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Posterior corneal astigmatism vital to calculating correct total astigmatism
by Erin L. Boyle EyeWorld Senior Staff Writer
Posterior corneal astigmatism
Baylor toric IOL nomogram Source (all): Douglas D. Koch, M.D., and Li Wang, M.D.
Not 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, M.D., professor and the Allen, Mosbacher, and Law Chair in ophthalmology, Cullen Eye Institute, Baylor College of Medicine, Houston, and Li Wang, M.D., 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 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.), measure it "moderately accurately," he said.
"I think that our measurements could improve," Dr. Koch said. "We do find that even the Galilei, which has a wonderful dual Scheimpflug mechanism for measuring the back, does not always seem to capture all of the posterior corneal astigmatism, and especially in patients [who have] with-the-rule astigmatism, it still seems to underestimate the amount of posterior corneal astigmatism based on our actual refractive outcomes."
Dr. Koch has created a nomogram that incorporates: 1) the mean posterior corneal astigmatism in eyes having either with-the-rule or against-the-rule astigmatism and 2) the effect of against-the-rule drift that occurs with age. He said that their data indicate that the new nomogram greatly improves accuracy with toric IOLs.
In addition, manufacturers are interested in providing clinicians with this information because they are finding similar results retrospectively in their data, he said. However, to disseminate a new nomogram themselves, they would have to validate it in a clinical trial with the U.S. FDA, which could slow the approval process.
Toric IOLs
Dr. Koch began examining posterior corneal astigmatism when he noticed that some patients had unexpected results with toric IOLs. Patients who had with-the-rule astigmatism were being overcorrected, while patients who had against-the-rule astigmatism were undercorrected.
"It has a huge impact on my decision making now in patients who are seeking astigmatic correction during cataract surgery," Dr. Koch said. "It's completely changed everything I'm doing with regard to both relaxing incisions and with regard to the selection of toric IOLs. "I have backed off on toric IOL power in patients who have with-the-rule astigmatism and conversely ramped it up for those patients with against-the-rule. For example, for someone who has 1 diopter with-the-rule astigmatism, I will not put a toric IOL in because I am likely to overcorrect him and leave him with against-the-rule astigmatism," he said. He cited a long-term study by K. Hayashi and colleagues that followed patients' astigmatic change after undergoing 3-mm clear corneal temporal incisions. The study also had a control group that did not undergo cataract surgery. Researchers found that both groups had a comparable change of against-the-rule shift after more than 10 years.
