High myopia induced by form deprivation is associated with altered corneal biomechanical properties in chicks

Spatial heterogeneity of corneal biomechanical properties in myopia at nanoscale: A preliminary study

PURPOSE

To investigate the spatial heterogeneity of the corneal biomechanical properties in individuals with non-high and high myopia.

METHODS

Atomic force microscopy was used to quantify the region-dependent elastic modulus (E) of 34 corneal lenticules from keratorefractive lenticule extraction surgery. The local E values of the central region, as well as the superior, inferior, nasal, and temporal points at the pericentral region, were measured. Differences between non-high myopia (-6.0 D < spherical equivalent [SE] ≤ -0.5 D) and high myopia (SE ≤ -6.0 D) were compared.

RESULTS

E was significantly higher in the non-high myopia group than in the high myopia group (P < 0.0001). In non-high myopia, the central cornea exhibited a higher E than its pericentral counterpart (P < 0.0001), and the pericentral region E was higher in the horizontal direction than in the vertical direction (P = 0.0393). However, these values converged to be similar in high myopia (P = 0.5973, P = 0.7799). No significant differences in E were found between the superior and inferior pericentral corneas, nor between the nasal and temporal in both non-high (P = 0.0931, P = 0.1800) and high myopia (P = 0.5154, P = 0.1007). The E values of central and pericentral cornea were positively correlated with the mean radius of the posterior corneal surface (r = 0.3747, P = 0.0290; r = 0.3961, P = 0.0204).

CONCLUSION

In non-high myopia, region-dependent corneal biomechanics revealed higher stiffness centrally than pericentrally, with pericentral cornea stiffer horizontally than vertically. High myopia exhibited a reduced E and a gradual loss of spatial heterogeneity. Emphasizing spatial heterogeneity is crucial for a comprehensive understanding of the biomechanical behavior in myopia.

Corneal Biomechanical Characteristics and Correlation Analysis in Children With Different Refractive States

Purpose: To investigate the correlation between corneal biomechanical characteristics and refractive status in adolescents aged 5-13 years. Methods: A cross-sectional study involved 339 children aged 5-13 with a spherical equivalent (SE) range from -6.00 to +2.00 diopters. Axial length (AL) was measured by IOL Master, corneal biomechanical parameters by Corvis ST, and anterior segment parameters by Pentacam. According to SE of right eye, the subjects were divided into moderate myopia, mild myopia, and emmetropia group. The correlation between AL and SE and corneal biomechanical parameters was analyzed. The corneal biomechanical parameters of the three groups were also compared. Results: The A2V value in the moderate myopia group was significantly lower than that in both the mild group and emmetropia group (p < 0.001). PD in the moderate group was higher than that in the mild group (p < 0.05), while PD in mild myopia was higher than that in emmetropia (p < 0.05). The SSI in the emmetropia group was significantly higher than that in the other two groups (all p < 0.001), and the SSI in the mild group was higher than that in the moderate group (p < 0.01). The A2V value in the 11-13 years old group was lower than that in the 5-7 years old group (p < 0.001) and 8-10 years old group (p < 0.01). PD in the 11-13 years old group was significantly higher than that in the 8-10 years old group (p < 0.001), and PD in the 8-10 years old group was significantly higher than that in the 5-7 years old group (p < 0.01). The SSI in the 5-7 years old group was significantly higher than that in the 8-10 years old group (p < 0.001), and the SSI in the 8-10 years old group was significantly higher than that in the 11-13 years old group (p < 0.05). AL was positively correlated with PD and negatively correlated with SSI and A2V. SE was positively correlated with A2V and SSI and negatively correlated with PD. Conclusions: Corneal stiffness seems to decrease with the increase of SE. The changes of SSI, PD, and A2V were statistically significant and can be predictors of myopia progression in adolescents aged 5-13 years.

Impact of Forms of Visual Attenuation on Short-Term Eye Changes Under Controlled Reading Visibility

Purpose

Animal studies have suggested that visual degradation impacts eye growth due to the attenuation of high spatial frequencies. However, the influence of perceptual visibility remains unclear in humans. The aim of this study was to investigate the impact of visibility on visual attenuation-related eye changes during reading.

Methods

Axial length (AxL) and choroidal thickness (ChT) changes associated with reading tasks were measured in two separate experiments. In the first experiment, the reading task was conducted under different forms of visual attenuation (contrast, resolution, defocus, noise, and crowding). For each form of visual attenuation, the text was set at a sub-threshold level of visibility, evaluated via prior measurement of reading performance, and kept constant via adaptive control of the intensity of the stimulation. Each sub-threshold reading condition was compared with a supra-threshold reading text, serving as control. In the second experiment, the effect of visibility on lens-induced defocus was further examined by comparing the effect of text stimulation with an equivalent dioptric of 5.5 D under sub- and supra-threshold levels of resolution.

Results

Near distance reading with supra-threshold texts caused eye elongation (AxL: +12.942 µm ± 2.147 µm; ChT: -3.192 µm ± 1.158 µm). Additional defocusing failed to exacerbate axial elongation under sub-threshold text visibility (mean difference: -0.135 µm ± 2.783 µm), revealing a clear inhibitory effect of lowering visibility on eye changes. Other forms of visual degradation, including crowding (mean difference: 6.153 µm ± 2.127 µm) and noise (mean difference: 5.02 µm ± 2.812 µm) also showed an inhibitory effect on eye elongation. The significant effect of crowding indicated that post-retinal mechanisms, involving attentional processes related to crowded characters, may play a role in the influence of visibility.

Conclusion

Although the featural composition of visual stimulation can drastically influence eye changes, this study revealed an important mediating role of visibility, previously underscored in chick studies, which warrants further explorations of the impact of post-retinal processes in eye growth.

