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Pre-LASIK measurements used to predict postop IOP

A statistical model was developed based on a constellation of preoperative conditions.

ByKatrina Altersitz

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A statistical model to predict the true IOP after myopic LASIK has been developed by researchers in Taiwan. The formula uses preoperative measurements and other information to predict the amount of underestimation of IOP after LASIK due to changes in the cornea.

Accurate measurement of IOP after LASIK is important for the detection of ocular hypertension, said Chia-Ching Yang, MD, and colleagues at several institutions in Taipei, who published a paper on their predictive model in the American Journal of Ophthalmology.

People who have LASIK early in life have the risk of developing ocular hypertension or glaucoma later in life, when information on their refractive surgical procedure may not be available to health care workers. Measurement of IOP may be unreliable in these patients because of changes in their corneal structure during LASIK.

Dr. Yang and coworkers used information such as age, preoperative IOP and preoperative central corneal thickness (CCT) to create a linear mixed model to predict postoperative IOP.

“Significant predictors for postoperative IOP after myopic LASIK procedures included age, gender, preoperative IOP, ablation depth, CCT and preoperative spherical equivalent refractive errors,” the study authors said. “The linear mixed model … explained 91% of the variation of postoperative IOP.”

The study

Dr. Yang and colleagues examined 386 eyes of 193 patients.

Preoperatively, IOP was recorded on three office visits over 3 months. Postoperatively, “at least two measurements of IOP for each eye were checked on the first day or during the first week immediately after LASIK,” the researchers said. Patients were followed for 3 months.

The LASIK procedures were standard, and all IOPs were measured with noncontact tonometry. The researchers said noncontact tonometry is particularly useful after LASIK because it is simple and safe.

“We averaged the three IOP measurements in each preoperative and postoperative visit to get the mean IOP for each eye for that visit,” the researchers said. “The mean IOPs of all preoperative visits were averaged again to get the mean preoperative IOP. … The mean postoperative IOP … was calculated the same way.”

The IOPs were then correlated with the qualities measured before LASIK. The IOP underestimation was determined by subtracting the predicted postoperative IOP from the preoperative IOP. That number was added to the patient’s underestimated IOP postoperatively to find the true IOP.

“There was a pretty good fit between the observed and the predicted postoperative IOP,” the study authors said.

Results

“Although the underestimation of IOP after LASIK has been reported, awareness of recording IOP immediately after LASIK is often neglected in Taiwan,” the researchers stated.

The decrease in IOP after LASIK was statistically significant in this study, as it was in others, they said.

Additionally, they said, “Ignorance of the underestimation of IOP resulting from LASIK procedure may not be a serious problem immediately after LASIK but will be particularly critical for subjects who undergo LASIK at a young age and have a high likelihood of developing glaucoma in old age.”

Based on age, gender, preoperative IOP, preoperative CCT, preoperative spherical equivalent refractive error and ablation depth, the researchers in this study concluded that they could accurately provide the predicted value of IOP immediately following LASIK.

For more information:

• Tony Hsiu-His Chen, PhD, the corresponding author of this study, can be reached at the Institute of Preventative Medicine, College of Public Health, National Taiwan University, Room 207, No 19, Syujhou Road, Jhongjheng District, Taipei City 100, Taiwan; e-mail: stony@episerv.cph.ntu.edu.tw.

Reference:

• Yang CC, Wang IJ, Chang YC, Lin LL, Chen TH. A predictive model for postoperative intraocular pressure among patients undergoing laser in situ keratomileusis (LASIK). Am J Ophthalmol. 2006;141(3):530-536.

• Katrina Altersitz is an OSN Staff Writer who covers all aspects of ophthalmology.