The prevalence of myopia in young adolescents has increased substantially over recent decades and is now approaching 10-25% and 60-80% in industrialized societies of Western and Eastern Asia, respectively;1 worldwide, the condition is considered to be the leading cause of visual impairment.2 In clinical terms, it is widely acknowledged that the myopic eye is a vulnerable eye, especially for myopia levels greater than 6.00 D, and one that is especially susceptible to a range of ocular pathologies.3-7 Several treatment therapies, including rigid contact lenses, bifocal and multifocal spectacle lenses as well as pharmaceutical agents, have been used in the past with relatively modest success to eliminate or, at least, reduce myopia progression.8,9 More recently, modern orthokeratology has claimed to be effective in slowing the progression of myopia in children. This technique is an effective treatment for the temporary reduction of up to -6.00D of myopia with the overnight use of reverse-geometry gas-permeable contact lenses.10 Usually, studies evaluating the effect of orthokeratology lens wear on myopia progression measure changes in the eye's axial length, the principal structural correlate of refractive error,11 due to the concomitant temporary reduction in myopia that occurs as a consequence of the corneal flattening induced by orthokeratology contact lens wear.12

Although a retrospective study13 and a case report14 on the subject were previously published, only two prospective studies have assessed the effect of orthokeratology contact lens wear on myopia progression in children.15,16

Over a two-year period, Cho et al.15 monitored the increase in axial length in 35 Hong-Kong Chinese children aged 7 to 12 who were fitted with orthokeratology lenses, and compared the rate of change of axial length with that observed in a historical control group made up of 35 children wearing single-vision spectacles. Both groups were matched for age, gender and baseline spherical equivalent refractive error. At the end of the 24 months, the increase in axial length was 0.29±0.27 mm and 0.54±0.27 mm for the orthokeratology lenses and single-vision spectacle groups, respectively. However, the study failed to recruit a prospective control group. Furthermore, the baseline level and progression of myopia observed among Chinese children are reported to be significantly greater than among white European children.8 In addition, differences in contact-lens-induced responses in the corneas of Asian and non-Asian subjects have also been previously observed.17

More recently, a study undertaken in the USA by Walline and co-workers16 compared the growth of the eye observed among myopic children wearing orthokeratology contact lenses with that observed in a historical control group of children wearing soft contact lenses. The groups consisted of children aged 8 to 11 with myopia ranging from 0.75 to 4.00 D and having less than 1.00 D of astigmatism. Over the two-year period, the axial length for the soft-contact-lens group increased, on average, 0.32 mm more than for the orthokeratology-lens group. However, the Walline et al.16 study was unable to recruit a prospective control group. Since Cho et al.15 and Walline et al.16 employed historical prospective control groups, subjects were not randomized into one modality of visual correction vs. another. Additionally, these two previous studies measured axial-length growth using A-scan ultrasonography.15,16 An alternative measuring method (the Zeiss IOLMaster) uses partial coherence interferometry to carry out non-contact measures of axial length with a dioptric resolution of 0.03 D (an order of magnitude better than the 10 Hz ultrasound technique).18

As with any treatment regimen, both efficacy and safety need to be assessed. Although case reports and case series of observations on undefined populations of participants wearing overnight orthokeratology contact lenses have been presented, there are no formal prospective reports on the incidence of adverse events associated with overnight orthokeratology contact lenses specifically used to treat myopia.19

This report introduces the study designated as MCOS (Myopia Control with Orthokeratology contact lenses in Spain) and outlines its design, methodology and baseline findings. The primary outcome measure of MCOS is to compare differences in axial length growth between white European myopic children wearing orthokeratology contact lenses (OK) and distance single-vision spectacles (SV) over a 2 year period. The secondary outcome is to record differences in the incidence of adverse events and discontinuations between the two study groups. To the authors’ knowledge, the MCOS study is the first prospective clinical trial to assess the safety and efficacy of overnight orthokeratology contact lens wear.

Sixty-nine subjects were recruited for the study between March 2007 and March 2008. Thirty-one children were prospectively allocated to OK and 30 to the SV correction modalities (Figure 2). Eight subjects could not be enrolled in the study because they failed to meet the inclusion criterion for refraction (Figure 2). A normal frequency distribution was found for all baseline demographics, refractive and biometric data in both groups (P>0.05), except for visual acuity and the cylindrical refractive component (P<0.01). Thus, parametric and non-parametric statistics were employed accordingly to assess differences between groups at baseline. Both study groups were well matched at baseline as no significant differences were found between the two groups neither in demographics, nor in refractive nor in biometric data (Table 4).

