EyeWorld India March 2013 Issue

March 2013 8 EWAP FEATURE Getting astigmatic cataract patients into corneal shape by Maxine Lipner Senior EyeWorld Contributing Writer Beefing up your astigmatic measurement and treatment routine W hile for decades practitioners neglected residual astigmatism when removing cataracts, many wouldn’t dream of it now, according to Stephen S. Lane, MD , adjunct professor of ophthalmology, University of Minnesota, Minneapolis, Minn., USA. “The way that I look at astigmatism in cataract surgery is the same way I’ve looked at it my entire career: You would never want to give patients glasses without the astigmatism correction in the glasses,” Dr. Lane said. “So why with cataract surgery would we essentially ignore astigmatism and just treat the spherical correction?” Evolving equipment Practitioners today have their pick of equipment for measuring astigmatism. Jack T. Holladay, MD , clinical professor of ophthalmology, Baylor College of Medicine, Houston, Texas, USA, noted that technology to measure astigmatism has been evolving for years, beginning with original manual keratometers. “Those devices used a circle and measured the principal meridians of the reflected image,” Dr. Holladay said. “If the reflected image was an oval that meant that you had astigmatism.” With this method, usually four points, located about 3.2 mm apart, were used to measure the principal radii. As automated keratometers emerged, the size of the ring was reduced, changing the area that was actually measured. Since the magnitude and axis of astigmatism is not always constant as one moves peripherally from the center, significant differences would result. Even today, the IOLMaster (Carl Zeiss Meditec, Jena, Germany) measures points that are 2.5 mm apart on a 44 D cornea, while the Lenstar (Haag- Streit, Mason, Ohio, USA) measures two rings, one with points 1.65 mm apart and the other with points 2.35 mm apart, and arrives at an average of the two. Meanwhile, topographers would measure a zone from 1-9 mm in diameter. They would measure thousands of points within this zone. “That’s when we began to find with topography that as you moved out from the center of the cornea, the magnitude and axis of astigmatism was not constant on many patients,” Dr. Holladay said. The development of tomographers, beginning with the Orbscan (Bausch + Lomb, Rochester, NY, USA), the Pentacam (Oculus, Lynwood, Wash., USA), and Galilei (Ziemer Ophthalmic Systems, Port, Switzerland), would allow practitioners to measure both surfaces and the thickness over a 9 mm area, including the center. These would measure all of the points within a zone and determine the best fit to the surfaces using a sophisticated algorithm. Dr. Holladay said measuring all of these points on both surfaces increases the accuracy, particularly when corneal irregularity is present. Taking the back surface into account also improves accuracy. “What we’re finding today when we begin to correct astigmatism with toric IOLs is that assuming that the back surface astigmatism is a constant fraction of the front surface is not always true,” Dr. Holladay said. A recent study by Douglas Koch, MD, showed that the back surface is becoming significant at the level of about ¼ or ½ a diopter in terms of fine- tuning the total astigmatism of the cornea, he said. Another group of devices, intraoperative wavefront aberrometers such as the ORA (WaveTec, Aliso Viejo, Calif., USA) and Clarity (Holos, Pleasanton, Calif., USA), now allow surgeons to directly take refractive measurements at the time of surgery. Since they use the cornea as a lens when measuring the refraction, they automatically take into account both surfaces and any irregularities. This is also something Dr. Holladay sees as enhancing accuracy. “You’re actually sending light through the cornea alone (for the aphakic measurement), through the cornea and IOL for the pseudophakic measurement, bouncing it off the retina, and having it come back like a refraction,” he said. This technique is more accurate than measuring individual curvatures and indices of refraction and trying to calculate the sphere and cylinder. However, when the intraoperative measurements determine that the spheroequivalent power or toricity of the IOL is different than the values predicted pre-op, the surgeon must bracket the IOL power and toricity, which may require bringing 9 IOLs (three SEQ powers and three toricities for each SEQ power), Dr. Holladay explained. Nevertheless, this is better than waiting until surgery and finding that the optimal IOL is unavailable and not in the inventory. Also, surgeons are reimbursed for pre-op biometry and would not abandon these measurements with all of the cutbacks until there are payments for intraoperative measurements that are completely up-charged to the patient. AT A GLANCE • Different devices consider varying numbers of points on the cornea in determining astigmatism. • The posterior surface is becoming significant in fine- tuning corneal astigmatism following cataract surgery. • In reconciling conflicting measurements, practitioners herald different approaches. Report from the Pentacam, which can measure the shape of the cornea and astigmatic regularity and magnitude Source: Jack T. Holladay, MD