Myopia is an ocular disorder associated with excessive axial elongation, which is believed to be both genetically based and environmentally influenced. The prevalence of myopia is high, reaching epidemic levels, e.g., 30% to 90%, in East Asian adolescents.1–4 Preventative treatment strategies, including pharmacological and optical means, have been developed to cease or slow the development of myopia and thus to lower the incidence of its severe complications. Muscarinic antagonists such as atropine5,6 and pirenzepine,7–9 albeit effective, have side effects (e.g., photophobia and cycloplegia) that are considered unacceptable for long-term use. Optical means such as bifocals10–12 and progressive addition lenses13–17 yielded statistically significant but not clinically meaningful treatment effects.
Among all the optical modalities being tested, orthokeratology (OK) has been found to have a strong inhibitory effect on axial elongation in myopic children. Modern myopic OK lenses are designed with a reverse geometry back surface that reshapes the cornea, mostly worn on a nightly basis, and thus correcting myopia during the day. Compared with control groups wearing single vision spectacles or soft contact lenses, OK lens wearers exhibited a reduction of axial elongation from 32% to 63%.18–24 However, the underlying anti-myopia mechanism associated with OK largely remains unknown. Recent studies25–28 have focused on OK’s effect on peripheral refraction, which is believed by many to mediate OK’s effect on eye growth.
Elongated myopic eyes result in relative hyperopic defocus on the peripheral retina.29–34 However, it is still unclear whether relative hyperopic defocus results in myopia development or is simply a result of eye elongation. Several animal experiments using custom-made dual-power lenses in rhesus monkeys35,36 or chicks37,38 have drawn consistent conclusions that peripheral hyperopic defocus promotes central myopic shift in infant animals. In humans, OK has been proved to be an effective method to induce both central hyperopic shift and peripheral myopic shift.25–28 Corneal topography has shown that OK decreases the central refractive power and increases the peripheral refractive power.25 Nevertheless, there has been no study investigating the relationship between the change in corneal refractive power distribution caused by OK and its relationship with myopia progression.
This prospective, longitudinal study was designed to investigate the relationship between corneal sagittal refractive power change along three axes (nasal, temporal, and inferior) after OK treatment and 2-year axial growth in children.
Study Design and Setting
This prospective, longitudinal study was performed at Eye & ENT Hospital (Fudan University, Shanghai, China) between April 2011 and May 2013.
In this study, 32 Chinese children aged from 9 to 14 who met the inclusion criteria were recruited within 2 months. The inclusion criteria required good ocular health condition and freedom from any ocular diseases, apical corneal refractive power between 41.5 D and 44.5 D, spherical refractive error between −1.00 D and −4.50 D, astigmatism no greater than −1.50 D, and monocular corrected distance visual acuity no worse than 20/20. To eliminate inter-eye correlation, only the right eye of the patients was included in the data analysis. All the possible risks were explained to the parents and written informed consents were signed before the study. This study conformed to the tenets of the Declaration of Helsinki.
The patients were fitted with OK lenses (Hiline Optics, China) in both eyes. The OK lenses were worn on an overnight basis, with no lens wearing during the day. Four-zone reverse geometry lenses composed of fluorosilicone acrylate (Boston XO), with an oxygen permeability (Dk) of 100 × 10−11 (cm2/s) (mL O2/mL·mm Hg) were used in the current study. The optical zone was 6.0 mm or 6.5 mm based on the pupil size, and the reverse curve width ranged from 0.6 mm to 0.8 mm. Lenses were fitted according to the instructions recommended by the manufacturer. In brief, for the first lens selection, the flat-K (K value along the flat meridian restricted to a 3-mm-diameter ring) and corneal asphericity coefficient (Q) generated by a corneal topographer (TMS-4, Japan) were provided to the manufacturer to determine the back optic zone radius of a lens. Slit-lamp biomicroscopy with corneal fluorescein staining tests were performed 1 and 2 hours after delivery of the lens. For a fit to be deemed acceptable, the contact lens had to be well centered on the cornea and move approximately 1 mm on a blink. The overall fluorescent pattern resembled a classic bull’s eye, with a central touch surrounded by a narrow and deep annulus of tears trapped in the reverse curve area. Total diameters of the lenses were 10.6 mm or 10.8 mm. The subjects were reevaluated for compliance, lens position, visual acuity, and corneal topography 1 day, 1 week, and 1 month after delivery of the lenses. They were instructed to wear their lenses for at least 8 hours per night and 6 days per week. Parents were requested to log their children’s OK lens wearing schedules and inform the researcher of the details at every face-to-face follow-up interview.
