Abstract
Objectives:
To assess the efficacy of orthokeratology in controlling the rate of myopia progression in children and investigate the factors associated with axial length (AL) growth rate with an average of 48 months of orthokeratology lens wear.
Methods:
As a retrospective study, 84 subjects underwent relatively complete ophthalmologic examinations. After initial lens wear, AL was measured on average every 12 months. The linear mixed-effects model (LMM) was used to compare the differences in AL growth rates at each time interval. The contribution of the independent variables to AL change was assessed using multiple linear regression.
Results:
In the LMM, there was a significant difference in the AL growth rate (P<0.001) at each follow-up. The growth rate of AL was associated with initial AL, spherical equivalent refractive errors (SERs) and diameter of lens (P=0.045, 0.003 and 0.037, respectively). When the baseline age was included as a factor, the influence of initial AL and SER became insignificant in the analysis, whereas age and diameter of lens were significantly correlated with the growth rate of AL (P<0.001 and P<0.001, respectively). There were significant differences in growth rates among different age groups.
Conclusions:
Results of the study demonstrated that the factors associated with lower growth rate in AL were older age and longer diameter of lens.
Key Words: Orthokeratology lens, Axial length, Myopia
Myopia is one of the most common refractive disorders worldwide and also one of the leading causes of vision loss. The incidence of myopia has been increasing substantially over the recent decades, which has attracted worldwide attention.1 The prevalence of myopia has been estimated to be approximately 20% to 50% in Western countries and up to 85% of the East Asia population, with the prevalence rates of up to 90% to 95% among Chinese university students.2,3 It has been forecast that myopia will affect around five billion people globally by the year 2050.4 Among myopic patients, high myopia is especially related to an increased risk of vision-threatening eye disorders including cataract, glaucoma, staphyloma, macular degeneration, retinal detachment and chorioretinal degeneration,5 and has significant economic and social impacts. Therefore, the control of myopic progression has received increased attention and has important implications for the overall population.
Various strategies have been applied to correct existing myopia and control the development of myopia, including glasses, contact lenses, atropine, orthokeratology, and refractive surgery.6,7 Of these, orthokeratology seems to be a popular and effective treatment option to slow down myopia progression for children.8 Currently, orthokeratology is well accepted by parents and children with a relatively low rate of adverse events and discontinuations.9,10 Many studies have demonstrated the efficacy of orthokeratology in controlling myopia progression compared with atropine,11 single-vision spectacle lenses,12–14 and soft or rigid gas-permeable contact lenses.15
Nowadays, identifying the children who are likely to benefit the most from myopic control with orthokeratology specifically is still a challenge in clinical practice. Several studies have been conducted to investigate the factors related to the efficacy of orthokeratology in myopic control, such as initial age,13 spherical equivalent refractive errors (SERs),16,17 axial length (AL),18 central corneal thickness (CCT),18 and parental refraction.19 However, different studies sometimes report contradictory results on the effect of various factors, especially the initial SER.13,17,20 In this study, we assessed the long-term efficacy of orthokeratology lenses in controlling myopia progression and evaluated the correlation between ocular parameters and AL increase over a 4-year period.
METHODS
Subjects
Subjects who visited Peking University Third Hospital for vision correction with orthokeratology lenses from November 2015 to March 2018 were recruited in the study. During the study period, the optician received a total of 204 patients who attended the hospital for OK treatment for the first time in the outpatient clinic. Two hundred four patients were invited to participate in this study. Only 84 patients met the inclusion criteria, whereas others were excluded because of not completing the required number of follow-up visits or meeting other exclusion criteria. On behalf of the minors/children, written informed consent was acquired from their caregivers or guardians. The study used routinely collected clinical data and no patient involvement was required. The research was approved by the Ethics Committee of the Peking University Third Hospital Review Board.
The subjects included in this study meet the following criteria: (1) aged 8 to 18 years at baseline; (2) spherical equivalent refraction from −0.50 to −8.00 D; (3) astigmatism ≤3.00 D; and (4) distant best-corrected visual acuity (BCVA) better than 0 log minimum angle of resolution units (20/20). The exclusion criteria include: (1) fail to achieve regular daily lens wear for at least 8 hr; (2) less than 48 months of follow-up or continuous lens wear; (3) ocular, systemic, or neurodevelopmental deviations that may affect refractive development; (4) use of medications that might affect refractive development; and (5) history of refractive surgery, laser surgery, or intraocular surgery.
