Efficacy of long-term orthokeratology treatment in children with anisometropic myopia

INTRODUCTION

Anisometropia is defined as the interocular difference of refraction. Anisometropia tends to occur over the course of a life span, namely, after birth, at the onset of myopia, and at the onset of presbyopia. Among school-aged children,the prevalence of anisometropia with a spherical equivalent refraction (SER) >0.5 diopter (D) is 34%, and it is 1%-10% when SER >1 D. Also, the prevalence of myopic anisometropia is significantly higher than that of emmetropic anisometropia.Both the prevalence and severity of anisometropia increase with increasing degree of myopia. Binocular vision can be impaired even with moderate refractive differences between eyes, including stereoacuity and contrast sensitivity.Anisometropia is thought to be closely related to aniseikonia and amblyopia. Without intervention, the difference between the two eyes will continue to increase in both mild and severe anisometropia among school-aged children.Thus, treating myopia and anisometropia at same time is seriously important.

The intraocular differences of AL were non-normally distributed.At beginning, it was 0.6 (0.46) mm. After monocular ortho-k treatment, it was significantly decreased to 0.22 (0.39) mm(=-5.402,<0.0001). However, after a year around binocular ortho‐k treatment, it was significantly increased to 0.30 (0.32) mm(=-4.086,<0.0001), but still significantly lower than baseline (=-4.716,<0.0001; Table 4).

A total of 50 children participated in this observation, 18 of whom were boys and 32 of whom were girls. The average age was 10.52±1.72y. At baseline, they only had one myopic eye (SER -1.80±0.82 D), and were applied with ortho-k lenses. The degree of SER anisometropia was 1.56±0.97 D (95%CI: 1.29, 1.84) at baseline. After different durations of monocular treatment, 12.20±6.94mo(95%CI: 10.23, 14.17), the contralateral eyes of these children developed myopia (SER -1.56±0.43 D) and applied with ortho-k lens, too. Then the patients started binocular ortho-k treatment.

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SUBJECTS AND METHODS

This study was in accordance with the ethical standards formulated in the Helsinki Declaration. All subjects signed an informed consent before treatment.

Anisometropia is a special form of refractive error that is defined as refractive differences between two eyes that are spherical, cylindric or both, but the criterion for anisometropia varies among studies from 0.5 to 1.5 D. Since the ortho‐k lens only applied to SER≤‐1.0 D, subjects in this study had anisometropia no less than SER 0.5 D, and the degree of anisometropia was SER 1.56±0.97 D at baseline. Although optical components such as corneal curvature, refraction of the crystalline lens, and AL all contribute to ocular refraction,AL is considered to be fundamental in creating interocular refraction differences. Thus, this study was to observe progression of axial anisometropia under ortho-k lens.

The average elongation of the AL per month(mm/mo) was calculated as the AL growth rate, as well as the difference of follow-up and baseline AL (mm) divided by duration (mo). In monocular period, the AL growth rate of Group B, untreated eyes, is 0.038±0.018 mm/mo (95%CI: 0.033,0.044). To explore the relationships, nine baseline factors were taken, including age, SER, mean corneal curvature(Km), anterior chamber depth (ACD), and AL of both eyes.In the multiple linear regression analysis, it was independent of observations (Durbin-Watson=1.901), and there was no multicollinearity. This regression model was statistically significant (=3.879,=0.001, adjusted=0.346). As shown in Table 2, among these factors, age (=0.005;: -0.004;95%CI: -0.007, -0.001), SER of Group A (=0.03;: 0.013;95%CI: 0.001, 0.024), AL of Group A (=0.006;: 0.032;95%CI: 0.01, 0.055), and AL of Group B (=0.011;: -0.035;95%CⅠ: ‐0.062, ‐0.009) were statistically significant.

For convenience, the eye that received the ortho-k lens first was regarded as Group A, and the other eye which received the ortho-k lens later was regarded as Group B. The AL and SER of the two groups of eyes at the baseline and endpoint of monocular period are shown in Table 1.During the whole treatment, there were no serious adverse events among these subjects.

All the subjects received baseline examinations, including autorefraction (RM 8800,Topcon, Japan), cycloplegic refraction (compound tropicamide eye drops, Sinqi, China), AL measurement (IOLMaster-500,Carl Zeiss, Germany), and corneal topography (Pentacam HR;Oculus, Germany). After no less than 3mo of treatment, the AL and corneal topography were evaluated at every 3mo or anytime subjects wanted.

