Astigmatism and its role in emmetropization

The effect of near work on the anterior eye and associations with myopia: a narrative review

The global prevalence of myopia has increased significantly in recent decades, and it is anticipated that half the population of the world will be myopic by 2050. The dramatic increase in myopia cannot be explained solely by genetic factors; hence, environmental factors such as near work may play an important role in myopia development. Near work activities involve accommodation, convergence, and pupil constriction, which lead to various mechanical changes that alter the structural and optical properties of the anterior eye. Mechanical changes associated with near work activities include ciliary body contraction, medial rectus contraction, lateral rectus relaxation, changes in the eyelid-cornea interaction, pupil size, and crystalline lens shape and position. These structural variations lead to optical changes including a change in spherical refractive power, astigmatism, accommodative convergence, higher order aberrations, and retinal image quality. Several differences in near work-related optical and structural changes have been observed between myopes and non-myopes. These differences elucidate mechanisms that potentially underpin near work-associated axial elongation and myopia development. This narrative review explores anterior segment structural and optical changes during near work and their potential mechanistic contribution to myopia development, while highlighting literature gaps that require further research.

Effect of induced astigmatism on vestibulo-ocular reflex

CLINICAL RELEVANCE

The vestibular-ocular reflex stabilises the retinal image and maintains balance during head movement. Astigmatism is one of the common refractive errors that can reduce the quality of visual inputs.

BACKGROUND

The purpose of this study was to investigate the effect of induced astigmatism on the function of the vestibular-ocular reflex.

METHODS

This prospective semi-experimental study was conducted on 48 participants aged 20 to 40 years who had no history of neurological, vestibular, or eye disorders. Simple myopic astigmatism was induced using three positive cylindrical lens powers of + 0.50D, +1.50D, and + 2.50D at three axes of 180, 45, and 90 degrees in both eyes. The vestibular-ocular reflex was evaluated by the ocular Vestibular Evoked Myogenic Potential (oVEMP) through acoustic stimulation (a 500 hz tone burst at an intensity of 95 dBnHL) of the ear, with potentials recorded from the lower oblique muscle. The latency of the N1 and P1 waves and the amplitude of N1-P1 were recorded for all participants in the baseline condition (without any induced astigmatism) and under different induced astigmatism conditions.

RESULTS

The latencies of N1 and P1 significantly increased with the increasing power of induced astigmatism; however, the powers of astigmatism had no effect on the amplitude of N1-P1. The axes of induced astigmatism did not have a significant effect on the latencies and amplitude of N1-P1.

CONCLUSIONS

Induced astigmatism due to retinal image blur can deteriorate the function of the vestibular-ocular reflex by increase in the latencies of N1 and P1 oVEMP waves.

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

Purpose

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

Methods

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

Results

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

Conclusion

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

Orthoptic findings in trigonocephaly patients after completed visual development

PURPOSE

Trigonocephaly is associated with suboptimal visual outcomes in young children. However, the presence of orthoptic abnormalities after completed visual development remains unknown. The purpose of this study is to assess the prevalence of orthoptic abnormalities in trigonocephaly patients, after completed visual development.

METHODS

A retrospective study among non-syndromic trigonocephaly patients aged 8 years and older was conducted at the Dutch Craniofacial center. Ophthalmologic records were reviewed for refractive errors and a history of strabismus and amblyopia. Refractive errors were classified as follows: myopia: spherical equivalent (SE) ≤ -0.50 diopters (D), emmetropia: SE -0.50 D to + 0.50 D, hyperopia: ≥ + 0.50 D, astigmatism: cylinder error ≥ 1.00 D. The history of strabismus and amblyopia was assessed retrospectively. Prevalence and cumulative incidence of these abnormalities in healthy age-matched populations were retrieved from literature.

RESULTS

We included 78 trigonocephaly patients with a median (interquartile range) age of 10.0 (9.0-12.0) years. Thirty-four (43.6%) of these patients presented with a refractive error after the age of 8 years. Specific findings in these 78 patients versus controls were hyperopia in 35.9% vs 8.4%; myopia in 6.4% vs 11.4%; astigmatism in 20.5% vs 11.9%; strabismus in 14.1% vs 2.65%; amblyopia in 6.4% vs 3.4%.