Figure 2.

Subjects recruited for the study.

(0.09MB).

Of the 31 subjects that were assigned orthokeratology contact lens wear at baseline, an optimum lens fit was obtained in 21 subjects with the first contact lens fitted in accordance with the Menicon Professional Easy Fit software. Ten subjects required a total of 35 adjustments to attain an optimum lens fit. Of these, 5 subjects required more than 2 contact lens fit changes per eye; 5 subjects required just 1 change each; 2 subjects required 1 change per eye each; 2 subjects required 2 changes per eye each; and 3 subjects required 3 changes each. The reasons for the changes were: undercorrection (13), lens decentration (12) and central island (10). Across the whole group, an average of 1.6 lenses per eye were required to attain an optimum fit. Two subjects broke their lenses (2) and one subject lost one lens in the interim between initial contact lens fitting and final enrolment in the study (Figure 1).

None of the subjects showed corneal staining and no adverse events were found in any of the two groups at baseline.

To the authors’ knowledge, the MCOS study is the first prospective clinical trial to assess the efficacy and safety of OK lens wear for myopia progression vs. a group of SV spectacle lens wearers. Subjects and parents engaged enthusiastically in the study and responded well to initial introduction of the study design and protocol. The number of contact lenses required to achieve an optimum fit were either lower than31 or similar to32 those reported in previous studies.

For the present study (MCOS), the subjects’ baseline refractive and biometric data were markedly similar to those from other studies assessing the effects of orthokeratology contact lens wear on myopia progression in children.15,16 Also, similar age groups and male/female ratios were employed in MCOS compared to a previous study;15 another study employed an older group and a higher percentage of female subjects.16

Although we are fully aware of the advantages of random allocation, to our knowledge no previous study on the effects of orthokeratology contact lens wear on myopia progression has used a randomized design. Subjects were not randomized into the two modalities of visual correction in MCOS, but both study groups were well matched, as shown by the similarity between the baseline demographics, refractive and biometric data collected in the two groups. Also, all subjects from both study groups in MCOS were monitored over the same time period. Previous studies have used historical prospective control groups that had not been monitored over the same time period; the latter feature might introduce a higher bias than that due to MCOS's design (e.g. it is possible that children from the historical control groups were monitored during times of different environmental exposure, such as greater levels of close work, compared to the experimental groups). Furthermore, the advantage of MCOS's approach is that it is apposite to actual clinical practice, where practitioners provide various options of visual correction and parents opt for a particular option with the child's approval.

A limitation of this study is that, in terms of statistical power, the sample size employed is theoretically too small to detect the absolute incidence rates of adverse events and discontinuations for each of the two modalities of visual correction under investigation. However, we envisage that it might be sufficiently powered to detect differences in incidence rate between the two groups, as previous studies with samples sizes similar to those used in the present study have effectively demonstrated differences in incidence rate for different contact lens types and wearing regimes.27,29

The primary outcome measure of this study is, however, to compare in white European myopic children axial length growth following OK and SV lens wear over a 2-year period. In this respect, the comparison of axial growth between the two groups is optimized by the use of non-contact partial coherence interferometry, which has a resolution that is an order of magnitude better than that of the 10 Hz ultrasound technique.18

Since the start of the MCOS, the importance of peripheral imagery in the etiology of myopia has been acknowledged both in animal and in human studies.33-35 Consequently, as an adjunct to the present study, peripheral axial length measures in the horizontal plane (using partial coherence interferometry) will be recorded at the 24-month follow-up visit for both study groups, at successive eccentricities at successive eccentricities from from 10° to 30° temporally and nasally.36

Another constraint of the MCOS study is that the investigator collecting clinical data (CV-C) was not masked with respect to the mode of visual correction. Full masking of data collection in clinical trials such as MCOS presents difficulties, in that the identity of the subject's group can be revealed by a variety of clinical observations such as, for example, limbal or conjunctival staining or corneal topography measurements. Nevertheless, the investigator collecting data was fully aware of the need to disregard where feasible the identity of the subject's group. Furthermore, data analysis was undertaken by an investigator (JS-R) who was masked with regard to the identity of the study groups.

Although some limitations in the MCOS study have been identified, the study offers a number of notable features: a prospective design; well-matched samples and high-resolution ocular biometry measures, which collectively should elucidate whether or not OK contact lens wear is a feasible and safe method for myopia progression control.