Corneal Biomechanical Characteristics in Myopes and Emmetropes Measured by Corvis ST: A Meta-Analysis

PURPOSE

To comprehensively identify the corneal biomechanical differences measured by Corvis ST between different degrees of myopia and emmetropia.

DESIGN

Systematic review and meta-analysis.

METHODS

Electronic databases, including PubMed, Embase, and Web of Science, were systematically searched for studies comparing the corneal biomechanics among various degrees of myopes and emmetropes using Corvis ST. The weighted mean differences and 95% confidence intervals were calculated. Meta-analysis was performed in high and nonhigh myopes and in myopes and emmetropes, respectively.

RESULTS

Eleven studies were included in this study. The meta-analysis among myopes and emmetropes included 1947 myopes and 621 emmetropes, and 443 high myopes and 449 nonhigh myopes were included in the meta-analysis among high and nonhigh myopia. Myopes showed the cornea with significantly longer time at the first applanation (A1t) and lower length at the second applanation (A2L) than emmetropes. High myopes showed significantly greater A1t, velocity at the second applanation (A2v), deformation amplitude at the highest concavity (HC-DA), and peak distance at the highest concavity (HC-PD) and decreased time at the second applanation (A2t) and radius of the highest concavity (HC-R).

CONCLUSIONS

Corneal biomechanics is different in myopia, especially in high myopia. Compared with nonhigh myopes, the corneas of high myopes deformed slower during the first applanation, faster during the second applanation, and showed greater deformation amplitude, indicating greater elasticity and viscidity.

The biomechanical proteins different between low myopic corneas and moderate to high myopic corneas in human

PURPOSE

To explore the biomechanical proteins different between low myopic corneas and moderate to high myopic corneas.

METHODS

A total of 27 myopic corneas were used for the Tandem Mass Tag (TMT) proteomics analysis. Differentially expressed proteins (DEPs) were clustered with fold changes > 1.20 or < 0.83 and p < 0.05. Proteins and Proteins Interactions (PPIs) were conducted to find hub proteins; Uniprot database was to screen proteins with biomechanical functions, and Parallel Reaction Monitoring (PRM) was performed to verify the TMT results. Pearson analysis was used to reveal the correlations between myopic degrees and biomechanical proteins. The Immunofluorescence (IF) staining was used to observe the protein distributions.

RESULTS

In total, 34 DEPs were observed between moderate myopic corneas and low myopic corneas; 103 DEPs were observed between high myopic corneas and low myopic corneas, 20 proteins overlapped. The PPIs analysis showed keratin 2, keratins 10 and PRSS1 were hub proteins. The Uniprot function analysis suggested keratin 2 and keratin 10 exhibited biomechanical functions. The PRM demonstrated keratin 2 and keratin 10 levels were significantly lower in moderate and high myopic corneas, which was consistent with the TMT proteomics results. IF staining also demonstrated keratin 2 and keratin 10 were less distributed in moderate and high myopic corneas than in low myopic corneas.

CONCLUSIONS

The levels of biomechanical proteins keratin 2 and keratin 10 are significantly lower in moderate and high myopic corneas than in low myopic corneas.

Corneal stress‒strain index in relation to retinal nerve fibre layer thickness among healthy young adults

BACKGROUND/OBJECTIVES

To determine the relationship between corneal stress-strain index (SSI) and retinal nerve fibre layer (RNFL) thickness.

SUBJECTS/METHODS

1645 healthy university students from a university-based study contributed to the analysis. The RNFL thickness was measured by high-definition optical coherence tomography (HD-OCT), axial length (AL) was measured by IOL Master, and corneal biomechanics including SSI, biomechanical corrected intraocular pressure (bIOP), and central corneal thickness (CCT) were measured by Corvis ST. Multivariate linear regression was performed to evaluate the relationship between the SSI and RNFL thickness after adjusting for potential covariates.

RESULTS

The mean age of the participants was 19.0 ± 0.9 years, and 1132 (68.8%) were women. Lower SSI was significantly associated with thinner RNFL thickness ( β =8.601, 95% confidence interval [CI] 2.999-14.203, P  = 0.003) after adjusting for age, CCT, bIOP, and AL. No significant association between SSI and RNFL was found in men, while the association was significant in women in the fully adjusted model. The association was significant in the nonhigh myopic group ( P for trend = 0.021) but not in the highly myopic group. Eyes with greater bIOP and lower SSI had significantly thinner RNFL thickness.

CONCLUSIONS

Eyes with lower SSI had thinner RNFL thickness after adjusting for potential covariates, especially those with higher bIOP. Our findings add novel evidence of the relationship between corneal biomechanics and retinal ganglion cell damage.

Higher intraocular pressure is associated with slower axial growth in children with non-pathological high myopia

OBJECTIVES

To investigate the association between intraocular pressure (IOP) and axial elongation rate in highly myopic children from the ZOC-BHVI High Myopia Cohort Study.

METHODS

162 eyes of 81 healthy children (baseline spherical equivalent: -6.25 D to -15.50 D) aged 7-12 years with non-pathological high myopia were studied over five biennial visits. The mean (SD) follow-up duration was 5.2 (3.3) years. A linear mixed-effects model (LMM) was used to assess the association between IOP (at time point t-1) and axial elongation rate (annual rate of change in AL from t-1 to t), controlling for a pre-defined set of covariates including sex, age, central corneal thickness, anterior chamber depth and lens thickness (at t-1). LMM was also used to assess the contemporaneous association between IOP and axial length (AL) at t, controlling for the same set of covariates (at t) as before.