The parents were fully instructed to help with the daily lens wearing procedures. In cases of severe scratch or abrasion affecting continuation of lens wearing or a lens breakage, a new lens of the same design was prescribed. Otherwise, should the monocular uncorrected distance visual acuity be equal to or worse than 20/25, or an unacceptable fitting continuously occurred after the 3-month visit, a new lens of modified curve parameters was prescribed and the patient excluded from the study because their topographic data collected at the 3-month visit were no longer useable as a result of lens modification.
Measurements and Follow-up Plans
All patients underwent ocular examinations including slit-lamp examination, manifest refraction, and axial length (AL) measurements at the time of enrollment and 6, 12, 18, and 24 months after the initial treatment. Corneal topography was measured at the time before enrollment and after 1 day, 1 week, 1 month, 3 months, and all the 6-month follow-up visits. AL was measured by partial coherence interferometry (IOLMaster; Carl Zeiss, Germany) five times and the data were automatically averaged. All these measurements were completed by one operator. Corneal topography was taken by the TMS-4 topographer (TOMEY, Japan), which has been tested for validity and repeatability compared with other topographers.39 The well-focused corneal photos were automatically taken by the topographer and the three best pictures were chosen for analysis. Corneal apical refractive power (Apex) was output automatically and corneal sagittal powers of multiple spots along the nasal, temporal, and inferior axes, e.g., 1 mm, 2 mm, 3 mm, and 4 mm away from the apex, were manually collected (N1-N4, T1-T4, I1-I4). (Fig. 1). To minimize the influence of diurnal variation, all measurements were conducted between 9 a.m. and 11 a.m., i.e., measurements were taken 2 to 4 hours after lens removal.
To track corneal power changes, differences in corneal sagittal refractive powers before and after OK treatment at various sites (N1-N4, T1-T4, I1-I4, and Apex) were calculated. The difference between any peripheral corneal power and apical power was defined as the relative refractive power (RRP). The maximum changes in RRP along the three axes after OK treatment were selected and defined as maxN, maxT, and maxI, with each being categorized into two levels, level 1 and level 2, depending on whether the value was below or above the mean value. Data from three measurements were averaged and used for analysis.
Because abundant evidence has shown that the corneal reshaping process following OK treatment is completed and stabilized in 7 to 10 days,40 the 3-month topographic outputs were taken as representative of the post-OK profile.
It was desired to have 90% power for detection of a 0.17-mm (approximately 0.50 D)6,41 difference in the repeated AL measurements, with significance at the two-sided 5% level and a referred standard deviation of 0.25 mm.18,19,21,22,24 Taking into account 20% loss to follow-up, 29 subjects were required.
