Axial Length Control Is Associated With a Choroidal Thickness Increase in Myopic Adolescents After Orthokeratology

Supplemental Digital Content is Available in the Text.

Abstract

Objective:

To investigate the changes in choroidal thickness and axial length after orthokeratology in adolescents with low-to-moderate myopia and to explore the relationship between choroidal thickness and axial length variation.

Methods:

Thirty eyes with low-to-moderate myopia were retrospectively studied, and optometric data were collected before and after 6 months of continuous orthokeratology. Axial length and choroidal and foveal thicknesses were measured using optical biometry and enhanced depth imaging–spectral domain optical coherence tomography, respectively.

Results:

Axial length in the low myopia group increased (P<0.001) after 6 months of orthokeratology, and the variation was greater than that in the moderate myopia group (P<0.05). The subfoveal choroidal thickness in low and moderate myopia groups increased (P<0.01), and the variation was greater in the moderate myopia group (P<0.05). Choroidal thickness in all seven measured spots increased, with the variation of subfovea, nasal 1 mm to fovea, and temporal 1 mm to fovea being statistically significant (P<0.001, P<0.05, and P<0.05). The change in axial length was negatively associated with subfoveal and average choroidal thicknesses (P<0.01).

Conclusion:

Adolescents with moderate myopia presented better axial length control after 6 months of orthokeratology. The choroidal thickness of low and moderate myopic eyes increased, and the variation was more significant in moderate myopic eyes. The axial length control effects can be associated with an increase in the subfoveal and average choroidal thickness.

Myopia is a global public health concern. Worldwide, the number of people with myopia has reached 2.6 billion in 2020, and the data on high myopia are also dramatically increasing.¹ Children with an early myopia onset are more likely to develop high myopia or pathologic myopia in the future and suffer from serious visual impairment.² Effective myopia control plays a significant role in improving adolescent visual health. Currently, the commonly accepted interventions for myopia control in adolescents are optical correction and drug therapy. Optical correction is represented by progressive addition spectacle lenses, peripheral defocus modifying contact lenses, and orthokeratology (OK). However, the effectiveness of OK varies greatly among different studies,³ and its mechanism for myopia control remains unclear.⁴ Recent studies indicated that increments of choroidal thickness occurred after OK application.^5,6 Then it is speculated that the underlying mechanism of myopia control using OK could be associated with the change in choroidal thickness. However, OK's axial control effects between different myopia degrees and the association between axial control and choroidal thickness variation still need further investigation. Moreover, there were still some inconsistencies among existing reports in regard to the change in choroidal thickness in different retinal locations.^7,8 This study aimed to measure choroidal thickness before and after OK application in adolescents with low-to-moderate myopia using spectral domain optical coherence tomography (enhanced depth imaging) (EDI-SD-OCT), compare the axial length (AL) control effect of OK with low-to-moderate myopia, analyze the relationship between changes in choroidal thickness and AL, and discuss the potential mechanism of OK in myopia control.

PARTICIPANTS AND METHODS

Inclusion and Exclusion Criteria

A retrospective analysis was performed on 30 eyes of 30 adolescents with low-to-moderate myopia who were admitted to the Optometry Center of the Fourth People's Hospital of Shenyang, China Medical University, from February 2021 to February 2022. All participants were prescribed with Vision Shaping Treatment (VST) designed night wear OK (Alpha Orthokeratology, Japan) for 8 to 10 hr every night with favorable lens fitting. Follow-ups were performed at 1 day; 1 week; and 1, 3, and 6 months after OK application. Replacement was required when serious contact lens abrasion occurred during follow-up. The degree of wear and time of replacement were determined by the same experienced optometrist after the examination. Inclusion criteria were age 8 to 16 years, considering the prescribed age for OK application and most children older than 8 years passed the visual development period⁹; data of the right eye from each participant were selected; spherical equivalent refraction (SER) was based on participants' cycloplegic refractive value and was set as −0.50 to −6.00D, with regular astigmatism of less than 1.50D; corneal flat keratometry (Kf) value was set as 40 to 46D; corneal E value greater than 0.2; and participants' monocular and binocular best-corrected visual acuity greater than or equal to 1.0. Participants with any active ocular inflammation status, systemic disease, or current use of other optical or medical interventions to control myopia were excluded. Participants were divided into the low myopia group (SER>−3.00) of 16 eyes and moderate myopia group (−6.00D<SER≤−3.00D) of 14 eyes, according to the SER at initial diagnosis. The current study was approved by the Institutional Review Board of the Fourth People's Hospital of Shenyang (2021-wjkt-002), China Medical University, and informed consent was obtained from all participants.

