Axes of astigmatism in fellow eyes show mirror rather than direct symmetry

Dioptric differences between clinically determined and metric-optimised refractions for adults with Down syndrome

PURPOSE

Refractions based on the optimisation of single-value wavefront-derived metrics may help determine appropriate corrections for individuals with Down syndrome where clinical techniques fall short. This study compared dioptric differences between refractions obtained using standard clinical techniques and two metric-optimised methods: visual Strehl ratio (VSX) and pupil fraction tessellated (PFSt), and investigated characteristics that may contribute to the differences between refraction types.

METHODS

Thirty adults with Down syndrome (age = 29 ± 10 years) participated. Three refractive corrections (VSX, PFSt and clinical) were determined and converted to vector notation (M, J0 , J45 ) to calculate the dioptric difference between pairings of each type using a mixed model repeated measures approach. Linear correlations and multivariable regression were performed to examine the relationship between dioptric differences and the following participant characteristics: higher order root mean square (RMS) for a 4 mm pupil diameter, spherical equivalent refractive error and Vineland Adaptive Behavior Scales (a measure of developmental ability).

RESULTS

The least squares mean estimates (standard error) of the dioptric differences for each pairing were as follows: VSX versus PFSt = 0.51 D (0.11); VSX versus clinical = 1.19 D (0.11) and PFSt versus clinical = 1.04 D (0.11). There was a statistically significant difference in the dioptric differences between the clinical refraction and each of the metric-optimised refractions (p < 0.001). Increased dioptric differences in refraction were correlated with increased higher order RMS (R = 0.64, p < 0.001 [VSX vs. clinical] and R = 0.47, p < 0.001 [PFSt vs. clinical]) as well as increased myopic spherical equivalent refractive error (R = 0.37, p = 0.004 [VSX vs. clinical] and R = 0.51, p < 0.001 [PFSt vs. clinical]).

CONCLUSIONS

The observed differences in refraction demonstrate that a significant portion of the refractive uncertainty is related to increased higher order aberrations and myopic refractive error. Methodology surrounding clinical techniques and metric-optimisation based on wavefront aberrometry may explain the difference in refractive endpoints.

Meridional binocular rivalry reveals a trace of uncorrected oblique input during development in the adult brain

Binocular rivalry (BR) is a visual perception phenomenon that occurs when each eye perceives different images and stimuli, causing alternating monocular dominance. To measure BR, many studies have used two monocular conflicting images to induce monocular alternations. Here we chose a group of participants with oblique astigmatism (OA) and who produced blur on the orthogonal oblique meridian in each eye, resulting in two conflicting images, which may enhance the stimulation of monocular alternations. Our results show that OA participants tend to have a high rate of BR when viewing natural images, whereas the control group does not have BR for the same images. We suggest that this low ability to fuse could indicate the presence of a trace due to uncorrected vision during the critical period, which could be retained in the adult brain.

Astigmatism profile in the elderly population: Tehran Geriatric Eye Study

PURPOSE

To determine the prevalence of astigmatism and its associated factors and examine astigmatism symmetry patterns in an elderly population.

STUDY DESIGN

Population based cross-sectional study.

METHODS

The present population-based cross-sectional study was conducted on an elderly population above 60 years of age in Tehran, Iran in 2019. The sampling was done using the stratified multistage random cluster sampling method. All study participants underwent a complete optometric examination and slit-lamp biomicroscopy.

RESULTS

The prevalence of astigmatism higher than -0.50, -1.00, and -2.00 D was 83% (95% CI: 81 -84), 52 % (95% CI: 50 -54), and 19% (95% CI: 17-20), respectively. These prevalence was 79%(95% CI: 77-81), 46(95% CI: 44-49) and 14(95% CI: 13-16) in subjects without a history of ocular surgery, respectively. Based on cylinder power worse than -1.00 D, 10% (95% CI: 9-12), 20% (95% CI: 18-22), and 21% (95% CI:19-23) of study participants had with the rule, against the rule, and oblique astigmatism, respectively. According to the results of the multiple regression model, male gender, older age, low education level, pure posterior subcapsular cataract, pseudophakia, and myopia were independent factors associated with astigmatism. The prevalence of anisorule astigmatism was 57 % (95% CI: 56-59), the most common anisorule astigmatism was against the rule-oblique type with a prevalence of 36% (95% CI: 34-38).