CONCLUSIONS

Trigonocephaly patients have a two-to-threefold higher risk of hyperopia, astigmatism, amblyopia, and strabismus, as compared to healthy populations. We recommend screening and, in case of abnormalities, regular follow-up before completed visual development. In addition, we recommend long-term surveillance of visual functioning by a pediatric ophthalmologist and/or orthoptist.

Impact of Astigmatism on Axial Elongation in School-Age Children: A Five-Year Population-Based Study in Tianjin, China

Purpose

To investigate the progression rates of axial length (AXL) among school-age children with baseline astigmatism and spherical ametropia.

Methods

Annual vision screenings were conducted at seven schools in Tianjin, China, from 2018 to 2022. Ocular biometry and non-cycloplegic autorefraction were collected. Children 5 to 16 years old without any myopia interventions were included and categorized by their baseline astigmatism magnitude (control, low, or high) and axis orientation (with the rule [WTR], against the rule [ATR], or oblique). Additionally, children were classified by baseline spherical ametropia (compound hyperopic, compound myopic, or other). Annual AXL progression rates of right eyes were calculated using regression models and compared across different types of astigmatism and spherical ametropia.

Results

A total of 10,732 Chinese children (baseline age, 9.26 ± 2.42 years; follow-up duration, 2.63 ± 1.01 years) were included and divided into a younger cohort (age < 11 years; n = 7880) and an older cohort (age ≥ 11 years; n = 2852). Across both age groups and all astigmatism magnitudes, ATR astigmatism exhibited the most rapid AXL progression, followed by oblique and WTR astigmatism. Two-way ANCOVA of the combined cohort revealed that both high-magnitude and ATR astigmatism were significantly associated with AXL progression (P ≤ 0.018). However, the impact of astigmatism on AXL progression varied depending on baseline spherical ametropia, as high-magnitude and ATR astigmatism increased AXL progression in compound myopic eyes but decreased progression in compound hyperopic eyes.

Conclusions

Both baseline magnitude and axis orientation of astigmatism are significantly associated with axial elongation in children. However, these associations may vary with spherical ametropia, with differential patterns being observed between compound hyperopic and myopic eyes.

Epidemiological Characteristics Associated with Astigmatism in a Population of 3- to 5-Year-Old Children in Los Angeles, CA

PURPOSE

To characterize the prevalence of astigmatism among the heretofore largest published sample of preschool children and to do so among different age, gender, and racial and ethnic subgroups.

METHODS

This study included data from 79,330 preschoolers (49.0% female, 79.4% Latino, mean age 4.29 ± 0.66 years) who were screened using a Retinomax autorefractor between 2012 and 2017. Cylindrical power and astigmatism defined as 1.50 D or more of cylinder were the dependent variables. Children were stratified by age, race/ethnicity, and gender. T-test and chi-square analyses were performed.

RESULTS

The average cylinder of the sample was found to be 0.65 ± 0.78 D and 11% of the children were astigmatic. Among all children, these values remain stable from age 3 (0.66 ± 0.76 D, 11.2%) to age 4 (0.66 ± 0.79 D, 11.4%, p = .53), but they decrease significantly from age 4 to age 5 (0.61 ± 0.80 D, 10.%, p < .01). Latinos have the highest average cylinder power and prevalence of astigmatism at 0.68 ± 0.82 D and 12.3%, and unlike most other race/ethnic subgroups, they do not experience an age-dependent decrease in astigmatism. While female and male students have equivalent cylindrical refractions broadly, Asian and African American females have higher levels of astigmatism than their male counterparts.

CONCLUSION

The data further demonstrate that Latino children in Los Angeles have significantly higher rates of astigmatism, which does not decrease during preschool years. These results emphasize the need for comprehensive school-based screening, especially among Latino enclaves in Los Angeles.

Higher order aberrations and retinal image quality during short-term accommodation in myopic and non-myopic children

INTRODUCTION

Despite the known associations between near work and myopia, and retinal image quality and eye growth, accommodation-induced changes in higher order aberrations (HOA's) and retinal image quality in children with different refractive errors are poorly understood.