Measurements

Routine Examination

All participants underwent visual acuity, cycloplegic manifest refraction, bilateral anterior segment slitlamp microscopy, noncontact tonometry, corneal topography, and direct fundoscopy at initial diagnosis to exclude other ophthalmic diseases. Follow-up visits included binocular uncorrected visual acuity, ocular movement inspection, automatic and comprehensive optometry, and visual function tests. Final refraction data were referred to the result of comprehensive optometry.

Axial Length

Optical biometry (IOLMaster 500; Carl Zeiss Meditec, Jena, Germany) was used under mesopic conditions to obtain AL data before OK and at 6 months after OK. The average value was obtained after five measurements with signal-to-noise value above 3. Cycloplegia was not performed during axial length or choroidal thickness measurements because cycloplegic agents may influence choroidal thickness in adolescents.¹⁰

Choroidal and Retinal Thickness

All participants underwent linear retinal scanning using the EDI mode in SD-OCT (Spectralis, Heidelberg Engineering) at initial diagnosis and after 6 months of continuous OK application. All the B-scan analysis achieved a quality index of greater than or equal to 25 dB.¹¹ The total retinal and choroidal thicknesses at the fovea and subfoveal choroidal thickness (SFChT) were measured using the linear scanning pattern along the horizontal line. The inner limit of the choroid was marked by the retinal pigment epithelium, and the outer limit of the choroid was marked by the choroid–sclera junction.¹² The fovea thickness (FT) was calculated by subtracting the choroidal thickness from the total thickness. Meanwhile, the choroidal thicknesses of 1 mm (N1), 2 mm (N2), and 3 mm (N3) to the nasal side and 1 mm (T1), 2 mm (T2), and 3 mm (T3) to the temporal side of the fovea were also measured, and the average choroidal thickness of these seven spots was calculated (Fig. 1). Automatic registration was applied for repeating scanning and the follow-up mode was selected to guarantee scanning on the same retinal spots during follow-ups. OCT inspection was conducted from 9 am to 12 am to avoid circadian variations in choroidal thickness by the same experienced investigator,¹³ and the average value was manually analyzed by two independent inspectors after three repeated measurements. The inspectors were both veteran OCT image analysts and were masked to the initial and follow-up measurements. All included participants cooperated well.

FIG. 1.

Choroidal thickness measurement using EDI-SD-OCT (Spectralis, Heidelberg Engineering). The green arrow indicates the OCT scanning direction.

Statistical Analysis

The R script (version 4.2.1) was used for statistical analysis and data visualization. The chi-square test was used to compare enumeration data between the groups. The Shapiro–Wilk test was performed for data distribution assessment before parametric tests. The t test was used for comparing between-group data conforming to a normal distribution, whereas the independent t test was used for equal variance, and the Welch t test was used for unequal variance. The Mann–Whitney U test was used for data that did not conform to normal distribution. A paired t test was performed to assess the in-group data difference over time with normal distribution, whereas the Wilcoxon signed-rank test was used if normal distribution was not satisfied. The data of AL, choroidal thickness, and FT changes were in accordance with a normal distribution, and the Pearson correlation test was used to analyze the correlation between them. Statistical significance was set as P less than 0.05.

