To trab or to tube: what is the best primary surgery for glaucoma?

Trabeculectomy: cut window seen under the scleral flap Source: Anand Sudhalkar, M.D.

Despite major advances in the development of novel drugs and laser techniques for IOP reduction in eyes with glaucoma, a significant proportion of glaucoma patients ultimately requires incision surgery to achieve adequate IOP control. Many novel surgical devices and techniques have been proposed in the past few years, but to date none have supplanted the two most common surgical procedures for IOP reduction: Trabeculectomy and tube-shunt surgery.
In the traditional stepped treatment algorithm, patients inadequately controlled with medical or laser interventions typically undergo trabeculectomy as the primary surgical procedure. Tube-shunt implantation is commonly reserved for trabeculectomy failures or complicated glaucoma that responds poorly to trabeculectomy, such as inflammatory or neovascular glaucoma.
But in recent years the glaucoma community has revisited this practice. At the recent Glaucoma Subspecialty Day at the 2009 American Academy of Ophthalmology meeting in San Francisco, two experts debated the merits of trabeculectomy versus tube-shunt implantation as the primary surgical intervention for glaucoma.

The case for tubes

“Historically, tubes as a primary surgical procedure have been reserved for very difficult glaucoma,” said Dale Heuer, M.D., professor, department of ophthalmology, Medical College of Wisconsin, Milwaukee. “But in recent years, there has been a trend toward greater use of tubes and reduced use of trabeculectomies by glaucoma specialists in the U.S.”
In response to this shift in preference and to address the need for high-quality comparative data between the two procedures, the Trabeculectomy Versus Tube study was conducted. Patients who had previously undergone either cataract surgery or failed trabeculectomy and required surgical intervention for IOP control were randomized to undergo either trabeculectomy with mitomycin-c or implantation of a non-restrictive Baerveldt glaucoma implant (Abbott Medical Optics, Santa Ana, Calif.). Three-year outcome data was recently reported.
“The mean IOP reductions in the two groups were dead even,” Dr. Heuer said. “But the failure rate of trabeculectomy in the TVT study was twice as high as the failure rate for tubes.” In fact, 30% of trabeculectomized eyes had failed within 3 years, compared to only 15% of tubes.
Failure in the study was defined as IOP being too high (>21 mm Hg) or too low (<5 mm Hg), the need for reoperation, or loss of light perception vision, he said.
The post-op complication profiles of the two procedures differed as well, he said. “In the trabeculectomy group, wound leaks and dysesthesia were more common, while in the tube group, post-operative diplopia was more common.”
Dr. Heuer, who was an investigator in the TVT study, feels that the study results support the choice of tube surgery over trabeculectomy for most patients requiring IOP-lowering surgery.

The case for trabeculectomy

“Both trabeculectomies and tubes for initial glaucoma surgery can be highly successful,” said David Greenfield, M.D., associate professor, Bascom Palmer Eye Institute, Miller School of Medicine, University of Miami, Miami. “Both can lead to serious complications. We have a quandary as clinicians. It is hard to know how to proceed.”
He acknowledged that the TVT study—in which he also participated as an investigator—demonstrated an advantage for tubes over trabeculectomy, but he pointed out that the study’s conclusions were based on safety outcomes and not efficacy outcomes.
“There were identical IOP outcomes in the two groups,” he said. “The difference favoring tubes was based on complication rates, not IOP reduction.” He said that post-op complications in TVT were seen in 39% of patients receiving the Baerveldt implant versus 60% of patients undergoing trabeculectomy.
He also felt that the criteria for failure may have been too stringent. “Eyes with IOP below 5 mm Hg were classified as failures, when in fact many eyes tolerate such low pressures quite well, with well-formed eyes, no maculopathy, and stable vision.”
Dr. Greenfield also pointed out that the TVT study’s results might not be the best basis for selecting a primary glaucoma procedure. “All of the eyes in the TVT study had undergone prior surgery—either cataract or failed trabeculectomy procedures. We cannot generalize the TVT results to patients requiring initial glaucoma surgery.”
Despite the TVT results, Dr. Greenfield prefers trabeculectomy over tube implantation for several reasons. First is the relative effect of the two procedures on the conjunctiva. “There is a limited amount of conjunctival real estate for the glaucoma surgeon to utilize,” he explained. “If a trabeculectomy fails, we lose a small amount of conjunctiva and can still typically do a subsequent tube. But if we do a primary tube and that fails, there is often not enough conjunctiva left to consider a trabeculectomy.”
Trabeculectomy affords two other key advantages, Dr. Greenfield continued. “We can get to single-digit IOP levels with trabeculectomy. In contrast, IOP following tubes usually settles somewhere in the high teens. With trabeculectomy, we are more likely to be able to reduce the need for topical medications post-operatively.”

One size does not fit all

Dr. Greenfield acknowledged that he does prefer primary tube implantation over trabeculectomy in some cases. “These include high myopes, contact lens wearers who are reluctant to discontinue their use, and patients who have failed a prior trabeculectomy in the fellow eye.”
In these cases, he often modifies his typical tube implantation technique. “I try to put them in the inferonasal quadrant, with the tube entering the eye at the 6 o’clock position. Also, I will use a corneal patch graft rather than a scleral patch graft, for cosmetic reasons.”

Editors’ note: Drs. Heuer and Greenfield did not indicate any financial interests related to their comments.

Contact information

Greenfield: dgreenfield@med.miami.edu
Heuer: dheuer@mcw.edu