Internal Astigmatism in Myopes and Non-myopes: Compensation or Constant?

PURPOSE To examine internal astigmatism (IA) in myopes and non-myopes using a new method to assess compensation of corneal astigmatism (CA) by IA, to look for predictors of high IA in young adult myopes, and to determine if as CA changes IA changes to reduce refractive astigmatism (RA) in an active compensatory process in myopes. METHODS Right eye keratometry and cycloplegic autorefraction were measured annually over 14 years in 367 myopes and once in 204 non-myopes age- (mean 21.91 ± 1.47 years), gender-, and ethnicity-matched to myopes at year 12. CA and RA at the corneal plane were expressed as J0, J45. IA = RA - CA. Inverse power transformation provided cylinder power and axis of IA for the compensation factor (IA/CA). Analyses included (1) paired and unpaired t-tests (refractive data), (2) chi-square tests (distributions of compensation factor), (3) logistic regression analysis (predictors of high IA), and (4) linear mixed models (time effect on RA, CA, and IA). RESULTS The magnitude of IAJ0 varied by refractive error (myopes -0.25 ± 0.24 vs. non-myopes -0.32 ± 0.21, p < 0.001). Compensation of CA by IA was poorer in myopes than non-myopes (χ p < 0.001). When matched by CA, compensation remained poorer in myopes than non-myopes (χ all p ≤ 0.04). Within each refractive group, compensation was better when CA was low than high (χ p < 0.001). When CA was low in myopes, high IA (≥1.00D) was less likely (p = 0.01). Longitudinal follow-up of myopes found no evidence for an active compensatory role for IA as CA increased over time. There were differences in IAJ0 by ethnicity over time (p < 0.0001). CONCLUSIONS In myopic and non-myopic eyes with low amounts of CA, IA may reduce CA's contribution to RA, but IA is not a constant. However, there is no evidence for an active compensatory role for IA reducing CA in myopes.