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EvaluatingRefractive Surgery
What Is the Appropriate Baseline?
MART?NEZ ET AL should be commended forattempting to analyze potential visual complaints associated with photorefractivekeratectomy (PRK). Their study is well performed and the article concise and clearlywritten. The reader, however, (as for all studies) needs to determine whether the studydesign is appropriate and if the conclusions are clinically relevant. In this article,Martínez et al compare the optical aberration of the anterior corneal surface afterPRK with a 5-mm optical zone with 3-and 7-mm pupil sizes to the preoperative cornea. Thereare many problems with the study design that may limit the clinical applicability.
It has become common to discuss the visual degradation and optical effects ofrefractive surgery. Often, as in this study, comparisons are made to the preoperativecornea. While the unaltered, preoperative cornea may perform well on an optical bench, ithas serious short-comings for the patient with a refractive error. Patients are notcomparing the risks and benefits of refractive surgery to their unaided vision, becausetheir unaided vision is unacceptable. It is not comforting to the 10.0-diopter myope toknow that their cornea has minimal spherical and coma-like optical aberration. What thepatient knows is that without glasses or contact lenses they cannot see, regardless of theoptical performance of their cornea. Both spectacles and contact lenses induce their ownvisual side effects. Contact lenses have defined optical zones, are often decentered, andmay rotate causing visual degradation in patients with astigmatic correction. Contactlenses are also associated with halos, glare, and night vision disturbances. Spectacleshave their own inherent optical problems, including minification, fogging, and peripheraldistortion (for the higher corrections). What needs to be analyzed is the opticalperformance 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 opticalaberration and that PRK compares favorably. Subjective data from VISX's PRK withastigmatism premarket approval showed that 31% of patients preoperatively complained ofdifficulty with night driving and that this number was reduced to 19% to 26%postoperatively (not statistically significant). These numbers are not used to suggestthat refractive surgery improves night driving, but to show that visual complaints arecommon to all forms of optical correction. Comparing the post-operative cornea with theunaided preoperative cornea is not germane, since patients who do not require a correctiondo not undergo PRK. I have no doubt that the patient surgically modified for emmetropiadoes not perform as well as the naturally occurring emmetrope. But this is an unfaircomparison. The amputee who walks with an artificial limb is probably willing to accept asmall limp in exchange for the freedom of ambulation.
Second, this is a retrospective study on a series of patients who had surgery with a5-mm optical zone (model 2020B, VISX, Santa Clara, Calif). Many of the studies citing ahigh percentage of complaints were with 5-mm optical zones and one study cited was with a4-mm optical zone. One of the benefits of our regulatory environment is that these smalleroptical zones were never approved in the United States and the results (with a 5-mmoptical zone) may not be applicable or even comparable. This is brought out by Martínezet al in their comments: “They also demonstrated that these effects were ameliorated tosome extent for the largest (6-mm) treatment zone”. Larger optical zones would logicallyappear preferable. However, larger diameters require a deeper ablation that may mitigatesome of its advantages. Multizone or aspherical ablation profiles may also offeradditional advantages or disadvantages. The optimal ablation diameter and profile remainsunknown. What is known, however, is that small ablation zones are associated withclinically significant patient complaints and that many of these complaints have beenameliorated by the use of 6-mm optical zone.
Additionally, Martínez et al should study more simulated pupil sizes than the3-and 7-mm sizes analyzed. The implied assumption here is that when the pupil enlargespast the optical zone (here 5.0 mm), that optical aberration increases substantially. Whatis unknown, is if this is a gradual increase or a sudden increase once you reach thetransition between the treated and nontreated cornea. The authors should study additionalpupil sizes to answer this question. This is clinically important. If the change issudden, than a larger optical zone or perhaps a transition zone may be advantageous. Ifthe change is more gradual as the pupillary size increases within the treatment zone, thanthe 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 elevationdata. While this is probably acceptable for the preoperative cornea, slope-derivedelevation 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 mmand the pupillary size up to 7.0 mm, it is possible that the peripheral computed elevationdata were associated with a significant degree of error.
Rarely has a medical procedure been more carefully studied than PRK. The Food andDrug Administration, to its credit, never approved the smaller optical zones previouslyavailable to our non-American counterparts. For many patients, the excimer laser is amiracle 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 , 百拇医药