EyeWorld Asia-Pacific June 2012 Issue

June 2012 18 EWAP FEATURE AT A GLANCE • SIA can be calculated by subtracting the pre-op K from the predicted post-op K • Moving incisions to be on axis instead of fixed will reduce SIA • Larger incision sizes create more SIA than smaller incisions • Lower power IOLs create less SIA than higher power IOLs About 37% of cataract incisions had a small Descemet’s detachment like this one post-op Source: Douglas D. Koch, MD, and Li Wang, MD Determining your surgically induced astigmatism by Michelle Dalton EyeWorld Contributing Editor Experts weigh in on how to define, calculate, and minimize SIA I nducing some astigmatism is to be expected during cataract surgery, but with typical cataract incisions becoming smaller and smaller, there may be a time when the discussion is moot. Until then, how surgically induced astigmatism (SIA) is caused and how to minimize it remains relevant. The increased use of toric IOLs has thrust the entire issue of SIA back into the limelight, experts say. The moment a keratome passes through the cornea, “the astigmatic characteristics of that cornea are different,” said Warren Hill, MD, East Valley Ophthalmology, Mesa, Ariz., USA. “We’re treating the post- op astigmatism by doing our best to predict it.” Jack Holladay, MD , clinical professor, Baylor College of Medicine (BCM), Houston, Texas, USA, said using refraction instead of post-op keratometry “can’t be done because the crystalline lens could contribute to the pre-op astigmatism.” Post-op Ks, however, “are the only measurements that tell you what you’ve done to change the cornea.” Operating temporally or superiorly “will give you very different results,” said Douglas D. Koch, MD , professor and the Allen, Mosbacher, and Law Chair in ophthalmology, Cullen Eye Institute, BCM, as will scleral vs. corneal incisions. “Scleral incisions are going to be more stable, but certainly very few of us do them. It’s why using smaller incisions is so beneficial to minimize SIA,” he said. What is SIA? SIA is influenced by the incision’s “size, location, corneal radius and corneal thickness, corneal rigidity, and the folded diameter of the lens as it passes through the eye,” Dr. Hill said. A 24-D lens inserted through a 2.2-mm incision “will exert a different influence on the incision than a 10-D lens,” he said. “Larger incisions will induce more astigmatism than smaller ones.” SIA “is not consistent. Eyes will always heal differently from each other,” and that also affects SIA, said Guy Kezirian, MD , founder, SurgiVision Consultants Inc., Scottsdale, Ariz., USA, an ophthalmic consulting firm. Corneal astigmatism differs from refractive astigmatism, and the “only way” to determine the SIA is “by running cross cylinder solution—what you have pre-op and what you have post-op and the vector difference between the two tells you what your SIA is,” Dr. Holladay said, adding the incisional length and incision location will be the two key determinants of SIA. “You’ll have more of an effect at 90 degrees than at 180,” he said. Moving incisions to be on-axis instead of fixed would help alleviate the majority of the issue, Dr. Kezirian said. Operating on the steep axis “will always cause flattening, which will always reduce the amount of astigmatism a person has,” Dr. Holladay said. “You get a bigger effect vertically than horizontally, but you’d still be reducing the astigmatism if on the steep axis.” Dr. Kezirian urges surgeons to adapt incision location to be on axis “because when you have to do a vector addition, you need to know both the amount and the placement of the astigmatism to add it to the patient’s corneal astigmatism to develop a solution.” Calculating your own SIA In the post-LASIK patient, “we can’t figure out the true corneal power so it’s next to impossible to determine the exact IOL power to place,” said Robert Brass, MD , founder, Brass Eye Center, Latham, NY, USA. Using a toric IOL can help address some of the residual astigmatism issues, he said. Dr. Holladay said the formula is simple—post-op Ks minus pre-op Ks will give you the SIA, and adding Dr. Kezirian: “Distribution of the standard deviations (SD) of the change in keratometric absolute cylinder amounts, by surgeon. Data from the SurgiVision DataLink IOL Edition software. The graph is based on data from 2,264 eyes from 55 surgeons who had entered 20 eyes or more having both preoperative and 3-month postoperative keratometry values. The data set excludes eyes that had LRIs or prior to surgery. The distribution plots the SD of the absolute change in keratometry cylinder amounts for each surgeon. One SD includes approximately 67% of eyes. “Only 6% of surgeons have SIA values with a standard deviation of 0.1 D or less, and only 37% of surgeons have induced SIA values with standard deviations of 0.30 D or less. The large variation in the induced astigmatism amounts may undermine the value of toric calculators, and speak toward the need for making surgical incisions on-axis or 90 degrees away from the preoperative cylinder axis location.” Source: Guy Kezirian, MD