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Contrasting the quality of refractive results
POSTOPERATIVE contrast sensitivity measurement following refractive surgery procedures provides a wealth of information about quality of vision. This information can be of immediate use to the operating surgeon, as well as providing useful data for researchers, Vikentia Katsanevaki MD PhD, told the XXI Congress of the ESCRS.Corneal topography does not provide enough data to interpret visual performance qualitatively, she said. And while routine measurement of contrast sensitivity can be time-consuming, a focused measurement of 6 and 12 cycles/degree provides excellent information about the patient's quality of vision without much added expense or consultation time, she pointed out."Contrast sensitivity is, in my opinion, the most sensitive indicator of optical performance after any kind of ophthalmic treatment," she stressed. Dr Katsanevaki explained that contrast sensitivity measures two variables defining quality of vision – the size of the image and contrast. Visual acuity testing measures only the size aspect of visual quality. Increasingly considered a key issue in refractive therapy, it is generally accepted that contrast sensitivity is altered after almost any kind of photorefractive treatment, she noted."As with everything else in refractive surgery, we don't know the exact cut off points, but at least for a few months after the procedure, contrast sensitivity is altered." Published reports of the times during which contrast sensitivity is altered following photorefractive treatments range from less than a week to more than six months. Photopic and mesopic contrast sensitivities also seem to return to normal at different speeds following some laser procedures, she noted."Although there is no hard proof, accumulated information indicates that contrast sensitivity returns slightly faster to eyes operated with PRK than with LASIK." The main sources of contrast sensitivity loss are optical aberration and haze. Defocus and other optical aberrations are expected to reduce contrast sensitivity at medium and high spatial frequencies, while haze is expected to attenuate contrast sensitivity uniformly across the full spectrum. Dr Katsanevaki presented the results of a prospective study of 43 eyes in which she estimated the change of ocular aberrations after myopic LASIK and correlated these changes to contrast sensitivity.
All eyes were treated with the Asclepion Meditec Mel 70. The ablation zone was 6.0 mm and the transition zone was 1.5 mm. All operations were uneventful and the postoperative refractive error was minor. However, Dr Katsanevaki recorded reduced contrast sensitivity in all patients. One month after the treatment, the mean contrast sensitivity was reduced to 85% of its preoperative values. The greatest decrease was noted on the spatial frequency of 6 cycles/degree. Both total and corneal aberrations increased post-surgery, in a proportional way – the higher the attempted correction, the greater the increase in aberrations. Total aberrations as were measured with the corneal aberrometer were increased by a factor of 1.44, whereas the calculated aberrations from the anterior corneal surface were increased by a factor of three. According to the videokeratography raw data, the MTF of the anterior corneal surface was reduced to 65% of its preoperative value. "There was a big discrepancy between the overall trend of aberrations and the corneal aberrations alone," she noted. "We found that the degradation of retinal image due to the anterior corneal surface alone (MTF) was worse than the actual decrease in contrast sensitivity. Total aberrations were better correlated to visual performance than corneal aberrations alone". Dr Katsanevaki and her team concluded that analysis of the aberrations of the anterior corneal surface does not provide sufficient information regarding visual performance after myopic LASIK. She suggested that this could be attributed to the lens-corneal aberration matching, which cannot be detected by corneal topography. Post-LASIK changes in corneal biomechanics involving protrusion of the posterior corneal surface may also play a role. Furthermore, contrast sensitivity is subject to neural editing which neither corneal topography nor aberrometers can detect. Her practical recommendation to her colleagues was that combined interpretation of corneal and total aberration is necessary to understand individual surgical outcomes that have a clear impact in visual performance. "Our clinical findings suggest that, one month after LASIK, contrast sensitivity is mainly decreased at 6 cycles/degree. We agree with previous investigators that reducing contrast sensitivity measurements to 6 and 12 cycles/degree provides sufficient information regarding visual quality. You can reduce the time needed to measure contrast sensitivity and still harvest all required information. Then you can add contrast sensitivity measurements to you routine service" Dr Katsanevaki's coauthors were N. Astyrakakis, Opt, H. Ginis, PhD and G. Kounis, Bsc. Vikentia Katsanevaki MD, PhD |
"We wanted to correlate total ocular aberrations to anterior corneal surface aberrations and to visual performance as described by contrast sensitivity," she said. The study measured total and 3rd and 4th order aberrations before, and one month after conventional myopic LASIK (attempted correction range: -2.5 to -8.7 D). Ocular aberrations were measured using laser ray tracing technique (Tracey Tec., Houston TX ) for a pupil aperture of 6 mm. "To calculate only anterior corneal surface aberrations we uploaded the raw data from the corneal topography to a customised optical software (ZEEMAX, Tuscon , Arizona , US ) and we calculated corneal aberrations from the anterior surface alone, not taking into account the posterior surface and the lens. Using the same optical software, we also calculated the modulation transfer function (MTF), which is the optical analogue of contrast sensitivity, for the anterior corneal surface. We compared the changes of contrast sensitivity as measured clinically one month after the myopic LASIK with the anterior corneal surface. MTF was calculated from corneal analysis raw data at the same postoperative interval. The clinical measurement of the contrast sensitivity was assessed with the CSV 1000 clinical testing," she explained.