Difference of the Accuracy between Autorefration and Subjective Refraction in Photorefractive Keratectomy(PRK) and LASIK-Treated Eyes

Authors decided to test whether differences occurred in automated and subjective refraction in untreated, PRK, and LASIK-treated eyes. Ninety six eyes of 50 patients(96 eyes) who underwent PRK(57 eyes) or LASIK(39 eyes) for myopia and myopic astigmatism were routinely autorefracted with the CANON RK-3 before subjective refraction was done, using several parameters. This two procedures were done preoperation and 3 months after operation. In comparing the postoperative(PRK, LASIK) results, automated refraction showed more myopia and higher cylinder power, without significant difference in refractive axis, than those of subjective measurement. The difference of cylinder power were found in the cases of large ablation depth(above 100micrometer), high degree of eccentric ablation(above 0.25 mm), LASIK operation. The significant differences of cylinder power and axis were found in the cases of high degree of eccentric ablation(above 0.25 mm). So we recommend the method of subjective refraction before PRK and LASIK operation instead of using automated refraction. Postoperatvely, we also should evaluate the postoperative status of the patient by subjective refraction. And we should decide the amount of ablation in case of retreatment by subjective refraction, instead of autorefraction. We have to be cautious of evaluating the automated refraction results after operation, especially in cases of large ablation depth, high degree of eccentric ablation, and LASIK.

A Study about the Accuracy of Automated Refraction

We analysed the differences between the values of noncycloplegicautomated refraction and manifest refraction by ophthalmologist 150 patients(274 eyes)were studied. We divided these patients into five groups according to the age; Group 1 included patients under 10 yea(n=50 eyes), Group 2 from 10 yrs to 19 yrs(n=74 eyes), Group 3 from 20 yrs to 39 yrs(n=66 eyes), Group 4 from 40 yrs to 49yrs(n=39 eyes), Group 5 over 50 yrs(n=46 eyes); We used Canon Auto Ref R-10 autorefractor and ophthalmologist's refractions were the results of both subjective refraction by retinoscope and objective refraction used by cross cylinder. Between the two examinations, the percentages of eyes of disagreement beyond +/-0.5D were 22%(group 1), 39.2%(group 2), 14.1%(group 3), 15.8%(group 4), 17.4%(group 5) in spherical power; 18%, 23%, 27.3%, 29%, 15.2% in cylindrical power respectively; 20%, 31.9%, 18.2%, 15.8%, 19.6% in spherical equivalent respectively and the percentages of eyes of disagreement beyond +/-10 degrees in cylindrical axis were 25%, 42.6%, 39.2%, 40%, 52.9% respectively. There was no statistic significance between the age groups when we compared the absolute differences of values between two examinations. Conclusively, this study revealed that there were large differences between the values of noncycloplegic automated refraction and manifest refraction by ophthalmologist in the large amout of patients and then our results indicated that automated refraction was dangerous to patient's eyes and subjective refinement was needed by ophthalmologist if patients wanted the spectacle or contact lens.