Eyeworld

EW FEATURE 64 IOL calculations February 2015 by Rich Daly EyeWorld Contributing Writer 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 is the modified double-K SRK/T formula based solely on the current mea- surement of the corneal peripheral power. It uses the estimated pre- LASIK K value, which is calculated based on the post-LASIK posterior corneal curvature of the central zone, instead of the actual pre-LASIK K value in the double-K method. In the modified A-P method, to prevent a hyperopic shift, it simply shifts the results using the A-P method to 0.98 D of myopia based on the third quartile of the hyperopic error of the A-P method (0.98 D). Corneal peripheral power approach A newer approach that Dr. Negishi has helped to develop is the central- peripheral corneal curvature method (C-P method), which is a modified double-K SRK/T formula based solely A growing number of power calculation options in myopic patients with previous refractive surgery has increased predictability of IOL placement in these patients T The development of photorefractive surgery in the mid-1990s has provided less dependence on specta- cles, but it has also created an ever-growing pool of complex patients. The unknown corneal refractive power changes from refractive surgery have drawn a growing number of IOL power calculation formulas specifically designed to address this problem. Developers of some of the latest IOL calculation formulas report progres- sively more reliable outcomes for the growing number of previously treated refractive surgery patients. However, critical limitations can still affect the accuracy of results. The ASCRS website, www.ascrs. org, offers an online post-refrac- tive surgery IOL calculator, which provides a variety of published calculation algorithms for eyes after refractive surgery. Harry Geggel, MD, head of the ophthalmology section, Virginia Mason Medical Center, 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 con- sensus 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 previ- ous refractive surgery history. "What's nice about my con- sensus 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 ap- proach 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 prop- er 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, as- sociate professor, Department of Ophthalmology, Keio University School of Medicine, Tokyo, primar- ily 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. Improving accuracy on the current measurement of the corneal peripheral power. This ap- proach uses the estimated pre-LASIK K value, which is calculated based on the post-LASIK peripheral corneal curvature. "The concept of this method is very similar to the A-P method," Dr. Negishi said. "It allows predictable outcomes of IOL power calculations in eyes that underwent corneal re- fractive laser surgery for myopia." Accurate measurement of the peripheral corneal data is important in the C-P method, and it should be strictly checked before calculating the IOL power. However, it is some- times hard to maintain the width of the palpebral fissure during measure- ment, especially in older patients with a narrow palpebral fissure. In such cases, Dr. Negishi shifts to the modified A-P method because it only needs the data of the central zone, which is easier to obtain. The modified A-P method provided greater accuracy than the original A-P method in published results.² The approach has provided Dr. Negishi even better results, with 63% of eyes within 0.5 D of target results and 91.3% within 1 D. 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. IOL power (D) Estimated postoperative spherical equivalent (D) Actual postop refraction after primary cataract surgery (D) Estimated refraction after IOL exchange (in the bag fixation) (D) 17.0 –2.55 N/A –1.42 16.5 –2.19 –1.06 16.0 –1.83 –0.70 15.5 –1.47 –0.34 15.0 –1.11 +0.02 +0.02 14.5 –0.76 N/A +0.37 14.0 –0.41 +0.72 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

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