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New Perspective on Myopia Control with Orthokeratology

Kang, Pauline; Swarbrick, Helen

Optometry and Vision Science: May 2016 - Volume 93 - Issue 5 - p 497–503
doi: 10.1097/OPX.0000000000000826
Original Articles

Purpose To compare peripheral refraction along both the horizontal and vertical retinal meridians before and after orthokeratology (OK) lens wear.

Methods Nineteen young adult myopic subjects (mean age, 28 ± 7 years) were fitted with OK lenses in both eyes. Central and peripheral refraction and corneal topography measurements were taken before and after 14 nights of OK. All measurements were taken with no correction or OK lens in place.

Results At baseline before OK, peripheral spherical equivalent refraction (M) across the horizontal meridian did not vary significantly from center. M across the vertical meridian was more myopic than the center (p < 0.05). After OK, there was a significant hyperopic shift in M (p < 0.001); both meridians now experienced myopic peripheral refraction. At baseline, J180 across the horizontal meridian was more negative than the center, and along the vertical meridian, it was more positive than the center (all p < 0.05). At baseline, J45 was more positive than center with increased eccentricity in the temporal and inferior retina and more negative than center with increased eccentricity in the nasal and superior retina. Orthokeratology caused greater rate of change of peripheral J180 across both retinal meridians (p < 0.001). Furthermore, compared with baseline, J45 became more positive in the nasal and superior retina and more negative in the temporal and inferior retina (all p < 0.05).

Conclusions Orthokeratology lenses induced significant changes in peripheral refraction along the horizontal and vertical meridians. As peripheral myopia was measured at baseline along the vertical meridian, the results of our study suggest that inducing greater degrees of myopic defocus on to the peripheral retina, more than habitually experienced, may be required for effective myopia control. Further investigation into the critical threshold of retinal area receiving myopic defocus and the impact of duration of exposure is necessary to improve the efficacy of current myopia control treatments.

*PhD, FAAO

School of Optometry and Vision Science, University of New South Wales, Sydney, New South Wales, Australia (both authors).

Pauline Kang School of Optometry and Vision Science University of New South Wales Sydney, New South Wales 2052 Australia e-mail: p.kang@unsw.edu.au

Orthokeratology (OK) is a specialized clinical technique involving overnight wear of rigid contact lenses for the correction of mild to moderate degrees of ametropia, most commonly myopia, through corneal reshaping. With improvements in technology and lens design, myopic OK has become a successful and predictable form of refractive error correction.1 Orthokeratology lenses are increasingly used in clinical practice for myopia control following numerous studies that have demonstrated reduced myopia progression with OK than with conventional single-vision spectacles or contact lenses.2–5 Corneal topography changes induced by myopic OK alter defocus on the peripheral retina; myopes, who typically experience peripheral hyperopic defocus along the horizontal meridian with conventional corrections,6–8 experience peripheral myopic defocus after OK.9–12 Following animal studies that have demonstrated the strong inhibitory effect of peripheral myopic defocus on axial length elongation or myopia development,13–15 it has been hypothesized that inducing myopic retinal defocus may slow down or stop the progression of myopia in children.16 Thus, changes in peripheral blur are hypothesized to be responsible, at least in some part, for the reduced myopia progression reported with OK.

Recent studies have explored peripheral refraction along the vertical meridian, and in contrast to the horizontal meridian, relative peripheral myopia has been measured along the vertical meridian in myopes.17–21 Differences in the direction of defocus along the horizontal and vertical meridians raise an interesting question about the current theory linking peripheral defocus and myopia development and control; if myopic defocus is already present along the vertical retinal meridian, how much more myopic defocus is required to induce a myopia control effect? The aim of this study was to measure peripheral refraction along both the horizontal and vertical meridians before and after OK lens wear, and to gain a better understanding of how peripheral defocus changes may play a role in myopia control reported with OK.

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MATERIALS AND METHODS

Study Design

Subjects were fitted with BE OK lenses (Capricornia Contact Lens, Australia) in both eyes according to manufacturer guidelines, and lenses were worn overnight for 2 weeks with no lens wear during the day. BE lenses have a total lens diameter of 11 mm and a 6-mm optic zone diameter. Lenses were fabricated from Boston XO2 material (Dk ISO/Fatt 141). Study visits were scheduled at baseline before lens wear and after 14 days of OK treatment. Measurements were taken from right eyes only.

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Subjects

Nineteen young adult myopic subjects were enrolled (6 men, 13 women; mean age, 28.26 ± 7.02 years). Central refraction was between −1.00 and −4.00D with ≤−1.50D of astigmatism. All subjects were screened before enrollment; inclusion criteria required subjects to have good ocular and general health and no previous rigid gas-permeable lens wear. Soft contact lens wearers were instructed to cease lens wear at least 24 hours before study commencement.