Data Collection
Subjects received complete ophthalmologic examinations including uncorrected visual acuity, BCVA, manifested refraction, cycloplegic refraction, topographic evaluation, pupil size, and AL measurement with IOL Master (Carl Zeiss, Jena, Germany). After initial lens wear, AL was measured on average every 12 months.
Statistical Analysis
Data analyses were performed using IBM SPSS Statistics 25.0 (Chicago, IL). The primary outcome measure of this study was the rate of AL growth, which was calculated by dividing the difference between the two AL measurements by the time interval (month) between the two visits. The linear mixed-effects model (LMM) and generalized estimating equation were conducted to compare the differences in AL growth rates at each time interval and the contribution of the independent variables on AL increase was assessed. Independent variables included gender, initial age, SER, astigmatism, the flat keratometry values (K1), diameter of pupil, baseline AL, brand of lens, and the diameter of the lens. If both eyes of an enrolled participant met all inclusion and exclusion criteria, the left eye was chosen for LMM. Pairwise comparison was conducted if applicable, with Bonferroni correction. The strength of association for significant variables is represented using beta values, 95% confidence intervals, corrected R2 values and P values. P<0.05 was considered to be statistically significant.
RESULTS
Patient Characteristics
The baseline characteristics of the children are presented in Table 1. The average age at the initiation of orthokeratology lens wear was 10.76±1.73 years old and the average duration of orthokeratology lens wear was 48.00±4.01 months. The average baseline SER was −3.13±1.47 Diopters (D). The baseline AL was 24.85±0.94 mm. The average axial growth of all patients during follow-up was 0.16±0.09 mm/year.
TABLE 1.
Demographic Characteristics and Pretreatment Optical Parameters of the Patients
| Variable | Mean±SD | Range |
| Age at initial lens wear (yrs) | 10.76±1.72 | 8–16 |
| Gender (Male: Female) | 39: 45 | |
| Sphere refraction (D) | −2.90±1.38 | −7.50 to −0.25 |
| Cylinder refraction (D) | −0.56±0.54 | −2.75 to 0.00 |
| Spherical equivalent refractive errors (D) | −3.16±1.47 | −7.50 to −0.50 |
| AL (mm) | 24.91±0.92 | 22.26–27.48 |
| Follow-up period (mo) | 48.00±4.01 | 39.87–61.2 |
| Flat keratometry values (K1, D) | 42.69±1.33 | 40.07–47.58 |
| Steep keratometry values (K2, D) | 43.95±1.59 | 40.29–48.89 |
| Diameter of the lens (mm) | 10.53±0.20 | 10.0–11.0 |
| Diameter of pupil (mm) | 3.96±0.69 | 2.4–6.9 |
AL, Axial Length.
Change of Axis Length Growth Rate
In the one-way analysis of variance (ANOVA), there was a significant difference in the AL growth rate (P<0.001) at each follow-up. Paired comparisons (Fig. 1) showed that the growth rate of AL in the first year was significantly faster than that in the fourth year (P<0.001). The growth rate in the second year was also significantly quicker than that in the third and fourth year (P=0.028 and P<0.001, respectively). And there was no significant difference between the first 2 years or the last 2 years.
FIG. 1.
Comparison of axial length growth rate in each follow-up interval. Paired comparisons showed that the growth rate of AL in the first year was significantly faster than that in the fourth year (P<0.001). The growth rate in the second year was also significantly faster than that in the third and fourth year (P=0.028 and P<0.001, respectively). (1) First 12 months; (2) 12–24 months; (3) 24–36 months; (4) 36–48 months. AL, axial length. *P<0.5. ***P<0.001.
Influencing Factors of Long-Term Axial Growth Rate
We calculated the growth rate of AL in 1, 2, 3 and 4 years, respectively, and analyzed the influencing factors of long-term AL growth rate by multiple linear regression (Table 2). The results of the analyses were consistent. The factors associated with lower growth rate in AL were older age, longer diameter of lens, and longer baseline AL.
TABLE 2.