Continuous variables are expressed as the means±standard deviation or median (interquartile range).Multiple linear regression was used to estimate the relationship between several independent ocular variables and the AL growth rate of emmetropic eyes at monocular period. The baseline characteristics and the average elongation of AL per month were evaluated by pairedtest. The comparation for interocular differences of AL were evaluated by Wilcoxon test. Statistical analysis was performed using the SPSS software package(SPSS for Windows, version 24, Chicago, IL, USA). The level of statistical significance was considered to be 0.05 (two‐sided).

RESULTS

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The ortho-k lenses used in this study were Paragon CRT lenses (Paragon Vision Sciences, USA) and LK lenses (Lucid, Korea). Ortho-k lenses were fitted to every subject according to the manufacturer’s recommended procedure. Lenses were selected by experienced optometrists based on corneal topographic variables and adjusted under sodium fluorescein and cobalt blue lighting. After treatment began, each subject was told to wear a lens overnight for approximately 8h and visit the hospital regularly and whenever ocular complaints occurred.

In order to explore the differences of AL changes between monocular and binocular periods, the AL of 12±1mo after beginning binocular ortho-k treatment was measured. TheAL was 24.89±0.76 mm (95%CI: 24.68, 25.11) of Group A, and 24.51±0.71 mm (95%CI: 24.31, 24.72) of Group B. The AL growth rates per month were shown in Table 3,the pairedtest was conducted. For the Group A, the AL growth rate was significantly lower in monocular period (0.008±0.022 mm/mo) than in binocular period(0.026±0.014 mm/mo;=-4.664,<0.0001). For the Group B, the AL growth rate was significantly higher in monocular period (0.038±0.018 mm/mo) than in binocular period(0.016±0.015 mm/mo;=7.826,<0.0001).

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The results are also shown in Table 3. As in Figure 1, during monocular period, the AL growth rate of the Group A (0.008±0.022 mm/mo)was significantly slower than that of the Group B(0.038±0.018 mm/mo;=-8.251,<0.0001). However,during binocular period, the AL growth rate of the Group A(0.026±0.014 mm/mo) was significantly faster than that of the Group B (0.016±0.015 mm/mo;=4.675,<0.0001).

An overnight orthokeratology (ortho-klens flattens the cornea by reverse geometry, which corrects the refractive error and provides clear vision during the daytime. Moreover,the ortho-k lens has been proven to be efficient in reducing myopia progression by nearly 45%, especially in patients with a higher degree of myopia. Although there is a risk of microbial keratitis, ortho-k lenses are thought to be a safe clinical option.Few studies have explored the effectiveness of ortho‐k lenses under certain circumstance. For example, in some children,after monocular ortho-k treatment, contralateral emmetropic eye develop myopia, too. Children with anisometropic myopia receive ortho-k treatment in the sequence of monocular and binocular. Our study was aim to analyze whether the ortho-k lens effect on the control of axial anisometropic myopia,especially the condition mentioned above. This study will contralaterally compare the axial length (AL) elongation characteristic under monocular and binocular ortho-k treatment, and find if there is an ocular parameter that related with the AL elongation of the eye myopic later.

DISCUSSION

This retrospective cohort study included children who visited Peking University Third Hospital for refraction correction from July 2015 to August 2020. Subjects met the following inclusion criteria: aged between 8-18y; monocular best corrected visual acuity not worse than 20/20 for both eyes; one eye with myopia (SER ≤‐1.0 D) received ortho‐k lens; the other eye being relatively emmetropic (SER <±0.5 D);after sometime of monocular ortho-k lens, the contralateral eye developed myopia (SER ≤‐1.0 D) too and began binocular ortho-k lens treatment. And the exclusion criteria are:astigmatism ≥2.0 D; history of corneal contact lens; history of atropine drops; eye disease or surgery; discontinuation of ortho-k lens use; ocular or systemic conditions that might affect.

The multiple analysis between the parameters and axial growth rate of Group B in monocular period showed significant relation (=0.001). Beside age (=0.005), AL of Group A and Group B (=0.006,=0.011) were significantly related to axial growth rate before developing myopia. This is in accordance with previous studies showing that myopia progression varies with AL elongation, besides age growth. And SER of Group A showed significant, too (=0.03). Which may suggest that the higher refractive error, the faster growth of the contralateral eye.During the monocular ortho-k lens period, the AL of the myopic eyes was prolonged by 0.008±0.022 mm/mo,compared to 0.038±0.018 mm/mo in the contralateral untreated eyes. Also, the interocular difference in AL was significantly smaller than that at baseline (<0.0001).

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