Monochromatic aberrations of the human eye and myopia

Associations between refractive error components and higher-order aberrations in simple myopia and compound myopic astigmatism

Aim

To investigate associations between refractive error components and higher-order aberrations (HOAs) in adult myopic subjects.

Methods

A total of 1370 myopia right eyes, aged 18-40, were included in a cross-sectional study. Subjective cycloplegic refractions and distance aberrometry measured with a Shack-Hartmann device were analyzed. Zernike components of horizontal coma (Z3 1), vertical coma (Z3 -1), oblique trefoil (Z3 3), vertical trefoil (Z3 -3), spherical aberration, and total root-mean-square (RMS) wave-front error for 6 mm pupil were analyzed. Pearson's correlations were calculated between sphero-cylindrical components and HOAs based on vector analysis for the astigmatism axis. Total subjects were divided into two subgroups: simple myopia (SMY, 648 eyes) and compound myopic astigmatism (CMA, 722 eyes). HOAs were compared between the two subgroups.

Results

Total RMS wave-front error correlates with spherical equivalent myopia (r = -0.1, P<0.05) and J45 (r = 0.1, P<0.001). J0 correlates positively with vertical coma (Z3 -1) (r = 0.1 p <0.001) and negatively with oblique trefoil (Z3 3) and vertical trefoil (Z3 -3), (r = -0.1, p < 0.001; r = -0.1, P < 0.05). The total RMS wavefront-error was larger in the CMA (|0.37| ± 0.18 µm) compared to the SMY (|0.34| ± 0.16 µm, P <0.001). The mean values of vertical coma (Z3 -1), vertical trefoil (Z3 -3), and oblique trefoil (Z3 3) differed between the two subgroups.

Conclusion

Total RMS wave-front error increases with increasing myopia and astigmatism. Increasing myopia power does not show a systematic correlation with HOAs components. A weak systematic correlation is suggested between astigmatism direction and third-order aberrations.

Longitudinal measures of peripheral optical quality in young children

PURPOSE

To assess longitudinal changes in optical quality across the periphery (horizontal meridian, 60°) in young children who are at high (HR) or low risk (LR) of developing myopia, as well as a small subgroup of children who developed myopia over a 3-year time frame.

METHODS

Aberrations were measured every 6 months in 92 children with functional emmetropia at baseline. Children were classified into HR or LR based on baseline refractive error and parental myopia. Zernike polynomials were calculated for 4 mm pupils, accounting for the elliptical shape of the pupil in the periphery. Various metrics were computed, including Strehl Ratios with only high-order aberrations (HO-SR). Primary spherical aberration (SA), horizontal coma and defocus were also analysed given their relevance in emmetropisation. The areas under the image quality metrics for various regions of interest were computed.

RESULTS

HO-SR were higher in children at HR and children with myopia, even when SA was removed from the Strehl Ratio (SR) calculation. SA was less positive in children at HR and children with myopia. Defocus was more negative in children at HR and children with myopia at all eccentricities and was even more negative when computed relative to the fovea, an effect that increased in the mid periphery. Relative peripheral defocus also became more negative over time in children at HR and children with myopia at the mid temporal retina. The other aberrations showed no significant changes in time overall.

CONCLUSIONS

This longitudinal study showed differences in HO-SR, SA and defocus in the central and near-peripheral retina (±20°) of young children at HR before they develop myopia compared with children at LR for myopia. The results may indicate these eccentricities are significant in providing signals for emmetropisation. The small changes noted over time may indicate that the differences are a cause of myopia development.

Corneal higher-order aberrations and their relationship with choroid in myopic patients

BACKGROUND

To investigate corneal higher-order aberrations (HOAs) and choroidal characteristics in myopic individuals and explore the association between HOAs and choroidal parameters.

METHODS

Myopic participants were categorized into three groups based on axial lengths (ALs). We compared corneal HOAs, including spherical (Z40), comatic (Z3 - 1 and Z31), and trefoil (Z3 - 3 and Z33) aberrations, as well as choroidal vascularity index (CVI) and choroidal thickness (CT). Linear regression analysis was used to assess the relationships among corneal HOAs, CVI, CT, spherical equivalent, and AL.

RESULTS

Groups 1, 2, and 3 included 105, 98, and 118 eyes, respectively. Group 3 exhibited lower spherical HOA root mean square and Z40 values than group 1(p < 0.05). Group 1 showed lower Z31 levels than other groups (p < 0.001). Groups 1 and 2 had higher mean, central, and I2 vertical CVIs than group 3 (p < 0.05). Group 1 had a larger vertical S1 CVI than group 3 (p < 0.05). Group 3 had smaller horizontal CVI values in all regions except N2 (p < 0.05). Both the mean and CT in all regions decreased as AL increased (p < 0.001). The comatic (Z31) and trefoil (Z33) components were predictors of mean horizontal CVI, and the comatic (Z31) component was correlated with both mean vertical and horizontal CT.

