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 astigmatisms (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 : 4,027 children, mean age 7.6 ± 1.0 years, were analysed. CMA was associated with male sex ( P  = 0.019), older age (β = -0.143, 95%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 possessed the greatest mean IA value whereas CMA and SMA had the least. There was 56.1% of CHA displayed over-compensation as opposed to 7.9% and 7.6% in CMA and SMA respectively. On the contrary, 55.6% and 52.9% of CMA and SMA under-compensated their astigmatism, compared to 8.2% in CHA. Conclusion : Children with hyperopic astigmatism over-compensate their CA, whereas children with myopia are more likely to suffer 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 myopia development of schoolchildren.