Eyeworld

Supported by an unrestricted educational grant from Johnson & Johnson Vision, Alcon, and Carl Zeiss Meditec NOVEMBER 2019 | SUPPLEMENT TO EYEWORLD | 3 WTR astigmatism of 1–1.5 D with relaxing incisions; for greater amounts, he uses a toric IOL. "You want to undercor- rect them to leave them with a little bit of WTR astigma- tism," he said. "Astigmatism drifts toward ATR over time, so if you leave a patient with 0.2 or 0.3 D of WTR astig- matism, they will have great vision, and it will last for many years." He recommends slight overcorrection of ATR astig- matism because it continues to drift further ATR over time. His threshold for ATR astigmatism is 0.4 or 0.5 D with a toric IOL. Dr. Koch explained that patients with oblique astigma- tism—unless they have always had it—most likely are mov- ing from having WTR to ATR astigmatism. "I treat them fully, but I place the IOL at the alignment or maybe even a little on the ATR side in hope that we will get them a good correction for a longer period of time," he said. "We understand what the anterior cornea does in terms of surgically induced astig- matism. We have measured it, and it is on average 0.1 D for a 2.4 mm clear corneal temporal incision," he said. "However, as Holladay's paper shows, we just do not have a good handle yet on all the factors that contribute to surgically induced astigmatism, but it is a combination of front of the cornea, back of the cornea, and IOL tilt," he said. 3 Some patients have signif- icant lenticular astigmatism. "Research by our group and others has shown that pre- operative crystalline lens tilt predicts postoperative IOL tilt," Dr. Koch said. 4 "If we could routinely measure preop crystalline lens tilt, we would improve our outcomes." Conclusion In addition to mastering astig- matism correction, surgeons should know how to manage postoperative astigmatism or refer patients to a colleague to manage it. "If patients are posterior cornea or perhaps lens tilt may have more influ- ence than we might suspect," he said. Assessing the posterior cornea Before surgery, the anterior cornea provides clues regard- ing the posterior cornea, Dr. Koch said. 1,2 "Because of this, we have been able to generate regression formulas based on measuring the anterior cornea to estimate posterior corneal astigmatism. As expected, however, some patients' cor- neas do not fit these mathe- matical models," he said. Dr. Koch explained that one swept-source optical bi- ometer provides total corneal astigmatism values and inserts them into a toric calculator with total keratometry. "This is very promising but needs further evaluation for valida- tion," he said. Otherwise, he said, regression-based toric cal- culators such as the Baylor nomogram, Barrett Universal Toric, Abulafia-Koch, and updated IOL manufacturers' toric calculators are required to incorporate estimated pos- terior corneal astigmatism into astigmatic planning. Managing astigmatism Dr. Koch usually manages going to pay upfront to treat their astigmatism, you want to take them across the finish line," he said. n References 1. Koch DD, et al. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg. 2012;38:2080–2087. 2. Koch DD, et al. Correcting astigma- tism with toric intraocular lenses: ef- fect of posterior corneal astigmatism. J Cataract Refract Surg. 2013;39:1803– 1809. 3. Holladay J, Pettit G. Improving toric intraocular lens calculations using total surgically induced astigmatism for a 2.5 mm temporal incision. J Cataract Refract Surg. 2019;45:272–283. 4. Wang L, et al. Evaluation of crystal- line lens and intraocular lens tilt using a swept-source optical coherence to- mography biometer. J Cataract Refract Surg. 2019;45:35–40. Dr. Koch is professor and Allen, Mosbacher and Law Chair in Ophthalmology, Cullen Eye Insti- tute, Baylor College of Medicine, Houston. He can be contacted at dkoch@bcm.edu or 713-798- 6443. The basics: WTR and ATR astigmatism " With-the-rule" astigmatism: + refractive astigmatism at 90 •In virgin eyes almost always because the anterior cornea is steep vertically, with the magnitude reduced by vertical steepness of the posterior cornea and cr ystalline lens tilt •Corrected with spectacle lens with vertical axis •Plano + 1.00 x 90: the cylinder power is at 180 "Against-the-rule" astigmatism: + refractive astigmatism at 180 •Due to some combination of horizontal steepness of the anterior cornea, vertical steepness of the posterior cornea, and cr ystalline lens tilt •Corrected with spectacle lens with horizontal axis •Plano + 1.00 x 180: the cylinder power is at 90

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