CONCLUSION

Longer AL myopic patients exhibited lower absolute values of spherical aberration and horizontal coma. Alterations in choroid in myopic patients correlated with corneal HOAs. Our results suggest a potential connection between the optical quality and ocular perfusion in myopia.

Central and Peripheral Ocular High-Order Aberrations and Their Relationship with Accommodation and Refractive Error: A Review

High-order aberrations (HOAs) are optical defects that degrade the image quality. They change with factors such as pupil diameter, age, and accommodation. The changes in optical aberrations during accommodation are mainly due to lens shape and position changes. Primary spherical aberration (Z(4.0)) is closely related to accommodation and some studies suggested that it plays an important role in the control of accommodation. Furthermore, central and peripheral HOAs vary with refractive error and seem to influence eye growth and the onset and progression of myopia. The variations of central and peripheral HOAs during accommodation also appear to be different depending on the refractive error. Central and peripheral high-order aberrations are closely related to accommodation and influence the accuracy of the accommodative response and the progression of refractive errors, especially myopia.

Higher-order aberrations and their association with axial elongation in highly myopic children and adolescents

BACKGROUND

Vision-dependent mechanisms play a role in myopia progression in childhood. Thus, we investigated the distribution of ocular and corneal higher-order aberrations (HOAs) in highly myopic Chinese children and adolescents and the relationship between HOA components and 1-year axial eye growth.

METHODS

Baseline cycloplegic ocular and corneal HOAs, axial length (AL), spherical equivalent (SE), astigmatism and interpupillary distance (IPD) were determined for the right eyes of 458 highly myopic (SE ≤-5.0D) subjects. HOAs were compared among baseline age groups (≤12 years, 13-15 years and 16-18 years). Ninety-nine subjects completed the 1-year follow-up. Linear mixed model analyses were applied to determine the association between HOA components, other known confounding variables (age, gender, SE, astigmatism and IPD) and axial growth. A comparison with data from an early study of moderate myopia were conducted.

RESULTS

Almost all ocular HOAs and few corneal HOAs exhibited significant differences between different age groups (all p<0.05). After 1 year, only ocular HOA components was significantly negative associated with a longer AL, including secondary horizontal comatic aberration (p=0.019), primary spherical aberration (p<0.001) and spherical HOA (p=0.026). Comparing with the moderate myopia data, the association of comatic aberration with AL growth was only found in high myopia.

CONCLUSION

In highly myopic children and adolescents, lower levels of annual ocular secondary horizontal comatic aberration changes, besides spherical aberrations, were associated with axial elongation. This suggests that ocular HOA plays a potential role in refractive development in high myopia.

Effect of orthokeratology on anisometropia control: A meta-analysis

BACKGROUND

The effectiveness of orthokeratology in retarding anisometropic progression has been investigated in several small-sample studies. This quantitative analysis aimed to elucidate the efficacy of orthokeratology for anisometropia control.

METHODS

We searched PubMed, Embase, and Cochrane databases for relevant studies through September 2020. Axial length (AL) data at baseline and final follow-up were extracted, and AL elongation and difference were calculated. Methodological quality was evaluated using the risk of bias in non-randomized studies of interventions (ROBINS-I) tool. Meta-analyses were performed using a fixed-effect model based on the heterogeneity.

RESULTS

A total of 10 cohort studies (nine retrospective studies; one prospective study) were included. The pooled results for the unilateral myopia group showed that the mean AL elongation difference between myopic and emmetropic eyes was -0.27 mm (95% CI, -0.31 to -0.22; p < 0.01) at the one-year follow-up (four studies) and -0.17 mm (95% CI, -0.33 to -0.02; p = 0.03) at the two-year follow-up (two studies). In the bilateral anisomyopic group, mean AL elongation difference between high and low myopic eyes was -0.06 mm (95% CI, -0.09 to -0.04; p < 0.01) at the one-year follow-up (seven studies) and -0.13 mm (95% CI, -0.21 to -0.06; p < 0.01) at the two-year followup (three studies).

CONCLUSION

This study demonstrated that orthokeratology can effectively retard myopic progression and reduce anisomyopic values. However, additional wellstructured randomized controlled trials or prospective studies with longer follow-up periods are warranted to address this topic in more detail.