CONCLUSION

The prevalence of astigmatism was high in the elderly population of Tehran. More than half of the participants in this study had anisorule astigmatism, and against the rule-oblique combination was more prevalent than other types. A posterior subcapsular cataract, a history of cataract surgery, and myopia were the associated factors of astigmatism in this study.

Long-term outcome of refractive errors in patients with congenital blepharoptosis who have undergone ptosis surgery

CLINICAL RELEVANCE

Ptosis surgery induces some changes in pre-existing refractive errors and astigmatism. Monitoring refractive outcomes, planning of vision rehabilitation, and amblyopia treatment may be required following ptosis surgery.

BACKGROUND

The few studies published on the outcomes of refractive error after ptosis surgery have been controversial. The aim of this study was to evaluate long-term outcomes of refractive error in patients with congenital blepharoptosis who had undergone ptosis surgery.

METHODS

Patients with congenital blepharoptosis who had undergone ptosis surgery were enrolled in the study. Data on pre- and post-operative refractive errors, marginal reflex distance 1 (MRD1), and levator function were obtained. The data from fellow eyes were used as the control.

RESULTS

The mean patient age of patients undergoing ptosis surgery was 19.8 ± 8 years. The mean follow-up time (time between the operation and post-up measurements) was 4.7 ± 2.1 years with a range of 2-10 years. There were no significant differences in mean refractive changes, being 0.53 ± 0.40D vs. 0.36 ± 0.45D in sphere (p = 0.19) and 0.48 ± 0.56 vs. 0.30 ± 0.23 in cylinder (p = 0.17) in operated and control eyes, respectively. Astigmatism change was, however, significantly higher in the operated eyes with an MRD1 change of ≥2.5 mm (0.84 ± 0.66 vs. 0.27 ± 0.39, p = 0.024). The changes in astigmatism in the operated eyes had a significantly positive correlation with the changes of MRD1 (r = 0.497, p = 0.019). Vectorial analysis showed no significant difference between the changes in astigmatism of the operated and control eyes (0.58 *48º vs. 0.45*53º, respectively).

CONCLUSION

Spherical power of the eyes of patients with congenital ptosis who undergo ptosis surgery does not change. A significant change in astigmatism can be anticipated in eyes with more than 2.5 mm change in MRD1. There is an association between change in astigmatism and MRD1 following ptosis surgery.

Prediction of postoperative refractive astigmatism before toric intraocular lens implantation

Background

To determine the preoperative factors influencing refractive astigmatism after cataract surgery for astigmatism correction by toric intraocular lens (IOL) implantation and to evaluate the prediction model using these factors.

Methods

Prospective, observational case series. The right eyes of forty consecutive patients with preoperative corneal astigmatism of the total cornea of 1.5 diopters (D) or more in magnitude and scheduled for implantation of a non-toric IOL during cataract surgery with a 2.4-mm temporal clear corneal incision were examined prospectively. The vertical/horizontal astigmatism component (J0) and oblique astigmatism component (J45) of refractive and corneal astigmatism were converted using power vector analysis. Multivariate regression analysis was performed with refractive astigmatism at three months postoperatively as the dependent variable, and preoperative parameters including age, sex, refractive astigmatism, corneal astigmatism, sphere, spherical equivalent, intraocular pressure, corneal thickness, anterior chamber depth, lens thickness, lens positions (tilt and decentration), axial length, and corneal higher order aberrations as independent variables. The root mean square (RMS) errors were calculated to express the regression model fit.

Results

The regression model for the J0 component was \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ Postoperative\kern0.34em refractive\kern0.2em J0=1.05\times Coneal\kern0.2em J0-0.14 $$\end{document}PostoperativerefractiveJ0=1.05×ConealJ0−0.14 (R² = 0.96, P < 0.001). The model for the J45 component was \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ Postoperative\kern0.34em refractive\kern0.2em J45=0.68\times Coneal\kern0.2em J45+0.19\times Preoperative\kern0.34em refractive\kern0.2em J45-0.06 $$\end{document}PostoperativerefractiveJ45=0.68×ConealJ45+0.19×PreoperativerefractiveJ45−0.06 (R² = 0.72, P < 0.001). The mean RMS errors for preoperative corneal astigmatism alone and the multivariate model were 0.58 D and 0.46 D, respectively. There was a statistically significant difference between them (P = 0.02).

