 days of recovery. The mRNA level for one gene, a disintegrin and metalloproteinase with thrombospondin motifs 1 (ADAMTS1), was upregulated in the treated eyes after 1 day of compensation. After 4 days, transforming growth factor beta receptor 3 (TGFBR3), transforming growth factor-beta-induced protein ig-h3 (TGFBI), and matrix metalloproteinase 14 (MMP14) mRNA levels were upregulated. Downregulated were mRNA levels for transforming growth factor beta-1 (TGFB1), transforming growth factor beta-2 (TGFB2), thrombospondin 1 (THBS1), tenascin (TNC), osteonectin (SPARC), osteopontin (SPP1), tissue inhibitor of metalloproteinases 3 (TIMP3), and a disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS5). After 11 days of lens wear, there was no differential expression. During recovery, after 1 day, treated-eye mRNA downregulation was found for TGFB2, TGFBR1, TGFBR2, TGFBR3, SPARC, ADAMTS1, ADAMTS5, syndecan 4 (SDC4), and collagen type VI, alpha 1 (COL6A1). After 4 days, TGFB1, TGFB2, TGFB3, THBS2, and TIMP3 mRNA levels were upregulated in the recovering eye. Significant downregulation, relative to normal eyes, was found in both the control and treated eyes for most genes after 1 day of compensation; a similar decrease was found, compared to lens-compensated eyes, after one day of recovery. Protein levels for THBS1 showed positive correlation with the differential mRNA levels and TGFBR3 showed a negative correlation. No differential protein expression was found for TGFB2, TGFBI, MMP14, and TIMP3.

CONCLUSIONS

The different patterns of differential mRNA expression during minus lens compensation (hyperopia) and recovery (myopia) show that scleral fibroblasts distinguish between "go" and "stop" conditions. There is evidence of binocular global downregulation of genes at the start of both lens wear and recovery. As additional information accumulates about changes in gene expression that occur during compensation and recovery the "signature" of differential changes may help us to understand in more detail how the sclera responds in "go" and "stop" conditions.

Nature and nurture: the complex genetics of myopia and refractive error

The refractive errors, myopia and hyperopia, are optical defects of the visual system that can cause blurred vision. Uncorrected refractive errors are the most common causes of visual impairment worldwide. It is estimated that 2.5 billion people will be affected by myopia alone within the next decade. Experimental, epidemiological and clinical research has shown that refractive development is influenced by both environmental and genetic factors. Animal models have showed that eye growth and refractive maturation during infancy are tightly regulated by visually guided mechanisms. Observational data in human populations provide compelling evidence that environmental influences and individual behavioral factors play crucial roles in myopia susceptibility. Nevertheless, the majority of the variance of refractive error within populations is thought to be because of hereditary factors. Genetic linkage studies have mapped two dozen loci, while association studies have implicated more than 25 different genes in refractive variation. Many of these genes are involved in common biological pathways known to mediate extracellular matrix (ECM) composition and regulate connective tissue remodeling. Other associated genomic regions suggest novel mechanisms in the etiology of human myopia, such as mitochondrial-mediated cell death or photoreceptor-mediated visual signal transmission. Taken together, observational and experimental studies have revealed the complex nature of human refractive variation, which likely involves variants in several genes and functional pathways. Multiway interactions between genes and/or environmental factors may also be important in determining individual risks of myopia, and may help explain the complex pattern of refractive error in human populations.