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Review Article

Myopia Control With Orthokeratology: A Review

Hiraoka, Takahiro M.D.

Author Information
Eye & Contact Lens: Science & Clinical Practice: March 2022 - Volume 48 - Issue 3 - p 100-104
doi: 10.1097/ICL.0000000000000867
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Abstract

Overnight orthokeratology (OK) uses specially designed and fitted contact lenses (CLs) during sleep to reshape the cornea and correct refractive errors, particularly in myopic patients. Uncorrected visual acuity improvement accompanied with temporary refractive error reduction is obtained by the flattening of the central corneal curvature using a specially designed rigid CL, termed as reverse-geometry lenses. These lenses can reduce patient dependence on daytime use of CLs or spectacles, providing acceptable uncorrected vision throughout the following day. Furthermore, the efficacy and safety of modern OK have been demonstrated in previous studies.1–3

Since the early 2000s, practitioners have discussed the utility of OK for myopia control. As a result, a number of studies showing the effectivity of OK in delaying myopia progression in children have been published.4–12 In addition, OK has recently been considered one of the most effective optical treatments for myopia control, showing efficacy similar to that of atropine (AT).13,14 In this article, we reviewed the literature on the efficacy of OK for myopia control.

POSSIBLE MECHANISM

It is generally considered that OK slows myopic progression based on the “peripheral refraction theory,”15,16 which states that peripheral hyperopic retinal blur is an important trigger for axial elongation and myopia progression.17–19 In fact, myopes usually have a more prolate (longer axially than equatorially) shape than emmetropes and hyperopes, and thus show greater relative peripheral hyperopia with respect to axial refraction, which has been proposed to accelerate axial elongation.17–19 However, OK induces an oblate shape cornea (flattened central cornea and steepened midperipheral cornea), leading to a circular area of increased peripheral myopia on the corneal surface. This reduces peripheral hyperopic defocus as a whole, thereby decreasing the visual feedback for eye elongation.

Recently, several researchers have suggested that higher-order aberrations (HOAs) play a role in slowing myopia progression.20–25 Particularly, Hiraoka et al.20 and Kim et al.21 have demonstrated an association between the increase in coma and third-order HOAs with axial elongation in OK-treated children, hypothesizing that elevated aberrations may provide a visual signal that slows eye growth. Moreover, Hiraoka et al.22 confirmed a similar relationship between HOAs and myopia progression even in myopic children wearing single-vision spectacles without special treatments, including OK, AT, or multifocal CLs. Lau et al.23 found a strong association between positive spherical aberration and slower axial elongation in OK-treated children, as well as confirmed a similar relationship in spectacle-wearing children with no special treatments.24 Furthermore, Vincent et al.25 observed a significant association between axial elongation and several HOA metrics in children undergoing combination therapy with OK and 0.01% AT. Given these findings, HOAs have been shown to have a substantial role in myopia progression in children; however, how these HOA components act in myopia onset and progression remains unknown. Further studies are needed to clarify this exact mechanism.

CASE REPORT

In 2004, Cheung et al.26 reported a case of an 11-year-old boy who received unilateral (left eye only) OK treatment. This case was significant because it was the first time that axial length elongation was slowed down by OK treatment. During the 2-year observation, minimal axial elongation in the left eye (0.13 mm) and a considerable axial elongation (0.34 mm) in the right eye, with corresponding myopic refractive error increase (0.75 D), were observed after OK treatment.

PILOT STUDY

The first pilot study was published in 2005 by Cho et al.27 Axial elongation in 35 children (7–12 years of age) undergoing OK treatment was monitored and compared with that of an earlier control group of 35 children wearing single-vision spectacles from a previous study. On assessment during the 2-year period, axial elongation was 0.29±0.27 mm and 0.54±0.27 mm in the OK and control groups, respectively, indicating that axial elongation was 46% slower in the OK-treated children.

Similarly, Walline et al.6 reported a significant change in axial length for 2 years in 28 children (8–11 years of age) treated with OK, as compared with that of an age-matched historical control group wearing monofocal soft CLs. In this study, the axial elongation was 0.25±0.22 mm and 0.57±0.51 mm in the OK and control groups, respectively, corresponding to a 55% decrease in axial elongation among children in the OK group (Table 1).