This study followed the tenets of the Declaration of Helsinki, and approval from the institutional Human Research Ethics Committee was obtained before study commencement. All subjects gave their written consent to participate in the study after being informed about the nature and possible consequences of study participation.

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Measurement Techniques

Objective Refraction

Noncycloplegic central and peripheral refraction measurements were taken with the Shin-Nippon NVision-K 5001 auto-refractor (Tokyo, Japan) at 10-degree intervals up to ±30 degrees along the horizontal and vertical meridians and also at ±35 degrees along the horizontal meridian. A device that projected a green monochromatic laser spot against a blank white wall 4.3 m away was used to present eccentric fixation targets for peripheral refraction measurements. The fixation device allowed rotation of the laser target in 5-degree intervals along both the horizontal and vertical meridians. A mirror system was used to view targets presented at ±10, ±20, and ±30 degrees on the superior and inferior retina. Subjects were instructed to turn their eyes to fixate on the eccentric targets.

Repeatability of the Shin-Nippon auto-refractor has been found to be high for both central, and to a lesser extent, peripheral cycloplegic M refraction measurements and after OK treatment.12,22 The auto-refractor has been used in a number of studies to characterize defocus changes after OK.9–11,23,24 The average of five refraction measurements at each location, converted into power vectors M, J180, and J45 using the equations derived by Thibos et al.,25 are reported.

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Corneal Topography

The Medmont E300 videokeratoscope (Medmont Pty. Ltd., Melbourne, Australia) was used to profile corneal topography, and data were analyzed using Medmont Studio 5. Apical radius of curvature (ro) and steep and flat K (in diopters) data were extracted from four maps and averaged for each eye at each study visit. The Medmont corneal topographer has been found to have very high repeatability and reproducibility when measuring both test surfaces26 and corneas.27

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Data Analysis

Normality of data was assessed using the Shapiro-Wilk test, and based on the results, paired t-tests or Wilcoxon tests were carried out on central refraction and corneal topography data to detect changes after lens wear (IBM SPSS v.22, Chicago, IL).

Linear mixed-model analysis was used to analyze refraction across the horizontal and vertical meridians and changes in peripheral refraction after OK lens wear. If significant, repeated-measures analysis of variance and planned contrasts or post hoc t-tests with Bonferroni correction were used to compare refraction values along the horizontal and vertical retinal meridians and to assess differences in refraction at each retinal position between study visits. A critical p - value of 0.05 was used to denote statistical significance.

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RESULTS

Table 1 shows central refraction and corneal topography parameters at baseline and after OK lens. There was a significant hyperopic shift in M after OK lens wear and no change in astigmatism components (Table 1). There was significant flattening of the cornea as shown by an increase in ro and decrease in flat K and steep K values (Table 1).

TABLE 1

TABLE 1

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Peripheral Refraction

Horizontal M

At baseline, M values did not vary significantly across the horizontal meridian (F = 0.84, p = 0.43) (Fig. 1A). There was a significant change in peripheral M profile after OK lens wear (F = 63.04, p < 0.001). Specifically, there was a hyperopic shift in M at all positions across the horizontal meridian (all p < 0.05) except at 30 and 35 degrees on the temporal retina (both p < 0.01) where there was a myopic shift and no change in M at 35 degrees on the nasal retina. After OK, M was relatively more myopic than the center at all positions (p < 0.05) except at 10 and 20 degrees on the nasal retina (Fig. 1B).

FIGURE 1

FIGURE 1

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Horizontal J180

At baseline, J180 values were more negative than the center at all positions along the horizontal meridian (all p < 0.05) (Fig. 2A). After OK lens wear, there was a significant change in J180 profile (F = 366.25, p < 0.001), with a significant negative shift in J180 at all positions (all p < 0.05) except at center and 10 degrees on the nasal retina (Fig. 2A). Peripheral J180 values were relatively more negative at all positions compared with center after OK lens wear (all p < 0.05).

FIGURE 2

FIGURE 2

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Horizontal J45

At baseline, J45 was significantly more positive at all positions on the temporal retina (all p < 0.05) and more negative at 30 and 35 degrees (both p < 0.001) on the nasal retina than at center (Fig. 2B). Orthokeratology lens wear caused significant changes in J45 peripheral refraction profile; there was a positive shift in J45 values at 20, 30, and 35 degrees on the nasal retina and a negative shift at all positions on the temporal retina (all p < 0.05). After OK lens wear, J45 was more negative compared with center at 10 degrees (p = 0.001) and 20 degrees (p = 0.038) and more positive at 35 degrees (p = 0.019) on the temporal retina.

