Longitudinal changes in corneal asphericity in myopia

Gene expression profile analyses to identify potential biomarkers for myopia

PURPOSE

Increasing evidence suggests myopia is not a simple refractive error, many other factors might also be involved. Here, we assessed myopic and normal corneas' gene expression profiles to identify possible diagnostic and therapeutic biomarkers for myopia.

MATERIALS AND METHODS

We obtained the expression profile of ten patients and seven normal control samples from the GSE112155 and GSE151631 datasets based on the Gene Expression Omnibus (GEO) database. We used the "limma" R package to determine the differentially expressed genes (DEGs) between myopic and normal corneas. Weighted gene co-expression network analysis (WGCNA) was used to identify critical co-expressed modules related to myopia, and enrichment analyses were used to annotate the function of genes encompassed in the compulsory module. We also validated these findings in two external datasets (GSE24641 and GSE136701).

RESULTS

We identified that the DEGs were significantly enriched in ultraviolet (UV) response, TNF-α signaling via NFκB, Angiogenesis, Myogenesis pathways, etc. We used 2095 genes to construct the co-expression gene modules and found five interesting modules because the eigengene expression of these modules was significantly differentially expressed between myopic and normal corneas. Notably, the enrichment analysis found that the genes encompassed in lightgreen module were significantly enriched in immune-related pathways. These findings were proved by subsequent analysis based on Xcell software. We found the component of B cells, CD4+ memory T cells, CD8+ central memory T cells, plasmacytoid dendritic cells, T helper 2 (Th2) cells, regulatory T cells (Tregs), etc. were significantly increased in myopic corneas, while CD8+ T cells, CD4+ T central memory cells, natural killer T (NKT) cells, and T helper 1 (Th1) cells were significantly decreased.

CONCLUSION

Our findings identified some markers that might detect diagnosis and treatment for myopia from cornea aspect. Future studies are warranted to verify the functional role of immune-related pathways in cornea during the pathogenesis or progression of myopia.

Analysis of Asphericity and Corneal Longitudinal Spherical Aberration of 915 Chinese Myopic Adult Eyes

Purpose

To analyze the corneal asphericity, longitudinal spherical aberration (LSA), and related factors in Chinese myopic adult eyes.

Methods

This was a retrospective study of myopic adult patients. The corneal asphericity and LSA were measured at 3.0, 4.0, 5.0, 6.0, and 7.0 mm diameter apertures using corneal tomography. Age and refractive power were recorded for correlation analysis.

Results

In total, 531 females and 384 males were included. At the above five diameter settings the corneal asphericity values (Q) of the anterior surface were -0.09±0.21, -0.14±0.16, -0.15±0.13, -0.17±0.11, and -0.20±0.11, and those of the posterior surface were 0.23±0.49, 0.06±0.29, -0.01±0.22, -0.07±0.16, and -0.08±0.15, respectively. The anterior corneal LSA values at these diameters were 0.39±0.19, 0.63±0.27, 0.97±0.36, 0.90±0.30, and 0.83±0.29 D, respectively and the whole corneal values were 0.26±0.20, 0.44±0.27, 0.70±0.36, 0.66±0.30, and 0.59±0.28 D, respectively. Corneal asphericity and peripheral LSA showed no or weak correlation with age or spherical equivalent (all r < 0.2).

Conclusion

Corneal anterior and posterior Q values in myopia patients are negatively correlated with corneal diameter. Corneal anterior and whole corneal LSA increased significantly with diameter up to 5 mm, then decreased slightly with diameter. Corneal asphericity and peripheral LSA showed very weak or no correlation with age or spherical equivalent.

Clinical Trial Registration Number

ChiCTR1800015985.

Three-year change in corneal asphericity in children at the age of emmetropisation

PURPOSE

To determine the 3-year changes in anterior and posterior values of corneal asphericity (Q) in 6- to 12-year-old children.

METHODS

The first and second phases of the study were conducted in 2015 and 2018, respectively. The target population in the first phase was 6- to 12-year-old students in Shahroud, Iran. Multistage cluster sampling was performed on urban students. Additionally, all rural students in Shahroud county were invited to participate. Corneal imaging and ocular biometry were performed using the Pentacam-HR and Allegro Biograph, respectively.

