Take-home message: One-year results are reviewed for use of a novel ab-interno gelatin stent as a stand-alone procedure for patients with mild-to-moderate primary open-angle glaucoma.
By Fred Gebhart
—New results on stand-alone implantation of a gel stent for glaucoma underscored existing data on combination glaucoma-plus-cataract surgery.
The latest data from a multicenter European study of the gelatin stent (Xen 45) showed a mean reduction in IOP of 42% 12 months after implantation. The stent was developed by AqueSys, which was acquired by Allergan in 2015, and is widely available outside the United States. It remains an investigational device in the United States
“This gel stent is simply the most exciting approach we have had for the surgical treatment of glaucoma for a long, long time,” said Rohit Varma, MD, MPH, interim dean of the Keck School of Medicine, the Grace and Emery Beardsley Professor and Chair of Ophthalmology and director, Roski Eye Institute, University of Southern California. “Once the pressure drops after implanting the stent, it stays low for a number of years.
Dr. Varma presented unpublished topline data from 12 months of follow up on 103 patients treated by 11 different surgeons across seven European countries.
Most prior data on the stent has come from combination procedures, Dr. Varma noted. This is one of the largest studies of stand-alone implantation in reducing IOP and medication use in patients primary open-angle glaucoma and no prior incisional surgery, he added.
Before implantation, patients had a mean best medicated IOP of 22.8 mm Hg. The mean best medicated IOP 12 months after implantation was 13.3 mm Hg, a 42% reduction. Patients in the trial were taking a mean of 3.3 IOP-lowering eye drops before implantation and 1.1 drops 12 months post-implantation, a 66% reduction in medication use.
“The problem is that glaucoma is a long-term disease and we are treating patients over a lifetime,” Dr. Varma said. “All we have at this point is drops, then laser, which is not particularly effective, then invasive surgery, trabeculectomy, and tubes. There is a real need for a treatment that is more effective than drops, is minimally invasive like laser, and produces clinically significant and consistent reduction in IOP over the long term. That’s where [this deviece] comes in.”
How it works
The stent is a 6-mm hydrophilic cylinder made of cross-linked porcine gelatin with a 45-μm lumen. With a 210-μm outer diameter, three stents can fit inside the lumen of a Baerveldt tube. A smaller and narrower device allows for ab interno placement that is less prone to hypotony and other complications than conventional tube stents. And because the device drains directly into the subconjunctival space, it can lower IOP as effectively as more invasive trabeculectomy.
About one-half of the transplants for the device done outside the United States have been stand-alone procedures and half done in conjunction with cataract surgery, Dr. Varma added. Three different models have been developed with lumens of 45, 63, and 140 μm. Testing outside the United States suggests the 45-μm model provides the optimal combination of improved IOP lowering and low rates of adverse events for most patients.
Device designers used Poiseuille’s law of laminar flow to exploit the known relationship between length and lumen to regulate outflow volume and avoid hypotony.
The stent is placed ab interno using a preloaded injector with a 27-gauge needle that is guided by hand through a clear-corneal incision about 1.6 mm in length. Once in place, the device hydrates and swells, adapting to the shape of the surrounding tissue to connect the anterior chamber directly with the subconjectival space.
Once implanted, 1 mm of the stent remains in the anterior segment, 3 mm are interscleral, and 2 mm extend into the subconjunctival space. The direct channel between the anterior chamber and the subconjunctival space avoids blockages in the physiologic drainage system, but sock-like scarring can block the tube tip in 10% to 30% of patients.
An application of mitomycin C at the time in insertion can prevent scarring or the scar can be needled in a follow-up office procedure if needed.
Other adverse events include minor bleeding at the time of insertion in fewer than 2% of cases and perforation of the conjunctiva. Erosion of the conjunctiva has been a very rare occurrence in the several hundred implantations that have been performed to date, Dr. Varma said.
“Because the [stent] is soft, it conforms to the shape of the tissue and is much less likely to erode compared with conventional stents,” he said. “And because the tube is soft, it avoids one of the major complications with harder silicone tubes, which can injure corneal epithelial cells, leading to corneal edema. We are not seeing corneal edema with [this] stent.”
The stent has a CE mark for both stand-alone placement and in combination with cataract surgery. Allergan recently completed a U.S. trial and is preparing data submission to the FDA for 510(k) marketing clearance.
Rohit Varma, MD, MPH
E: [email protected]
This article was adapted from Dr. Varma’s presentation at the 2016 meeting of the American Society of Cataract and Refractive Surgery. He is a consultant for AqueSys.