METHODS

Ocular HOA's were measured using a Hartmann-Shack wavefront sensor (COAS-HD, Wavefront Sciences) in 18 myopic and 18 age- and sex-matched non-myopic children during short-term accommodation tasks (four demands of 0, 3, 6 and 9 D) presented using a Badal optometer. Eighth order Zernike polynomials were fitted across a 2.3 mm pupil diameter to determine refractive power vectors (M, J₁₈₀ and J₄₅ ) and the accommodation error, and a 4 mm pupil was used for HOA analyses. Retinal image quality was examined using the visual Strehl ratio based on the optical transfer function (VSOTF) for third to eighth radial orders only.

RESULTS

Most refractive error group differences were observed for the 6 and 9 D demands. Myopic children underwent greater changes in with-the-rule astigmatism (J₁₈₀ ), higher order and third order RMS values, primary vertical ( C 3 - 1 $$ {C}_3^{-1} $$ ) and horizontal coma ( C 3 1 $$ {C}_3^1 $$ ), and several other individual Zernike coefficients compared with non-myopic children (all refractive error group by demand interaction p-values of ≤0.02). Non-myopic children exhibited a greater negative shift in primary ( C 4 0 $$ {C}_4^0 $$ ) and positive shift in secondary spherical aberration ( C 6 0 $$ {C}_6^0 $$ ) (both refractive error group by demand interaction p-values of ≤0.002). The VSOTF degraded for the 6 and 9 D demands in both groups, but the myopic children underwent a greater mean (SE) reduction from 0 D of -0.274 (0.048) for the 9 D demand, compared with -0.131 (0.052) for the non-myopic children (p = 0.001).

CONCLUSION

These results may have implications for the association between near work, accommodation and myopia development, particularly related to the use of short working distances during near tasks.

Refractive development I: Biometric changes during emmetropisation

PURPOSE

Although there are many reports on ocular growth, these data are often fragmented into separate parameters or for limited age ranges. This work intends to create an overview of normal eye growth (i.e., in absence of myopisation) for the period before birth until 18 years of age.

METHODS

The data for this analysis were taken from a search of six literature databases using keywords such as "[Parameter] & [age group]", with [Parameter] the ocular parameter under study and [age group] an indication of age. This yielded 34,409 references that, after screening of title, abstract and text, left 294 references with usable data. Where possible, additional parameters were calculated, such as the Bennett crystalline lens power, whole eye power and axial power.

RESULTS

There were 3422 average values for 17 parameters, calculated over a combined total of 679,398 individually measured or calculated values. The age-related change in refractive error was best fitted by a sum of four exponentials (r²  = 0.58), while all other biometric parameters could be fitted well by a sum of two exponentials and a linear term ('bi-exponential function'; r² range: 0.64-0.99). The first exponential of the bi-exponential fits typically reached 95% of its end value before 18 months, suggesting that these reached genetically pre-programmed passive growth. The second exponentials reached this point between 4 years of age for the anterior curvature and well past adulthood for most lenticular dimensions, suggesting that this part represents the active control underlying emmetropisation. The ocular components each have different growth rates, but growth rate changes occur simultaneously at first and then act independently after birth.

CONCLUSIONS

Most biometric parameters grow according to a bi-exponential pattern associated with passive and actively modulated eye growth. This may form an interesting reference to understand myopisation.

Measuring Retinal Thickness and Visual Acuity in Eyes with Different Types of Astigmatism in a Cohort of Hong Kong Chinese Adults

Purpose

The purpose of this study was to investigate optical coherence tomography (OCT)-measured retinal thickness (RT) and best-corrected distance visual acuity (BCDVA) in eyes with different types of astigmatism.

Methods

This is a case-control study of 101 participants stratified into With-The-Rule (WTR; n = 41), Against-The-Rule (ATR; n = 25), and control (n = 35) groups by noncycloplegic subjective refraction. Inclusion criteria were ages between 18 and 45 years, spherical-equivalent (SE) refraction ≥-10.00 diopters (D), negative cylindrical power (CYL) ≤-0.75 D with axes of 0 to 30 degrees/150 to 180 degrees for WTR and 60 to 120 degrees for ATR, or CYL ≥-0.25 D for controls. Participants suffering from ocular diseases related to retinal defects, having a history of ocular surgery, with BCDVA >0.10 LogMAR, or poor OCT imaging quality were excluded. Fovea-centered scans were performed using spectral-domain OCT (SD-OCT), and RT automatically measured by the inbuilt software. Only right eyes were analyzed. Groups were matched for age, gender, SE, axial length, and corneal curvature.

