Corneal Reshaping Influences Myopic Prescription Stability
Corneal Reshaping Influences Myopic Prescription Stability
The key finding of this retrospective study is that OK was associated with a greater degree of myopic prescription stability when compared with a spectacle-wearing control group. OK eyes had a significantly (P<0.05) lower rate of manifest myopic refractive change for all treatment intervals up to 8 years. All control eyes demonstrated some degree of increase in their myopic refraction, whereas 64% of OK eyes showed apparent stability of their myopic refraction.
The CRIMPS study is one of the first peer-reviewed, case–control studies to describe the effect of OK on childhood myopic refraction over an extended period. Single case reports have lent support to a growing clinical impression that myopic stability associated with OK treatment may prevail for longer periods. There is currently only one other study in the literature that has sought to determine whether such an effect does occur. In this study, undertaken by Mok and Chung, manifest myopic subjective refraction was shown to increase substantially less in an OK-treated group of Chinese children over a 7-year period. The CRIMPS study also demonstrated that an apparent reduction in progressive increases to myopic refractive errors with OK is not limited to a short-term effect; the relative benefit of OK in apparently stabilizing the manifest refractive error persisted up to and including an 8-year treatment interval.
Our data support previous studies 32–38,44,45 that report a decrease in the rate of myopic progression with OK in children and adolescents. Overall, our findings would suggest a more significant degree of apparent attenuation in myopic refractive change than is frequently described with OK. A number of factors may contribute to this result. In this study, OK lenses were fitted and reviewed by a single, experienced contact lens specialist, ensuring adoption of consistent lens prescribing criteria and eliminating interpractitioner variability in lens fitting. Variable fitting efficacy may contribute to the smaller treatment effect reported elsewhere. We speculate that different proprietary brands of OK lens design may also inherently confer varying degrees of myopic attenuation.
A further consideration is the difference in patient demographics between the present investigation and other OK studies (Table 4). Younger age of onset has been identified as the strongest independent risk factor for myopic progression. Children in the CRIMPS study were significantly older and had higher baseline myopia than these earlier investigations.
The mean rate of myopic refractive change in control eyes over the first 2 years of treatment (−0.46 ± 0.06 D/year) was consistent with the myopic progression rate of spectacle wearers reported in other studies. After the 2-year treatment interval, there was a general trend toward slower rates of myopic refractive change in control eyes. This is likely caused by age-related differences in refractive error development (i.e., children tend to show less myopic progression in their late teenage years). It may also reflect the fact that our control population consisted of patients with a heterogeneous range of spectacle corrections, including single-vision distance and progressive addition lenses, the latter of which have reported potential benefits in subpopulations of juvenile-onset myopia in reducing myopic progression.
A greater understanding of the genesis and temporal development of myopia has emerged from laboratory-based form deprivation experiments. Using foveal ablation in primates, the paracentral retina, rather than the central macular region, has been identified as the likely contributor to the process of axial elongation in response to hyperopic defocus. Paracentral hyperopic defocus has an established role in driving axial elongation in both animal models of myopia and human myopic eyes.
The leading theory to explain how OK may regulate the rate of myopic progression links the positive spherical aberration induced in the OK treatment zone to a presumed effect on manipulating the midperipheral visual experience. Where myopic correction with spectacle lenses creates hyperopic defocus in the peripheral retina, the OK treatment zone acts to focus midperipheral light rays anterior to the retina, bending the image plane anteriorly within the globe and potentially creating a myopic retinal image shell. Further work to elucidate the precise role of the peripheral optic profile in ocular growth regulation is indicated.
A further outcome of this article is the identification of a factor that appeared to be associated with myopic refractive stability in individual eyes treated with OK. It has previously been noted that there can be substantial variation in axial length changes among children undergoing OK; however, the effect was considered unpredictable for individual patients. This study suggests that the concept of "risk factors" for changes to the myopic prescription during OK treatment is valid; confirmation of the reported observation within a larger population may empower practitioners to make more reliable assessments regarding an individual's likelihood for refractive stability. Our data indicate a previously unreported potential association between refractive stability and the symmetry of the vertical meridian of the baseline corneal topography.
