Power vector analysis of refractive, corneal, and internal astigmatism in an elderly Chinese Population: The Shihpai Eye Study

Purpose. To investigate age-related trends in refractive, corneal, and internal astigmatism and to assess the association between internal astigmatism and lens opacity in an elderly Chinese population. Methods. A population-based study was conducted among 1360 inhabitants aged 65 years and older in Taipei, Taiwan. Participants underwent measurements of refraction, corneal dioptric power, and slit lamp biomicroscopy with lens grading. A total of 2084 eyes were included in power vector analyses of Cartesian astigmatism (J ₀) and oblique astigmatism (J ₄₅) components of refractive, corneal, and internal astigmatism. Results. The crude prevalence of refractive astigmatism (defined as ≥0.75 diopters) was 73.0% based on the right eyes and 76.4% based on the left eyes. The vector values in both refractive J ₀ and corneal J ₀ tended to be more negative with increasing age (P < 0.001), indicating the trend toward against-the-rule (ATR) astigmatism. Corneal J ₀ alone accounted for 54% of the variability in refractive J ₀. Refractive J ₄₅ increased with age in the right eyes (P < 0.001) and decreased slightly with age in the left eyes (P = 0.012). Cortical opacity was associated with internal J ₀ (P = 0.025), but the association was weak. Conclusions. Astigmatism affects approximately three quarters of the Chinese population aged 65 years and older in Taiwan. With increasing age, the prevalence of astigmatism increases, and refractive and corneal astigmatism shift toward ATR. Continuous corneal changes appear to be responsible for the age trend in refractive astigmatism. The severity of lens opacity plays only a minor role in the change of internal astigmatism.