EyeWorld Asia-Pacific March 2011 Issue

19 EW REFRACTIVE March 2011 Understanding straylight by Matt Young EyeWorld Contributing Editor New research reveals factors influencing the refractive phenomenon W hile mystery still shrouds the causes of intraocular straylight, new research has found some answers about what improves versus what exacerbates the condition. Although straylight does change after LASIK (with some reports showing improvement), according to a study published in the March issue of Current Eye Research, increased straylight could occur if anterior corneal cell density is diminished. “Subjects in whom intraocular straylight improved also showed less cell loss and this correlation was more significant at 6 months after refractive surgery,” reported Amelia Nieto-Bona, OD, Department of Optics II, University School of Optics, Universidad Complutense de Madrid, Madrid, Spain. Corneal injury may be important because it could affect light scattering in the cornea. “Both clinical and experimental data have indicated that corneal keratocytes may show intense light scattering in injured corneas,” Dr. Nieto-Bona noted. Understanding straylight causes Intraocular straylight worsens quality of vision by reducing contrast of an image on the retina, according to Dr. Nieto-Bona. “In the theoretical eye, there is no intraocular straylight since the ocular media are optically perfect,” Dr. Nieto-Bona noted. “In the real eye, many of its structures, such as the cornea, iris and sclera, the eye lens, and the fundus, can cause light scattering.” In this study, straylight improved after LASIK, although questions remain about whether factors like haze after refractive surgery could worsen straylight. Results were based on an analysis of 25 eyes of 30 patients who underwent LASIK. Dr. Nieto-Bona analyzed structural changes in the cornea as well as straylight using a third generation van den Berg straylight meter. “After LASIK, AKD [anterior keratocyte density] was reduced by 19.12 and 11.72% at 15 days and 6 months compared to before surgery,” Dr. Nieto-Bona noted. PKD (posterior keratocyte density) also decreased after LASIK, but the difference was only statistically significant 15 days post-op. Meanwhile, mean log(s) straylight values were 1.03±0.26 pre-op, 0.87±0.14 at 15 days post- op, and 0.88±0.13 at 6 months, indicating improvement after refractive surgery. “A positive correlation was detected between reduced anterior keratocyte density and the ratio log(s) straylight15 days or log(s) straylight6 months,” Dr. Nieto-Bona reported. “This correlation indicates that subjects whose intraocular straylight improved underwent less cell loss and this correlation was more significant 6 months after refractive surgery.” Furthermore, in patients whose log(s) straylight ratio improved, no activated keratocytes could be found. Patients with less myopic refractive correction, meanwhile, were found to have a worsening of straylight after LASIK during both follow-up periods. Neither higher-order aberrations nor haze could be linked to straylight in this study. “Increases in HOA [higher order aberrations] are among the main causes of reduced visual performance after LASIK,” Dr. Nieto-Bona noted. “We thought that it would be necessary to investigate if changes in straylight values after LASIK are correlated with changes in contrast sensitivity and HOA in order to analyze possible relations. Finally, the formation of haze after LASIK could be an important factor in producing the worsening straylight value. However, straylight values improved in our patients and, in addition, no significant amount of haze was detected in the slit lamp examination to suggest any worsening of straylight.” If you’re still puzzled about the causes and effects of straylight, you’re not alone. “The literature lacks information on corneal cell loss and its effects on straylight,” Dr. Nieto-Bona reported. “We will continue working in this direction to identify the structures that enhance straylight although we were unable to detect a relationship between any of the qualitative variables examined here and straylight measurements.” However, the following conclusions from this study may help advance knowledge about this condition: • AKD decreased six months post-op. • The straylight value did not deteriorate six months after LASIK (which is different from in previous research). • Straylight was correlated with a loss of corneal cells. John D. Sheppard, MD, professor of ophthalmology, microbiology, and immunology, Eastern Virginia Medical School, Norfolk, Va., USA, agreed that elimination of capsular opacities, reduction of glare, and other or for an injection at the end of the case,” she said. “It’s not dissimilar in some ways to mixing antibiotics for subconjunctival injection. There are a lot of different medications that can be supplied one way by a pharmacy but need to be mixed by your operating room staff. It’s not a new idea and doesn’t require a different skill set; it’s something they do every day.” However, according to Dr. Mamalis, the ultimate solution would be a commercially available antibiotic. The problem is that none currently exist, and getting approval for such a product would require an incredible amount of funding and a massive study of hundreds of thousands of patients. Even so, the demand from cataract surgeons is there. “If we had a commercial available, sterile, non-preserved single-use dosage than we’d all use intracameral injections because it would eliminate the compliance issues with patients, and ensure we’d get the correct dose into the eye at the conclusion of the case,” said Dr. Mamalis. “That would certainly take care of the problems.” EW Editors’ note: The physicians interviewed did not indicate financial interests related to their comments. Contact information Awdeh: richard.awdeh@gmail.com Devgan: devgan@ucla.edu Gimbel: hvgimbel@gimbel.com Mamalis: nick.mamalis@hsc.utah.edu Oetting: thomas-oetting@uiowa.edu Talley-Rostov: atalleytostov@nweyes.com optics-related problems are essential to achieving superior visual quality. He believes the future of refining corneal refractive surgery lies in understanding and improving corneal biomechanics. “There are very few things we’re unaware of,” when it comes to understanding what hinders vision in the cornea, Dr. Sheppard said. “But the unpredictable patient may simply be unable to achieve the architecture we wish or might develop problems with ectasia.” He likened managing ectasia to trying to manage astigmatism after cataract surgery with mixed success due to unpredictability. “This is also a byproduct of the wide range of biomechanical properties,” Dr. Sheppard said. EW Editors’ note: Dr. Nieto-Bona has no financial interests related to this study. Dr. Sheppard has no financial interests related to his comments. Contact information Nieto-Bona: amnop@fis.ucm.es Sheppard: 757-622-2200, docshep@hotmail.com Using continued from page 17