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Nomogram refined for IOL power calculation in post-LASIK eyes
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Using a new data set, originators of the Diehl-Miller nomogram for calculating IOL power in post-LASIK eyes have validated and refined their equation, according to one of the formula’s creators.
“The first study was a regression analysis, for which the polynomial equation we generated worked for the data set from which it was derived,” co-author Kevin M. Miller, MD, said. “But in order to validate that the formula works on patients in general, we needed to repeat the study using a fresh data set.”
Miller, along with Joseph W. Diehl, MD, developed the nomogram. The slightly refined equation combines both the original and new data set and is called the Diehl-Date-Miller nomogram. Both studies appeared in the Journal of Cataract & Refractive Surgery.
“We have a difficult time calculating lens implant power in patients who have had corneal refractive procedures such as LASIK, PRK and RK,” Miller told Ocular Surgery News. “Various people have proposed methods for improving the predictability. However, many of them have never gone back and conducted a validation study.”
Since the first published study, Miller has operated on a number of eyes with a history of prior LASIK.
“We went back and basically redid the study,” he said. “We generated a new second-order polynomial regression equation. We then compared the new equation to the previous equation and found that the two equations were essentially the same. This means that our regression formula works in general and not just on the data set from which it was derived.”
Miller is the Kolokotrones Professor of Clinical Ophthalmology at the Jules Stein Eye Institute, David Geffen School of Medicine at UCLA. Diehl was a medical student at UCLA when the original Diehl-Miller nomogram was formulated. Rishabh C. Date, who co-authored the validation study, is currently a UCLA medical student.
“In the original study, we reviewed patient records and figured how much change in refraction occurred as a result of the LASIK,” Miller said. “Then we looked at the results of their cataract surgery. Back then, I was basically guessing where I should target the lens power to achieve an acceptable outcome.”
The Diehl-Miller nomogram relates LASIK correction to the amount of myopia to be targeted in lens power calculation to attain emmetropia. Of the 23 eyes of 18 patients who were evaluated in the follow-up study, Miller said that 90% of eyes achieved postoperative refractive errors within 1 D of emmetropia and 70% achieved errors within 0.5 D.
In the follow-up study, axial lengths were obtained using an IOLMaster device (Carl Zeiss Meditec), and K values were obtained using simulated keratometry readings from the Pentacam Scheimpflug device (Oculus). In the original study, simulated keratometry was obtained using an EyeSys topography system.
“Both the EyeSys and Pentacam provided similar results,” Miller said.
Miller is impressed with the advances that have been made over the years in calculating IOL power after LASIK to achieve emmetropia.
“However, our nomogram only works on eyes for which the historical information is available, because we need to know the amount of myopic correction achieved by LASIK,” Miller said.
Miller is hopeful that by looking at the peripheral corneal curves, he may be able to predict the central corneal power before LASIK and use that number to calculate the change in corneal power. – by Bob Kronemyer