Astigmatism Types Effects on Internal Astigmatism and Compensation in Children: Hong Kong Children Eye Study.

INTRODUCTION: Astigmatism is highly prevalent among schoolchildren in Hong Kong. Children demonstrate distinct patterns of internal astigmatism (IA) to compensate for corneal astigmatism (CA). This study aims to investigate the associations among astigmatism subtypes, IA and compensation factors (CF) in schoolchildren with a minimum of 0.5 dioptre(D) of refractive astigmatism (RA). METHODS: RA was assessed following cycloplegia and classified into simple myopic (SMA), compound myopic (CMA), mixed (MA), simple hyperopic (SHA) and compound hyperopic astigmatism (CHA) based on the location of the focal planes. IA was calculated through vector components. CF were determined by dividing IA by CA in respective vectors. RESULTS: A total of 4027 children, mean age 7.6 ± 1.0 years, were analysed. CMA (n = 633) was associated with male sex (p = 0.02), older age (β = -0.143, 95% Confidence Interval (CI) -0.201 to -0.085, p < 0.001), longer axial length (β = -0.143, 95% CI -0.158 to -0.129, p < 0.001) and deeper anterior chamber (β = -0.019, 95% CI -0.022 to -0.015, p < 0.001). CHA (n = 1207) showed the greatest mean IA value whereas CMA and SMA (n = 104) had the smallest value. Some 56.1% of those with CHA displayed over-compensation as opposed to 7.9% and 7.6% in CMA and SMA, respectively. To the contrary, 55.6% and 52.9% of CMA and SMA had under-compensation of their astigmatism, compared with 8.2% in CHA. CONCLUSION: Children with hyperopic astigmatism over-compensate for their CA, whereas children with myopia are more likely to exhibit a greater amount of RA because of lower IA and under-compensation. The differential compensation between hyperopic and myopic children may play a determining role in the development of myopia in school children.