RESULTS

Higher IOP was associated with slower axial growth (β = -0.01, 95% CI -0.02 to -0.005, p = 0.001). There was a positive contemporaneous association between IOP and AL (β = 0.03, 95% CI 0.01-0.05, p = 0.004), but this association became progressively less positive with increasing age, as indicated by a negative interaction effect between IOP and age on AL (β = -0.01, 95% CI -0.01 to -0.003, p = 0.001).

CONCLUSIONS

Higher IOP is associated with slower rather than faster axial growth in children with non-pathological high myopia, an association plausibly confounded by the increased influence of ocular compliance on IOP.

Synchronous myopia development induced by bilateral form deprivation in chicks

Form deprivation (FD) is a widely employed experimental paradigm, typically used to induce unilateral myopia in animal models. This model is weakened by potential influence upon the FD eye from vision in the freely-viewing contralateral eye, which could be eliminated by imposing FD in both eyes; but while a few previous studies have explored the feasibility of inducing bilateral FD in chicks, substantial discrepancies in treatment outcomes were noted. Consequently, this study aimed to establish a bilateral FD myopia model in chicks, with validation by investigating the associated ocular growth patterns, feeding, and social behavior. Six-day-old chicks were treated with bilateral (n = 21) or unilateral (n = 10) FD for 12 days; the fellow untreated eyes in the unilateral FD group served as controls. Refractive error, corneal power, and ocular axial dimensions were measured at 4-day intervals after the onset of form deprivation, with a Hartinger refractometer, a custom-made videokeratography system, and a high-resolution A-scan ultrasonographer, respectively. Body weight was monitored to assess the chick's physical development. Our results showed that birds treated with bilateral FD grew as robustly as the unilaterally form-deprived chicks, with similar or slightly heavier body weights and mortalities. Unilateral FD induced significantly higher myopia in the treated eye, with stronger corneal power, deeper anterior and vitreous chambers, and longer axial length. Moreover, either bilaterally or unilaterally FD eyes developed similar refractive error (bilateral FD, left: -28.03 ± 9.06 D, right: -28.44 ± 9.45 D; unilateral FD: -29.48 ± 8.26 D) and ocular biometric changes; but choroidal thickness was thicker in bilaterally FD eyes, rather than thinner as in unilaterally FD eyes. In addition to the highly synchronized (symmetrical, parallel) development reported previously in bilateral FD, we found in this study that the correlations between bilaterally form-deprived eyes were highest for ocular biometric parameters directly contributing to myopia development, including corneal power (r = 0.74 to 0.93), anterior chamber depth (r = 0.60 to 0.85), vitreous chamber depth (r = 0.92 to 0.94), and axial length (r = 0.90 to 0.96). The remarkably synchronized growth pattern confirmed the feasibility of the bilateral FD paradigm for future research on myopia.

Refractive associations with corneal biomechanical properties among young adults: a population-based Corvis ST study

PURPOSE

To assess the associations of corneal biomechanical properties as measured by the Corvis ST with refractive errors and ocular biometry in an unselected sample of young adults.

METHODS

A total of 1645 healthy university students underwent corneal biomechanical parameters measurement by the Corvis ST. The refractive status of the participants was measured using an autorefractor without cycloplegia. Ocular biometric parameters were measured using the IOL Master.

RESULTS

After adjusting for the effect of age, sex, biomechanical-corrected intraocular pressure and central corneal thickness, axial length was significantly associated with A1 velocity (A1v, β = -10.47), A2 velocity (A2v, β = 4.66), A2 deflection amplitude (A2DeflA, β = -6.02), HC deflection amplitude (HC-DeflA, β = 5.95), HC peak distance (HC-PD, β = 2.57), deformation amplitude ratio max (DA Rmax, β = -0.36), Ambrósio's relational thickness to the horizontal profile (ARTh, β = 0.002). For axial length / corneal radius ratio, only A1v (β = -2.01), A1 deflection amplitude (A1DeflA, β = 2.30), HC-DeflA (β = 1.49), HC-PD (β = -0.21), DA Rmax (β = 0.07), stress-strain index (SSI, β = -0.29), ARTh (β < 0.001) were significant associates. A1v (β = 23.18), HC-DeflA (β = -15.36), HC-PD (β = 1.27), DA Rmax (β = -0.66), SSI (β = 3.53), ARTh (β = -0.02) were significantly associated with spherical equivalent.

CONCLUSION

Myopic eyes were more likely to have more deformable corneas and corneas in high myopia were easier to deform and were even softer compared with those in the mild/moderate myopia.

Corneal stress-strain index in myopic Indian population

AIM

The purpose is to study the corneal stress-strain index (SSI) in myopic refractive error among Indian subjects.

METHODS

A retrospective study where young myopic subjects aged between 11 and 35 years who had undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2021 were enrolled. Subjects with central corneal thickness (CCT) <500 μ, intraocular pressure (IOP) >21 mmHg, history of any systemic and ocular disease or any previous ocular surgery, high astigmatism, corneal disease such as keratoconus were excluded. Subjects with missing data or having poor quality scan were excluded. Corneal biomechanical properties and corneal SSI were assessed using Corvis ST. For statistical purposes, eyes were divided into four different groups and were analyzed using one-way ANOVA.

RESULTS

Nine hundred and sixty-six myopic eyes with mean ± standard deviation age, IOP, and CCT of 26.89 ± 4.92 years, 16.94 ± 2.00 mmHg, and 540.18 ± 25.23 microns, respectively, were included. There were 311, 388, 172, and 95 eyes that were low, moderate, severe, and extreme myopic. Deformation amplitude ratio at 1 mm and 2 mm were similar across different myopic groups. A significant increase in max inverse radius, ambrosia relational thickness, biomechanically corrected IOP, integrated radius was noted with an increase in myopic refractive error. Corvis biomechanical index, corneal SSI was found to be decreased significantly with an increase in myopic refractive error. We noted a significant positive association between myopic refractive error and SSI (P < 0.001).