RESULTS

A total of 4961 right eyes were analysed in this report. The mean (95% confidence interval) anterior and posterior Q values for an 8-mm chord diameter were -0.38 ± 0.11 (-0.39 to -0.38) and -0.32 ± 0.12 (-0.33 to -0.32) in the first phase of the study, respectively. The mean 3-year changes in anterior and posterior Q values were 0.00 ± 0.08 (95% CI: 0.00-0.00) and -0.01 ± 0.06 (95% CI: -0.01 to -0.01), respectively. Based on a multiple regression model, advancing age (β = -0.002; [-0.003 to -0.001]), 3-year increase in mean keratometry (β = -0.04; [-0.06 to -0.02]), central corneal thickness (β = -0.001; [-0.001 to -0.000]), lens thickness (-0.05; [-0.11 to -0.003]) and anterior chamber depth (-0.09; [-0.15 to -0.03]) showed a statistically significant association with increased anterior Q prolation. Increased axial length was associated with a decrease in Q (β = 0.02; [0.004 to 0.03]). In addition, female gender (β = -0.004; [-0.007 to -0.001]) was significantly associated with a more prolate posterior Q value, while rural residence (β = 0.005; [0.002 to 0.009]) was significantly associated with a more oblate posterior Q value.

CONCLUSION

Three-year changes in anterior and posterior Q values were very small. The cornea tends to become more prolate with increasing age.

Effect of Corneal Tilt on the Determination of Asphericity

Purpose: To quantify the effect of levelling the corneal surface around the optical axis on the calculated values of corneal asphericity when conic and biconic models are used to fit the anterior corneal surface. Methods: This cross-sectional study starts with a mathematical simulation proving the concept of the effect that the eye's tilt has on the corneal asphericity calculation. Spherical, conic and biconic models are considered and compared. Further, corneal asphericity is analysed in the eyes of 177 healthy participants aged 35.4 ± 15.2. The optical axis was determined using an optimization procedure via the Levenberg-Marquardt nonlinear least-squares algorithm, before fitting the corneal surface to spherical, conic and biconic models. The influence of pupil size (aperture radii of 1.5, 3.0, 4.0 and 5.0 mm) on corneal radius and asphericity was also analysed. Results: In computer simulations, eye tilt caused an increase in the apical radii of the surface with the increase of the tilt angle in both positive and negative directions and aperture radii in all models. Fitting the cornea to spherical models did not show a significant difference between the raw-measured corneal surfaces and the levelled surfaces for right and left eyes. When the conic models were fitted to the cornea, changes in the radii of the cornea among the raw-measured corneal surfaces' data and levelled data were not significant; however, significant differences were recorded in the asphericity of the anterior surfaces at radii of aperture 1.5 mm (p < 0.01). With the biconic model, the posterior surfaces recorded significant asphericity differences at aperture radii of 1.5 mm, 3 mm, 4 mm and 5 mm (p = 0.01, p < 0.01, p < 0.01 & p < 0.01, respectively) in the nasal temporal direction of right eyes and left eyes (p < 0.01, p < 0.01, p < 0.01 & p < 0.01, respectively). In the superior-inferior direction, significant changes were only noticed at aperture radii of 1.5 mm for both right and left eyes (p = 0.05, p < 0.01). Conclusions: Estimation of human corneal asphericity from topography or tomography data using conic and biconic models of corneas are affected by eyes' natural tilt. In contrast, the apical radii of the cornea are less affected. Using corneal asphericity in certain applications such as fitting contact lenses, corneal implant design, planning for refractive surgery and mathematical modelling when a geometrical centre of the eye is needed should be implemented with caution.

Corneal asphericity and related factors in the geriatric population: A population-based study

PURPOSE

To determine the distribution of the corneal asphericity coefficient (Q value) and related factors in an Iranian geriatric population.

METHODS

This population-based study was conducted in 2019 in Tehran, using stratified multistage random cluster sampling. The study population was ≥60 years of age. Participants underwent corneal imaging using a Pentacam HR. Mean keratometry, corneal astigmatism, central corneal thickness, anterior chamber depth and the overall anterior and posterior Q values (for 8 mm chord diameter) were recorded. Axial length measurements were performed using the IOL Master 500.

RESULTS

2457 eyes of 2457 individuals were analysed. The mean age was 67.3 ± 5.82 years and 1479 (60.2%) were female. The mean Q value for the anterior corneal surface was -0.35 ± 0.17 (95% CI: -0.35 to -0.34). The anterior Q value showed a statistically significant inverse relationship with axial length and mean keratometry, and a significant direct association with anterior chamber depth and corneal astigmatism. The mean posterior Q value was -0.41 ± 0.15 (95% CI: -0.42 to -0.40). The posterior Q value had a significant direct relationship with age, anterior chamber depth, mean keratometry and corneal astigmatism.