Habitual higher order aberrations affect Landolt but not Vernier acuity

To assess whether the eye's optical imperfections are relevant for hyperacute vision, we measured ocular wave aberrations, visual hyperacuity, and acuity thresholds in 31 eyes of young adults. Although there was a significant positive correlation between the subjects' performance in Vernier- and Landolt-optotype acuity tasks, we found clear differences in how far both acuity measures correlate with the eyes' optics. Landolt acuity thresholds were significantly better in eyes with low higher order aberrations and high visual Strehl ratios (r² = 0.22, p = 0.009), and significantly positively correlated with axial length (r² = 0.15, p = 0.03). A retinal image quality metric, calculated as two-dimensional correlation between perfect and actual retinal image, was also correlated with Landolt acuity thresholds (r² = 0.27, p = 0.003). No such correlations were found with Vernier acuity performance (r² < 0.03, p > 0.3). Based on these results, hyperacuity thresholds are, contrary to resolution acuity, not affected by higher order aberrations of the eye.

Effect of Orthokeratology on Axial Length Elongation in Anisomyopic Children

SIGNIFICANCE

Anisomyopia is a natural experimental paradigm that compares dose response between fellow eyes. This study is the first to explore whether orthokeratology (ortho-k) has a dose-response effect on axial length growth and reduces the interocular difference in axial length in anisomyopic children.

PURPOSE

The purpose of this study was to compare the effect of ortho-k on axial length elongation between the fellow eyes of anisomyopic children.

METHODS

In this retrospective study, 49 anisomyopic children who wore ortho-k lenses were assigned to the anisomyopic ortho-k group. Based on the one-to-one match principle (same age and proximate spherical equivalent), high-isomyopic and low-isomyopic groups each enrolled 49 isomyopic children who wore ortho-k lenses with spherical equivalent similar to that of the more myopic eye and the less myopic eye in the anisomyopic ortho-k group, respectively. Forty-nine anisomyopic children who wore spectacles were enrolled in the anisomyopic spectacle group. At baseline and at 1- and 2-year visits, axial length was measured. Axial length elongation and interocular difference in axial length were compared.

RESULTS

In the anisomyopic ortho-k group, the less myopic eyes exhibited more axial length elongation than did the more myopic eyes during 1- and 2-year treatment periods (P < .01). However, there was no significant difference in axial length elongation between the fellow eyes in the isomyopic groups and anisomyopic spectacle group. At the 2-year visit, the interocular difference in axial length of children in the anisomyopic ortho-k group significantly decreased from 0.72 ± 0.34 to 0.56 ± 0.38 mm (P < .05). In contrast, ortho-k lens-wearing isomyopic children or spectacle-wearing anisomyopic children did not show a significant change in interocular difference in axial length.

CONCLUSIONS

Orthokeratology could reduce the amount of anisomyopia in children primarily through stronger myopia control in the more myopic eye.

Higher order aberrations, refractive error development and myopia control: a review

Evidence from animal and human studies suggests that ocular growth is influenced by visual experience. Reduced retinal image quality and imposed optical defocus result in predictable changes in axial eye growth. Higher order aberrations are optical imperfections of the eye that alter retinal image quality despite optimal correction of spherical defocus and astigmatism. Since higher order aberrations reduce retinal image quality and produce variations in optical vergence across the entrance pupil of the eye, they may provide optical signals that contribute to the regulation and modulation of eye growth and refractive error development. The magnitude and type of higher order aberrations vary with age, refractive error, and during near work and accommodation. Furthermore, distinctive changes in higher order aberrations occur with various myopia control treatments, including atropine, near addition spectacle lenses, orthokeratology and soft multifocal and dual-focus contact lenses. Several plausible mechanisms have been proposed by which higher order aberrations may influence axial eye growth, the development of refractive error, and the treatment effect of myopia control interventions. Future studies of higher order aberrations, particularly during childhood, accommodation, and treatment with myopia control interventions are required to further our understanding of their potential role in refractive error development and eye growth.

Elucidation of the more myopic eye in anisometropia: the interplay of laterality, ocular dominance, and anisometropic magnitude

This study reveals how, in a myopic anisometrope, the odds of an eye being more myopic are related to laterality, ocular dominance, and magnitude of anisometropia. In 193 subjects, objective refraction was performed with cycloplegia. Sighting, motor, and sensory dominance were determined with the hole-in-the-card test, convergence near-point test, continuous flashing technique, respectively. Multiple logistic regression was used for probability analysis. Seventy percent of the subjects had a right eye that was more myopic, while 30% of them had a more myopic left eye. When the right eye was the sensory dominant eye, the probability of the right eye being more myopic increased to 80% if the anisometropia was less than 3.0 D, and decreased below 70% if anisometropia was beyond 3.0 D. When the left eye was the sensory dominant eye, the probability of the left eye being more myopic increased to above 40% if the anisometropia was less than 4.0 D and decreased below 30% if the anisometropia was beyond 4.0 D. Therefore, between the two eyes of anisometropes, laterality tilts the chance of being more myopic to the right. Being the sensory dominant eye increases an eye’s probability of being more myopic by another 10% if the magnitude of anisometropia is moderate.

