EyeWorld Asia-Pacific March 2013 Issue

18 March 2013 EWAP FEATURE Views from Asia-Pacific Sri GANESH, MD Chairman and Managing Director, Nethradhama Hospitals Pvt. Ltd. 26/14, Kanakapura Main Road, 7th Block Jayanagar Bangalore 560082 India Tel. no. +91-80-26088000/+91-98451294740 Fax no. +91-80-26633770 chairman@nethradhama.org A s cataract and refractive surgeons, we are constantly in pursuit of ensuring that our patients attain maximum uncorrected visual acuity after our interventions. Residual refractive astigmatism contributes significantly to the refractive outcomes of the surgery. Not measuring posterior corneal astigmatism may be one of the reasons for unexpected postoperative astigmatism, especially after toric IOLs. Despite correct IOL calculation with the standard parameters, IOL placement and alignment, surgeons encounter a refractive surprise. There may be overcorrection of with-the-rule astigmatism and undercorrection of against-the-rule astigmatism. This can be attributed to the posterior corneal astigmatism. Anterior corneal astigmatism in younger individuals is with-the-rule and in older individuals it is against-the-rule. In contrast, the posterior cornea has a steeper vertical axis which effectively causes against-the-rule astigmatism, since the posterior cornea tends to act as a minus lens. Therefore, the anterior with-the-rule astigmatism is reduced and the anterior against-the-rule astigmatism is enhanced due to the posterior corneal astigmatism in most cases. As pointed out by the authors, estimation of posterior corneal astigmatism should be done routinely in toric IOL patients in addition to the standard parameters. If there is no access to the available devices to estimate the posterior astigmatism, then the data available in Dr. Koch’s nomogram or the Baylor’s nomogram can be used to estimate the mean posterior corneal astigmatism to calculate the toric IOL power and axis. One more important fact pointed out by the authors is that it is essential to keep in mind the effect of against-the-rule drift that occurs with age. Hence, it is always better to leave the toric IOL patient with some amount of residual with-the-rule astigmatism to compensate for the against-the-rule drift which occurs with aging. Imaging the posterior corneal astigmatism can be done on all patients posted for refractive cataract procedures in particular toric IOLs and toric multifocal IOLs thereby enhance the outcome. The posterior corneal surface acts like a negative lens due to the relative change in the corneal thickness from periphery to center. This may be the reason that some eyes having with-the-rule astigmatismmay have a higher overcorrection than others following toric IOL implantation. It would probably be a good idea to assess the effect of posterior corneal astigmatism in relation to the relative change in corneal thickness or progression of corneal thickness from periphery to center. Depending upon the central corneal thickness an average correction factor could be derived to compensate for the posterior corneal astigmatism and this could be incorporated into the formula for calculating the power of the toric IOL. This could help practices that do not have Scheimpflug devices to measure the posterior corneal astigmatism. Editors’ note: Dr. Ganesh is a consultant for Abbott Medical Optics (Santa Ana, Calif., USA), Carl Zeiss (Jena, Germany), Hoya Surgical Optics (Chino Hills, Calif., USA), Bausch + Lomb (Rochester, NY, USA), and Schwind eye-tech-solutions (Kleinostheim, Germany), but has no financial interests related to his comments. “I think that our measurements could improve,” Dr. Koch said. “We do find that even the Galilei, which has a wonderful dual Scheimpflug with against-the-rule astigmatism,” he said. He cited a long-term study by K. Hayashi and colleagues that followed patients’ astigmatic change after undergoing 3-mm clear corneal temporal incisions. The study also had a control group that did not undergo cataract surgery. Researchers found that both groups had a comparable change of against-the-rule shift after more than 10 years. “You would think that a corneal incision temporarily might weaken the cornea such that the cornea would not steepen along the horizontal meridian over time. But in fact it does,” Dr. Koch said. “So in planning for our patients, I believe that we need to leave our patients a little bit on the with-the- rule side in order to compensate for the fact that they’re going to drift to against-the-rule over time. This will provide them with better uncorrected acuity over a much longer period of time and perhaps serve them well for 20 or more years.” EWAP References 1. Hayashi K, Hirata A, Manabe S, Hayashi H. Long-term change in corneal astigmatism after sutureless cataract surgery. Am J Ophthalmol. 2011 May;151(5):858-65. 2. Koch DD, Ali SF, Weikert MP, Shirayama M, Jenkins R, Wang L. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg. 2012 Oct 12. [Epub ahead of print] Editors’ note: Dr. Wang has financial interests with Ziemer. Dr. Koch has financial interests with Ziemer, Alcon (Fort Worth, Texas, USA/Hünenberg, Switzerland), Abbott Medical Optics (Santa Ana, Calif., USA), and OptiMedica (Sunnyvale, Calif., USA). Contact information Wang: 713-798-7946, liw@bcm.tmc.edu Koch: 713-798-6443, dkoch@bcm.tmc.edu mechanism for measuring the back, does not always seem to capture all of the posterior corneal astigmatism, and especially in patients [who have] with-the- rule astigmatism, it still seems to underestimate the amount of posterior corneal astigmatism based on our actual refractive outcomes.” Dr. Koch has created a nomogram that incorporates: 1) the mean posterior corneal astigmatism in eyes having either with-the-rule or against-the-rule astigmatism and 2) the effect of against-the-rule drift that occurs with age. He said that their data indicate that the new nomogram greatly improves accuracy with toric IOLs. In addition, manufacturers are interested in providing clinicians with this information because they are finding similar results retrospectively in their data, he said. However, to disseminate a new nomogram themselves, they would have to validate it in a clinical trial with the U.S. FDA, which could slow the approval process. Toric IOLs Dr. Koch began examining posterior corneal astigmatism when he noticed that some patients had unexpected results with toric IOLs. Patients who had with- the-rule astigmatism were being overcorrected, while patients who had against-the-rule astigmatism were undercorrected. “It has a huge impact on my decision making now in patients who are seeking astigmatic correction during cataract surgery,” Dr. Koch said. “It’s completely changed everything I’m doing with regard to both relaxing incisions and with regard to the selection of toric IOLs. “I have backed off on toric IOL power in patients who have with- the-rule astigmatism and conversely ramped it up for those patients with against-the-rule. For example, for someone who has 1 diopter with- the-rule astigmatism, I will not put a toric IOL in because I am likely to overcorrect him and leave him Posterior - from page 17