[Research on the visual performance and effects of higher order aberration after Epi-LASIK for myopia]

To investigate changes of visual performance and effects of higher order aberration on visual performance after Epi-LASIK for myopia.

In this prospective study, 102 patients were included. The trial group had 52 patients with Epi-LASIK, and 50 cases with LASIK as the control group. Ocular higher order aberrations were measured with the Hartman-shack wavefront analyzer, and high (100%), moderate (25%), low contrast (10%) and very low contrast (5%) visual acuity with and without glare were measured with Multi-Function Visual Acuity Tester under mesopic condition preoperatively and 4 - 6 months after surgery. 35 subjects in Epi-LASIK and 33 cases in LASIK were followed up 4 - 6 months later. Paired t test were used to analyze different contrast visual acuity before and after Epi-LASIK. Multiple linear regression analysis was performed to find the explanatory aberrations which contributed to the visual acuity. The Pearson correlation was used to explore the correlation between high- and low-contrast visual acuity and the explanatory higher order aberrations. The independent t test was used to analyze the difference of refraction power and visual acuity between Epi-LASIK and LASIK.

There were significantly statistical differences at 5% and 10% contrast level compared to preoperation with glare and with no glare. With no glare, the mean logMAR visual acuity at 5% and 10% contrast level were 0.32 + or - 0.14 and 0.21 + or - 0.12 preoperatively, and 0.47 + or - 0.15 and 0.33 + or - 0.16 postoperatively (t = -4.863, 3.950; P = 0.000, 0.000). With glare, the mean logMAR visual acuity were 0.37 + or - 0.20 and 0.22 + or - 0.13 preoperatively, and 0.46 + or - 0.18 and 0.30 + or - 0.17 postoperatively (t = -1.949, -2.283; P = 0.047, 0.029). Using stepwise multiple linear regressions to evaluate which higher order aberration was significantly associated with each contrast VA. For each order RMS, S(4) was positive correlated with 5% contrast visual acuity (r(2) = 0.282;P = 0.025); S(4) was positive and S(5) was negative correlated with 10% contrast visual acuity (r(2) = 0.192;P = 0.033). None of the higher order RMS can significantly affect the 25% and 100% contrast visual acuity. For each Zernike coefficients, C(4)(0) and C(6)(4) were positive correlated with 5% contrast visual acuity (r(2) = 0.233; P = 0.024); C(4)(-4) was negative, C(4)(0) and C(5)(-5) were positive correlated with 10% contrast visual acuity (r(2) = 0.289; P = 0.013); None of Zernike coefficients had a significant correlation with 25% and 100% contrast visual acuity. The Pearson correlated coefficients between S(4), C(4)(0) and each contrast visual acuity increased as the contrast level decreased. S(4) and C(4)(0) were associated significantly with 5% contrast visual acuity, the correlation coefficients were 0.378 and 0.390 (P = 0.025 and 0.021). The visual acuity under 100% contrast with no glare was a little poorer than in LASIK.

At the early stage after Epi-LASIK, the visual acuity for lower contrast level was worse compared to preoperative with glare and with no glare. Spherical aberration and the spherical-like aberration may contribute to this.