Conclusions

Refractive astigmatism after implantation of a toric IOL can be predicted by the regression model more accurately than by corneal astigmatism alone. However, the prediction of oblique astigmatism remains a challenge.

Distorted optical input affects human perception

Collinear facilitation, the mechanism for grouping contour elements, is a process involving lateral interactions that improve the detectability of a target by the presence of collinear flankers. It was shown that the development of collinear facilitation is experience dependent and that it may be impaired when the visual input is distorted in one meridian (meridional amblyopia). In oblique astigmatism, the blurring is on the opposite oblique meridian in both eyes, resulting in two conflicting images, which may affect the development of binocular vision. We hypothesized that the collinear facilitation of adults with oblique astigmatism is reminiscent of the abnormal development of the lateral facilitation of meridional amblyopia. We explored the perception of binocular vision and collinear facilitation in cases of both distorted and non-distorted vision. Fully corrected participants that tested for the target contrast detection of Gabor patches and two collinear flankers, presented for 80 ms, were positioned at different orientations (0° (180°), 45°, 90°, and 135°) and for different eyes (monocular, binocular). The results show a significant anisotropy for monocular collinear facilitation between the blured and the clear meridians, being lower in the blurriest meridian than in the clearest meridian, resembling the meridional amblyopia results. Collinear facilitation results in poor binocular summation between the monocular channels. Our results indicate that the perceptual behavior was similar to that of meridional amblyopic subjects having an anisotropy of collinear facilitation between cardinal meridians in oblique astigmatic subjects.

Multivariate Regression Analysis to Predict Postoperative Refractive Astigmatism in Cataract Surgery

Purpose

To assess the correlation between postoperative refractive astigmatism and preoperative parameters in cataract surgery.

Methods

Left eyes of 100 consecutive patients scheduled for cataract surgery with a 2.4 mm clear corneal incision were examined prospectively. Refractive astigmatism was measured using an autokerato/refractometer. Corneal astigmatism of the total cornea was calculated using a Scheimpflug camera. The vertical/horizontal component (J0) and oblique component (J45) of refractive and total corneal astigmatism were determined using power vector analysis. Refractive astigmatism at 8 weeks postoperatively was estimated using multivariate linear regression analysis. Independent variables analyzed included age, sex, refractive astigmatism, total corneal astigmatism, sphere, intraocular pressure, corneal thickness, anterior chamber depth, lens thickness, axial length, and pupil diameter.

Results

Multivariate regression analysis identified total corneal J0 and age as significant contributors to postoperative refractive J0 (P < 0.001 and P=0.029, respectively). The standard partial regression coefficients in the multiple regression analysis were 0.59 and −0.16 for total corneal J0 and age, respectively. Significant contributors to postoperative refractive J45 were total corneal J45 and lens thickness (P < 0.001 and P=0.015, respectively). The standard partial regression coefficients were 0.79 and −0.15 for total corneal J45 and lens thickness, respectively.

Conclusion

These results suggest that preoperative total corneal astigmatism is the most significant predictor of postoperative refractive astigmatism when performing astigmatism correction in cataract surgery.

Interocular Symmetry of Fixation, Optic Disc, and Corneal Astigmatism in Bilateral High Myopia: The Shanghai High Myopia Study

Purpose

We investigate the interocular symmetry of fixation, optic disc, and corneal astigmatism in bilateral high myopia, and evaluate the predictive relationships between them.

Methods

We enrolled 202 cases with bilateral high myopia. Fixation, in terms of the bivariate contour ellipse area (BCEA), was evaluated with the Macular Integrity Assessment microperimetry. Optic disc features, including orientation, tilt, and rotation, were evaluated with ultrawide-field retinal photographs. Corneal topography was performed with Pentacam. Interocular symmetry of fixation, optic disc, and corneal astigmatism was assessed, and the predictive relationships between these parameters were investigated.

