EyeWorld India December 2013 Issue

December 2013 15 EWAP FEAturE Measuring posterior corneal astigmatism a valuable tool for surgeons by Ellen Stodola EyeWorld Staff Writer continued on page 16 AT A GLANCE • Measuring posterior corneal astigmatism helps to account for the astigmatism on the back of the cornea, rather than just the front. Without these measurements, it is sometimes necessary to rely on estimates. • Knowing the posterior corneal astigmatism can help a surgeon choose which IOL is appropriate for a patient. • The back of the cornea astigmatism does not generally change with age, although against-the-rule astigmatism becomes more common with age. Case sample with posterior corneal astigmatism of –0.51 D Source: Mitchell Weikert, MD These tests can help surgeons account for the back of the cornea and get a total measurement of a patient’s astigmatism M easuring posterior corneal astigmatism (PCA) can be an important step in determining a patient’s overall astigmatism, and its value in toric IOL use is being discussed, with a number of studies and devices used for this. Being able to see and measure the back of the cornea, as well as the front, is important for obtaining accurate measurements, rather than just estimations and approximations. Mitchell P. Weikert, MD , associate professor of ophthalmology, and director of the residency program, Cullen Eye Institute, Baylor College of Medicine, and medical director, Lion’s Eye Bank of Texas, Houston, Texas, USA, and D. Rex Hamilton, MD , associate clinical professor of ophthalmology, Jules Stein Eye Institute, and director, UCLA Laser Refractive Center, Los Angeles, Calif., USA, commented on measuring posterior corneal astigmatism, why it is important, and factors that need to be considered when looking at this. How to measure PCA Dr. Weikert said there are currently several tests for measuring posterior corneal astigmatism. “You need an imaging technology that can obviously show both the front surface and the back surface of the cornea,” he said. He cited the Galilei Dual Scheimpflug Analyzer (Ziemer Ophthalmic Systems, Port, Switzerland) as a test with this capability. He said that although optical coherence tomography and very high frequency ultrasound-based instruments allow for viewing the back of the cornea, they currently lag behind Scheimpflug technology in calculating the cornea’s overall astigmatism. “In addition to being able to image the front surface and the back surface of the cornea, that image needs to be accurately analyzed to determine the curvature of the cornea,” Dr. Weikert said. This measurement will help identify the focusing power. Dr. Hamilton said that he uses the Galilei Dual Scheimpflug Analyzer. “It turns out that the back surface of the cornea does contribute to the overall astigmatism,” Dr. Hamilton said. “It’s pretty much ignored in conventional keratometry measurements, such as those obtained with the IOLMaster [Carl Zeiss Meditec, Jena, Germany].” He said rotating Scheimpflug devices that can measure posterior corneal astigmatism use a slit beam of light and rotate it around the center of the cornea, imaging it from multiple angles. This allows for a view of both the front and back surface of the cornea. Who has this technology Dr. Weikert said posterior corneal astigmatism technology is available to a number of doctors. Currently the most common measurement for this is with a Scheimpflug device. Dr. Hamilton indicated that not many surgeons are using posterior corneal astigmatism at this time. “It’s not something that is generally used right now,” he said. However, he believes the technology will be useful in the future. There has been quite a bit of buzz recently about it. Dr. Hamilton said that not a lot of cataract surgeons have this more sophisticated topography technology; it’s mainly refractive surgeons who have it. Why PCA is important Posterior corneal astigmatism is important when it comes to choosing an appropriate lens for a patient, especially when toric IOLs are involved. “Most devices only measure the front surface of the cornea and then adjust that measurement to take into account the contribution of the back surface of the cornea,” Dr. Weikert said. Though this can be helpful for approximations, it can lead to assumptions. “Sometimes those assumptions are correct and sometimes those assumptions lead to errors.” Dr. Hamilton said posterior corneal astigmatism measurements could be useful to “fine-tune our results for toric lenses for cataract surgery.” Without imaging the back of the cornea as well as the front,