EyeWorld Asia-Pacific March 2015 Issue

IOL Calculations March 2015 24 EWAP SECONDARY FEATURE Improving accuracy by Rich Daly EyeWorld Contributing Writer A growing number of power calculation options in myopic patients with previous refractive surgery have increased the predictability of IOL placement in these patients D evelopers of some of the latest IOL calculation formulas report progressively more reliable outcomes for the growing number of previously treated refractive surgery patients. However, critical limitations can still affect the accuracy of results. The development of photorefractive surgery in the mid- 1990s has provided less dependence on spectacles, but it has also created an ever-growing pool of complex patients. However, the unknown corneal refractive power changes from refractive surgery have drawn a growing number of IOL power calculation formulas AT A GLANCE • An average of multiple IOL calculation formulas may help increase accuracy in IOL surgery in patients who have previously undergone refractive surgery for myopia. • Encourage IOL candidates to track down medical records from previous refractive procedures. • Develop a plan for recalculating the correct power to address any instance of surprise error that necessitates IOL replacement. Figure 1. Example of the calculation for IOL exchange. The area surrounded with a square is an IOL calculation sheet for primary cataract surgery. The IOL power for IOL exchange is calculated on the basis of hyperopic shift (1.13 D) in the primary cataract surgery. Source: Kazuno Negishi, MD, PhD specifically designed to address this problem. The ASCRS website, www.ascrs. org, offers an online post-refractive surgery IOL calculator, which provides a variety of published calculation algorithms for eyes after refractive surgery. Harry Geggel, MD , head of ophthalmology section at the Virginia Mason Medical Center in Seattle, has found that choosing the proper IOL power in former myopic refractive surgery patients undergoing routine cataract surgery was challenging using standard accepted formulas. That is because many reported methods rely on pre-refractive data or the verified change in spherical equivalent refraction. “It is not infrequent that patients come in with no data available from the type of surgery or type of treatment that was done,” Dr. Geggel said. Dr. Geggel has developed a consensus formula, which is an average of 6 formulas—Geggel, Shammas, Haigis-L, Latkany average, Savini, and Seitz (Hoffer Q). Three of those formulas— Geggel, Shammas, and Haigis-L—do not require any previous refractive surgery history. “What’s nice about my consensus formula is that each of the formulas I’m using tries to solve this puzzle in a different way,” Dr. Geggel said. “Savini plays around with the refractive index of the cornea; the Seitz (Hoffer Q) employs the Hoffer equation and alters the K reading of the cornea by how much laser treatment was done; and the Latkany formula uses the SRK/T formula with a modification.” In addition to published results that show the consensus formula provides refractive outcomes for 70% within 0.5 D and 96% within 1 D of the intended result, the approach avoids overcorrections.¹ “Surgeons want a technique that minimizes hyperopia postop, and if the patient is a little bit nearsighted they can still do some reading,” Dr. Geggel said. “All patients are told upfront that we lose a little bit of precision in the picking of the proper implant in such eyes. If they don’t have any data then we’re averaging 3 formulas, and if they have data we’re averaging 6 formulas.” Another averaged approach Kazuno Negishi, MD, PhD, associate professor, Department of Ophthalmology, Keio University School of Medicine, Tokyo, primarily uses an approach known as the modified anterior-posterior corneal curvature method (A-P method). As another approach that does not require historical data to calculate IOL power for eyes after LASIK, Dr. Negishi and her colleagues presented on the modified A-P method at the 2014 ARVO meeting. Dr. Negishi uses an average of the modified A-P method, the Haigis-L, and Camellin-Calossi, if, after comparing the results, there is more than 1 D of difference. The original A-P method