The Relationship Between Corneal and Refractive Astigmatism: Effects of Gender, Ethnicity and Refraction

Abstract

Abstract: : Purpose: Javal’s original and modified rule for predicting total (TA) from corneal astigmatism (CA) do not consider effects of gender, ethnicity, refraction, and exact astigmatic axis. We examined the effects of these variables on CA, TA, and residual astigmatism (RA) using exact astigmatic axis. Methods: The Sydney Myopia Study is a population–based study of refraction and eye health in school children from Sydney, Australia. Keratometry (Zeiss IOLMaster) and cycloplegic (cyclopentolate 1%) autorefraction (Canon RK–F1) were performed on 1725 children aged 6 years during 2003–4. Right eye data was analysed using J₀ (with– and against–the–rule astigmatism) and J₄₅ (oblique astigmatism) power vectors. Results: Mean age was 6.7 years (range 5.5–8.4). Most children were European Caucasian (64.4%), East Asian (17.2%), Middle Eastern (5.0%), or Indian/Pakistani (2.3%). Mean CA was 0.82D (CI 0.77–0.87), and mean TA 0.29D (CI 0.26–0.32). There were no gender differences. East Asian and Indian/Pakistani children had greater CA and TA than Caucasian children. Gender (p>0.8) and ethnicity (p>0.3) did not significantly affect the relationship between corneal (CJ₀) and total J₀ (TJ₀). The regression slope was flatter (p<0.006) in low hyperopic (SE 0.5–2.0D) than in more hyperopic (SE>2.0D) children. The regression slope between corneal (CJ₄₅) and total J₄₅ (TJ₄₅) was higher in girls (p<0.01), and in East Asian (p<0.9) compared to Caucasian children, but only for CJ₄₅<–0.2. The regression slope was flatter (p<0.0001) for low hyperopic (SE 0.5–2.0D) than in more hyperopic (SE>2.0D) children. The effect of refraction on the relationship between corneal and total J₀ (p<0.005) and J₄₅ (p<0.0001) remained significant when adjusted for age, gender, and ethnicity. Mean RA (vector difference between CA & TA) was significantly (p<0.0001) lower in boys (0.72D; CI 0.70–0.74) than in girls (0.79D; CI 0.77–0.82). There were no significant differences in RA between children from the major ethnic groups compared to Caucasian children. RA was significantly (p<0.015) lower (0.76D; CI 0.73–0.79) for SE 0.5–2.0D than in children with SE>2.0D (0.83D; CI 0.78–0.89). Conclusions:Of factors examined, the most important variable determining the relationship between CA and TA is the refraction. Gender and ethnicity do not appear to be important. Variations in residual astigmatism were relatively small.

Keywords: astigmatism • refraction