Data were analyzed using SPSS 20.0 statistical software (SPSS Inc., Chicago, IL, USA). The Kolmogorov-Smirnov test was used to test the normality of the data. The baseline variables in the subgroups were tested by independent-sample t test, except for gender, which was tested by chi-square test. To determine the repeatability of the topographic plots measured at the 3-month visit, repeated measures ANOVA was used. The within-subject standard deviation (Sw) was calculated. The test-retest repeatability (TRT) was defined as 2.77Sw, an interval within which 95% of the differences between measurements were expected to fall. The within-subject coefficient of variation (COV) was calculated as the ratio of the Sw to the overall mean. To compare the refractive power distribution, paired t test and one-way ANOVA were used. To determine if time and maximum RRP changes have a significant impact on axial growth, repeated two-way ANOVA test was used, in which repeated visits were treated as within-subject effects and corneal max RRP changes along the three axes were taken as between-subject effects. The two-way interactions between corneal powers and repeated visits were also tested. In addition, age, baseline AL, and baseline spherical equivalent refractive error (SER) were tested as covariates. Bonferroni corrections were applied for post hoc pairwise comparisons. Pearson correlation was used to investigate the correlation between the corneal max RRP changes (maxN, maxT, and maxI) and AL change at the 24-month visit.
One patient dropped out at the third month because of persistent poor distant uncorrected visual acuity. Another one dropped out at the 12th month because of emigration. Two patients dropped out at the 18th month because their distance visual acuity dropped below 20/25 and were in the need of new lenses. The last dropout patient failed to follow up at the 24th month. In total, 27 patients completed the whole study and their data were analyzed. The dropout rate was 15.6%.
The cohort that completed the study included 14 males and 13 females. The mean age, SER, AL, and apical corneal power were 10.37 ± 1.18 years, −2.57 ± 0.90 D, 24.50 ± 0.60 mm, and 43.46 ± 1.02 D, respectively. There were no significant differences in baseline age, SER, AL, or apical corneal power between the subgroups of N level, T level, and I level (p > 0.05, Table 1).
Repeatability of the Topography Plots
There were no significant differences in Apex, maxN, maxT, and maxI among the three sets of repeated data (p ≥ 0.93 for all the variables, repeated measures ANOVA). TRTs were 0.06 D, 0.27 D, 0.18 D, and 0.44 D for Apex, maxN, maxT, and maxI, respectively. COVs were 0.05%, 0.22%, 0.16%, and 0.36% for Apex, maxN, maxT, and maxI, respectively.
Corneal Peripheral Refractive Power Distribution
Three months after OK lens wear, corneal apical refractive power decreased significantly (p = 0.031, paired t test). The power data showed significant steepening at the mid-peripheral sites compared with the central cornea and peaked at approximately 2 or 3 mm off the apex along the three axes (Fig. 2). A statistically significant increase in corneal power was observed at the nasal 2 mm (p = 0.001, Bonferroni correction) and 3 mm (p < 0.001, Bonferroni correction); temporal 3 mm (p = 0.002, Bonferroni correction); and inferior 2 mm (p < 0.001, Bonferroni correction), 3 mm (p < 0.001, Bonferroni correction), and 4 mm (p = 0.015, Bonferroni correction) locations compared with the apical center.
Corneal Relative Refractive Power and Axial Elongation
Mean axial elongation at 24 months was 0.30 ± 0.21 mm. Within-subject tests indicated that AL increased significantly throughout the 24-month study period (F = 41.45, p < 0.001). Between-subject tests showed that N level (F = 16.17, p = 0.001), T level (F = 7.92, p = 0.011), and I level (F = 15.64, p = 0.001) significantly affected axial elongation. Specifically, axial growth in level 2 was 31% to 46% of that in level 1 at each visit (Table 2, Fig. 3). However, age, baseline AL, or SER did not affect AL growth (p > 0.05).
Based on Pearson correlation, maxN (F = 21.32, p < 0.001, r 2 = 0.46, Fig. 4A), maxT (F = 30.01, p < 0.001, r2 = 0.55, Fig. 4B), and maxI (F = 39.18, p < 0.001, r2 = 0.61, Fig. 4C) were negatively correlated with 2-year axial growth.
Smith et al42 reported that, in infant monkeys, myopia induced by diffusers or defocusing lenses in eyes with intact retina are similar to that in eyes with laser-ablated foveas. These experiments demonstrated that peripheral visual signals might have a greater influence on emmetropization than previously thought. Liu et al43 and Tepelus et al44 applied custom-designed bifocal spectacle lenses to young chickens and also showed that peripheral defocus could influence both peripheral and central refractive development.