CONCLUSION

The corneal Q values in this geriatric Iranian population were more negative than the values reported in most previous studies. Corneal asphericity was greater affected by ocular biometry and corneal curvature than demographic factors and refractive status.

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.

Age distribution and associated factors of cornea biomechanical parameter stress-strain index in Chinese healthy population

Background

To investigate the new cornea biomechanical parameter stress-strain index (SSI) in Chinese healthy people and the factors associated with SSI.

Methods

A total of 175 eyes from 175 participants were included in this study. Axial length was measured with the Lenstar LS-900. Pentacam measured curvature of the cornea and anterior chamber volume (ACV). Cornea biomechanical properties assessments were performed by corneal visualization Scheimpflug technology (Corvis ST). Student’s t-test, one-way ANOVA, partial least square linear regression (PLSLR) and linear mixed effects (LME) model were used in the statistical analysis.

Results

The mean (±SD) SSI was 1.14 ± 0.22 (range, 0.66–1.78) in all subjects and affected by age significantly after age of 35 (P < 0.05). In LME models, SSI was significantly associated with age (β = 0.526, P < 0.001), axial length (AL) (β = − 0.541, P < 0.001), intraocular pressure (IOP) (β = 0.326, P < 0.001) and steepest radius of anterior corneal curvature (RsF) (β = 0.229, P < 0.001) but not with ACV, biomechanical corrected intraocular pressure (bIOP), flattest radius of anterior corneal curvature (RfF) or central corneal thickness (CCT) (P > 0.05 for each).

Conclusions

SSI increased with age after the age of 35. In addition to age, SSI was positively correlated with RsF and IOP, while negatively correlated with AL.

Predictors of Myopic Regression for Laser-assisted Subepithelial Keratomileusis and Laser-assisted in Situ Keratomileusis Flap Creation with Mechanical Microkeratome and Femtosecond Laser in Low and Moderate Myopia

Purpose: To determine the predictive factors of postoperative myopic regression in subjects who have undergone laser-assisted subepithelial keratomileusis (LASEK), laser-assisted in situ keratomileusis (LASIK) with flaps created using a mechanical microkeratome (MM) or LASIK with flaps created using a femtosecond (FS) laser for -0.50 D to -6.0 D myopia.Materials and Methods: This study was designed as a retrospective comparison. We included 236 eyes in the LASEK group, 1,178 eyes in the MM-LASIK group and 1,333 eyes in the FS-LASIK group. Refractive outcomes were recorded at one day; one week; and one, three, six and 12 months postoperatively. Predictors affecting myopic regression and other covariates were estimated using a Cox proportional hazards model for the three methods of surgery.Results: At 12 months, the survival rates (no myopic regression) were 63.36%, 74% and 77% in the MM-LASIK, LASEK, and FS-LASIK groups, respectively. Risk factors for myopic regression were indicated for patients with greater preoperative central corneal thicknesses (CCT) (P = .02), female patients (P < .01), patients with aspherical ablations (P = .01) and those with larger transitional zones (TZ) (P < .01). Among the three surgeries, MM-LASIK had a significantly higher probability of postoperative myopic regression (P < .01). Longer durations of myopia (P = .03), steeper corneal curvatures (K_max) (P < .01) and larger optical zones (OZ) (P < .01) were protective factors.Conclusion: MM-LASIK had the highest risk for myopic regression. OZ, TZ, aspherical ablations, preoperative corneal curvatures and being a female patient had significant influence on myopic regression in -0.50 D to -6.0 D myopia.

Group Analysis of Q Values Calculated with Tangential Radius of Curvature from Human Anterior Corneal Surface

Objective

To calculate the Q values from the human anterior corneal surface with the tangential radius of curvature and analyze its distribution characteristics in different age and refractive status groups.

Methods

Tangential power maps of the anterior cornea from Orbscan II were acquired for 201 subjects' right eyes. They were divided into groups of adults and children and then divided further into subgroups according to the refraction status. The Q values of each semimeridian were calculated by the tangential radius with a linear regression equation. The Q value distribution in both the nasal cornea and temporal cornea were analyzed.

Results

The mean temporal Q values of the emmetropia group of adults and all children's groups were significantly different from the mean nasal Q value. The mean nasal corneal Q values were more negative in children. The adult group showed differences only in the low myopia group. The mean Q value of the nasal cornea among different refractive groups of children was significantly different, and so was the temporal cornea between the adult myopia and emmetropia group.

