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Zone of Clear Single Binocular Vision in Myopic Orthokeratology
Abstract
Purpose:
To examine the zone of clear single binocular vision (ZCSBV) in myopic children and young adults after 12 months of orthokeratology (OK) wear, in comparison with single-vision soft contact lens (SCL) wear.
Methods:
Twelve children (8-16 years) and 8 adults (18-29 years) were assessed with a series of near-point binocular vision tests when myopia was corrected using single-vision SCLs and again after 1 and 12 months of OK wear, and axial length was measured. The ZCSBV was constructed for baseline SCL wear and after 12 months of OK wear.
Results:
After 1 month of OK wear, increased accommodative responses were noted in children (C) and adults (A) as increased binocular amplitude (C:P=0.03, A:P=0.04) and reduced accommodative lag (C:P=0.01, A:P=0.01). Divergence reserves improved after 1 month in both groups (P<0.04), and a near exophoric shift was evident at 12 months (C:P=0.01, A:P=0.04). All changes at 1 month maintained stability at 12 months. An increase in accommodation and vergence responses without reduction in range resulted in an expansion of the ZCSBV in both age groups. Axial length did not significantly change in either children (P=0.25) or adults (P=0.72).
Conclusion:
In both pediatric and young adult myopes, the ZCSBV expands toward a more divergent, increased accommodation response in OK compared with SCL wear. This occurs without a corresponding loss of convergence or accommodation deactivation, indicating improved depth of focus. These findings are relevant to visual acceptance and possible mechanisms of OK's efficacy for myopia control.
... Одним з перспективних напрямків у консервативному лікуванні аномалій рефракції є ортокорнеальна терапія -застосування жорстких газопроникних контактних лінз (ОК-лінз) з метою зміни рефракції ока, що дозволяє не лише досягти еметропізації, але й покращити акомодаційну спроможність ока та відновити бінокулярні функції [3,7,8]. Однак у дітей, що народилися передчасно, цей метод майже не застосовується: висока кривизна рогівки (понад 47,0 Дптр) є протипоказанням до застосування більшості ОКлінз, оскільки в такому випадку потрібно змінювати параметри першої пробної лінзи, зменшувати діаметр та збільшувати глибину поворотної зони, що потребує індивідуального підходу. ...
... У свою чергу, нормалізація гостроти зору сприятливим чином впливає на розвиток фузії -злиття двох зорових зображень в одне ціле та на здатність взаємодії двох очей у зоровому процесі [7,8]. Завдяки підвищенню зорових функцій стійке відновлення бінокулярного зору спостерігалося нами у всіх дітей основної та 83% дітей контрольної групи. ...
... 22/ Том XXVII / 3 впливу ортокератологічної терапії на стан акомодації, бінокулярного зору та гостроти зору в дітей та осіб молодого віку [7,8]. ...
The study included 62 children aged 6 to 14 years with mild to moderate myopia, born at 28-34 weeks of gestation with a birth weight of 970 to 2200 g. Selection criteria: corneal refractive power >46.00 D, no keratoconus or macular degeneration. The main group – 32 children (64 eyes), who were assigned orthokeratologic lenses, the control group – 30 children (60 eyes) – glasses users. Differences between the groups were not significant. Observation period was 3 years. There were no complications. Corrected visual acuity in the main group increased from 0.63±0.08 to 0.98±0.06, in the control – from 0.61±0.05 to 0.73±0.05; p<0.005. Stable restoration of binocular vision was observed in all children of the study group and 83% of children in the control one. Reserves of absolute accommodation in the main group during the observation period increased by 6.7±0.38 D, and in the control group – by 2.3±0.42 D (p<0.001), which is explained by the active use of accommodation by children of the main group. After 3 years of observation in the main group, the anteroposterior size of the eyeball, according to echobiometry data, almost did not change (from 22.32±0.9 to 24.02±1.1 mm, p>0.2), and in the control group, the eye elongation was more pronounced: from 22.45±0.8 to 25.94±0.9 mm (p<0.01). Complete stabilization of myopia was observed in 30 children (93.75%) of the study group. Orthokeratological lenses MoonLens can be used in patients with high corneal curvature; their use in prematurely babies with a high refractive power of the cornea (>46.0 D) allowed to obtain better results of the treatment.
... Recent studies have shown that binocular vision changes after wearing orthokeratology lenses or multifocal soft contact lenses [8][9][10]. For example, the fusional range decreases [8], and ocular alignment shows an exophoric shift [8][9][10]. ...
... Recent studies have shown that binocular vision changes after wearing orthokeratology lenses or multifocal soft contact lenses [8][9][10]. For example, the fusional range decreases [8], and ocular alignment shows an exophoric shift [8][9][10]. In these studies, subjects were always visually healthy with stable binocular vision; however, clinical concerns have been raised about whether those interventions can also be applied to children with abnormal binocular vision. ...
... Previous studies proved that the positive fusional vergence (PFV) and fusion reserve ratio were correlated with the ability to maintain ocular alignment [13][14][15], and PFV was lower in IXT than in visually normal children [16,17], which could make it difficult for the fusional maintenance system to control ocular deviation. Since contact lenses for myopia control may influence the vergence and fusion systems in normal myopic children [8][9][10], therefore, it is worth exploring whether specialized spectacle lenses for myopia control will also affect binocular vision in myopic children, especially those with IXT. ...
Purpose:
To evaluate the influence of spectacle lenses with highly aspherical lenslets (HAL) on binocular vision and accommodation in myopic children with intermittent exotropia (IXT) and compare the changes after wearing HAL in binocular vision and accommodation in myopic children with or without IXT.
Method:
Forty myopic subjects aged 8-12 years were recruited: 20 with IXT and 20 visually normal children. Stereoacuity, phoria, accommodative facility, fusional vergence, vergence facility, near point of convergence, amplitude of accommodation, and accommodative response (AR) were measured by wearing HAL or single vision spectacle lenses (SVL) in a random order after adapting for 20 minutes. Accommodative microfluctuation (AMF) was defined as the standard deviation of AR. Changes in binocular vision and accommodation after wearing HAL were compared between the two groups.
Results:
No significant differences were found in binocular vision after wearing HAL versus SVL in either group (all P > 0.05). A greater AMF was found after wearing HAL than after wearing SVL in both groups (0.04 D, 95% confidence interval (CI), 0.03 to 0.05 D, P < 0.001 for the IXT group; 0.05 D, 95% CI, 0.03 to 0.07 D, P < 0.001 for the visually normal group); however, the other accommodation parameters did not change significantly (all P > 0.05). There were no differences in the changes after wearing HAL in any parameter between the two groups (all P > 0.05).
Conclusion:
HAL did not significantly change the binocular vision and accommodation for myopic children with or without IXT except for AMF in the short term.
... Since near work and accommodation have long been associated with myopia development [330], and accommodation is influenced by spherical aberration [331], several studies have examined the effect of ortho-k upon accommodation function ( Table 7). The most consistent finding across a range of study designs is that ortho-k for the correction of myopia typically results in a reduction in a lag of accommodation (on average ~0.50 D or less in children [312,332,333] and ~1.00 D in adults [333]). Most studies have assessed accommodative accuracy using dynamic retinoscopy, without specifically controlling for pupil size over time or when comparing between treatment groups. ...
... Since near work and accommodation have long been associated with myopia development [330], and accommodation is influenced by spherical aberration [331], several studies have examined the effect of ortho-k upon accommodation function ( Table 7). The most consistent finding across a range of study designs is that ortho-k for the correction of myopia typically results in a reduction in a lag of accommodation (on average ~0.50 D or less in children [312,332,333] and ~1.00 D in adults [333]). Most studies have assessed accommodative accuracy using dynamic retinoscopy, without specifically controlling for pupil size over time or when comparing between treatment groups. ...
... Most studies have assessed accommodative accuracy using dynamic retinoscopy, without specifically controlling for pupil size over time or when comparing between treatment groups. Other studies have also reported improvements in positive [333] or negative relative accommodation [334] and accommodative facility [332], but these findings are less consistent. Further studies that quantify the accommodation response using objective automated techniques while controlling for pupil diameter are still required to understand if ortho-k significantly alters the accommodation response in children and adults, and if this contributes to the myopia control effect of ortho-k. ...
Orthokeratology (ortho-k) is the process of deliberately reshaping the anterior cornea by utilising specialty contact lenses to temporarily and reversibly reduce refractive error after lens removal. Modern ortho-k utilises reverse geometry lens designs, made with highly oxygen permeable rigid materials, worn overnight to reshape the anterior cornea and provide temporary correction of refractive error. More recently, ortho-k has been extensively used to slow the progression of myopia in children.
This report reviews the practice of ortho-k, including its history, mechanisms of refractive and ocular changes, current use in the correction of myopia, astigmatism, hyperopia, and presbyopia, and standard of care. Suitable candidates for ortho-k are described, along with the fitting process, factors impacting success, and the potential options for using newer lens designs. Ocular changes associated with ortho-k, such as alterations in corneal thickness, development of microcysts, pigmented arcs, and fibrillary lines are reviewed. The safety of ortho-k is extensively reviewed, along with an overview of non-compliant behaviours and appropriate disinfection regimens. Finally, the role of ortho-k in myopia management for children is discussed in terms of efficacy, safety, and potential mechanisms of myopia control, including the impact of factors such as initial fitting age, baseline refractive error, the role of peripheral defocus, higher order aberrations, pupil size, and treatment zone size.
... Prior myopia progression status was not quantified, nor considered a criterion for participation. The primary outcomes of this study have been reported elsewhere [31] with sample size calculations undertaken to appropriately power that study -the outcomes reported here were secondary measures. Twelve out of 16 children and 8 out of 11 young adults had RPR measures undertaken over the duration of the study. ...
... The most significant limitation of this study is number of participantsa larger study may have elicited relationships further between RPR and axial length. This study was powered to detect differences for other outcome measures which have been previously reported [31], and as such the lack of relationship between RPR and axial length changes should not be taken as absence of a correlation. The myopia control effect of OK in young adult myopes has not been studied, and the comparatively older mean age of the children group investigated here of 13.2 ± 2.1 years could also lead to less observable myopia progression over a year. ...
Purpose:
Orthokeratology (OK) is known to alter relative peripheral refraction (RPR) with this presumed to be its key myopia control mechanism. A prospective, longitudinal study was performed to examine stability of OK-induced RPR changes in myopic children and young adults.
Methods:
RPR of twelve children (C)(8-16 years) and eight adults (A)(18-29 years) with spherical equivalent refraction of -0.75 to -5.00D were measured unaided and while wearing single vision soft contact lenses (SCL). Measurements were repeated after 1, 6 and 12 months of OK wear. RPR was measured using an open-field Shin Nippon SRW-5000 autorefractor at 10, 20 and 30 degrees nasally (N) and temporally (T), converted into power vectors M, J0 and J45. On-axis refractions and axial lengths (IOL Master) were also measured.
Results:
Compared to the unaided state, 1-month of OK wear shifted the RPR in the myopic direction at 30 T (C: p = 0.023; A:, p = 0.002) and 30 N (C&A, p = 0.003) and was stable thereafter, with similar changes compared to SCL wear. J0 showed a myopic shift in comparison to both unaided and SCL correction in children but not adults, and J45 did not change in either group. The on-axis OK correction was predictive of the RPR shift in both children and adults at 30 T (C: r=-0.58, p = 0.029; A: r=-0.92, p < 0.001) and 30 N (C: r=-0.60, p = 0.024; A: r=-0.74, p = 0.013) with symmetry of RPR shifts (C: r = 0.67, p = 0.008; A: r = 0.85, p = 0.004). No relationships between changes in RPR and axial length were found after twelve months of OK wear; level of myopia was stable in both groups.
Conclusion:
Relative to both unaided and single vision SCL correction, OK shifted the RPR in the myopic direction; the RPR was stable from 1 to 12 months. The RPR shift in OK wear varied with the degree of myopia but was not correlated with myopia progression.
... 18 OrthoK also tends to increase exophoria in young adult myopes, making it a suitable option for children with esophoria. 19 However, patients with IXT or high exophoria may not be ideal candidates. Evaluating near base-out vergence ranges during initial follow-ups ensures Sheard's criterion is met, optimizing treatment outcomes. ...
