Glaucoma: Trabecular micro-bypass device aids IOP reduction in prospective study

September 1, 2009

Implantation of a trabecular micro-bypass (iStent, Glaukos Corp.) in patients with open-angle glaucoma who are undergoing cataract surgery represents an alternative surgical approach that can provide clinically significant reductions in IOP and use of ocular hypotensive medications.

Key Points

This study and a similar ongoing trial in the United States are part of the search to find ways to lower IOP without creating a filtering bleb, said Thomas W. Samuelson, MD. He described results of the multi-country evaluation, which took place in Austria, Germany, the Netherlands, Spain, and Turkey, at the annual meeting of the American Society for Cataract and Refractive Surgery.

Dr. Samuelson is affiliated with Minnesota Eye Consultants, Minneapolis. He was not an investigator in the trial but was familiar with the findings through his role as an investigator in a similar study currently in progress in the United States as part of the FDA approval process.

The stent bypasses the juxtacanalicular meshwork to gain direct access to Schlemm's canal. The objective is to bypass only the portion of the trabecular meshwork that is obstructed and lower IOP simply by making the natural system work better. In contrast, the strategy behind approaches such as trabeculectomy or tube shunts is to bypass the meshwork altogether.

"The primary advantage of the [trabecular micro-bypass] is safety in that there's no external reservoir of aqueous-that is, a filtering bleb-that could lead to infection, foreign body sensation, endophthalmitis, or other problems down the line," Dr. Samuelson said. "Another advantage is that it's performed through the same clear corneal incision as cataract surgery, or it could be done as a stand-alone procedure through a clear corneal incision."

The prospective, randomized study is evaluating trabecular micro-bypass placement in patients with open-angle glaucoma who are undergoing cataract surgery compared with cataract extraction alone. The results have not yet been made public, thus the data from the European experience are the best source of information on the safety and efficacy of the stent in patients undergoing both cataract and glaucoma surgery, according to Dr. Samuelson.

Results show IOP improvement

The results he presented showed improvement in IOP above and beyond what is customary with cataract surgery alone. Dr. Samuelson cautioned that it cannot be definitively stated that the pressure-reducing effect and reduction in number of medications needed to manage glaucoma were produced entirely by the use of the stent. It is becoming increasingly clear that cataract surgery alone also lowers IOP. Because the IOP reductions in this study exceeded historic levels cited in the literature, however, it is likely that the trabecular micro-bypass had a favorable effect on IOP, Dr. Samuelson said.

In the study, 59 patients underwent clear cornea phacoemulsification followed by ab interno gonioscopically guided implantation of the trabecular micro-bypass through the same 2.5- to 3.5-mm temporal incision used to extract the cataract. The primary outcome was IOP at all time points, and the secondary outcome was the number and type of glaucoma medications used pre- and postoperatively.

Inclusion criteria were a diagnosis of open-angle glaucoma, including pseudoexfoliation glaucoma or pigmentary glaucoma; current use of at least one glaucoma medication; IOP ≥ 18 mm Hg; and a concurrent diagnosis of cataract that requires cataract surgery and implantation of an IOL. Most patients were female (66%) and Caucasian (98%), and the mean age was 75 years.

The mean IOP at baseline was 21.5 mm Hg; at 24 months, 45 of 53 patients had an IOP of ≤ 18 mm Hg with a mean IOP of 15.8 ± 2.1 mm Hg. Among the 45 patients, 26 (57.8%) had a mean IOP of 15.6 ± 1.8 mm Hg and were taking no medications. The average IOP decrease from medicated baseline was 5.1 ± 4.2 mm Hg (p < 0.001).

Implantation of the stent also resulted in a statistically significant decrease in the number of ocular hypertensive medications required to control IOP. At baseline, the mean number of medications was 1.7 ± 0.9, and at 24 months the mean was 0.5 ± 0.7, a mean reduction of 1.1 ± 0.8 medications (p < 0.0001).

In all, the European study demonstrated significant reductions in IOP and number of medications necessary following simultaneous cataract surgery and trabecular micro-bypass.

Additive effect

"One of the things that I like about the concept is that the mechanism of IOP reduction with the [trabecular micro-bypass] is additive to the effect that cataract surgery has on IOP," he said. "If we do [a stent] procedure and a cataract procedure, the favorable effects from each are harmonious; they work in the same strategy, improving trabecular outflow.

"If, on the other hand, we take a cataract out and do a trabeculectomy at the same time, they're opposite strategies, and they're not additive," he concluded. "A trabeculectomy bypasses the very same drain that the cataract surgery is enhancing; therefore, once you've done a trabeculectomy, attempts at enhancing trabecular outflow are far less likely to be effective. The bottom line is that the [trabecular micro-bypass] is a procedure that's additive not only in its effect on pressure reduction but also on its mechanism of pressure reduction."