OK eyes that were nonprogressors were observed to have a significantly (P<0.05) higher degree of vertical symmetry on baseline corneal topography compared with eyes that demonstrated myopic progression. Asymmetric axial power manifested as either relative inferior or superior corneal steepening measured within a radius of 4 mm centered on the corneal apex (Fig. 2). Regional topographical differences will create localized variations in the lens–cornea relationship across the cornea, potentially affecting the OK lens fit. Treatment forces may be compromised because of fluid escape through a nonsealing spherical landing zone resting on an underlying toric peripheral cornea, resulting in poorer lens centration, lens flexure, and/or undertreatment. A quadrant-specific peripheral design OK lens will likely overcome such compromise in treatment effect.
In the context of the published literature, the present study is unique owing to a review period of up to 8 years. The CRIMPS study benefited from appropriate matching of baseline demographic (i.e., age) and ocular (i.e., spherocylindrical refractive error) parameters between control and OK eyes. Furthermore, there were no significant differences (P>0.05) in these parameters across any of the time intervals; thus, despite a reducing sample size, there was no bias in the patient population over the 8-year investigation period.
We acknowledge a number of important limitations to this work. The study is retrospective and thus has the potential for both practitioner and investigator bias as neither the examiners nor the patients were masked to the treatment groups. Patients in each treatment group also derived from different optometric practices; therefore, there is potential for practitioner-driven bias with regard to different measurement techniques and practice protocols. Other known factors influencing myopic progression, such as the significance of a parental history of myopia, near esophoria, and accommodative lag, were also not explored. As axial length and vitreous chamber depth were not measured, we cannot be certain that the reported differences in myopic refractive change correlate to real differences in ocular growth. Nonetheless, the novel approach used in this study of using ROL in situ as a key determinant of change in the ocular refraction may have advantages over traditional biometric methodologies that determine individual components of the eye because of its simple, all-embracing nature. Subjective refraction is also the primary technique used to measure myopic progression in the clinic, and therefore, it is a valuable functional marker for myopic change in optometric practice.
Discussion
Orthokeratology Treatment has an Apparent Stabilizing Effect on the Manifest Refractive Prescription in Myopic Children
The key finding of this retrospective study is that OK was associated with a greater degree of myopic prescription stability when compared with a spectacle-wearing control group. OK eyes had a significantly (P<0.05) lower rate of manifest myopic refractive change for all treatment intervals up to 8 years. All control eyes demonstrated some degree of increase in their myopic refraction, whereas 64% of OK eyes showed apparent stability of their myopic refraction.
The CRIMPS study is one of the first peer-reviewed, case–control studies to describe the effect of OK on childhood myopic refraction over an extended period. Single case reports have lent support to a growing clinical impression that myopic stability associated with OK treatment may prevail for longer periods. There is currently only one other study in the literature that has sought to determine whether such an effect does occur. In this study, undertaken by Mok and Chung, manifest myopic subjective refraction was shown to increase substantially less in an OK-treated group of Chinese children over a 7-year period. The CRIMPS study also demonstrated that an apparent reduction in progressive increases to myopic refractive errors with OK is not limited to a short-term effect; the relative benefit of OK in apparently stabilizing the manifest refractive error persisted up to and including an 8-year treatment interval.
Our data support previous studies 32–38,44,45 that report a decrease in the rate of myopic progression with OK in children and adolescents. Overall, our findings would suggest a more significant degree of apparent attenuation in myopic refractive change than is frequently described with OK. A number of factors may contribute to this result. In this study, OK lenses were fitted and reviewed by a single, experienced contact lens specialist, ensuring adoption of consistent lens prescribing criteria and eliminating interpractitioner variability in lens fitting. Variable fitting efficacy may contribute to the smaller treatment effect reported elsewhere. We speculate that different proprietary brands of OK lens design may also inherently confer varying degrees of myopic attenuation.
A further consideration is the difference in patient demographics between the present investigation and other OK studies (Table 4). Younger age of onset has been identified as the strongest independent risk factor for myopic progression. Children in the CRIMPS study were significantly older and had higher baseline myopia than these earlier investigations.
