MART?NEZ ET AL should be commended for attempting to analyze potential visual complaints associated with photorefractive keratectomy (PRK). Their study is well performed and the article concise and clearly written. The reader, however, (as for all studies) needs to determine whether the study design is appropriate and if the conclusions are clinically relevant. In this article, Martínez et al compare the optical aberration of the anterior corneal surface after PRK with a 5-mm optical zone with 3-and 7-mm pupil sizes to the preoperative cornea. There are many problems with the study design that may limit the clinical applicability.

It has become common to discuss the visual degradation and optical effects of refractive surgery. Often, as in this study, comparisons are made to the preoperative cornea. While the unaltered, preoperative cornea may perform well on an optical bench, it has serious short-comings for the patient with a refractive error. Patients are not comparing the risks and benefits of refractive surgery to their unaided vision, because their unaided vision is unacceptable. It is not comforting to the 10.0-diopter myope to know that their cornea has minimal spherical and coma-like optical aberration. What the patient knows is that without glasses or contact lenses they cannot see, regardless of the optical performance of their cornea. Both spectacles and contact lenses induce their own visual side effects. Contact lenses have defined optical zones, are often decentered, and may rotate causing visual degradation in patients with astigmatic correction. Contact lenses are also associated with halos, glare, and night vision disturbances. Spectacles have their own inherent optical problems, including minification, fogging, and peripheral distortion (for the higher corrections). What needs to be analyzed is the optical performance not of the unaided cornea, but the optically corrected, preoperative cornea. It may be that spectacles and/or contact lenses exhibit a similar increase in optical aberration and that PRK compares favorably. Subjective data from VISX's PRK with astigmatism premarket approval showed that 31% of patients preoperatively complained of difficulty with night driving and that this number was reduced to 19% to 26% postoperatively (not statistically significant). These numbers are not used to suggest that refractive surgery improves night driving, but to show that visual complaints are common to all forms of optical correction. Comparing the post-operative cornea with the unaided preoperative cornea is not germane, since patients who do not require a correction do not undergo PRK. I have no doubt that the patient surgically modified for emmetropia does not perform as well as the naturally occurring emmetrope. But this is an unfair comparison. The amputee who walks with an artificial limb is probably willing to accept a small limp in exchange for the freedom of ambulation.

Second, this is a retrospective study on a series of patients who had surgery with a 5-mm optical zone (model 2020B, VISX, Santa Clara, Calif). Many of the studies citing a high percentage of complaints were with 5-mm optical zones and one study cited was with a 4-mm optical zone. One of the benefits of our regulatory environment is that these smaller optical zones were never approved in the United States and the results (with a 5-mm optical zone) may not be applicable or even comparable. This is brought out by Martínez et al in their comments: “They also demonstrated that these effects were ameliorated to some extent for the largest (6-mm) treatment zone”. Larger optical zones would logically appear preferable. However, larger diameters require a deeper ablation that may mitigate some of its advantages. Multizone or aspherical ablation profiles may also offer additional advantages or disadvantages. The optimal ablation diameter and profile remains unknown. What is known, however, is that small ablation zones are associated with clinically significant patient complaints and that many of these complaints have been ameliorated by the use of 6-mm optical zone.

Additionally, Martínez et al should study more simulated pupil sizes than the 3-and 7-mm sizes analyzed. The implied assumption here is that when the pupil enlarges past the optical zone (here 5.0 mm), that optical aberration increases substantially. What is unknown, is if this is a gradual increase or a sudden increase once you reach the transition between the treated and nontreated cornea. The authors should study additional pupil sizes to answer this question. This is clinically important. If the change is sudden, than a larger optical zone or perhaps a transition zone may be advantageous. If the change is more gradual as the pupillary size increases within the treatment zone, than the benefit of further enlarging the optical zone, beyond a certain point, may be limited.

Finally, it seems as if the authors' calculations are based on derived elevation data. While this is probably acceptable for the preoperative cornea, slope-derived elevation data have been shown to have a high error in the postsurgical cornea, particularly outside the ablation zone. Since in this study the optical zone was 5.0 mm and the pupillary size up to 7.0 mm, it is possible that the peripheral computed elevation data were associated with a significant degree of error.

Rarely has a medical procedure been more carefully studied than PRK. The Food and Drug Administration, to its credit, never approved the smaller optical zones previously available to our non-American counterparts. For many patients, the excimer laser is a miracle of modern medical science. Most miracles come with some strings attached.

投稿日期截止到1999年1月5日前