RESULTS

Baseline Characters

The baseline age average in the low myopia group was 10.31±1.08 years and 6 (37.5%) were boys. The baseline age average in the moderate myopia group was 10.79±2.58 years and 5 (35.7%) were boys. There were no significant differences in age or sex between the low and moderate myopia groups (P=0.507 and P=0.939, respectively). SER and Kf in the moderate myopia group (−3.96±0.70D; 44.16±0.88D) were higher than those in the low myopia group (−1.69±0.67D; 43.11±0.71D), and the differences were statistically significant (P<0.001 and P<0.01) (Fig. 2A,B).

FIG. 2.

(A, B) Baseline characters of SER and Kf (independent sample t test; **P<0.01; ***P<0.001). (C) AL difference before and after OK application (paired sample t test; ***P<0.001). (D) SFChT difference before and after OK application (Wilcoxon signed-rank sum test; **P<0.01). (E) Average choroidal thickness difference before and after OK application (paired sample t test; **P<0.01). (F) FT difference before and after OK application (paired sample t test; *P<0.05). (G) Changes in AL, SFChT, average choroidal thickness, and FT before and after OK application (Mann–Whitney U test; *P<0.05).

AL Difference before and after OK Application

In the low myopia group, the baseline AL was 24.16±0.57 mm, and the AL increased to 24.27±0.62 mm after 6 months of OK (t=4.811, P<0.001). In the moderate myopia group, the baseline AL was 24.57±0.47 mm, and the AL was 24.56±0.43 mm after 6 months OK (t=0.151, P=0.882) (Table 1; Fig. 2C).

TABLE 1.

Characteristics of the Study Objects Between the Low and Moderate Myopia Groups Before and After OK Application

	AL (mm)a			SFChT (μm)b			Average ChT (μm)c			FT (μm)d
(mean±SD)	Low Myopia	Moderate Myopia	P'	Low Myopia	Moderate Myopia	P'	Low Myopia	Moderate Myopia	P'	Low Myopia	Moderate Myopia	P'
Before OK	24.16±0.57	24.57±0.47		253.94±39.54	264.5±51.94		263.56±40.69	270.29±49.46		217.06±16.63	209.21±19.48
6 months after OK	24.27±0.62	24.56±0.43		267.06±45.83	289.36±60.95		270.94±46.96	288.64±56.51		220.88±17.51	210±19.50
P	0.000***	0.882		0.004**	0.008**		0.152	0.004**		0.028*	0.856
Changes before and after OKe	110.63±91.98 μm	−5±124.08 μm	0.011*	13.13±15.68	24.86±22.56	0.030*	7.38±19.55	18.36±19.51	0.077	3.81±6.26	0.79±15.91	0.632

a, paired sample t test; ***P<0.001; b, Wilcoxon signed-rank sum test; **P<0.01; c, paired sample t test; **P<0.01; d, paired sample t test; *P<0.05; e, Mann–Whitney U test; *P'<0.05.

AL, axial length; FT, foveal thickness; OK, orthokeratology; SFChT, subfoveal choroidal thickness.

SFChT Difference before and after OK Application

In the low myopia group, the baseline SFChT was 253.94±39.54 μm, and the SFChT increased to 267.06±45.83 μm after 6 months of OK (P<0.01). In the moderate myopia group, the SFChT at baseline was 264.5±51.94 μm, and the SFChT increased to 289.36±60.95 μm after 6 months of OK (P<0.01; Table 1; Fig. 2D).

Average Choroidal Thickness Difference before and after OK Application

In the low myopia group, the average choroidal thickness was 263.56±40.69 μm at baseline and 270.94±46.96 μm after 6 months of OK (t=1.509, P=0.152). In the moderate myopia group, the average choroidal thickness was 270.29±49.46 μm at baseline and increased to 288.64±56.51 μm after 6 months of OK (t=3.520, P<0.01; Table 1; Fig. 2E).

FT Difference before and after OK Application

In the low myopia group, the FT was 217.06±16.63 μm at baseline and increased to 220.88±17.51 μm after 6 months of OK (t=2.435, P<0.05). In the moderate myopia group, the FT was 209.21±19.48 μm at baseline and 210±19.50 μm after 6 months of OK (t=0.185, P=0.856; Table 1; Fig. 2F).

