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Up Front | Jan 2002

Deciphering Wavefront Higher-Order Aberrations

New research offers some unexpected and provocative wavefront data.

Last year, we conducted a study that examined patients with “supernormal” vision. The location of the study was the US Naval Air Station in Fallon, Nevada, the home of Top Gun, chosen due to the likelihood of the subjects having excellent vision. The goal was to understand the aberrations, particularly higher-order aberrations, of eyes with excellent vision. We all understand lower-order aberrations, classified as Zernike polynomials of defocus and two terms for astigmatism, and we can manipulate them regularly with glasses, contact lenses, intraocular lenses, or refractive surgery. However, with the advent of wavefront devices that can quantify all of the optical properties of the eye, our understanding of higher-order aberrations in its infancy. This is a field full of unanswered and even as yet unposed questions, but also incredible and exciting potential clinical applications.

STUDY PARAMETERS
We studied 142 eyes of 73 aviators, both pilots and non-pilot aviators, such as radar intercept officers, at the Navy Fighter Weapon School in Fallon. Supernormal vision was defined as 20/12.5 or better on a 4 m ETDRS eye chart. The best, or lowest vision that we could measure on this eye chart was 20/10. Every subject had a BCVA of 20/20 or better but there were differences in UCVA. Pilots tended to have an average UCVA of better than 20/16, while non-pilots were generally worse than 20/20. Wavefront measurements were performed on the subjects using the VISX® WaveScanTM Wavefront System (Santa Clara, CA), and Zernike terms were then extracted into a database. When analyzing wavefront data there are many factors that must be taken into consideration, but one of the most critical is pupil size. We used a standard 6-mm entrance pupil to derive the Zernike terms. This limited the analysis to those subjects with a pupil size of 6 mm or larger (we included 81 eyes of 48 subjects).

ACADEMIC HYPOTHESES VS. TEST RESULTS
Current approved forms of refractive surgery generally achieve significant improvement in uncorrected vision with conventional refraction. The visual benefit is derived by correcting only three of the Zernike aberration terms (defocus and two astigmatic modes). Another way to say this is that the goal is achieved by reducing or eliminating the lower-order wavefront aberrations. It would be logical to assume that the higher-order aberrations may also impair vision. Patients might derive added benefit from correcting not just their lower-order, but also their higher-order aberrations. Yet, the interesting result of our study suggests that it may not be possible to achieve supernormal vision by correcting higher-order aberrations in otherwise healthy eyes. Our findings showed that there was considerable variation in the amount of higher-order aberrations irrespective of vision. Subjects with excellent vision did not tend to have lower root mean square (RMS) values of higher-order aberrations compared to those with what we consider average vision. Most of our comparisons showed no difference between the groups. One interesting analysis is a pairing of vision categories that showed just the opposite. Subjects with excellent UCVA, 20/12.5 or better, had more higher-order aberrations, on average, than those who had a UCVA of 20/20 or worse. We then deconstructed the higher-order wavefront patterns of this pairing into Zernike polynomials to identify specific terms that may be characteristic of people who have better UCVA.

REFINING THE MEANING OF TEST RESULTS
We conducted further analysis to better understand this somewhat counterintuitive UCVA result. The lower-order terms conformed to conventional wisdom. With the exception of hyperopes and the ability of this population to accommodate, subjects with worse UCVA had greater amounts of defocus. We then looked closely at the higher-order terms, labeled Z-6 and above. With one notable exception, all of the terms (spherical aberration, trefoil, etc.) were similar between the two groups. However, one third-order aberration, called vertical coma, was statistically different in the two groups, and it was elevated in the supernormal vision group. Figure 1 shows the average absolute value of each term.

Of course, the next logical question is why. Perhaps a small amount of vertical coma might help a person read the high contrast eye chart in the vertically oriented English optotypes. Thus we are selecting patients with more vertical coma who, by definition, tend to have supernormal vision. Or, perhaps our results were due to sampling error. There is considerable ?subject to subject? variation in higher-order aberrations, much the same way there is variation in lower-order aberrations, which increases the chance of a sampling error. Another possible explanation is the difference in aberrations between the pupil size when the vision was tested and the wavefront derivation using a 6-mm pupil. In this study, Zernike terms were derived using a 6-mm entrance pupil while the subjects were being imaged by the wavefront device. This ensured that all comparisons were performed with the same standard pupil size. In addition, there was no difference in average pupil size between the two groups measured with a pupillometer under room light conditions. However, the actual pupil diameter for a given subject when they viewed the eye chart could have been larger or smaller than 6 mm. For instance, a subject could have a 4-mm pupil as they viewed the eye chart under room light conditions, but a 6-mm pupil when their wavefront was measured. The patient could have excellent vision with his 4-mm pupil, but also have higher levels of aberrations when the pupil dilated to 6 mm for wavefront measurements. This is a confounding variable in our study and one that requires further investigation.

IN CONCLUSION
We did not find that people with supernormal vision have lower levels of higher-order aberrations compared to those with average vision. This suggests that supenormal vision is not primarily determined by the optics of the eye. One analysis suggests that a certain amount of vertical coma is associated with improved UCVA. But our definition of supernormal vision did not consider other important aspects of vision, such as contrast sensitivity. In addition, subtle higher-order aberrations likely play a more significant role in night vision when the pupil is dilated. As our exploration of the higher-order aberrations of the US Navy aviators demonstrates, much more work is needed to understand the role of higher order aberrations in visual perception.

CDR Steven C. Schallhorn, MD, is the Director of Cornea and Refractive Surgery at the Naval Medical Center, San Diego, California. Dr. Schallhorn may be reached at (619) 532-6700; scschallhorn@nmcsd.med.navy.mil Material extracted from Dr. Schallhorn's presentation at the AAO meeting's ”Best of American Society of Cataract and Refractive Surgery” program, delivered on November 12, 2001 in New Orleans, Louisiana.
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