Conclusion

The method using the tangential radius of curvature combined with linear regression to obtain anterior surface Q values for both adults and children was stable and reliable. When we analyzed the anterior corneal Q value, area division was necessary.

The analysis of corneal asphericity (Q value) and its related factors of 1,683 Chinese eyes older than 30 years

Purpose

To determine corneal Q value and its related factors in Chinese subjects older than 30 years.

Design

Cross sectional study.

Methods

1,683 participants (1,683 eyes) from the Handan Eye Study were involved, including 955 female and 728 male with average age of 53.64 years old (range from 30 to 107 years). The corneal Q values of anterior and posterior surfaces were measured at 3.0, 5.0 and 7.0mm aperture diameters using Bausch & Lomb Orbscan IIz (software version 3.12). Age, gender and refractive power were recorded.

Results

The average Q values of the anterior surface at 3.0, 5.0 and 7.0mm aperture diameters were -0.28±0.18, -0.28±0.18, and -0.29±0.18, respectively. The average Q value of the anterior surface at the 5.0mm aperture diameter was negatively correlated with age (B = -0.003, p<0.01) and the refractive power (B = -0.013, p = 0.016). The average Q values of the posterior surface at 3.0, 5.0, and 7.0mm were -0.26±0.216, -0.26±0.214, and -0.26±0.215, respectively. The average Q value of the posterior surface at the 5.0mm aperture diameter was positively correlated with age (B = 0.002, p = 0.036) and the refractive power (B = 0.016, p = 0.043).

Conclusion

The corneal Q value of the elderly Chinese subjects is different from that of previously reported European and American subjects, and the Q value appears to be correlated with age and refractive power.

Analysis of age, refractive error and gender related changes of the cornea and the anterior segment of the eye with Scheimpflug imaging

PURPOSE

To assess age, refractive error and gender related changes occurring in the cornea and the anterior segment of the eye using a Scheimpflug system.

METHODS

The study included 666 healthy eyed subjects with a mean age of 39.3±19.7 years (range: 3-85 years). All analyses were based on the right eyes of the patients as all measured parameters correlated well between the right and left eyes. Each parameter was correlated with age and the right eye's spherical equivalent (SE) using Pearson correlations. Univariate linear regression models were constructed for analyses of parameters.

RESULTS

The anterior corneal surface asphericity showed significant positive correlations whereas posterior corneal surface asphericity showed significant negative correlations with age. Anterior chamber depth (ACD), volume (ACV) and angle (ACA) showed significant negative correlations with age and SE. Age explained 25% of the variance in anterior corneal surface asphericity, 22% of variance in posterior corneal surface asphericity, 26% of variance in ACV, 27% of variance in ACD, and 19% of variance in ACA. In the SE model SE was identified to account for 25% of variance in ACV, 22% of variance in ACD, each, and 17% of variance in ACA. Significant differences were detected in anterior and posterior keratometry values, ACV, ACD and ACA among gender groups (p<0.01).

CONCLUSIONS

The cornea shows a tendency for a decrease in anterior corneal surface asphericity and an increase in posterior corneal surface asphericity with advancing age. Men have flatter corneas and women have shallower anterior chambers and narrower anterior chamber angles.

Corneal power change is predictive of myopia progression in orthokeratology

PURPOSE

This study aims to investigate the relationship between corneal refractive power change along three axes (nasal, temporal, and inferior) after orthokeratology (OK) treatment and 2-year axial growth in children.

METHODS

Thirty-two Chinese children aged from 9 to 14 were fitted with OK. When corneal reshaping process following OK treatment was completed and stabilized, the 3-month topographic outputs were taken as the post-OK data. Corneal refractive powers along the nasal, temporal, and inferior axes were collected over an 8-mm-diameter ring in 1-mm steps using the sagittal power map. The maximum power change along each axis was selected and divided into two subcategories, level 1 and level 2, depending on whether the value was below or above the average. Axial length (AL) was measured every 6 months during a 24-month period. The relationship between the maximum power changes and 2-year axial elongation were analyzed.

RESULTS

Twenty-seven subjects completed the 24-month study. After OK treatment, statistically significant steepening (p < 0.05) was observed at the nasal 2 mm and 3 mm; temporal 3 mm; and inferior 2 mm, 3 mm, and 4 mm locations compared with the apical center. AL increased significantly throughout the 24-month observation period (p < 0.001). Changes in corneal refractive power significantly affected axial elongation (nasal, p = 0.001; temporal, p = 0.011; inferior, p = 0.001). Two-year axial elongation in patients with larger corneal power changes (level 2) was reduced by 54% to 69% compared with those with smaller corneal power changes (level 1). Maximum power changes along the three axes were negatively correlated (p < 0.05) with 2-year axial growth.

