IOL Choice and Calculations in a Post-RK Patient
You are scheduling a patient for cataract surgery who has undergone eight-incision RK in both eyes. How do you advise the patient on IOL choice and accuracy,and what is your preoperative regimen for performing an IOL power calculation?
DAVID R. HARDTEN, MD
The complex scenario of cataract surgery in a post-RK patient is fortunately becoming less common. Many of these patients have already had cataract surgery, and the more common scenario is that of a patient’s undergoing cataract surgery after LASIK or PRK.
My first step is to inquire about the stability of refractive error throughout the day. Instability is fairly uncommon for a medium-zone, eight-incision RK patient. However, those who have had optical zones of less than 3 mm, incisions all the way to the limbus, or crossing T-cut incisions may have fluctuation of refractive error throughout the day, which makes management at the time of cataract surgery much more complex. Some investigators have started to discuss corneal collagen cross-linking as a helpful procedure for stabilizing corneas in eyes that have a large amount of refractive fluctuation throughout the day.1
I evaluate the amount of irregular corneal astigmatism using corneal topography and tomography. If there is a significant amount of irregular astigmatism from the prior RK, then results will be more unpredictable (Figure 1). That information helps me to set realistic expectations for the patient.
My default position in patients with a history of RK is to suggest a monofocal IOL, typically targeted for distance vision. I implement the same formulas that I use to calculate IOL powers after LASIK, even though there are some theoretical reasons to use different methods. The post-LASIK formulas have worked well in my hands. I try several formulas and then select the one with the result that seems to make the most sense (Figure 2). I do not use formulas based on history, which is often unreliable in this subset of patients. Sometimes, a significant scar will render the results of the Orbscan topographer (Bausch + Lomb) or Pentacam Comprehensive Eye Scanner (Oculus Optikgeräte GmbH) less reliable. Typically, because there is poor agreement between the methods, selection of the IOL power is difficult, and significant ametropia is not uncommon after the cataract surgery.
An IOL exchange or PRK enhancement 3 to 4 months postoperatively (after hyperopia from corneal edema resolves) may be an option if the targeted refractive outcome is not achieved. I have used presbyopia-correcting IOLs in RK patients, but the amount of postoperative work and the need for lesser expectations cannot be overemphasized.
WARREN E. HILL, MD
Cataract surgery in the setting of previous RK is always a challenge. The triad of uniformly absent prior records, variable amounts of hyperopic drift, and frequent cases of lens-induced myopia precludes the use of historical methods for estimating the central corneal power. Additionally, the flattening of the central cornea renders conventional methods for measuring corneal power inaccurate. Unmodified theoretical formulas uniformly underestimate IOL power, and the potential for inaccuracy looms large unless a specific approach is followed.
I begin the calculation process by averaging the 1-mm through 4-mm ring powers of the Atlas topographer (Carl Zeiss Meditec Inc.). Other methods have been published, but I find that this approach gives the most consistent results.
For the calculation of IOL power, a double-K modified formula is necessary to remove the calculation artifact of iatrogenically flat keratometry (K) readings. This can be done by checking the box on the Holladay II formula (Holladay Consulting, Inc.) that reads, “Prior Rk, Lasik, Alk, etc.” Another method is to use the American Society of Cataract & Refractive Surgeons’ online calculator at http://iol.ascrs.org, which employs an Aramberri double-K method, a modified version of the Holladay 1 formula. I select a refractive target of between -0.50 and -0.75 D for two reasons: (1) hyperopic errors are common and (2) if hyperopic drift continues, the refractive error will change toward something better (eg, emmetropia).
I place a small corneal incision between the eight radial incisions at the limbus. I use an aspheric IOL, with the addition of negative spherical aberration to offset a typically elevated value of anterior corneal spherical aberration.
Initially, the refractive error will shift from hyperopic to myopic but will then stabilize 6 to 12 weeks after surgery.