Profile of off-axis higher order aberrations and its variation with time among various refractive error groups

Peripheral higher order aberrations (HOA) of 646 children at 30° temporal, nasal and inferior visual field were measured under cycloplegia (5 mm pupil diameter) using a commercially available Shack-Hartmann aberrometer in the Sydney Myopia Study [age, 12.7 ± 0.4 years (mean ± standard deviation)] and five years later in the Sydney Adolescent Vascular and Eye Study. At baseline, 176 eyes were emmetropic, 95 were myopic and 375 were hyperopic. Coma, 3rd order and RMS of coma increased with eccentricity for all eyes and no difference was observed for 4th order and RMS of C(4,0) among refractive error groups. More positive C(4,0) was observed for hyperopic eyes at periphery. At follow up, 26% had 'myopic change' and 70% had 'no change' in refractive error. At follow-up, horizontal coma became more negative at nasal field and more positive at temporal field for all eyes. More positive C(4,0) for hyperopic eyes at baseline may indicate variation in optical characteristics of peripheral cornea and crystalline lens. An increase in horizontal coma with time, irrespective of refractive error change, may be attributed to variation in the shape factor of peripheral cornea and crystalline lens and/or misalignment of optical surfaces/components relative to the visual axis (angle kappa) as the eye grows in axial length.

Reliability of Ocular Aberration Measurements in Children with Moderate and Low Myopia under Scotopic Conditions

Purpose

To investigate the reliability of ocular aberration measurement in myopic children under scotopic conditions and to validate the mathematical Zernike pupil scaling-down technique.

Methods

Ocular aberrations of 45 myopic children were examined under scotopic conditions via iTrace aberrometer. The intra- and intersession repeatability was evaluated for both the measured values with the true pupil sizes and the estimated ones that were determined by scaling down the pupil sizes to the largest integer value across all measurements.

Results

The intra- and intersession difference of clinically measured aberration was generally insignificant, and the ICCs for each aberration component exhibited good to excellent reliability (ICCs > 0.4). Similar results were found for the estimated aberration using the scaling-down technique. Although the majority of the estimated Zernike components were comparable with the corresponding measured one, the estimated values of defocus, coma, and the corresponding total aberrations were found significantly smaller than the measured values (all P < 0.01).

Conclusions

The ocular aberration measurements in myopic children under the circumstances described are reliable. The scaling-down technique is a useful option for comparing the results obtained from different pupil sizes, but the estimated Zernike coefficients were not always comparable with the corresponding measured values.

Real-Time Measurement of Ocular Wavefront Aberrations in Symptomatic Subjects

The purpose of this work was to study the real-time changes of the optical properties of the eye with accommodation in subjects with symptoms of accommodative disorders. From ocular aberrations, it is possible to compute several parameters like the response and lag of accommodation. The ocular aberrations were measured in 4 subjects, with different accommodative disorders, during several cycles of accommodation/disaccommodation and for different accommodative stimuli. The measurement was done continuously and in real time during different accommodative stimuli. It was possible to see the changes in accommodative response during the several stimuli of accommodation. Subjects with accommodative disorders showed different accommodative responses. The use of wavefront ocular aberrations can be a tool to diagnose accommodative disorders. In some subjects with complaints, this method showed irregularities even when the results of the usual clinical exams were normal.

Characteristics of higher-order aberrations and anterior segment tomography in patients with pathologic myopia

PURPOSE

To investigate prospectively the characteristics in the higher-order aberrations and anterior segment tomography in patients with pathologic myopia.

METHODS

One hundred and twelve consecutive highly myopic patients (mean age 43.4 ± 9.3 years, spherical equivalent of refractive error ≥8 D and an axial length ≥26.5 mm) were studied. Thirty-seven emmetropic individuals (mean age 37.0 ± 14.5 years, spherical equivalent of refractive error ≤ ±1 D) were analyzed as controls. The ocular and cornea higher-order aberrations were measured using a Hartmann-Shack wavefront sensor (KR-1W; Topcon Corporation, Tokyo, Japan). The crystalline lens rise, the angle-to-angle, and the white-to-white values were measured using anterior segment OCT (SS-1000; Tomey Corporation, Nagoya, Japan). The mean curvature of the anterior corneal surface, the thickness at the thinnest central corneal point, the location of the central corneal point, the corneal volume, the anterior chamber volume, and the anterior chamber depth were measured using the Pentacam HR (Oculus, Inc., Wetzlar, Germany).