Influencing Factors of Long-Term Axial Growth Rate
| Year | Parameter | B | Standard Error | Exp(B) | P | CI | |
| 1 | Age | −0.005 | 0.0009 | 0.995 | <0.001 | 0.993 | 0.997 |
| Diameter of lens | −0.033 | 0.0059 | 0.967 | <0.001 | 0.956 | 0.978 | |
| 2 | Age | −0.005 | 0.0007 | 0.995 | <0.001 | 0.993 | 0.996 |
| Diameter of lens | −0.028 | 0.0055 | 0.973 | <0.001 | 0.962 | 0.983 | |
| Baseline AL | 0.006 | 0.0030 | 1.006 | 0.039 | 1.000 | 1.012 | |
| Diameter of lens | 0.002 | 0.0011 | 1.002 | 0.037 | 1.000 | 1.005 | |
| 3 | Age | −0.005 | 0.0006 | 0.995 | <0.001 | 0.994 | 0.996 |
| Diameter of lens | −0.019 | 0.0041 | 0.981 | <0.001 | 0.973 | 0.989 | |
| Baseline AL | 0.004 | 0.0021 | 1.004 | 0.036 | 1.000 | 1.008 | |
| 4 | Age | −0.004 | 0.0004 | 0.996 | <0.001 | 0.995 | 0.996 |
| Diameter of lens | −0.015 | 0.0035 | 0.985 | <0.001 | 0.978 | 0.992 | |
| Baseline AL | 0.005 | 0.0017 | 1.005 | 0.005 | 1.001 | 1.008 | |
AL, axial length.
Factors Affecting the Growth Rate of Axial Length
The LMM was conducted to assess the contribution of each independent variable on the growth rate of AL. Because the influence of age on the growth rate of AL has been widely confirmed by several studies, and there is a correlation between age and ophthalmic parameters, the variables included in our first LMM analysis did not include age. The results showed that the AL growth rate was associated with initial AL, SER, and diameter of lens (Table 3). The growth rate of AL slowed down with a longer initial AL (P=0.045). Subjects with a greater SER and longer diameter of lens demonstrated faster growth rate in AL elongation (P=0.003 and 0.037, respectively). Nevertheless, gender, year of follow-up, astigmatism, K1, and diameter of pupil did not show a significant effect on the growth rate of AL.
TABLE 3.
Factors Affecting the Growth Rate of AL Without the Inclusion of Age as an Independent Variable
| t | P | 95% CI | ||
| Lower | Upper | |||
| Year of follow-up | 0.128 | |||
| Brand of lens | 0.281 | |||
| Gender | 1.335 | 0.183 | −0.0001221 | 0.006354 |
| Astigmatism | 0.422 | 0.674 | −0.003233 | 0.004994 |
| Diameter of pupil | 0.452 | 0.652 | −0.002123 | 0.003387 |
| SER | −3.045 | 0.003 | −0.005109 | −0.001095 |
| Baseline AL | −2.013 | 0.045 | −0.009101 | −0.000098 |
| K1 | −1.733 | 0.084 | −0.004192 | 0.000268 |
| Diameter of lens | −2.103 | 0.037 | −0.020101 | −0.000656 |
AL, axial length; K1, the flat keratometry values; SER, spherical equivalent refractive error.
To explore whether the effect of the initial AL is age-related, age was added as a factor into the model (Table 4). The effect of initial AL became insignificant, whereas age and diameter of lens were significantly correlated with the growth rate of AL (P<0.001 and P<0.001, respectively).
TABLE 4.
Factors Affecting the Growth Rate of AL With the Inclusion of Age as an Independent Variable
| t | 95% CI | |||
| P | Lower | Upper | ||
| Age | <0.001 | |||
| Year of follow-up | 0.134 | |||
| Brand of lens | 0.497 | |||
| Gender | 0.409 | 0.683 | −0.002831 | 0.004314 |
| Astigmatism | 0.091 | 0.927 | −0.003712 | 0.004072 |
| Diameter of pupil | 0.868 | 0.386 | −0.001430 | 0.003680 |
| SER | −0.459 | 0.647 | −0.002521 | 0.001568 |
| Baseline AL | 0.773 | 0.440 | −0.002838 | 0.006501 |
| K1 | −0.166 | 0.868 | −0.002357 | 0.001990 |
| Diameter of lens | −3.691 | <0.001 | −0.027233 | −0.008277 |
AL, axial length; K1, the flat keratometry values; SER, spherical equivalent refractive error.
