Safety of long-term ortho-k (SOLO) study : corneal and ocular microbiome changes in children undergoing orthokeratology treatment and concerns of children, parents and practitioners about the treatment

Abstract

Background: The rapidly increasing prevalence of myopia is an issue of global concern. Orthokeratology (Ortho-k) has been shown to be an effective myopia control treatment in children. Current evidence from clinical studies shows that the treatment is safe for children provided there is good compliance with proper usage and care by all parties involved in the treatment, i.e. children, parents and practitioners. Occurrence of microbial keratitis in the community implies a certain degree of non-compliance from the parties involved in the treatment. As microbial keratitis can have serious consequences, prevention of this and other possible adverse effects that may be associated with orthokeratology should be assessed to determine if they can be avoided or ameliorated. The following gaps in knowledge regarding the safety of ortho-k lens wear in children were investigated in this thesis: 1. Changes in corneal endothelium associated with ortho-k lens wear or normal ageing 2. Contamination is a risk for microbial keratitis, however, sources of contaminants have not been investigated 3. Compliance of patients is affected by the attitudes of users/parents towards the treatment, but little attention has been given to this area 4. Safe ortho-k practice for myopia control could be improved by careful patient selection; however, there are no guidelines for myopia control treatment for practitioners Objectives: 1. To investigate changes (if any) on the corneal endothelium of children wearing ortho-k lenses or spectacles for two years 2. To compare the ocular microbiota of children with and without ortho-k lens wear and to determine the sources of contaminations of their accessories 3. To evaluate the attitudes towards ortho-k of parents seeking myopia control treatment for their children 4. To develop a decision tree for patient selection for practitioners using ortho-k for myopia control Methods: Four studies were conducted with different experimental designs to address the above objectives. Study 1. A retrospective cohort study to compare the changes in corneal endothelial morphology in the central and superior segments of subjects aged six to 12 before and two years after wearing orthokeratology and single-vision spectacles. Study 2. A cross-sectional study to compare the carriage rates of ocular microbiota in the peri-orbital tissues and the accessories of two groups of subjects aged seven to less than 15, one group using orthokeratology over a year and the other single-vision spectacles. The associations between carriage of Staphylococcus aureus and Gram-negative bacteria and contamination of accessories were determined. Study 3. A telephone interview with parents seeking myopia control for their myopic children was conducted to assess their attitudes towards the use of orthokeratology, the use of daily wear soft contact lenses, and spectacles for myopia control. Study 4. A prospective study to investigate the effect of history of myopia progression and age on the efficacy of myopia control by comparing the rate of axial elongation after switching from seven months of spectacle-wear to another seven months of orthokeratology-wear in subjects aged six to less than 16. A protocol for myopia treatment based on the individual characteristics was derived from the results. Results: Study 1. Ortho-k retarded reduction in endothelial cell density in the central cornea (p = 0.024). Otherwise, it had little influence on polymegathism and pleomorphism induced by normal ageing (p > 0.105). Study 2. Bacterial count in the conjunctiva was reduced after orthokeratology (p = 0.009), but the diversity of the bacteria in other skin tissues was not affected. The diversity of bacteria of the accessories was similar to that found in the skin (p > 0.122). However, association between carriage of bacteria in the skin tissues and in the accessories was only significant for Staphylococcus aureus in the control subjects (p = 0.029). Studies 3. Parents were conservative about children using contact lenses for refractive correction, but were more receptive to use the lenses for myopia control. Confidence in the treatment was affected by prior knowledge of myopia control strategy. Study 4. More significant reduction in axial elongation was observed in subjects with a history of rapid myopia progression, whereas no beneficial effect was observed in those displaying slow elongation. The history of progression was affected by age, thus, a decision tree for myopia control based on age and history of progression was proposed. Conclusions: The safety of orthokeratology was confirmed as it exerted minimal stress on the cornea. Risk of contamination could be reduced by regular review of handling procedures to identify patients/parents with poor hygiene or poor handling habits, whereas compliance with correct ortho-k practice was affected by positive exposure to this treatment. Myopia control was not indicated for non-progressing myopic children. Safety of the treatment could be further improved with the aid of the proposed guideline for selecting candidates for whom the benefits outweigh the risks.