Uncorrected astigmatism can cause visual

Spatial Vision and Astigmatism Correction What is the minimum amount of astigmatism that should be corrected? By Eloy A. Villegas, PhD; and Pablo Artal, PhD Uncorrected astigmatism can cause visual quality to deteriorate significantly (Figure 1). Spectacle lenses typically correct defocus and astigmatism with an accuracy of ±0.25 D; however, contact lenses with a toric component lack rotational stability, thereby reducing the overall efficacy of astigmatism correction. Today, toric contact lenses are typically available in cylindrical powers of 0.75 D or higher, in 0.50 D steps. On the other hand, in laser refractive surgery, errors in astigmatism correction are approximately 0.50 D or higher. Therefore, the correction of cylinder errors of 0.75 D or less poses a dilemma for surgeons. In cataract surgery, toric IOLs are an option for pseudophakic patients with astigmatic corneas. But the effects of surgically induced astigmatism from the corneal incision and rotational and tilt errors during lens positioning limit the efficacy of these lenses for correcting small amounts of astigmatism. For this reason, the lowest cylindrical powers available in modern IOL designs are still greater than 1.00 D. SPATIAL VISION One common option to minimize the visual impact of residual astigmatism is to leave residual defocus in order to reach a null value of the spherical equivalent that is, to place the so-called circle of least confusion on the retinal plane. One practical question that has not been completely answered is this: What is the minimum amount of astigmatism that has a significant impact on spatial vision? If this could be determined, a lower limit for astigmatism correction could be set a limit that obviously would also be affected by the particular accuracy of each correcting procedure. In this context, we recently studied how small amounts of natural astigmatism (below 0.50 D) and their correction affect visual acuity (VA). We measured astigmatism and higher-order aberrations with a custom wavefront sensor in 54 healthy, young eyes with astigmatism ranging from 0.00 to 0.50 D. Astigmatism was corrected for natural pupil diameters using an experimental crossed-cylinder device. VA was measured for high and low contrast stimuli, at best subjective focus with the natural and corrected astigmatism. There was no significant correlation between the amount of astigmatism and VA. That is to say, when astigmatism was less than 0.50 D in these normal eyes, VA did not depend on the precise value of astigmatism. Additionally, despite some individual variability, the correction of anything less than 0.30 D of astigmatism did not produce any improvement in VA. Some patients even experienced a mild reduction in VA after correction (Figure 2). OPTIMAL CORRECTION The results of our study have practical implications for determining the optimal correction of astigmatism equal to or less than 0.50 D with optical or surgical approaches. According to our results, it is safe to expect a relative improvement in Figure 1. Simulated retinal images of the impact of astigmatism on image quality. (Continued on page 43) June 2014 Cataract & Refractive Surgery Today Europe 39

What is the Minimum Amount of Astigmatism That You Correct? CRST Europe Editorial Board members weigh in on this debate. FRANCESCO CARONES, MD In my hands, the question of the amount of astigmatism to be corrected has different answers depending on the attempted surgical procedure. With a laser refractive procedure, I tend to correct all the preexisting refractive astigmatism in younger patients but only the preoperative against-the-rule astigmatism (ATR) in patients of the presbyopic age who have less than 0.50 D of with-the-rule (WTR) astigmatism. This is because younger patients will have sharper vision with no residual astigmatism, whereas the trend in older patients is the progressive formation of some ATR astigmatism with age. With lens-based surgery, I make a distinction between monofocal and multifocal IOLs. I implant a toric monofocal IOL when the expected residual astigmatism from my calculation is greater than 0.75 D, and I aim for plano or minor WTR astigmatism (less than 0.50 D). With multifocal IOLs, my experience has led me to get as close to plano as possible in all cases, because the presence of even negligible astigmatism affects distance visual acuity significantly. Therefore, I always try to leave less than 0.25 D of astigmatism after lens implantation. Francesco Carones, MD, is the Cofounder and Medical Director of the Carones Ophthalmology Center in Milan, Italy. Dr. Carones may be reached at tel: +39 02 76318174; e-mail: fcarones@carones.com. ARTHUR B. CUMMINGS, MB ChB, FCS(SA), MMed(Ophth), FRCS(Edin) As with all things in life and with ophthalmology in particular, in my opinion, you have to go for it. Therefore, I prefer to target plano or zero astigmatism in cataract surgery, as well as in refractive surgery. If astigmatism is not addressed, there is the potential for it to worsen. For example, there are two outcomes of targeting plano in a patient with -0.50 D of cylinder at 180º: Either you nail it, or the patient is left with some residual astigmatism. Either way, the patient is better