Results

Axial length differences between the two eyes were: ≥0 to ≤1 mm, 67.8%; 1 to ≤2 mm, 20.3%; 2 to ≤3 mm, 9.4%; and >3 mm, 2.5%. Axial length, 95% BCEA, and magnitude of corneal astigmatism showed good interocular symmetry, whereas the optic disc tilt, rotation, and axis of corneal astigmatism (mirror axes) showed less symmetry (all P < 0.05). No interocular symmetry was observed in the direction of the fixation ellipse. In both eyes, the corneal steep meridian more often was consistent with the optic disc orientation than inconsistent (right eye [OD], P < 0.001; left eye [OS], P = 0.029).

Conclusions

As different parameters presented different degrees of symmetry, cautions are needed when including both eyes or only one lateral eye in cases of bilateral high myopia for clinical investigations. The optic disc orientation, to some extent, may indicate the steep meridian of the cornea.

Translational Relevance

Our study provided evidences for selection of eye laterality in clinical investigations of highly myopic eyes.

Enantiomorphism and rule similarity in the astigmatism axes of fellow eyes: A population-based study

PURPOSE

To evaluate the relationship patterns between astigmatism axes of fellow eyes (rule similarity and symmetry) and to determine the prevalence of each pattern in the studied population.

METHODS

This population-based study was conducted in 2015 in Iran. All participants had tests for visual acuity, objective refraction, subjective refraction (if cooperative), and assessment of eye health at the slit-lamp. Axis symmetry was based on two different patterns: direct (equal axes) and mirror (mirror image symmetry) or enantiomorphism. Bilateral astigmatism was classified as isorule if fellow eyes had the same orientation (e.g. both eyes were with-the-rule) and as anisorule if otherwise.

RESULTS

Of the total cases of bilateral astigmatism, 80% were isorule, and in the studied population, the prevalence of isorule and anisorule astigmatism was 14.89% and 3.53%, respectively. The prevalence of isorule increased with age (p<0.001). The prevalence of both isorule and anisorule increased at higher degrees of spherical ametropia (p<0.001). Median inter-ocular axis difference was 10° in mirror symmetry and 20° in direct symmetry with no significant difference between two genders (p>0.288). Both symmetry patterns reduced with age (p<0.001). Among cases of bilateral astigmatism, 15.5% and 19.8% had exact direct and mirror symmetry, respectively.

CONCLUSION

Bilateral astigmatism is mainly isorule in the population and anisorule astigmatism is rare. The enantiomorphism is the most common pattern in the population of bilateral astigmatism.

Assessment of Pattern and Shape Symmetry of Bilateral Normal Corneas by Scheimpflug Technology

Purpose: The aim of this study was to assess bilateral symmetry in normal fellow eyes by using optical and geometric morphometric parameters. Methods: All participants underwent complete biocular examinations. Scheimpflug tomography data from 66 eyes of 33 patients were registered. The interocular symmetry was based on five patterns: morphogeometric symmetry, axial symmetry at the corneal vertex, angular-spatial symmetry, direct symmetry (equal octants), and enantiomorphism (mirror octants). Results: No statistically significant differences were found between right and left eyes in corneal morphogeometric (p ≥ 0.488) and aberrometric parameters (p ≥ 0.102). Likewise, no statistically significant differences were found in any of the axial symmetry parameters analyzed (p ≥ 0.229), except in the surface rotation angle beta (p = 0.102) and translation coordinates X0 and Y0 (p < 0.001) for the anterior corneal surface, and the rotation angle gamma (p < 0.001) for the posterior surface. Similarly, no statistically significant differences were identified for direct symmetry (p ≥ 0.20) and enantiomorphism (p ≥ 0.75), except for some elevation data in the posterior surface (p < 0.01). Conclusions: The level of symmetry of both corneas of a healthy individual is high, with only some level of disparity between fellow corneas in rotation and translation references. Abnormalities in this pattern of interocular asymmetry may be useful as a diagnostic tool.

Corneal astigmatism in cataract surgery patients from Bosnia and Herzegovina

PURPOSE

To determine corneal astigmatism prevalence, its correlations with age and symmetry pattern in fellow eyes of patients undergoing cataract surgery.