TABLE 1. - Summary of Representative Studies
Reference Country Design Period Randomization Group Age Number of Patients Myopia (D) Astigmatism (D) Axial Elongation (mm) Inhibition Ratio (%)
Cho et al.27 Hong Kong Pilot 2 years None OK/SV 7–12 35/35 −0.25 to −4.50 <2.00 0.29/0.54 46
Walline et al.6 US Pilot 2 years None OK/SCL 8–11 28/− −0.75 to −4.00 <1.00 0.25/0.57 55
Kakita et al.28 Japan Prospective 2 years None OK/SV 8–16 42/50 −0.50 to −10.00 ≤1.50 0.39/0.61 36
Santodomingo-Rubido et al.29 Spain Prospective 2 years None OK/SV 6–12 29/24 −0.75 to −4.00 ≤1.00 0.47/0.69 32
Cho et al.4 Hong Kong Prospective 2 years Yes OK/SV 6–10 37/41 −0.50 to −4.00 ≤1.25 0.36/0.63 43
Charm et al.30 Hong Kong Prospective 2 years Yes OK/SV 8–11 12/16 −5.00 to −8.00 ≤2.00 0.19/0.51 63
Chen et al.31 Hong Kong Prospective 2 years None OK/SV 6–12 35/23 −0.50 to −5.00 1.25–3.50 0.31/0.64 52
Hiraoka et al.5 Japan Prospective 5 years None OK/SV 8–12 22/21 −0.50 to −5.00 ≤1.50 0.99/1.41 30
Santodomingo-Rubido et al.33 Spain Prospective 7 years None OK/SV 6–12 14/16 −0.75 to −4.00 ≤1.00 0.91/1.35 33
Hiraoka et al.34 Japan Prospective 10 years None OK/SCL 8–16 53/39 −0.50 to −7.00 ≤1.25 −/−
Kinoshita et al.36 Japan Prospective 1 year Yes OK+AT/OK 8–12 20/20 −1.00 to −6.00 ≤1.50 0.09/0.19 53
Kinoshita et al.37 Japan Prospective 2 year Yes OK+AT/OK 8–12 38/35 −1.00 to −6.00 ≤1.50 0.29/0.40 28
Tan et al.38 Hong Kong Prospective 1 year Yes OK+AT/OK 6–11 29/30 −1.00 to −4.00 <2.50 0.07/0.16 56
AT, 0.01% atropine solution; OK, orthokeratology; SCL, soft contact lens; SV, single-vision spectacles.

NONRANDOMIZED CLINICAL STUDY

In the recent decades, studies have been performed using an appropriate control group with matched baseline characteristics. Kakita et al.28 conducted a 2-year, non-randomized, prospective study that compared the axial elongation between the OK-treated and single-vision spectacles-wearing groups (42 vs. 50 children), finding that axial elongation was 36% slower in the OK-treated children (0.39±0.27 and 0.61±0.24 mm, respectively).

A similar study was performed by Santodomingo-Rubido et al.29 in Spain, showing that axial elongation over a 2-year period was 32% slower in the OK-treated children, as compared with children wearing spectacles (0.47 and 0.69 mm, respectively).

These nonrandomized studies confirmed that OK significantly reduced axial elongation in comparison with single-vision spectacles in children (Table 1).

RANDOMIZED CONTROLLED TRIAL

The first randomized clinical trial, which was called the retardation of myopia in orthokeratology (ROMIO) study, was conducted in Hong Kong by Cho et al.4 In this study, participants (6–10 years of age) were randomly assigned to wear either OK lenses or single-vision spectacles for a period of two years. Seventy-eight patients (37 in the OK group and 41 in the control group) completed the study, and a significant difference in the mean axial elongation was found between the groups (0.36±0.24 and 0.63±0.26 mm, respectively), indicating that axial elongation was 43% slower in the OK group (Table 1).

APPLICATION TO HIGHER MYOPIA AND ASTIGMATISM

Myopia control in high myopic children was attempted by Charm et al.30 In their study, they used a combination of partial reduction by OK and spectacles for residual refractive errors rather than full correction by OK lens alone, because the correction of high myopic refractive errors by OK often results in severe corneal staining, heavy lens binding, and lens decentration. Specifically, a 4.00 D myopic reduction was attempted using OK, and a pair of single-vision spectacles was prescribed for residual refractive error correction for daytime wear. A total of 28 participants (12 in the partial reduction OK group and 16 in the control group) with subjective refraction of 5.50-8.25 D completed the 2-year study, showing a mean axial elongation of 0.19±0.21 and 0.51±0.32 mm, respectively. On comparing the two groups, axial elongation was 63% slower in the partial reduction OK group, showing the highest retardation rate of axial elongation so far.