CONCLUSION

Corneal SSI was found to be reduced in extreme myopic eyes.

Comparisons of the protein expressions between high myopia and moderate myopia on the anterior corneal stroma in human

PURPOSE

To investigate the differentially expressed proteins (DEP) between high myopia and moderate myopia on the anterior corneal stroma.

METHODS

Tandem mass tag (TMT) quantitative proteomics was utilized to reveal proteins. DEPs were screened by the multiple change of more than 1.2 times or less than 0.83 and the P value < 0.05. The DEPs were functional annotated by Gene Ontology (GO) terms. Proteins and protein interaction (PPI) networks were conducted with String online tool. Parallel reaction monitoring (PRM) data processing was used to verify the TMT proteomics results.

RESULTS

There are 36 DEPs between high myopia and moderate myopia on the anterior corneal stroma, of which 11 proteins are upregulated, 25 proteins are downregulated. The GO analysis demonstrated keratinocyte migration and structural constituent of cytoskeleton that are significantly changed with most of the proteins decreased in high myopic corneas. Keratin 16 (KRT16) and erythrocyte membrane protein band 4.1-like protein 4B are the only two proteins involved in both functions. The PPI analysis showed keratin type II cytoskeletal 6A (KRT6A) and KRT16 that have strong connections. Immunoglobulin lambda variable 8-61(IGLV8-61) and nicotinamide phosphoribosyl transferase (NAMPT) have consistent results with the TMT.

CONCLUSIONS

The high myopic corneas have 36 DEPs compared to the moderate myopic corneas on the anterior corneal stroma. Keratinocyte migrations and structural constituent of cytoskeleton are weakened in high myopic corneas, which may partly account for the lower corneal biomechanics in high myopic eyes. The lower expressed KRT16 plays important roles in high myopic corneas.

Complement-mediated inflammation and mitochondrial energy metabolism in the proteomic profile of myopic human corneas

Myopia is one of the most common causes of visual impairment worldwide. To identify proteins related to myopiagenesis, data-independent acquisition proteomic analysis was performed using corneal lenticules of myopic patients who underwent small incision lenticule extraction surgery. A total of 19 lenticules from 19 age and sex-matched patients were analyzed, 10 in high refractive error (HR, spherical equivalent over -6.00 D) group and 9 in low refractive error (LR, spherical equivalent between -3.00 and - 1.00 D) group. Differentially expressed proteins (DEPs) were identified by comparing the corneal proteome between the two groups. Functional analyses were performed to explore the biological pathways and interactions of the DEPs. 107 DEPs (67 upregulated and 40 downregulated in HR group, compared to LR) were identified from 2138 quantified proteins. Functional analyses indicated that upregulated proteins were primarily involved in the complement pathways and extracellular matrix (ECM) remodeling, while downregulated proteins were involved in mitochondrial energy metabolism. Western blot analysis confirmed increased complement C3a and apolipoprotein E in HR samples, further supporting the proteomics data. In conclusion, this proteomic study reveals that proteins associated with the complement system, ECM remodeling, and mitochondrial energy metabolism might be key effectors in myopiagenesis. SIGNIFICANCE: Myopia has become one of the most prevalent causes of visual impairment, especially in Asia. The underlying mechanism of myopia development is still up for debate. This study compares the proteomic profiles of high and low myopic corneas, identifying differentially expressed proteins associated with the complement system, ECM remodeling, and mitochondrial energy metabolism. The findings of this study could provide novel insights into the pathogenesis of myopia. The complement system and mitochondrial energy metabolism may provide potential therapeutic targets in the treatment and prevention of myopia.

Effect of corneal stiffness decrease on axial length elongation in myopia determined based on a mathematical estimation model

Purpose: To investigate the relationship between the corneal material stiffness parameter stress-strain index (SSI) and axial length (AL) elongation with varying severities of myopia, based on a mathematical estimation model. Methods: This single-center, cross-sectional study included data from healthy subjects and patients preparing for refractive surgery in the Qingdao Eye Hospital of Shandong First Medical University. Data were collected from July 2021 to April 2022. First, we performed and tested an estimated AL model ( A L M o r g a n ) based on the mathematical equation proposed by Morgan. Second, we proposed an axial increment model ( Δ A L ) corresponding to spherical equivalent error (SER) based on A L e m m e t r o p i a ( A L M o r g a n at SER = 0) and subject's real AL. Finally, we evaluated the variations of Δ A L with SSI changes based on the mathematical estimation model. Results: We found that AL was closely associated with A L M o r g a n (r = 0.91, t = 33.8, p < 0.001) with good consistency and SER was negatively associated with Δ A L (r = -0.89, t = -30.7, p < 0.001). The association of SSI with AL, A L e m m e t r o p i a , and Δ A L can be summarized using the following equations: A L = 27.7 - 2.04 × S S I , A L e m m e t r o p i a = 23.2 + 0.561 × S S I , and Δ A L = 4.52 - 2.6 × S S I . In adjusted models, SSI was negatively associated with AL (Model 1: β = -2.01, p < 0.001) and Δ A L (Model 3: β = -2.49, p < 0.001) but positively associated with A L e m m e t r o p i a (Model 2: β = 0.48, p < 0.05). In addition, SSI was negatively associated with Δ A L among subjects with AL ≥ 26 mm (β = -1.36, p = 0.02). Conclusion: AL increased with decreasing SSI in myopia.