Results

One-way ANOVA showed a significant difference in both BCDVA (P = 0.039) and macular RT (P = 0.028) among the three groups. Bonferroni's post hoc test revealed statistically significant between-group differences in BCDVA (WTR vs. controls, P = 0.041), as well as in RT at inner-nasal (WTR vs. ATR, P = 0.034) and outer-temporal subfields (WTR vs. controls, P = 0.042). BCDVA was positively associated with macular RT (r = 0.206, P = 0.041) after adjusting for age, gender, and axial length.

Conclusions

Greater RT and poorer BCDVA were found in eyes with WTR astigmatism. Our findings suggest that the effect of astigmatism on retinal thickness and BCDVA may vary depending on not only magnitude, but also axis of astigmatism.

Meridional Attentional Asymmetries in Astigmatic Eyes

Purpose

To investigate the impact of attention orientation in young myopic adults with astigmatism.

Methods

The effect of attention on foveal meridional performance and anisotropy was measured in corrected myopes with various levels of astigmatism (with-the-rule astigmatism ≤ -0.75D, Axis: 180 ± 20) using orientation-based attention. Attention was manipulated by instructing subjects to attend to either the horizontal or the vertical line of a central pre-stimulus (a pulsed cross) along separate blocks of trials. For each attention condition, meridional acuity and reaction times were measured via an annulus Gabor target situated remotely from the cross and presented at random horizontally and vertically in a two-alternative forced-choice employing two interleaved staircase procedures (one-up/one-down). Attention modulations were estimated by the difference in performance between horizontal and vertical attention.

Results

Foveal meridional performance and anisotropy were strongly affected by the orientation of attention, which appeared critical for the enhancement of reaction times and resolution. Under congruent orienting of attention, foveal meridional anisotropy was correlated with the amount of defocus for both reaction time and resolution, demonstrating greater vertical performance than horizontal performance as myopia increased. Compatible with an attentional compensation of blur through optimal orienting of attention, vertical attention enhanced reaction times compared to horizontal attention and was accompanied by an increase in overall acuity when myopia increased. Increased astigmatism was associated with smaller attention effects and asymmetry, suggesting potential deficits in the compensation of blur in astigmatic eyes.

Conclusion

Collectively, attention to orientation plays a significant role in horizontal-vertical foveal meridional anisotropy and can modulate the asymmetry of foveal perception imposed by the optics of the eye in episodes of uncorrected vision. Further work is necessary to understand how attention and refractive errors interact during visual development. These results may have practical implications for methods to enhance vision with attention training in myopic astigmats.

Experimentally induced myopia and myopic astigmatism alter retinal electrophysiology in chickens

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

Correlation between ocular residual astigmatism and anterior corneal astigmatism in children with low and moderate myopia

BACKGROUND

To assess the correlation between ocular residual astigmatism and anterior corneal astigmatism in children with low and moderate myopia.

METHODS

Refractive astigmatism was determined by subjective manifest refraction. Anterior corneal astigmatism was determined by IOL Master. Thibos vector analysis was used to calculate ocular residual astigmatism. Correlation analysis was used to assess the relationship between the amounts of ocular residual astigmatism and anterior corneal astigmatism. The relationship between the vectors of ocular residual astigmatism and anterior corneal astigmatism was evaluated by a physical method.