Myopia is a growing problem worldwide and is particularly common among young people. Myopia is a disease that affects the retina responsible for detail in the middle eye and can lead to permanent vision loss. Myopia, a prevalent refractive error causing distant objects to appear blurred, is increasingly affecting global populations, particularly children. By 2050, it's projected that half the world's population will be myopic, largely due to genetic predispositions and environmental factors like excessive near work and limited outdoor activities. A promising approach to managing myopia is binocular vision training, which involves using both eyes to create a single, clear image, potentially slowing myopia progression. Understanding the benefits of this method is critical as we move forward, especially considering the surge in myopia cases during the COVID-19 pandemic due to increased digital device use. Research underscores the importance of addressing lifestyle and promoting outdoor activities and mitigating the impact of near work can help manage myopia progression and associated binocular vision disorders. Binocular vision training shows promise for myopia control by enhancing eye coordination, potentially slowing progression. Key factors include accommodation, AC/A ratios, and phoria, which are essential for effective diagnostics and therapy. Recent research emphasizes a holistic assessment of these elements for precise management of myopia and related binocular vision disorders in children. Understanding the mechanisms and innovative approaches of binocular vision in controlling
... Fusiform vergence is also higher in those with progressive myopia [49], and on the other hand, myopic children show less convergent changes in vergence adaptation compared to emmetropes, which may be attributed to higher accommodative adaptation (assessed by changes in tonic accommodation) [50]. When myopia is compensated with spectacle correction for distance, a child's area of clear single binocular vision becomes more divergent, and accommodative responses increase relative to that measured with correction with mono-focal glasses [51]. A limitation of our work is that examiners enter absolute values of eyeball deviation without marking the direction of exo-or esophoria. ...
Background: Determination of the number of pupils at risk of developing pre-myopia and selected ophthalmic parameters in a group of 1155 children aged 8. Material: Ophthalmic examinations were performed in Polish 8-year-old, /1518 individuals/; 1155 of whom presented complete data for analysis. There was a total of 554 (47.9%) girls and 602 (52.1%) boys. Examination of the anterior and posterior segment of the eye, evaluation of accommodation, convergence, heterophoria, alignment of the eyeball, muscular balance with ocular mobility in 9 directions of gaze, and spatial vision were tested. Refraction was obtained under cycloplegia. Refractions (spherical equivalent, SE). were categorized as pre-myopia (−0.50 D–+0.75 D), myopia (≤−0.5 D), emmetropia (>−0.5 D to ≤+0.5 D), mildly hyperopia (>+0.5 D to ≤+2.0 D) and hyperopia (>+2.0 D). Data analysis was performed using Statistica 13.5 software: chi-squared, Pearson’s, t-Student, and U Mann–Whitney tests. p-values of <0.05 were considered statistically significant. Results: Pre-myopia was diagnosed in as many as 704 subjects (60.9%) with a similar frequency among both girls—328 (46.6%)—and boys with 376 (53.4%). Conclusions: Current data indicates that the growing group of myopic individuals in many industrialized countries is the sixth most common cause of blindness. Further research is crucial to understand the factors underlying accommodative and binocular mechanisms for myopia development and progression and to make recommendations for targeted interventions to slow the progression of myopia in a group of early school children.
... Several previous studies have investigated the accommodative changes following orthokeratology treatment. Gifford et al. [52,53] discovered that patients treated with orthokeratology had better accommodative responses and less accommodative lag than those treated with single-vision contact lenses. Han et al. [54] found that accommodative accuracy and facility were improved with long-term orthokeratology. ...
Purpose
To investigate the relationship between relative corneal refractive power shift (RCRPS) and axial length growth (ALG) in bilateral myopic anisometropes treated with orthokeratology.
Methods
A total of 102 children with myopic anisometropia in this prospective interventional study were randomly assigned to the spectacle group and orthokeratology group. Axial length (AL) and corneal topography was measured at baseline and the 12-month follow-up visit. ALG was defined as the difference between the two measurements, and RCRPS profiles were calculated from two axial maps obtained.
Results
In the orthokeratology group, the ALG in the more myopic eye (0.06 ± 0.15 mm) was significantly smaller than that in the less myopic eye (0.15 ± 0.15 mm, p < 0.001), and the interocular difference in AL significantly decreased following 1-year treatment, from 0.47 ± 0.32 to 0.38 ± 0.28 mm (p < 0.001). However, in the spectacle group, the ALG was similar between the two eyes, and the interocular difference in AL did not change significantly over one year (all p > 0.05). The interocular difference in ALG in the orthokeratology group was significantly correlated with the interocular difference in RCRPS (dRCRPS, β=−0.003, p < 0.001) and the interocular difference in baseline AL (β=−0.1179, p < 0.001), with R² being 0.6197.
Conclusion
Orthokeratology was effective in decreasing the magnitude of anisometropia. The interocular variation in RCRPS is an important factor accounting for the reduction of interocular ALG difference in anisomyopic children post-orthokeratology. These results provide insight into establishing eye-specific myopia control guidelines during orthokeratology treatment for myopic anisometropes.
... Dual-focus SCL also had a minimal impact on phoria and accommodative response. [228][229][230] New data on ortho-k propensity to increase the accommodation response 231,232 and create an exophoric shift in children at near 232 are in agreement with results from previous studies 233,234 and appear linked to a greater myopia control. 231 In contrast, aspheric MFSCLs were found to reduce the accommodation response and yet induce a small exophoric shift in children 235,236 and young adults, 228,237,230,237 with different peripheral add powers having no differential impact on these responses. ...
Myopia is a dynamic and rapidly moving field, with ongoing research providing a better understanding of the etiology leading to novel myopia control strategies. In 2019, the International Myopia Institute (IMI) assembled and published a series of white papers across relevant topics and updated the evidence with a digest in 2021. Here, we summarize findings across key topics from the previous 2 years. Studies in animal models have continued to explore how wavelength and intensity of light influence eye growth and have examined new pharmacologic agents and scleral cross-linking as potential strategies for slowing myopia. In children, the term premyopia is gaining interest with increased attention to early implementation of myopia control. Most studies use the IMI definitions of ≤-0.5 diopters (D) for myopia and ≤-6.0 D for high myopia, although categorization and definitions for structural consequences of high myopia remain an issue. Clinical trials have demonstrated that newer spectacle lens designs incorporating multiple segments, lenslets, or diffusion optics exhibit good efficacy. Clinical considerations and factors influencing efficacy for soft multifocal contact lenses and orthokeratology are discussed. Topical atropine remains the only widely accessible pharmacologic treatment. Rebound observed with higher concentration of atropine is not evident with lower concentrations or optical interventions. Overall, myopia control treatments show little adverse effect on visual function and appear generally safe, with longer wear times and combination therapies maximizing outcomes. An emerging category of light-based therapies for children requires comprehensive safety data to enable risk versus benefit analysis. Given the success of myopia control strategies, the ethics of including a control arm in clinical trials is heavily debated. IMI recommendations for clinical trial protocols are discussed.
... Preventive strategies for myopia control are important, and many studies suggest orthokeratology (Ortho-K) contact lenses are an effective method to mitigate myopia progression in adults. 36,37 However, orthokeratology contact lenses may be a more difficult option during the COVID-19 situation. Thus, another way to manage COVID-19-related myopia progression in adults is to wear bifocal or multifocal eyeglasses. ...
Purpose
The COVID-19 pandemic has necessitated specific public health measures, resulting in the alteration of lifestyles, such as increased digital screen time and fewer outdoor activities. Such conditions have increased the progression of myopia in children. However, no investigation of myopia progression in early adulthood has been conducted during this period. Consequently, this study aimed to evaluate the outbreak of COVID-19-related myopia progression among adults at an optometry clinic during the COVID-19 pandemic.
Materials and Methods
This was a retrospective cohort study in which participants aged 18–25 years who first visited (baseline) the optometry clinic between June 2019 and March 2020 were recruited for follow-up from November 2021 to March 2022. Spherical equivalent refraction (SER), uncorrected distance visual acuity (UCDVA), and binocular cross cylinder (BCC) were recorded at baseline and a follow-up visit. Using questionnaires, a survey was conducted to assess the lifestyle changes that transpired during the COVID-19 pandemic.
Results
In total, 37 participants with a mean age of 22.5±1.4 years were enrolled, of which 89.2% were female. Following the outbreak of the COVID-19 pandemic, most participants self-reported increased daily use of digital devices (89.2%), online education (86.5%), and spending more time at home (94.6%), which increased by approximately 7.6±3.2 hours, 5.9±1.7 hours, and 13.2±7.5 hours, respectively. There were statistically significant differences between SER and BCC at baseline and after approximately 2 years of the COVID-19 pandemic (p < 0.05). The mean two-year myopia progression was −0.59±0.67 D (Maximum = 0.00 D, Minimum = −3.38 D).
Conclusion
This study revealed that myopia could progress during adulthood among those who have lived under public health measures intended to address the COVID-19 pandemic.
... [10][11][12] Some researchers observed that accommodative accuracy improved after OK treatment and proposed that it plays a role in myopia control by reducing hyperopic defocus. [13][14][15][16][17] However, there are no longitudinal studies to explore the possible effects of these accommodative changes on myopia control in OK-treated eyes. ...
Purpose
This study aimed to investigate the effect of orthokeratology (OK) on accommodative function and aberrations, to explore the correlations between them and determine what role they play in myopia control.
Methods
In this prospective case-controlled study, 61 children were divided into an OK (n = 30) and a single-vision spectacles (SVS) (n = 31) group. Accommodation and ocular wavefront aberrations in the OK group were measured at baseline and after 1, 3, 6, 9 and 12 months of OK wear, and again at 1 month after stopping OK (13th month). The same procedure was performed in the SVS group at baseline and at 3, 6 and 12 months. Axial length (AL), accommodative lag area and aberrations including spherical aberration (SA), coma and total higher-order aberrations (HOAs) were analysed.
Results
During OK wear, the accommodative lag area at each visit was lower than the baseline level (all p < 0.01); all aberrations at each visit were higher than pre-treatment (all p < 0.001). After 1 month of OK treatment, changes in accommodative lag area and SA did not show significant correlation (p = 0.16), but after OK cessation these changes were correlated (p = 0.01). In the OK group, multivariate regression analysis showed changes in accommodative lag area were associated with AL progression in the first 6 months but not in the 1-year analysis. For the SVS group, there were no significant changes in the accommodative lag area or any aberrations during the study period.
Conclusions
Increased HOAs and improved accommodative accuracy were observed during OK treatment, but began to regress after the cessation of OK. A significant positive correlation between improved accommodative accuracy and slowed axial elongation was only observed during the first 6 months of treatment.
... 23 A reduced accommodation demand could be expected to increase the measured accommodative amplitude and reduce accommodative lag, which is the case with orthokeratology. 20,24 This has not been observed with MFCLs, for which there is evidence for no amplitude change 10 but there is an increase in measured accommodative lag. 10,11 An increased amplitude of accommodation in orthokeratology treated children has been related to greater myopia control efficacy. ...
Purpose:
Prolonged nearwork has been implicated in myopia progression. Accommodation responses of young-adult myopes wearing different multifocal contact lenses were compared.
Methods:
Twenty adults, 18-25 years, with myopia (spherical equivalent refraction -0.50 to -5.50 D, mean -2.1 ± 1.6 D) wore five lens types in random order: Proclear single vision distance (SV), MiSight concentric dual-focus +2.00 D Add (MS), Biofinity aspheric centre distance +1.50 D Add (CD1) and +2.50 D Add (CD2) (all Coopervision), and NaturalVue aspheric (Visioneering Technologies) (NVue). Using a Grand-Seiko WAN-5500 autorefractor with binocular correction and viewing right eye accommodative responses were measured after a 10 min adaptation period at 4.0, 1.0, 0.5, 0.33 and 0.25 m distances. Dynamic measurements were taken for 4 s at 6 Hz. Accommodative stimuli and responses were referenced to 4 m (i.e., refraction differences between 4 m and nearer distances). Accommodation lags and refraction instabilities (standard deviations of dynamic responses) were determined. For comparison, results were obtained for an absolute presbyopic eye, where trial lenses counteracted the accommodation stimulus.
Results:
For SV and MS, accommodation responses were similar to the stimulus values. For aspheric lenses CD1, CD2 and NVue, accommodation responses were approximately 1.0 D lower across the stimulus range than with SV and MS, and rates of change were approximately 0.84 D per 1 D stimulus change. MS produced greater refraction instabilities than other lenses. For the presbyope, changes in refraction matched the trial lenses, indicating that corrections due to measurement through the different lenses were not needed.
Conclusion:
Reductions in accommodation response occurred in young myopes wearing aspheric multifocal contact lenses independent of the labelled 'add' power. The concentric dual-focus MS lens produced minimal lags but had greater instability than the other lenses. The results indicate that the mechanism of multifocal contact lenses slowing myopia progression is unlikely to be through relaxing accommodation, at least in young adults.