off than he or she was before. However, if you choose to ignore it, the best that you can do for the patient is to achieve unchanged astigmatism, but it may end up worse. Even though the effect of a small amount of astigmatism may be inconsequential, 1 the drive to obliterate it should not wane. Obviously there are times when a small amount of astigmatism is beneficial, but I am ignoring them for the principle of this argument: I always target the best-possible outcome. I do not want to have a less-than-ideal outcome because I chose not to target the full error. My strategies are these: With laser vision correction, I target the full correction; with cataract surgery, I use incision size and placement to correct astigmatism less than 1.25 D and toric IOLs to correct astigmatism greater than 1.25 D. Arthur B. Cummings, MB ChB, FCS(SA), MMed(Ophth), FRCS(Edin), is a Consultant Ophthalmologist at the Wellington Eye Clinic and UPMC Beacon Hospital in Dublin, Ireland, and an Associate Chief Medical Editor of CRST Europe. Dr. Cummings may be reached at e-mail: abc@wellingtoneyeclinic.com. JACK T. HOLLADAY, MD, MSEE, FACS Villegas et al 1 should be congratulated on an important study. I agree that correcting less than 0.30 D of astigmatism is not clinically relevant. However, as the authors point out, neural adaptation may play a role in astigmatism correction, as it takes time for the brain to readjust to the new optical system. This point is also supported by the lack of correlation of the imagequality metrics and the measured visual acuities in their study. Based on their results, there was an improvement in measured visual acuity when correcting astigmatism greater than 0.30 D. Their argument for not correcting 0.30 to 0.50 D of astigmatism was that the axis orientation of the correction may not be perfect. They gave the example that, if the correction was off by 10º for -0.50 D of astigmatism, it would result in a residual refraction of +0.09-0.18 X 45º to the original axis and a possible breakdown of neural adaptation. I would argue that, even though this may be true initially, within a few weeks the neural adaptation would readjust and result in a significant improvement over the original -0.50 D of astigmatism. The lowest available toric IOL currently includes 1.00 D of toricity, which on average corrects 0.66 D of astigmatism in patients with normal eyes (ie, normal corneal power, normal crystalline lens or IOL power, and normal effective lens position). I agree with these authors that, due to the levels of precision of current keratometers, refractometers, and topographers, attempting to correct less than 0.50 D of astigmatism is not prudent. What I glean from their study is that the goal must be to achieve less than 0.30 D of residual astigmatism for the patient to have an optimal result, and that it may take a few weeks of neural adaptation before the best visual acuity is attained. Jack T. Holladay, MD, MSEE, FACS, is a Clinical Professor of Ophthalmology at the Baylor College of Medicine in Houston, Texas. Dr. Holladay may be reached at e-mail: holladay@docholladay.com. ERIK L. MERTENS, MD, FEBOphth My training as a refractive surgeon made me aware 40 Cataract & Refractive Surgery Today EUROPE June 2014

What is the Minimum Amount of Astigmatism That You Correct? (continued) that astigmatism correction is of the utmost importance, and this lesson has translated to my experience in cataract surgery. Especially when we implant multifocal IOLs, the final astigmatic refractive result should be 0.50 D of cylinder or less. I correct up to 1.00 D of corneal astigmatism by placing the incision at the steepest meridian, and I use a toric IOL for more than 1.00 D of cylinder. I acknowledge, however, that even small errors in IOL position can significantly affect the patient s final visual acuity with a toric IOL, more so than with a spherical IOL. For every 1º of error in a toric IOL s rotational alignment, there is a 3.3% decrease in the amount of astigmatism correction. The most common reasons for residual astigmatism are preoperative measurement errors, incorrect marking of the reference points on the cornea, incorrect IOL placement, and failure to take into account the impact of surgically induced astigmatism (SIA). Incorrect IOL power at the corneal plane due to the patient s posterior corneal curvature or measurement variability can also affect the amount of residual astigmatism. My postoperative outcomes in patients with astigmatism who receive toric IOLs have improved since integrating the Optiwave Refractive Analysis System (ORA; WaveTec Vision) into my standard operative workflow. This intraoperative aberrometer is used to verify the magnitude and axis of refractive astigmatism and to determine the optimal cylinder power and axis for the IOL to be implanted. After the final lens selection, ORA is used for pseudophakic measurements and to rotate the IOL to its optimal axis. Erik L. Mertens, MD, FEBOphth, is Medical Director of Medipolis, Antwerp, Belgium. Dr. Mertens states that he has no financial interest in the products or companies he mentions. He is a Co-Chief Medical Editor of CRST Europe. He may be reached at tel: +32 3 828 29 49; e-mail: e.mertens@medipolis.be. SIMONETTA MORSELLI, MD; AND