RESULTS

The ocular total higher-order aberration for 4-mm pupil, the ocular spherical aberrations, and internal spherical aberration for 6-mm pupil were significantly higher in highly myopic eyes than in the emmetropic controls. The crystalline lens rise was significantly smaller in highly myopic eyes than in the emmetropic controls. The anterior chamber depth and the anterior chamber volume were significantly larger in highly myopic eyes than in the emmetropic controls.

CONCLUSION

Highly myopic eyes had higher-order aberrations than emmetropic eyes because of the increasing internal aberrations.

Higher order aberrations of the eye: Part two

This article is the second in a series of two articles, which provides a discussion of the factors that may possibly contribute to variable results when wavefront aberrations of the human eye are measured. Some of the factors discussed in this article are the influences that refractive errors (specifically myopia and astigmatism), pupil diameter, accommodation of the crystalline lens, age, mydiatric drops and the integrity of the tear film may have on these wavefront measurements. The first article in the series explained the general principles of higher order aberrations (HOAs), as well as HOAs of importance in the eye and the measuring apparatus used to measure HOAs of the eye.Keywords: wavefront aberrations; aberrometry

Higher order aberrations of the eye: Part one

This article is the first in a series of two articles that provide a comprehensive literature review of higher order aberrations (HOAs) of the eye. The present article mainly explains the general principles of such HOAs as well as HOAs of importance, and the measuring apparatus used to measure HOAs of the eye. The second article in the series discusses factors contributing to variable results in measurements of HOAs of the eye.Keywords: Higher order aberrations; wavefront aberrations; aberrometer

Interocular Difference of Peripheral Refraction in Anisomyopic Eyes of Schoolchildren

Purpose

Refraction in the peripheral visual field is believed to play an important role in the development of myopia. The purpose of this study was to investigate the differences in peripheral refraction among anisomyopia, isomyopia, and isoemmetropia for schoolchildren.

Methods

Thirty-eight anisomyopic children were recruited and divided into two groups: (1) both eyes were myopic (anisomyopic group, AM group) and (2) one eye was myopic and the contralateral eye was emmetropic (emmetropic anisomyopic group, EAM group). As controls, 45 isomyopic and isoemmetropic children were also recruited with age and central spherical equivalent (SE) matched to those of the AM and EAM groups. The controls were divided into three groups: (1) intermediate myopia group (SE matched to the more myopic eye of AM group), (2) low myopia group (SE matched to the less myopic eye of AM group and the more myopic eye of EAM group), and (3) emmetropia group (SE matched to the less myopic eye of EAM group). Peripheral refraction at 7 points across the central ±30° on the horizontal visual field with a 10° interval was measured with an autorefractor. Axial length (AL), corneal curvature (CC), and anterior chamber depth (ACD) were also determined by using the Zeiss IOL-Master.

Results

The relative peripheral spherical equivalent [RPR(M)] and relative peripheral spherical value [RPR(S)] of the more myopic eye was shifted more hyperopically than the contralateral eye in both the AM and the EAM groups (both p<0.0001). The RPR(M, S) of the less myopic eyes in the AM and EAM groups showed a relatively flat trend across the visual field and were not significantly different from the emmetropia group. The RPR(M, S) of less myopic eyes in the AM group were shifted less hyperopically than in the isomyopic low myopia group and the more myopic eye of the EAM group [RPR(M), p = 0.007; RPR(S), p = 0.001], although the central SEs of the three groups were not significantly different from each other. However, RPR(M, S) of the more myopic eyes were not different from the corresponding isomyopic groups. There was also no significant difference in the relative peripheral astigmatism [RPR(J0, J45)] between the more and the less myopic eyes in either the AM or the EAM group.

Conclusion

Refraction of anisomyopia differs between the two eyes not only at the central visual field but also at the off-axis periphery. The relative peripheral refraction of the more myopic eye of anisomyopia was shifted hyperopically, as occurs in isomyopia with similar central subjective SE values. Less myopic eyes were much less hyperopically shifted in relative peripheral refraction than the corresponding isomyopic eyes, but are comparable to emmetropic eyes. This emmetropia-like relative peripheral refraction in less myopic eyes might be a factor responsible for slowing down the progression of myopia.