... 275 Orthokeratology lens wear has also been shown to increase exophoria in young adult myopes. 269 However, unlike soft multifocal lenses, orthokeratology lenses have been found to lower accommodative lags at near, prompting some to suggest that these lenses may be a better strategy to slow reduce myopia progression in adults with binocular vision disorders. 276 Studies in children show reduced accommodation response and an increase in exophoria while wearing center-distance soft bifocal 277 or multifocal contact lens 278 compared with single vision contact lenses, suggesting that perhaps children resort to using the relative plus power in an attempt to relax accommodation. ...
The role of accommodation in myopia development and progression has been debated for decades. More recently, the understanding of the mechanisms involved in accommodation and the consequent alterations in ocular parameters has expanded. This International Myopia Institute white paper reviews the variations in ocular parameters that occur with accommodation and the mechanisms involved in accommodation and myopia development and progression. Convergence is synergistically linked with accommodation and the impact of this on myopia has also been critiqued. Specific topics reviewed included accommodation and myopia, role of spatial frequency, and contrast of the task of objects in the near environment, color cues to accommodation, lag of accommodation, accommodative-convergence ratio, and near phoria status. Aspects of retinal blur from the lag of accommodation, the impact of spatial frequency at near and a short working distance may all be implicated in myopia development and progression. The response of the ciliary body and its links with changes in the choroid remain to be explored. Further research is critical to understanding the factors underlying accommodative and binocular mechanisms for myopia development and its progression and to guide recommendations for targeted interventions to slow myopia progression.
... The zone of clear single binocular vision (ZCSBV) is the range of vergence and accommodative stimuli for which the viewer has sharp, single vision (Hofstetter, 1945;Peli, 1995;Gifford, Gifford, Hendicott, & Schmid, 2020). Outside of the zone, the percept is blurred and/or diplopic. ...
From measurements of wavefront aberrations in 16 emmetropic eyes, we calculated where objects in the world create best-focused images across the central 27 (diameter) of the retina. This is the retinal conjugate surface. We calculated how the surface changes as the eye accommodates from near to far and found that it mostly maintains its shape. The conjugate surface is pitched top-back, meaning that the upper visual field is relatively hyperopic compared to the lower field. We extended the measurements of best image quality into the binocular domain by considering how the retinal conjugate surfaces for the two eyes overlap in binocular viewing. We call this binocular extension the blur horopter. We show that in combining the two images with possibly different sharpness, the visual system creates a larger depth of field of apparently sharp images than occurs with monocular viewing. We examined similarities between the blur horopter and its analog in binocular vision: the binocular horopter. We compared these horopters to the statistics of the natural visual environment. The binocular horopter and scene statistics are strikingly similar. The blur horopter and natural statistics are qualitatively, but not quantitatively, similar. Finally, we used the measurements to refine what is commonly referred to as the zone of clear single binocular vision.
... The same author, in a 12-month prospective study, reported improved accommodative responses with OK. The accommodative lag decreased after one month of OK in children and after 12 months in young adults [48]. The strongest evidence of changes in the accommodative response with OK is present in a study conducted by Han et al. [49]. ...
This study aimed to evaluate the effects of two months of orthokeratology (OK) treatment in the accommodative response of young adult myopes. Twenty eyes (21.8 ± 1.8 years) were fitted with the Paragon CRT® 100 LENS to treat myopia between −1.00 and −2.00 D. Low- and high-contrast visual acuity (LCDVA and HCDVA), central objective refraction, light disturbance (LD), and objective accommodative response (using the Grand Seiko WAM-5500 open-field autorefractometer coupled with a Badal system) were measured at baseline (BL) before lens wear and after 1, 15, 30, and 60 nights of OK. Refractive error correction was achieved during the first fifty days of OK lens wear, with minimal changes afterwards. LD analysis showed a transient increase followed by a reduction to baseline levels over the first 30 nights of treatment. The accommodative response was lower than expected for all target vergences in all visits (BL: 0.61 D at 1.00 D to 0.96 D at 5.00 D; 60 N: 0.36 D at 1.00 D to 0.79 D at 5.00 D). On average, the accommodative lag decreases over time with OK lens wear. However, these differences were not statistically significant (p > 0.050, repeated-measures ANOVA and Friedman test). This shows that overnight OK treatment does not affect objectively measured the accommodative response of young, low myopic eyes after two months of treatment stabilization.
... However, Gifford et al. 58 found a lower accommodative lag in orthokeratology subjects compared with control subjects wearing single-vision contact lenses in a retrospective study. In a subsequent 12-month prospective study, Gifford et al. 59 reported improved accommodative responses with orthokeratology. They measured negative relative accommodation, positive relative accommodation, and accommodative lag in myopic children and young adults. ...
Orthokeratology has undergone drastic changes since first described in the early 1960s. The original orthokeratology procedure involved a series of lenses to flatten the central cornea and was plagued by variable results. The introduction of highly oxygen‐permeable lens materials that can be worn overnight, corneal topography, and reverse‐geometry lens designs revolutionised this procedure. Modern overnight orthokeratology causes rapid, reliable, and reversible reductions in refractive error. With modern designs, patients can wear lenses overnight, remove them in the morning, and see clearly throughout the day without the need for daytime refractive correction. Modern reverse‐geometry lens designs cause central corneal flattening and mid‐peripheral corneal steepening that provides clear foveal vision while simultaneously causing a myopic shift in peripheral retinal defocus. The peripheral myopic retinal defocus caused by orthokeratology is hypothesised to be responsible for reductions in myopia progression in children fitted with these lenses. This paper reviews the changes in orthokeratology lens design that led to the reverse‐geometry orthokeratology lenses that are used today and the optical changes these lenses produce. The optical changes reviewed include changes in refractive error and their time course, high‐ and low‐contrast visual acuity changes, changes in higher‐order aberrations and visual quality metrics, changes in accommodation, and changes in peripheral defocus caused by orthokeratology. The use of orthokeratology for myopia control in children is also reviewed, as are hypothesised connections between orthokeratology‐induced myopic peripheral defocus and slowed myopia progression in children, and safety and complications associated with lens wear. A better understanding of the ocular and optical changes that occur with orthokeratology will be beneficial to both clinicians and patients in making informed decisions regarding the utilisation of orthokeratology. Future research directions with this lens modality are also discussed.
Orthokeratology lenses are effective methods for controlling myopia progression. This paper describes the principles and effects of orthokeratology lenses, focusing on randomized controlled trials investigating their ability to prevent myopia progression. While the short-term effects are widely accepted, further well-designed studies are required to assess the persistence of long-term benefits and the possibility of a rebound phenomenon after lens discontinuation. Additionally, both physicians and patients should prioritize safety, particularly in preventing complications such as infectious keratitis.
Purpose: The study aimed to investigate the changes in binocular vision (BV) and accommodation in myopic children following long-term wear of Defocus Incorporated Multiple Segments (DIMS) spectacle lenses for 24 months. Methods: Twenty-three Malay myopic children aged 7–12 years were enrolled in this prospective, self-controlled study. DIMS spectacle lenses were prescribed, and assessments were conducted at baseline, 12, and 24 months. Assessments included visual acuity (VA), cycloplegic refraction, axial length (AL), and a range of BV measures including stereopsis, near point of convergence (NPC), phoria, positive/negative fusional vergence (PFV/NFV), amplitude of accommodation (AA), accommodative lag, positive/negative relative accommodation (PRA/NRA), and accommodative convergence to accommodation (AC/A) ratio. Results: Statistically significant changes were observed after 24 months, including AL elongation and myopia progression ( P <0.001), receded NPC ( p <0.001), reduced AA ( p =0.002), increased distance PFV ( p =0.026), and improvements in accommodative lag ( p =0.002), NRA ( p =0.004), stereopsis ( p <0.001), and the AC/A ratio ( p <0.001). Additionally, there were statistically significant improvements in distance and near VA ( p <0.01). Conclusion: Wearing DIMS spectacle lenses for 24 months significantly altered the BV and accommodation of myopic children, particularly by improving the accommodation-convergence interaction. Regular monitoring of NPC, as well as distance and near PFV and NFV, is recommended to optimize the effectiveness of DIMS lenses in controlling myopia progression and maintaining BV stability.
Objective
The study aimed to compare the visual performance of contact lenses with and without negative spherical aberration (SA) over 5 days of wear.
Methods
At baseline, 32 myopic participants (aged 18–33 years) were fitted in a randomized order with two lenses (test lens with minimal or no SA and 1-Day Acuvue Moist designed with negative SA) for 5 days (minimum 6 hours wear/day). Participants returned for a follow-up visit. This consisted of on-axis SA measurements; high- and low-contrast visual acuities at 6 m; high-contrast acuities at 70 and 40 cm; low-illumination, low-contrast acuity at 6 m; stereopsis at 40 cm; horizontal phorias at 3 m and 33 cm; and ±2.00 D monocular accommodative facility at 33 cm. Participants also rated (1–10 scale) vision quality (clarity and lack of ghosting for distance, intermediate, near, driving vision and vision stability during day- and night-time), overall vision satisfaction, ocular comfort, and willingness to purchase (yes/no response).
Results
1-Day Acuvue Moist induced significantly (p<0.05) more negative SA at distance (Δ=0.078 μm) and near (Δ=0.064 μm) compared to the test lens, for a 6 mm pupil. There were no significant differences (p>0.05) in acuity, binocular vision, and all subjective metrics except vision stability between lenses where the test lens was rated to provide more stable vision (p<0.05).
Conclusion
Contrary to expectations, incorporating negative SA in single vision soft contact lenses did not improve visual performance in non-presbyopic adult myopes.
Purpose:
To analyze the binocular function changes produced on subjects undergoing overnight orthokeratology (OK) treatment over short-term (3 months) and long-term (3 years) wear.
Methods:
A prospective, longitudinal study on young adult subjects with low to moderate myopia was carried out. Binocular function was assessed by the following sequence of tests: Distance and near horizontal phoria (Von Graefe technique), distance and near horizontal vergence ranges (Risley rotary prisms), accommodative convergence/accommodation (AC/A) ratio (gradient method) and the near point of convergence (standard push-up technique). The short-term sample consisted of: 21 subjects in the control group, 26 in a corneal refractive therapy (CRT) treatment lenses group and 25 in a Seefree treatment lenses group. Those subjects were evaluated at baseline and at a 3-month follow-up visit. Twenty one subjects were old CRT wearers that attended a 3-year follow-up visit (long-term group).
Results:
A statistically significant difference over the 3-month treatment was found for divergence at distance: the break point decreased 1.4 Δ (p = 0.0006) in the CRT group and the recovery point increased 1.2 Δ (p = 0.001) in the Seefree group. Also, the Seefree group had an exophoric trend of 2.3 Δ at near (p = 0.02) and a base-out break decrease of 2.3 Δ (p = 0.03). For the long-term group, only the base-out break point at distant vision showed a statistically significant difference of 4.9 Δ (p = 0.02).
Conclusions:
OK induces minimal changes in the binocular function for either short-term or long-term periods, apart from a near exophoric trend over the short-term period.
To evaluate the clinical treatment effects of orthokeratology to slow the progression of myopia.
Several well-designed controlled studies have investigated the effects of orthokeratology in school-aged children. We conducted this meta-analysis to better evaluate the existing evidence. Relevant studies were identified in the Medline and Embase database without language limitations. The main outcomes included axial length and vitreous chamber depth reported as the mean ± standard deviation. The results were pooled and assessed with a fixed-effects model analysis. Subgroup analyses were performed according to geographical location and study design.
Of the seven eligible studies, all reported axial length changes after 2 years, while two studies reported vitreous chamber depth changes. The pooled estimates indicated that change in axial length in the ortho-k group was 0.27 mm (95% confidence interval [CI]: 0.22, 0.32) less than the control group. Myopic progression was reduced by approximately 45%. The combined results revealed that the difference in vitreous chamber depth between the two groups was 0.22 mm (95% confidence interval [CI]: 0.14, 0.31). None of the studies reported severe adverse events.
The overall findings suggest that ortho-k can slow myopia progression in school-aged children.
Higher myopic refractive errors are associated with serious ocular complications that can put visual function at risk. There is respective interest in slowing and if possible stopping myopia progression before it reaches a level associated with increased risk of secondary pathology. The purpose of this report was to review our understanding of the rationale(s) and success of contact lenses (CLs) used to reduce myopia progression.
A review commenced by searching the PubMed database. The inclusion criteria stipulated publications of clinical trials evaluating the efficacy of CLs in regulating myopia progression based on the primary endpoint of changes in axial length measurements and published in peer-reviewed journals. Other publications from conference proceedings or patents were exceptionally considered when no peer-review articles were available.
The mechanisms that presently support myopia regulation with CLs are based on the change of relative peripheral defocus and changing the foveal image quality signal to potentially interfere with the accommodative system. Ten clinical trials addressing myopia regulation with CLs were reviewed, including corneal refractive therapy (orthokeratology), peripheral gradient lenses, and bifocal (dual-focus) and multifocal lenses.
