| Abstract |
Purpose: Visual acuity for many years has been the standard procedure for assessing visual performance in optometric practice. Visual acuity involves the measurement of the recognition of small, high-contrast letters, which change in size. Unfortunately, conventional letter acuity is limited because, despite proper refractive correction, some visual problems, which mainly affect low contrast detection, may not be well demonstrated with conventional letter charts. Moreover, because the real world is composed of objects of varying sizes and contrasts, visual acuity is a simplistic assessment of visual performance for every day visual tasks. Contrast sensitivity function (CSF) is a measure of contrast threshold for a range of objects sizes and is conventionally measured by finding the contrast threshold of sine-wave gratings of varying spatial frequencies. As a result, contrast sensitivity measures may allow a more complete investigation of visual function and can be used to detect visual problems at early stage. The purpose of this study was to evaluate changes in visual performance after refractive surgery, comparing contrast sensitivity, visual acuity and high order aberrations for the two mostly used techniques: photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). Moreover, the effect of pupil size and retinal illuminance on contrast sensitivity was investigated. Methods: Twenty-three subjects volunteered to participate in the study. The mean age was 25.4 ± 5.1 SD). The dominant eyes of 10 LASIK and 13 PRK myopic patients, corrected with the Laser Allegretto (Wavelight Technologies). Nine more subjects served as the control group. Best-corrected contrast sensitivity (C.S) was measured preoperatively, 1 and 3 months after refractive surgery. C.S was evaluated using Gabor patches of gratings displayed on a Sony GDM F-520 CRT display (frame rate of 120 Hz) by means of a VSG2/5 stimulus generator card (CRS, Rochester, UK). Gabor patches, measured 100 pixels in diameter at half height in the centre of the display, with a standard deviation subtending 1.2 deg at a 2 m distance. Six spatial frequencies, 1, 2, 4, 8, 12 and 16 c/deg were tested. The average of three thresholds was taken. Threshold was determined using a binary-search staircase with a contrast resolution of 1dB (0.05 log units). An artificial pupil was used to isolate the ocular high order aberrations for 3mm and 6mm diameter pupil. A COAS (Complete Ophthalmic Analysis System, Wavefront Sciences, Ltd) aberrometer was used for measuring the aberrations. The luminance of the Sony display was 30 and 7.5 cd/m2, respectively, so that the retinal illuminance was equal in the two conditions. Moreover, the contrast sensitivity was evaluated for 30 cd/m2 and for 6mm diameter pupil in order to evaluate the effect of the luminance. Results: For equal amounts of retinal luminance, contrast sensitivity was higher at 3mm diameter pupil compared to 6mm. Contrast sensitivity was for 6mm was depressed especially for high spatial frequencies. When luminance was reduced, contrast sensitivity was decreased especially for low and medium spatial frequencies. Preoperatively, and at 1 and 3 months, the mean decimal BCVA was 1.06±0.21 (SD), 1.06±0.10 and 1.09±0.21 respectively for LASIK subjects and 1.15±0.23 (SD), 1.02±0.13 and 1.15±0.13 for PRK subjects. Contrast sensitivity increased 1 and 3 months after refractive surgery. While the measurements take place without correction 3 months post-operatively, contrast sensitivity decreased especially for 6mm diameter pupil. Contrast sensitivity also improved for the control group. Αt the second visit there was a significant improvement especially for the high spatial frequencies (2 to 5 dB). This difference was more obvious for 7.5 cd/m2 grating luminance. The root mean square (RMS) values of ocular high-order aberrations after refractive surgery were significantly greater one month post-operatively, decreasing three months following refractive surgery. The root mean square (RMS) values of ocular higher-order aberrations after LASIK and PRK were significantly greater one month following the surgery than before (the biggest increase was observed for the spherical aberration). At three months later there was a decrease in high order aberrations. Aberrations were higher for the LASIK group. There was no significant correlation among high order aberrations and contrast sensitivity. Conclusions: An increase in contrast sensitivity in post-PRK and post-LASIK eyes was observed, but this may be due to the training effect. When this was taken into account a depression in contrast sensitivity (mainly the higher spatial frequencies) one month post-operatively, was observed. Contrast sensitivity recovered to pre-operative levels at 3 months. Corneal surface irregularities after laser vision correction induce significant ocular aberrations apart from scattering errors. When retinal iluminance is steady, pre-operatively, bigger amounts of contrast sensitivity were found for 3mm diameter pupil. The decreased performance at 6mm may be attributed to the increased levels of ocular aberration, although no significant correlation was found between contrast sensitivity and ocular aberrations. When the measurements took place at 3 months following refractive surgery without the best corrected refraction a decrease in contrast sensitivity was evident, especially for the 6mm diameter pupil.
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Αξιολόγηση Ευαισθησίας Φωτεινής Αντίθεσης μετά από Διαθλαστική Χειρουργική
