Trab-shunt debate continues

April 1, 2010

In this point-counterpoint overview, David S. Greenfield, MD, speaks in favor of trabeculectomy, whereas Dale K. Heuer, MD, argues the merits of tube shunts.

Dr. Greenfield is professor of ophthalmology at the Bascom Palmer Eye Institute, University of Miami Miller School of Medicine, and Dr. Heuer is professor and chairman of ophthalmology, Medical College of Wisconsin, and director of the Froedtert & The Medical College of Wisconsin Eye Institute, Milwaukee.

Although observing that initial glaucoma surgery offers an important opportunity for successful glaucoma management and that clinicians should choose the procedure wisely, Dr. Greenfield posed several arguments in favor of primary trabeculectomy.

Dr. Greenfield said that a panel of experts from the American Academy of Ophthalmology's Ophthalmic Technology Assessment Panel concluded that trabeculectomy results in lower postoperative IOP compared with tubes, after which the pressure often settles in the high teens, which in turn is likely to reduce rates of optic disc and visual field progression.

In addition, patients who have undergone primary trabeculectomy are more likely to discontinue topical glaucoma medication and have lower prescription drug expenses.

According to Dr. Greenfield, mid- to long-term outcome studies have shown that a high percentage of patients discontinue glaucoma therapy. Based on recent data concerning the yearly costs of glaucoma medication, patients who discontinue medical therapy could save from $150 to $875 annually.

Patients undergoing primary trabeculectomy also may experience improvement in ocular surface disease and quality-of-life measures. Another advantage is the opportunity to achieve a single-digit target IOP goal, an outcome that is not achievable with tube-shunt surgery despite the use of adjunctive medical therapy, Dr. Greenfield said.

Patients who have had a trabeculectomy also have greater flexibility for subsequent surgery. Primary tube shunt surgery is associated with conjunctival scarring, increasing the risk for subsequent trabeculectomy failure and possibly precluding further filtering surgery.

According to Dr. Greenfield, the results of the Tube versus Trabeculectomy (TVT) Study do not provide guidance for primary glaucoma surgery. Although the TVT study was an excellent trial conducted with rigorous methodology, it reported that the cumulative probability of failure was nearly twice as high in the trabeculectomy group compared with the tube group (30.7% versus 15.1%), and surgeons should not falsely believe that these conclusions apply to eyes being considered for initial glaucoma surgery.

The TVT study enrolled eyes with prior surgery, which increased the risk of trabeculectomy failure, and enrolled many eyes with a prior failed trabeculectomy, both of which will bias success in favor of the tube group. The surgical techniques and definition of success may further bias the results in favor of the tube group.

Another aspect of the study, the long duration of mitomycin-C (MMC) application (4 minutes), contributed to the high rate of wound leakage (11%) and subsequent bleb failure. Also, eyes with a postoperative IOP below 6 mm Hg were considered to be a failure despite the fact that such eyes often have excellent vision and stabilization of their glaucoma, Dr. Greenfield said.