WILLIAM B. TRATTLER, MD
Patients with a history of RK who require cataract surgery are a challenge in regard to calculating the optimal IOL power. When I perform a consultation for a patient with RK, I carefully evaluate the ocular surface. Dry eye syndrome is very common in patients scheduled for cataract surgery, and it can affect my ability to get optimal K readings. I also perform corneal topography to determine whether there is any corneal asymmetry from the RK that may lead to reduced postoperative BCVA. In general, I avoid a multifocal IOLs in RK patients, because the corneal shape typically does not provide satisfactory optics for these types of lenses.
Unlike in LASIK, the cornea is not thinned by RK, so central keratometry is an accurate measurement that can be used in IOL calculation formulas. However, some IOL formulas (such as the SRK-T or SRK-II formulas) assume a relationship between keratometry and anterior chamber depth. Because RK patients were once myopic but now have flat corneas, they typically have a deeper anterior depth than patients with similarly flat K readings who do not have a history of myopic refractive surgery. Therefore, more advanced formulas may be more accurate for RK patients. For example, the Holladay II or Haigis formulas take into consideration measurements of the anterior depth and the final effective lens position in relation to the cornea.
Additionally, I find that the very central K values—in the central 2 mm, for example—are the most useful for IOL calculation formulas. The Atlas, Pentacam, and other similar technologies can provide this value.
Postoperatively, it is important to be patient, because the initial refractive results may be inaccurate. It is not uncommon for 2 months or more to elapse before the cornea reaches its final, stable shape after cataract surgery. Before reacting to an off-target result, it is therefore prudent to wait and reevaluate the patient. If the outcome remains off target, surface ablation or piggyback IOLs are safe and effective procedures for enhancing post-RK eyes.
R. BRUCE WALLACE III, MD, AND ROBERT T. CROTTY, OD
When counseling post-RK patients about cataract surgery, we point out the lack of predictability of IOL power calculations due to the alteration of the corneal curvature after RK. We also explain that variability in UCVA is common for weeks to months after cataract surgery due to the temporary expansion of the RK incisions. Once corneal stability returns, UCVA can be assessed. If the result is not as expected, a piggyback IOL may be more appropriate than an IOL exchange. We inform patients that this procedure, if needed, is not covered by insurance.
For IOL calculations after RK, we have employed many different methods. For the past few years, we have been using one introduced to us in an article written by Dr. Hill.2 In it, he stressed understanding the ratio between the posterior and anterior corneal radii. This ratio decreases in corneas that have undergone ablation for myopic keratorefractive surgery, but it increases in eyes that have undergone RK. The ratio may allow us to use elevation data to estimate central corneal power. With the Atlas, we are able to use an average of the 1-, 2-, 3-, and 4-mm ring values to determine an estimated central corneal power to be used in our IOL power formula. Using the Holladay II formula, we enter the surgeon’s K value for the IOL power calculations. This method of finding the central corneal power has proven to be the most successful in our clinic.
Section Editor R. Bruce Wallace III, MD, is the medical director of Wallace Eye Surgery in Alexandria, Louisiana. Dr. Wallace is also a clinical professor of ophthalmology at the Louisiana State University School of Medicine and an assistant clinical professor of ophthalmology at the Tulane School of Medicine, both located in New Orleans. He acknowledged no financial interest in the products or companies he mentioned. Dr. Wallace may be reached at (318) 448-4488; firstname.lastname@example.org.
Robert T. Crotty, OD, is the clinical director at Wallace Eye Surgery in Alexandria, Louisiana. He is on the speakers’ bureau for Allergan, Inc., and Bausch + Lomb. Dr. Crotty may be reached at (318) 448-4488.
David R. Hardten, MD, is the director of refractive surgery at Minnesota Eye Consultants in Minneapolis. He acknowledged no financial interest in the products or companies he mentioned. Dr. Hardten may be reached at (612) 813-3632; email@example.com.
Warren E. Hill, MD, is in private practice at East Valley Ophthalmology in Mesa, Arizona. He acknowledged no financial interest in the products or companies he mentioned. Dr. Hill may be reached at (480) 981-6130; firstname.lastname@example.org.
William B. Trattler, MD, is the director of cornea at the Center for Excellence in Eye Care in Miami and the chief medical editor of Eyetube.net. He acknowledged no financial interest in the products or companies he mentioned. Dr. Trattler may be reached at (305) 598-2020; email@example.com