CONCLUSIONS

Subjects with larger magnitude of corneal relative peripheral power change along specific axes after OK treatment experienced slower axial elongation by the end of 24 months. This effect might be mediated by the induction of greater amount of relative myopic defocus on the peripheral retina. Our study lends weight to potential OK lens designs for myopia control in children.

Variations in corneal asphericity (Q value) between African-Americans and whites

PURPOSE

To search for differences in corneal asphericity on the basis of ethnicity between African-American and white populations.

METHODS

A prospective cohort design was used to analyze corneal asphericity (Q value) data obtained by Pentacam HR (Oculus, Wetzlar, Germany) on right eyes from African-American (n = 80) and white (n = 80). Subjects were stratified by ethnicity, age, and spherical equivalent (SE) refractive error. Q values were obtained from each quadrant (superior, nasal, inferior, and temporal) and two meridians (horizontal and vertical).

RESULTS

The mean Q values were African-Americans -0.26 ± 0.19 and whites -0.20 ± 0.12, indicating that the eyes of African-Americans were significantly more prolate (p = 0.003) than those of whites. There was a significant difference between mean Q values for ethnic groups only in the 30- to 39-year olds (p = 0.01) and there was a lack of correlation with age in both ethnic groups. Q value contrasts by gender were only significant between males (p = 0.01). There was a lack of correlation between Q value and SE for either ethnic group. Age group contrasts between ethnic groups found significant differences for those with SE greater than 0.00 D to -3.00 D (p = 0.05) and greater than 0.00 D to +3.00 D (p = 0.05). Comparison of mean Q values in opposing meridians within and across ethnic groups were significant, although neither group showed significant differences between horizontal and vertical meridians.

CONCLUSIONS

Corneal asphericity as represented by mean Q value varies significantly between African-Americans and whites. The greatest differences are evident in opposing quadrants and appear to be little influenced by age, gender, or SE.

Corneal shapes of Chinese emmetropes and myopic astigmats aged 10 to 45 years

PURPOSE

Myopia and astigmatism are highly prevalent in the Hong Kong Chinese. This study aimed to determine the effects of age and myopic astigmatism (MA) on the corneal shape factors in the Hong Kong Chinese.

METHODS

One hundred subjects with compound MA or emmetropia (EM) were recruited from three age groups: 10 to 15 years (n = 32), 20 to 25 years (n = 37), and 40 to 45 years (n = 31). Refractive errors were measured by noncycloplegic subjective refraction. Corneal astigmatism and corneal shape factors were measured by the Scheimpflug-based Pentacam. The effects of age and refractive errors on the whole corneal shape (mean-P) and the semimeridian corneal shapes (semi-Ps) at the nasal, temporal, superior, and inferior corneal quadrants (from corneal apex to 3 mm peripheral cornea) were analyzed.

RESULTS

Age had significant effects on the mean-P and semi-Ps (both p < 0.001), with both EM and MA showing less prolate corneal shapes in older age groups. Partial correlation analyses adjusted for age showed that mean-P and semi-Ps were correlated with multiple refractive-error components (Pearson r = -0.30 to -0.78, all p < 0.05), with higher correlations found along the horizontal semi-Ps in MA (Pearson r = +0.37 to -0.78, all p < 0.01). Compared with EM, MA had more prolate temporal semi-Ps in all the three age groups (p < 0.05). Strikingly, age and refractive errors also had significant impacts on the asymmetry of the corneal shape along the horizontal meridian.

CONCLUSIONS

Corneal shapes were influenced by age and MA in the Hong Kong Chinese. These results highlight the importance of controlling these factors when designing a study on corneal shape.

Time course of Q value after myopic laser-assisted in situ keratomileusis

OBJECTIVE

To assess the time course of Q value after myopic laser-assisted in situ keratomileusis (LASIK) and preliminarily evaluate the determinants of the difference of Q value between before and after LASIK.

METHODS

We performed a retrospective, longitudinal investigation on patients undergoing wavefront optimized LASIK therapy for emmetropization. A total of 418 eyes from 222 cases were examined preoperatively, and partly followed up at one week (172 eyes), one month (134 eyes) and three months (51 eyes) after surgery. The horizontal, vertical and total Q values of cornea were calculated from eccentricity measured at the central 6-mm corneal zones respectively. Potential determinants of the change of Q value were analyzed using multiple linear regressions.

