s THE LITERATURE IOL CALCULATIONS FOR PATIENTS WITH KERATOCONUS Work continues to improve refractive accuracy in this patient population. BY ALICE ROTHWELL, MBCHB, AND ANDREW M.J. TURNBULL, BM, PGCERTMEDED, PGDIPCRS, FRCOPHTH INTRAOCULAR LENS POWER CALCULATION TABLE 1. CLASSIFICATION OF KERATOCONUS SEVERITY IN EYES WITH KERATOCONUS Stage Keratometry Reading Savini G, Abbate R, Hoffer KJ, et al1 1 ≤ 48.00 D Industry support: K.J.H. licenses 2 > 48.00 D registered trademark name Hoffer to various companies 3 > 53.00 D ABSTRACT SUMMARY spherical equivalent. Myopic and stage 1 disease. Accuracy decreased Savini and colleagues compared hyperopic surprises were indicated by with more advanced keratoconus, with the prediction errors (PEs) of negative and positive PEs, respectively. a MedAE of greater than 2.50 D in all five standard formulas: Barrett Mean error (ME), median absolute stage 3 eyes. Universal II (BUII), Haigis, Hoffer Q, error (MedAE), mean absolute error, Holladay 1, and SRK/T. The study and percentage of eyes achieving within DISCUSSION included 41 consecutive keratoconic ±0.50 D, ±0.75 D, and ±1.00 D of the Keratoconus presents multiple eyes undergoing phacoemulsification refractive target were also calculated. challenges to IOL selection. First, and IOL implantation. Eyes were A hyperopic ME was found across all the standard keratometric index classified by disease severity (Table 1). five formulas. Across the whole dataset, cannot reliably be applied to these A subjective refraction was obtained the lowest ME (0.91 D) and MedAE eyes because this index depends on for each eye at 1 month postoperatively. (0.62 D) and the highest percentage a normal ratio between the anterior The PE for each eye was calculated by (36%) of eyes within ±0.50 D of target and posterior corneal surfaces, but subtracting the predicted spherical were achieved with the SRK/T formula. this ratio is disrupted in keratoconus. equivalent from the actual postoperative Outcomes were best among eyes with Second, keratometry assumes a degree of consistency in corneal curvature along any given meridian, which is not true for keratoconic eyes with STUDY IN BRIEF asymmetrical corneal curvature. s Third, standard IOL formulas predict A multicenter retrospective study compared the prediction errors of five standard formulas the effective lens position based on for IOL power calculation in eyes with keratoconus. Even with the most accurate of these theoretical model eyes and data formulas, refractive outcomes were significantly worse than those achieved in healthy eyes. obtained from normal eyes. The increased corneal curvature, axial WHY IT MATTERS length, and anterior chamber depth IOL power calculations are less accurate for eyes with keratoconus, and hyperopic refractive typical of keratoconic eyes and the surprises are commonplace. This is the first study to assess the accuracy of the Barrett abnormal relationships between these Universal II for keratoconic eyes. parameters limit the applicability of standard formulas. Fourth, tear film irregularities restrict the repeatability 16 CATARACT & REFRACTIVE SURGERY TODAY | MAY 2021 s THE LITERATURE of corneal measurements, thereby in nonkeratoconic eyes, but it failed through serendipity rather than design introducing an additional source in this study to match the accuracy of that the SRK/T achieves superiority. of error. the SRK/T formula in keratoconic eyes. Aside from its small sample size As a consequence, hyperopic This study corroborates three (N = 41), other limitations of this study refractive surprises after cataract surgery conclusions drawn by other researchers: must be acknowledged. Disparate are commonplace among keratoconic • There is a strong tendency toward biometry methods and IOL models eyes when IOL selection is based on hyperopic PE in keratoconic eyes;2 were employed across the different traditional formulas.2 The accuracy of • Greatest accuracy in this population sites. Importantly, some preoperative IOL calculations in keratoconus remains is achieved by using the SRK/T data (eg, lens thickness) were lacking, significantly worse than in normal eyes, formula;3 and precluding an investigation of formulas of which 80% to 90% achieve within • IOL formula accuracy is inversely that require this information. Although ±0.50 D of the refractive target when proportional to keratoconus severity.4 the BUII may be used with just two or the latest methods are used. Savini et al offer a possible three parameters, it performs best with Several formulas have been shown explanation for the greater accuracy its full complement of five (keratometry, to provide greater accuracy than the of the SRK/T formula: In healthy eyes axial length, anterior chamber depth, older SRK/T formula for IOL power with steep corneas, this formula is lens thickness, and white-to-white calculations in normal eyes and less accurate and overestimates IOL distance). This is particularly true in those with abnormal axial lengths power, leading to myopic surprises. abnormal eyes, for