Glaucoma
A new look at glaucoma drainage devices

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Traditionally, candidates for glaucoma drainage devices have been chosen simply because they are not good candidates for trabeculectomies. Although glaucoma drainage implants are considered to be a second surgical treatment choice for lowering pressure in glaucoma, they have recently been gaining a more prominent role.
“There’s no question that trabeculectomy is still the most effective way to lower intraocular pressure,” said Garry Condon, M.D., associate professor of ophthalmology, Drexel University College of Medicine, Philadelphia, and director of the glaucoma service, Allegheny General Hospital, Pittsburgh, “but the late-term problems are sometimes really difficult to handle, and so with more variations on tube shunts and evolving techniques on using tube shunts, some people have gone to using them more early on in the surgical steps for managing glaucoma.”

Changing indications for use

The Tube Versus Trabeculectomy study, the results of which were recently published online in the American Journal of Ophthalmology, is one factor influencing how the views on glaucoma drainage devices have changed.
The study, which was coordinated by the Bascom Palmer Eye Institute, Miami, compared the safety and efficacy of a 350 mm2 Baerveldt glaucoma implant and trabeculectomy with mitomycin C among 212 patients who had previously had intraocular surgery. At the one year follow-up, patients who had the tube shunt device were less likely to experience post-op complications (P=0.001), less likely to experience failure (P=0.017), and more likely to take more glaucoma medications (P<0.001). Surgical complications and IOP showed statistically similar results between the two groups.
“Based on the Tube versus Trab trial, patients who are good candidates for basic trabeculectomies may now also be considered good candidates for shunts,” said Dr. Condon. “Now, there may be more supporting evidence to consider tube shunts earlier in the stages of glaucoma surgery, because of the results of the Tube versus Trab study. The clinical trial hasn’t convinced me, per se … but it does make me more comfortable going to a tube shunt in cases where I might have waffled before on whether to go with a trabeculectomy or to go with a tube shunt.”

Types of tubes and evolving techniques

The four commonly used types of glaucoma drainage devices fall into two major categories: valved (Ahmed valve and the Krupin implant) and nonvalved (Baerveldt implant and the Molteno implant). The more common ones in use today are the Ahmed valve and the Baerveldt implant.
Because the nonvalved design may be associated with hypotony immediately after surgery, a valve design was created as a way to prevent excess fluid drainage through the tubes. According to Ike K. Ahmed, M.D., assistant professor of ophthalmology, University of Toronto, Ontario, “I think that the valve does its job, particularly in the early postoperative period. In the long term, it probably functions primarily as a tube, as a regular device does.”
Dr. Ahmed said that retrospective studies have shown that the Baerveldt and Ahmed implants appear to be virtually equivalent in lowering pressure over the long term. “I use both [devices] pretty equally,” he said. “For patients who we’ve driving for a very low pressure, we often have stuck with the Baerveldt, but that’s basically based on anecdotal reasons.”
Surgical manipulation is required to overcome the problem of postoperative hypotony associated with the nonvalved drainage tubes. Dr. Condon said, “We do have a number of techniques that allow us to use nonvalved shunts in a way that mimics the intention of the valve. These primarily involve absorbable ligature sutures and fenestrating the tube anterior to the ligature. The valved shunts, in my experience, don’t work any better than nonvalved at providing a predictable outflow or pressure in the early postoperative period. So I don’t rely on the valve.”
Dr. Ahmed noted that implanting the drainage devices is more involved than performing a trabeculectomy. “The surgical time is longer, and it does involve placing an implant both in the eye and on the outer surface of the eye.” He also said, “There does appear to be higher rates of corneal decompensation related to the tube and its effect on the corneal endothelium. It’s one of the big concern with tubes, plus the fact it’s a pretty large implant. You’re putting the plate in between the muscles at the equator of the globe. There’s concern about its effect on the extraocular muscle function, as well as eye lid droop, ptosis for example, and also just long-term concerns about erosion—there could be erosion of the tube to the conjunctival and there could be risk for infection and other related problems to erosion.”

Future roles of devices

“I think that they [glaucoma drainage devices] have played an increasing role. More and more we’re using them earlier in the disease and more and more we’re using these implants,” said Dr. Ahmed. “I think it’s been an enormous benefit to these high-risk patients, and we’re probably going to continue and evolve the designs to make them easier-to-use, implant, follow postoperatively, and perhaps a bit more predictable, which is hard, because there is quite a variable type of patients that we’re using them on.”
Dr. Condon sees new ideas for the glaucoma drainage devices on the horizon. “Certainly there are some new ideas that are being tried in the realm of the valve, per se,” he said.
Ideally there needs to be a better way to create a predictable resistance to fluid outflow, so that we can just plug these in and from the outset, achieve the desired amount of outflow for stability in the early post-op period. And right now, there are none currently in the market that we can really rely on for that predictability.”

Editors’ note: Dr. Condon has no financial interests related to his comments. Dr. Ahmed is consultant to Advanced Medical Optics (AMO, Santa Ana, Calif.) for the Baerveldt device and speaks for an honorarium for New World on the Ahmed valve.

Contact Information
Condon: 412-359-6298, garlinda@usaor.net
Ahmed: 905-459-0088, ike.ahmed@utoronto.ca