EyeWorld India March 2013 Issue

17 March 2013 EWAP FEATURE Posterior corneal astigmatism vital to calculating correct total astigmatism by Erin L. Boyle EyeWorld Senior Staff Writer N ot measuring the posterior corneal astigmatism could result in incorrect estimation of total corneal astigmatism, hindering toric IOL selection through overcorrection in with-the-rule astigmatism and undercorrection in against-the-rule astigmatism, researchers found. Douglas D. Koch, MD , professor and the Allen, Mosbacher, and Law Chair in ophthalmology, Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA, and Li Wang, MD , associate professor, Cullen Eye Institute, Baylor College of Medicine, Houston, are researching the effect of posterior corneal astigmatism and toric IOL selection in cataract surgery cases. Dr. Wang said both posterior and anterior corneal astigmatism measurements are important to all cases undergoing cataract surgery. “It would be best to measure posterior corneal astigmatism,” she said. “The magnitude of posterior corneal astigmatism cannot be predicted based on the amount of anterior corneal astigmatism. If there is no access to a device that measures the posterior corneal astigmatism, the average value of the posterior corneal astigmatism may be used.” Drs. Koch and Wang and colleagues published study results on the topic in the Journal of Cataract & Refractive Surgery. They evaluated 715 corneas of 435 consecutive patients, calculating total corneal astigmatism using ray tracing, corneal astigmatism from simulated keratometry, anterior corneal astigmatism, and posterior corneal astigmatism. They found that toric IOL selection based on anterior corneal measurements only could lead to problems. “Patients who have anterior Posterior corneal astigmatism Baylor toric IOL nomogram Source (all): Douglas D. Koch, MD, and Li Wang, MD with-the-rule astigmatism—in other words, the cornea is steep at 90 degrees anteriorly—tend to have, on average, 0.5 diopter (D) of steepness vertically along the posterior cornea, and because the posterior cornea is a minus lens, steepness vertically translates into power horizontally or against- the-rule effect refractive power at 180,” Dr. Koch said. “So you might measure a patient who has 2 D on the anterior cornea. And when all is said and done, that patient may only have 1.3 or 1.4 D on the total corneal power because the posterior cornea throws in about 0.5 or 0.6 D in the other direction.” Measuring devices Measuring posterior corneal astigmatism is a challenge, Dr. Koch said. Two devices on the market, the Galilei Dual Scheimpflug Analyzer (Ziemer, Port, Switzerland) and the Pentacam (Oculus, Lynnwood, Wash., USA), measure it “moderately accurately,” he said. continued on page 18