EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
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34 EW GLAUCOMA February 2011 August 2012 Glaucoma editor's corner of the world Glaucoma on the cutting edge by Maxine Lipner Senior EyeWorld Contributing Writer The latest on MIGS, outflow, and more he much-anticipated revolution in glaucoma surgery may have finally arrived with the recent FDA approval of the iStent—the first available trabecular bypass device. Other new devices—the Hydrus and the CyPass—are in FDA trials and may be up for approval in the near future. These new glaucoma implants may lower IOP and reduce the burden of medical therapy through procedures that are much faster and safer than trabeculectomies or tube shunts. But with new technology comes the need for new terminology. Filling this void is MIGS—micro-invasive glaucoma surgery. This term was developed by Ike Ahmed, M.D.—the surgeon with perhaps the most experience with all the MIGS procedures. But this has become a contro- versial area as many other procedures and devices seek to claim MIGS status. We are very fortunate this month to have Steve Vold, M.D., and Rick Lewis, M.D., steer us through this controversy. T As the MIGS procedures gain traction in our surgical arsenal, it is increasingly important to understand how they work and also why in some cases they may not lower IOP as much as desired. What is needed is an angiogram of aqueous outflow. But this has been an elusive goal since ocular outflow is relatively slow and staining/ tracking aqueous has been quite difficult. Fortunately, these challenges have not dis- couraged Malik Kahook, M.D., and Murray Johnstone, M.D., from pursuing novel approaches to assessing ocular outflow. They share their insights this month. The iStent is a breakthrough technol- ogy in glaucoma surgery. Hopefully it is part of a rising arc of MIGS innovation that will lift glaucoma surgery into a zone of efficacy, efficiency, and safety that has not been pos- sible with trabs or tubes. The iStent also will bring glaucoma surgery into the sphere of the comprehensive cataract surgeon. This will bring greater numbers of surgeons and companies into the realm of glaucoma surgical treatment and increase the chances that we can solve difficult problems like an angiogram for aqueous outflow. Reay Brown, M.D., glaucoma editor S pearheaded in part by innovative glaucoma procedures dubbed MIGS (micro-invasive glaucoma surgery) as well as unique ways of measuring outflow facility, new hope is burgeoning for better controlling this chronic, sight- threatening condition. Mixing up MIGS The term MIGS was first coined by Ike Ahmed, M.D., and initially largely referred to micro-stent tech- nologies, according to Steven D. Vold, M.D., Fayetteville, Ark. How- ever, many glaucoma technologies have attempted to crowd under what is seen as the attractive MIGS umbrella, with some controversy as to whether all really belong. "The lexicon of marketing all of these dif- ferent products for glaucoma surgery has resulted in everything but tube shunts and trabeculectomies now being classified as micro-invasive or minimally invasive glaucoma surger- ies," Dr. Vold said. "The problem is that we're getting things that are fairly extensive surgeries now being classified as minimally invasive or micro-invasive, and it's quite confus- ing for the doctors, and for the patients as well." He thinks that the approach used as well as the nature of the surgery have an important bearing here. "What we're looking at are things that we're doing from an ab interno approach—very minimal damage to the tissue structure, a high level of safety, fast recovery, and at least some efficacy with the procedure." Richard A. Lewis, M.D., Sacra- mento, Calif., agreed that damage with MIGS is kept to a minimum. "There's no suturing; they're short procedures," he said. The Trabectome (NeoMedix, Tustin, Calif.), the newly FDA-approved iStent (Glaukos, Laguna Hills, Calif.), the Hydrus Intracanalicular Implant (Ivantis, Irvine, Calif.), and the AqueSys (Irvine, Calif.) would all be considered MIGS procedures. How- ever, the EX-PRESS Glaucoma Filtra- tion Device (Alcon, Fort Worth, Texas), despite its size, is not a MIGS procedure because you have to go through the conjunctiva, so it is ab externo. This is all still evolving, Dr. Lewis believes. He thinks that a bet- ter way to define MIGS is by how the device works. "Some work in the canal space, some work in the supra- choroidal space, others work in the subconjunctival space," Dr. Lewis said. "That's probably a more impor- tant MIGS definition because the space that you work in is going to be an important component here." In Dr. Vold's mind the simplest way to categorize MIGS would have been to limit it to just the micros- tent technologies. "This would be the CyPass [Transcend Medical, Menlo Park, Calif.], the Hydrus Intracanalicular Implant, the iStent, and possibly the AqueSys device," Dr. Vold said. He sees the advantages of hav- ing MIGS technology as important. "I think that safety is a huge deal be- cause when we talk about standard filtration surgery, we're talking about failures of almost 50% by 5 years and 1% risk per year for devel- oping infection after filtration sur- gery," Dr. Vold said. "That means that if you live another 20 years you potentially could have up to a 20% chance of having an infection." Also, MIGS procedures are associated with faster recovery compared with filtration procedures. "We're talking 4-6 weeks with filtration surgery," Dr. Vold said. "With MIGS proce- dures, within the first week most of these people are seeing quite well." Zoning in on outflow In addition to MIGS technology now bolstering the armamentarium, glaucoma treatment is poised to get a leg up thanks to more accurate as- sessment of the condition. IOP has been how practitioners assessed out- flow, according to Malik Y. Kahook, M.D., professor of ophthalmology, University of Colorado, Aurora. Dr. Kahook sees devices such as the iStent or the Hydrus that address the outflow system pathology as helping to bring to the fore the need for new measurements. These bypass stents get around the area thought to harbor the pathology. With all of these devices, it is thought that if you can get past this area of obstruction, the pressure should be improved. However, while this has helped, the bypass approach has not turned out to be a panacea. "What we are learning is that removing the trabec- ular meshwork or just bypassing it doesn't have a dramatic effect on intraocular pressure, so it doesn't dramatically decrease it, but it does decrease it to a certain degree," he said. "That's bringing up a new de- bate as to whether most of the out- flow obstruction is in the trabecular meshwork or are there other areas of the distal outflow system that are also pathologic within glaucoma." One new technology that Dr. Kahook is hopeful can help is spec- tral OCT, the brainchild of Joel S. Schuman, M.D., University of Pitts- burgh. "With the use of special soft- ware, the team at the University of Pittsburg is able to measure aqueous outflow in Schlemm's canal and col- lector channels, but also to image the anatomy of Schlemm's canal and collector channels, which was previously impossible to do," Dr. Kahook said. Likewise, Murray A. Johnstone, M.D., clinical professor, Department of Ophthalmology, University of Washington, Seattle, pointed out that with current OTC it's hard to see much detail. "We can envision the micron range, which is about 10-6," he said. He views this as a pixal-based limitation. To push this limit Dr. Johnstone took the problem to Ruikang K. Wang, Ph.D., professor, Department of Bioengi- neering, University of Washington, who was able to develop a new con- ceptual framework based on examin- ing the way the tissue moves—a motion-based OTC. Dr. Wang was already using the no-touch, non-in- vasive technology in the 10-12 picometer range, able to measure the vibration of hair cells in the cochlear. To tackle the motion of the trabecular meshwork, he was further able to enhance the technology's capability, bringing this down to the 10 -9–20 nanometer range. With the technology, Dr. Johnstone finds that it is possible to watch the internal motion of the meshwork as the fluid propagates outward to the external wall of Schlemm's canal. "We've also devel- oped intensity maps, which allow us to measure the intensity of the pres- sure wave as it hits the surface of the