Eyeworld

32 | EYEWORLD | AUGUST 2019 R EFRACTIVE by Liz Hillman EyeWorld Senior Staff Writer Dr. Khandelwal pays special attention to eyes with long axial lengths. In these eyes, a hyperopic outcome would be a worst-case sce- nario because these patients have always been myopic. Meanwhile, she added, anterior cham- ber depth cannot be overemphasized, especially if the patient has a shallow anterior chamber. These patients' effective lens position is often tough to determine so the refractive error can be unpredictable especially with higher diopter lenses, she said. Dr. Holladay said that for eyes longer than 24.0 mm, one should use either the Wang-Koch linear regressions or the Holladay non-linear equations to avoid hyperopic errors. "The most likely sources of error in IOL calculations were shown by Sverker Norrby," 1 Dr. Holladay said. "In order of the error in nor- mal patients is: effective lens position, [postop] spectacle refraction, axial length, keratometry, and pupil size. These are the sources of the pre- diction error, and choosing the correct IOL for- mula and looking at the quality of axial length and keratometry can make the most significant improvement in results." Dr. Holladay said standard deviation for three measurements of optical keratometry would be <0.20 D (or 0.030 mm or 30 µm), and axial length should have a signal-to-noise ratio of >2.0. He said that symmetry between eyes is also good for screening measurements. If the axial length is >0.3 mm, the Ks >1.0 D, or the IOL powers >1.0 D, the measurements must be repeated. "The values are on every optical biometer and identify the patients who are most likely to be outliers. Repeating the measurements and go-to topography for the cornea when irregular reduces the chance of a refractive surprise," he explained. With more modern biometry systems, Dr. Khandelwal said she is using immersion biometry less. She currently uses the IOLMaster 700 (Carl Zeiss Meditec), which reads through posterior subcapsular cataracts and long axial lengths. However, both she and Dr. Holladay see a continued role for immersion biometry, T here are many factors that affect the accuracy of IOL powers chosen for patients. Which formula to use? What variables to consider for IOL calcula- tions? Which biometer to use? When to adjust calculations for specific situa- tions? Physicians also have to consider effective lens position estimates and more. Jack Holladay, MD, and Sumitra Khandelw- al, MD, shared their thoughts on this topic, of- fering young eye surgeons insights into effective IOL power calculations that can help lead to more accurate outcomes. The two biggest mistakes in IOL power calculations Dr. Holladay sees young eye sur- geons making today are: (1) not looking at the quality of optical keratometry and axial length measurements and (2) not personalizing the lens constant for the primary IOL. Dr. Khandelwal expressed a similar sentiment. "The biggest mistake I see is young sur- geons sticking to what they were taught in train- ing rather than what the updated recommenda- tions are," she said. "I also see young surgeons by default choosing –0.50 as their target. Once a surgeon is in practice, one priority should be to optimize their surgeon factor so that they can start aiming closer to plano and be able to reach the goals of the patient." Quality calculations start with quality mea- surements, paying special attention to keratom- etry, axial length, and anterior chamber depth. Both Drs. Khandelwal and Holladay empha- sized the importance of accurate and repeatable keratometry. Obtaining this, however, can be difficult in some patients. "These include dry eye patients, cornea pathology like anterior basement membrane dystrophy or keratoconus, or contact lens warpage," Dr. Khandelwal said. "The key with keratometry is to recognize any issues, pretreat the patient, and counsel them that the irregular keratometry may affect their outcome. If that is not mentioned before surgery, any issues after including residual refractive errors will feel like something the surgeon did rather than how the patient's eye always was." IOL power calculations for the young eye surgeon In the era of refractive cataract surgery, intraocular lens technologies continue to evolve toward maximizing spectacle independence. The accuracy of intraocular lens power calculations has never been so relevant as in the current times, when patients are invested both financially and emotionally in maxi- mizing spectacle indepen- dence, and the intraocular lens technologies demand precise and predictable out- comes. Yet, as always, our expertise is warranted to discern what is best for each individual patient among the overwhelming amount of options, and intraocular lens calculation formulas and biometry measurements are not exempt. Here we offer some pearls on intraocular lens calculations from Dr. Khandelwal and Dr. Holladay to maximize refractive outcomes after cataract surgery. Claudia Perez-Straziota, MD, YES Connect Co-editor YES CONNECT

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