EyeWorld India March 2015 Issue

IOL Calculations March 2015 28 EWAP SECONDARY FEATURE Adjusting practitioners’ view of long eyes by Maxine Lipner EyeWorld Senior Contributing Writer Tamping down hyperopic surprise W hile patients often expect excellent outcomes following cataract 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 talk to other patients who have excellent vision without glasses [following cataract surgery] and that becomes the standard to which they aspire,” Dr. Koch said, adding that 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 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. unique calculation 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 formulas, 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 postoperative hyperopia, Dr. Wang said. Optimizing axial length Such was the impetus for the development of the Wang-Koch adjustment. “It was Li’s original idea,” Dr. Koch said. “We were struggling with how to get better results in long 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.” IOLMaster axial length was 29.51 mm, and a 2.0 D MA60MA predicted refrac- tion of –0.53 D. The optimized axial length was 28.73mm, and a 4.0 D MA60MA predicted refraction of –0.54 D. A 4.0 D MA60MA was implanted. 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 (all): Li Wang, MD, PhD 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 refraction 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 postoperatively, Drs. Koch and Wang developed several formulas. These could be applied to 4 different IOL calculations—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 =