ANTONIO TOSO, MD Medium to high residual astigmatism can be disturbing to patients. The amount of postoperative astigmatism that a patient accepts depends on the type of surgery planned. For a refractive procedure, the minimum amount of astigmatism that we correct is 0.50 D, and the target is always plano. For cataract surgery with monofocal IOL implantation in an elderly patient, a small amount of ATR astigmatism (0.50 0.75 D) can improve depth of focus. In patients with more than 1.50 or 2.00 D of astigmatism preoperatively, we implant a toric IOL. If the patient has a dense cataract and used an astigmatic spectacle lens before surgery, he or she usually will not complain about postoperative astigmatism. In our hospital, we use toric IOLs in carefully selected patients only. When we plan to implant a multifocal IOL, we correct even small amounts of astigmatism (0.75 D) and carefully study the anterior and posterior corneal astigmatism with Scheimpflug imaging. If the amount of astigmatism is more than 1.00 D, we implant a toric multifocal IOL. In some patients, total corneal astigmatism can be corrected with femtosecond LASIK, thereby enhancing the results of multifocal IOL implantation. If this is the chosen strategy, we prefer to correct the astigmatism after IOL implantation, as the cataract surgery incision affects corneal astigmatism. Simonetta Morselli, MD, is Head of the Ophthalmology Department, S. Bassiano Hospital, Bassano del Grappa, Italy. Dr. Morselli may be reached at e-mail: simonetta.morselli@gmail.com. Antonio Toso, MD, is Head of the Vitreoretinal Surgery Unit, Ophthalmology Department, S. Bassiano Hospital, Bassano del Grappa, Italy. Dr. Toso may be reached at e-mail: antonio.toso@gmail.com. KJELL U. SANDVIG, MD, PhD In reimbursed cataract patients, I usually discuss toric IOLs if the corneal astigmatism is 1.50 D or more and is reproducible with at least two different instruments or methods, unless there are contraindications. Correction of lower degrees of astigmatism with a toric IOL is often less accurate, and the SIA plays a relatively larger role. Thus, the refractive postoperative cylinder is less predictable in these cases, which makes it more difficult to charge patients the extra expense for premium IOLs. As a result of these considerations, I implant toric IOLs in 10% to 14% of my cataract patients. In patients who undergo refractive lens exchange, I implant a toric IOL to correct 0.75 D or more of corneal astigmatism and adjust it later with LASEK if necessary. In primary excimer laser surgeries, I correct refractive astigmatism of 0.25 D or more. Kjell U. Sandvig, MD, PhD, practices at Oslo Eye Center in Norway. Dr. Sandvig may be reached at tel: +47 22 93 12 60; fax: +47 22 93 12 70; e-mail: kusandvig@gmail.com. KARL G. STONECIPHER, MD Surgeons love to believe that the next technology is going to take them one step further to the prize of perfection. We look at our outcomes and we monitor our nomograms, and yet we still have not quite been able to fit the human cornea or ocular anatomical system into our box of perfection. With regard to corneal refractive surgery, my current enhancement rate is 0.71% (n=4,029) in corrections from -1.00 to -12.00 D with up to 5.00 D of June 2014 Cataract & Refractive Surgery Today Europe 41

What is the Minimum Amount of Astigmatism That You Correct? (continued) cylinder. Including only enhancements for premium IOLs, the percentage is even less. I can only hope that future technologies will help me approach my LASIK levels in cataract surgery. Currently, intraoperative aberrometry and a laser-assisted cataract surgery (LACS) technique are helping me to achieve 93% within ±0.50 D and 87% within ±0.25 D of intended astigmatic correction. Are my patients happy? Currently, if I look at patient satisfaction rates with LASIK and cataract surgery, patients are not requesting additional intervention until cylinder reaches the 0.75 D level. (The number for me is 0.68 D on average.) With LACS, 82% of patients achieve outcomes of 20/20 or better, and 48% achieve 20/16. I was intrigued when I read the study by Villegas et al, 1 which confirmed outcomes I had already seen in my practice. I currently suggest an astigmatic intervention at 0.75 D or higher in cataract patients and correct down to 0.25 D of astigmatism in laser refractive surgery. With regard to intraocular technology, at 1.00 D of cylinder or higher I use a toric IOL, and with 0.75 to 1.00 D I use a laser astigmatic relaxing incision at the time of surgery. Can we do better? I sure hope so. Karl G. Stonecipher, MD, is Director of Refractive Surgery at The Laser Center, in Greensboro, North Carolina. Dr. Stonecipher may be reached at tel: +1 336 288 8823; e-mail: stonenc@aol.com. KHIUN F. TJIA, MD I started using toric IOLs in 2008 in a copayment regimen in the Netherlands. After implanting more than 1,800 of these lenses, I feel confident in my guidelines for patient counseling. Every cataract patient is informed of the available IOL options, which include standard monofocal, multifocal (if reduced spectacle dependence is desired), and monofocal toric lenses. I emphasize enhanced quality of vision more than spectacle independence for distance. The current thresholds