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Vertical M

At baseline, M was more myopic than the center at 30 degrees on the inferior retina and at all positions on the superior retina (all p < 0.05). Orthokeratology lens wear caused significant changes in peripheral M profile along the vertical meridian (F = 171.23, p < 0.001); there were significant hyperopic M shifts at all positions except at 30 degrees on the inferior retina (all p < 0.05) (Fig. 3A). The M was relatively more myopic than the center at all positions (all p < 0.05) except at 10 degrees on the superior retina after OK lens wear where there was no change (Fig. 3B).

FIGURE 3

FIGURE 3

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Vertical J180

At baseline, J180 was significantly more positive than the center at all positions along the vertical meridian (all p < 0.05) (Fig. 4A). Similar to changes along the horizontal meridian, there was a significant change in J180 peripheral refraction profile along the vertical meridian after OK lens wear (F = 46.99, p < 0.001). Positive shifts in J180 at 20 and 30 degrees on the inferior retina (both p < 0.001) were measured. J180 was more positive than the center at all positions along the vertical meridian after OK treatment (all p < 0.05) (Fig. 4A).

FIGURE 4

FIGURE 4

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Vertical J45

At baseline, J45 was significantly more positive than the center at all positions on the superior retina (all p < 0.05) and more negative than the center at all positions on the inferior retina (all p < 0.05) (Fig. 4B). Orthokeratology changed the vertical J45 profile (F = 4.67, p = 0.03). There was a positive shift at 30 degrees on the superior retina (p = 0.01) and a negative shift at 20 degrees (p = 0.01) and 30 degrees (p = 0.001) on the inferior retina. Compared with center, after OK, J45 became more positive on the superior retina and more negative on the inferior retina (all p < 0.05) (Fig. 4B).

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DISCUSSION

This study explored peripheral refraction changes across the horizontal and vertical retinal meridians before and after OK treatment because it has been hypothesized that peripheral optical changes induced by lens wear may be responsible for the myopia control effects reported with OK.3,5,23,28 Orthokeratology lenses caused significant hyperopic M shifts on the central retina consistent with changes in corneal topography and induced peripheral myopic defocus along both the horizontal and vertical retinal meridians.

There are only a handful of studies that have explored peripheral refraction along the vertical meridian,17–21 and in agreement with previous studies, relative to the center, we measured myopic defocus on the superior and inferior retina in this myopic study population. Compared with the single-vision control group in the study reported by Berntsen et al17 (−0.43D at 20 degrees inferior retina and −0.40D at 30 degrees superior retina), the current study measured greater amounts of peripheral myopic defocus (−0.48D at 30 degrees inferior retina and −0.87D at 30 degrees superior retina). Individual variations and differences in baseline M between the two studies (−2.36 ± 0.97D compared with −1.54 ± 0.77D in the current study) may explain to some degree the differences found.

Similar to previous studies, J180 along the vertical meridian was more positive than the center.17–19,21 There are inconsistent reports on J45 values along the vertical meridian. In agreement with the current study, some studies have measured a positive increase in J45 values with a positive increase in positions on the retina,19,21 whereas the opposite effect has also been measured.18 However, J45 values overall varied only between ±0.50D.

Confirming previous reports, BE OK lenses caused significant hyperopic M shifts on the central retina and negative shifts in J180 along the horizontal meridian.9–12,23,24,28 Similarly, there were significant hyperopic M shifts on the central retina along the vertical meridian (Fig. 3A). In agreement with Ticak and Walline,29 the peripheral retina experienced myopic defocus along both the horizontal and vertical meridians after OK. Orthokeratology lens wear changed astigmatic components only in the peripheral retina; there was a positive shift in J180 at 20 and 30 degrees on the inferior retina and a positive shift in J45 at 30 degrees on the superior retina and a negative shift at 20 and 30 degrees on the inferior retina, in accordance with Mathur and Atchison.11

Studies have attributed temporal-nasal asymmetries in peripheral refraction after OK to temporal lens decentration commonly seen in OK patients.9,24,30 This results from regional variations in corneal curvature and asphericity across the horizontal corneal meridian.31–34 In contrast, no asymmetry in central and paracentral tangential curvature changes have been found after OK along the vertical meridian, implying absence of vertical lens decentration.33 Thus, the cause of asymmetric peripheral refraction changes along the vertical meridian remains unclear and requires further investigation.