RESULTS

The study analysed 241 right eyes of 241 children aged 8 to 18 years old. In this study, the median magnitude of ocular residual astigmatism was 1.02 D, with an interquartile range was of 0.58 D. Against-the-rule ocular residual astigmatism was seen in 232 eyes (96.3%). There was a significant and moderate correlation between ocular residual astigmatism and anterior corneal astigmatism (r = 0.50, P < 0.001). Ocular residual astigmatism compensated for anterior corneal astigmatism in 240 eyes (99.6%). The mean compensation value was 1.00 ± 0.41 D (range 0.02 D to 2.34 D). Based on this effect, 37 eyes had a different axial classification of anterior corneal astigmatism and refractive astigmatism. In contrast, one eye (0.4%) had oblique ocular residual astigmatism and the ocular residual astigmatism superimposed with-the-rule anterior corneal astigmatism.

CONCLUSIONS

The magnitude of ocular residual astigmatism was relatively large in myopic children and predominantly compensated for anterior corneal astigmatism. Ocular residual astigmatism should be assessed in patients before fitting them with orthokeratology lenses.

Bi-directional Refractive Compensation for With-the-Rule and Against-the-Rule Astigmatism in Young Adults

Purpose

The purpose of this study was to investigate the short-term effect of imposing astigmatism on the refractive states of young adults.

Methods

Nineteen visually healthy low-astigmatic young adults (age = 20.94 ± 0.37 years; spherical-equivalent errors [M] = -1.47 ± 0.23 diopters [D]; cylindrical errors = -0.32 ± 0.05 D) were recruited. They were asked to wear a trial frame with treated and control lenses while watching a video for an hour. In three separate visits, the treated eye was exposed to one of three defocused conditions in random sequence: (1) with-the-rule (WTR) astigmatism = +3.00 DC × 180 degrees; (2) against-the-rule (ATR) astigmatism = +3.00 DC × 90 degrees; and (3) spherical defocus (SPH) = +3.00 DS. The control eye was fully corrected optically. Before and after watching the video, non-cycloplegic autorefraction was performed over the trial lenses. Refractive errors were decomposed into M, J0, and J45 astigmatism. Interocular differences in refractions (treated eye - control eye) were analyzed.

Results

After participants watched the video with monocular astigmatic defocus for an hour, the magnitude of the J0 astigmatism was significantly reduced by 0.25 ± 0.10 D in both WTR (from +1.53 ± 0.07 D to +1.28 ± 0.09 D) and 0.39 ± 0.15 D in ATR conditions (from -1.33 ± 0.06 D to -0.94 ± 0.18 D), suggesting an active compensation. In contrast, changes in J0 astigmatism were not significant in the SPH condition. No compensatory changes in J45 astigmatism or M were found under any conditions.

Conclusions

Watching a video for an hour with astigmatic defocus induced bidirectional, compensatory changes in astigmatic components, suggesting that refractive components of young adults are moldable to compensate for orientation-specific astigmatic blur over a short period.

Increased Risk of Refractive Errors and Amblyopia among Children with Ptosis: A Nationwide Population-Based Study

Background: This study aimed to investigate the risk of refractive errors (astigmatism, myopia, and hyperopia) and amblyopia in children with ptosis and association between age at diagnosis of ptosis and subsequent risks of vision problems. Methods: Retrospective claims data from the Taiwan National Health Insurance Research Database (NHIRD) were analyzed. We identified 1799 children aged 0−18 years who were newly diagnosed with ptosis between 2000 and 2012 and 7187 individuals without the disease. Both cohorts were followed up until 2013 to estimate the incidence of refractive errors and amblyopia. Results: Children with ptosis had 5.93-fold, 3.46-fold, 7.60-fold, and 13.45-fold increases in the risk of developing astigmatism, myopia, hyperopia, and amblyopia, respectively, compared with the control cohort (astigmatism: adjusted hazard ratio, aHR = 5.93, 95% confidence interval, CI = 5.16−6.82; myopia: aHR = 3.46, 95% CI = 3.13−3.83; hyperopia: aHR = 7.60, 95% CI = 5.99−9.63; amblyopia: aHR = 13.45, 95% CI = 10.60−17.05). Children diagnosed with ptosis at an age older than 3 years old had a higher risk of myopia than patients diagnosed with ptosis before age 3. There was no significant difference of the risk of astigmatism, amblyopia, and hyperopia between age groups. Conclusions: Children with ptosis may exhibit a higher risk of astigmatism, myopia, hyperopia, and amblyopia than children without ptosis. The risk of myopia is higher in children with ptosis diagnosed at >3 years than those diagnosed at ≤3 years.

