The components of adult astigmatism and their age-related changes

Purpose To study the corneal and internal astigmatism and the age-related changes underlying the known refractive shift with-the-rule (WTR) towards against-the-rule (ATR) astigmatism. Methods Refractive and corneal biometry data were collected for a total of 1195 healthy Caucasian subjects, recruited by ophthalmological centres across Europe. After conversion of refractive and corneal surface astigmatism to power vectors J(0) and J(45), the total corneal and internal astigmatism were calculated. Results Both refractive power vectors had leptokurtic distributions with a narrow peak at zero, and a broader, secondary distribution at its base, corresponding to eyes without balanced components. This may be fitted with a bigaussian function (J(0): r(2) = 0.87; J(45): r(2) = 0.98). Statistically significant changes in these distributions are seen with age: for refractive J(0) the narrow peak shortens progressively, while the broad peak shifts positive from to negative values, corresponding to increased astigmatism and a shift from with-the-rule towards against-the-rule. For J(45) the narrow peak shortens with age as well, while the broad peak flattens. These changes in refractive J(0) result from statistically significant negative shifts in both the corneal and internal components with age (Kruskal-Wallis, p < 0.05). Conclusion In young participants corneal and internal astigmatism partially cancel each other out. This balance between both is gradually lost after age 50, leading to larger amounts of refractive astigmatism, as well as a reorientation of the axis. Given the small amplitude of these changes, they are unlikely to have repercussions for long term stability toric cataract or refractive surgery outcomes.