CLs were reported to be well accepted, consistent, and safe methods to address myopia regulation in children. Corneal refractive therapy (orthokeratology) is so far the method with the largest demonstrated efficacy in myopia regulation across different ethnic groups. However, factors such as patient convenience, the degree of initial myopia, and non-CL treatments may also be considered. The combination of different strategies (i.e., central defocus, peripheral defocus, spectral filters, pharmaceutical delivery, and active lens-borne illumination) in a single device will present further testable hypotheses exploring how different mechanisms can reinforce or compete with each other to improve or reduce myopia regulation with CLs.
To evaluate the impact of amplitude of accommodation on controlling the development of myopia in orthokeratology.
Forty-nine children aged 7 to 14 years were enrolled in this prospective clinical study.Orthokeratology was performed to correct the refractive errors of these children after measurement of refraction, corneal topography, amplitude of accommodation and axial length. Axial length (AL) and amplitude of accommodation was measured after theatment. The average amplitude of accommodation was calculated and was used as the cutting point for dividing the cohort into "amplitude of accommodation above average" vs. "amplitude of accommodation below average". Data were analyzed by paired t-test, independent t-test, repeated measures-ANOVAs and Pearson correlation analysis.
The AL before and after 1- year and 2-year treatment was (24.98 ± 0.75) mm, (25.13 ± 0.74) mm and (25.32 ± 0.78) mm, respectively. AL increased significantly throughout the observed 24-month period (F = 75.848, P < 0.001) . Amplitude of accommodation increased from (13.68 ± 2.65) D to (16.12 ± 2.41) D in 2 years (t = -6.461, P < 0.001) and amplitude of accommodation significantly affected axial growth (F = 7.395, P = 0.009) . The axial growth of subjects with below average amplitude of accommodation and those with above average amplitude of accommodation was (0.23 ± 0.25) and (0.44 ± 0.30) mm, indicating a statistically difference(t = -2.719, P = 0.009). AL change in subjects with below average amplitude of accommodation was 55.81% that of the subjects with above average amplitude of accommodation. Baseline amplitude of accommodation was positively correlated to axial growth at 24-month visit (r = 0.502, P < 0.001). Linear regression analysis was used between baseline amplitude of accommodation and 2-year axial growth: Axial growth = 0.055·Baseline amplitude of accommodation-0.409(F = 15.806, P < 0.001). The change of amplitude of accommodation for subjects with below average amplitude of accommodation and those with above average amplitude of accommodation after 2-year was (4.04 ± 2.16) D and (0.91 ± 2.15) D, indicating statistically difference (t = 5.084, P < 0.001).
Myopic control effect would be more beneficial to lower amplitude of accommodation children than that to higher amplitude of accommodation children in orthokeratology. The enhancement of accommodation provides some basis for slowing myopia progression with orthokeratology.
Purpose:
The Bonferroni correction adjusts probability (p) values because of the increased risk of a type I error when making multiple statistical tests. The routine use of this test has been criticised as deleterious to sound statistical judgment, testing the wrong hypothesis, and reducing the chance of a type I error but at the expense of a type II error; yet it remains popular in ophthalmic research. The purpose of this article was to survey the use of the Bonferroni correction in research articles published in three optometric journals, viz. Ophthalmic & Physiological Optics, Optometry & Vision Science, and Clinical & Experimental Optometry, and to provide advice to authors contemplating multiple testing.
Recent findings:
Some authors ignored the problem of multiple testing while others used the method uncritically with no rationale or discussion. A variety of methods of correcting p values were employed, the Bonferroni method being the single most popular. Bonferroni was used in a variety of circumstances, most commonly to correct the experiment-wise error rate when using multiple 't' tests or as a post-hoc procedure to correct the family-wise error rate following analysis of variance (anova). Some studies quoted adjusted p values incorrectly or gave an erroneous rationale.
Summary:
Whether or not to use the Bonferroni correction depends on the circumstances of the study. It should not be used routinely and should be considered if: (1) a single test of the 'universal null hypothesis' (Ho ) that all tests are not significant is required, (2) it is imperative to avoid a type I error, and (3) a large number of tests are carried out without preplanned hypotheses.
Measurements obtained from the right and left eye of a subject are often correlated whereas many statistical tests assume observations in a sample are independent. Hence, data collected from both eyes cannot be combined without taking this correlation into account. Current practice is reviewed with reference to articles published in three optometry journals, viz., Ophthalmic and Physiological Optics (OPO), Optometry and Vision Science (OVS), Clinical and Experimental Optometry (CEO) during the period 2009-2012.
Of the 230 articles reviewed, 148/230 (64%) obtained data from one eye and 82/230 (36%) from both eyes. Of the 148 one-eye articles, the right eye, left eye, a randomly selected eye, the better eye, the worse or diseased eye, or the dominant eye were all used as selection criteria. Of the 82 two-eye articles, the analysis utilized data from: (1) one eye only rejecting data from the adjacent eye, (2) both eyes separately, (3) both eyes taking into account the correlation between eyes, or (4) both eyes using one eye as a treated or diseased eye, the other acting as a control. In a proportion of studies, data were combined from both eyes without correction.
It is suggested that: (1) investigators should consider whether it is advantageous to collect data from both eyes, (2) if one eye is studied and both are eligible, then it should be chosen at random, and (3) two-eye data can be analysed incorporating eyes as a 'within subjects' factor.
Background:
Approximately one in ten students aged 6 to 16 in Ontario (Canada) school boards have an individual education plan (IEP) in place due to various learning disabilities, many of which are specific to reading difficulties. The relationship between reading (specifically objectively determined reading speed and eye movement data), refractive error, and binocular vision related clinical measurements remain elusive.
Methods:
One hundred patients were examined in this study (50 IEP and 50 controls, age range 6 to 16 years). IEP patients were referred by three local school boards, with controls being recruited from the routine clinic population (non-IEP patients in the same age group). A comprehensive eye examination was performed on all subjects, in addition to a full binocular vision work-up and cycloplegic refraction. In addition to the cycloplegic refractive error, the following binocular vision related data was also acquired: vergence facility, vergence amplitudes, accommodative facility, accommodative amplitudes, near point of convergence, stereopsis, and a standardized symptom scoring scale. Both the IEP and control groups were also examined using the Visagraph III system, which permits recording of the following reading parameters objectively: (i) reading speed, both raw values and values compared to grade normative data, and (ii) the number of eye movements made per 100 words read. Comprehension was assessed via a questionnaire administered at the end of the reading task, with each subject requiring 80% or greater comprehension.
Results:
The IEP group had significantly greater hyperopia compared to the control group on cycloplegic examination. Vergence facility was significantly correlated to (i) reading speed, (ii) number of eye movements made when reading, and (iii) a standardized symptom scoring system. Vergence facility was also significantly reduced in the IEP group versus controls. Significant differences in several other binocular vision related scores were also found.
Conclusion:
This research indicates there are significant associations between reading speed, refractive error, and in particular vergence facility. It appears sensible that students being considered for reading specific IEP status should have a full eye examination (including cycloplegia), in addition to a comprehensive binocular vision evaluation.
To determine the types of contact lenses prescribed for infants (aged 0 to 5 years), children (6 to 12 years), and teenagers (13 to 17 years) around the world.
Up to 1000 survey forms were sent to contact lens fitters in each of 38 countries between January and March every year for 5 consecutive years (2005 to 2009). Practitioners were asked to record data relating to the first 10 contact lens fits or refits performed after receiving the survey form.
Data were received relating to 105,734 fits [137 infants, 1,672 children, 12,117 teenagers, and 91,808 adults (age ≥ 18 years)]. The proportion of minors (<18 year old) fitted varied considerably between nations, ranging from 25% in Iceland to 1% in China. Compared with other age groups, infants tend to be prescribed a higher proportion of rigid, soft toric, and extended wear lenses, predominantly as refits for full-time wear, and fewer daily disposable lenses. Children are fitted with the highest proportion of daily disposable lenses and have the highest rate of fits for part-time wear. Teenagers have a similar lens fitting profile to adults, with the main distinguishing characteristic being a higher proportion of new fits. Orthokeratology fits represented 28% of all contact lenses prescribed to minors.
Patterns of contact lens prescribing to infants and children are distinctly different to those of teenagers and adults in a number of respects. Clinicians can use the data presented here to compare their own patterns of contact lens prescribing to minors.
The steady-state accommodative responses of emmetropes and late-onset myopes was measured for an array of numbers located at -1, -3 and -5 dioptres using an objective infra-red optometer. Responses were compared for passive (reading numbers) and active (adding numbers) conditions. For the passive condition, the late-onset myopes showed a significantly lower accommodative response than the emmetropic group. No significant differences were found between the two groups for the active condition. Ocular biometric characteristics were also measured in emmetropes, late-onset myopes and early-onset myopes using keratometry and ultrasonography. No significant differences in corneal curvature, anterior chamber depth and crystalline lens thickness were found between the groups. Late-onset myopes exhibited significantly deeper vitreous chambers than emmetropes, which more than accounted for the difference in refractive error between the two refractive groups. We conclude that, while significant differences exist in the accommodative responses of late-onset myopes and emmetropes, late-onset myopia is due predominantly to elongation of the vitreous chamber.
An objective infrared optometer was used to investigate the influence of cognitive demand on tonic accommodation (TA) for two groups of young emmetropes (N = 15) and late-onset myopes (N = 15). TA was measured under a passive condition and while subjects carried out a reverse counting task. The TA of myopes under the passive condition (mean = 0.81 D, SD = 0.46 D) was shown to have lower values than the TA of emmetropes (mean = 1.14 D, SD = 0.46 D). For the myopic group the counting task induced a positive shift in TA (mean = +0.35 D, SD = 0.31 D) that was significantly higher than that for the emmetropic group (mean = +0.07 D, SD = 0.27 D). The implications for theories of refractive error development are discussed.
To determine whether the use of progressive addition spectacle lenses reduced the progression of myopia, over a 2-year period, in Hong Kong children between the ages of 7 and 10.5 years.
A clinical trial was carried out to compare the progression in myopia in a treatment group of 138 (121 retained) subjects wearing progressive lenses (PAL; add +1.50 D) and in a control group of 160 (133 retained) subjects wearing single vision lenses (SV). The research design was masked with random allocation to groups. Primary measurements outcomes were spherical equivalent refractive error and axial length (both measured using a cycloplegic agent).
There were no statistically significant differences between the PAL and the SV groups for of any of the baseline outcome measures. After 2 years there had been statistically significant increases in myopia and axial length in both groups; however, there was no difference in the increases that occurred between the two groups.
The research design used resulted in matched treatment and control groups. There was no evidence that progression of myopia was retarded by wearing progressive addition lenses, either in terms of refractive error or axial length.
Recent work has shown that differences exist in the accommodation response characteristics of myopes and emmetropes, although the exact nature of the differences has yet to be ascertained. This may in part be due to the classification of the refractive error employed in many studies. Previously it has been suggested that inaccuracies in the accommodation response occur predominantly during the progression of myopia.(1) To test this hypothesis we measure nearwork induced transient myopia (NITM) in progressing myopes (PMs), stable myopes (SMs) and emmetropes (EMMs).
A total of 41 young (18-27 years) subjects participated in the study (13 PMs, 14 SMs, 14 EMMs). Following a 10 minute near task (4 D) the change in accommodation response back to distance viewing was measured over a two minute time period.
Grouped mean data revealed a significant nearwork after-effect in PMs in comparison to SMs and EMMs after 10 seconds (p < 0.01) and 30 seconds (p < 0.01) post-task. Significantly longer time constants were also found in the PMs group.
Our results suggest that NITM is manifest during the progressive phase of myopia development.
The purpose of the Correction of Myopia Evaluation Trial (COMET) was to evaluate the effect of progressive addition lenses (PALs) compared with single vision lenses (SVLs) on the progression of juvenile-onset myopia.
COMET enrolled 469 children (ages 6-11 years) with myopia between -1.25 and -4.50 D spherical equivalent. The children were recruited at four colleges of optometry in the United States and were ethnically diverse. They were randomly assigned to receive either PALs with a +2.00 addition (n = 235) or SVLs (n = 234), the conventional spectacle treatment for myopia, and were followed for 3 years. The primary outcome measure was progression of myopia, as determined by autorefraction after cycloplegia with 2 drops of 1% tropicamide at each annual visit. The secondary outcome measure was change in axial length of the eyes, as assessed by A-scan ultrasonography. Child-based analyses (i.e., the mean of the two eyes) were used. Results were adjusted for important covariates, by using multiple linear regression.
