Glaucoma drainage devices forging new pathways

Reviewing the latest options for glaucoma surgeons

The Gold Micro Shunt after implantation Source: Ike Ahmed, M.D.

Glaucoma drainage devices play a key role in treatment today. In a recent article published in the October 2009 issue of Expert Review of Ophthalmology, investigators took a closer look at the design and surgical success of such devices as well as some new things on the horizon, according to Ramesh S. Ayyala, M.D., associate professor of ophthalmology, Tulane University, New Orleans.
“All of the devices that we have had in the past are based on the concept of diverting the fluid from the anterior chamber into the subconjunctival space,” Dr. Ayyala said. He finds that the problem with traditional drainage devices such as the Molteno (Molteno Ophthalmic Limited, Dunedin, New Zealand) and the Ahmed valve (New World Medical, Rancho Cucamonga, Calif.) is wound healing. “The body wants to heal and you don’t want it to heal,” Dr. Ayyala said. “That’s the reason why no matter how good a surgeon is, as time goes on, a surgeon’s success rate will go down—there’s a 10% failure rate every year.” As a result, at the end of 10 years only 30% of valves now being injected will survive.
Newer devices like the Ex-PRESS shunt (Optonol, Kansas City, Mo.) have the same problem. “They’re doing the same thing, which is essentially diverting the fluid into the sub-choroidal space,” Dr. Ayyala said. “We’re bound by the same rules of the game, which is wound healing response, so it is not ideal.”
Shifting pathways

Emerging surgeries have shifted the focus a bit. “We are doing more canal surgeries and canal directed surgeries, all into the super-choroidal space,” Dr. Ayyala said. He pointed out two devices that are being used, the Gold Shunt (SOLX, Medway, Mass.) and the Aquashunt (Opko Health, Miami). “I think these will have limitations, too, because I have to imagine that scar tissue will find its way into the super-choroidal space also,” he said. “But at least we’ll have another option when the subconjunctival tissue is completely scarred.”
While this is another pathway and does offer an alternative, once again scar tissue is likely to be a limitation. With this in mind, Dr. Ayyala and colleagues have devised a long-release mitomycin device (New World Medical), which is attached to the Ahmed valve. This releases mitomycin antifibrotic agents in a slow moving fashion for two to three weeks. Animal results with the device have been promising. “What we showed in the animal model is that the device releases the mitomycin slowly over two to three weeks, cutting the immediate scar tissue-forming response after the surgery,” Dr. Ayyala said. “In the animal model we have had zero complications and great results in the sense that fibrosis was reduced by almost 70%-80%.” If these results translate into humans, Dr. Ayyala thinks that there will be a significant decrease in fibrosis, which he hopes will translate into better circulation of the aqueous across the fibrous tissue, resulting in better intraocular pressure.
“My hope is that in the next couple of years we’ll get a prototype out that we can test in humans,” Dr. Ayyala said. “We are already working on a second generation slow-release device using biodegradable polymers that I think will take us to the next level that can help even the super-choroidal devices.”

Targeting Schlemm’s canal

Schlemm’s canal directed work, such as the Trabectome procedure (NeoMedix, Tustin, Calif.), canaloplasty, and the iStent (Glaukos, Laguna Hills, Calif.), are also making inroads. “The Trabectome essentially chews up a portion of the trabecular meshwork and the inner wall of Schlemm’s canal, opening the Schlemm’s canal to the face of the aqueous so the aqueous can access the canal directly,” Dr. Ayyala said. Average results for the Trabectome procedure have been between 15 mm Hg and 16 mm Hg.
Meanwhile, with the canaloplasty procedure practitioners are essentially inserting a microcatheter into Schlemm’s canal and then using a viscoelastic material like Healon GV (Abbott Medical Optics, Santa Ana, Calif.) to enlarge the drainage channel. Then a prolene stent is inserted inside the trabecular meshwork to tighten the canal. In Dr. Ayyala’s practice they recently compared canaloplasty results to those from trabeculectomy. “The average IOP in the trabeculectomy group was about 11 mm Hg, while in the canaloplasty group it was approximately 10.5 mm Hg,” he said. “This is very similar, and the failure rate of about 10% at one year is also similar in the two groups.”
The new device, the iStent, is inserted into the trabecular meshwork. “At one end it will go into the Schlemm’s canal and at the other it opens up into the anterior chamber,” Dr. Ayyala said. “This seems to have some limitations because it will give access to only one segment of the Schlemm’s canal whereas canaloplasty appears to dilate 360 degrees of the canal.”
Overall, Dr. Ayyala thinks that these are exciting times to be a glaucoma surgeon and envisions great progress on the horizon. “I foresee a 5- or 10-minute outpatient procedure with a bypass stent,” he said. “The ideal glaucoma surgery should be able to overcome the natural obstruction and let the fluid access the canal and beyond and re-establish the natural drainage pathway. If we can do that successfully I think it will be a great benefit to all of our patients.”

Editors’ note: Dr. Ayyala has financial interests with New World Medical (Rancho Cucamonga, Calif.).

Contact information

Ayyala: rAyyala@tulane.edu