Optical changes and apparent emmetropization in a patient with a peripheral unilateral lens coloboma

Lens coloboma is a developmental defect resulting from abnormalities of the zonules and ciliary body. It may present as an isolated pathology or be accompanied by anomalies in different ocular structures. We report the case of a 20-year-old man referred for evaluation of anisometropic amblyopia in the right eye. Manifest refraction was -2.25 +3.00 ×35 in the right eye; corrected distance visual acuity, 20/50. Corneal topography revealed regular astigmatism of +2.46 D at 124°, and wavefront aberrometry revealed an irregular internal astigmatism of +6.27 D at 35°. Only after full pupillary dilation was a peripheral lens coloboma observed. This case demonstrates that even minor distortions of clear and normally positioned lenses may lead to amblyopia and raises the possibility that corneal changes may occur developmentally through the process of emmetropization partly to compensate for lenticular astigmatism arising from the coloboma.

Corneal and Ocular Residual Astigmatism in School-Age Children

Purpose

To determine the distribution of residual and corneal astigmatism (CA) in children aged 6-18 years and their relationship with age, sex, spherical equivalent, and biometric parameters.

Methods

In this cross-sectional study, multi-stage stratified cluster sampling was done to select students from Dezful, a city in Southwestern Iran. Examinations included the measurement of visual acuity with and without optical correction, refraction with and without cycloplegia, and biometry using the Biograph (Lenstar, Germany). The main outcomes in this report were corneal and residual astigmatism. The CA was measured by Biograph (difference between k1 and k2), and residual astigmatism was calculated using Alpine method. The power vector method was applied to analyze the data of astigmatism.

Results

Of 864 students that were selected, 683 (79.1%) participated in the study. The mean residual and CA were -0.84 diopter (D) and -0.85 D, respectively. According to the results of J0 and J45 vectors, residual astigmatism was -0.33 D and 0.04 D, and CA was 0.38 D and 0.01 D, respectively. With-the-rule (WTR), against-the-rule (ATR), and oblique astigmatism were seen in 3.4%, 66.8%, and 4.5% of the children with residual astigmatism and 67.94%, 1.3%, and 1.5% of the children with CA. Residual astigmatism decreased with an increase in spherical refractive error, whereas CA increased with an increase in spherical refractive error.

Conclusion

The results of the present study showed a high prevalence and amount of residual astigmatism with ATR pattern among the 6-18-year-old population and the compensatory effect of this type of astigmatism on CA that mostly followed a WTR pattern.

Early Astigmatism Can Alter Myopia Development in Chickens

Purpose

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

Methods

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

Results

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

Conclusions

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

Chick Eyes Can Recover from Lens Compensation without Visual Cues

SIGNIFICANCE

This study shows that nonvisual mechanism(s) can guide chick eyes to recover from myopia or hyperopia bidirectionally to regain their age-matched length. Because eye growth control is phylogenetically conserved across many species, it is possible that, in general, emmetropization mechanisms are not exclusively based on a local visual feedback system.

PURPOSE

Across species, growing eyes compensate for imposed defocus by modifying their growth, showing the visual controls on eye growth and emmetropization. When the spectacle lens is removed, the eyes rapidly recover back to a normal size similar to that in the untreated eyes. We asked whether this recovery process was dependent on visual feedback or whether it might be guided by intrinsic nonvisual mechanisms.

METHODS

Chicks wore either a +7 (n = 16) or -7 D (n = 16) lens over one eye for 4 to 7 days; the fellow eye was left untreated. After lens removal, half were recovered in darkness and half in white light. Refractive error and ocular dimensions were measured before and after lens treatment and after recovery with a Hartinger refractometer and A-scan biometer, respectively.

RESULTS

Whereas chick eyes completely recovered from prior lens treatment under normal light after 2 days, they also partially recovered from prior hyperopia (by 60%) and myopia (by 69%) after being kept in darkness for 3 days: a +7 and -7 D lens induced a difference between the eyes of +7.08 and -4.69 D, respectively. After recovery in darkness, the eyes recovered by 3.18 and 2.88 D, respectively.