Of the 469 children (mean age at baseline, 9.3 +/- 1.3 years), 462 (98.5%) completed the 3-year visit. Mean (+/-SE) 3-year increases in myopia (spherical equivalent) were -1.28 +/- 0.06 D in the PAL group and -1.48 +/- 0.06 D in the SVL group. The 3-year difference in progression of 0.20 +/- 0.08 D between the two groups was statistically significant (P = 0.004). The treatment effect was observed primarily in the first year. The number of prescription changes differed significantly by treatment group only in the first year. At 6 months, 17% of the PAL group versus 30% of the SVL group needed a prescription change (P = 0.0007), and, at 1 year, 43% of the PAL group versus 59% of the SVL group required a prescription change (P = 0.002). Interaction analyses identified a significantly larger treatment effect of PALs in children with lower versus higher baseline accommodative response at near (P = 0.03) and with lower versus higher baseline myopia (P = 0.04). Mean (+/- SE) increases in the axial length of eyes of children in the PAL and SVL groups, respectively, were: 0.64 +/- 0.02 mm and 0.75 +/- 0.02 mm, with a statistically significant 3-year mean difference of 0.11 +/- 0.03 mm (P = 0.0002). Mean changes in axial length correlated with those in refractive error (r = 0.86 for PAL and 0.89 for SVL).
Use of PALs compared with SVLs slowed the progression of myopia in COMET children by a small, statistically significant amount only during the first year. The size of the treatment effect remained similar and significant for the next 2 years. The results provide some support for the COMET rationale-that is, a role for defocus in progression of myopia. The small magnitude of the effect does not warrant a change in clinical practice.
To examine baseline measurements of accommodative lag, phoria, reading distance, amount of near work, and level of myopia as risk factors for progression of myopia and their interaction with treatment over 3 years, in children enrolled in the Correction of Myopia Evaluation Trial (COMET).
COMET enrolled 469 ethnically diverse children (ages, 6-11 years) with myopia between -1.25 and -4.50 D. They were randomly assigned to either progressive addition lenses (PALs) with a +2.00 addition (n = 235) or single vision lenses (SVLs; n = 234), the conventional spectacle treatment, and were observed for 3 years. The primary outcome measure was progression of myopia by autorefraction after cycloplegia with 2 drops of 1% tropicamide. Other measurements included accommodative response (by an open field of view autorefractor), phoria (by cover test), reading distance, and hours of near work. Independent and interaction analyses were based on the mean of the two eyes. Results were adjusted for important covariates with multiple linear regression.
Children with larger accommodative lags (>0.43 D for a 33 cm target) wearing SVLs had the most progression at 3 years. PALs were effective in slowing progression in these children, with statistically significant 3-year treatment effects (mean +/- SE) for those with larger lags in combination with near esophoria (PAL - SVL progression = -1.08 D - [-1.72 D] = 0.64 +/- 0.21 D), shorter reading distances (0.44 +/- 0.20 D), or lower baseline myopia (0.48 +/- 0.15 D). The 3-year treatment effect for larger lags in combination with more hours of near work was 0.42 +/- 0.26 D, which did not reach statistical significance. Statistically significant treatment effects were observed in these four groups at 1 year and became larger from 1 to 3 years.
The results support the COMET rationale (i.e., a role for retinal defocus in myopia progression). In clinical practice in the United States children with large lags of accommodation and near esophoria often are prescribed PALs or bifocals to improve visual performance. Results of this study suggest that such children, if myopic, may have an additional benefit of slowed progression of myopia.
The Study of Progression of Adult Nearsightedness (SPAN) is a 5-year observational study to determine the risk factors associated with adult myopia progression. Candidate risk factors include: a high proportion of time spent performing near tasks, performing near tasks at a close distance, high accommodative convergence/accommodation (AC/A) ratio, and high accommodative lag.
Subjects between 25 and 35 years of age, with at least -0.50 D spherical equivalent of myopia (cycloplegic autorefraction), were recruited from the faculty and staff of The Ohio State University. Progression is defined as an increase in myopia of at least -0.75 D spherical equivalent as determined by cycloplegic autorefraction. Annual testing includes visual acuity, noncycloplegic autorefraction and autokeratometry, phoria, accommodative lag, response AC/A ratio, cycloplegic autorefraction, videophakometry, ultrasound, and partial coherence interferometry (IOLMaster). Participants' near activities were assessed using the experience sampling method (ESM). Subjects carried a pager for two 1-week periods and were paged randomly throughout the day. Each time they were paged, they dialed into an automated telephone survey and reported their visual activity at that time. From these responses, the proportion of time spent performing near work was estimated.
Three-hundred ninety-six subjects were enrolled in SPAN. The mean (+/- standard deviation) age at baseline was 30.7 +/- 3.5 years, 66% were female, 80% were white, 11% were black, and 8% were Asian/Pacific Islander. The mean level of myopia (spherical equivalent) was -3.54 +/- 1.77 D, the mean axial length by IOLMaster was 24.6 +/- 1.1 mm, and subjects were 1.7 +/- 4.0 Delta exophoric. Refractive error was associated with the number of myopic parents (F = 3.83, p = 0.023), and the number of myopic parents was associated with the age of myopia onset (chi2 = 13.78, p = 0.001). In a multivariate analysis, onset of myopia (early vs. late) still had a significant effect on degree of myopia (F = 115.1, p < 0.001), but the number of myopic parents was no longer significant (F = 0.65, p = 0.52). For the ESM, the most frequently reported visual task was computer use (mean, 18.9%; range, 0-60.0%) and, overall, subjects reported near work activity 34.1% of the time (range, 0-67.3%).
The design of SPAN and the baseline characteristics of the cohort have been described. Parental history of myopia is related to the degree of myopia at baseline, but this effect is mediated by the age of onset of myopia.
Purpose:
To investigate the effects of short-term orthokeratology (OK) on accommodation and binocular visual function in young adults.
Methods:
Twenty-four myopes (18 to 38 years) were fitted with OK lenses in both eyes. Best corrected distance visual acuity (VA), subjective and objective refractions, corneal topography and a series of binocular vision tests were measured at baseline (BL) before lens wear and then repeated after 28 nights of OK. Data from 15 subjects who demonstrated successful OK lens fit are reported.
Results:
Corneal flattening and hyperopic shifts in spherical equivalent refractive error (all p < 0.001) after 28 nights of OK indicated myopic correction. Improvement in best corrected distance VA was measured after OK (right eye p = 0.021; left eye p = 0.014). Although there was no significant change in mean distance and near phorias and stereoacuity scores after OK compared to BL, there was a significant reduction in standard deviation (SD) and range of data (distance p = 0.01; near p = 0.02; stereoacuity p < 0.001). While there appeared to be an improvement in distance accommodative facility after OK, this failed to reach statistical significance (p = 0.053). Furthermore, there was no change in AC/A gradients with ±1 D and ±2 D lenses after OK compared to BL.
Conclusions:
Binocular vision remained unchanged after OK, although variability of phoria and stereoacuity measures reduced. This suggests that OK improves or maintains accommodative and binocular vision function in young adult myopes who achieve good vision with OK. Myopes with phorias outside normal ranges and/or poor distance accommodative facility may benefit most with OK, in binocular and accommodative function.
Objective:
To observe and compare changes in accommodative response between myopic children wearing ordinary frame glasses (OFG), Mouldway orthokeratology lenses (M-OK), and medcall lenses (ML).
Methods:
A total of 240 myopic children were divided into three groups: OFG [n=90]; M-OK [n=90]; and ML [n=60]). The diopter, accommodative lag, and binocular accommodative facility before wearing glasses (T0) and 1-year after wearing glasses (T1) were compared among the three groups.
Results:
Commercially available software was used to perform statistical analysis, and the data were expressed as mean±SD. There were no significant differences among the three groups at T0. The accommodative lags in M-OK and ML at T1 were significantly lower than those at T0; this finding was most evident in M-OK. Although accommodative facility increased in all three groups, the increase was most evident in M-OK and ML.
Conclusion:
Mouldway orthokeratology lenses and ML can reduce the accommodative lag and increase the accommodative facility in myopic children. Compared with ML, M-OK showed considerably more marked effects to myopia progression in children.
Purpose:
To compare near point binocular vision function of young adult myopes wearing orthokeratology (OK) lenses to matched single vision soft disposable contact lens (SCL) wearers.
Methods:
A retrospective clinical record analysis of all OK wearers (18-30 years) presenting over an 18 month period was undertaken. Data was extracted for 17 OK wearers, with 17 SCL wearers matched for age, refractive error and duration of contact lens wear. Binocular vision data included horizontal phoria (phoria), horizontal base-in (BIFR) and base-out fusional reserves (BOFR) and accommodation accuracy (AA).
Results:
The OK group was 25.8±3.2years, with a duration of wear of 45.7±25months and refractive error of R -2.09±1.23D, L -2.00±1.35D. Compared to matched SCL wearers the OK group were significantly more exophoric (OK -2.05±2.38Δ; SCL 0.00±1.46Δ, p=0.005) and had better accommodation accuracy (OK 0.97±0.33D; SCL 1.28±0.32D, p=0.009). BIFR and BOFR were not different in the two groups. Frequency histograms showed that more SCL wearers had high lags of accommodation (AA≥1.50D: 8 SCL,2 OK) and esophoria (≥1Δ: 5 SCL,1 OK) than OK wearers. A positive correlation was found between refraction and phoria in the SCL group (r=0.521, p=0.032).
Conclusion:
Young adult myopes wearing OK lenses display more exophoria and lower accommodative lags at near compared to matched single vision SCL wearers. Young adult myopes with specific binocular vision disorders may benefit from OK wear in comparison to single vision SCL wear. This has relevance to both the visual acceptance of OK lenses and in managing risk factors for myopia progression.
: The growing incidence of pediatric myopia worldwide has generated strong scientific interest in understanding factors leading to myopia development and progression. Although contact lenses (CLs) are prescribed primarily for refractive correction, there is burgeoning use of particular modalities for slowing progression of myopia following reported success in the literature. Standard soft and rigid CLs have been shown to have minimal or no effect for myopia control. Overall, orthokeratology and soft multifocal CLs have shown the most consistent performance for myopia control with the least side effects. However, their acceptance in both clinical and academic spheres is influenced by data limitations, required off-label usage, and a lack of clear understanding of their mechanisms for myopia control. Myopia development and progression seem to be multifactorial, with a complex interaction between genetics and environment influencing myopigenesis. The optical characteristics of the individual also play a role through variations in relative peripheral refraction, binocular vision function, and inherent higher-order aberrations that have been linked to different refractive states. Contact lenses provide the most viable opportunity to beneficially modify these factors through their close alignment with the eye and consistent wearing time. Contact lenses also have potential to provide a pharmacological delivery device and a possible feedback mechanism for modification of a visual environmental risk. An examination of current patents on myopia control provides a window to the future development of an ideal myopia-controlling CL, which would incorporate the broadest treatment of all currently understood myopigenic factors. This ideal lens must also satisfy safety and comfort aspects, along with overcoming practical issues around U.S. Food and Drug Administration approval, product supply, and availability to target populations. Translating the broad field of myopia research into clinical practice is a multidisciplinary challenge, but an analysis of the current literature provides a framework on how a future solution may take shape.
Purpose:
A paper published by the author in 1988 in this journal provided some important findings about the lack of precision of visual acuity (VA) measures made with commonly used Snellen charts and the advantages of using letter charts designed using the principles proposed by Bailey and Lovie in 1976. That 1988 paper has been cited a number of times since, mostly supporting the findings. The purpose of this review is to examine the changes that have occurred in VA measurement in research and clinical practice since that earlier study.
Recent findings:
While precise measures of VA using Bailey-Lovie or ETDRS charts are now commonly used in major, multi-centre research studies, it is disappointing to see that many research papers still report VA measured with Snellen charts and even use Snellen fractions, invalidly converted to logMAR notation, in parametric analyses of VA. Many studies have examined the test-retest variability (TRV) of VA measures in groups and individuals, but it is difficult to determine if clinicians or researchers determine patients' individual TRVs to more accurately detect real changes in VA for each individual.
Summary:
This paper summarises the findings of the 1988 study: (1) Snellen charts and VA notations are not appropriate for accurate clinical and research measures of VA; (2) Charts employing the Bailey-Lovie design principles should be used to provide precise measures of VA. (3) Test-retest variability should be used to determine the limit for detecting significant change in VA. This author suggests that it is time for Snellen charts, Snellen fractions and decimal notation to be confined to the teaching of the history of VA measurement. A request is also made to stop the use of the redundant term 'best corrected' VA (BCVA). Recommended procedures are given for precise measures of VA and accurately monitoring changes in VA in clinical practice and research.
