March 2015

 

RESIDENTS

 

EyeWorld journal club

Myopic eyes and cataract surgery: Weve come a long way


by Dagny Zhu, MD, Lloyd Cuzzo, MD, and Vivek Patel, MD

 
 

Vivek Patel, MD

Vivek Patel, MD, residency program director, USC Eye Institute, Keck School of Medicine

Dagny Zhu, MD

Dagny Zhu, MD

Lloyd Cuzzo, MD

Lloyd Cuzzo, MD

We all recognize that IOL selection for extremely myopic eyes is challenging. I asked the USC residents to review this paper comparing multiple strategies and formulae in this months JCRS.

David F. Chang, MD, chief medical editor

 
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.

Cataract surgery is increasingly becoming a refractive procedure, with patients and surgeons alike expecting 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 measurements and has thereby increased the predictability of IOL power calculation formulas.1 Specifically, the use of this technology achieves a benchmark 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 refractive extremes. High axial myopic eyes prove to be especially challenging. Standard third-generation formulas (e.g., Holladay 1, SRK/T, Hoffer Q, and Haigis) using standard optical constants often yield postoperative hyperopic errors in long eyes.35 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 adjustments in high axial myopic eyes. The authors retrospectively analyzed 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 complications 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 formulas 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 clinical 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 appear 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 Universal II formula, however, accounts for these variables via paraxial ray tracing (Gaussian/thick lens).

Another explanation for postoperative hyperopic error in longer eyes may be due to the difficulty in obtaining accurate axial length measurements. Optical coherence biometry results in a systematic error that increases linearly and overestimates true AL.4 In this study, AL adjustment 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 explained above. In sum, the practice of arbitrarily targeting slightly myopic refractions 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 two groups was drawn at 6 D in this study, but it is unclear whether a different power IOL would have yielded similar results. Overall, the number of patients with low power IOLs is rare and negative power IOL even rarer (only 5 patients in this study). Future studies with larger sample sizes should be repeated for this specific population. Nevertheless, it appears that unadjusted standard formulas are sufficient for the majority of high axial myopes, and several options are available for even the most extreme myopic eyes.

References

1. Haigis W, Lege B, Miller N, Schneider B. Comparison of immersion ultrasound biometry and partial coherence interferometry for intraocular lens calculation according to Haigis. Graefes Arch Clin Exp Ophthalmol 2000; 238:765773. 2. Behndig A, Montan P, Stenevi U, Kugelberg M, Zetterstrom C, Lundstrom M. Aiming for emmetropia after cataract surgery: Swedish National Cataract Register study. J Cataract Refract Surg 2012; 38:11811186.

3. Wang JK, Hu CY, Chang SW. Intraocular lens power calculation using the IOLMaster and various formulas in eyes with long axial length. J Cataract Refract Surg 2008; 34:262267.

4. Wang L, Shirayama M, Ma XJ, Kohnen T, Koch DD. Optimizing intraocular lens power calculations in eyes with axial lengths above 25.0 mm. J Cataract Refract Surg 2011; 37:20182027.

5. Zaldivar R, Shultz MC, Davidorf JM, Holladay JT. Intraocular lens power calculations in patients with extreme myopia. J Cataract Refract Surg 2000; 26:668674.

6. User Group for Laser Interference Biometry. Optimized constants for the Zeiss IOLMaster. Accessed November 1, 2012, www.augenklinik.uni-wuerzburg.de/ulib/c1.htm. 7. Haigis W. Intraocular lens calculation in extreme myopia. J Cataract Refract Surg 2009; 35:906911.

Contact information

Patel
: Vivek.Patel@med.usc.edu

Myopic eyes and cataract surgery Myopic eyes and cataract surgery
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