CONCLUSIONS

In the absence of visual cues, anisometropic eyes can modify and reverse their growth to regain a similar length to their fellow untreated eye. Because eye growth control is phylogenetically conserved across many species, it is possible that nonvisual mechanisms may contribute more generally to emmetropization and that recovery from anisometropic refractive errors may not be wholly visually controlled.

Astigmatismus

Der Astigmatismus ist der weltweit häufigste Refraktionsfehler vor Hypermetropie und Myopie. Man unterscheidet den äußeren vom inneren Astigmatismus. Der äußere Astigmatismus kann weiter in „mit der Regel“, „gegen die Regel“ und „schräg“ unterteilt werden. Die Summierung des äußeren und inneren Astigmatismus ergibt den refraktiven Zylinder. Astigmatismus wurde lange als zweidimensionales Phänomen gesehen, doch erst die dreidimensionale Betrachtung hat den Blick auf bestehende Analyseverfahren (Topo- und Tomographie) erweitert. Die Vektoranalyse nach Alpins ist hierbei eine bekannte Methode zur Therapieplanung. Zur Therapie des Astigmatismus stehen konservative Optionen wie Brille oder torische Kontaktlinse sowie unterschiedlichste chirurgische Verfahren wie photorefraktive Keratektomie, Femtosekundenlaser-assistierte Keratotomie, Laser-in-situ-Keratomileusis, „small-incision lenticule extraction“ und die Implantation torischer Intraokularlinsen zur Verfügung.

IMI - Report on Experimental Models of Emmetropization and Myopia

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

[Astigmatism]

Astigmatism is the most frequent refractive error worldwide followed by hyperopia and myopia. Internal astigmatism has to be differentiated from external astigmatism. Furthermore, external astigmatism can be divided into "with the rule", "against the rule" and "oblique". The summation of internal and external astigmatism results in the refractive cylinder. Astigmatism has for a long time been regarded as a two-dimensional phenomenon; however, only a three-dimensional consideration expanded the view on existing analytical methods (topography and tomography). Alpins' vector analysis is a commonly used method for treatment planning. Multiple options exist for treatment of astigmatism with conservative approaches, such as eyeglasses or toric contact lenses as well as various surgical procedures, such as photorefractive keratectomy, femtosecond laser-assisted keratotomy, laser in-situ keratomileusis, small incision lenticule extraction and toric intraocular lens implantation.

Astigmatism and its components in 12-year-old Chinese children: the Anyang Childhood Eye Study

Purpose

To determine prevalence of refractive (RA), corneal (CA) and internal astigmatism (IA), including variation with gender and spherical equivalent refraction (SE), in a population of 12-year-old Chinese children.

Methods

A total of 1783 students with a mean age of 12.7 years (range 10.0–15.6 years) completed comprehensive eye examinations in the Anyang Childhood Eye Study. Data of cycloplegic refraction and corneal curvature were analysed.

Results

Prevalences of RA, CA and IA ≥1.0 D were 17.4% (95%CI 15.6% to 19.2%), 52.8% (50.5% to 55.1%)%) and 20.9% (19.0% to 22.8%), respectively. With different limits of astigmatism axes classification, including ±15°, ±20° and ±30°, RA and CA axes were mainly ‘with-the-rule’ (WTR) (ie, correcting axis of negative cylinders at or near 180°), while those for IA axes were mainly ‘against-the-rule’ (ATR) (ie, correcting axis of negative cylinders at or near 90°). RA was not different between the genders, but girls had higher prevalence and greater means of CA and IA. RA and CA increased in students with higher ametropia (more myopia and more hyperopia) and were the highest in a high myopic group (SE≤−6 D), while IA was stable across refraction groups. Children with RA higher than 0.50 D were more likely to have lens corrections (51%, 57%, 61% and 69% for magnitudes of ≥0.50 D, ≥0.75 D, ≥1.0 D and ≥1.5 D, respectively).

Conclusions

Prevalence of RA in the Chinese 12-year-old children was relatively high compared with other studies. RA and CA had mainly ‘WTR’ astigmatism, while IA was mainly ATR and partially compensated for CA. Girls had greater means and prevalences of CA and IA than did boys. Both RA and CA, but not IA, increased with refractive errors away from emmetropia.

