Aqueous shunts aid glaucoma management

Boston-Aqueous shunts can offer a very effective modality for controlling IOP in challenging glaucoma cases, and their therapeutic index has improved thanks to innovations in technique and technology introduced based on clinical experience.

However, a number of questions remain to be resolved regarding the use of aqueous shunts in glaucoma management, said Dale K. Heuer, MD, at Innovations in Glaucoma. The continuing medical education activity was jointly sponsored by The New York Eye and Ear Infirmary and Ophthalmology Times with an unrestricted grant from Pfizer Ophthalmics.

"Currently, there are still no clear answers about which shunt to use and when," said Dr. Heuer, professor and chairman, department of ophthalmology, Medical College of Wisconsin, Milwaukee.

"The one caveat in patients in whom filtering surgery with mitomycin-C failed is that if a localized bleb is present but just not working well enough, it is reasonable to consider needle bleb revision with mitomycin-C before going on to a shunt," said Dr. Heuer, who is also director, Froedtert & Medical College Eye Institute, Milwaukee, WI. "However, if no bleb is present, then shunt surgery is the better approach, at least in my hands."

In other clinical situations, there is more controversy about using an aqueous shunt. Patients who are in that "gray zone" include those with prior cataract and/or filtering surgery. The multicenter, 5-year Tube versus Trabeculectomy study was designed in the hope of shedding some light on whether in those cases, there might be some benefit for implanting an aqueous shunt instead of performing trabeculectomy with mitomycin-C. Patient enrollment in that trial has been completed and 1-year results are expected to be reported over the next year. Patients randomly assigned to the shunt group are receiving the Baerveldt 350-mm² implant.

"With the prolific use of mitomycin-C and the high probability of hypotony, bleb infection, and bleb leaks accompanying that practice, one has to question whether patients may be better served by performing shunt surgery earlier rather than later," Dr. Heuer said.

Which shunt? Regarding the issue of which shunt to use, decisions largely reflect individual surgeon preference. However, there are certain features to consider because they may influence efficacy and safety.

Dr. Heuer pointed out that surface area plays a definite role because data from various studies demonstrate that the smaller single-plate implants are associated with a lower percent reduction in IOP and a lower long-term success rate than the currently available intermediate and large-surface-area implants.

Comparing valved and non-valved shunts, Dr. Heuer noted the valved devices provide immediate IOP reduction and fewer problems in the short term with hypotony, choroidal expansion, and shallow chambers. However, there is another school of thought favoring use of the non-valved implants, especially in eyes with neovascular glaucoma or uveitis, since with their use, encapsulation of the endplate occurs in an aqueous-free environment.

"Research performed years ago by Dr. Tony Molteno of New Zealand, as well as some results from more recent wound healing studies, suggest that a capsule formed in the presence of aqueous may be relatively thicker, and that may be particularly true in eyes with inflammatory-type glaucomas where the aqueous may be a brew of pro-inflammatory cytokines and growth factors," Dr. Heuer said. "Since the fibrous reaction around the endplate influences long-term success of shunt surgery, those findings provide indirect evidence to support use of the non-valved implants. However, any true differences relating to this mechanism remain to be proven in a clinical study."