which the additional and other abnormal biometric This flaw is counterbalanced by the parameters improve accuracy. Thus, this parameters. The BUII, for example, is tendency toward hyperopic surprises study might have underestimated the one of the best-performing formulas in keratoconic eyes. It is therefore efficacy of the BUII. ACCURACY OF INTRAOCULAR LENS TABLE 2. RECOMMENDED ADJUSTMENTS TO THE REFRACTIVE TARGET WITH POWER FORMULAS MODIFIED FOR THE KANE KERATOCONUS FORMULA PATIENTS WITH KERATOCONUS Stage Predicted Refraction 5 Kane JX, Connell B, Yip H, et al 1 No adjustment Industry support: None 2 -0.75 to -1.50 D ABSTRACT SUMMARY 3 -2.00 to -3.00 D This retrospective study compared the accuracy of two IOL power formulas Haigis, Hoffer Q, Holladay 1, Holladay 2, percentage (50%) of eyes within modified specifically for patients Kane, and SRK/T). (Editor’s note: J.X.K. ±0.50 D of the refractive target across with keratoconus (Holladay 2 with devised the Kane and Kane-KC formulas.) all stages of keratoconus using the keratoconus adjustment and Kane The Kane-KC achieved the lowest same classification system as Savini et al keratoconus formula [Kane-KC]) to mean absolute error (0.81 D, P < .01) (shown in Table 1).1 This was statistically seven normal IOL power formulas (BUII, and MedAE (0.50 D) and the highest significant when compared to the other formulas. Furthermore, the Kane-KC achieved an ME of just 0.04 D, whereas the other formulas returned hyperopic MEs ranging from 0.31 D (SRK/T) to STUDY IN BRIEF 0.78 D (Hoffer Q). s A retrospective consecutive case series compared the accuracy of IOL power formulas Despite the greater accuracy of the modified specifically for patients with keratoconus to normal IOL power calculations. One of Kane-KC, the accuracy of outcomes did the modified formulas was found to be the most accurate for this patient population. not approach that expected in normal eyes. The Holladay 2 with keratoconus adjustment was the least accurate of WHY IT MATTERS the formulas investigated. All normal This is the first study to evaluate IOL power formulas modified specifically for patients with formulas resulted in a hyperopic ME that keratoconus. worsened with increasing keratometry. 18 CATARACT & REFRACTIVE SURGERY TODAY | MAY 2021 s THE EVOLVING OSD PIPELINE s THE LITERATURE formula, devised by Graham D. Barrett, DISCUSSION MB BCh SAf, FRACO, FRACS, and SECTION EDITOR EDWARD MANCHE, MD This study supports previous available for free from the Asia-Pacific n Director of Cornea and Refractive Surgery, findings that the SRK/T is the most Association of Cataract & Refractive Stanford Laser Eye Center, Stanford, California accurate of the traditional formulas Surgeons (www.apacrs.org), is therefore n Professor of Ophthalmology, Stanford University for eyes with keratoconus, followed by of great interest. In a recent personal School of Medicine, Stanford, California the BUII and original Kane formulas, communication to one of the authors n [email protected] which returned similar outcomes. (A.M.J.T.), Dr. Barrett stated that early n Financial disclosure: None These remain the best of a bad bunch, results suggest that this modified however, because only approximately formula holds great promise. ALICE ROTHWELL, MBCHB 40% to 45% of keratoconic eyes were To further optimize outcomes and n Ophthalmology Trainee, Royal Bournemouth Eye within ±0.50 D of target compared avoid hyperopic errors after cataract Unit, University Hospitals, Dorset, United Kingdom to an anticipated 80% or more in surgery on eyes with keratoconus, n [email protected] normal eyes. Kane et al recommend adjusting n Financial disclosure: None This study found the Kane-KC to be a the refractive target when using the better option than any of the standard Kane-KC (Table 2). n ANDREW M.J. TURNBULL, BM, PGCERTMEDED, formulas investigated. Although the PGDIPCRS, FRCOPHTH Kane-KC represented a welcome, 1. Savini G, Abbate R, Hoffer KJ, et al. Intraocular lens power calculation in eyes n Consultant Ophthalmic Surgeon, Cornea, Cataract, with keratoconus. J Cataract Refract Surg. 2019;45(5):576-581. and Refractive Surgery, Bournemouth Eye Unit, statistically significant improvement, 2. Kamiya K, Iijima K, Nobuyuki S, et al. Predictability of intraocular lens results were far inferior to the accuracy power calculation for cataract with keratoconus: a multicenter study. Sci Rep. University Hospitals, Dorset, and Optegra Eye 2018;8(1):1312. Hospital, Hampshire, United Kingdom achieved in normal eyes. Further work 3. Hashemi H, Heidarian S, Seyedian MA, Yekta A, Khabazkhoob M. Evaluation of to improve the accuracy of IOL power the results of using toric IOL in the cataract surgery of keratoconus patients. n Honorary Lecturer, Ulster University, Eye Contact Lens. 2015;41(6):354-358. United Kingdom calculation for eyes with keratoconus 4. Watson MP, Anand S, Bhogal M, et al. Cataract surgery outcome in eyes with keratoconus. Br J Ophthalmol. 2014;98(3):361-364. n [email protected]; Twitter @the_iSurgeon is necessary.