This article is more than 5 years old. Information may no longer be current.
Formula may help calculate IOL power after hyperopic LASIK
The formula for calculating IOL power after hyperopic LASIK does not require knowledge of the pre-refractive surgery keratometry.
A recently developed regression formula may be the best way to calculate IOL power after hyperopic refractive surgery, according to one investigator.
Amit Chokshi, MD, and colleagues compared the accuracy of seven methods of IOL calculation after hyperopic LASIK. He presented the results of their study at the American Society of Cataract and Refractive Surgery meeting earlier this year.
“The traditional methods of IOL calculations after hyperopic refractive surgery have been shown to be inaccurate,” he said. “Using our regression formula to make adjustments to traditional methods, the practitioner can certainly be more accurate than they have been before with IOL calculations after hyperopic refractive surgery.”
Dr. Chokshi said the advantage of the regression formula they derived over other methods of calculating IOL power after hyperopic LASIK is that for this method the pre-refractive-surgery keratometry is not necessary.
“Only prior refraction — perhaps an old pair of glasses — needs to be known to provide accurate results,” he said.
Formulas
Using the SRK/T formula, the investigators evaluated seven IOL calculations retrospectively in 20 eyes of 13 patients who had undergone uncomplicated hyperopic LASIK followed by uncomplicated cataract surgery. The seven calculations were compared to an “exact” IOL that would have resulted in emmetropia. The “exact” IOL value was back-calculated from the surgical result. The seven calculations were also compared to the pre-refractive-surgery manifest refraction using linear regression, Dr. Chokshi said.
“Regarding patients who had undergone hyperopic refractive surgery, we tried to determine the effect on IOL power calculation, compare established methods of IOL calculation, determine how pre-refractive-surgery spherical equivalent affects the deviation of IOL power calculation and provide alternative formulas for IOL power calculations,” Dr. Chokshi said in his presentation.
The seven methods of IOL calculation evaluated in the study were these: The clinical history method, the clinical history at the spectacle plane, the vertex method, the back-calculated method, calculation based on average keratometry (K), calculation based on steepest K, and the double-K method.
All of these calculations were done for all 20 eyes of 13 patients.
“If you take the post-cataract-surgery spherical equivalent (of these patients) and you subtract it from the aim of the IOL, the average patient turned out to be –0.69 D spherical equivalent,” Dr. Chokshi explained during his presentation. “Every method overestimated IOL calculation, producing a myopic result.”
The findings showed that the vertex and the back-calculated methods were the most accurate calculations. Average K was the most inaccurate, Dr. Chokshi said. But because that is a method that is commonly used for IOL calculations, he said, the authors developed a regression formula based on the average K method that produced accurate results.
“We made our IOL calculations based on traditional methods using the SRK/T formula. We looked at all of our results and we saw if there was any variation based on the amount of hyperopic LASIK correction that they had,” Dr. Chokshi explained. “We used regression analysis and found that there was a statistically significant linear regression. For each diopter of hyperopic LASIK treatment, there was a certain predictable amount of IOL power adjustment that we have to make.”
From that analysis, Dr. Chokshi and colleagues devised a formula with which, if the amount of pre-refractive-surgery hyperopic spherical equivalent is known, the physician can adjust the IOL power: 0.27x + 1.53.
“IOL average K is accurate if it is adjusted by taking 0.27x, where x is the pre-refractive-surgery hyperopic spherical equivalent, adding 1.53, and bringing down the IOL power by that total amount.”
Putting the study into action
Dr. Chokshi explained that IOL calculations after refractive surgery, especially for hyperopia, are difficult for a number of reasons.
He explained in an interview with OSN, “There’s been a lot of work done on IOL calculations after myopic LASIK. Unfortunately, the results that we get after myopic LASIK can’t just be transferred to hyperopic LASIK.”
This study, the authors believe, was the first to examine results in patients who had undergone both hyperopic refractive surgery and cataract surgery, and the largest study on the topic to date.
“At least it will give us more of an idea how to calculate IOLs in this situation,” Dr. Chokshi said.
He said although this information may be helpful, it is based on retrospective data and cannot be validated without conducting a prospective study in which the formula developed by the authors is used to determine IOL power.
“Certainly, this is a step in the right direction,” Dr. Chokshi told OSN. “But until a prospective trial with an adequate control group is done, we won’t know the validity of our results. Based on anecdotal data after our study was completed, it appears very effective.”
For more information:
- Amit Chokshi, MD, can be reached at the University of Florida, Fl 2, 580 West 8th St., Jacksonville, FL 32209-6533; 904-244-9361; fax: 904-244-9391; e-mail: amit.chokshi@jax.ufl.edu.
- Katrina Altersitz is an OSN Staff Writer who covers all aspects of ophthalmology.