EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/87458
56 EW CATARACT October 2012 Is cataract surgery an effective glaucoma operation? by Tony Realini, M.D. I n the search for a safer alterna- tive to trabeculectomy for IOP reduction in glaucoma, are we overlooking an effective candi- date—cataract surgery? "Several studies have evaluated the effect of cataract surgery on IOP in patients with suspected or established glaucoma," said Anne L. Coleman, M.D., University of Cali- fornia-Los Angeles. These data are mixed, with some studies showing significant and enduring IOP reduc- tion and others showing little long- term benefit to IOP control. "In general," said Steven Mansberger, M.D., Devers Eye Institute, Portland, Ore., "prior studies have suggested that there is about a 1.5 mm Hg reduction in IOP after cataract surgery." One study is the exception. "In the iStent Trabecular Micro-Bypass [Glaukos, Laguna Hills, Calif.] study, the control group underwent cataract surgery alone without im- plantation of the device, and the mean IOP reduction in control eyes was 8.5 mm Hg," said Thomas Samuelson, M.D., Minneapolis. He attributed this dramatic change to the study design, which required a high baseline IOP to qualify for en- rollment. Earlier studies have shown that a higher pre-op IOP generally results in greater IOP reduction fol- lowing cataract removal. Even so, the strict eligibility criteria may have predisposed study eyes to exhibit "regression to the mean" following enrollment. Additionally, the IOP reduction in the control arm of the iStent trial represents a medicated IOP value. Indeed, the efficacy of the Treating continued from page 54 avoid damage from excessive phaco energy. "Advanced phaco technol- ogy such as torsional ultrasound, burst, and pulse modes can decrease the total amount of energy delivered inside the eye," she said. EW Editors' note: Dr. Henderson has financial interests with Alcon (Fort Worth, Texas) and ISTA Pharmaceuti- cals (Irvine, Calif.). The other physi- cians have no financial interests related to this article. Contact information Afshari: 919-681-3937, Natalie.afshari@duke.edu Henderson: 800-635-0489, bahenderson@eyeboston.com Safran: 609-896-3931, safran12@comcast.net Verdier: 616-949-2001, daverdier@aol.com iStent in the trial was based on the fact that less medication was needed in the iStent arm of the trial as compared to cataract surgery alone. Not all data in agreement At the annual meeting of the Ameri- can Glaucoma Society in New York, Ta Chen Peter Chang, M.D., Bas- com Palmer Eye Institute, University of Miami, presented the results of a retrospective study to evaluate cataract surgery's effect on IOP, using the phakic fellow eye as a con- trol. "We identified 29 patients with either ocular hypertension or open- angle glaucoma who had previously undergone unilateral phacoemulsifi- cation with the fellow eye remaining phakic for at least 3 years post-oper- atively," he said. Some patients were using IOP-lowering medications, he said. Mean IOP before unilateral surgery was comparable between groups, he said, and there was no statistically significant difference in the mean IOP of the groups at 1, 2, or 3 years post-op. "There was also no significant difference in the num- ber of IOP-lowering medications needed post-operatively," he said. While this study was strength- ened by the use of the fellow eye as a control, its retrospective nature makes it difficult to draw conclu- sions that can be implemented into clinical practice. What did OHTS find? To more closely examine the effect of cataract surgery on IOP, Dr. Mansberger presented a post hoc analysis of data from the Ocular Hypertension Treatment Study (OHTS). While the OHTS was not designed specifically to answer this question, the prospective nature of the study coupled with its well-de- signed protocol lent weight to its findings. "We identified 63 eyes of 42 patients in the observation arm of the OHTS that underwent cataract surgery," Dr. Mansberger said. Another 743 eyes in the same obser- vation arm that did not undergo cataract surgery served as a control group. The mean of three IOP meas- urements before surgery and three more after surgery represented the change in IOP in the active group. This was compared to three corre- sponding IOP measurements in the control group, he explained. "Mean IOP was similar in the two groups before surgery, and there was a larger drop in IOP in eyes un- dergoing cataract surgery compared to controls," he reported. The aver- age drop in IOP was 16.5%, from a mean baseline of 23.9 mm Hg to 19.8 mm Hg, a 4.1 mm Hg reduc- tion, he said. "The mean IOP of the group was still below baseline at 36 months, and 40% of operated eyes enjoyed an IOP reduction of 20% or more," he added. Possible mechanisms How might cataract surgery lower IOP? It likely is related to changes in anterior chamber depth and angle configuration. Murray Johnstone, M.D., Seattle, explained. "As the crystalline lens shifts forward with age, the ciliary body does, too, and this shallows the angle," he said. "After cataract surgery the ciliary body shifts backward, a behavior known to open the intertrabecular spaces and Schlemm's canal. This backward movement of the ciliary body provides a mechanism to explain improvement in aqueous outflow following cataract surgery alone," Dr. Johnstone said. "This is why eyes with narrow angles seem to have greater IOP reductions after surgery," Dr. Mansberger said. "The capsular tension created by the capsular bag 'shrink-wrapping' around the intraocular lens implant may pull the meshwork open as well," Dr. Mansberger said. He added that another explanation may be biologic with increased outflow from mechanisms similar to trabeculo- plasty or administration of a prostaglandin analogue. "These are the mechanisms that explain and account for the IOP reduction seen after cataract surgery alone," Dr. Johnstone said. Clinical implications Not every glaucoma patient under- going cataract surgery benefits, and there are risks to cataract surgery for eyes with glaucoma. "There is a 2.4- fold higher risk of a post-operative IOP spike in glaucomatous eyes compared to normal," Dr. Coleman said. Dr. Mansberger pointed out that significant IOP reductions in the range of 20% or greater only occurred in about 40% of OHTS participants. "We should consider who is most likely to benefit," said Dr. Samuelson. "If we could identify those patients most likely to show improved IOP with cataract surgery alone, we can limit their exposure to move invasive and riskier opera- tions. Virtually all of the ancillary effects of cataract surgery are favor- able, such as improved refractive error, deeper anterior chamber, and for many, lower IOP." Dr. Mansberger agreed. "The next step is to predict the glaucoma patients who will have enough IOP lowering from cataract surgery alone." EW Editors' note: Drs. Chang, Coleman, and Johnstone have no financial interests related to this article. Dr. Samuelson has financial interests with Glaukos, Ivantis (Irvine, Calif.), AqueSys (Aliso Viejo, Calif.), Endo Optiks (Little Silver, N.J.), Alcon (Fort Worth, Texas), and Abbott Medical Optics (Santa Ana, Calif.). Contact information Chang: t.chang@med.miami.edu Coleman: doctor_coleman@yahoo.com Johnstone: johnstone.murray@gmail.com Mansberger: smansberger@deverseye.org