EyeWorld Asia-Pacific June 2012 Issue

June 2012 12 EWAP FEATURE Calculating IOL powers by Michelle Dalton EyeWorld Contributing Editor Experts weigh in on why that’s not as easy as it sounds I t sounds simple enough— someone presents with a cataract and the surgeon needs to determine what IOL power lens to implant. But add in the complexity of which formula to use, whether one formula alone can suffice, the impact of surgically induced astigmatism on outcomes, and whether or not the eye has undergone previous refractive surgery, and all of a sudden the concept isn’t simple at all. For most surgeons, calculating IOL powers in virgin eyes is not as challenging as calculating them in post-refractive eyes, and those eyes are not as challenging as eyes without any historical data or those that have undergone radial keratotomy (RK). Just look at the refractive outcomes in cataract surgery vs. customized LASIK: in the former, 58% of surgeons are within 0.5 D and 85-90% are within 1 D of emmetropia. In the latter, 87% are within 0.5 D and 98% are within 1 D, said Amin Ashrafzadeh, MD , in private practice, Modesto and Turlock, Calif., USA. The numbers bear themselves out. Michael B. Raizman, MD , Ophthalmic Consultants of Boston, Mass., USA, said a “high number” of his patients are referred in and now need IOL exchanges. As a result, he takes a “unique approach” in his IOL calculations. “It’s often impossible to make an accurate calculation in post- surgical eyes, but we’re getting very good at getting close with historical data,” Dr. Raizman said. “Once the implant is there but outcomes are unacceptable, an IOL exchange is highly likely to give the patient the desired outcome. Surgeons need to set up realistic expectations for those patients.” At a presentation at the 2012 Hawaiian Eye meeting in Maui this past January, Dr. Ashrafzadeh noted several reasons for the differences in virgin eye accuracy, including IOL measurements—axial length, keratometry, and effective lens position are all dynamic— ocular surface conditions, and manufacturing tolerances of the IOL itself, among others. “A 1-mm error in axial length equates to three diopters in post- op refraction,” he said. “A 1-D error in K readings leads to 1 D in post-op refraction, and a 0.5-mm displacement in IOL position ends up with 1 D in post-op refraction.” He added that surgeons who are not using multiple formulas may be doing a disservice to their patients and that several online IOL calculators can help make the decision a bit easier. Personally, in virgin eyes he recommends using the Hoffer Q for small eyes (<22.5 mm) and the Holladay and SRT/T for long eyes (24.5 mm or longer); he also advises using the Haigis formula on every eye and averaging the formulas for a consensus. “We use the IOLMaster [Carl Zeiss Meditec, Dublin, Calif., USA] with the Holladay II formula for almost every eye,” said J.E. “Jay” McDonald II, MD , McDonald Eye, Fayetteville, Ark., USA. “It works particularly well on short eyes. We used to read just standard Ks, but now we use the K readings off the IOLMaster, and we’ve noticed much more consistent and reasonable readings. I also can’t say enough about the Masket formula. It’s a simple regression number, but we’ve found it to be really accurate.” Dr. McDonald also has one technician and one optometrist look at every calculation before the final decision is made on which IOL to use. “The three of us have to sign off on lens choice before we move ahead with surgery,” he said, noting that someone finds a discrepancy with the initial calculations “every few weeks”. Post-refractive surgery If only 40% of surgeons are within 0.5 D in virgin eyes, what about the post-refractive surgery patient? “Use all historical information available, plus all the current measurements you can get,” said Mark Packer, MD , clinical associate professor, ophthalmology department, Oregon Health & Science University, Portland, Ore., USA. He uses both the EyeSys (Houston, Texas, USA) and Atlas AT A GLANCE • Fewer than 60% of cataract surgeons are within 0.5 D of emmetropia • Using multiple IOL calculations in virgin eyes can improve those numbers • In post-laser vision patients, IOL calculations are more difficult, but not impossible • In post-RK patients, IOL calculations are extremely difficult A post-RK eye presenting for cataract surgery Because of the way an RK eye was ablated, IOL calculations are much trickier Source: Uday Devgan, MD