EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/146899
February 2011 July 2013 Cataract challenges EW FEATURE 41 Cataract surgery in post-refractive surgery patients by Michelle Dalton EyeWorld Contributing Writer Performing complicated surgery doesn't have to be excruciating—if you are meticulous in preoperative stages W ith the popularity of laser vision correction, it is extremely unlikely cataract urgeons will never encounter a patient who has undergone refractive surgery. But because the corneas of these patients have changed, planning for cataract surgery is a bit more complicated. "Most of our machines measure just outside this central area and then extrapolate values for the central cornea. This works well for virgin corneas, but not so well in eyes with prior LASIK, PRK, or RK," said Uday Devgan, MD, in private practice, Los Angeles, and chief of ophthalmology, Olive View – UCLA Medical Center. (See "Calculating IOL power in post-RK eyes" for more on that topic.) Plus, surgeons need to consider both the anterior and posterior corneal curvatures—the relationship between each changes depending on what type of refractive surgery the patient had, said Samuel Masket, MD, in private practice, Advanced Vision Care, Los Angeles. In myopic photoablative procedures, the anterior cornea is flattened but the posterior cornea remains unchanged (except in cases of ectasia), causing the preop near-parallel curves to take on a higher value than the assumed –6 D in an unoperated eye, he said. "When we read with a standard keratometer, we overestimate the corneal power in an eye that has undergone myopic photoablation," he said. Conversely, those same machines will underestimate the central corneal value in eyes that underwent hyperopic photoablation, Dr. Devgan said, and those values may be off by as much as 2 D. "While manual and automated keratometry can provide reasonable starting values, we still need to use calculations to adjust for the prior keratorefractive surgery. Some machines like dual Scheimpflug imaging can give a more accurate central corneal power value, but even still the surgeon needs to factor in the prior keratorefractive surgery," he said. Barry Schechter, MD, in practice, Florida Eye Microsurgical Institute, Boynton Beach, Fla., uses the Pentacam (Oculus, Wetzlar, Germany) to generate K readings on both the anterior and posterior cornea. "I also use the Holladay formula, and it's been accurate about 85-90% of the time," Dr. Schechter said. For Barbara Bowers, MD, in private practice, Innovative Ophthalmology, Paducah, Ky., "I'm a little obsessed with preop testing. I take readings from the Lenstar [Haag-Streit, Koniz, Switzerland], the Pentacam and AR Ks on all my cases, including the post-refractive patients," she said. "I've found the Lenstar to be the most accurate of any measurement. It's nice when they all line up and agree, but when they don't, I use the Lenstar." There's been "some dispute in the literature" about the reliability of "true" corneal power measurements obtainable with the Orbscan (Bausch + Lomb, Rochester, N.Y.) and the Pentacam in post-refractive eyes, Dr. Masket said. AT A GLANCE • Patient education is crucial for those who have had previous refractive surgery. • Consider both anterior and posterior corneal curvatures when assessing eyes that have already undergone laser vision correction. • Choose IOL formulas based on available patient information. • Multifocal lenses may not be an appropriate choice in someone with significant post-LVC higher order aberrations. continued on page 42 Comparing IOL calculation methods S amuel Masket, MD, and Nicole Fram, MD, presented a comparison of the Haigis-L, Masket Regression Formula, optical coherence tomography-based IOL calculations (as can be done with the Optovue True Corneal Power), and intraoperative aberrometry (as can be done with the ORA system) to determine which formula was most accurate in predicting outcomes in post-laser vision correction eyes scheduled for cataract surgery. The study was presented during this year's ASCRS•ASOA Symposium & Congress. This retrospective review included 25 eyes (19 patients) with a mean age of 56 years. Only 13 eyes had available history of prior laser vision correction. A total of 18 eyes were previously myopic, while seven were hyperopic; the range of original refractive errors was +4.21 D to –5.25 D. All patients underwent uncomplicated phaco with a 2.2 mm incision and an Alcon SN series IOL implanted. "IOL prediction error is the difference between the predicted IOL power and actual based on the spherical equivalent of actual refraction post-cataract surgery," Dr. Masket said. The refractive mean absolute error was: Haigis-L, 0.36 (n=24); ORA, 0.37 (n=25); Optovue, 0.33 (n=25); and Masket, 0.28 (n=13). (The Masket formula must have previous history; hence the lower number of included eyes.) See Figure 1. With the exception of the Optovue, which revealed that 88% of eyes were within 0.75 D, all other formulas produced refractive prediction errors at more than 90% within 0.75 D; the Masket formula was best within 0.5 D (85% of eyes), with the other formulas between 71-76%. The Optovue produced the best predictions within 0.25 D (60%), followed by the Masket, ORA, and Haigis-L. See Figure 2. "What this means is that intraoperative aberrometry and OCT-based formulas compare well with prior established methods and show promise in cases with no historical data," Dr. Masket said. Figure 1. Refractive mean absolute error Figure 2. Distribution of refractive prediction error Source (all): Samuel Masket, MD