Vector analysis of evolutive corneal astigmatic changes in keratoconus.

PURPOSE To evaluate by vector analysis the corneal astigmatic changes occurring in keratoconic corneas during a 3-year follow-up and to determine the relationship between these changes and other clinical changes. METHODS Keratoconic eyes (n = 114) of 75 patients ranging in age from 14 to 70 years were retrospectively reviewed in four different centers. In all cases, a 3-year follow-up was completed after the diagnosis of keratoconus. Visual, refractive, keratometric, aberrometric, internal astigmatism (IA), and pachymetric changes were evaluated during the follow-up. In addition, corneal astigmatic changes were evaluated by examining the following parameters, using a modification of the Alpins vectorial method: evolutive astigmatism (EA) and angle of error (AE). RESULTS An increase in the magnitude of refractive (P = 0.02) and corneal astigmatism (P = 0.05) was found. The mean magnitude of EA was 1.21 ± 0.97 D at 3 years, with no significant changes at each annual visit (P ≥ 0.52). Mean absolute AE increased significantly by the end of the follow-up (P < 0.01). Absolute AE and the increase in corneal astigmatism were found to correlate at 2 years (r = 0.675, P < 0.01). This correlation became poorer at 3 years (r = 0.352, P = 0.02). The magnitude of the EA was also found to be significantly correlated with central corneal thinning (r = -0.441, P = 0.02). Multiple regression analysis revealed that the magnitude of EA at 3 years correlated significantly with the baseline sphere and IA (R² = 0.86, P < 0.01). CONCLUSIONS Corneal vector astigmatic changes are related to some signs of keratoconus progression and are therefore predictive.