METHODS

This is a clinical-based retrospective cross-sectional study. Keratometry measurements of patients undergoing cataract surgery assigned to University Clinical Center Tuzla, Bosnia and Herzegovina, between January 2011 and June 2012 were recorded and analyzed retrospectively.

RESULTS

The study consisted of 4080 eyes of 2205 consecutive cataract surgery patients with a mean age of 68.24 years ± 9.25 (SD) (range 32-84 years), and 54.0% of the patients were women. Mean corneal astigmatism was 0.72 ± 0.61 D (range 0-6.5 D). The prevalence of corneal astigmatism 1.0 D or more was in 1291 eyes (31.64%), 1.5 D or more in 736 eyes (18.03%), 2.0 D or more in 396 eyes (9.71%) and 3.0 D or more in 108 eyes (2.65%). There was no significant difference in the magnitude of astigmatism between age groups (p = 0.10), male and female (p = 0.29) or right and left (p = 0.75) eyes. The prevalence and amount of astigmatism increased with age (p < 0.05). Gradual shift from with-the-rule astigmatism toward against-the-rule astigmatism was observed (p = 0.03). Patients with higher amount of astigmatism in one eye are more likely to have significant astigmatism in fellow eye (p < 0.01). Symmetry in pairs of eyes is present in eyes with astigmatism greater than 2.5 D (p < 0.01).

CONCLUSION

This study provides useful reference data for cataract surgeons and patients from Bosnia and Herzegovina.

An alternative clinical routine for subjective refraction based on power vectors with trial frames

PURPOSE

Subjective refraction determines the final point of refractive error assessment in most clinical environments and its foundations have remained unchanged for decades. The purpose of this paper is to compare the results obtained when monocular subjective refraction is assessed in trial frames by a new clinical procedure based on a pure power vector interpretation with conventional clinical refraction procedures.

METHODS

An alternative clinical routine is described that uses power vector interpretation with implementation in trial frames. Refractive error is determined in terms of: (i) the spherical equivalent (M component), and (ii) a pair of Jackson Crossed Cylinder lenses oriented at 0°/90° (J₀ component) and 45°/135° (J₄₅ component) for determination of astigmatism. This vector subjective refraction result (VR) is compared separately for right and left eyes of 25 subjects (mean age, 35 ± 4 years) against conventional sphero-cylindrical subjective refraction (RX) using a phoropter. The VR procedure was applied with both conventional tumbling E optotypes (VR1) and modified optotypes with oblique orientation (VR2).

RESULTS

Bland-Altman plots and intra-class correlation coefficient showed good agreement between VR, and RX (with coefficient values above 0.82) and anova showed no significant differences in any of the power vector components between RX and VR. VR1 and VR2 procedure results were similar (p ≥ 0.77).

CONCLUSIONS

The proposed routine determines the three components of refractive error in power vector notation [M, J₀ , J₄₅ ], with a refraction time similar to the one used in conventional subjective procedures. The proposed routine could be helpful for inexperienced clinicians and for experienced clinicians in those cases where it is difficult to get a valid starting point for conventional RX (irregular corneas, media opacities, etc.) and for refractive situations/places with inadequate refractive facilities/equipment.

Comprehensive profile of bilateral astigmatism: rule similarity and symmetry patterns of the axes in the fellow eyes

PURPOSE

To study rule similarity (isorule or anisorule) and symmetry patterns of axes (direct and mirror) in bilateral astigmatism.

METHODS

Six years of data were collected from four ophthalmology clinics in Tehran. After applying the exclusion criteria (having eye disease or history of ocular surgery), final analyses were performed on data of 160 608 cases whose mean age was 39.2 ± 15.4 years (10-80 years) and 53.7% female. All subjects had autorefraction, retinoscopy, and subjective refraction data. Rule similarity was categorised as isorule (with, against or oblique astigmatism) if the orientation type in fellow eyes was the same and anisorule if they were different.

