Early in March 2011, Glaucoma Today held the third annual Cutting Edge: Innovative Glaucoma Surgery Symposium. Iqbal Ike Ahmed, MD, and Richard Lewis, MD, served as the course directors of this 1-day event that brought together clinicians, scientists, and members of industry with the goal of advancing glaucoma surgery. The catch phrase of the meeting was minimally invasive glaucoma surgery, and this year's focus was on procedures that use an ab interno approach.
COMPARING TARGETED OUTFLOW SPACES
In his opening comments, Dr. Ahmed broadly discussed the risk profiles and the IOP-lowering effects of procedures that target the subconjunctival space, Schlemm's canal, or the suprachoroidal space.
John Samples, MD, divided the physiologic outflow of the eye into pressure-dependent (trabecular meshwork) and pressure-independent (uveoscleral) pathways. Addressing circumferential flow through the three targeted spaces, he noted little difference in drainage for 360ยบ subconjunctivally if there has been no prior surgery, whereas he said that only several clock hours of Schlemm's canal work at any given time. The circumferential flow in the suprachoroidal space is unknown at this time. Dr. Samples highlighted the work of Kate Keller, PhD, who has found areas of low, medium, and high outflow from Schlemm's canal.1 He posits that, one day, our surgical techniques will be able to address the high-flow areas directly. Dr. Samples also highlighted the recent work of Mike Fautsch, PhD, who found that the collector channels of Schlemm's canal are more evenly distributed than we previously thought.2
Carol Toris, PhD, commented on outflow percentages to the trabecular meshwork and uveoscleral pathway. She stated that, in healthy individuals, outflow to both is almost equal, which contradicts the classic teaching that very little pressure-independent outflow is active.
WOUND-HEALING CONSIDERATIONS FOR NEW GLAUCOMA PROCEDURES
Malik Kahook, MD, concentrated on intraoperative techniques that modulate wound healing. He cited research in which Mark Sherwood, MD,3 compared several agents to mitomycin C (MMC) in terms of the bleb's survival. The agents were not as effective as MMC when used separately, but the rate of bleb survival approached that with MMC when the agents were used in a combined fashion. Dr. Kahook also mentioned his own research on bleb survival.4 In his study, eyes either received the antivascular endothelial growth factor ranibizumab intravitreally and MMC at the time of trabeculectomy surgery or MMC alone. Six months after surgery, vascularization was much less prevalent among eyes that received both ranibizumab and MMC. The IOP control was equivalent between the two groups.
Deepak Edward, MD, reported variation in the size of Schlemm's canal in eyes with advanced glaucomatous damage, even in the same patient (Figure 1). He acknowledged the work of Lütjen-Drecoll,5 in which electron microscopy was performed on eyes that had undergone goniotomy and trabeculotomy surgery. Lütjen-Drecoll found a lower likelihood of a fibrotic response when the outer wall of Schlemm's canal was preserved than when it was damaged. According to Dr. Edward, because its phenotypic features are most characteristic of venous-like tissue, we need to think of Schlemm's canal as an out-pouching of the collector channels. Placing a Prolene suture (Ethicon Inc., Somerville, NJ) in Schlemm's canal, such as during canaloplasty, produces breaks in the canal's endothelial cells, which he said might be more important to the surgical outcome than the canal's dilation.
Murray Johnstone, MD, used a sinusotomy specimen as the basis for his comments on wound healing. Sinusotomy entails the opening up of Schlemm's canal from an ab externo approach, during which a block of the canal's outer wall is excised. This was the first nonpenetrating filtering surgery described by Krasnov6 in 1962. Dr. Johnstone noted that normal flow pathways were absent in the specimen, but very small vessels connecting to intrascleral vessels were present, leading to a direct communication between the juxtacanalicular space and the episcleral veins. He surmised that this may explain the improvement in IOP observed with nonpenetrating procedures.
THE METRICS FOR AND FUTURE OF MINIMALLY INVASIVE GLAUCOMA SURGERY
Dr. Lewis noted President Obama's recent acknowledgment that the FDA is in need of modernization. Dr. Lewis then stated that patients gain access to new technology earlier in Europe than in the United States. He wondered if the United States is still on the cutting edge, given regulatory constraints. In the open discussion that ensued, attendees debated what metrics should be used in studies of new technologies. Overall, the group felt that peerreviewed publications are necessary to help to define acceptable standards for new devices but that we have to decide what those standards will be. Dr. Lewis mentioned that we might be able to improve the process of advancing new technology by lobbying through our national societies.
William Freeman, MD, of Market Scope drew on the results of several surveys to paint a picture of the glaucoma market. He said that, currently, the glaucoma industry is 93% medications and 2% surgical devices. In ophthalmology, a total of 31 million surgical procedures are performed worldwide on an annual basis, 19 million of which are cataract procedures. Fewer than 500,000 surgical glaucoma procedures are performed on an annual basis globally, he said. Dr. Freeman predicted that, by 2016, however, 1.1 million glaucoma surgical procedures will be performed annually as the techniques become safer.
Leon W. Herndon, MD, is an associate professor of ophthalmology at the Duke University Eye Center in Durham, North Carolina. He acknowledged no financial interest in the product or company mentioned herein. Dr. Herndon may be reached at (919) 684-6622; leon.herndon@duke.edu.
