EyeWorld Asia-Pacific September 2013 Issue

September 2013 21 EWAP FEAturE changes depending on what type of refractive surgery the patient had, said Samuel Masket, MD , in private practice, Advanced Vision Care, Los Angeles. In myopic photoablative procedures, the anterior cornea is flattened but the posterior cornea remains unchanged (except in cases of ectasia), causing the preop near- parallel curves to take on a higher value than the assumed –6 D in an unoperated eye, he said. “When we read with a standard keratometer, we overestimate the corneal power in an eye that has undergone myopic photoablation,” he said. Conversely, those same machines will underestimate the central corneal value in eyes that underwent hyperopic photoablation, Dr. Devgan said, and those values may be off by as much as 2 D. “While manual and automated keratometry can provide reasonable starting values, we still need to use calculations to adjust for the prior keratorefractive surgery. Some machines like dual Scheimpflug imaging can give a more accurate central corneal power value, but even still the surgeon needs to factor in the prior keratorefractive surgery,” he said. Barry Schechter, MD , in practice, Florida Eye Microsurgical Institute, Boynton Beach, Fla., USA, uses the Pentacam (Oculus, AT A GLANCE • Patient education is crucial for those who have had previous refractive surgery. • Consider both anterior and posterior corneal curvatures when assessing eyes that have already undergone laser vision correction. • Choose IOL formulas based on available patient information. • Multifocal lenses may not be an appropriate choice in someone with significant post- LVC higher order aberrations. Views from Asia-Pacific Michael LAWLESS, MD Vision Eye Institute 4/270 Victoria Ave., Chatswood, NSW, Australia Tel. no. +61-2-9494-9999 Michael.lawless@visioneyeinstitute.com.au T he authors have raised many issues for surgeons having to deal with cataract calculations in patients who have had previous laser corneal surgery. Each of the authors has their own individual advice, which can be very confusing. E.g. using Ks based on Pentacam, Lenstar, etc. It needs to be understood that some devices are robust, and this includes the IOLMaster, Lenstar and manual and automated keratometry. Devices such as Pentacam and Orbscan are not as robust and make more assumptions, which can be misleading in post-refractive cases, and need to be used with caution in this setting. Some of the information they give is valuable, such as helping to confirm the axis of astigmatism. The authors rightly point out that not all post laser refractive patients are the same. Those treated for higher refractive errors with older technology will have a different corneal higher-order aberration profile, and this needs to be considered when deciding which intraocular lens to use. The general rule is that the more corneal HOAs the less likely you are to use a multifocal lens. The ASCRS website is very helpful for the practicing ophthalmologist. It has three settings to choose from: firstly, where there is clinical history data available, secondly, where there is some history available (i.e. the change in manifest refraction after laser treatment) and, thirdly, where there is no prior data. If you have some prior data, make sure you can trust it. It is worth looking at the article by Wang et al. 1 which clearly shows that the clinical history methods are not as accurate as the methods which rely on change in manifest refraction such as the modified Masket. These are motivated patients, but we should not let their motivation for spectacle independence interfere with our clinical advice, which should be to maintain good visual quality and not compromise these patients for the sake of spectacle independence. We also need to have a way out of refractive surprises, even more so than in normal patients, so determine that they would be suitable for an enhancement prior to cataract surgery. Of interest here is that about 75% of patients in this category will end up within half a diopter of intended on spherical equivalent, which means approximately one in four patients will need an enhancement of some sort. Results within half a diopter of intended also do not tell the full story, as some of these patients can be within this range on sphere, but have a troubling 0.75 or 1.00 D of astigmatism remaining, which means that the results are not quite as good as the spherical data would suggest. These patients are being seen in increasing numbers; they form part of all of our practices and the authors are to be congratulated for helping us deal with them. Reference 1. Wang, Hill, and Koch. Evaluation of intraocular lens power prediction methods using the American Society of Cataract and Refractive Surgeons Post-Keratorefractive Intraocular Lens Power Calculator. J Cataract Refract Surg. 2010;36:1466–1473. Editors’ note: Dr. Chan has no financial interests related to her comments. continued on page 22