RESULTS

The prevalence of isorule astigmatism was 82.9% and was similar in men and women (p = 0.44). The prevalence of isorule astigmatism decreased with age and increased with greater spherical ametropia (p < 0.001). The median of the absolute inter-ocular axis difference from exact symmetry was 10° under the mirror symmetry model (mirror axes). Under the direct symmetry model (equal axes), the median of the absolute inter-ocular axis difference was significantly higher, at 20° (p < 0.001). 16.4% and 10.8% had exact mirror and direct symmetry, respectively. In addition, 68.0% of cases with mirror symmetry and 47.1% with direct symmetry were within ±15 degrees of exact symmetry.

CONCLUSION

The results of the present study showed that bilateral astigmatism was mostly isorule. Bilateral symmetry of the astigmatism axes was highly prevalent and mostly of the mirror pattern. High astigmatic cases showed better symmetry.

Astigmatism among myopics and its changes from childhood to adult age: a 23-year follow-up study

PURPOSE

To study the prevalence of and changes in astigmatism from the onset of myopia at school age.

METHODS

Two hundred and forty myopic schoolchildren (mean age 10.9 years), with no previous spectacles, were recruited during 1983-1984 to a randomized 3-year clinical trial of bifocal treatment of myopia. Three annual examinations with subjective cycloplegic refraction were performed for 237-238 subjects. Subsequent examinations were performed at the mean ages of 23.2 and 33.9 years for 178 and 163 subjects, and the last examination, including data from prescriptions of different ophthalmologists, for 32 subjects. Corneal topography was studied at baseline, at the 3-year follow-up and at the two adulthood follow-ups. Prevalence and changes in refractive astigmatism (RA), in its polar values J0 and J45, and corneal astigmatism (CA) were studied.

RESULTS

Mean RA of the right eye increased during follow-up from 0.26 D (SD) ± 0.30 to 0.79 D ± 0.74. Mean CA was 1.07 D ± 0.74 at study end. The prevalence of RA ≥0.25 or ≥1.00 D increased from 54.9 and 3.8% to 83.4 and 34.4%, respectively. The main direction of the axis of RA and its polar value J0 and CA changed mainly through sphericity, from against the rule (ATR) to with the rule during the follow-up. There was a negative correlation between RA and spherical refraction in the ATR group at end of follow-up. Changes in RA were associated with increase in myopia and with changes in CA.

CONCLUSIONS

The prevalence and mean amount of RA associated with CA increased, and the axis of astigmatism changed among myopics during the 23-year follow-up.

[The cornea in high axial myopia]

PURPOSE

To compare corneal characteristics of eyes with high myopia with those of eyes with no spherical ametropia using Orbscan and ultrasonic pachymetry.

METHODS

Orbscan and ultrasonic pachymetry values were prospectively recorded in a study group of 105 patients with high myopia (i.e., axial length greater than 26mm in both eyes) and in a control group of 105 patients with no spherical ametropia (absolute value of spherical equivalent less than 1.25D regardless of cylinder value). Astigmatism data were expressed by rectangular coordinates in a dioptric plane. Axis was decomposed in 2 components (WTR/ATR and oblique) which were analyzed by Cos2axis and Sin2axis trigonometric functions. Enantiomorphism (mirror-image symmetry) between fellow eyes was quantified by a Euclidean distance for the location of the thinnest point and by the difference (in absolute value) between 180° and the sum of both axes for astigmatism.

RESULTS

In the study group, the mean axial length and subjective spherical equivalent were, respectively, 27.82±2.14mm (26.00 to 34.06) and -9.00±3.46D (-4.71 to -19.82). The mean corneal astigmatism was +0.92D×91.3° in the study group and +0.65D×89.3° in the control group. The mean corneal cylinder was higher in the study group (1.44D versus 0.91D; P<0.001) whereas axis showed no significant differences between both groups. The mean maximal keratometry was steeper in the study group (44.53D versus 44.13D; P=0.03) whereas the mean keratometry and minimal keratometry displayed no significant differences between both groups. No significant differences in central corneal thickness (540.2μm versus 546.9μm; P=0.10), peripheral corneal thickness, corneal diameter, corneal irregularity, asphericity, and irregular astigmatism were found between both groups. There were no significant differences in enantiomorphism parameters between both groups. In the study group, correlation with axial length was significant only for spherical equivalent (r=-0.86; P<0.001) and corneal cylinder (r=0.16; P=0.04).

