System improves accuracy of refractions before, after LASIK

November 1, 2004

San Diego-The WaveScan System (VISX) may be a valuable method to measure refractions accurately and objectively before LASIK.

San Diego-The WaveScan System (VISX) may be a valuable method to measure refractions accurately and objectively before LASIK.

"A number of things have changed in treatments: incorporation of the wavefront refraction into treatments, the ability to modify and use the physician adjustment, and the ability to change the diameter and profile of the ablation if desired," said Sandy Feldman, MD, at the annual meeting of the American Society of Cataract and Refractive Surgery.

This study was conducted in normal eyes before patients underwent LASIK to validate the accuracy of wavefront-derived refractive error and to compare the results obtained with the WaveScan System with those obtained from manifest and cycloplegic refractions.

The preoperative refraction in this patient group ranged from mild to severe myopia. The mean postoperative refraction, which included three eyes corrected for monovision, was about -0.43 D; when the three monovision eyes were excluded the postoperative refraction was -0.29 D. All treatments were conventional, because the study was performed before customized treatment was approved.

"The most common cases I encounter in my practice are the patients who are dissatisfied, not because of night glare or subjective complaints, but those with decreased Snellen acuity because they need enhancements," Dr. Feldman said. "This is what prompted me to undertake this study before we had the ability to treat patients with customized treatments.

"When we evaluated the manifest and cycloplegic refractions, we found that the manifest refraction was slightly more myopic," Dr. Feldman said. "When comparing the manifest and the wavefront refractions, the manifest refractions were more myopic, and that depended on the pupil diameter of the WaveScan reading. Smaller pupil sizes were more minus."

Real-world measurements Dr. Feldman also pointed out that this is the first time refractions could be measured as the pupil size changes in a dynamic situation.

"This is important because this is what happens in real-world situations," Dr. Feldman emphasized.

"The cycloplegic refractions also were more minus compared with the wavefront refractions," Dr. Feldman said. "The wavefront refractions seemed to approximate the cycloplegic refractions. Smaller pupils on the WaveScan System again were more minus."

The most differences were found between the 6- and the 3-mm pupil sizes, as expected. As the pupil size decreased, the amount of myopia increased.

When the investigators evaluated astigmatism, they found that the manifest refraction had more astigmatism compared with the wavefront-derived refraction. All measurements fell within about 0.25 D. There were some outliers in the 3-mm pupil size at all levels tested. The cycloplegic astigmatism was also slightly greater than the WaveScan level of astigmatism, but all were within 0.25 D. Preoperatively, the astigmatism readings were clustered, with some outliers at the 3-mm pupil size.

Another evaluation When they evaluated the data from another WaveScan system preoperatively, the investigators found that the WaveScan-derived refractions were similar to the manifest refractions for all pupil sizes tested and that the results were slightly less myopic than the manifest refraction. The astigmatism values were also clustered and there was one outlier with greater than 0.5 D of difference.

Postoperatively, the results differed slightly, with the WaveScan refraction within 0.5 D in most eyes. There were, however, a few eyes with more accommodation, Dr. Feldman noted, and he stressed the importance of avoiding accommodation during the measurements.

When they evaluated the manifest refraction compared with the WaveScan refraction with various pupil sizes, they found that the results were similar postoperatively with all pupil sizes. The postoperative astigmatism also was within 0.5 D in all patients.

"The WaveScan has excellent accuracy in predicting refractions, both before and after LASIK. We need to watch for accommodation in all eyes, especially after LASIK," Dr. Feldman reported.