To examine the efficacy of toric design OK lenses for myopia control, Chen et al.31 also conducted the TO-SEE (myopia control using toric orthokeratology) study in myopic children with moderate-to-high astigmatism. A total of 35 OK and 23 control subjects (6–12 years) with astigmatism of 1.25 to 3.50 D completed the study, with a 2-year mean axial elongation of 0.31±0.27 and 0.64±0.31 mm, respectively. Based on these results, axial elongation was 52% slower in children treated with toric OK than in children who wore spectacles. This indicated that toric design OK lenses can also delay myopic progression in myopic children with moderate-to-high astigmatism (Table 1).

TWIN STUDY

The inhibitory effect of OK on myopia progression was also investigated in a pair of identical twins. Chan et al.32 compared the 2-year axial elongation between Twin A, who was treated with OK, and Twin B, who wore single-vision spectacles. Results showed that the axial elongations from baseline were 11% and 48% in the right and left eyes, respectively, in Twin A, whereas the corresponding values were 87% and 67%, respectively, in Twin B. OK was more effective in controlling axial elongation than single-vision spectacles, in identical twins.

Meta-analysis

From 2015 to 2016, four meta-analyses on OK were published. Si et al.8 conducted a meta-analysis of seven studies (two randomized controlled trials and five nonrandomized controlled trials) involving 435 subjects (218 patients in the OK group and 217 patients in the control group) aged 6 to 16 years. They found a −0.26 mm (95% confidence interval [CI]: −0.31 to −0.21) weighted mean difference for axial elongation between the OK and control groups in a 2-year period follow-up. It was concluded that OK may slow myopia progression in children, but further large-scale studies are needed to substantiate these results.

Wen et al.9 also performed a systematic review and meta-analysis of eight studies, involving a total of 769 subjects. They reported a significant difference in axial length change between the OK and control groups, indicating a weighted mean difference of −0.25 mm (95% CI: −0.30 to −0.21) at a 2-year follow-up. It was concluded that OK with careful education and monitoring was effective and acceptable for slowing myopic progression in children.

Sun et al.10 in another meta-analysis evaluated the effectiveness of OK and assessed its effects in slowing myopia progression with seven eligible studies. Their results found a 0.27 mm (95% CI: 0.22–0.32) decrease in the 2-year axial elongation of the OK group, as compared with that of the control group, indicating an approximately 45% reduction in myopia progression. Consistent with the other meta-analyses, this study concluded that OK can slow myopia progression in school-aged children.

In another meta-analysis, Li et al.11 used data from nine studies (three randomized controlled trials and six cohort studies) with 667 children aged 6−16 years and showed that the 2-year mean axial elongation differences in the randomized clinical trials (−0.28 mm; 95% CI: −0.35 to −0.20) was like those in the cohort studies (−0.27 mm; 95% CI: −0.32 to −0.22) and that the effect was greater in children with moderate-to-high myopia in comparison with children with low myopia (−0.35 vs. −0.25 mm). Given these results, they concluded that OK had significantly greater efficacy in controlling axial elongation in children than spectacle correction and showed a greater myopia control effect in children with higher baseline myopia.

Long-Term Study

Hiraoka et al.5 performed a long-term study demonstrating the 5-year data of 43 Japanese subjects (22 in the OK group and 21 in the single-vision spectacles group). In their study, axial elongation during the 5-year study period was 0.99±0.47 mm and 1.41±0.68 mm for the OK and control groups, respectively, indicating a 30% decrease in axial elongation in the OK group. Furthermore, patients in the OK group showed a consistently lesser annual axial elongation relative to the spectacle control group over three years; however, no statistically significant benefit was observed in the fourth and fifth year.

Another long-term study was conducted by Santodomingo-Rubido et al.,33 which reported the 7-year follow-up data of 30 White subjects (14 in the OK group and 16 in the single-vision spectacles group). After 7 years of lens wear, axial elongation in the OK group was 0.44 mm lower than the spectacles control group, corresponding to a reduction rate of 33%.

Recently, Hiraoka et al.34 reported the results of a retrospective 10-year study in a total of 92 patients (53 in the OK group and 39 in the soft CL group) with baseline ages from 8 to 16 years. In their study, they compared the rates of myopia progression and adverse events between the OK and soft CL groups over a 10-year period, finding that myopia progression was significantly slower in the OK group than in the soft CL group at all baseline ages except at 16 years. In addition, no significant differences were observed in the number of adverse events between the two groups. These findings support the long-term efficacy and safety of OK lenses in reducing myopia progression among school children (Table 1).

INFLUENCE ON RAPID PROGRESSION

Notably, Cho et al.35 reanalyzed the combined data from the ROMIO and TO-SEE studies to investigate the influence of OK on rapid myopia progression, defined as an axial elongation of more than 0.36 mm/year. On assessment in this study, the relative risk (RR) of rapid progression did not reach statistical significance in the older age group (9–12 years) (RR: 0.35; 95% CI: 0.07–1.72), whereas statistical significance was reached in the younger age group (6–8 years) (RR: 0.11; 95% CI: 0.03–0.44). Based on these findings, OK could considerably prevent younger patients from having rapid progression over the 2-year period of treatment, and it is suggested that OK treatment should be started in younger myopic children (6–8 years).

