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Barbara Boughton in San Francisco, US

PREOPERATIVE myopia is more predictive of a superior outcome following LASIK than preoperative K value, reported Canadian researchers at the annual ASCRS Symposium on Cataract, IOL and Refractive Surgery.

Investigators at Montreal’s McGill University measured the two-year postoperative vision quality of 51 eyes which had undergone myopic LASIK procedures (Bausch & Lomb 217c).

They divided the patients into four groups based on their preoperative myopia and K values. The four groups were: patients with high K values (44 or above) and lower myopia (less than -5.0 D); high K values and higher myopia (at least -5.0 D); lower K (less than 44) and lower myopia; and lower K and higher myopia.

All patients in the study achieved 20/20 vision after their LASIK surgeries as measured by Snellen acuity testing. But there were clear differences among patients in terms of their contrast sensitivity scores as measured by their ability to distinguish different size sine-wave gratings, reported Leon Solomon MD.

"Our study indicates that a patient who walks into the office at -8.0 D has a lesser chance of getting normal contrast sensitivity after LASIK than those who walk in at -3.0 D," Dr Solomon said.

Patients with higher K values and lower myopia achieved the best contrast sensitivity scores - 93% of patients had contrast sensitivity equivalent to 20/20 or better. A total of 75% of those with lower K and lower myopia attained contrast sensitivity equivalent or better than 20/20.

But the two groups with higher myopia had the worst outcomes. Only 28% to 36% of those patients achieved contrast sensitivity equivalent to 20/20 or better. However, a majority of these patients - about 65% to 70% - achieved 20/30 vision.
"These are issues ophthalmologists should be aware of in order to counsel their patients," Dr Solomon said.

"If a patient comes in at -8.0 D or -9.0 D and asks for LASIK, you should probably tell them they will have to sacrifice a little contrast sensitivity. But they will probably not suffer much. The will just lose a little contrast sensitivity - they will not see as well in dull light."
None of the patients in Dr Solomon’s study have complained of contrast sensitivity problems, he added.

Dr Solomon stressed that preoperative K values had no predictive value for superior visual outcomes. Some 61% of those with higher K had contrast sensitivity equivalent to 20/20 or better, compared with 57% of those with lower K.
"Preoperative K didn’t make a difference in visual function after LASIK. Rather it was the degree of myopia that was important," he said.

"This is exactly what our findings have been using the Allegretto laser. Degree of myopia is the important predictive value for visual function after LASIK," agreed researcher Guy M Kezirian MD, US.

Contrast sensitivity testing has come to the fore recently as a measure of quality of vision. The Snellen visual acuity test is valuable for obtaining refractive values, but doesn’t reliably measure contrast or quality of vision. Contrast sensitivity results tend to correlate better with patient satisfaction after LASIK, Dr Solomon emphasised.
In fact, in his own study, all patients achieved BCVA of 20/20 or better using Snellen, but only 59% had contrast sensitivity scores equivalent to 20/20 or better.

"Different values in sine wave gratings show us the patient’s abilities in daily vision tasks. Thus, contrast sensitivity is a simple method of determining visual function. It is an under-utilised test, but it is simple and can be done in your office in two minutes," Dr Solomon said.

The sine wave grating evaluation uses a repeated number of blurry dark and white cycles, with the amount of cycles of a grating over a specified visual angle being equal to a spatial frequency.

The higher the spatial frequency, the smaller the image size expressed in cycles per degree. The patient’s ability to distinguish different size sine wave gratings is indicative of better quality vision - the capability to distinguish objects of different sizes and at different levels of contrast, he explained.

Leon Solomon MD
McGill University, Montreal, Canada
Email: Jeanne_young41@hotmail.com

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