above which we discuss a toric IOL as an option are 1.25 D for WTR astigmatism, 1.00 D for oblique astigmatism, and 0.80 D for ATR astigmatism. Our standard incision is located superiorly. I still feel reluctant to correct small degrees of cylinder, as my SIA in a consecutive series of 300 cases in 2010 was 0.30 D on average (0.30 D standard deviation; at least 3 months postoperatively). I anticipate repeating the SIA study, as we have switched to a new torsional phaco tip technology with the potential to reduce wound-induced change. I also look forward to IOL injection innovations, as we know that the greatest amount of wound stretch is caused by IOL delivery into the eye. Nevertheless, I have the impression that toric IOL patients are among the most satisfied in our practice. I cannot understand why some cataract surgeons have not embraced toric IOL technology, other than for financial reasons. Khiun F. Tjia, MD, is an Anterior Segment Specialist at the Isala Clinics, Zwolle, Netherlands. Dr. Tjia is Editor Emeritus of CRST Europe. He may be reached at e-mail: kftjia@planet.nl. PAOLO VINCIGUERRA, MD When I hear colleagues stating that patients do not perceive a small amount of residual astigmatism, my answer is this: Try to tell my refractive surgery patients that leaving ±0.50 D of astigmatism uncorrected makes no difference to their postoperative outcomes especially if one eye is perfectly corrected and the other has residual astigmatism. Letters on an eye chart do not tell much about patients perceived quality of vision. Additionally, if residual astigmatism is present, the perceived quality of vision changes significantly when the pupil is dilated. Since recently adopting the Verion Image Guided System (Alcon) for toric IOL alignment, I have started to correct as little as 0.50 or 0.75 D of astigmatism. In patients who previously had the fellow eye treated without cylinder correction, and that eye has a 0.50 to 0.75 D residual astigmatic error, they notice the difference clearly. Many questions about residual astigmatism remain: How many surgeons spend time measuring the different aspects of astigmatism? How many calculate how much astigmatism comes from the anterior cornea, the posterior cornea, the lens, and the total vectorial addition of anterior and posterior corneas? How many learn how to separate cylinder from coma or other higher-order aberrations? How many understand that the effects of astigmatism change with pupil size? I believe that, if one does not perform the previously specified measurements, one is not always able to detect how much astigmatism must be treated or identify the correct axis. Approximations of measurements can lead to the decision to treat only higher astigmatism. Regarding refractive surgery, 100% of my treatments are custom ablations with a corneal wavefront-guided profile and full astigmatism correction, even ±0.25 D. Consider that part of the astigmatism in a precise analysis frequently comes from a higher-order error (eg, coma): If one does not perform a custom ablation, many patients will have some residual astigmatism. n Paolo Vinciguerra, MD, is the Chairman of the Department of Ophthalmology, Istituto Clinico Humanitas, Milan, Italy. Dr. Vinciguerra may be reached at e-mail: paolo.vinciguerra@humanitas.it. 1. EA Villegas, E Alcón, P Artal. Minimum amount of astigmatism that should be corrected. J Cataract Refract Surg. 2014;40(1):13-19. 42 Cataract & Refractive Surgery Today EUROPE June 2014

(Continued from page 39) Figure 2. Differences in visual acuity before and after correction of astigmatism. VA when correcting amounts of astigmatism higher than 0.30 D, as long as the axis orientation of the correction is perfect. However, an axis error of 10º leaves residual astigmatism of 35% of the cylinder correction with a 40º change in orientation and an additional defocus equal to 50% of the remaining astigmatism. For example, a 10º error for a correction of -0.50 D of cylinder leaves 0.17 D of residual astigmatism with the axis rotated 40º and induced sphere of 0.09 D. Such errors in the correction of small amounts of astigmatism could adversely affect VA in two ways: (1) due to potential breakdown in the neural adaptation process 2 to the original orientation of astigmatism, and (2) due to the addition of positive spherical power, thereby inducing myopia. CONCLUSION Our results provide an argument to leave small values of natural astigmatism, typically below 0.50 D, uncorrected in refractive and cataract surgery procedures. n Pablo Artal, PhD, is a Professor at the Laboratorio de Optica, Universidad de Murcia, Campus de Espinardo, Spain. Professor Artal states that he has no financial interest in the material presented in this article. He may be reached at tel: +34 968357224; e-mail: pablo@um.es. Eloy A. Villegas, PhD, is a researcher at the Laboratorio de Optica, Universidad de Murcia, Campus de Espinardo, Spain. Dr. Villegas states that he has no financial interest in the material presented in this article. 1. EA Villegas, E Alcón, P Artal. Minimum amount of astigmatism that should be corrected. J Cataract Refract Surg. 2014;40(1):13-19. 2. Artal P, Chen L, Fernández EJ, et al. Neural adaptation for the eye s optical aberrations. J Vis. 2004;4:281-287.