Lack of data fuels glaucoma shunt study

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A prospective study randomly assigned 64 patients with open-angle glaucoma to surgery with a miniature glaucoma shunt or standard trabeculectomy, both with mitomycin-C.

A surgical technique showing the glaucoma shunt implanted under a scleral flap. (Video courtesy of Yvonne M. Buys, MD)

Toronto-Results from 1 year of follow-up in a prospective, randomized study comparing guarded implantation of a miniature glaucoma shunt (Ex-PRESS P50, Alcon Laboratories) to standard trabeculectomy show no between-group differences in IOP reduction or safety outcomes.

The device-based procedure was associated with faster visual recovery, but its use carried a higher cost, according to Yvonne M. Buys, MD.

The study enrolled 64 consecutive patients with open-angle glaucoma who were scheduled to undergo filtration surgery because their IOP was uncontrolled on maximally tolerated medical therapy.

The miniature shunt was implanted under a scleral flap, and both surgeries were performed using intraoperative mitomycin-C 0.4 mg/ml for 2 minutes. Sixty-one of the 64 patients were evaluated at 1 year.

“The miniature glaucoma shunt has been available since 2002, and the surgical procedure for its implantation has several potential advantages, including a short-learning curve, consistency, and less tissue manipulation,” said Dr. Buys, professor, Department of Ophthalmology and Vision Sciences, University of Toronto, Toronto.

Study motivation

This shunt has been available since 2002. However, there is a lack of evidence-based data on its outcomes, and so researchers were motivated to undertake this study, she said.

“In our study, only cost emerged as a major factor differentiating between the two procedures through 1 year,” Dr. Buys said. “However, the faster visual recovery after the miniature shunt procedure compared with trabeculectomy is an interesting difference that deserves further study, and longer follow-up is certainly needed.

IOP was analyzed as the primary efficacy outcome measure for the randomized trial. Mean IOP was similar in the miniature glaucoma shunt and trabeculectomy groups at baseline (22.6 and 22 mm Hg, respectively) and at all follow-up visits, which were scheduled at 1 day, 1 and 2 weeks, and 1, 2, 3, 4, 6, and 12 months.

At 1 year, mean IOP was 11 mm Hg in the miniature glaucoma shunt group and 10 mm Hg in the trabeculectomy eyes.

The rate of complete success, defined as IOP between 5 and 18 mm Hg with a ≥20% reduction from baseline without medication, was higher in the miniature shunt group compared with the trabeculectomy eyes, 71% versus 57%, but the difference was not statistically significant.

Mean logMAR visual acuity decreased significantly immediately after surgery in both groups and then began to improve.

In the trabeculectomy group, best mean visual acuity after surgery was not achieved until 2 months after surgery, but it worsened thereafter. Mean visual acuity was significantly below the baseline level at all follow-up visits.

In the miniature shunt group, mean visual acuity never returned to baseline, but it did recover to a level that was no longer significantly different from baseline by 1 month after surgery and was stable thereafter.

At 1 year, both groups had a median visual acuity loss from baseline of 1 Snellen line. However, the proportion of eyes with a >2 line loss of visual acuity was significantly higher in the trabeculectomy group than in the miniature shunt eyes, 30% versus 3%.

Four of the nine eyes in the trabeculectomy group that lost more than 2 Snellen lines were due to reasons not directly related to the surgery, Dr. Buys noted.

Other secondary outcomes included number of glaucoma medications, complications, need for additional procedures, bleb morphology, central corneal thickness, endothelial cell count, and surgical time. There were no significant differences between study groups in any of these endpoints.

Cost considerations

The economic analysis took into account the costs of the surgery and of follow-up care through 1 year. Postoperative cost was lower for the miniature shunt group than for the trabeculectomy eyes by about $125.

However, due to the cost of the device, the total cost per case through 1 year was nearly $1,000 more for the miniature shunt procedure.

Dr. Buys noted that at the time the study was launched, there was a single published report of a randomized controlled study comparing the miniature glaucoma shunt with trabeculectomy. While it reported significantly greater success in the device group after 1 year, by 3 years and continuing through 5 years of follow-up, there was no difference between groups.

“Subsequently, another prospective, randomized study was published that compared the two procedures in fellow eyes of patients with bilateral glaucoma,” Dr. Buys said. “While this study found the success rate was significantly higher for the miniature shunt after a mean follow-up of almost 2 years, it enrolled only 15 patients of which 3 dropped out.

“Our study enrollment surpassed our target sample size of 52 patients, which was the number needed to detect a 2-mm Hg difference in IOP between groups with 80% power. Now we are looking at our data from follow-up at 2 and 3 years,” she said.

Yvonne M. Buys, MD

E: y.buys@utoronto.ca

Dr. Buys received some of the devices used in the study at no charge from Alcon Canada and I-Med, and has received speaking fees from Alcon for unrelated topics.