Experimentally induced myopia and myopic astigmatism alter retinal electrophysiology in chickens

Myopia (or "short-sightedness") and astigmatism are major causes of visual impairment worldwide. Significant amounts of astigmatism are frequently observed in infants and have been associated with myopia development. Although it is well established that both myopia and astigmatism are associated with ocular structural changes from anterior to posterior segments, very little is known on how these refractive errors alter retinal functions. This study investigated the effects of experimentally induced myopia and myopic-astigmatism on retinal electrophysiology by using an image-guided, multifocal global flash stimulation in chickens, a widely used animal model for refractive error development. Myopia and myopic-astigmatism were experimentally induced, respectively, by wearing spherical (- 10 D, n = 12) and sphero-cylindrical lenses (- 6.00 DS/- 8.00 DCx90: Hyperopic With-The Rule, H-WTR, n = 15; - 6.00 DS/- 8.00 DCx180: Hyperopic Against-The-Rule, H-ATR, n = 11) monocularly for a week (post-hatching day 5 to 12). An aged-matched control group without any lens treatment provided normal data (n = 12). Multifocal electrophysiological results revealed significant regional variation in the amplitude of induced component (IC) (central greater than peripheral; both p < 0.05) in the normal and H-ATR groups, but not in the - 10 D and H-WTR groups. Most importantly, for the first time, our results showed that both H-WTR and H-ATR groups exhibited a significantly longer implicit time of the inner retinal response at the central region when compared to the normal and - 10 D groups, highlighting a significant role of astigmatism in retinal physiology.

Biomechanical analysis of ocular diseases and its in vitro study methods

Ocular diseases are closely related to the physiological changes in the eye sphere and its contents. Using biomechanical methods to explore the relationship between the structure and function of ocular tissue is beneficial to reveal the pathological processes. Studying the pathogenesis of various ocular diseases will be helpful for the diagnosis and treatment of ocular diseases. We provide a critical review of recent biomechanical analysis of ocular diseases including glaucoma, high myopia, and diabetes. And try to summarize the research about the biomechanical changes in ocular tissues (e.g., optic nerve head, sclera, cornea, etc.) associated with those diseases. The methods of ocular biomechanics research in vitro in recent years are also reviewed, including the measurement of biomechanics by ophthalmic equipment, finite element modeling, and biomechanical analysis methods. And the preparation and application of microfluidic eye chips that emerged in recent years were summarized. It provides new inspiration and opportunity for the pathogenesis of eye diseases and personalized and precise treatment.

Assessment of corneal biomechanics in anisometropia using Scheimpflug technology

Purpose: To investigate the relationship between corneal biomechanical and ocular biometric parameters, and to explore biomechanical asymmetry between anisometropic eyes using the corneal visualization Scheimpflug technology device (Corvis ST). Methods: 180 anisometropic participants were included. Participants were divided into low (1.00≤△Spherical equivalent (SE) < 2.00D), moderate (2.00D≤△SE < 3.00D) and high (△SE ≥ 3.00D) anisometropic groups. Axial length (AL), keratometry, anterior chamber depth (ACD) and corneal biomechanical parameters were assessed using the OA-2000 biometer, Pentacam HR and Corvis ST, respectively. Results: The mean age of participants was 16.09 ± 5.64 years. Stress-Strain Index (SSI) was positively correlated with SE (r = 0.501, p < 0.001) and negatively correlated with AL (r = -0.436, p < 0.001). Some other Corvis ST parameters had weak correlation with SE or AL. Corneal biomechanical parameters except for time of first applanation (A1T), length of second applanation (A2L), deformation amplitude (DA), first applanation stiffness parameter (SPA1) and ambrosia relational thickness-horizontal (ARTh) were correlated with ametropic parameters (SE or AL) in multiple regression analyses. A1T, velocity of first applanation (A1V), time of second applanation (A2T), A2L, velocity of second applanation (A2V), corneal curvature radius at highest concavity (HCR), peak distance (PD), DA, deformation amplitude ratio max (2 mm) (DAR), SPA1, integrated radius (IR), and SSI showed significant differences between fellow eyes (p < 0.05). There was no significant difference in asymmetry of corneal biomechanics among the three groups (p > 0.05). Asymmetry of some biomechanical parameters had weak correlation with asymmetry of mean corneal curvatures and ACD. However, asymmetry of corneal biomechanical parameters was not correlated with asymmetry of SE or AL (p > 0.05). Conclusion: More myopic eyes had weaker biomechanical properties than the contralateral eye in anisometropia. However, a certain linear relationship between anisometropia and biomechanical asymmetry was not found.

Unraveling the mechanobiology of cornea: From bench side to the clinic

The cornea is a transparent, dome-shaped structure on the front part of the eye that serves as a major optic element and a protector from the external environment. Recent evidence shows aberrant alterations of the corneal mechano-environment in development and progression of various corneal diseases. It is, thus, critical to understand how corneal cells sense and respond to mechanical signals in physiological and pathological conditions. In this review, we summarize the corneal mechano-environment and discuss the impact of these mechanical cues on cellular functions from the bench side (in a laboratory research setting). From a clinical perspective, we comprehensively review the mechanical changes of corneal tissue in several cornea-related diseases, including keratoconus, myopia, and keratectasia, following refractive surgery. The findings from the bench side and clinic underscore the involvement of mechanical cues in corneal disorders, which may open a new avenue for development of novel therapeutic strategies by targeting corneal mechanics.

Measuring Human Corneal Stromal Biomechanical Properties Using Tensile Testing Combined With Optical Coherence Tomography

Purpose: To investigate the ex vivo elastic modulus of human corneal stroma using tensile testing with optical coherence tomography (OCT) imaging and its correlation with in vivo measurements using corneal visualization Scheimpflug technology.

