Wavefront analysis has an expanding role in diagnosis

Early results are promising for this emerging technology, which may one day become a standard diagnostic tool.

ByRichard L. Lindstrom, MD

Wavefront analysis has become a key part of the ophthalmic lexicon. Many of us are now taking a closer look at this emerging technology and gaining a better understanding of how it can serve our patients. Here at Minnesota Eye Consultants, we have had the opportunity to work with the Visx WaveScan. In addition to the potential for improving LASIK outcomes, this technology can provide practitioners with valuable insight to aid on the diagnostic front.

Wavefront analysis has already assisted us with numerous diagnostic dilemmas, proving helpful with patients who have come in with symptoms of unknown cause. We further believe that wavefront also holds great promise in the therapeutic area, for helping us to bring patients whose acuity has suffered as a result of early refractive surgery back up to visual par and beyond.

Wavefront analysis

Wavefront systems can provide physicians with precise measurement of both higher- and lower-order aberrations. The Visx Star S3 uses Shack-Hartmann principles, which allows for the measurement and analysis of higher-order aberrations. Aberrations are measured in light scattered from the retina.

With this technology, an infrared laser places a spot on the retina, which is reflected through the eye and focused by a lenslet array. The resulting image is then compared to a uniform spot pattern to determine what deviations are present in each portion of the eye and to what degree. The more uniform the pattern, the less the wavefront error. If the Shack-Hartmann sensor receives scattered light, which is dull, distorted and nonuniform, this will account for a higher wavefront error.

What we are hoping to accomplish with wavefront-guided refractive surgery is to correct this wavefront so it will produce a uniform light path with a flat wavefront.

Today’s usage

We have not yet reached the point where the majority of physicians can apply this technology to LASIK. The question for many now is if there is any reason to have this instrument on hand if they are not currently participating in one of the Food and Drug Administration clinical trials. We believe that the answer is yes. We have found this technology to be very useful, from both a screening and a diagnostic perspective. Most of the time we see patients who have minimal higher-order aberrations and we have felt comfortable treating those patients with a standard LASIK approach.

However, there are other patients who come in with acuity that can only be best corrected to 20/25. When we evaluate them with the Shack-Hartmann WaveScan sensor, we can determine that they have a large amount of higher-order aberrations. We believe that physicians can best serve these patients by suggesting they wait for upcoming wavefront-driven custom ablation.

We have found that wavefront can also be invaluable in the diagnostic evaluation of patients with problem eyes who return to the ophthalmic office with complaints of halos or inadequate night vision. These patients have often consulted with several different ophthalmologists, all of whom were unable to determine the cause of their complaints.

With wavefront, we can measure the higher-order aberrations and determine what percentage of the patient’s problems are related to residual astigmatism and spherical error and what percentage are linked to these higher-order aberrations. In our experience, patients find it extremely reassuring when we can pinpoint the problem and tell them they have a certain amount of coma or a certain amount of trefoil. Just being able to determine for the patient which of the higher-order aberrations correlates to what complaint can make an enormous difference for them, even if we cannot yet correct the problem.

Therapeutic re-treatment

However, some inroads are being made regarding treatment of these patients who have had difficulties resulting from early refractive surgery. Julian Stevens, MD, at Moorfields Hospital in London, has been studying therapeutic wavefront-guided ablation, using the Star S3 to re-treat a small number of patients who have experienced loss of best corrected visual acuity after undergoing LASIK. This preliminary study included five eyes of five patients, with major decentrations, who have lost two or more lines of BCVA.

After undergoing re-treatment with the Star S3, these patients experienced a significant improvement in their visual function and acuity. At the 3-month mark, three of these patients showed improvement of two or more lines of BCVA.

Manifest refractions

The technology is also doing well in normal subjects, showing itself to have a high degree of accuracy as an objective refractor. Douglas Koch, MD, recently conducted a study comparing Wave-Scan results from the Star S3 with manifest refraction, considered to be the gold standard here. The study included 69 eyes of 42 patients. These patients included individuals with normal eyes, as well as those who had undergone refractive procedures such as LASIK, photorefractive keratectomy, Intacs and limbal corneal relaxing incisions.

Thirty-five eyes underwent three consecutive WaveScan measurements. These measurements showed themselves to be very reproducible. There was a mean difference of 0.13 D spherical equivalent, 0.14 D sphere and 0.08 D cylinder.

When WaveScan results were compared to manifest refraction, the Star S3 also showed itself to have a high degree of accuracy. There was a mean difference just –0.26 D spherical equivalent, –0.12 D sphere and just –0.28 D cylinder. The only difficulties were in obtaining measurements in cases involving patients with abnormal corneal surfaces or with those who had previously undergone keratoplasty and who were left with large amounts of irregular astigmatism. Dr. Koch and colleagues were pleased by the accuracy and reproducibility of these results obtained with the Star S3 WaveScan device.

We also believe that early results are promising. We think wavefront is likely to become a very useful diagnostic tool, similar to the role that topography has played. Initially with topography, it was only the occasional consultative surgeons who acquired the units. Once everyone recognized the benefits this technology could provide, however, its role expanded dramatically. I think we are now in the beginning of this same type of transition period for wavefront. Very soon, we are all likewise going to find these wavefront analysis systems to be indispensable to our practices.

For Your Information:

Richard L. Lindstrom, MD, can be reached at Minnesota Eye Consultants, 710 E. 24th St., Suite 106, Minneapolis, MN 55404 U.S.A.; +(1) 612-813-3600; fax +(1) 612-813-3660; e-mail: rllindstrom@mneye.com. Dr. Lindstrom is a paid consultant for Visx.

Visx Inc., manufacturer of the Star S3 ActiveTrak laser, can be reached at 3400 Central Expressway, Santa Clara, CA 95051-7122 U.S.A.; +(1) 408-733-2020; fax: +(1) 408-773-7278.