Update and clinical pearls from the TVT study

April 18, 2008

The Tube versus Trabeculectomy (TVT) Study found that nonvalved tube shunt surgery was more likely to maintain IOP control and avoid persistent hypotony and reoperation for glaucoma than trabeculectomy with mitomycin C (MMC) during the first year of follow-up.

The Tube versus Trabeculectomy (TVT) Study found that nonvalved tube shunt surgery was more likely to maintain IOP control and avoid persistent hypotony and reoperation for glaucoma than trabeculectomy with mitomycin C (MMC) during the first year of follow-up. Tube shunt surgery and trabeculectomy with MMC produced similar pressure reduction at 1 year, but there was less need for supplemental medical therapy with trabeculectomy.

Summarizing the key points of the TVT study, Steven J. Gedde, MD, presented clinical pearls from the TVT study during Glaucoma Subspecialty Day at the American Academy of Ophthalmology annual meeting. Dr. Gedde also said that there were a large number of surgical complications during the first year of follow-up, but most were transient and self-limited. The incidence of postoperative complications was higher following trabeculectomy with MMC than nonvalved tube shunt surgery, but the rate of serious complications resulting in vision loss or reoperation was similar for both surgical procedures.

The TVT study was a multicenter, randomized trial designed to evaluate the safety and efficacy of nonvalved tube shunt surgery to trabeculectomy with MMC in patients with previous ocular surgery. Enrolled patients were randomly assigned to treatment with a 350-mm2 Baerveldt glaucoma implant placed superotemporally with a complete restriction of flow through the tube at the time of implantation or a superior trabeculectomy with MMC (0.4 mg/ml for 4 minutes). Dr. Gedde is professor of ophthalmology, Bascom Palmer Eye Institute, University of Miami, School of Medicine, Miami.

Outcome measures were IOP, visual acuity, visual field, quality of life, reoperations, complications, and need for supplemental medical therapy. Failure criteria were prospectively defined as IOP >21 mm Hg; or not reduced by 20% below baseline on two consecutive visits after 3 months; IOP ≤5 mm Hg on two consecutive visits after 3 months; additional glaucoma surgery; or loss of light perception vision.

The study was carried out at 17 clinical centers across the United States as well as Moorfields Eye Hospital in London. A total of 212 eyes in 212 patients were enrolled, including 107 in the tube group and 105 in the trabeculectomy group. The average age was about 71 years; there was a slight majority of males in the trabeculectomy group and females in the tube group. The mean baseline IOP was 25.4 ± 5.3 mm Hg, 81% of patients had primary open-angle glaucoma, and they were using an average of 3 medications. The average mean deviation was –16.0 ± 10.2 dB.

"There was no significant difference in any of the baseline demographic or ocular characteristics between the tube group and the trabeculectomy group, suggesting that randomization was very effective in creating two balanced treatment groups," Dr. Gedde said.

Describing the outcomes, he noted that there was a significantly greater reduction in IOP among the patients who underwent trabeculectomy at all timepoints during the first 3 months of the study. However, at 6 months and 1 year there was no difference between the treatment groups. He added that there was excellent retention during the first year of the study, with 96% of possible follow-up visits completed.

The mean IOP in the two treatment groups was similar at baseline (25.1 mm Hg in the tube group and 25.6 mm Hg in the trabeculectomy group), and medication use was also nearly identical (3.2 and 3.0 medications, respectively). At one year, IOP levels had dropped considerably to a similar level in both groups; the mean pressure in the tube group was 12.4 mm Hg compared with 12.7 mm Hg in the patients who underwent trabeculectomy. However, there was a significantly greater need for adjunctive medical therapy in the tube group at 1 year (1.3 versus 0.5, p < 0.001), Dr. Gedde said.

Using Kaplan-Meier survival analysis, the cumulative probability of failure at 1 year was 3.9% in the tube group and 13.5% in the trabeculectomy group, a difference that was statistically significant (p = 0.017).

"We recognized that a pressure ≤ 21 mm Hg may not be adequate for many of our patients with glaucoma," Dr. Gedde said. "For that reason, we did a post-hoc analysis to see if the results would change if more stringent pressure criteria were applied to define success and failure. If we defined failure as a pressure above 17 mm Hg or failure at a pressure even above 14 mm Hg, there was still a significantly higher failure rate in the trabeculectomy group compared with the tube group at 1 year."

There were a large number of surgical complications during the first year of follow-up. The incidence of intraoperative complications such as conjunctival buttonholes and hyphema was 7% in the tube group and 10% in the trabeculectomy group. The rate of postoperative complications was significantly higher in the trabeculectomy group (60%) than in the tube group (36%), but most were transient and self-limited, such as shallowing of the anterior chamber or choroidal effusions.

Recognizing that all complications are not equal in severity, the TVT investigators defined serious complications as those that were associated with reoperation to manage the complication or loss of 2 or more lines of Snellen visual acuity. The frequency of serious complications was similar: 18% in the tube group and 28% in the trabeculectomy group.

The results of this study suggest that the role of tube shunts should be expanded beyond the management of refractory glaucoma, Dr. Gedde said. The TVT study does not demonstrate clear superiority of one glaucoma operation over the other, however, and other factors must be considered when selecting a surgical procedure. Those include the surgeon's skill, experience with both operations, and the patient's medication tolerance, availability, and compliance with therapy.

Additional follow-up data are needed to assess fully the risks of tube shunt surgery and trabeculectomy with MMC in managing medically uncontrolled glaucoma in similar patient groups, Dr. Gedde said.