Orthokeratology, binocular co-ordinationand myopia control

This work investigates two key questions: ; how myopia progression control treatments impact accommodation and binocular vision function and if accommodation and binocular vision function prior to myopia control interventions has any association with the efficacy of treatment. The work is made up of four main studies. Study 1 was a review and re-analysis of a previously completed study to investigate whether there are any possible associations between accommodation and binocular vision status prior to lens wear which may impact the myopia control efficacy of orthokeratology lens treatment. The data of 26 children were used. The study involved using an orthokeratology lens in one eye, with the contralateral eye used as a rigid gas permeable lens wearing control. Given the unusual nature of this modality, accommodation and binocular vision status was measured at baseline and monitored throughout the study to ensure that there were no adverse responses to lens wear. In a novel analysis of pre-existing data, the accommodation and binocular vision profiles of those participants that responded the best to orthokeratology lens treatment in terms of inhibiting ocular axial length growth were compared to those that did not respond to treatment. A similar comparison was made between those participants who progressed the most to the least in the control eye. Results from this analysis suggested that accommodation and binocular vision status prior to orthokeratology lens wear may be associated with treatment effect. Orthokeratology lens treatment worked best for myopia control when accommodative facility was higher and closer to population norms, AC/A ratio was lower and closer to population norms and accommodative lag was higher. None of these associations reached statistical significance, however further investigation appeared warranted. Interestingly, baseline near phoria did not appear to have an influence on response to treatment with orthokeratology lens wear. This is different to bifocal spectacle lens wear studies that show that initial near phoria has an impact on the efficacy of myopia control treatment. Study 2 was a review of clinical records of 37 children and young adult patients seen in two private optometric practices in Australia. Accommodation and binocular vision function prior to orthokeratology lens wear was compared to during lens wear in patients who were fitted with orthokeratology lenses between 2010 and 2012. The results of this study showed that there was a statistically significant change in mean near phoria in the exo direction with lens wear. Mean positive relative accommodation increased, mean negative relative accommodation decreased and accommodative facility increased. While the mean distance phoria remained unchanged there was a statistically significant reduction in the standard deviation of this variable. There was a slight reduction in mean lag of accommodation, but it failed to reach statistical significance. The study showed that the binocular vision status including accommodative and vergence measures changed during orthokeratology lens wear. The changes were in a direction closer to population norms. Study 3 was a prospective study of the impact of short-term orthokeratology lens wear on binocular vision in 12 young adults. Measurements of binocular vision status were taken at baseline and after one month of lens wear. The results of this study were similar to Study 2. There was no statistically significant change in mean near phoria. Distance accommodative facility increased. There was a slight, but not statistically significant, change in mean near accommodative facility Again, while there was no change in mean distance phoria, there was a significant reduction in the standard deviation of this variable. Additional variables of interest in this study included stereopsis, which was unchanged, and fixation disparity at distance and near which were unchanged. This short-term study showed that orthokeratology lens wear alters binocular vision status including accommodative and vergence measures. Again, the changes in binocular vision were in a direction closer to population norms. Study 4 was a record review of patients seen in the Myopia Control Clinic at the University of New South Wales, Sydney, Australia. The accommodation and binocular vision function of myopic children treated with orthokeratology lens wear or low dose atropine were reviewed. A total of 9 children treated with orthokeratology were followed for 3 to 6 months. Mean near phoria moved in the exo direction with orthokeratology lens wear and gradient AC/A ratio moved to more normal values with lens wear. There was an association between annualised axial length growth and gradient AC/A ratios and stereopsis, suggesting that baseline accommodation and binocular vision function may influence treatment. A total of 19 children were treated with low dose atropine and data were available from 3 to 6 months of treatment. Low dose atropine led to a small but not statistically significant decrease in amplitude of accommodation. Patients who were the worst responders to low dose atropine had lower baseline amplitude of accommodation compared to the best responders. Although speculative, the reduction in amplitude of accommodation may have a detrimental impact on the accommodation and binocular vision function and increase blur. Alternatively, those patients with high amplitude of accommodation may benefit from a subtle change in the accommodation and vergence relationship Both accommodation and vergence measures of binocular vision status appear to change with orthokeratology lens wear. The changes in binocular vision move in a direction closer to population norms; this is a novel finding of this thesis and is not reported elsewhere. Binocular vision that is abnormal has been associated with onset of myopia and progression. The changes in binocular vision associated with orthokeratology lens wear may contribute to the myopia progression control effect. Close monitoring of accommodation and binocular vision during myopia progression treatment is warranted.