MEASUREMENT OF IRREGULAR ASTIGMATISM USING SEMIMERIDIAN DATA FROM VIDEOKERATOGRAPHS

BACKGROUND: Irregular astigmatism is one of the most troublesome side effects of corneal surgery. We attempted to quantify irregular astigma tism in a simple way, using semimeridian data from videokeratographs o f corneas with different amounts of irregular astigmatism. METHODS: We studied the corneal topography of 30 normal corneas to be used as con trols, 30 corneas from patients after radial keratotomy, 30 corneas fr om patients after nonfreeze myopic keratomileusis, and 30 corneas with clinically apparent keratoconus. We developed two diagnostic formulas , a refractive power symmetry index and an angle symmetry index, using the absolute-scale, color-coded map, and semimeridian data from the E yeSys Corneal Analysis System, and used them to compare the four group s of corneas. RESULTS: In the corneas with irregular astigmatism, valu es for the refractive power symmetry index and angle symmetry index we re significantly higher than in the normal corneas. The refractive pow er symmetry index confidence interval set at 95% was 0.58 diopters (D) (range, 0.00 D to 0.58 D) in the control group. Using these limits, m ost values in the radial keratotomy, keratomileusis, and keratoconus g roups were abnormal. Spectacle-corrected visual acuity correlated nega tively with the refractive power symmetry index (rs=-0.58, p<.001). Th e refractive power symmetry index sensitivity and specificity were res pectively 97% and 66%. Spectacle-corrected visual acuity did not corre late with the angle symmetry index. CONCLUSIONS: Semimeridian data fro m videokeratographs can be used to quantify irregular astigmatism.