Toric orthokeratology for slowing eye elongation (TO-SEE) in myopic and astigmatic children

Abstract

The prevalence of myopia is high in East Asian countries, such as Hong Kong, China, Taiwan, Japan and Korea, and has also been reported to be increasing gradually in the western countries. Preventing or slowing myopic progression has attracted the interest of many clinicians and researchers. Spherical design ortho-k has been shown to be effective for reducing myopia and for myopic control but they are not effective in reducing astigmatism. With higher corneal astigmatism (> 1.50 D), lens decentration is the most common problem which can lead to induced astigmatism and poor vision. Hence spherical design ortho-k lenses are only recommended for children with low astigmatism. However, many myopic children also have significant astigmatism and considering the high prevalence of astigmatism in myopic children, there is a need for a myopic control treatment for myopic children with astigmatism. Toric reverse geometry design lenses have therefore been developed and introduced to improve lens centration as well as for astigmatic correction. However, except for a few case reports or conference presentations, there was no published study on the use of toric design ortho-k for myopic control in children with moderate to high amount of astigmatism. There was therefore a need to investigate the effectiveness of a toric design ortho-k for myopic and astigmatic reduction as well as for myopic control in this group of children. OBJECTIVES: The objectives of this PhD study were: 1. To determine the clinical performance of a toric design ortho-k lens and the safety of ortho-k lens wear in terms of corneal staining and corneal binding 2. To determine the effectiveness of the toric design ortho-k lenses for myopic and astigmatic reduction and control 3. To investigate long-term ortho-k lens wear on anterior and posterior corneal curvatures, and corneal thickness 4. To determine the treatment zone sizes and their correlations with the refractive changes in the ortho-k subjects 5. To investigate long-term ortho-k effects on corneal biomechanics in terms of corneal hysteresis (CH) and corneal resistance factor (CRF). METHODS: Eighty subjects (6 - 12 years old) (ortho-k: 43; control: 37), with myopia of -0.50 to -5.00 D and with-the-rule astigmatism of -1.25 to -3.50 D and unremarkable ocular and general conditions were enrolled. Data collection, including visual acuity, subjective and objective refraction, axial length, corneal topography, biomicroscopy examination, and corneal biomechanical properties were performed every six months during the 2-year study period. Axial length (AL) was monitored with the IOLMaster and the anterior and posterior corneal curvatures, central corneal thickness (CCT), CH, and CRF were performed with the Pentacam and Ocular Response Analyser (ORA), respectively. Results from the right eye or the eye with higher astigmatism were reported. RESULTS: A total of 35 ortho-k and 23 control subjects successfully completed the study. The mean ± SD myopia was -2.53 ± 1.31 D at baseline. Myopia was significantly reduced to -1.33 ± 0.80 D (42% reduction) at the 1-overnight visit and to -0.41 ± 0.43 D (81% reduction) at the 1-month visit. The mean ± SD refractive (with-the-rule) astigmatism reduced from -1.86 ± 0.64 D at baseline to -0.88 ± 0.59 D (54% reduction) and -0.40 ± 0.39 D (79% reduction) at the 1-overnight and 1-month visits, respectively. Corneal toricity reduced from -2.28 ± 0.53 D at baseline to -2.01 ± 0.61 (13%) at the 1-overnight visit and -1.28 ± 0.52 D (44%) at the 1-month visit. Subjects in both ortho-k and control groups demonstrated axial elongation. At the end of study, the average axial elongation was 0.31 ± 0.27 mm in the ortho-k group and 0.64 ± 0.31 mm in the control group. Axial elongation was significantly slower in the ortho-k group than in the control group at every 6-month visit. The levels of reduction of myopia progression compared to the spectacle-wearing control group were 61%, 58%, 53%, and 52% (cumulative elongations) after 6, 12, 18 and 24 months of ortho-k lens wear. Axial elongation was significantly correlated with the initial age of the subjects and treatment assigned. ANCOVA analysis was conducted to test the effect of ortho-k treatment on axial length increase adjusted for age and the small difference in age did not bias the effect of the treatment. Moreover, axial length elongation was not affected by gender, initial myopia, initial refractive cylinder or initial corneal toricity. Significant changes in the anterior Sim Kflat and Sim Ksteep were observed in the ortho-k group, but only at the 1-month visit. No significant changes were observed in the subsequent visits with ortho-k lens wear. In the control group, no significant changes in anterior Sim Kflat and Sim Ksteep were observed during the study period. There were no significant changes in the posterior Sim Kflat and Sim Ksteep at all visits in both ortho-k and control groups. In the ortho-k group, significant CCT thinning of 7 ± 9 μm was observed at the 1-month visit (p = 0.004). No significant differences in CCT were observed in the subsequent visits. No significant differences in CCT were observed at any visit during the 2-year study period in the control group. No significant changes were observed in CH at any visit for both groups of subjects. In the ortho-k group, CRF significantly reduced from an average of 10.7 ± 1.4 mmHg at baseline to 10.0 ± 1.4 mmHg after 1-month of lens wear (p = 0.003). Again, no significant differences were found in the subsequent visits with continued ortho-k lens wear. In the control group, no significant change in CRF was found during the study. CONCLUSIONS: The toric design ortho-k lenses used were effective for myopic reduction and astigmatic reduction. The results also demonstrated the strong potential for the toric ortho-k lenses to control myopic progression by about 50%. Myopia reduction from ortho-k treatment involved flattening of the anterior corneal curvatures with no involvement of the posterior curvatures. Corneal biomechanical properties, in terms of CH and CRF, were not useful parameters to predict the ortho-k response.