CONCLUSION

High myopia exhibits corneal characteristics similar to corneas of eyes with no spherical ametropia, except for toricity. While the posterior segment elongates, development of high myopia appears not to affect corneal characteristics. Corneal toricity may be associated with evolution toward high myopia by perturbing mechanisms of emmetropization.

Interocular symmetry analysis of bilateral eyes

The purpose of this study was to evaluate the interocular symmetry of several biometric parameters between both eyes. The symmetry between the right and left eye of 397 subjects in 14 biometric parameters, spherical equivalent of refractive error (SE), Jackson crossed cylinder power of refractive error astigmatism with axes at 90° and 180° (RJ0) and at 45° and 135° (RJ45), best-corrected visual acuity (BCVA), average corneal curvature (CC), Jackson crossed cylinder power of corneal astigmatism (CJ0 and CJ45), corneal asphericity coefficient (Q), intraocular pressure (IOP), central corneal thickness (CCT), axial length (AL), anterior chamber depth (ACD), lens thickness (LT) and vitreous chamber depth (VCD), was assessed by comparative data analysis. Aside from RJ0 (p = 0.00), RJ45 (p = 0.02) and Q (p = 0.00), the overall interocular differences of other biometric parameters between fellow eyes were not significant (p > 0.05). The interocular correlation and Bland-Altman plots showed a good agreement between fellow eyes in 14 biometric parameters. Correlations between interocular differences in SE and that in RJ0 (p = 0.03), CC (p = 0.00), AL (p = 0.00) and VCD (p = 0.00) were statistically significant. There were similar strong linear relationships between refractive error astigmatism vectors and corneal astigmatism vectors in bilateral eyes. There were negative correlations of RJ45 and CJ45 between bilateral eyes. A potentially clinically important interocular symmetry in SE, BCVA, CC, CJ0, CJ45, IOP, CCT, AL, ACD, LT and VCD is found in this research, while the differences of RJ0, RJ45 and Q between left and right eyes seem a bit large. The negative interocular relationships of RJ45 and CJ45 demonstrate moderate mirror symmetry exists among fellow eyes. High interocular symmetry in bilateral eyes may be helpful in intraocular lens power calculation, intraocular pressure evaluation, post-operative visual acuity and refraction prediction at the time the fellow eye is undergoing refractive surgery.

Evaluation of the shape symmetry of bilateral normal corneas in a Chinese population

PURPOSE

To investigate the bilateral symmetry of the global corneal topography in normal corneas with a wide range of curvature, astigmatism and thickness values.

DESIGN

Cross-Sectional Study.

METHODS

Topography images were recorded for the anterior and posterior surfaces of 342 participants using a Pentacam. Elevation data were fitted to a general quadratic model that considered both translational and rotational displacements. Comparisons between fellow corneas of estimates of corneal shape parameters (elevation, radius in two main directions, Rx and Ry, and corresponding shape factors, Qx and Qy) and corneal position parameters (translational displacements: x0, y0 and z0, and rotational displacements: α, β and γ) were statistically analyzed.

RESULTS

The general quadratic model provided average RMS of fit errors with the topography data of 1.7±0.6 µm and 5.7±1.3 µm in anterior and posterior corneal surfaces. The comparisons showed highly significant bilateral correlations with the differences between fellow corneas in Rx, Ry, Qx and Qy of anterior and posterior surfaces remaining insignificantly different from zero. Bilateral differences in elevation measurements at randomly-selected points in both corneal central and peripheral areas indicated strong mirror symmetry between fellow corneas. The mean geometric center (x0, y0, z0) of both right and left corneas was located on the temporal side and inferior-temporal side of the apex in anterior and posterior topography map, respectively. Rotational displacement angle α along X axis had similar distributions in bilateral corneas, while rotation angle β along Y axis showed both eyes tilting towards the nasal side. Further, rotation angle γ along Z axis, which is related to corneal astigmatism, showed clear mirror symmetry.

CONCLUSIONS

Analysis of corneal topography demonstrated strong and statistically-significant mirror symmetry between bilateral corneas. This characteristic could help in detection of pathological abnormalities, disease diagnosis, measurement validation and surgery planning.