To determine the relationship between binocular vision (BV) disorder and dry eye symptoms and the frequency of BV disorders in subjects with contact lens-induced dry eye symptoms.
Subjects recruited for a larger dry eye study (n = 104) completed the Ocular Surface Disease Index (OSDI) and Convergence Insufficiency Symptom Survey (CISS) to determine if symptoms assessed on these two surveys were related. Also, myopic soft contact lens wearers (n = 29) with self-reported dry eye symptoms were recruited. Subjects completed the OSDI and CISS to assess severity of dry eye and BV disorder symptoms. Basic BV and dry eye testing was performed on each subject.
Severity of symptoms assessed on the OSDI and CISS was found to be significantly correlated in the larger subject group (ρ = 0.68, p = 0.0001). This significant correlation warranted further investigation of both symptoms and clinical signs. In the group of myopic soft contact lens wearers, 48.3% had a BV disorder. This proportion appeared to be higher than previously reported prevalence estimates of BV disorders. Accommodative lag greater than or equal to 1.00 diopter was the most common BV disorder sign encountered (48.3%), and pseudo-convergence insufficiency was the most common BV disorder (31.0%).
Symptoms related to dry eye and BV disorders overlap. Subjects with symptoms of discomfort while wearing soft contact lenses may be experiencing a concurrent or stand-alone BV disorder. Accommodative insufficiency and pseudo-convergence insufficiency were common in the sample of myopic soft contact lens wearers. Clinicians should screen symptomatic contact lens-induced dry eye patients for BV disorders. Dry eye studies should assess basic BV function.
To conduct a meta-analysis on the effects of orthokeratology in slowing myopia progression.
A literature search was performed in PubMed, Embase, and the Cochrane Library. Methodological quality of the literature was evaluated according to the Jadad score. The statistical analysis was carried out using RevMan 5.2.6 software.
The present meta-analysis included seven studies (two randomized controlled trials and five nonrandomized controlled trials) with 435 subjects (orthokeratology group, 218; control group, 217) aged 6 to 16 years. The follow-up time was 2 years for the seven studies. The weighted mean difference was -0.26 mm (95% confidence interval, -0.31 to -0.21; p < 0.001) for axial length elongation based on data from seven studies and -0.18 mm (95% confidence interval, -0.33 to -0.03; p = 0.02) for vitreous chamber depth elongation based on data from two studies.
Our results suggest that orthokeratology may slow myopia progression in children. Further large-scale studies are needed to substantiate the current result and to investigate the long-term effects of orthokeratology in myopia control.
To investigate the impact of simulated hyperopia and sustained near work on children's ability to perform a range of academic-related tasks.
Fifteen visually normal children (mean [±SD] age, 10.9 [±0.8] years; 10 male and 5 female) were recruited. Performance on a range of standardized academic-related outcome measures was assessed with and without 2.50 diopters of simulated bilateral hyperopia (administered in a randomized order), before and after 20 minutes of sustained near work, at two separate testing sessions. Academic-related measures included a standardized reading test (the Neale Analysis of Reading Ability), visual information processing tests (the Coding and Symbol Search subtests from the Wechsler Intelligence Scale for Children), and a reading-related eye movement test (the Developmental Eye Movement test).
Simulated bilateral hyperopia and sustained near work each independently impaired reading, visual information processing, and reading-related eye movement performance (p < 0.001). A significant interaction was also demonstrated between these factors (p < 0.05), with the greatest decrement in performance observed when simulated hyperopia was combined with sustained near work. This combination resulted in performance reductions of between 5 and 24% across the range of academic-related measures. A significant moderate correlation was also found between the change in horizontal near heterophoria and the change in several of the academic-related outcome measures, after the addition of simulated hyperopia.
A relatively low level of simulated bilateral hyperopia impaired children's performance on a range of academic-related outcome measures, with sustained near work further exacerbating this effect. Further investigations are required to determine the impact of correcting low levels of hyperopia on academic performance in children.
To evaluate short-term (3 months) and long-term (3 years) accommodative changes produced by overnight orthokeratology (OK).
A prospective, longitudinal study on young adult subjects with low to moderate myopia was carried out. A total of 93 patients took part in the study. Out of these, 72 were enrolled into the short-term follow-up: 21 were on a control group, 26 on a Paragon CRT contact lenses group, and 25 on a Seefree contact lenses group. The other 21 patients were old CRT wearers on long-term follow-up. Accommodative function was assessed by means of negative and positive relative accommodation (NRA / PRA), monocular accommodative amplitude (MAA), accommodative lag, and monocular accommodative facility (MAF). These values were compared among the three short-term groups at the follow-up visit. The long- and short-term follow-up data was compared among the CRT groups.
Subjective accommodative results did not suffer any statistically significant changes in any of the accommodative tests for any of the short-term groups when compared to baseline. There were no statistically significant differences between the three short-term groups at the follow-up visit. When comparing the short- and long-term groups, only the NRA showed a significant difference (p = 0.0006) among all the accommodation tests.
OK does not induce changes in the ocular accommodative function for either short-term or long-term periods.
Purpose:
The purpose of this study was to investigate the optical zone power profile of the most commonly prescribed soft contact lenses to assess their potential impact on peripheral refractive error and hence myopia progression.
Methods:
The optical power profiles of six single vision and ten multifocal contact lenses of five manufacturers in the powers -1.00 D, -3.00 D, and -6.00 D were measured using the SHSOphthalmic (Optocraft GmbH, Erlangen, Germany). Instrument repeatability was also investigated.
Results:
Instrument repeatability was dependent on the distance from the optical centre, manifesting unreliable data for the central 1mm of the optic zone. Single vision contact lens measurements of -6.00 D lenses revealed omafilcon A having the most negative spherical aberration, lotrafilcon A having the least. Somofilcon A had the highest minus power and lotrafilcon A the biggest deviation in positive direction, relative to their respective labelled powers. Negative spherical aberration occurred for almost all of the multifocal contact lenses, including the centre-distance designs etafilcon A bifocal and omafilcon A multifocal. Lotrafilcon B and balafilcon A seem to rely predominantly on the spherical aberration component to provide multifocality.
Conclusions:
Power profiles of single vision soft contact lenses varied greatly, many having a negative spherical aberration profile that would exacerbate myopia. Some lens types and powers are affected by large intra-batch variability or power offsets of more than 0.25 dioptres. Evaluation of power profiles of multifocal lenses was derived that provides helpful information for prescribing lenses for presbyopes and progressing myopes.
Purpose:
To determine the between-visit repeatability of peripheral autorefraction measurements using the Grand Seiko WAM-5500 in normal eyes.
Methods:
Cycloplegic autorefraction of the right eye was measured on 25 myopic young adults using a modified Grand Seiko autorefractor. Measurements were made centrally (along the line of sight) and ±20, ±30, and ±40 degrees from the line of sight in the horizontal meridian at two visits separated by 1 to 15 days. Five autorefraction measurements at each location were converted to vector space and averaged. Relative peripheral refraction (RPR) was calculated as the difference between the peripheral and central spherical equivalent. Between-visit repeatability was evaluated by plotting the difference versus the mean of the measurements at the two visits (bias) and by calculating the 95% limits of agreement (LoA).
Results:
The mean (±SD) age and spherical-equivalent refractive error centrally (at visit 1) were 24.0 ± 1.3 years and -3.45 ± 1.42 diopters (D), respectively. There was no significant between-visit bias for any refractive component evaluated (M, J0, J45, and RPR) at any location measured (all p > 0.05). The 95% LoA of defocus (M) was ±0.21 D centrally and increased with increasing eccentricity to ±0.73 and ±0.88 D at 40 degrees nasally and temporally on the retina, respectively. The 95% LoA of RPR increased with increasing eccentricity to ±0.67 and ±0.82 D at 40 degrees nasally and temporally on the retina, respectively.
Conclusions:
In normal eyes, the repeatability of cycloplegic autorefraction was best centrally and decreased as eccentricity increased; however, repeatability in the far periphery was still better than previously reported between-visit repeatability for foveal cycloplegic subjective refraction. With clear knowledge of the repeatability of on- and off-axis cycloplegic autorefraction with the Grand Seiko, peripheral measurements can be properly interpreted in longitudinal studies.
The Canon Autoref R-1 is an ‘open-field’ autorefractor which has been widely used for research purposes for the past 20 years, but is no longer manufactured. A new autorefractor, the Shin-Nippon SRW-5000, is now available, and if measures using this instrument are shown to be equally accurate and reliable, is likely to replace the R-1. Here we report on the accuracy and reliability (repeatability and reproducibility) of refraction measures in a paediatric population (from 4 to 8 years of age). Subject numbers were 44 for cycloplegic measures and 53 for non-cycloplegic measures. As would be expected, agreement with cycloplegic refraction and reliability were better when SRW-5000 measures were taken using cycloplegia. Repeatability results from the SRW-5000 autorefractor, both with and without cycloplegia were similar to those reported for the Canon R-1.
Purpose:
To investigate relationships between changes to corneal and ocular aberrations induced by orthokeratology (OK) and their influence on visual function.
Methods:
Eighteen subjects (aged 20 to 23 years) were fitted with OK lenses (BE Enterprises Pty Ltd, Australia), manufactured in Boston XO material (Bausch & Lomb Boston, Wilmington, MA), and worn overnight for seven nights. Corneal and ocular aberrations were simultaneously captured (Discovery, Innovative Visual Systems, Elmhurst, IL), and contrast sensitivity function was measured on days 1 and 7, within 2 and 8 hours after lens removal on waking. Data from the eye achieving the higher myopic correction were analyzed for changes over time.
Results:
There was a significant refractive effect at all visits. Orthokeratology induced an increase in corneal and ocular root mean square higher order aberrations (HOAs) and a positive shift in spherical aberration (SA) on day 1, with further increases by day 7. Increases in root mean square coma became significant by day 7. Changes to corneal and ocular SA were similar on day 1; however, by day 7, there was a greater increase in corneal than ocular SA, indicating a change in internal SA. Orthokeratology led to an overall decrease in contrast sensitivity function, which was isolated to spatial frequency changes on day 1 at 1 cycle per degree and on day 7 at 1 and 8 cycles per degree.
Conclusions:
A greater positive shift in corneal compared with ocular SA on day 7 suggests a negative shift in internal SA, which would be consistent with an increased accommodative response. Lack of any difference on day 1 indicates that this may be an ocular adaptation response toward neutralizing induced positive SA, rather than a direct effect of SA changes on the accommodation mechanism.
In children little is known about the relationship between the AC/A ratio and the development of myopia, although they have been linked in adults. The purpose of this study was to investigate the interaction between accommodation and convergence and its relationship to refractive errors in children. Accommodation was measured for the right eye using the Canon R-1 autorefractor, and concomitant changes in vergence were assessed using a Maddox rod and a Risley prism before the left eye. Thirty-three myopic and 68 emmetropic children were tested wearing best subjective correction while looking at a distant (4.0 m) letter array and a near (0.33 m) one through additional plus and minus lenses. Lens-induced and distance-induced response AC/A ratios were calculated from the data. Both types of AC/A ratios are elevated in myopic children, who show reduced accommodation and enhanced accommodative convergence. Myopic children with esophoria underaccommodate at near. This suggests that a child who is esophoric must relax accommodation to reduce accommodative convergence and maintain single binocular vision. The reduction in accommodation could produce blur during near work, which could induce myopia as in animal models.
Several models of myopia predict that growth of axial length is stimulated by blur. Accommodative lag has been suggested as an important source of blur in the development of myopia and this study has modeled how cross-link interactions between accommodation and convergence might interact with uncorrected distance heterophoria and refractive error to influence accommodative lag.
Accommodative lag was simulated with two models of interactions between accommodation and convergence (one with and one without adaptable tonic elements). Simulations of both models indicate that both uncorrected hyperopia and esophoria increase the lag of accommodative and uncorrected myopia and exophoria decrease the lag or introduce a lead of accommodation in response to the near (40 cm) stimulus. These effects were increased when gain of either cross-link, accommodative convergence (AC/A) or convergence accommodation (CA/C), was increased within a moderate range of values while the other was fixed at a normal value (clamped condition). These effects were exaggerated when both the AC/A and CA/C ratios were increased (covaried condition) and affects of cross-link gain were negated when an increase of one cross-link (e.g. AC/A) was accompanied by a reduction of the other cross-link (e.g. CA/C) (reciprocal condition). The inclusion of tonic adaptation in the model reduced steady state errors of accommodation for all conditions except when the AC/A ratio was very high (2 MA/D).
Combinations of cross-link interactions between accommodation and convergence that resemble either clamped or reciprocal patterns occur naturally in clinical populations. Simulations suggest that these two patterns of abnormal cross-link interactions could affect the progression of myopia differently. Adaptable tonic accommodation and tonic vergence could potentially reduce the progression of myopia by reducing the lag of accommodation.
