The influences on mesopic visual function are many and range from lighting conditions to optics to the quality of the ocular media and health of the retina. LASIK's influences can be traced to its effects on the eye's optical properties, which, in turn, are greatly affected by pupil size.
Understanding the relationship between pupil size and visual function is crucial to surgeons' ability to manage patients' expectations preoperatively. The optical issues are well understood, and recent studies provide clinical correlates that show how these principles can be applied in practice. For instance, results from the FDA trials of the Allegretto excimer laser (WaveLight Laser Technologie AG, Erlangen, Germany) presented at the ASCRS meeting in April 20031 may be used to demonstrate the roles of pupil size, optical zones, and aberrations in subjective glare complaints.
First, large pupils worsen mesopic vision after LASIK if significant aberrations, especially spherical aberration, are present. Treating eyes with newer lasers that minimize the amount of induced spherical aberrations can improve mesopic function. By contrast, with older lasers that induce significant spherical aberration,2 mesopic complaints have increased with the higher treatment amounts.
Second, glare and mesopic visual complaints are always worse when the patient has residual refractive error, regardless of his pupil size. Larger pupils aggravate the problem, however. Oddly enough, larger pupils may result in better Snellen acuity in these eyes (as explained later), but the bottom line is that surgeons should consider enhancing eyes with refractive errors (even for small amounts) when patients complain of glare.
Third, hyperopic corrections result in more postoperative spherical aberration than do myopic treatments. Spherical aberration may impart better reading vision after hyperopic LASIK due to the multifocal effect of the aberrations, but patients should be cautioned about contrast loss in mesopic conditions.
For more than a century, ophthalmologists have been aware that defocus depends on pupil size and the location of aberrations on the cornea.3 When pupils dilate in dim lighting, aberrations induced by refractive surgery that are located in the midperiphery are able to enter the visual system and can worsen visual function.
Large pupils improve mesopic visual function by increasing the amount of light entering the eye. In the presence of aberrations (particularly spherical aberrations), large pupils can also improve image clarity in eyes with small refractive errors. For instance, an eye that is slightly hyperopic in the central cornea may be emmetropic toward the midperiphery in the presence of spherical aberration. A dilated pupil allows light rays to enter through the emmetropic portion of the eye. This situation decreases low contrast acuity, however, because only a portion of the entering light rays are in focus. Defocused rays do not contribute to visual acuity but may result in the perception of glare. In short, aberrations can improve acuity, but they decrease contrast.4DIFFERENCES BETWEEN MYOPES AND HYPEROPES
The SurgiVision Refractive Consultants, LLC's (Paradise Valley, AZ) clinical trials of the Allegretto laser were conducted over the past 2 years under strictly controlled conditions for both myopia and hyperopia. The trials included the use of a standardized questionnaire through which patients provided their subjective evaluations of night vision and glare.
For both myopes and hyperopes, the amount of overall glare decreased after treatment with the Allegretto laser, both in the presence of a glare source and during nighttime driving. These individuals actually possessed superior mesopic visual function after LASIK.
Although the mean scores improved, some scores worsened. We analyzed those subjects with worsened scores in an effort to determine any correlations. The most significant correlation for increased postoperative glare was residual refractive error, particularly astigmatism—for both hyperopic and myopic treatments.
Regarding eyes with emmetropic outcomes, pupil size did not correlate with complaints of worsened glare in myopic treatments. In hyperopic treatments, however, pupil size did correlate with worsened scores, and affected eyes were significantly more likely to have larger pupils than eyes with improved scores, even with emmetropic outcomes.
Explaining the Differences
In myopes, the Allegretto laser creates a true 6.5-mm optical zone and uses a “wavefront-optimized” profile that places extra treatment spots in the periphery to reduce spherical aberration (Figure 1). Because any blending occurs peripherally to the optical zone, a patient who achieves an emmetropic result basically has little or no spherical aberration within the 6.5-mm optical zone. The glare findings in this report suggest that that a 6.5-mm zone is adequate to preserve or improve mesopic visual function in most eyes undergoing myopic treatment.
During a hyperopic LASIK treatment, the optical zone is labeled as 6.5 mm, but the monofocal optical zone only extends to approximately 4.5 or 5.0 mm (Figure 2). Blending occurs beyond 6.5 mm, but corneal topography reveals that some blending starts within the 6.5-mm optical zone. Eyes generally have more spherical aberration following hyperopic treatment within the 6.5-mm zone. As explained earlier, pupil dilation in the presence of spherical aberrations results in dimmed vision and may explain the correlation of larger pupils with increased glare complaints in hyperopes.
Based on these results, it seems reasonable to counsel myopic patients who will be treated with an excimer laser that delivers a true 6.5-mm optical zone that their nighttime vision will likely improve provided they achieve a good refractive result.
Patients who complain of glare and have even small amounts of residual refractive error should consider undergoing an enhancement procedure. When performing LASIK enhancements with this sort of laser, surgeons can also be fairly confident that myopic patients will achieve nighttime vision superior to that which they possessed preoperatively.
By contrast, ophthalmologists need to counsel hyperopic patients who have pupils that dilate larger than 5 or 6 mm at night that they will likely experience some decrease in mesopic visual function due to the presence of spherical aberrations postoperatively. This decrease may be accompanied by better near visual performance owing to a multifocal cornea. It is not known at this point whether wavefront-driven ablations will be able to reduce induced spherical aberration in hyperopic treatments.
Refractive surgeons should understand the difference between clarity of vision and mesopic visual function and realize that they may work at cross-purposes. The presence of aberrations can improve depth of focus but may worsen mesopic vision. Many refractive surgeons are now intentionally using smaller optical zones in presbyopic patients in order to create spherical aberrations that will improve patients' reading vision. These physicians must be aware of the fact that this technique will have an equal and opposite effect on patients' nighttime vision and provide appropriate preoperative counseling.Guy M. Kezirian, MD, FACS, is a partner in SurgiVision Refractive Consultants, LLC, which serves as the sponsor of WaveLight's US FDA trials. Dr. Kezirian may be reached at (480) 348-9299; firstname.lastname@example.org.
1. Kezirian GM. Pupil size, optical zones, and aberrations: determinants of mesopic visual function. Paper presented at: ASCRS/ASOA Symposium on Cataract, IOL and Refractive Surgery; April12, 2003; San Francisco, CA.
2. Haw WW, Manche EE. Effect of preoperative pupil measurements on glare, halos, and visual function after photoastigmatic refractive keratectomy. J Cataract Refract Surg. 2001;27:907-916.
3. Holladay JT, Lynn MJ, Waring GO 3d, et al. The relationship of visual acuity, refractive error, and pupil size after radial keratotomy. Arch Ophthalmol. 1991;109:70-76.
4. Holladay JT, Dudeja DR, Chang J. Functional vision and corneal changes after laser in situ keratomileusis determined by contrast sensitivity, glare testing, and corneal topography. J Cataract Refract Surg. 1999;25:663-669.