EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
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40 EW FEATURE February 2011 Diagnostic technologies for cataract surgery August 2013 Measuring posterior corneal astigmatism a valuable tool for surgeons by Ellen Stodola EyeWorld Staff Writer 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 AT A GLANCE 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; and D. Rex Hamilton, MD, associate clinical professor of ophthalmology, Jules Stein Eye Institute, and director, UCLA Laser Refractive Center, Los Angeles, 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 • 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. 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 Monthly Pulse T he survey results for treating 1.25 diopters of astigmatism seem to clearly reflect a perception of superior outcomes with the use of toric IOLs vs. astigmatic cuts of any kind for even relatively low levels of astigmatism. As more toric IOLs become available I'm sure we'll see the percentage of their use rise. The fact that only 1.7% less surgeons would treat 1.25 with-the-rule vs. 1.25 against-the-rule makes me wonder if this difference would go up sharply if the question was asked regarding 0.75 diopters of astigmatism. Our understanding that the posterior cornea is generally against-the-rule should hopefully have some influence on what surgeons would consider doing. I am now reluctant to treat small amounts of with-therule astigmatism at the time of cataract surgery for fear of inducing refractive astigmatism that would otherwise not be there. I prefer to wait and see if it's necessary in these small with-the-rule cases and also tend to treat with-the-rule conservatively and against-the-rule aggressively in general. This also makes sense when you consider that most patients will tend to drift against-the-rule over time. Case sample with posterior corneal astigmatism of –0.51 D 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 Keeping a Pulse on Ophthalmology It is interesting to see that 2/3 of physicians would opt for manual phaco, and it's possible this percentage would have been even higher if the option of manual phaco plus LRIs was offered, as that is what I would do for most patients in this situation and would likely choose to have done on myself. Less than 10% of surgeons would opt for femto-assisted surgery with femto LRI, which I think is revealing. More than half the surgeons felt that intraoperative aberrometry needs more published outcomes data, and almost three times as many have no interest vs. are planning to purchase within the next year. It appears that this technology still has some obstacles to overcome. Steven Safran, MD, Cataract Editorial Board member Source: Mitchell Weikert, MD