Methods: Twenty-four corneal specimens extracted from stromal lenticules through small incision lenticule extraction were cut into strips for uniaxial tensile tests. In vivo corneal biomechanical responses were evaluated preoperatively using the corneal visualization Scheimpflug technology (CorVis ST). The correlation of the elastic modulus with clinical characteristics and dynamic corneal response parameters were analyzed using Spearman’s correlation analysis.

Results: The mean low strain tangent modulus (LSTM) of the human corneal stroma was 0.204 ± 0.189 (range 0.010–0.641) MPa, and high strain tangent modulus (HSTM) 5.114 ± 1.958 (range 2.755–9.976) MPa. Both LSTM (r = 0.447, p = 0.029) and HSTM (r = 0.557, p = 0.005) were positively correlated with the stress-strain index (SSI). LSTM was also positively correlated with the A1 deflection length (r = 0.427, p = 0.037) and A1 deflection area (r = 0.441, p = 0.031). HSTM was positively correlated with spherical equivalent (r = 0.425, p = 0.038).

Conclusions: The correlation of corneal elastic modulus with A1 deflection parameters and SSI may indicate a relationship between these parameters and tissue elasticity. The HSTM decreased with the degree of myopia. Combining tensile test with OCT may be a promising approach to assess corneal biomechanical properties.

Alteration of EIF2 Signaling, Glycolysis, and Dopamine Secretion in Form-Deprived Myopia in Response to 1% Atropine Treatment: Evidence From Interactive iTRAQ-MS and SWATH-MS Proteomics Using a Guinea Pig Model

Purpose: Atropine, a non-selective muscarinic antagonist, effectively slows down myopia progression in human adolescents and several animal models. However, the underlying molecular mechanism is unclear. The current study investigated retinal protein changes of form-deprived myopic (FDM) guinea pigs in response to topical administration of 1% atropine gel (10 g/L).

Methods: At the first stage, the differentially expressed proteins were screened using fractionated isobaric tags for a relative and absolute quantification (iTRAQ) approach, coupled with nano-liquid chromatography-tandem mass spectrometry (nano-LC-MS/MS) (n = 24, 48 eyes) using a sample pooling technique. At the second stage, retinal tissues from another cohort with the same treatment (n = 12, 24 eyes) with significant ocular changes were subjected to label-free sequential window acquisition of all theoretical mass spectra (SWATH-MS) proteomics for orthogonal protein target confirmation. The localization of Alpha-synuclein was verified using immunohistochemistry and confocal imaging.

Results: A total of 1,695 proteins (8,875 peptides) were identified with 479 regulated proteins (FC ≥ 1.5 or ≤0.67) found from FDM eyes and atropine-treated eyes receiving 4-weeks drug treatment using iTRAQ-MS proteomics. Combining the iTRAQ-MS and SWATH-MS datasets, a total of 29 confident proteins at 1% FDR were consistently quantified and matched, comprising 12 up-regulated and 17 down-regulated proteins which differed between FDM eyes and atropine treated eyes (iTRAQ: FC ≥ 1.5 or ≤0.67, SWATH: FC ≥ 1.4 or ≤0.71, p-value of ≤0.05). Bioinformatics analysis using IPA and STRING databases of these commonly regulated proteins revealed the involvement of the three commonly significant pathways: EIF2 signaling; glycolysis; and dopamine secretion. Additionally, the most significantly regulated proteins were closely connected to Alpha-synuclein (SNCA). Using immunostaining (n = 3), SNCA was further confirmed in the inner margin of the inner nuclear layer (INL) and spread throughout the inner plexiform layer (IPL) of the retina of guinea pigs.

Conclusion: The molecular evidence using next-generation proteomics (NGP) revealed that retinal EIF2 signaling, glycolysis, and dopamine secretion through SNCA are implicated in atropine treatment of myopia in the FDM-induced guinea pig model.

The Effect of Axial Length Elongation on Corneal Biomechanical Property

Background: To investigate the correlation between the corneal biomechanical parameter stress-strain index (SSI) and axial length (AL) in moderately elongated eye (MEE) and severely elongated eye (SEE).

Methods: This study included 117 eyes from 117 participants. Among them, 59 (50.4%) had MEE (AL<26 mm) and 58 (49.6%) had SEE (AL≥26 mm). AL was measured using Lenstar LS-900, and central corneal thickness (CCT) and anterior chamber volume (ACV) were measured using Pentacam. SSI was measured via corneal visualisation Scheimpflug technology (Corvis ST). Kolmogorov-Smirnov test, Student’s t-test, and Pearson and partial correlation analyses were used for statistical analyses.

Results: The mean (±SD) SSI was 1.08 ± 0.15 in the MEE group and 0.92 ± 0.13 in the SEE group (p < 0.01). SSI was positively correlated with age (MEE: r = 0.326, p < 0.05; SEE: r = 0.298, p < 0.05) in both groups; it was negatively correlated with AL (r = −0.476, p < 0.001) in the MEE group but not in the SEE group (p > 0.05). CCT was negatively correlated with AL (r = −0.289, p < 0.05) and ACV positively correlated with AL (r = 0.444, p < 0.001) in the MEE group. Neither CCT nor ACV was correlated with AL (p > 0.05) in the SEE group.

Conclusion: Corneal biomechanical parameter SSI, which represents the stiffness of corneal tissue, was lower in the SEE group than in the MEE group. When analyzed separately, SSI was negatively correlated with AL in the MEE group, but not in the SEE group, which may provide insight into different ocular growth patterns between lower myopia and higher myopia.