Higher Order Aberration and Astigmatism in Children with Hyperopic Amblyopia

Purpose

To investigate the changes in corneal higher-order aberration (HOA) during amblyopia treatment and the correlation between HOA and astigmatism in hyperopic amblyopia children.

Methods

In this retrospective study, a total of 72 eyes from 72 patients ranging in age from 38 to 161 months were included. Patients were divided into two groups based on the degree of astigmatism. Corneal HOA was measured using a KR-1W aberrometer at the initial visit and at 3-, 6-, and 12-month follow-ups. Correlation analysis was performed to assess the association between HOA and astigmatism.

Results

A total of 72 patients were enrolled in this study, 37 of which were classified as belonging to the higher astigmatism group, while 35 were assigned to the lower astigmatism group. There was a statistically significant difference in success rate between the higher and lower astigmatism groups. In both groups, all corneal HOAs were significantly reduced during amblyopia treatment. When comparing the two groups, a significant difference in coma HOA at the 12-month follow-up was detected (p = 0.043). In the Pearson correlation test, coma HOA at the 12-month follow-up demonstrated a statistically significant correlation with astigmatism and a stronger correlation with astigmatism in the higher astigmatism group than in the lower astigmatism group (coefficient values, 0.383 and 0.284 as well as p = 0.021 and p = 0.038, respectively).

Conclusions

HOA, particularly coma HOA, correlated with astigmatism and could exert effects in cases involving hyperopic amblyopia.

Ocular Aberrations and Corneal Shape in Adults with and without Astigmatism

PURPOSE

To characterize and compare the corneal shapes and monochromatic aberrations in Chinese myopic adults with and without astigmatism.

METHODS

Forty-six Hong Kong Chinese aged 50 to 70 years with compound against-the-rule myopic astigmatism (n = 18) or simple myopia (n = 28) were recruited. Corneal shapes were measured by a Scheimpflug-based corneal topographer: the semimeridian corneal shape factors at the nasal, temporal, inferior, and superior corneal quadrants measured from the corneal apex to 3 mm midperiphery were analyzed. The ocular aberrations were measured by the COAS (Complete Ophthalmic Analysis System) Shack-Hartmann wavefront aberrometer; the corneal aberrations were computed using the corneal topographic map data measured by the Medmont E300 corneal topographer; and the internal aberrations were calculated from the ocular and corneal aberrations.

RESULTS

Compared with simple myopia, myopic astigmatism had more oblate nasal and temporal corneal shapes and showed significantly more negative Y trefoil and more positive vertical coma. The asymmetry in corneal shape along the vertical principal meridian (inferior - superior) was significantly associated with the Y trefoil and vertical coma of the cornea, suggesting that this regional asymmetry in corneal shape may contribute to the ocular aberrations.

CONCLUSIONS

The significant relationships found between astigmatism, corneal shapes, and monochromatic aberrations underscore the importance of taking corneal shape into account when correcting the optical defects in myopic Chinese adults with astigmatism.

Crystalline lens and refractive development

Individual refractive errors usually change along lifespan. Most children are hyperopic in early life. This hyperopia is usually lost during growth years, leading to emmetropia in adults, but myopia also develops in children during school years or during early adult life. Those subjects who remain emmetropic are prone to have hyperopic shifts in middle life. And even later, at older ages, myopic shifts are developed with nuclear cataract. The eye grows from 15 mm in premature newborns to approximately 24 mm in early adult years, but, in most cases, refractions are maintained stable in a clustered distribution. This growth in axial length would represent a refractive change of more than 40 diopters, which is compensated by changes in corneal and lens powers. The process which maintains the balance between the ocular components of refraction during growth is still under study. As the lens power cannot be measured in vivo, but can only be calculated based on the other ocular components, there have not been many studies of lens power in humans. Yet, recent studies have confirmed that the lens loses power during growth in children, and that hyperopic and myopic shifts in adulthood may be also produced by changes in the lens. These studies in children and adults give a picture of the changing power of the lens along lifespan. Other recent studies about the growth of the lens and the complexity of its internal structure give clues about how these changes in lens power are produced along life.

Progression of lower and higher-order aberrations: a longitudinal study

BACKGROUND

The aim of the present study was to investigate the effect of near-work on lower and higher-order aberrations, and its progression over a 9-month period during the school year.

METHODS

Data from 24 young-adult myopic eyes, and 24 non-myopic eyes were used in this investigation. The lower-order aberrations, coma, spherical aberration (SA), and total root mean square (RMS) of higher order aberrations (total HOA) were measured using an open-field iTrace aberrometer, at both the initial baseline evaluation, and then at the follow-up visits over a 9-month period. Pupil size of 4 mm was used for the aberration measurements.