Because the baseline age showed the strongest relationship with AL growth rate, the growth rate at different ages were compared (Table 5). The axial growth rate of 9-year-old subjects was faster than that of over 11. Subjects aged 10 and 11 grew their AL faster than children aged 13, 14, 15, and 16. There was no significant difference among subjects aged 13 to 17.
TABLE 5.
Comparison of the Growth Rate of AL in Children of Different Ages Based on Mixed Linear Model With Bonferroni Correction
| Age (n, Mean±SD) | Age (n, Mean±SD) | Differences in the Average Values (Mean±SD) | P |
| 9 (22, 0.017±0.006) | 12 (58, 0.013±0.002) | 0.015±0.003 | 0.001 |
| 13 (51, 0.009±0.002) | 0.019±0.004 | <0.001 | |
| 14 (41, 0.008±0.002) | 0.019±0.004 | <0.001 | |
| 15 (27, 0.004±0.003) | 0.024±0.004 | <0.001 | |
| 16 (12, 0.004±0.004) | 0.024±0.005 | <0.001 | |
| 17 (4, 0.001±0.006) | 0.027±0.007 | 0.009 | |
| 10 (35, 0.028±0.003) | 13 | 0.012±0.003 | 0.004 |
| 14 | 0.013±0.004 | 0.012 | |
| 15 | 0.018±0.004 | <0.001 | |
| 16 | 0.018±0.005 | 0.010 | |
| 11 (50, 0.019±0.002) | 13 | 0.009±0.003 | 0.027 |
| 14 | 0.010±0.003 | 0.049 | |
| 15 | 0.015±0.003 | 0.001 | |
| 16 | 0.015±0.004 | 0.038 |
AL, Axial Length.
DISCUSSION
In our work, the subjects showed a mean AL increase rate of 0.16±0.11 mm/year, which was consistent with long-term and short-term studies.13,16,18,21,22 Reported in a previous survey of children treated with orthokeratology, AL increased over a 2-year period range from 0.29 to 0.45 mm,14,17,18,20,23 to which our results were considered to be comparable. Recent studies and meta-analysis studies have confirmed that there is a significantly smaller growth in wearers of orthokeratology versus controls using spectacles.24,25 In these literature, there was a 40% to 60% mean reduction in the rate of refractive change compared with the control groups.15,26,27 However, no comparison with other treatments in retarding myopia progression was made in this study. Instead, we mainly focus on the growth rate of AL and its influencing factors during the follow-up period.
Our results showed that the growth rate of AL showed a downward trend with the extension of follow-up time (P<0.001), although the differences between the first and second years and between the third and fourth years were not significant. The follow-up period of most previous studies was limited to 2 years.17,28 Although some studies were followed up for 5, 7, or even 12 years, they did not compare the trend of AL growth rate of children wearing orthokeratology.14,29,30 Therefore, this trend has not been shown in previous studies. In the study followed up for 7 years, the increases in AL over the first 24 months were similar to the increase rate between 24 and 84 months in the orthokeratology lenses group, which indicated the axial elongation rate decreases regardless of the visual correction being worn.29 The reduction of AL growth rate with long periods of orthokeratology wear in this study could be explained by the natural history of myopia progression, and the reduced rate of axial elongation is associated with older age.8 However, our results showed that despite the influence of age, the effect of orthokeratology in the first two years has not decreased yet.