Association Between Severity of Myopia and Deformation Characteristics of the Cornea Based on Propensity Score Matching Analysis

PURPOSE

To investigate the relationship between severity of myopia and corneal deformation characteristics after removing confounding factors using propensity score matching (PSM) analysis.

METHODS

A prospective study was conducted from April 1 to December 30, 2019 in Tianjin Eye Hospital. Participants were divided into low (spherical equivalent [SE] > -6.00 diopters) and high (SE ⩽ -6.00 diopters) myopia groups. Corneal deformation parameters were obtained using corneal visualization Scheimpflug technology. PSM analysis was conducted to minimize the effect of confounding factors (age, intraocular pressure, and corneal thickness) on corneal deformation. Correlation analysis and logistic regression models were applied to investigate the relationships between SE and corneal deformation parameters.

RESULTS

A total of 2,126 eyes from 1,063 patients with a mean age of 23.81 ± 5.68 years were enrolled. The left and right eyes were analyzed separately. After PSM analysis, logistic regression indicated that the peak distance was a significant indicator for high myopia in both eye models (left: odds ratio [OR] = 1.978, 95% CI: 1.303 to 3.004; right: OR = 2.089, 95% CI: 1.362 to 3.202; P < .001). The maximum amplitudes of deformation and deflection and peak distance were significantly negatively correlated with SE in both eyes, whereas the whole eye movement was significantly positively correlated with SE (P < .01), as well as the deformation amplitude at the second applanation (P < .001).

CONCLUSIONS

Eyes with high myopia exhibited larger deformation and deflection amplitude compared to eyes with low myopia. The high distance between bending points of the cornea at the highest concavity (peak distance) may be a feature of high myopia. [J Refract Surg. 2021;37(5):344-350.].

Biophysical properties of corneal cells reflect high myopia progression

Myopia is a common ocular disorder with significant alterations in the anterior ocular structure, including the cornea. The cell biophysical phenotype has been proposed to reflect the state of various diseases. However, the biophysical properties of corneal cells have not been characterized during myopia progression and their relationship with myopia remains unknown. This study characterizes the biophysical properties of corneal cells in normal, myopic, and recovered conditions, using two classical myopia models. Surprisingly, myopic corneal cells considerably reduce F-actin and microtubule content and cellular stiffness and generate elevated traction force compared with control cells. When myopia is restored to the healthy state, these biophysical properties are partially or fully restored to the levels of control cells. Furthermore, the level of chromatin condensation is significantly increased in the nucleus of myopic corneal cells and reduced to a level similar to healthy cells after recovery. These findings demonstrate that the reversible biophysical alterations of corneal cells reflect myopia progression, facilitating the study of the role of corneal cell biophysics in myopia.

IMI 2021 Yearly Digest

Purpose

The International Myopia Institute (IMI) Yearly Digest highlights new research considered to be of importance since the publication of the first series of IMI white papers.

Methods

A literature search was conducted for articles on myopia between 2019 and mid-2020 to inform definitions and classifications, experimental models, genetics, interventions, clinical trials, and clinical management. Conference abstracts from key meetings in the same period were also considered.

Results

One thousand articles on myopia have been published between 2019 and mid-2020. Key advances include the use of the definition of premyopia in studies currently under way to test interventions in myopia, new definitions in the field of pathologic myopia, the role of new pharmacologic treatments in experimental models such as intraocular pressure-lowering latanoprost, a large meta-analysis of refractive error identifying 336 new genetic loci, new clinical interventions such as the defocus incorporated multisegment spectacles and combination therapy with low-dose atropine and orthokeratology (OK), normative standards in refractive error, the ethical dilemma of a placebo control group when myopia control treatments are established, reporting the physical metric of myopia reduction versus a percentage reduction, comparison of the risk of pediatric OK wear with risk of vision impairment in myopia, the justification of preventing myopic and axial length increase versus quality of life, and future vision loss.

Conclusions

Large amounts of research in myopia have been published since the IMI 2019 white papers were released. The yearly digest serves to highlight the latest research and advances in myopia.

Ocular growth and metabolomics are dependent upon the spectral content of ambient white light

Myopia results from an excessive axial growth of the eye, causing abnormal projection of remote images in front of the retina. Without adequate interventions, myopia is forecasted to affect 50% of the world population by 2050. Exposure to outdoor light plays a critical role in preventing myopia in children, possibly through the brightness and blue-shifted spectral composition of sunlight, which lacks in artificial indoor lighting. Here, we evaluated the impact of moderate levels of ambient standard white (SW: 233.1 lux, 3900 K) and blue-enriched white (BEW: 223.8 lux, 9700 K) lights on ocular growth and metabolomics in a chicken-model of form-deprivation myopia. Compared to SW light, BEW light decreased aberrant ocular axial elongation and accelerated recovery from form-deprivation. Furthermore, the metabolomic profiles in the vitreous and retinas of recovering form-deprived eyes were distinct from control eyes and were dependent on the spectral content of ambient light. For instance, exposure to BEW light was associated with deep lipid remodeling and metabolic changes related to energy production, cell proliferation, collagen turnover and nitric oxide metabolism. This study provides new insight on light-dependent modulations in ocular growth and metabolomics. If replicable in humans, our findings open new potential avenues for spectrally-tailored light-therapy strategies for myopia.