Changes of AL, SFChT, Average Choroidal Thickness, and FT before and after OK Application

The AL growth of the moderate myopia group (−5±124.08 μm) was slower than that of the low myopia group (110.63±91.98 μm), and the change was statistically significant (P<0.05). The increase in SFChT in the moderate myopia group (24.86±22.56 μm) was greater than that in the low myopia group (13.13±15.68 μm), and the change was statistically significant (P<0.05). The average choroidal thickness of the moderate myopia group (18.36±19.51 μm) increased more than that of the low myopia group (7.38±19.55 μm), but the change was not statistically significant (P=0.077). The change in FT between the moderate myopia group (0.79±15.91 μm) and the low myopia group (3.81±6.26 μm) was not statistically significant (P=0.632; Table 1; Fig. 2G).

To further explore the relationship between AL change and choroidal thickness change, the participants were divided into the AL shortening group (change ≤0, 12 eyes) and AL growth group (change >0, 18 eyes) according to the AL change observed after 6 months of OK (see Table, Supplemental Digital Content 1, http://links.lww.com/ICL/A275). The baseline age average in the AL shortening group was 11.25±2.63 years and 7 (58.3%) were girls. The baseline age average in the AL growth group was 10.06±1.06 years and 12 (66.7%) were girls. There was no significant difference in age and sex between the groups at baseline (P=0.158, P=0.643). The SER and Kf of the AL shortening group (−3.58±1.32D; 44.14±1.01D) were higher than those of the AL growth group (−2.19±1.05D; 43.24±0.72D), and the difference was statistically significant (P<0.01; Fig. 3A,B).

FIG. 3.

(A, B) Baseline characters of SER and Kf (independent sample t test; **P<0.01; ***P<0.01). (C) SFChT difference before and after OK application (paired sample t test; *P<0.05; ***P<0.001). (D) Average choroidal thickness difference before and after OK application (paired sample t test; ***P<0.001). (E) Changes in SFChT, average choroidal thickness, and FT before and after OK application (independent sample t test; **P<0.01).

SFChT Difference before and after OK

In the AL shortening group, the baseline SFChT was 262.17±62.92 μm, and SFChT after 6 months of OK increased to 293.42±72.51 μm (t=7.419, P<0.001). In the AL growth group, the baseline SFChT was 256.67±30.28 μm, and SFChT after 6 months of OK increased to 266.83±34.80 μm (t=2.338, P<0.05; Fig. 3C).

Average Choroidal Thickness Difference before and after OK

In the AL shortening group, the average choroidal thickness was 271.25±58.32 μm at baseline and 295.17±65.84 μm after 6 months of OK, and the difference was statistically significant (t=4.775, P<0.001). In the AL growth group, the average choroidal thickness was 263.67±33.55 μm at baseline and 268.56±37.70 μm after 6 months of OK, but the difference was not statistically significant (t=1.136, P=0.272; Fig. 3D).

Changes of SFChT, Average Choroidal Thickness, and FT before and after OK Application

The increase in SFChT in the AL shortening group (31.25±14.59 μm) was greater than that in the AL growth group (10.17±18.45 μm), and the change was statistically significant (t=−3.320, P<0.01). The average choroidal thickness of the AL shortening group (23.92±17.35 μm) increased more than that of the AL growth group (4.89±18.25 μm), and the difference was statistically significant (t=−2.852, P<0.01). The change in FT between the AL shortening group (0.25±15.93 μm) and the AL growth group (3.83±7.91 μm) was not statistically significant (t=0.819, P=0.419; Fig. 3E).

Choroidal Thickness Differences at Different Spots in the Choroid before and after OK Application

For all the enrolled participants in general, the choroidal thickness at each spot increased after 6 months of OK (Table 2). N1 (before/6 months [247.54±50.24/258.57±55.23 μm], SFChT [257.21±46.42/275.36±54.96 μm], and T1 [272.25±41.92/285.25±55.55 μm]) had a statistical significance (t=2.452, 4.708, 2.589; P<0.05, 0.001, 0.05, respectively; Fig. 4A,B).