RESULTS

The group mean (SD) of the subjects (mean age: 23.6 +/- 3.4 years) at the initial and follow-up visit was 0.47D (0.47D) and 0.31D (0.41D), in the non-myopes and -3.58D (2.08D) and -3.86D (2.14D) in the myopes, respectively. Significant increases in myopic refraction were observed. The group mean (SD) total HOA at the initial and final visit was 0.12 (0.08) and 0.11 (0.06) microns, in the non-myopes, and 0.15 (0.08) and 0.15 (0.08) microns, in the myopes, respectively. The group mean RMS of the coma at the initial and final visit was 0.06 (0.04) and 0.07 (0.05), in the non-myopes, and 0.08 (0.06) and 0.09 (0.06) microns, in the myopes, respectively. The group mean SA of the subjects at the initial and last visit was 0.04 (0.04) and 0.03 (0.03), in the non-myopes, and 0.04 (0.04) and 0.04 (0.04) microns, in the myopes, respectively.

CONCLUSIONS

There was a significant difference in myopic refraction, over the 9-month assessment period. However, no significant difference in total HOA, SA, and coma between the initial and follow-up visits in both the myopes and the non-myopes was observed.

Relationship between higher-order aberrations and myopia progression in schoolchildren: a retrospective study

AIM

To investigate the relationship between higher-order aberration (HOA) and myopic progression in school children.

METHODS

Between April 23, 2011 and August 29, 2011 in the children's myopia outpatient clinic of the West China Hospital of Sichuan University, 148 eyes of 74 schoolchildren were reviewed. HOAs for a 6-mm pupil were measured with an aberrometer. Myopic progression rate was defined according to the change in spherical equivalent refraction (SER) divided by the time span (years). Subjects with myopic progression rate of ≥0.50 diopters (D) were classified as the 'fast' group and the subjects with myopic progression rate of <0.50D were classified as the 'slow' group. A retrospective study was conducted to compare HOA between the two groups, using root mean square (RMS) values and Zernike coefficients.

RESULTS

The RMS values of HOA (t=2.316, P=0.02), HOA without Z4 (0) (t=2.224, P=0.03), third-order aberrations (t'=2.62, P=0.01), and coma (t'=2.49, P=0.01) were significantly higher in the fast group than those in the slow group. The individual Zernike coefficients of Z3 (-1) (t=-2.072, P=0.04) and Z5 (1) (Z =-2.627, P=0.01) displayed statistically significant differences between the two groups. Significant correlations were found between the RMS values of HOA (r=0.193, P=0.019), RMS values of HOA without Z4 (0) (r=0.23, P =0.005), RMS values of coma (r=0.235, P=0.004), RMS values of third-order aberrations (r=0.243, P =0.003), and the progression rate.

CONCLUSION

Our results provide evidence of a relationship between HOA and myopic progression. In a future prospective longitudinal study, we aim to verify whether HOA is a risk factor for myopic progression.

The effect of simulated normal and amblyopic higher-order aberrations on visual performance

PURPOSE

To study the effect of simulated amblyopic and normal higher-order aberrations on visual performance of normal eyes.

METHODS

To assess visual function, an adaptive optics visual simulator was used to compensate volunteers' ocular aberrations and simulate the wavefront aberration patterns found in healthy and amblyopic eyes in 7 healthy individuals. Visual acuity for high (100%), medium (50%), and low (10%) contrast and contrast sensitivity at 10, 20, and 25 cycles per degree (cpd) were measured after simulation of both pattern aberrations. The modulation transfer function and the point spread function were computed based on the aberration data. All measures were taken for 3 and 5.5 mm pupil sizes.

RESULTS

No statistically significant differences in visual acuity and contrast sensitivity were found between both groups for any analyzed contrast level, spatial frequency, and pupil size values. Mean logMAR visual acuity for a 3 mm pupil was -0.11 ± 0.04 for 100% contrast levels, -0.06 ± 0.06 for 50%, and 0.17 ± 0.07 for 10%. For a 5.5 mm pupil, the values were -0.06 ± 0.04 (100%), 0.00 ± 0.05 (50%), and 0.21 ± 0.06 (10%). Mean contrast sensitivity for a 3 mm pupil was 1.9 ± 0.2 for 10 cpd, 1.2 ± 0.15 for 20 cpd, and 0.9 ± 0.1 for 25 cpd. For a 5.5 mm pupil, contrast sensitivity was 1.4 ± 0.2 (10 cpd), 0.9 ± 0.2 (20 cpd), and 0.6 ± 0.2 (25 cpd). MTFs and PSFs were comparable in the two groups for both pupils.

