Eyeworld

EW FEATURE 68 IOL calculations February 2015 by Maxine Lipner EyeWorld Senior Contributing Writer longer than 25 mm, the program will automatically calculate the IOL power, with the adjustment formula." While using these formulas should help most prevent hyper- opic surprise, Dr. Wang stressed that practitioners may want to fine-tune these for their own patients. "If [practitioners] have a num- ber of [long] eyes, it would be good to optimize their lens constant," she said. "That might improve the accuracy." Meanwhile, Dr. Koch foresees a day when such a regression formula will no longer be needed. "Ideal- ly what we're going to have is a situation where we can accurately measure each compartment of the eye; with that in mind, this modifi- cation will no longer be necessary," he said. "We're all working toward that goal." EW Editors' note: Drs. Koch and Wang have no financial interests related to their comments. Contact information Koch: dkoch@bcm.tmc.edu Wang: liw@bcm.tmc.edu adjustment should only clinically be used for eyes 25 mm or longer, Dr. Wang advised. Practical pearls When using the adjustment, Dr. Koch recommended picking the IOL that is just on the minus side of zero. "Also, you should advise patients that calculations [in long eyes] are more challenging," he said. "The only other factor to consider is that if you're using a toric lens in these patients, you have to bear in mind that the effective toric power of a lens implant is less when you have a low-powered IOL that's going to fit deep in the eye." It can be as much as 0.5 D less, Dr. Koch noted. "You need to think about adjusting your toricity if you're adding that to your calculation." "The Holladay 2 also includes the adjustment of the effective IOL toricity as a function of anterior chamber depth and axial length," Dr. Koch added. Dr. Wang pointed out that the Holladay 2 consultant program now includes the adjustment. "The Holladay 2 consultant program incorporated our formula," she said. "So if users select that for eyes eyes, and we were finding that optimizing the lens constant still wasn't getting us close enough, so she came up with a brilliant idea of optimizing the axial length." As Dr. Koch began to consider this, Dr. Wang's idea to adjust axial length in these long eyes made sense to him. "What occurred to me is that we're using the same index of refraction for all axial length measurements regardless of the medium through which the light of partial coherence interferometry is traveling," Dr. Koch said. "We're using the same index of refrac- tion for the aqueous, the lens, the cornea, and the vitreous." However, in the case of long eyes, there is an asymmetrically large vitreous cavity or posterior segment. "Putting that together, it seemed to me that we were miscalculating that long part of the eye in back and anticipating that it was actually longer than it was," Dr. Koch said. "What Li's original concept did was isolate that segment in a way that enabled us to adjust for that." To help compensate for the flaw that resulted in often leaving patients hyperopic postoperative- ly, Drs. Koch and Wang developed several formulas. These could be applied to 4 different IOL calcula- tions—the Holladay 1, the Haigis, the Hoffer Q, and the SRK/T, Dr. Koch noted. The formulas are as follows with the IOLMaster (Carl Zeiss Meditec, Jena, Germany): With the Holladay 1 nomogram, they determined that the revised axial length = 0.8289 x IOLMaster AL + 4.2663. With the Haigis, this revised axial length = 0.9286 x IOLMaster AL + 1.562. With the SRK/T, the revised axial length = 0.8544 x IOLMaster AL + 3.7222. Finally, the Hoffer Q optimized axial length = 0.853 x IOLMaster AL + 3.5794. Using the formulas, practi- tioners first take traditional axial length measurements, Dr. Wang explained. "Then we use the formula to adjust the measured axial length to get a new axial length—that is the optimized axial length." The opti- mized axial length is then plugged into the biometry machine. "The user runs the calculation again and gets a new outcome for IOL power," she said. This axial length Adjusting practitioners' view of long eyes AT A GLANCE • With traditional IOL formulas, patients with long eyes often end up with hyperopic surprise. • The Wang-Koch adjustment optimizes axial length with nomograms for 4 IOL calculation formulas. • With toric lenses, toricity must also be adjusted to compensate for lower effective toricity when the anterior chamber is deep and IOL power is lower. W hile patients often expect excellent out- comes following cata- ract surgery, for those with particularly long eyes, the results can be a challenge, according to Douglas D. Koch, MD, professor and the Allen, Mosbacher, and Law chair in ophthalmology, Cullen Eye Institute, Baylor College of Medicine, Houston. Patients often expect to have excellent vision without glasses following cataract surgery. Dr. Koch said it takes some education for many to understand that a bevy of complexities, including unusual ocular characteristics, can interfere with results. However, at least in the case of long eyes, a unique calcu- lation known as the Wang-Koch adjustment can bring practitioners closer to the desired mark, he said. Unfortunately, traditional calculations do not work as well in long eyes as in average ones, explained Li Wang, MD, PhD, associate professor, Baylor College of Medicine. Drs. Wang and Koch devised the adjustment together. "With our modern IOL formu- las, currently on [virgin] eyes with an average length of 23–25 mm, IOL calculations are pretty accurate," she said. "But for longer eyes and shorter eyes, it's still a challenge." In the case of longer eyes, formulas tend to select IOL powers that leave patients with postop hyperopia, Dr. Wang said. Optimizing axial length Such was the impetus for the devel- opment of the Wang-Koch adjust- ment. "It was Li's original idea," Dr. Koch said. "We were struggling with how to get better results in long IOLMaster axial length was 29.51 mm, and a 2.0 D MA60MA predicted refraction of –0.53 D. The optimized axial length was 28.73 mm, and a 4.0 D MA60MA predicted refraction of –0.54 D. A 4.0 D MA60MA was implant- ed. At 3 weeks postop, the uncorrected visual acuity was 20/20, and manifest refraction was –0.25 D sphere with best corrected visual acuity of 20/20. If the 2.0 D MA60MA had been used, the eye would have ended up about 1 D hyperopic. Source: Li Wang, MD, PhD

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