Accurate IOL power calculations are critical in preventing a post-operative refractive surprise. For patients who have previously undergone corneal refractive surgery, this remains a challenging task. The difficulty lies in measuring the true central corneal power. The traditional instruments we have to measure corneal power (i.e.,
keratometer,
corneal topographer) make several assumptions to predict the central power. In a normal cornea, these devices work well, but in those that have been altered by refractive surgery, these devices simply don’t work. Readings from corneas that have been flattened with myopic treatments are falsely steep, while those after hyperopic treatments are falsely flat. Thus, if these inaccurate corneal values are used in standard IOL calculations, the predicted IOL power will be too weak or too strong, respectively.
Not only is this issue frustrating to the cataract surgeon, but it is also a source of concern to the patient who initially underwent refractive surgery in order to have excellent uncorrected visual acuity. Although glasses and contact lenses will correct a postop refractive error, these patients in particular are less accepting of wearing lenses. It is particularly important to discuss this topic with the patient pre-operatively so that if a significant refractive error does occur after surgery, it is not unexpected. It is the “surprise” that can cause a problem. In fact, wrong IOL power is a significant cause of ophthalmic malpractice claims. One third of all ophthalmic claims are for cataract surgery and ¼ of these are IOL-related, mainly due to wrong IOL power.
Newer instruments to measure the cornea as well as numerous formulas and correction factors have improved our ability to select the appropriate IOL, but we still need a method that is more exact. There are many reports in the literature describing methods of obtaining the correct IOL power. These include using historical information, rigid contact lens over-refraction, EffRP from the Holladay diagnostic CVK map, refraction derived methods, correction factors, regression formulas, and various nomograms. Many of the available formulas only apply to calculations after myopic refractive surgery. A recent paper by Drs. Chokshi, Latkany, et al, provides a simple method for calculating the IOL power after hyperopic refractive surgery (Ophthalmology 2007;114:2044). There are also several websites that allow quick calculation of IOL power:
- iol.ocularmd.com: developed by Dr. Goldsberry, this program uses 10 different methods to calculate the correct IOL power.
- iol.ascrs.org: developed by Drs. Hill, Wang, and Koch to calculate IOL power for those who have previously had laser vision correction (myopic or hyperopic) or RK.
- eyelab.com: developed by Drs. Hoffer and Savini, this is an Excel spreadsheet that instantly calculates the IOL power using all published methods depending on the information you input.
Better corneal imaging techniques should enable accurate measurement of the central corneal power, so that standard IOL calculations can be used to calculate the correct lens power. In the meantime, I recommend discussing this unpredictability issue with the patient pre-operatively; obtaining as much pre-op and post-op keratometry, topography, and refraction information as possible; using multiple methods to calculate the IOL power; and having a plan to address a significant post-operative surprise. Preparing patients in advance makes everybody happier.
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