EyeWorld India March 2019 Issue

• September 2018 EWAP FEATURE 9 March 9 identified as regular, Dr. Gupta proceeds with biometry, currently using the IOLMaster 700 and the LENSTAR. “I like to use two devices because I like to compare the repeatability and accuracy of the measurements,” she said. Multiple devices These doctors’ routines for preop evaluation indicate that no single instrument is enough. According to Dr. Raviv, the routine use of multiple devices to measure astigmatism prior to surgery “speaks to the limitations of our technologies.” “No keratometry measurement is perfectly repeatable, and inter-device agreement varies considerably,” he said. While he said that the accuracy and repeatability of optical biometry has dramatically improved their refractive outcomes, as have the latest generation of IOL formulas such as the Barrett and Hill-RBF, Dr. Raviv admitted that “[t]he variability in our K measurements and inability to accurately measure the posterior cornea still remain a challenge.” “In fact, Graham Barrett’s latest formula improves its refractive outcomes by allowing multiple device K readings instead of just one,” he said. Dr. Raviv routinely uses three different devices—the LENSTAR, Cassini Total Corneal Astigmatism (Cassini Technologies, The Hague, the Netherlands), and a Topcon autorefractor/keratometer (Topcon Medical Systems, Oakland, New Jersey)—and more, if indicated. “For irregular eyes, we’ll also utilize a Placido disc-based topographer.” Each device has its strengths and weaknesses. “For IOL calculations, LENSTAR or IOLMaster 700 Ks are the best, auto-Ks are pretty accurate,” Dr. Raviv said. “Sim Ks from Placido-based devices are more approximate and shouldn’t be plugged into formulas except in extreme cases such as keratoconus when other keratometers fail. I find that Ks from Scheimpflug topographers are the least ideal for IOL calculations.” “In an ideal situation, all of these measurements will be consistent,” Dr. Donaldson said. “However, in some cases … the measurements will differ. In those cases, we need to determine why there is a discrepancy and treat the underlying cause. The most common cause is ocular surface disease.” Ocular surface matters “As part of our standard cataract evaluation, my technicians perform an Ocular Surface Disease Index questionnaire, tear osmolarity, and MMP-9 test,” Dr. Donaldson said. “This allows us to assess both signs and symptoms of ocular surface disease in preparation for surgery. In addition, if measurements are inconsistent or if irregularities are detected during topography or tomography, the patient should be brought back for additional measurements to ensure reliability before surgery.” In case of ocular surface disease, Dr. Donaldson initiates treatment and brings the patient back for follow-up evaluation a few weeks later. “I think it’s important when you’re assessing astigmatism to not forget the ocular surface,” Dr. Gupta said. “At my clinic I routinely look at osmolarity and meibography, but a simple thing that you can also do is use a little fluorescein strip to highlight any corneal staining that might alter the pattern of the astigmatism.” Dr. Thompson also cautioned against using measurements from untreated dry eye, but added that surgeons and their technicians should think about acquisition time; as noted earlier, the technique used to measure astigmatism using any device may introduce variables. “It’s not only about the ocular surface but also about technician comfort and speed of acquiring information so the tear film doesn’t break up and negatively affect our measurements,” he said. Again, the use of multiple devices can highlight the presence of ocular surface problems. Dr. Raviv noted that variable measurements among different devices may indicate one of three possibilities: “poor wettability” of the cornea from ocular surface disease or dry eye; an irregular corneal surface that destabilizes the overlying tear film; or a subtle ptosis, enough to create variability between blinks. “I address variable Ks from ocular surface disease by priming the ocular surface (with preservative-free tears, steroid drops, LipiFlow [Johnson & Johnson Vision, Santa Ana, California], IPL [intense pulsed light], immunomodulator, as indicated) and repeating measurements a few weeks later,” he said. “With epithelial basement membrane dystrophy patients, there is the option to perform a superficial keratectomy, especially if an advanced technology IOL is requested.” Adjustability and true measurements In the future, Dr. Donaldson looks hopefully toward IOLs that are adjustable postoperatively and that will be less dependent on preoperative measurements. The Light Adjustable Lens (RxSight, Aliso Viejo, California), for instance, already allows some surgeons to fine-tune their postoperative refractive results. Postop adjustability certainly makes preop astigmatism measurement “less impactful, but at the same time we want to go into surgery with the most accurate measurement so we either don’t need to adjust IOL power postoperatively or it’s a smaller adjustment,” said Dr. Thompson, who has experience with the LAL as a principle investigator in the RxSight United States FDA monitored clinical trials. Dr. Gupta also thinks that preop astigmatism measurement will remain valuable despite the ability to adjust postop. Among other things, LALs are only currently available as a monofocal. Preop astigmatism management will thus remain critical for patients who need presbyopia-correcting IOLs. In addition, there may be patients who are unable to come back and forth for additional measurements and for the lock-in procedure after LAL implantation. More critical developments may come in terms of how physicians measure astigmatism through the entire cornea. “There’s a key concept called total corneal astigmatism, which is a combination of the anterior and posterior corneal astigmatism,” Dr. Gupta said. “We are still looking for that device that gives us true total corneal astigmatism … potentially even have devices that measure the true value as opposed to an approximated value.” At the moment, devices only provide posterior corneal astigmatism measurements based on demographic assumptions, Dr. Thompson said. Future technologies will hopefully give surgeons true, direct measurements of both the anterior and posterior corneal astigmatism. Dr. Raviv said that he sees work toward improving posterior Continued on page 10