EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/78716
84 EW FEATURE February 2011 Refractive March 2012 Calculating IOL powers by Michelle Dalton EyeWorld Contributing Editor 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 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 suf- fice, the impact of surgically induced astigmatism on outcomes, and whether or not the eye has under- gone previous refractive surgery, and all of a sudden the concept isn't sim- ple at all. For most surgeons, calcu- lating 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 out- comes in cataract surgery vs. cus- tomized 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, M.D., in private prac- tice, Modesto and Turlock, Calif. The numbers bear themselves out. Michael B. Raizman, M.D., Ophthalmic Consultants of Boston, 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 calcu- lations. "It's often impossible to make an accurate calculation in post-surgi- cal eyes, but we're getting very good at getting close with historical data," Dr. Raizman said. "Once the implant is there but outcomes are unaccept- able, an IOL exchange is highly likely to give the patient the desired outcome. Surgeons need to set up How accurate are we? by Warren E. Hill, M.D., East Valley Ophthalmology, Mesa, Ariz. W hen ophthalmic surgeons get together, the topic of refractive accu- racy frequently comes up, and I am often surprised by what I hear. The reality is that very few physicians actually track their results, and outcomes may not be quite as accurate as imagined. Here is a quick sanity check. In 2007, Gale and others published in Eye what has come to be known as the United Kingdom National Health Service benchmark standard for cataract surgery refractive outcomes. The study recommended the adoption of a benchmark standard for refractive accuracy as being a final spherical equivalent within 1 D for 85% of cases of the predicted figure and 55% of patients within 0.5 D. Of course, we know that we are all much better than that, but by how much? For many years I have reviewed upper-tier optical biometry physician databases of 250 cases or more for lens-constant optimization as a free serv- ice as outlined at www.doctor-hill.com/physicians/download. htm. What has emerged from this work is that even after the removal of outliers, less than 1% of databases are within ±0.5 D at a 90% level. Only 6% of data- bases are in the mid 80% range or above, and the majority of surgical practices have outcomes at a ±0.5 D level between 70% and 82%. The bottom line is the only way to truly know your refractive accuracy is to track outcomes. What you discover may be some- what less than imagined. Please watch for Dr. Hill's follow-up story on tracking and improving refractive outcomes in the April issue of EyeWorld magazine. Gale RP, Saldana M, Johnston RL, et al. Benchmark standards for refractive outcomes after NHS cataract surgery. Eye. 2007. www.nature.com/eye/journal/v23/n1/full/6702954a.html. 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, ker- atometry, and effective lens position are all dynamic—ocular surface con- ditions, and manufacturing toler- ances 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 D-mm displace- ment 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 calcula- tors 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 Hol- laday 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 consen- sus. "We use the IOLMaster (Carl Zeiss Meditec, Dublin, Calif.) with the Holladay II formula for almost every eye," said J.E. "Jay" McDonald II, M.D., McDonald Eye, Fayetteville, Ark. "It works particu- larly 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 con- sistent and reasonable readings. I also can't say enough about the Masket formula. It's a simple regres- sion number, but we've found it to be really accurate." Dr. McDonald also has one tech- nician and one optometrist look at every calculation before the final de- cision 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? Because of the way an RK eye was ablated, IOL calculations are much trickier Source (all): Uday Devgan, M.D. "Use all historical information available, plus all the current meas- urements you can get," said Mark Packer, M.D., clinical associate pro- fessor, ophthalmology department, Oregon Health & Science University, Portland. He uses both the EyeSys (Houston, Texas) and Atlas (Carl Zeiss Meditec) topographers "in all the formulas on the ASCRS website. Once I have a range of IOL powers— usually about 2-2.5 D—I bracket these IOL powers and have them all available in the OR (there's actually a plastic basket full of IOL boxes on the back table)." About 33% of post-refractive surgery patients who need cataract surgery present without any histori- cal records, said Uday Devgan, M.D., chief of ophthalmology, Olive View-University of California, Los Angeles Medical Center, Sylmar, Calif. "If they have no records, do your best to get as much pre-op data as possible. Ask a spouse if there are old contact lenses or spectacles around. If not, ask [patients] if they remember how far away they had to hold a book to read it clearly." In these eyes, Dr. Devgan also suggested using the ASCRS calculator, but warned surgeons need to clearly explain to patients they'll be estimat- ing on lens power in post-surgical eyes with no historical data. "The [pa- A post-RK eye presenting for cataract surgery