RESULTS

The mean Q value was -0.17 +/- 0.13 preoperatively, and 0.99 +/- 0.70, 0.97 +/- 0.66, and 0.86 +/- 0.41 one week, one and three months postoperatively, respectively. One way analysis of variance (ANOVA) demonstrated significant differences between measurements made before surgery and at all postoperative times (at one week, one and three months; all P<0.0001, Bonferroni post hoc), but no significant differences were found among postoperative groups. Significant differences of Q values between horizontal and vertical meridians were found before surgery and at all postoperative times (all P<0.0001). Multiple regression analysis revealed that change of Q value significantly correlated with manifest refraction spherical equivalent (r=0.116, P<0.0001) and axial length (r=0.264, P<0.0001).

CONCLUSIONS

Over the study period, the primary changes in Q value occur within 1 week after surgery, and then become slightly decreased and nearly stable. Manifest refraction spherical equivalent and axial length play a significant role in the change of postoperative Q value.

Topography-based assessment of anterior corneal curvature and asphericity as a function of age, sex, and refractive status

PURPOSE

To assess corneal asphericity (Q) and evaluate potential factors influencing the shape of the anterior corneal surface.

SETTING

Medical Optics Research Group, Institute of Medical Physics, University of Erlangen-Nuremberg, Erlangen, Germany.

METHODS

In this cross-sectional consecutive study, 3 topographic measurements were taken. Eyes were grouped by age in years (A: <or=29; B: 30 to 39; C: 40 to 49; D: 50 to 59; E: 60 to 69; F: >or=70), sex, and refraction.

RESULTS

The study comprised 487 eyes (205 men, 288 women; age 17 to 81 years). The mean Q of the anterior corneal surface was -0.22 +/- 0.14 (SD) overall, -0.21 +/- 0.12 in Group A, -0.25 +/- 0.11 in Group B, -0.21 +/- 0.15 in Group C, -0.23 +/- 0.14 in Group D, -0.19 +/- 0.17 in Group E, -0.20 +/- 0.15 in Group F, -0.23 +/- 0.13 in men, -0.21 +/- 0.14 in women, -0.19 +/- 0.14 in hyperopes (n = 166; >+0.50 to +6.50 diopters [D]), -0.23 +/- 0.13 in emmetropes (n = 162; -0.50 to +0.50 D), and -0.23 +/- 0.15 in myopes (n = 165; <-0.50 to -8.00 D). There was no significant correlation between Q and age; Q differed significantly between men and women (P = .005), hyperopes and emmetropes (P<.0001), and hyperopes and myopes (P = .001).

CONCLUSIONS

There were high interindividual variations in the Q value. Thus, proper correction of spherical aberration with intraocular lenses (IOLs) requires sophisticated selection of the asphericity of IOL surfaces based on biometric data and individual corneal Q values.

A pilot study on the differences in wavefront aberrations between two ethnic groups of young generally myopic subjects

A comparative population-based cross-sectional study design was used to examine the prevalence of wavefront patterns in two different ethnic groups, and the relationship of these patterns with ocular biometrics and gender. The Shin-Nippon SRW5000 open field autorefractor, the Wavefront Analysis Supported Customized Ablation (WASCA) wavefront analyser and the IOLMaster were used to determine wavefront aberrations, mean spherical equivalent (SE) refractive error and axial length (AL). Seventy-four eyes from 74 young healthy subjects (44 British Asians, 30 Caucasians; 36 men, 38 women; mean age 22.51 +/- 3.89 years) with mean SE averaging -1.90 +/- 2.76 D (range -10.88 to +2.19 D) were examined. Relationships between ethnicity, gender, AL and SE, against the wavefront high-order root mean square, and aberration components up to the fifth order, were assessed by using multiple regression and correlation analysis. AL on its own accounted for 4.7% of the variance in trefoil component Z(-3)(3) (F(1,72) = 4.602; p = 0.035), 13.7% of coma component Z(1)(3) (F(1,72) = 12.536; p = 0.001), 6.1% of trefoil component Z(3)(3) (F(1,72) = 5.705; p = 0.020) and 9.8% of coefficient Z(-2)(4) (F(1,72) = 8.908; p = 0.004). A significant model emerged (F(2,71) = 6.164; p = 0.003) for ethnicity and axial length, accounting for 12.4% of variance in primary spherical aberration with ethnicity accounting for 8.4% of that variance. For Caucasian subjects, a significant correlation was found between axial length and (Pearson's correlation coefficient -0.500; p = 0.005) and Z(0)(4) (Pearson's correlation coefficient -0.423; p = 0.020). For British Asian subjects, AL was only correlated with coefficient Z(-2)(4) (Pearson's correlation coefficient -0.358; p = 0.017). Ethnicity is a factor to be considered in the variability of wavefront aberration, particularly spherical aberration. Relationship between AL and wavefront aberrations seems to vary between ethnicities. If higher order aberrations play a role in the emmetropization process, this may be different for different populations.

