Factors influencing the reliability of autorefractometry after LASIK for myopia and myopic astigmatism.

Purpose: To study the factors influencing the reliability (accuracy) of autorefractometry before and after laser in situ keratomileusis (LASIK) for myopia and myopic astigmatism.

Design: Retrospective case series.

Methods: A total of 250 consecutive eyes (132 patients, mean age 37 years) were included from the Department of Ophthalmology, J.W. Goethe University, Frankfurt, Germany. The mean preoperative spherical equivalent of the subjective refraction (SR) was -6.59 diopters (D) (-1.38 to -15.13). The patient data were reviewed before and 1 month after LASIK, including SR, objective automated refraction (AR), and visual acuity. Subgroup analysis was performed with regard to the preoperative myopia and the excimer laser optical zone (OZ). The Holm-Sidak and Wilcoxon matched-pairs tests were used for statistical analysis.

Results: The correlation coefficient between AR und SR is r = 0.98 before LASIK versus r = 0.79 afterwards (P < .001). The mean difference between the spherical equivalents (DSE) is +0.13 ± 0.51 D preoperatively versus -0.30 ± 0.58 D after LASIK (P < .001). With a small optical zone size (5.0-5.5 mm) the postoperative difference is -0.61 D, versus -0.36 D for OZ 5.6-6.0, versus -0.16 for OZ 6.1-7.0 mm. With a rising preoperative amount of myopia, the postoperative AR results become increasingly more myopic than the SR.

Conclusions: Following LASIK, autorefractometry is less accurate than in nonoperated eyes. The reliability of the AR is influenced by the OZ and the preoperative amount of myopia, with a small OZ and high myopia resulting in a greater difference between AR and SR and with the AR determining more myopic results.