CONCLUSIONS

The higher-order aberrations seen in idiopathic amblyopic eyes alone do not appear to contribute to the decreased visual function.

Relationship between ocular wavefront aberrations and refractive error in Chinese school children

BACKGROUND

The relationship between ocular wavefront aberrations and refractive error in children's eyes remains controversial. The purpose of this study is to re-examine this relationship in Chinese school children under natural distance accommodation.

METHODS

Ocular wavefront aberrations were measured in 86 Chinese children with spherical equivalent refraction (SER) between +0.5 D and -6.0 D and astigmatism less than -1.00 D. Wavefront aberrations were calculated using an objective method based on the Hartmann-Shack principle. Refractive error was obtained using a phoropter after cycloplegia. Subjects were categorised into three groups based on the mean SER: emmetropia (SER from -0.50 D to +0.50 D), mild myopia (SER greater than -0.50 D to -3.00 D) and moderate myopia (SER greater than -3.00 D to -6.00 D). Of the 86 participants, 22 were emmetropic, 43 were mildly myopic and 21 were moderately myopic. The root mean square (RMS) values of higher-order aberrations, Zernike coefficients (third-, fourth- and fifth-order aberrations) and R(j) (the ratio of third-, fourth- or fifth-order aberrations to total higher-order aberrations) were compared across the three refractive groups.

RESULTS

No significant correlations were found between the RMS values of total higher-order aberrations, third-order aberrations, fourth-order aberrations, fifth-order aberrations, spherical aberration or coma and SER. No significant differences in the RMS values of total higher-order aberrations or R(j) were observed among the groups. The difference in fifth-order aberrations was statistically significant among the groups (p = 0.022); no other differences in higher-order aberration were found. Aside from C (3,1), no other differences were observed for Zernike coefficients.

CONCLUSION

Ocular wavefront aberrations are similar among Chinese school children with different refractive errors under natural accommodation for a distance target. There is no evidence that myopes have a different amount of ocular higher-order aberrations than emmetropes.

Optical and biometric characteristics of anisomyopia in human adults

PURPOSE

To investigate the role of higher order optical aberrations and thus retinal image degradation in the development of myopia, through the characterization of anisomyopia in human adults in terms of their optical and biometric characteristics.

METHODS

The following data were collected from both eyes of 15 young adult anisometropic myopes and 16 isometropic myopes: subjective and objective refractive errors, corneal power and shape, monochromatic optical aberrations, anterior chamber depth, lens thickness, vitreous chamber depth, and best corrected visual acuity. Monochromatic aberrations were analyzed in terms of their higher order components, and further analyzed in terms of 31 optical quality metrics. Interocular differences for the two groups (anisomyopes vs isomyopes) were compared and the relationship between measured ocular parameters and refractive errors also analyzed across all eyes.

RESULTS

As expected, anisomyopes and isomyopes differed significantly in terms of interocular differences in vitreous chamber depth, axial length and refractive error. However, interocular differences in other optical properties showed no significant intergroup differences. Overall, higher myopia was associated with deeper anterior and vitreous chambers, higher astigmatism, more prolate corneas, and more positive spherical aberration. Other measured optical and biometric parameters were not significantly correlated with spherical refractive error, although some optical quality metrics and corneal astigmatism were significantly correlated with refractive astigmatism.

CONCLUSIONS

An optical cause for anisomyopia related to increased higher order aberrations is not supported by our data. Corneal shape changes and increased astigmatism in more myopic eyes may be a by-product of the increased anterior chamber growth in these eyes; likewise, the increased positive spherical aberration in more myopic eyes may be a product of myopic eye growth.

Corneal spherical aberration of eyes with cataract in a Japanese population

PURPOSE

To investigate the distribution of corneal spherical aberration in Japanese eyes with cataract for implantation of aspheric intraocular lenses (IOLs).

METHODS

Corneal spherical aberration (Zernike coefficient Z04) in the central 6-mm zone was measured with a wavefront analyzer (KR9000PW, Topcon) in 257 eyes of 168 Japanese patients with cataract. Axial length was also measured for each eye.

RESULTS

Mean corneal spherical aberration was 0.203+/-0.100 microm (range: -0.103 to 0.497 microm). A significant negative correlation was found between axial length and corneal spherical aberration (r =/-0.135, P=.036).

CONCLUSIONS

Corneal spherical aberration varied among cataract patients and correlated with axial length. Japanese patients showed a relatively smaller corneal spherical aberration than previous studies of Caucasians. Preoperative measurement of wavefront aberrations is thus important in using aspheric IOLs.