Fluctuations in intraocular pressure and the potential effect on aberrations of the eye

OBJECTIVE

To investigate the fluctuations in intraocular pressure during the day and to see if these are associated with changes in corneal shape and in the patterns of ocular aberrations.

METHODS

Intraocular pressure, corneal curvature, refractive error, spherical equivalent and aberrations (defocus (sphere); cylinder (astigmatism); coma, trefoil and third order spherical aberration) were measured in 17 healthy subjects three times during the day. The first measurement was made between 9:00 and 9:30, the second at midday (12:30-13:00) and the third in the afternoon (17:00-17:30). Aberrations, corneal shape, refractive error and pupil size (for which correction was made) were measured with an Irx3 Dynamic Wavefront Aberrometer. Intraocular pressures were measured using a non-contact tonometer (Cambridge Instruments Inc.) and calibrated with the Goldmann applanation tonometer.

RESULTS

Variations in intraocular pressures were unrelated to age or refractive error. Statistically significant differences in intraocular pressure between morning and midday as well as between midday and afternoon were found. Intraocular pressure variations between midday and afternoon were associated with changes in spherical equivalent, corneal radius of curvature and aberrations (defocus, cylinder, coma, trefoil and spherical aberration) over the same time period. Aberration patterns varied between individuals, and no association was found between two eyes of the same subject.

CONCLUSIONS

Changes in intraocular pressure have no noticeable effect on image quality. This could be because the eye has a compensating mechanism to correct for any effect of ocular dynamics on corneal shape and refractive status. Such a mechanism may also affect the pattern of aberrations or it may be that aberrations alter in a way that offsets any potentially detrimental effects of intraocular pressure change on the retinal image. Variations in patterns of aberrations and how they may be related to ocular dynamics need to be investigated further before attempts at correction are made.

Optical analysis of an accommodating intraocular lens

PURPOSE

To model the feasibility of an accommodating intraocular lens (IOL) that allows near vision by means of an anterior translation within the capsular bag.

SETTING

University of Waterloo, Waterloo, Ontario, Canada.

METHODS

Model eyes were constructed and analyzed based on experimental data using Code V, a computerized optical design tool. The potential near vision of IOLs of different powers was calculated as they were moved anteriorly within the capsular bag. The conditions under which a spherical lens performs well and when an aspheric design should be considered were determined.

RESULTS

Accommodation (the dioptric change from the far to the near point) varies linearly with lens movement and is sensitive to corneal and IOL powers. Simple equations were derived and accurately predicted induced accommodation. Retinal image quality varies significantly with pupil size and IOL power. However, image quality is minimally affected by the amount of induced accommodation.

CONCLUSIONS

From an optical standpoint accommodation with adequate image quality can be achieved by anterior movement of a spherical IOL within the capsular bag.

Aberrations and myopia

It has been suggested that high levels of axial aberration or specific patterns of peripheral refraction could play a role in myopia development. Possible mechanisms involving high levels of retinal image blur caused by axial aberrations include form deprivation through poor retinal image quality in distance vision, enhanced accommodative lags favouring compensatory eye growth, and an absence of adequate directional cues to guide emmetropization. In addition, in initially emmetropic eyes, hyperopia in the retinal periphery may result in local compensatory eye growth, which induces axial myopia. Evidence in support of these ideas is reviewed and it is concluded that, for any fixed pupil diameter, evidence for higher levels of axial aberration in myopes in comparison with other refractive groups is weak, making involvement of axial aberrations in myopization through image degradation at the fovea unlikely. If, however, some potential myopes had unusually large pupil diameters, their effective aberration levels and associated retinal blur would be larger than those of the rest of the population. There is stronger evidence in favour of differences in patterns of peripheral refraction in both potential and existing myopes, with myopes tending to show relative hyperopia in the periphery. These differences appear to be related to a more prolate eyeball shape. Longitudinal studies are required to confirm whether the retinal defocus associated with the peripheral hyperopia can cause patterns of eyeball growth which lead to axial myopia.