TABLE 2.

Choroidal Thickness Differences at Different Spots in the Choroid Before and After OK Application

(mean±SD; μm)	N3	N2	N1	SFChT	T1	T2	T3
Before OK	234.18±46.89	241.61±50.03	247.54±50.24	257.21±46.42	272.25±41.92	288.93±56.76	316.93±74.48
6 months after OK	238.93±59.81	250.39±54.89	258.57±55.23	275.36±54.96	285.25±55.55	301.82±64.22	330.07±77.80
P	0.574	0.175	0.021*	0.000***	0.015*	0.069	0.168

Paired sample t test; *P<0.05; ***P<0.001.

OK, orthokeratology; SFChT, subfoveal choroidal thickness.

FIG. 4.

(A, B) Choroidal thickness differences at different spots in the choroid before and after OK application (paired sample t test; *P<0.05; ***P<0.001).

Correlation Analysis of Initial AL, Kf, AL Changes, and Choroidal Thickness Changes

The positive correlation between the initial AL and SFChT changes was not significant (Pearson correlation test, r=0.316, P=0.089). The positive correlation between the initial Kf and SFChT changes was not significant (Pearson correlation test, r=0.358, P=0.052) (Fig. 5A). There was a moderate positive correlation between the initial AL and average choroidal thickness changes (Pearson correlation test, r=0.382, P=0.037). A correlation between the initial Kf and average choroidal thickness changes was not revealed (Pearson correlation test, r=0.220, P=0.243) (Fig. 5B). There was a significant negative correlation between AL changes and SFChT changes before and after 6 months of OK (Pearson correlation test, r=−0.532, P=0.003) (Fig. 5C). There was a significant negative correlation between AL changes and average choroidal thickness changes before and after 6 months of OK (Pearson correlation test, r=−0.508, P=0.004) (Fig. 5D). A correlation between AL and FT changes, and SFChT and FT changes was not revealed (Pearson correlation test, r=0.106, P=0.578; r=−0.134, P=0.481; see Figure, Supplemental Digital Content 1, http://links.lww.com/ICL/A274).

FIG. 5.

(A) Correlation analysis between initial AL, Kf, and SFChT changes (Pearson correlation test, r=0.316, P=0.089; Pearson correlation test, r=0.358, P=0.052). (B) Correlation analysis between initial AL, Kf, and average choroidal thickness changes (Pearson correlation test, r=0.382, P=0.037; Pearson correlation test, r=0.220, P=0.243). (C) Correlation analysis between AL and SFChT changes (Pearson correlation test, r=−0.532, P=0.003). (D) Correlation analysis between AL and average choroidal thickness changes (Pearson correlation test, r=−0.508, P=0.004).

DISCUSSION

This study retrospectively analyzed AL and choroidal thickness variations in adolescent participants with low-to-moderate myopia before and after OK application. The results demonstrated that, in general, choroidal thickness increased after 6 months of OK, and a more significant increase was shown in the participants with moderate myopia. Increases in SFChT and average choroidal thickness can be associated with AL shortening. With the increase in educational burden and the shortened time required for outdoor activities, the global incidence of myopia is on the rise.^14,15 OK has been applied to the control of juvenile myopia for a relatively long time, and its effectiveness has been extensively accepted worldwide. A large number of randomized controlled trials have shown that OK presents a better AL and refractive diopter control effect than low-concentration atropine or monofocal spectacle lenses.^16,17 Choroidal thickness is closely related to the development of myopia. Previous studies have observed that high myopic eyes accompanied by peripapillary choroidal crescents have a significantly thinner choroid.¹⁸ Experimental studies have shown that in the process of myopia development, the continuous growth of the eyeball induces mechanical stretching and thinning of the retina and choroid and causes retinochoroidal vascular disorders.¹⁹