Abstract— Changes in the visual system following prolonged near work were investigated. Fifteen young, normal subjects undertook a severe, two hour long, binocular near visual task at 20 cm without any breaks. Fusional stress was assessed by near ‘phoria change. Refractive change was measured with an auto-refractometer to investigate whether transient myopia occurred and its subsequent recovery. The near task caused vergence adaptation which was primarily due to the fusional stress of the task (accounting for 67% of its variance). The ’phoria change was to a lesser extent (40%) dependent on the accommodative “stress” of the task. Fatigue of the accommodative system resulted in increased accommodative innervation to maintain the same accurate response. Increased innervation can continue after the task on subsequent distance viewing, resulting in transient myopia (mean 0.29 DS). This transient myopia was found to be due to a transient regression of the far point towards the subject's tonic accommodation level. This can be accounted for by a shift of the tonic level as well as an increased bias towards the pre-task tonic level.
Purpose: Caucasian children with myopia have elevated response accommodative vergence to accommodation (AC/A) ratios. The purpose of this study was twofold: to determine if response AC/A ratios vary with refractive error and with myopic progression rate in Hong Kong Chinese children, and to determine the effect of beta-adrenergic antagonism with topical timolol application on AC/A ratios.
Methods: Thirty children aged eight to 12 years participated in the study. All refractive errors were corrected with spectacle lenses. Accommodative responses were measured using a Shin-Nippon autorefractor and concurrent changes in vergence were assessed using a vertical prism and a Howell-Dwyer card at three metres and 0.33 metre. Accommodative demand was altered using plus or minus two dioptre lenses and lens- and distance-induced response AC/A ratios were calculated. Measurements were repeated 30 minutes after the instillation of topical timolol maleate (0.5 per cent).
Results: AC/A ratios appeared higher in progressing myopic children but the difference was not statistically significant. Timolol application reduced accommodative convergence (AC) in the stable myopes (reduction = -3 ± 1.14A) but not in the emmetropes (0.69 ± 0.9P) or progressing myopes (0.16 ± 0.43A) and this difference between refractive groups was statistically s ignificant (F2,27= 3.766; P= 0.036). However, timolol did not produce a significant change in the accommodative response to positive or negative lenses or response AC/A ratios.
Conclusions: We did not find that AC/A ratios in myopic Chinese children were elevated and therefore, it is unlikely that elevated AC/A ratios are responsible for the high levels of myopia that occur in Hong Kong. The finding that timolol reduced AC in the stable myopes suggests that the autonomic control of accommodative convergence in these children may be different from that in emmetropic children and those with progressing myopia.
The lack of predictability in orthokeratology has always been seen as one of the major drawbacks of the procedure. Being able to assess the likely degree of myopia reduction would be a valuable clinical tool in that those patients presenting for orthokeratology who would not be viable candidates could be advised against proceeding with a course of treatment. In this study the pre- and post-orthokeratology refraction, corneal eccentricity, keratometry, and apical corneal power changes were measured to see whether a correlation exists between the pre- and post-treatment corneal shape and refractive changes that could be used as a predictive tool. A good correlation was found between refractive change and corneal eccentricity change (r2 = 0.83), apical corneal power change and corneal eccentricity change (r2 = 0.84), and refractive change and apical corneal power change (r2 = 0.91). A poorer correlation was found between keratometry change and refractive, apical corneal power, and eccentricity change. The shape changes induced in the cornea by orthokeratology were also studied by topographical analysis. The final corneal shape is typically that of a central spherical zone (4.00–5.00 mm chord) surrounded by a mid-peripheral steep zone (5.00–7.50 mm chord) that tends to flatten in curvature as the periphery (8.00 mm chord) is approached.
To conduct a meta-analysis on the rates of myopia progression in urban children of Asian and predominately European ethnicities who are corrected with traditional single-vision spectacles.
A search of the National Library of Medicine's PubMed literature database for articles on myopia progression was conducted using the terms "myopi*progression" and MeSH terms "myopia" and "disease progression," and limited to publications from January 1990 and only for articles reporting data for humans <16 years of age. Studies were excluded if they were non-randomized, did not use cycloplegic autorefraction, had a sample size <30 individuals, examined high myopia (worse than -6.0 D) or special subject groups, presented myopia as part of a syndrome or condition, were retrospective, or used controls wearing optical corrections other than spectacles.
Of 175 articles identified, 20 remained after applying the exclusion criteria. The estimated myopia progression at a mean age of 9.3 years after 1 year of follow-up was -0.55 D [95% confidence interval (CI), -0.39 to -0.72 D] for populations of predominantly European extraction and -0.82 D (95% CI, -0.71 to -0.93 D) for Asians. The estimated progression rates were dependent on baseline age, with decreasing progression as age increased. The rates also varied with gender. For an average baseline age of 8.8 years, estimated annual progression (combined ethnicities) was -0.80 D/yr for females (95% CI, -0.51 to -1.10), and a significantly slower (p < 0.01) -0.71 D/yr for males (95% CI, -0.42 to -1.00).
In children wearing single-vision spectacles, higher myopia progression rates were found in urban Asians compared with urban populations of predominantly European descent. Younger children and females demonstrated greater annual rates of progression of myopia.
To identify longitudinal changes in fusional vergence ranges and their relationship to other clinical measures in young myopic subjects.
Measurements were collected annually for 10 years on 114 subjects from the University of Houston Correction of Myopia Evaluation Trial cohort. Subject age was 7 to 13 years at year 1 of follow-up. Measurements included refractive error, distance and near phoria, interpupillary distance (IPD), prism bar fusional vergence ranges, and nearpoint of convergence (NPC). Multilevel modeling was used to determine baseline and rate of change for fusional vergence ranges and the impact of phoria, IPD, and NPC on these measures.
Year 1 mean distance base-out (BO) break was 20 prism diopters (pd) and decreased 5.6 pd over 10 years (p < 0.001). Mean near BO break was 30 pd at year 1 and decreased 9.4 pd over 10 years (p < 0.001). Greater esophoria was significantly related to greater BO break (p < 0.02) and receded NPC was significantly related to lower magnitude BO break at near (p < 0.001). Distance IPD increased 3 mm over 10 years (p < 0.001) but was unrelated to the magnitude of the BO ranges (p > 0.2). Mean distance base-in (BI) break was 7 pd at year 1 and increased 0.5 pd in 10 years (p = 0.04). Mean near BI break was 13 pd at year 1 and did not significantly change. Mean distance phoria was 0.1 pd exophoria at year 1 and did not change, whereas near phoria was 2.4 pd esophoria at year 1 and became more exophoric (4 pd in 10 years, p < 0.001).
These results suggest that for myopic children convergence ranges decrease for both distance and near viewing during the school years as near phoria becomes more exophoric. These findings could have clinical implications given that compensating convergence ranges decrease as near phoria becomes more divergent.
Clinical measurement of the accommodative response (AR) identifies the focusing plane of a subject with respect to the accommodative target. To establish whether a significant change in AR has occurred, it is important to determine the repeatability of this measurement. This study had two aims: First, to determine the intraexaminer repeatability of AR measurements using four clinical methods: Nott retinoscopy, monocular estimate method (MEM) retinoscopy, binocular crossed cylinder test (BCC) and near autorefractometry. Second, to study the level of agreement between AR measurements obtained with the different methods.
The AR of the right eye at one accommodative demand of 2.50 D (40 cm) was measured on two separate occasions in 61 visually normal subjects of mean age 19.7 years (range 18-32 years). The intraexaminer repeatability of the tests, and agreement between them, were estimated by the Bland-Altman method. We determined mean differences (MD) and the 95% limits of agreement [coefficient of repeatability (COR) and coefficient of agreement (COA)].
Nott retinoscopy and BCC offered the best repeatability, showing the lowest MD and narrowest 95% interval of agreement (Nott: -0.10 +/- 0.66 D, BCC: -0.05 +/- 0.75 D). The 95% limits of agreement for the four techniques were similar (COA = +/- 0.92 to +/-1.00 D) yet clinically significant, according to the expected values of the AR. The two dynamic retinoscopy techniques (Nott and MEM) had a better agreement (COA = +/-0.64 D) although this COA must be interpreted in the context of the low MEM repeatability (COR = +/-0.98 D).
The best method of assessing AR was Nott retinoscopy. The BCC technique was also repeatable, and both are recommended as suitable methods for clinical use. Despite better agreement between MEM and Nott, agreement among the remaining methods was poor such that their interchangeable use in clinical practice is not recommended.
To evaluate the effectiveness of progressive addition lenses (PALs), with a near addition of +1.50 D, on the progression of myopia in Chinese children.
We enrolled 178 Chinese juvenile-onset acquired myopes (aged 7-13 years, -0.50 to -3.00 D spherical refractive error), who did not have moderately or highly myopic parents, for a 2-year prospective study. They were randomly assigned to the PAL group or single vision (SV) group. Primary measurements, which included myopia progression and ocular biometry, were performed every 6 months. Treatment effect was adjusted for important covariates, by using a multiple linear regression model.
One hundred and forty-nine subjects (75 in SV and 74 in PAL) completed the 2-year study. The myopia progression (mean +/- S.D.) in the SV and PAL groups was -1.50 +/- 0.67 and -1.24 +/- 0.56 D, respectively. This difference of 0.26 D over 2 years was statistically significant (p = 0.01). The lens type (p = 0.02) and baseline spherical equivalent refraction (p = 0.05) were significant contributing factors to myopia progression. Mean increase in the depth of vitreous chamber was 0.70 +/- 0.40 and 0.59 +/- 0.24 mm, respectively. This difference of 0.11 mm was statistically significant (p = 0.04). Age (p < 0.01) was the only contributing factor to the elongation of vitreous chamber. Different near phoria (p < 0.01) and gender (p = 0.02) caused different treatment effects when wearing SV lenses. However, there were no factors found to influence the treatment effect of wearing PALs.
Compared with SV lenses, myopia progression was found to be retarded by PALs to some extent in Chinese children without moderately or highly myopic parents, especially for subjects with near esophoria or females.
This study was designed to determine the repeatability of fusional vergence ranges measured using the rotary prisms in the phoropter and in free space using the prism bar. The level of agreement between the two methods was also investigated.
In two separate sessions, negative and positive fusional vergence ranges (NFV and PFV, respectively) were measured at distance and near in 61 young adults (mean age 19.74, S.D. 2.5 years) who were unfamiliar with the methods used. Base-in and base-out blur, break and recovery points were sequentially determined. Both sets of measurements were obtained by the same examiner. At each distance, NFV was determined first and then PFV. The repeatability of the tests and agreement between measurements made with the phoropter rotary prisms and the prism bar were estimated by the Bland and Altman method.
For both the phoropter rotary prisms and prism bar, NFV measurements showed better repeatability than PFV at both near and distance. Mean differences recorded for the NFV break and recovery points were non-significant (under 0.5Delta), while those observed for PFV were generally greater than 2Delta. When agreement between the two tests was assessed, it was found that break points were higher when determined using the phoropter rotary prisms, while recovery points were generally higher for the prism bar method. In clinical terms, according to the expected values of the NFV and PFV, agreement between the two techniques can be described as fair, because although mean differences were never greater than 5.5Delta, 95% agreement intervals were as wide as +/-8.00Delta for NFV and +/-13.19Delta for PFV.
The two methods used to measure fusional vergences showed fairly good inter-session repeatability for measuring NFV but repeatability was reduced for PFV measurements. The level of agreement observed between the two methods was such that their interchangeable use in clinical practice is not recommended.
Proximal stimuli to accommodation were assessed by comparing the open-loop accommodative responses (AR's) to stimuli placed at viewing distances of 5 m (0.2 D) and 0.33 m (3D) in 10 late-onset myopes (LOM's) and 10 emmetropes. The accommodation and vergence loops were opened by subjects viewing targets monocularly through 0.5-mm pinholes. Pre- and post-task levels of tonic accommodation (TA) were used to assess the accommodative adaptation induced by the task. For the 0.2 D condition there was no significant difference between pre-task TA, within-task AR, or post-task TA, indicating that accommodation was open-loop throughout this distal condition. When viewing the 3 D target both refractive groups exhibited significant increases in AR, emmetropes showing higher levels of proximally induced accommodation (PIA) when compared with LOM's. The data indicate that target proximity will influence AR even when both blur and vergence cues have been stabilized.