Control Group
The mean rate of myopic refractive change in control eyes over the first 2 years of treatment (−0.46 ± 0.06 D/year) was consistent with the myopic progression rate of spectacle wearers reported in other studies. After the 2-year treatment interval, there was a general trend toward slower rates of myopic refractive change in control eyes. This is likely caused by age-related differences in refractive error development (i.e., children tend to show less myopic progression in their late teenage years). It may also reflect the fact that our control population consisted of patients with a heterogeneous range of spectacle corrections, including single-vision distance and progressive addition lenses, the latter of which have reported potential benefits in subpopulations of juvenile-onset myopia in reducing myopic progression.
What is the Mechanism Underlying the Apparent Myopic Refractive Stability Observed With Orthokeratology?
A greater understanding of the genesis and temporal development of myopia has emerged from laboratory-based form deprivation experiments. Using foveal ablation in primates, the paracentral retina, rather than the central macular region, has been identified as the likely contributor to the process of axial elongation in response to hyperopic defocus. Paracentral hyperopic defocus has an established role in driving axial elongation in both animal models of myopia and human myopic eyes.
The leading theory to explain how OK may regulate the rate of myopic progression links the positive spherical aberration induced in the OK treatment zone to a presumed effect on manipulating the midperipheral visual experience. Where myopic correction with spectacle lenses creates hyperopic defocus in the peripheral retina, the OK treatment zone acts to focus midperipheral light rays anterior to the retina, bending the image plane anteriorly within the globe and potentially creating a myopic retinal image shell. Further work to elucidate the precise role of the peripheral optic profile in ocular growth regulation is indicated.
A Potential Predictive Factor for Myopic Refractive Stability
A further outcome of this article is the identification of a factor that appeared to be associated with myopic refractive stability in individual eyes treated with OK. It has previously been noted that there can be substantial variation in axial length changes among children undergoing OK; however, the effect was considered unpredictable for individual patients. This study suggests that the concept of "risk factors" for changes to the myopic prescription during OK treatment is valid; confirmation of the reported observation within a larger population may empower practitioners to make more reliable assessments regarding an individual's likelihood for refractive stability. Our data indicate a previously unreported potential association between refractive stability and the symmetry of the vertical meridian of the baseline corneal topography.
OK eyes that were nonprogressors were observed to have a significantly (P<0.05) higher degree of vertical symmetry on baseline corneal topography compared with eyes that demonstrated myopic progression. Asymmetric axial power manifested as either relative inferior or superior corneal steepening measured within a radius of 4 mm centered on the corneal apex (Fig. 2). Regional topographical differences will create localized variations in the lens–cornea relationship across the cornea, potentially affecting the OK lens fit. Treatment forces may be compromised because of fluid escape through a nonsealing spherical landing zone resting on an underlying toric peripheral cornea, resulting in poorer lens centration, lens flexure, and/or undertreatment. A quadrant-specific peripheral design OK lens will likely overcome such compromise in treatment effect.
Strengths and Limitations
In the context of the published literature, the present study is unique owing to a review period of up to 8 years. The CRIMPS study benefited from appropriate matching of baseline demographic (i.e., age) and ocular (i.e., spherocylindrical refractive error) parameters between control and OK eyes. Furthermore, there were no significant differences (P>0.05) in these parameters across any of the time intervals; thus, despite a reducing sample size, there was no bias in the patient population over the 8-year investigation period.
We acknowledge a number of important limitations to this work. The study is retrospective and thus has the potential for both practitioner and investigator bias as neither the examiners nor the patients were masked to the treatment groups. Patients in each treatment group also derived from different optometric practices; therefore, there is potential for practitioner-driven bias with regard to different measurement techniques and practice protocols. Other known factors influencing myopic progression, such as the significance of a parental history of myopia, near esophoria, and accommodative lag, were also not explored. As axial length and vitreous chamber depth were not measured, we cannot be certain that the reported differences in myopic refractive change correlate to real differences in ocular growth. Nonetheless, the novel approach used in this study of using ROL in situ as a key determinant of change in the ocular refraction may have advantages over traditional biometric methodologies that determine individual components of the eye because of its simple, all-embracing nature. Subjective refraction is also the primary technique used to measure myopic progression in the clinic, and therefore, it is a valuable functional marker for myopic change in optometric practice.
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