Refractive and Biometric Changes With Silicone Hydrogel Contact Lenses

PURPOSE

This study reports data from an 18-month longitudinal study of neophyte contact lens wearers and compares changes in ocular refraction and biometry induced by daily wear and continuous wear of two different silicone hydrogel (SiH) materials.

METHODS

Forty-five subjects were enrolled in the study and randomly assigned to wear one of the two silicone hydrogel materials: Lotrafilcon A or Balafilcon A lenses on either a daily or continuous wear basis. Measurements of objective refraction, axial length, anterior chamber depth, corneal curvature, and the rate of peripheral corneal flattening were performed before and 1, 3, 6, 12, and 18 months after initial fitting.

RESULTS

Mean spherical equivalent refractive error increased in the myopic direction in all contact lens groups across time (p < 0.001). Axial length was the main biometric contributor to the development of myopia. After 18 months of lens wear, subjects in the Lotrafilcon A group showed the greater mean increase in myopia (i.e., -0.50 D).

CONCLUSIONS

The results of this study show that increases in myopia, similar if not higher than those found to occur normally in young adult noncontact lens wearers, still occur with silicone hydrogel contact lens wear. The main biometric contributor to the progression of myopia was an increase in axial length. Differences between our results and those of previous studies with silicone hydrogel contact lenses could be attributed to the differing populations used in which both age and occupation may have played a role.

Relationship between refractive error and monochromatic aberrations of the eye

PURPOSE

To examine the relationship between ametropia and optical aberrations in a population of 200 normal human eyes with refractive errors spanning the range from +5.00 to -10.00 D.

METHODS

Using a reduced-eye model of ametropia, we tested the hypothesis that the optical system of the eye is uncorrelated with the degree of ametropia. These predictions were evaluated experimentally with a Shack-Hartmann aberrometer that measured the monochromatic aberrations across the central 6 mm of the dilated pupil in well-corrected, cyclopleged eyes.

RESULTS

Optical theory predicted, and control experiments on a model eye verified, that Shack-Hartmann measurements of spherical aberration will vary with axial elongation of the eye even if the dioptric components of the eye are fixed. Contrary to these predictions, spherical aberration was not significantly different from emmetropic eyes. Root mean square of third-order aberrations, fourth-order aberrations, and total higher aberrations (third to 10th) in myopic and hyperopic eyes were also uncorrelated with refractive error. Astigmatic eyes tended to have larger total higher-order aberrations than nonastigmatic eyes.

CONCLUSIONS

We conclude that a reduced-eye model of myopia assuming fixed optical parameters and variable axial length is not tenable.

The effect of accommodation on ocular shape

PURPOSE

Ocular shape is altered in myopia, and accommodation during nearwork is a proposed risk factor for myopia. Using relative peripheral refractive error (RPRE), ocular shape was assessed before, during, and after a period of sustained nearwork to determine whether accommodation affects ocular shape.

METHODS

Measurements of RPRE at 30 degrees in the nasal visual field were obtained using the spherical equivalent calculated from Canon R-1 autorefraction. The RPRE of 41 young adults was measured on two separate occasions separated by at least 1 week to assess RPRE repeatability. Later, the RPRE of 22 young adults was measured at a 0 D accommodative stimulus and then at a 3 D stimulus level at 0, 1, and 2 h during which subjects performed sustained nearwork at 33 cm. After 2 h of nearwork, subjects had RPRE measured at prescribed time intervals over a 1-h period in which they looked in the distance (0 D stimulus).

RESULTS

The measurement of RPRE had adequate repeatability (mean difference +/- SD, -0.05 +/- 0.35 D) with +/- 0.68 D as the 95% limits of agreement. The onset of accommodation produced an immediate hyperopic shift of RPRE relative to baseline (+0.37 +/- 0.44 D; p = 0.0007), indicating that ocular shape had become more prolate. This shape remained unchanged after 1 h of sustained accommodation (RPRE difference from baseline, +0.25 +/- 0.55 D; p = 0.04) and then returned to baseline dimensions after 2h of accommodation (RPRE difference from baseline, +0.11 +/- 0.39 D; p = 0.21). At the 0 D stimulus level one minute after the period of nearwork, RPRE became more myopic relative to baseline (RPRE difference from baseline, -0.28 +/- 0.50 D; p = 0.016). Ocular shape returned to baseline dimensions after 45 min of accommodative relaxation.

CONCLUSIONS

Accommodation induced the ocular shape to become more prolate. The opposite occurred after accommodation was relaxed, namely a change toward a more oblate ocular shape. The transient nature of these changes suggests that tension on the choroid and choroidal hysteresis may play a role in influencing ocular shape.