STUDIES ON COMBINATION TREATMENTS

To assess the additive effect of OK with 0.01% AT, Kinoshita et al.36 randomly allocated children aged 8 to 12 years to the combination of OK and 0.01% AT group or OK monotherapy group. On assessment, a total of 40 subjects (20 vs. 20 in each group) completed the 1-year follow-up, with an axial elongation of 0.09±0.12 mm in the combination group and 0.19±0.15 mm in the monotherapy group, showing a significant difference between the two groups. The same researchers also reported the 2-year results,37 wherein 38 subjects in the combination group and 35 subjects in the monotherapy group completed the whole examination. Here, axial elongations were 0.29±0.20 mm and 0.40±0.23 mm in the combination and monotherapy groups, respectively, showing a significant difference between the groups. Interestingly, in the subgroup analysis, the additive effect of the combination therapy for slowing axial elongation was found to be greater in children with low initial myopia (−1.00 to −3.00 D) than those with moderate myopia (−3.01 to −6.00 D).

Similarly, Tan et al.38 conducted a randomized controlled trial to examine the additive effect of OK with 0.01% AT. In this study, a total of 59 patients (29 subjects in the combination of OK and 0.01% AT group and 30 subjects in the OK monotherapy group) completed the 1-year visit. Axial elongation during the 1-year follow-up was significantly slower in the combination of OK and 0.01% AT group than in the OK monotherapy group (0.07±0.16 mm vs. 0.16±0.15 mm, respectively; P=0.03), which was comparable with the 1-year results observed by Kinoshita et al.36 The study concluded that there was an additive effect between OK and 0.01% AT in slowing axial elongation over one year (Table 1).

ADVANTAGES AND DISADVANTAGES

There are various advantages in OK. In addition to the promising efficacy in controlling axial elongation as explained above, overnight wearing modality can be convenient for patients who are unwilling to wear spectacles or CLs during the day, or want to participate actively especially in sports, by providing acceptable vision during waking hours without the need for optical correction. In addition, parents can have more control and supervision over the insertion and removal of OK lenses at home for their children, making it possible to start the OK earlier than other daily CLs. However, OK use can be associated with several disadvantages. Corneal irregular astigmatism and HOAs increase following OK even in clinically successful cases, depending on the magnitude of myopic correction.39 Similarly, reduction in contrast sensitivity associated with HOAs was also reported in eyes undergoing OK.40 A systematic review by Liu and Xie7 on the safety of OK reported corneal staining as the most common ocular finding, followed by lens binding. Other clinically insignificant complications including epithelial iron deposit, fibrillary lines, and transient changes in corneal biomechanical properties were also reported.7 In addition, as in all forms of CLs, overnight wearing brings with it the increased risk of infectious keratitis. A large-scale multicentered retrospective study reported the estimated incidence rate of infectious keratitis as 7.7 per 10,000 OK lens wearers every year, which was similar to other overnight wear CLs.41 Another systematic review including 173 eyes with OK lens-related keratitis by Kam et al.42 showed that the culture positivity rate was 69.4%, with Pseudomonas aeruginosa and Acanthamoeba species being the most common causative agents. The study also demonstrated that most of the patients with positive microbiology resulted in the formation of corneal scars and approximately 10% of eyes required surgical procedure despite early intervention and treatment. Thus, practitioners must provide proper lens prescription and education for patients and avoid excessive corrections by OK. In addition, patients should strictly follow lens care regimen and regular follow-ups to prevent severe complications and possible vision loss.

CONCLUSION AND FUTURE ISSUES

OK is an effective and promising treatment option to control myopia in children, although it cannot halt myopia progression completely. Generally, the 2-year inhibitory effect on axial elongation based on the aforementioned studies ranged from 32% to 63%, which were relative to single-vision spectacles and CLs. In addition, Williams et al.43 stated that OK should be presented to all young patients with progressive myopia as the safest and most effective strategy to reduce myopia progression; however, it should also be noted that this procedure is not perfect. Furthermore, the possibility of a rebound phenomenon in myopia progression after OK discontinuation and the determination of the treatment duration for maximum effect remain unclear. Thus, further research with longer follow-up periods in more diverse populations is needed.

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Keywords:

Orthokeratology; Myopia control; Axial length elongation; Myopia progression; Combination therapy

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