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.

Effects of optically imposed astigmatism on early eye growth in chicks

PURPOSE

To determine the effects of optically imposed astigmatism on early eye growth in chicks.

METHODS

5-day-old (P5) White Leghorn chicks were randomly assigned to either wear, monocularly, a "high magnitude" (H: +4.00DS/-8.00DC) crossed-cylindrical lens oriented at one of four axes (45, 90, 135, and 180; n = 20 in each group), or were left untreated (controls; n = 8). Two additional groups wore a "low magnitude" (L: +2.00DS/-4.00DC) cylindrical lens orientated at either axis 90 or 180 (n = 20 and n = 18, respectively). Refractions were measured at P5 and after 7 days of treatment for all chicks (P12), whereas videokeratography and ex-vivo eyeshape analysis were performed at P12 for a subset of chicks in each group (n = 8).

RESULTS

Compared to controls, chicks in the treatment groups developed significant amounts of refractive astigmatism (controls: 0.03 ± 0.22DC; treatment groups: 1.34 ± 0.22DC to 5.51 ± 0.26DC, one-way ANOVAs, p ≤ 0.05) with axes compensatory to those imposed by the cylindrical lenses. H cylindrical lenses induced more refractive astigmatism than L lenses (H90 vs. L90: 5.51 ± 0.26D vs. 4.10 ± 0.16D; H180 vs. L180: 2.84 ± 0.44D vs. 1.34 ± 0.22D, unpaired two-sample t-tests, both p ≤ 0.01); and imposing with-the-rule (H90 and L90) and against-the-rule astigmatisms (H180 and L180) resulted in, respectively, steeper and flatter corneal shape. Both corneal and internal astigmatisms were moderately to strongly correlated with refractive astigmatisms (Pearson's r: +0.61 to +0.94, all p ≤ 0.001). In addition, the characteristics of astigmatism were significantly correlated with multiple eyeshape parameters at the posterior segments (Pearson's r: -0.27 to +0.45, all p ≤ 0.05).

CONCLUSIONS

Chicks showed compensatory ocular changes in response to the astigmatic magnitudes imposed in this study. The correlations of changes in refractive, corneal, and posterior eyeshape indicate the involvement of anterior and posterior ocular segments during the development of astigmatism.

Genome-wide association study for refractive astigmatism reveals genetic co-determination with spherical equivalent refractive error: the CREAM consortium

To identify genetic variants associated with refractive astigmatism in the general population, meta-analyses of genome-wide association studies were performed for: White Europeans aged at least 25 years (20 cohorts, N = 31,968); Asian subjects aged at least 25 years (7 cohorts, N = 9,295); White Europeans aged <25 years (4 cohorts, N = 5,640); and all independent individuals from the above three samples combined with a sample of Chinese subjects aged <25 years (N = 45,931). Participants were classified as cases with refractive astigmatism if the average cylinder power in their two eyes was at least 1.00 diopter and as controls otherwise. Genome-wide association analysis was carried out for each cohort separately using logistic regression. Meta-analysis was conducted using a fixed effects model. In the older European group the most strongly associated marker was downstream of the neurexin-1 (NRXN1) gene (rs1401327, P = 3.92E-8). No other region reached genome-wide significance, and association signals were lower for the younger European group and Asian group. In the meta-analysis of all cohorts, no marker reached genome-wide significance: The most strongly associated regions were, NRXN1 (rs1401327, P = 2.93E-07), TOX (rs7823467, P = 3.47E-07) and LINC00340 (rs12212674, P = 1.49E-06). For 34 markers identified in prior GWAS for spherical equivalent refractive error, the beta coefficients for genotype versus spherical equivalent, and genotype versus refractive astigmatism, were highly correlated (r = -0.59, P = 2.10E-04). This work revealed no consistent or strong genetic signals for refractive astigmatism; however, the TOX gene region previously identified in GWAS for spherical equivalent refractive error was the second most strongly associated region. Analysis of additional markers provided evidence supporting widespread genetic co-susceptibility for spherical and astigmatic refractive errors.