In this study, the variations in AL and choroidal thickness were analyzed before and after 6 months of OK application in adolescents with low-to-moderate myopia. Previous studies are controversial on the AL control effects regarding different degrees of myopia. Some studies speculated that no difference existed in the AL control effects between low and moderate myopia,^20,21 whereas other studies discovered that a better control effect was achieved with greater SER.²² Albeit another study managed to reveal the choroidal thickness variation among different SER after OK, the axial control effects were not shown.²³ In the current study, moderate myopia was found to be superior to low myopia in axial control, possibly because participants with higher baseline SER experienced greater diopter reduction and greater peripheral defocus after OK. It is reported that choroidal thickening after OK can appear in the early stage of shaping, and the choroidal thickness remains relatively stable after 1 year.¹¹ In this study, SFChT and average choroidal thickness increased after 6 months of OK, and the SFChT thickening in the moderate myopia group was greater than that in the low myopia group, and the AL growth in the moderate myopia group was less than that in the low myopia group. In a previous experimental study performed on chicks,²⁴ eyes with thicker choroids were found to grow slower than those with thinner choroids, indicating that choroidal thickness could predict axial elongation. A reasonable hypothesis is that the decrease in choroid thickness could stimulate the release and diffusion of growth factors. The reduction in choroidal blood perfusion may lead to scleral hypoxia and remodeling of scleral extracellular matrix, resulting in excessive elongation of the eyeball, and exacerbate myopia progression.²⁵ It can be speculated that choroidal thickening after OK may have the potential to be one of the predictive indicators of long-term myopia control effect.²⁶ Meanwhile, the FT in the low myopia group presented a significant trend of thickening after 6 months of OK, indicating the interaction between choroid and retina. Former studies did not reveal an absolute correlation between refractive diopter and corneal curvature.²⁷ In the current study, the baseline Kf of the moderate myopia group is higher than that of the low myopia group. A possible explanation could be the VST design, which indicates that myopic children with higher SER and lower Kf might experience less satisfactory corneal shaping and myopia control effects, or other potential risks, such as poor uncorrected visual acuity, after OK. Therefore, children with higher SER and lower Kf tended to choose other intervention methods. Hence, the included participants with higher baseline SER had relatively higher Kf values.²⁸ However, the results of correlation analysis did not show a significant correlation between baseline Kf and choroidal thickness changes, indicating that initial Kf may not be a prominent factor that affects choroidal thickness changes in this study.

Our study attempted to further analyze the relationship between AL and choroidal thickness change and conducted a novel grouping method by dividing the participants into the AL shortening and AL growth group. The underlying mechanisms of AL shortening after OK could be as follows: Previous studies implied that periretinal myopic defocus could effectively delay myopia progression in both human and animal eyes.¹² In addition, it was speculated that myopia control by OK might be related to periretinal myopic defocus.²⁹ The increase in choroidal thickness may result from the molecular signal shift induced in the myopic defocus after OK and affect oxygen supply and metabolic process of retina or choroid, thus altering the biosynthesis of scleral extracellular matrix and delaying the elongation of eyeball.¹¹ The current study showed that after 6 months of OK, choroid thickened at different locations and AL shortening also occurred, and the increase of SFChT and average choroidal thickness in the AL shortening group was greater than that in the AL growth group, indicating that the AL shortening could be related to the increase of choroidal thickness, which was consistent with the correlation analysis. In addition, the inverse geometric design of OK increased tear viscosity at night. The vacuum and sucker effect may affect choroidal homeostasis and lead to choroidal thickening.⁸ Furthermore, the shape and thickness of the central cornea changed during OK. It is reported that a higher corneal curvature could demonstrate a better axial control effect,³⁰ which is in accordance with our results. This study found that the AL shortening group had higher baseline Kf and SER than the AL growth group. The cornea with a greater Kf value has a steeper shape, which could allow more central epithelium to migrate to the periphery after shaping, and the shaping force is stronger. Meanwhile, corneas with higher baseline SER could experience greater diopter reduction, resulting in more extensive myopic defocus. These combined functions may delay AL growth.

