Seasonal variation in axial elongation in children with orthokeratology treatment

Purpose: To investigate seasonal variations in changes of axial length (AL) among myopic children wearing orthokeratology (ortho-K) lenses. Methods: The data of 600 children receiving ortho-K treatment, aged 7-13 years, were collected retrospectively. Data were classified as 'summer' or 'winter' based on the midpoint of the 6-month period between visits. For each interval, AL from the right eye was used for statistical analysis. Paired sample t-tests were used to compare the seasonal differences in AL. Furthermore, the relationship between seasonal differences in AL, age and initial AL was analysed using Pearson correlation and multiple regression analysis, respectively. Results: The AL at 1 year after the initial ortho-K treatment was 24.96 +/- 0.85 mm, representing an increase of 0.19 +/- 0.15 mm. The semi-annual axial elongation was 0.07 +/- 0.09 and 0.12 +/- 0.09 mm for the summer and winter, respectively (p < 0.001). Linear regression analysis revealed that the control of axial growth in the summer was better than in the winter (beta = -0.05, p < 0.001). A similar seasonal pattern was found among children aged 7-12 years as well as for those with an initial AL < 26 mm (but not in children aged 13 years). Conclusions: The inhibition of axial growth by ortho-K treatment in the summer months is superior to that seen in the winter. Seasonal differences decrease with age and the initial AL.