Eyeworld

54 T he belief that LASIK should- n't be performed on thin corneas because of the risk of sight-threatening ectasia isn't scientifically valid, ac- cording to one researcher. Abnormal corneal topography is the most important pre-op risk fac- tor for the post-surgical complica- tion, said William B. Trattler, M.D., director, cornea, Center for Excel- lence in Eye Care, Miami. "Most patients who have thin corneas are not abnormal," Dr. Trattler said. Corneal ectasia, which is a pro- gressive bulging of the cornea, much like that of keratoconus, can severely reduce both uncorrected and best corrected visual acuity. Researchers have preliminarily identified risk factors for the complication, al- though a list of true risk factors is still being hotly debated. Some pro- posed risk factors include forme fruste keratoconus, residual stromal bed less than 250 µm, high myopia, and pre-op corneal thickness less than 500 µm. Dr. Trattler cited a retrospective review of 1,700 patients who were scored using the Randleman scoring system. "We found that if the topogra- phies are normal, the Randleman criteria aren't that helpful for pre- dicting who's at risk for ectasia," he said. "I think that the bottom line answer is that corneal topography is reason numbers 1-10 as a sign for who is at an increased risk for ecta- sia." Dr. Trattler said if the patient's corneas are biomechanically weak, they will warp and develop kerato- conus. "When patients have asymme- try and signs of early kerataconus, you know that those corneas are al- ready weakened, and that is differ- ent from patients who are younger or who have thin corneas," he said. "There is no evidence that thin equals weak." Dr. Trattler said results from corneal crosslinking prove his point, since crosslinked corneas become more compact. "As they become more compact, the corneas stiffen and get stronger," he said. "The same cornea before and after crosslinking is stronger, but also thinner." It is common knowledge that with age, corneas become stiffer. Thickness, however, is a different story. "There's no relationship be- tween aging and thickening or thin- ning. It stays about the same," Dr. Trattler said. "If you have a patient who is 500 µm and 18 years old, the cornea is going to be about the same thickness but much stiffer when the patient is 70." African-American patients, too, naturally have thin corneas, but are not at increased risk for ectasia after LASIK or for developing kerato- conus. "If thickness was related to weakening, we would expect more African Americans to develop kera- toconus than Caucasians, but this has never been shown to be the case," Dr. Trattler said. He cited a study presented at the 2011 ASCRS•ASOA Symposium and Congress in San Diego in which he evaluated the pre-op topogra- phies of 104 patients with post- LASIK ectasia. Only 15 of those patients had pre-op pachymetry measurements of less than 500 µm, and 12 of those 15 patients had either keratoconus, forme fruste keratoconus, or pellucid marginal degeneration. "The point here is that most pa- tients who develop ectasia have pre- op corneal thickness of more than 500 µm. For those patients with thin corneas, the ones that I have seen typically have pre-op topographic abnormalities that would exclude them from surgery in 2011," Dr. Trattler said. Dr. Trattler said three of the eyes in the series did have thin corneas and bilateral normal pre-op topogra- phy and developed ectasia, however, "these three eyes were in patients with very high myopia, and they did not have intraoperative pachymetry when they had their surgery more than 10 years ago. It is likely that these cases of ectasia were caused by the metal microkeratome creating a thicker-than-expected flap." More tissue is often removed by the laser to treat the refractive error in high myopes with a thinner cornea, which results in a patient having a residual stromal bed that is too thin—below 250 µm. "Since these cases occurred more than a decade ago, we do not know the actual flap thickness," Dr. Trattler said. "But if thin corneas were an independent risk factor, we would expect many more cases of ectasia to be popping up in patients who had thin pre-op corneas and normal topography." Dr. Trattler said corneal topogra- phy measurements can identify pa- tients who may have signs of subclinical keratoconus or other corneal abnormalities. Those pa- tients would not be good candidates for LASIK, he said. EW Editors' note: Dr. Trattler has financial interests with Abbott Medical Optics (Santa Ana, Calif.). Contact information Trattler: 305-598-2020, wtrattler@gmail.com EW REFRACTIVE SURGERY 54 May 2011 by Jena Passut EyeWorld Staff Writer Don't exclude thin cornea patients from LASIK for fear of ectasia Corneal thickness is not directly related to corneal strength. This patient had a thick cornea but was obviously weak biomechanically, which can be seen on topography as FFKC pre-op. This patient developed ectasia 27 months after LASIK Source: William B. Trattler, M.D. Bascom Palmer teams with Keio University for rescue mission B ascom Palmer recently sent its Vision Van, a fully equipped, 40-foot converted bus that contains a comprehensive examination room, three screening stations, a waiting room, and ophthalmic equip- ment, to Sendai, Japan, an area that was decimated in March's massive earthquake and tsunami. The Vision Van was also stocked with 1,000 pairs of eyeglasses to replace those lost by victims as they fled the earthquake, the ensuing tsunami, and multiple aftershocks. Richard Lee, M.D., a glaucoma specialist with Bascom Palmer Eye Insti- tute, traveled to Japan as a volunteer trainer for Japanese care providers. The van will allow local ophthalmologists to screen victims for eye injuries, infections, and inflammations from contaminated water, as well as replace eyeglasses lost during the crisis. The overall effort will last 3 months.

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