OCT 2012

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

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64 EW GLAUCOMA February 2011 October 2012 Surgeons say adjusting IOP based on CCT doesn't work by Matt Young EyeWorld Contributing Writer Formulae that do so are no better in predicting glaucoma risk, researchers find L ogically, it makes sense that central corneal thickness (CCT) could impact accurate tonometry measurements of IOP. Businessweek.com laid the logic out back in 2004 in an article titled "Getting A Better Fix On Glaucoma." "Ophthalmologists have long known that an unusually thin or thick cornea can skew results from the pressure test," according to the article. "Just as it takes less pressure to indent a balloon than an automo- bile tire—regardless of the air pres- sure inside—a thinner cornea is more easily pushed in, tricking the tonometer into registering a lower pressure. A patient with a thin cornea may seem to have normal eye pressure even if it's high. Con- versely, one with a thick cornea may have a high reading, leading an ophthalmologist to treat a problem that may not exist." But more and more, ophthal- mologists and researchers are realiz- ing that what some have "long known" may have been false wisdom, at least to some extent. After all, if this logic were cor- rect, then adjusting IOP based on CCT would help to calculate actual risk for primary open-angle glaucoma (POAG). That is simply Central corneal thickness (CCT) is a biomaker for structural or physical factors involved in the pathogenesis of primary open-angle glaucoma, according to James D. Brandt, M.D. not the case, according to newer research. "The calculation of individual risk for developing POAG in ocular hypertensive individuals is simpler and equally accurate using IOP and CCT as measured, rather than apply- ing an adjustment formula to correct IOP for CCT," reported James D. Brandt, M.D., director, Glaucoma Service, UC Davis Eye Services, Sacramento, Calif., in a study published in Ophthalmology. Formulae developed by various researchers to correct IOP for CCT were put to the test but did no better than a model that did not adjust IOP for CCT. SLT, medical therapy results relatively similar as first-line treatment S elective laser trabeculoplasty (SLT) has been suggested as a first-line treatment for patients with open-angle glaucoma or ocular hypertension, and a new study in the September issue of Journal of Glaucoma supports the option. L. Jay Katz, M.D., director, Glaucoma Services, Wills Eye Institute, Jefferson Medical College, Philadelphia, and colleagues prospectively randomized 127 eyes (69 patients) to receive either SLT or a prostaglandin analog. The SLT group (100 applications 360 degrees) were retreated with SLT if the target IOP range was not achieved in any of the six visits over the 1-year study time frame. The prostaglandin group received additional medical therapies if their target IOPs were not achieved. Mean IOP (both eyes) at last follow-up was 18.2 mm Hg (6.3 mm Hg reduction) in the SLT arm and17.7 mm Hg (7.0 mm Hg reduction) in the medical arm. By last follow-up, 11% of eyes received additional SLT, 27% required additional medication. McGee Eye Institute, and clinical associate professor, University of Oklahoma, but he agreed that it does not often do so in a major way. "What I do is look at the pres- sure and corneal thickness, but un- less the corneal thickness is a major outlier, I don't let it influence [my assessment]," Dr. Sarkisian said. "It needs to be greater than 600 mi- crons or less than 500 microns for me to think GAT is going to be sig- nificantly altered to the point that I have to adjust the target range in controlling glaucoma." There is still a following of physicians that adjust for IOP though, Dr. Sarkisian said. "I have seen docs who have re- Source: Karl Brasse, M.D. Dr. Brandt reasoned that CCT is only one component of many mate- rial properties of the cornea that might influence tonometry. Current formulae do not take certain other biomechanical properties into ac- count, he said. Further, the "influ- ence of CCT on glaucoma risk is caused by more than just tonometry artifact. … CCT is a biomarker for structural or physical factors in- volved in the pathogenesis of POAG," he reported. Glaucoma specialists agree Adam Reynolds, M.D., in private practice, Intermountain Eye Clinic, Boise, Idaho, agreed that formulae to correct IOP for CCT are flawed. "You cannot construct an accu- rate algorithm to adjust IOP for cen- tral corneal thickness," Dr. Reynolds said. "IOP is a measurement that has too much variability for other rea- sons, and many of those variables we don't understand well yet." Dr. Reynolds added that a cornea made thin from refractive surgical intervention is completely different from a cornea that is natu- rally thin. Further, even when pressure is adjusted after Goldmann applana- tion tonometry (GAT), "We're talk- ing about 1-2 mm Hg in most cases," Dr. Reynolds said, "not a sig- nificant amount compared to other risk factors." CCT does influence GAT, said Steven R. Sarkisian Jr., M.D., Glaucoma Fellowship director, Dean ferred me patients with an adjusted IOP," Dr. Sarkisian said. "They write, 'Adjusted IOP based on corneal thickness.' There are people doing that for sure." Dr. Sarkisian said ophthalmolo- gists are a long way off from an ab- solute formula for adjusting IOP. "However, I think it's important to be aware of this issue and take it into consideration," he said. Barbara A. Smit, M.D., in private practice, Spokane Eye Clinic, Spokane, Wash., said, "I personally have never been a big believer in adjusting for corneal thickness." CCT correction formulae provide a false sense of accuracy in determining true IOP, she said. "I take CCT into account in a more general sense," she said. "If the patient has a thick cornea I pay at- tention to that and think probably he's at lower risk of glaucoma. A thin cornea might mean higher risk. These are factors to flavor my deci- sions rather than [make me] think I can calculate what the [true] pres- sure is. If an eye has a pressure of 35 and the cornea is fairly thick, the patient may be in a lower risk cate- gory so I will observe him. If a thin cornea were involved, I might treat him sooner." EW Editors' note: Drs. Reynolds, Sarkisian, and Smit have no financial interests related to this article. Contact information Reynolds: 208-373-1200, adamreynolds@cableone.net Sarkisian: 405-271-1093, Steven-Sarkisian@dmei.org Smit: 509-456-0107, barbsmit@hotmail.com

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