EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/111385
50 EW FEATURE February 2011 Refractive/astigmatism February 2013 Getting astigmatic cataract patients into corneal shape by Maxine Lipner Senior EyeWorld Contributing Writer 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. Bee���ng 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, M.D., adjunct professor of ophthalmology, University of Minnesota. ���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, M.D., clinical professor of ophthalmology, Baylor College of Medicine, Houston, 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 pe- ripherally 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 (HaagStreit, Mason, Ohio) 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, N.Y.), the Pentacam (Oculus, Lynwood, Wash.), 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, M.D., showed that the back surface is becoming significant at the level of about �� or �� 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.) and Clarity (Holos, Pleasanton, Calif.), 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 in- ventory. 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 upcharged to the patient. Optimizing outcomes How can practitioners best use devices to optimize results for astigmatic cataract patients? Dr. Lane stressed that it���s important to begin by distinguishing lenticular from corneal astigmatism. ���Obviously the astigmatism associated with the lens will be absent following the removal of the cataract,��� he said. ���So you need to have an idea of what the post-operative corneal astigmatism will be.��� He finds that���s best accomplished pre-op with the aid of different available tools. ���Some of them How devices measure up Practitioners today are fortunate to have a variety of devices for measuring astigmatism at the ready. Here���s what���s available: Manual keratometry determines the quantity of astigmatism and the axis, according to Dr. Trattler. ���It���s good for planning cataract surgery, but it doesn���t help us to figure out whether the cornea is regular or irregular,��� he said. The IOLMaster considers corneal shape, using three measurements for astigmatism. ���It���s a very rudimentary method, but it���s very accurate as far as helping us plan for the right intraocular lens power,��� Dr. Trattler said. This will tell if astigmatism is present, how steep the cornea is, and help with surgical planning but will not identify irregular astigmatism. Corneal topography uses imaging technology to get a sense of the magnitude of the astigmatism and the shape of the cornea, Dr. Trattler explained. It can tell if the cornea is regular or irregular and if the patient has a condition such as keratoconus. The Pentacam measures the shape of the cornea and the magnitude and regularity of the astigmatism. ���You press it back and can determine the shape of the cornea and if there���s any regularity or irregularity,��� Dr. Trattler said. The Galilei gives measurements for both anterior and posterior corneal curvatures. This can be helpful in considering what posterior astigmatism contributes, which has gained importance thanks to Dr. Koch���s new nomogram for implanting toric IOLs, which uses both measurements, Dr. Trattler explained. The Clarity and the ORA offer intraoperative wavefront measurements of astigmatism. These allow practitioners during surgery to measure the cornea through the power of the astigmatism. ���It helps you to fine-tune your planning,��� Dr. Trattler said. Devices such as the iDesign (Abbott Medical Optics, AMO, Santa Ana, Calif.), the iTrace (Tracey Technologies, Houston), and the OPD (Marco, Jacksonville, Fla.) can analyze a combination of topography and wavefront measurements at the same time. These units can give corneal shape and also determine whether the astigmatism is symmetrical or asymmetrical.