The mechanism of orthokeratology on myopia progression has not been completely elucidated. There has been much speculation about the potential mechanism, and the predominant hypothesis is that orthokeratology slows myopic progression based on the “peripheral refraction theory.” The theory states that orthokeratology reduces stimuli for axial elongation via decreasing peripheral hyperopic defocus and increasing peripheral myopic defocus.31 It is generally believed that the length of the eyeball in the axial direction in myopes is longer than that of emmetropes and hyperopes, resulting in greater relative hyperopia of the peripheral visual field. Meanwhile, peripheral hyperopia and blurred retinal imaging are crucial inducements of axial elongation and myopia. Wearing orthokeratology lenses could induce the midperipheral cornea to steepen and the central cornea flattened, which results in the myopia of the surrounding cornea and prevent peripheral hyperopia from defocusing. Therefore, the further reduced visual feedback of eye axis elongation delays the progress of myopia.27 This hypothesis has been confirmed in animal models and human studies. In experiments of animal models, it was found that the induction of hyperopia defocus on the peripheral retina could induce the growth of AL and the development of myopia, whereas the relative peripheral myopia defocus reduced the rate of AL growth and hyperopia.32 In humans, corneal remodeling after orthokeratology lenses wear has also been shown to lead to myopic defocus on the peripheral retina.33
In this study, we found that age was correlated with the growth rate of AL and younger myopic children demonstrated a greater growth rate than older. This trend has been consistently reported in previous studies.13 At the same time, the results of this study were consistent with the results of a meta-analysis; that is, the change of myopia is the largest in children aged 9 to 11.34
Previous studies have generally believed that the effect of age is mainly because of the natural process and development of myopia, which means the AL of younger children grows faster, and the progress of myopia in children gradually slows down with the increase of age.8 For older children with late-onset myopia, they may have experienced a natural decline in the growth rate of AL.14 Although the AL of younger children grows faster, studies have shown that orthokeratology lenses are more beneficial for myopia control in younger patients.22 Therefore, for children with early-onset myopia, early use of orthokeratology lenses can make them obtain the most benefits and delay the progress of myopia so as to reduce the prevalence of high myopia.
It is worth noting that age is related to many parameters that may affect the growth rate of AL. The younger the age, the shorter the initial AL and the smaller the SER.18 Therefore, the inclusion of age factors in regression analysis may weaken the role of other relevant factors. This hypothesis can also explain why the initial AL, which was significant without considering the age factor, became insignificant in the analysis including the age factor. Likewise, in one study, baseline AL was significantly correlated with AL growth rate in univariable analysis, but became insignificant in multivariable analysis.18
The current study also showed a significantly negative correlation between diameter of lens and growth rate of AL. The lens diameter is influenced by the pupil diameter of children. In Chen et al.35’s study on the impact of pupil diameter on AL growth in orthokeratology, there were consistent results that children with larger pupils showed significantly less myopia progression during the 24 months visit. This could be explained by the fact that the larger pupil allows more light to pass through and fall on the peripheral retina, which induces more myopic displacement in the peripheral retina.
We demonstrated no significant association between baseline SER and AL growth rate. In some previous studies, the same conclusion had been reported.12,13,17,26,36 In contrast, other studies obtained conflicting results that higher baseline myopia before orthokeratology use was associated with slower AL elongation versus control. Furthermore, Santodomingo-Rubido et al.19 observed that patients with lower SER showed slower AL increase.
This discrepancy may be caused by the different design of orthokeratology lenses. The different optical designs of orthokeratology lenses may affect the defocus of the peripheral cornea. Zhu et al.37 compared the difference in AL growth rate within 1 year in patients with orthokeratology using three different optical designs (different total diameter, refractive index, wetting angle and central thickness). The results showed that there was a significant difference in the growth rate of AL among the three groups. In this study, we included children wearing different brands of orthokeratology lenses and did not take the lens design as a separate factor. The exact relationship between baseline SER and AL elongation is still unclear and further investigation is required.
Limitations
There were some limitations in this study. First, we did not include a control group to compare the AL growth rate with in this retrospective study. Second, some other factors that may be relevant to myopia progression, such as anterior chamber depth, CCT, age of myopia onset, corneal power, and parental refraction, were not included in the measurements and analysis.
CONCLUSION
In conclusion, in this study, we conducted a long-term study to assess the efficacy of orthokeratology in controlling the rate of myopia progression in children, and to investigate the factors associated with the AL growth rate with an average of 48 months of orthokeratology lens wear. The results showed the factors associated with lower growth rate in AL were older age and longer diameter of lens.
Footnotes
The author has no funding or conflicts of interest to disclose.
Research data supporting this publication are available from corresponding authors.
H. Lv, Z. Liu, and J. Li contributed equally.
Contributor Information
Huibin Lv, Email: 1710301216@pku.edu.cn.
Yuexin Wang, Email: l18810625357@163.com.
Yulin Tseng, Email: 1071464560@pku.edu.cn.
Xuemin Li, Email: lxmlxm66@sina.com.
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