Eyeworld

EW RESIDENTS 152 March 2015 by Dagny Zhu, MD, Lloyd Cuzzo, MD, and Vivek Patel, MD C ataract surgery is increas- ingly becoming a refractive procedure, with patients and surgeons alike expect- ing to achieve predictable postoperative refractions. Given this trend, optimal intraocular lens (IOL) selection remains an essential component of this process. The development of partial coherence interferometry technology allows for more accurate biometric measure- ments and has thereby increased the predictability of IOL power calcula- tion formulas. 1 Specifically, the use of this technology achieves a bench- mark maximum absolute deviation from the target refraction of ±1.0 diopters (D) in 93% of eyes and only ±0.5 D in 71% of eyes. 2 However, not surprisingly, this predictability breaks down at refrac- tive extremes. High axial myopic eyes prove to be especially chal- lenging. Standard third-generation formulas (e.g., Holladay 1, SRK/T, Hoffer Q, and Haigis) using standard optical constants often yield post- operative hyperopic errors in long eyes. 3–5 Fortunately, various methods have successfully decreased mean error in these eyes, including new generation formulas (e.g., Holladay 2, Olsen, and Barrett Universal II) and standard formulas employing IOL constant (e.g., ULIB) and/or axial length (AL) adjustments. 4,6 In the March 2015 issue of the Journal of Cataract & Refractive Surgery (JCRS), Abulafia et al. became the first group to report on the relative predictability of specific IOL power calculation formulas and adjust- ments in high axial myopic eyes. The authors retrospectively ana- lyzed a total of 106 eyes among 86 patients with AL >26 mm at a single eye center in Tel-Aviv, Israel over a 31-month period. Patients with postoperative corrected distance visual acuity of 20/40 or better were included in the study, while patients with previous ocular surgery and intra- or postoperative complica- tions were excluded. Patients were further divided into two groups for sub-group analysis: those implanted with IOL power >6 D (n=76) and those with IOL power <6 D (n=30). As expected, almost all IOL formulas erred toward postoperative hyperopia, but the magnitude of error varied widely between formu- las and patient group. For myopic eyes with IOL >6 D, a multitude of formulas appear viable. In particular, the standard formulas SRK/T, Hoffer Q, and Haigis met the benchmark criteria described above without the use of any adjustments. In fact, the study revealed that AL adjustment actually resulted in postoperative myopic errors, suggesting overly aggressive compensation with this method. The only adjustment that significantly decreased error was using ULIB constants for Haigis. It is somewhat surprising that the Hoffer Q performed so well since it is usually reserved for hyperopic eyes. Although this issue was not specifically addressed by the study, the authors did recommend using a higher constant with Hoffer Q to further decrease error in the clini- cal setting. Benchmark outcomes were also achieved with the new generation formulas Holladay 2, Olsen, and Barrett Universal II for this group of myopes with relatively high IOL power. In contrast, fewer options ap- pear to exist for high axial myopic eyes with IOL power <6. The only formulas that met benchmark criteria were AL-adjusted Holladay 1, AL-adjusted Haigis, and Barrett Universal II. Adjustments using ULIB constants also decreased mean error, but were not sufficient to reach benchmark. Barrett Universal II, the only new generation formula that met benchmark criteria in this group, resulted in the overall lowest mean error (though not statistically significant). These observations provide invaluable insight into why other new generation formulas (and most formulas in general) may result in postoperative hyperopic errors in long eyes with lower power IOLs. Part of the explanation seems to be in the varying geometry of different power IOLs. 7 For example, low and negative power IOLs adopt meniscus curvatures, while higher power IOLs are biconvex. The subsequent effect of this variation on the principal planes and effective location of the IOL are not adequately addressed by most formulas. The Barrett Univer- sal II formula, however, accounts for these variables via paraxial ray tracing (Gaussian/thick lens). Another explanation for post- operative hyperopic error in longer eyes may be due to the difficulty in obtaining accurate axial length mea- surements. Optical coherence biom- etry results in a systematic error that increases linearly and overestimates true AL. 4 In this study, AL adjust- ment successfully decreased mean error for high axial myopic eyes with IOL <6 D. Interestingly, however, AL adjustment resulted in myopic mean errors for those eyes with IOL >6 D. The reason for this remains unclear, but may also be related to IOL power and the geometric differences ex- plained above. In sum, the practice of arbitrari- ly targeting slightly myopic refrac- tions for long, myopic eyes is no longer necessary. This study provides novel clinical guidance in selecting IOL power calculation formulas for high axial myopic eyes, especially for those with low or negative power IOLs. The boundary dividing the Myopic eyes and cataract surgery: We've come a long way We all recognize that IOL selection for extremely myopic eyes is chal- lenging. I asked the USC residents to review this paper comparing multi- ple strategies and formulae in this month's JCRS. –David F. Chang, MD, chief medical editor Lloyd Cuzzo, MD Vivek Patel, MD, residency program director, USC Eye Institute, Keck School of Medicine Dagny Zhu, MD EyeWorld journal club " This study provides novel clinical guidance in selecting IOL power calculation formulas for high axial myopic eyes, especially for those with low or negative power IOLs. "

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