Differences in peripheral refraction profile along the horizontal meridian compared with the vertical meridian and changes after OK lens wear raise questions about the current theory that links peripheral defocus and myopia development and control. Animal studies have revealed the significant impact of the peripheral retina in refractive error development,35,36 and it has been proposed that hyperopic defocus may stimulate axial length elongation and myopia development; the eye will grow in axial length to bring the peripheral retina in focus with the peripheral image. Thus, it has been hypothesized that treatments such as OK3,5,23 and multifocal soft contact lenses37 exert their myopia control effects by correcting peripheral defocus or inducing peripheral myopic defocus. This theory has been further supported by animal studies that have shown that peripheral positive or myopic defocus appears to inhibit axial length elongation, whereas the opposite has been demonstrated with negative or hyperopic defocus.13,14 However, the aforementioned studies exploring myopia control interventions have characterized changes in peripheral defocus along the horizontal meridian, and defocus along the vertical meridian has been largely ignored. Modifications to instrumentation have allowed refraction to be measured along the vertical meridian and, confirming previous reports,17–21 at baseline before any lens wear, peripheral myopic defocus was evident along the vertical meridian. This challenges the current myopia control theory as myopic eyes appear to already experience myopic defocus in some parts of the retina. Therefore, if peripheral retinal defocus is in some part a driver for refractive error development in humans, we propose that the induction of myopic defocus greater than the eye is habitually exposed to may be necessary for myopia control. This aligns with Berntsen et al17 who proposed that larger amounts of defocus above a threshold value may be required to have an effect on myopia progression.

A further aspect of the peripheral defocus myopia control theory that requires further clarification is the threshold of critical retinal area receiving myopic defocus to provide effective myopia control and the impact of duration of exposure. It has been proposed that the greater the extent of retinal exposure to myopic defocus, the more effective the myopia control.16 This concept has been supported by chick studies that demonstrated greater inhibition of lens-induced myopia with greater proportion of retina exposed to myopic defocus.38 Pupil size appears to impact myopia progression; the greater the pupil size during OK lens wear, the larger the myopia control effects. It was proposed that larger pupil sizes increased the area of retinal exposure to peripheral myopic defocus, resulting in enhanced myopia control effects.39

It has also been proposed that longer duration or more consistent retinal exposure to myopic defocus across various fixation distances will result in more effective myopia control.16 Animal studies have demonstrated that refractive error development involves complex temporal integration of visual signals, and the frequency and duration of lens wear influence refractive error development. Exposing the retina to intermittent viewing without any occluders or lenses reduced the development of experimentally induced refractive errors, although changes induced by positive defocus were more resilient.40 Similarly, in humans, a temporal dose-response relationship was found with custom-made bifocal soft contact lenses developed specifically for myopia control. Greater myopia control effects were experienced in children who wore these lenses for longer periods during the day.41 One of the key advantages claimed for OK for myopia control in children is that the retina is consistently exposed to significant myopic defocus across various fixation distances.16

Only one previous study has reported peripheral refraction along the vertical meridian after OK.29 The study reported here has more comprehensively characterized peripheral refraction before and after OK lens wear along both the horizontal and vertical meridians. At baseline, myopic eyes were found to have similar levels of defocus across the horizontal meridian compared with the center and relatively myopic defocus compared with the center across the vertical meridian. Confirming previous reports, OK lenses induced hyperopic shifts in peripheral refraction along both the horizontal and vertical meridians in the central retina, corresponding to changes in corneal topography; eyes now experienced myopic defocus in both the horizontal and vertical retinal meridians. Because the myopic eye typically experiences varying signs of defocus across the peripheral retina including myopic defocus in the vertical meridian with conventional correction,17 we propose that greater degrees of myopic defocus than the eye habitually experiences with traditional correction are required for effective myopia control. In agreement with Smith16 and Berntsen et al,17 we also propose that greater extent and duration of retinal exposure to myopic defocus will enhance the efficiency of myopia control modalities including OK. To be able to further improve the effectiveness of current myopia control treatments, studies aimed at better understanding the spatial and temporal integration of visual signals across the retina are required.

Pauline Kang

School of Optometry and Vision Science

University of New South Wales

Sydney, New South Wales 2052

Australia

e-mail: p.kang@unsw.edu.au

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ACKNOWLEDGMENTS

Supported under the Australian Research Council Linkage Project Scheme with industry collaborators Bausch + Lomb Boston (Wilmington, MA), BE Enterprises (Brisbane, Queensland, Australia), and Capricornia Contact Lens (Brisbane, Queensland, Australia). We thank Bausch + Lomb (Australia) for providing contact lens solutions.

Portions of these data were presented in poster form at the 2013 International Myopia Conference (Asilomar, CA) and at the 2014 Annual Meeting of the Association for Research in Vision and Ophthalmology (Orlando, FL).

Received June 1, 2015; accepted November 4, 2015.

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Keywords:

orthokeratology; peripheral refraction; myopia; myopia control; corneal topography

© 2016 American Academy of Optometry