Characteristics of astigmatism as a function of age in a Hong Kong clinical population

PURPOSE

To characterize astigmatism as a function of age in a Hong Kong clinical population.

METHODS

All records from new clinical patients at a university optometry clinic in the year 2007 were used for the study. Only data from subjects with corrected visual acuity ≥6/9 in both eyes and with completed subjective refraction were analyzed. The subjects were divided into seven age groups by decade (i.e., 3 to 10 years, 11 to 20 years, …, >60 years). Refractive errors were decomposed into spherical-equivalent refractive error (M), J0, and J45 astigmatic components for analyses. Internal astigmatism was calculated by subtracting corneal astigmatism from refractive astigmatism (RA).

RESULTS

Of the 2759 cases that fulfilled our selection criteria, 58.9% had myopia (M ≥-0.75 D) and 28.4% had RA (Cyl ≥ 1.00 D). The prevalence of RA increased from 17.8% in the 3 to 10 years age group to 38.1% in the 21 to 30 years age group. It then dipped to 25.8% in 41 to 50 years age group but increased again to 41.8% in the >60 years age group. Among the astigmats, almost all 3- to 10-year-old children (92.6%) had with-the-rule (WTR) astigmatism, but a majority of the elderly (>60 years) had against-the-rule (ATR) astigmatism (79.7%). For a subset of subjects who had both subjective refraction and keratometric readings (n = 883), RA was more strongly correlated with corneal (r = 0.35 to 0.74) than with internal astigmatism (r = 0.01 to 0.35). More importantly, the magnitudes of both refractive and corneal J0 were consistent with synchronized decrements (-0.15 and -0.14 D per 10 years, respectively) after the age of 30 years, indicating that the shift toward more ATR astigmatism was related to corneal change.

CONCLUSIONS

In this Hong Kong Chinese clinical population, the prevalence rates of both myopia and astigmatism increased during the first three decades and shared a similar trend before the age of 50 years. The manifest astigmatism was mainly corneal in nature, bilaterally mirror symmetric in axis, and shifted from predominantly WTR to ATR with age.

[Relationships between refractive parameters: sphere, cylinder and axis]

PURPOSE

To analyze the relationships between refractive parameters as well as the relationship between fellow eyes in a normal population.

METHODS

Both subjective refraction and auto-refractometry data of 500 patients were prospectively recorded. Refraction data were analyzed on three rectangular coordinates in a dioptric space. WTR/ATR ratio of axis was quantified by the Cos2axis function. Enantiomorphism (mirror-image symmetry) between fellow eye axes was quantified by the absolute value of the difference between 180° and the sum of both axes.

RESULTS

Mean refraction and mean cylinder were -1.74D (+0.28D × 91.5°) and 0.81 ± 0.89D respectively. The spherical component had no significant influence on refractive astigmatism (r(s)≤ 0.07, P ≥ 0.07) except for high spherical ametropia. Eyes with spherical equivalent greater than 4D (in absolute value) demonstrated higher cylinder (1.15D vs 0.84D, P<0.001). Cylinder influenced the WTR/ATR ratio (r(s)=-0.25, P<0.001) and the enantiomorphism (r(s)=0.36, P<0.001). Age also influenced the WTR/ATR ratio (r(s)=0.27, P<0.001) and the enantiomorphism (r(s)=0.14, P<0.001). Axes were more likely WTR and enantiomorphic when the cylinder was high and the subject young. Oblique axes were less enantiomorphic (35.5° vs 20.6°, P<0.001) and were associated with lower cylinder (0.56D vs 0.98D, P<0.001). Correlation between fellow eyes was significant for cylinder (r(s)=0.66, P<0.001) and for spherical equivalent (r(s)=0.96, P<0.001). Gender had no significant influence on refraction (P>0.12) except for spherical equivalent (relative hyperopia of +0.17D, P=0.04 in females).

CONCLUSION

The spherical component of the refraction appears to be independent of the refractive astigmatism except for high spherical ametropia. Cylinder influences somewhat the WTR/ATR ratio of axis and the enantiomorphism. Relationships between refractive parameters are weak in comparison to the fellow eye relationships.