Recent animal studies45,46 further indicated that lens-induced relative myopic defocus of the peripheral retina could significantly reduce the developmental rate of lens-induced myopia in chickens. Based on the results from animal experiments, numerous clinical trials have attempted to impose relative myopic defocus on the peripheral retina via different optical designs. The rotationally symmetric or asymmetric spectacles47 and bifocal or multifocal contact lenses48,49 yielded satisfactory results on myopia control, being more effective than traditionally designed bifocal or progressive addition spectacle lenses,10–17 probably because of the relative myopic defocus presented on a larger area of the retina than in the former cases.
Studies25–28 with the assistance of open-field autorefractor have indicated that OK reverses the pattern of peripheral refraction, converting the relative peripheral hyperopia typically observed in myopic eyes to relative peripheral myopia. Coupled to the change in peripheral refractive pattern associated with OK is the finding that the corneal epithelium can be redistributed by reverse geometry contact lens design, evidenced by corneal topography,50 optical coherence tomography,51,52 and confocal microscopy,52,53 resulting in a relatively thin and flat central cornea combined with a relatively thick and steep peripheral cornea. The advantage of OK for peripheral refraction manipulation comes from its consistent reshaping profile that does not change with blinking or eye movements, problems encountered in other spectacle or contact lens modalities.
Despite the fact that the weighted average reduction in axial elongation by OK for previous studies18–24 was 41.7%,54 the highest among all the optical strategies, no study up to date has directly targeted cornea changes, and thus peripheral refractive power changes, as predictive factors for future myopia progression following OK therapy. The current study calculated the adjusted peripheral refractive power for central refractive power on the cornea and found that changes in peripheral corneal power along all three directions were negatively correlated with myopic eye growth, indicating that the induced relative peripheral myopic defocus might be protective against myopia progression. In light of this finding, future strategies may concentrate on modifying the curve design of OK lenses, e.g., widening the reverse curve or adjusting its curvature, to facilitate relative peripheral myopic defocus. Narrowing the spherical-design optical zone could be another alternative.
All the subjects enrolled in the current study were characterized by well-centered cornea reshaping with OK, with lens displacement no greater than 1 mm. Nevertheless, corneal refractive power distribution (Fig. 3) showed that most patients had a detectable temporal displacement, which is likely a result of the temporally located corneal apex relative to the pupillary axis. The sites with maximum power change along the three axes appeared approximately 2 or 3 mm away from the apex on the sagittal power maps. The superior axis was not included because the palpebral fissures in Chinese children were generally smaller than those in Caucasians and difficulty has been encountered in collecting far superior corneal contours in these subjects. A comprehensive model covering the entire cornea with continuous data sites deserves future efforts, i.e., collecting the total peripheral refractive power change of the front cornea associated with OK.
One of the limitations of the current study is the relatively small sample size. Although several covariates were included to adjust the results, a larger sample size is still needed to confirm the findings of this study.
In summary, the current study documented that subjects with a larger magnitude of relative corneal peripheral power change along specific axes after OK lens wear experienced slower axial elongation by the end of 24 months. We speculate that this effect was mediated by the induction of a greater amount of relative myopic defocus on the peripheral retina. Potential OK lens designs are warranted accordingly to better control myopia progression in children.
No. 19, Baoqing Road
Authors YZ and ZC contributed equally to this work and are considered co-first authors.
We thank Chun Gong, Alan Lu, and Mariana Garcia for the revision of the article. This work was supported by the Key Project of Science and Technology of Shanghai, Grant No. 11JC1402000. The authors have no financial interest in any of the products mentioned in the article.
Received July 8, 2013; accepted December 20, 2013.
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Keywords:© 2014 American Academy of Optometry
orthokeratology; myopia; axial length; corneal sagittal refractive power; peripheral refraction