Systems for the classification of myopia are usually based on etiological dichotomies such as hereditary vs. environmental, physiological vs. pathological, structural vs. functional, or axial vs. refractive. The purpose of this paper is to propose a system for the classification of myopia based not on assumed etiological factors but on readily available and easily verifiable information, including age-related prevalence and age of onset. The proposed system classifies myopia into four categories: congenital, youth-onset, early adult-onset, and late adult-onset. Paradoxically, such a classification, being devoid of etiological assumptions, may help to make possible a better understanding of the etiology of the various categories of myopia.
It has recently been demonstrated that intersubject variations in tonic (dark focus) levels of accommodation are related to corrected refractive state (McBrien and Millodot, 1987). The aim of the present study was to investigate the effect of sustained visual tasks on the tonic level of accommodation in different refractive groups. Eleven hyperopes, 16 emmetropes, ten early onset myopes and ten late onset myopes had their tonic accommodation measured with the objective infrared optometer Canon Autoref R-1 before and after a 15 min sustained visual counting task. The post-task tonic accommodation level was monitored for 15 min to assess the decay rate of any observed task-induced changes in tonic accommodation. Subjects repeated the experimental procedure for four task locations (6 m, pre-task tonic position, 37 cm and 20 cm). Late onset myopes showed significant positive (myopic) changes in their tonic level of accommodation at both near viewing distances, which showed no evidence of decay during the 15 min post-task monitoring period. Hyperopes, however, underwent transient "counteradaptive" decreases in their tonic level of accommodation after sustained near viewing. Emmetropes and early onset myopes showed little change in tonic levels at the two near distances. Differences between groups were also obtained at tonic and far viewing distances. Post-task changes in tonic accommodation demonstrated only a weak negative correlation with pre-task tonic accommodation levels at each task distance. It is proposed that the observed differences in adaptation of tonic accommodation among refractive groups may be related to variations in autonomic innervation of the ciliary muscle.
Many theories of myopia etiology suggest differences in accommodation and convergence in children who become myopic. Several studies have found differences in accommodation and convergence in myopes and emmetropes. Only one previous study examined accommodation and convergence before youth onset myopia.
A cohort of initially emmetropic children was given eye and vision examinations at 6-month intervals for 3 years. The data collected included fusional vergence ranges at 4 m and 40 cm, and relative accommodation findings with a target at 40 cm. These clinical accommodation and vergence test findings were compared in children who became myopic (became-myopic group) to children who did not (remained-emmetropic group). The mean test findings were used to construct a zone of clear single binocular vision (ZCSBV) for each group.
The midpoint between the 40-cm fusional vergence range blur points was more convergent in the became-myopic group (+5.8 delta compared to +3.2 delta in the remained-emmetropic group; p < 0.004). The positive relative accommodation (PRA) finding was less in magnitude in the became-myopic group (-1.46 D) than in the remained-emmetropic group (-2.04 D), the difference being significant at he 0.02 level.
The ZCSBV near blur point were more convergent in the children who became myopic as indicated by the more convergent position of the midpoint of the near fusional vergence range. The lower value for the PRA test in the became-myopic group duplicates the same result reported in a previous study for retrospective longitudinal private practice data.
The Howell phoria card, a new modification of the Prentice design, has no previously published validity or reliability data. The purpose of this study was to compare the interexaminer repeatability of the Howell phoria card with established tests.
Experienced optometrists measured the near heterophoria of 72 subjects. Heterophoria was measured in a pseudo-random order using five standardized procedures: (1) Howell phoria card continuous presentation, (2) Howell phoria card flashed presentation, (3) free-space von Graefe method, 4) Bernell Muscle Imbalance Measure (MIM) card continuous presentation, and (5) MIM card flashed presentation.
The von Graefe method and the Howell phoria card flashed presentation found significantly less exophoric results than the MIM card. There was no consistent tendency for different examiners to find more exo- or more esophoric results with the phoria measurement techniques used in this study (p = 0.28). Both the Howell phoria card continuous presentation and the MIM card had significantly less variability in interexaminer differences than the Howell phoria card flashed presentation and the von Graefe method (p < 0.05).
Results from this study suggest that the Howell phoria card used in a continuous presentation method has an interexaminer repeatability that is not significantly different from the MIM card but greater than the von Graefe method.
Background: Clinicians frequently assess the accommodative convergence to accommodation (AC/A) ratio using near phoria measurements and accommodative stimuli. The purpose of this study was to evaluate the repeatability of AC/A ratio measurements and to compare the response AC/A ratio to stimulus AC/A ratios determined two different ways.
Methods: Heterophorias and accommodative responses to different stimuli were measured simultaneously on eight subjects at two visits. A Canon Auto Ref R-1 autorefractor was used to measure accommodative response, while vergence posture was measured using the modified Thorington heterophoria technique. These results were used to calculate response-slope, stimulus-slope and stimulus-gradient AC/A ratios and the means of these ratios for each session were compared. Correlations between the response AC/A ratios and the stimulus AC/A ratios were calculated.
Results: The mean response AC/A ratios for the two sessions were 3.8 prism dioptres per dioptre for each session. The correlation between the slope-determined ratios was 0.56, and between the response-slope and the stimulus-gradient AC/A ratios the correlation was -0.03. The regression equation for the slope-determined AC/A ratios was calculated.
To determine which clinical tests are useful in orthokeratology aftercare examination, and to examine the objective and subjective characteristics of a group of orthokeratology lens wearers.
Thirty orthokeratology subjects (8-19 years) who had been wearing orthokeratology lenses for over 12 months were recruited. Autorefraction, corneal topography, retinoscopy, subjective refraction and biomicroscopy were performed. Only left eyes results were analysed. Subjective ratings of symptoms and problems experienced by subjects were obtained using a questionnaire.
Autorefraction yielded higher residual sphere and residual cylinder by -0.54 D and -0.39 D respectively while retinoscopy yielded higher residual sphere and residual cylinder by -0.20 D and -0.03 D respectively. Corneal toricity measured by autokeratometry and corneal topography overpredicted the residual cylinder by -2.02 D and -2.08 D respectively. The mean +/- SD residual spherical equivalent refractive error was -0.11 +/- 0.57 D and the mean +/- SD unaided postorthokeratology visual acuity was 0.08 +/- 0.14 logMAR. The unaided visual acuity was significantly related to the residual cylinder. Pigmented arc was present in 16 corneas (53%). The most common problems/symptoms experienced by the subjects were lens binding (73%), ocular discharge in the morning (69%) and blur distance vision (47%). Over 80% of the subjects found lens handling troublesome in varying degree. All, except two subjects (who disliked the lens handling), wanted to continue the treatment.
History taking, subjective refraction, biomicroscopy and corneal topography are important in a routine orthokeratology aftercare examination. Corneal pigmented arc, ocular discharge in the morning and lens binding were the most common sign, symptom and problem respectively observed/ reported. Most orthokeratology lens wearers with low to moderate myopia and low astigmatism enjoyed reasonably good unaided post-orthokeratology vision in the daytime.
To evaluate corneal higher-order aberrations induced by overnight orthokeratology for myopia.
Prospective, noncomparative, consecutive, interventional case series.
A prospective study was conducted in 64 eyes of 39 patients with overnight orthokeratology for myopia, who were followed up for at least 3 months and attained uncorrected visual acuity of 20/20 or better. Corneal height data were obtained with computerized videokeratography (TMS-2N, Tomey), and wavefront aberration was derived using Zernike polynomials. Higher-order aberrations of the cornea were calculated for 3- and 6-mm pupils.
Orthokeratology significantly reduced manifest refraction from -2.60 +/- 1.13 (mean +/- SD) diopters to -0.17 +/- 0.31 diopters (P < .0001, paired t test). Root-mean-square (RMS) of third-order (coma-like) aberrations significantly increased by orthokeratology for both 3-mm (P < .0001, paired t test) and 6-mm (P < .0001) pupils. Fourth-order RMS (spherical-like) aberrations increased significantly by the treatment for both 3-mm (P < .0001) and 6-mm (P < .0001) pupils. Vertical coma significantly changed from positive to negative for both 3-mm (P = .0323) and 6-mm (P < .0001) pupils. Horizontal coma significantly increased to the positive direction for both 3-mm (P < .0001) and 6-mm (P < .0001) pupils. Increases in the third- and fourth-order RMS showed significant positive correlations with the amount of myopic correction for 3-mm (Pearson correlation coefficient, r = .452, P = .0001 for third-order RMS, r = .381, P = .0017 for fourth-order RMS) and 6-mm (r = .499, P < .0001, r = .455, P = .0001) pupils.
Corneal higher-order aberrations significantly increased, even in clinically successful orthokeratology cases. The increases in the higher-order aberrations correlated with the magnitude of myopic correction.
The purpose of this study was to investigate accommodation, accommodative convergence, and AC/A ratios before and at the onset of myopia in children.
Refractive error, accommodation, and phorias were measured annually over a period of 3 years in 80 6- to 18-year-old children (mean age at first visit = 11.1 years), including 26 who acquired myopia of at least -0.50 D and 54 who remained emmetropic (-0.25 to + 0.75 D). Refraction was measured by noncycloplegic distance retinoscopy. Concomitant measures of accommodation and phorias were taken for letter targets at 4.0 m and 0.33 m using the Canon R-1 open field-of-view autorefractor with an attached motorized Risley prism and Maddox rod. The accommodation and phoria measurements were used to calculate response AC/A ratios.
Compared with children who remained emmetropic, those who became myopic had elevated response AC/A ratios at 1 and 2 years before the onset of myopia, in addition to at onset and 1 year later (t's = -2.97 to -4.04, p < 0.01 at all times). The significantly higher AC/A ratios in the children who became myopic are a result of significantly reduced accommodation. Accommodative convergence was significantly greater in myopes only at onset.
These findings suggest that the abnormal oculomotor factors found before the onset of myopia may contribute to myopigenesis by producing hyperopic retinal defocus when a child is engaged in near-viewing tasks.
We assessed the extent to which different accommodative functions are correlated and whether accommodative functions predict the refractive error or the progression of myopia over a 12 month period in 64 young adults (30 myopes and 34 non-myopes). The functions were: amplitude of accommodation; monocular and binocular accommodative facility (6 m and 40 cm); monocular and binocular accommodative response to target distance; AC/A and CA/C ratios, tonic accommodation (dark focus and pinhole), accommodative hysteresis, and nearwork-induced transient myopia. Within groups of related accommodative functions (such as facility measures or open-loop measures) measurements on individuals were generally significantly correlated, however correlations between functions from different groups were generally not significant. Although accommodative amplitude and pinhole (open loop) accommodation were significantly different in myopes than in non-myopes, these functions were unrelated to myopia progression. Facility of accommodation and accommodative lag was independent predictors of myopia progression.
Our aim was to determine the repeatability of measurements of axial length (AL) and anterior chamber depth (ACD) made with two ultrasonic biometers and the IOLMaster in a group of non-orthokeratology (ortho-k) adult subjects and to investigate the agreement among instruments in children undergoing ortho-k therapy and in children wearing spectacles.
To determine repeatability, AL and ACD were measured twice in 22 non-ortho-k young adults using two A-scan ultrasonic biometers (A-5500 and A-2500) and the IOLMaster. To determine agreement, AL and ACD were measured with the same instruments in 30 children undergoing ortho-k therapy and 30 spectacle-wearing children.
In the adult subjects, there were no significant differences in ACD and AL measurements obtained from the three instruments (repeated measures ANOVAs, p > 0.05). There was also no significant between-measurement difference for each instrument. The between-measurement agreement was better for the IOLMaster (95% limits of agreement (LA): -0.04 and +0.05 mm for both AL and ACD) than for the two A-scan ultrasonic biometers (95% LA: -0.12 and +0.11 mm for AL; -0.22 and +0.27 mm for ACD). Among the children, AL measurements with all three instruments were not significantly different from each other for both the children undergoing ortho-k therapy and those wearing spectacles (repeated measures ANOVAs, p > 0.05). The 95% LA of differences obtained from any two instruments were also comparable for both groups of subjects (within -0.20 mm and +0.20 mm). ACD measurements of the children were significantly different among the three instruments (repeated measures ANOVAs, p < 0.05). No significant differences in ACD measurements were found between A-5500 and A-2500 for both groups of children (paired t tests, p > 0.017).
The repeatability of AL and ACD measurements with the IOLMaster was very good, and was better than with the A-scan ultrasonic biometers. The agreements in AL measurements between A-scan ultrasonic biometers and IOLMaster were comparable in both the ortho-k and the spectacle-wearing subjects, and were comparable to the repeatability of the A-scan ultrasonic biometers. ACD measurements between A-scan ultrasonic biometry and the IOLMaster were not comparable. AL measurements with the IOLMaster can replace the measurements from the two A-scan ultrasonic biometers used, however, the reverse is not true. AL and ACD measurements with all three instruments were unaffected by the flattened cornea following ortho-k lens wear.