Corneal proteome and differentially expressed corneal proteins in highly myopic chicks using a label-free SWATH-MS quantification approach

Myopia, or short-sightedness, is a highly prevalent refractive disorder in which the eye's focal length is too short for its axial dimension in its relaxed state. High myopia is associated with increased risks of blinding ocular complications and abnormal eye shape. In addition to consistent findings on posterior segment anomalies in high myopia (e.g., scleral remodeling), more recent biometric and biomechanical data in myopic humans and animal models also indicate anterior segment anomalies (e.g., corneal biomechanical properties). Because the cornea is the anterior-most ocular tissue, providing essential refractive power and physiological stability, it is important to understand the biochemical signaling pathway during myopia development. This study first aimed to establish the entire chicken corneal proteome. Then, using the classical form deprivation paradigm to induce high myopia in chicks, state-of-the-art bioinformatics technologies were applied to identify eight differentially expressed proteins in the highly myopic cornea. These results provide strong foundation for future corneal research, especially those using chicken as an animal model for myopia development.

Early Astigmatism Can Alter Myopia Development in Chickens

Purpose

To determine the effects of optically imposed astigmatism on myopia development in chickens.

Methods

Chicks were randomly assigned to wear either spherical (-10D, "LIM", n = 14) or sphero-cylindrical lenses (n ≥ 19 in each group) monocularly for a week from 5 days of age. All lenses imposed the same magnitude of spherical-equivalent hyperopic defocus (-10D), with the two astigmatic magnitudes (-8D or -4D) and four axes (45°, 90°, 135°, or 180°) altered to simulate four subtypes of clinical astigmatism. At the end of the treatment, refractive state was measured for all birds, whereas ocular axial dimensions and corneal curvature were measured for subsets of birds.

Results

Sphero-cylindrical lens wear produced significant impacts on nearly all refractive parameters (P < 0.001), resulting in myopic-astigmatic errors in the treated eyes. Compared to LIM, the presence of astigmatic blur induced lower myopic error (all except L180 group, P < 0.001) but with higher refractive astigmatism (all P < 0.001) in birds treated with sphero-cylindrical lenses. Distributions of the refractive, axial, and corneal shape parameters in the sphero-cylindrical lens-wear groups indicated that the astigmatic blur had directed the eye growth toward the least hyperopic image plane, with against-the-rule (ATR) and with-the-rule (WTR) astigmatisms typically inducing differential biometric changes.

Conclusions

The presence of early astigmatism predictably altered myopia development in chicks. Furthermore, the differential effects of WTR and ATR astigmatisms on anterior and posterior segment changes suggest that the eye growth mechanism is sensitive to the optical properties of astigmatism.

Biomechanics of C2C12 Cells Observed with Cellular Resolution Scanning Acoustic Microscope Combined with Optical Microscope

Biomechanics of the cell indicates the inner structure and viability of the cell. Mechanical properties are represented by acoustic properties such as speed of sound (SOS) or acoustic impedance. In the present study, cellular resolution scanning acoustic microscope combined with optical microscope (OptSAM) is developed to observe the change of mechanical properties in cell differentiation. Main part of the OptSAM was consisted of 350 MHz ultrasound transducer mechanically scanned by a piezo-actuator. Thickness, SOS, acoustic impedance, density and elastic bulk modulus of the cell were deduced by the ultrasound responses in both time domain and frequency domain. C2C12 cell changing its form from myoblast to myotube was observed by OptSAM. The value of bulk modulus slightly increased in response to differentiation process. OptSAM non-invasively provides important information on biomechanics of cells without contact or staining.

Comparison of Corneal Biomechanics Between Low and High Myopic Eyes-A Meta-analysis

PURPOSE

To compare the corneal biomechanical difference between the low myopic eyes and high myopic eyes.

DESIGN

Systematic review and meta-analysis.

METHODS

Data sources, including PubMed, Medline, EMBASE, Web of Science, and Chinese databases including Wanfang and China National Knowledge Infrastructure, were searched to find the relevant studies. Primary outcomes were corneal hysteresis (CH), corneal resistance factor (CRF), corneal-compensated intraocular pressure (IOPcc), Goldmann-correlated intraocular pressure (IOPg), and central corneal thickness (CCT) in high myopic eyes and low myopic eyes.

RESULTS

Eleven studies were enrolled in this study. CH and CRF were significantly higher in the low myopic eyes. The mean difference of CH was 0.73 mm Hg, 95% confidence interval (CI) [0.53 to 0.93], P < .001. The mean difference of CRF was 0.20 mm Hg, 95% CI [0.04 to 0.37], P = .02. The IOPcc and IOPg were significantly lower in the low myopic eyes. The mean difference of IOPcc was -2.53 mm Hg, 95% CI [-3.24, -1.83], P < .01. The mean difference of IOPg was -1.42 mm Hg, 95% CI [-2.26, -0.58], P = .0009. There was no significant difference between the 2 groups on CCT; the mean difference was -2.85 μm, 95% CI [-9.64.3.93], P = .41.

CONCLUSION

Corneal biomechanics are different in the high myopic eyes. Low CH and CRF and high IOPcc and IOPg are suggested to be associated factors for high myopia. Future studies are needed to investigate the underlying corneal structure difference that causes the low CH and CRF value in the high myopic eyes.

Data on corneal proteome and differentially expressed corneal proteins in highly myopic chicks using a data independent quantification approach

Myopia is an abnormal refractive status, explained by an excessive ocular lengthening mostly in posterior segments. Although growing evidence of anterior segments, specifically altered corneal geometries with biomechanical properties in myopes have been reported, the mechanism behind is poorly understood. We hereby prepared experimentally induced highly myopic chicks to investigate the molecular basis of corneal remodeling by applying a novel proteomic approach integrated with information dependent acquisition (IDA) and data independent quantification (SWATH-MS) analysis. As a result, differentially expressed protein biomarkers that might be involved in structural changes were screened based on the first of its kind unique chicken corneal proteome. All generated raw data from IDA and SWATH-MS are accessible at Peptide Atlas public repository (http://www.peptideatlas.org/PASS/PASS01410) for general release.