The choroid appears thicker on the temporal side, followed by the fovea, and thinner on the nasal side. This difference may be related to the anatomic asymmetry of the ciliary artery and its neurolocalization.²³ In addition, after 6 months of OK, the choroidal thickness at SFChT, T1, and N1 significantly increased compared with baseline data. The results demonstrated that although the retinal defocus induced by OK mainly functioned on the peripheral retina, it can also cause remarkable subfoveal choroid thickening. A previous study reported that in the process of axial elongation and choroid thinning, owing to a higher metabolic demand in the fovea, the more obvious choroid thinning occurred in the peripheral retina.¹⁸ This is consistent with our results. SFChT presented a more significant increase than the peripheral locations, and the differences of choroidal thickness varied in different spots. During choroidal thickness alteration, the intrinsic choroidal neurons and nonvascular smooth muscle cells were hypothesized to contribute to the fovea stabilization.^18,31 Neuroregulation of the choroid is controlled by both sympathetic and parasympathetic nerves. Parasympathetic nerve excitation causes choroidal vasodilation and increases choroidal blood flow, whereas sympathetic nerve excitation causes constriction of innervated vessels and decreases choroidal blood flow.³² Neuroregulation disorders of choroidal blood circulation might further lead to chorioretinopathy and dysfunction. The retinal pigment epithelium can transmit growth regulatory signals from the retina to the choroid and sclera and can regulate scleral growth. Growth regulatory defocus signals could probably be contained in the RPE and might exist in the choroid to some extent. The choroid is the vascular layer of the eyeball, providing oxygen and nutrients to the outer retina while adjusting its thickness mechanically to accommodate the retina to the ocular focal plane in response to the imposed optical defocus.³³

The study further conducted correlation analyses between the initial AL and choroidal thickness changes, and AL and choroidal thickness changes before and after 6 months of OK application. A moderate positive correlation was revealed between the initial AL and average choroidal thickness changes. This finding may be associated with the better AL control effect discovered in the moderate myopia group. Moreover, significant negative correlations were shown between the change in AL and SFChT, and the change in AL and average choroidal thickness. Previous studies have discovered a significant correlation between the change in choroidal thickness and AL. Choroid thinning was usually accompanied by AL growth. Animal experiments further confirmed that the variation in choroidal thickness and AL growth was significantly negatively correlated.³⁴ The increases in choroidal thickness could be generated by the recovery of scleral homeostasis or the improvement of scleral hypoxia.³⁵ Moreover, the choroidal thickness increase may slow axial elongation by inhibiting the diffusion of growth factors or increasing scleral mechanical buffering, and the correlation between choroidal thickness and AL growth persistently exists after defocus.²⁵ However, the axial control effects of OK could not be fully explained by the increment of choroidal thickness. In addition, the current study did not manage to reveal the correlation between FT and AL or SFChT changes. The specific relationship between AL, choroidal, and retinal thickness still need further efforts to discover.

There were several limitations in the current study. This midterm study included a relatively small sample size. A long-term study with larger sample size will be performed to further validate existing results. The choroidal thickness superior or inferior to the macula was not measured in this study so as to avoid eyelid hindrance.⁷ What is more, because our study did not included eyes with high myopia, magnification correction was not applied during OCT measurements.

CONCLUSION

Above all, the choroidal thickness of adolescent participants with low-to-moderate myopia generally increased after 6 months of OK application, and the amount of increase was more significant in participants with moderate myopia. The SFChT increased the most after OK, and the increase in SFChT and average choroidal thickness was associated with AL shortening. Long-term studies with a larger sample size will be conducted to confirm the results of this study and to further investigate the effects of different optical interventions on refractive diopter, AL, and choroidal thickness. The specific mechanism of choroidal thickening, the changes in the sclera in this process, and the implications on axial length control still need to be further investigated.

Supplementary Material

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