Portal de Periódicos da CAPES

Night vision after LASIK: the pupil proclaims innocence

2004; Elsevier BV; Volume: 111; Issue: 1 Linguagem: Inglês

10.1016/j.ophtha.2003.09.021

1549-4713

Stephen D. Klyce,

Glaucoma and retinal disorders

Seldom do we question our intuitive notions, particularly when we have the advantage of high-end technology with which to reach obvious conclusions. But one of our paradigms in refractive surgery may be about to fall. The current standard of care for refractive surgery includes the measurement of pupil size, as a large pupil size has been said to put patients at a higher risk for night vision disturbances. In a comprehensive literature review of radial keratotomy, photorefractive keratectomy (PRK), and LASIK issues, Fan-Paul et al1Fan-Paul N.I Li J Miller J.S Florakis G.J Night vision disturbances after corneal refractive surgery.Surv Ophthalmol. 2002; 47: 533-546Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar reported that the peer-reviewed literature overwhelmingly cites a large pupil coupled with a small clear zone (radial keratotomy) or optical zone (PRK, LASIK) as a dominant factor leading to increased night vision disability. However, recent evidence suggests that large daytime pupil size is much less critical than previously supposed. In this issue, Pop and Payette2Pop M, Payette Y. Risk factors for night vision complaints after LASIK for myopia. Ophthalmology 2004;111:3–10Google Scholar present a comprehensive evaluation of night vision complaints (NVCs) in nearly 1500 LASIK procedures; they found no correlation between NVCs and pupil size or NVCs and the difference between pupil and optical zone sizes. In this editorial, these findings are examined in detail in an effort to emphasize their clinical relevance. Pop and Payette used a psychometric test designed to determine the severity of symptoms often associated with night vision: halos, starburst, and acuity reduction. Patients were asked to grade their symptoms as none, mild, moderate, or disturbing. The study separated the patients into 2 groups: those with clinically important NVCs with moderate or disturbing symptoms and those with milder symptoms. Variables included pupil size; optical zone size; transition zone size; patient age; degree of attempted correction; preoperative average keratometry; postoperative spherical equivalent; and the difference variables derived from pupil, treatment, and optical zone sizes. Data were collected at 1 day and 1, 3, 6, and 12 months after surgery. The prevalence of NVCs dropped from 25.6% at 1 month to 4.7% at 12 months. Compared with the asymptomatic group, the NVC group had significantly higher attempted correction, older age, smaller optical zone size, and a postoperative spherical equivalent greater than ±0.5 diopters (D). Importantly, there was no significant difference at baseline in mesopic pupil size between the 2 groups (6.3 mm NVC vs. 6.6 mm asymptomatic; P = 0.18). In contrast to most of the earlier studies, even when the data were stratified to include only those patients with large pupil sizes (≥7 mm) and greater attempted correction (>5 D), age over 50, or smaller optical zones, pupil size was still not a risk factor for NVCs. Although multiple regression analysis revealed a strong correlation between age and pupil size, pupil size did not correlate with increased risk of NVCs after LASIK. It seems curious that age correlated with pupil size and NVCs, but that pupil size did not. Must we assume that the substantial body of prior evidence to the contrary was wrong? Not at all. We may be detecting procedural evolution. Third-generation laser algorithms, such as used in the Pop and Payette study, may have solved the red ring problem seen at the transition zone with tangential power maps on topography from early renditions of PRK and LASIK profiles. The newer lasers not only attempt to maximize the optical zone, but also produce a transition zone to blend the principal curvature of the optical zone smoothly into the curvature of the peripheral cornea. As a result, it is quite possible that the former importance of pupil size no longer obtains. One caveat: until pupil size has been convincingly shown not to be a risk factor in NVCs for all refractive surgical procedures, we need to remember that preoperative measurement of pupil size is still the standard of care—don't put that pupillometer into storage just yet! It is interesting that there were more than a quarter of the patients in the NVC group 1 month after surgery, but only 5% at 1 year. The question is whether the residual 5% of NVC cases were due to the surgery or simply represent the prevalence of NVCs in an age-matched nonsurgical population. Most people find it more difficult to drive at night than during the day, and it could well be that given the same questionnaire, 5% of untreated patients would complain of night vision disturbances. Is age really more a risk factor for NVCs after LASIK, or is the risk the same without the surgery? The reduction in NVCs from 1 month to 1 year was quite substantial, and mirrors the reduction in spherical-like aberrations Endl et al3Endl M.J Martinez C.E Klyce S.D et al.Effect of larger ablation zone and transition zone on corneal optical aberrations after photorefractive keratectomy.Arch Ophthalmol. 2001; 119: 1159-1164Crossref PubMed Scopus (94) Google Scholar measured from corneal topography using a 7-mm pupil to simulate night-time conditions. These reductions probably correlate with slow shape changes in the transition zone. The reduction in NVCs may also be due in part to the thesis put forth by Wilson4Wilson S.E Wave-front analysis are we missing something?.Am J Ophthalmol. 2003; 136: 340-342Abstract Full Text Full Text PDF PubMed Scopus (13) Google Scholar that, when aberrations are induced by refractive surgery, there can be a slow adaptation by the central nervous system. Although neural plasticity of the visual system may vary among individuals, central compensation may be responsible for some of the reduction in NVCs with time after refractive surgery. Finally, Pop and Payette conclude that future wavefront studies characterizing higher order aberrations might be helpful in understanding the sources of NVCs. The authors have the corneal topography data to do this. Because the plan for LASIK includes only modification of the cornea, ray-tracing programs such as VOL-CT (Sarver and Associates, Celebration, FL) can be used to provide wavefront information from corneal topography including root mean square error, Zernike coefficients, and the point spread function. In addition, it would be helpful to measure postoperative flap centration and size, as there is a growing consensus among refractive surgeons that flap parameters play a role in NVCs.5Ambrósio R, Schallhorn SC, Wilson SE. The importance of pupil size in refractive surgery. Refract Surg Outlook 2002 Winter:1–4Google Scholar Ablation centration and corneal surface optical quality indexes available from topography should be correlated to NVCs, particularly because night vision disturbances correlate so strongly with attempted correction. In summary, pupil size does not seem to be a risk factor for NVCs after LASIK, at least with modern laser algorithms that optimize optical and transition zone sizes. Age still appears to have an important role in this arena, but is the risk really attributable to LASIK, or does the general population have the same 5% prevalence of NVCs? What roles do changes in the transition zone and central neural adaptation play in the slow decrease in NVCs? What is the contribution of flap size and centration issues to NVCs? Like many other careful studies, the article by Pop and Payette may leave us with more questions than answers, and further study of risk factors for NVCs in the rapidly evolving field of laser refractive surgery is essential.