Advances in Glaucoma Filtration Surgery
Two devices in development make use of Poiseuille’s law.
The trabeculectomy has been the so-called gold standard of glaucoma filtration surgery. When one trabeculectomy failed, surgeons would often perform a second or even third, followed by some form of cycloablation when these filters failed as well. In the latter part of the 20th century, glaucoma drainage implants, starting with the Molteno (Katena Products), became a viable option. Currently, drainage devices are more often used as the next surgery in the glaucoma treatment algorithm when one filter fails. This practice is supported by the results of the Tube Versus Trabeculectomy (TVT) Study, which demonstrated that tube shunts may hold an advantage in terms of efficacy and fewer complications when performed in cases of refractory glaucoma where previous surgery has failed.1
Glaucoma drainage devices are not without unique complications, however, such as diplopia, tube/plate exposure, retinal breaks/scarring from deep scleral fixation sutures, plate encapsulation, and failure.2 Consequently, new technology is being developed to minimize the complications from both tubes and trabeculectomy. Among the devices in development—and the focus of this article—are the Xen Gel Stent from AqueSys and the InnFocus MicroShunt from InnFocus. Both manufacturers are trying to control flow and minimize hypotony by applying Poiseuille’s law of laminar flow to create a tube that is sufficiently long and narrow to restrict and control outflow while allowing for minimal biofouling, scarring, and inflammation.
THE XEN GEL STENT
The Xen is a soft, collagen-based gelatin implant. Because it is injected into the subconjunctival space via an ab interno route through a clear corneal incision (Figure 1), the technology is categorized as microinvasive glaucoma surgery (MIGS). The Xen is the only MIGS procedure, however, that uses a space typically accessed in an ab externo fashion with the goal of an external, subconjunctival fistula. It is therefore an interesting and unique “hybrid” that could conceivably be performed as a primary procedure in the early stages of glaucoma without eliminating the option of standard filtration surgery in the future. Nor would the device preclude future treatment with ab interno MIGS procedures such as the iStent Trabecular Micro-Bypass Stent (Glaukos), CyPass Micro-Stent (Transcend Medical; not available in the United States), Trabectome (NeoMedix), or Hydrus (Ivantis; not available in the United States). Future endoscopic cyclophotocoagulation would also be an option.
Additional advantages of the Xen compared with glaucoma surgeries that filter to a subconjunctival space are the absence of any incision in the conjunctiva and the minimal ab interno dissection that occurs.
Currently, the implant is under investigation in the United States, and a prospective, multicenter clinical trial is underway. Early results from Europe, Canada, and South America demonstrate an IOP in the midteens after up to 2 years of follow-up. Data were presented at the World Glaucoma Congress and at the American Society of Cataract and Refractive Surgery annual symposium last year.3-5 In a representative example from the presentations at the 2013 meetings, Paul Palmberg, MD, reported results from a prospective, noncomparative study of 109 patients who received the Xen. The average preoperative IOP was 21.8 mm Hg on 2.8 medications, and 86 patients reached the 1-year follow-up. At 1 year, the average IOP was 15.3 mm Hg with a mean medication use of 1.0. Of the patients who completed 1 year of follow-up, 66% achieved an IOP less than or equal to 16 mm Hg and/or a 30% drop in IOP.5
THE InnFocus MICROSHUNT
The InnFocus Microshunt (formerly the MIDI Arrow) is made of a proprietary material called poly(styrene-block-isobutylene-block-styrene) or SIBS. This novel material has been used in cardiac stents and has been shown to cause minimal inflammation.6 The device is 8.5 mm long and has a 70-μm lumen.
Conceptually, the InnFocus Microshunt resembles a conventional tube, but there is no large plate that will cause scarring and become encapsulated (Figure 2). Another difference between the device and a conventional tube shunt is that, with the InnFocus Microshunt, mitomycin C (MMC) is used to modulate wound healing. Research has shown MMC not to be effective at lowering IOP with conventional glaucoma drainage implants.7 The InnFocus Microshunt is placed in the anterior chamber through an ab externo scleral needle track tunneled posteriorly to the limbus. The implantation of the device is not technically MIGS, because it must be implanted with a fornix-based conjunctival incision.
No data on this device have been published, but the implant is under investigation in the United States. A multicenter, randomized, prospective clinical trial comparing the InnFocus Microshunt with MMC to standard trabeculectomy with MMC is currently recruiting patients. Like the Xen implant, the InnFocus Microshunt does not preclude the use of future ab interno MIGS procedures. Unlike the Xen implant, implantation of the InnFocus Microshunt requires significant conjunctival dissection, making future filtration surgery more difficult.
A continuing shift of filtration surgeries to smaller-incision “stenting” procedures is likely. Since the release of the Ex-Press Glaucoma Filtration Device (Alcon) over 10 years ago, surgeons have sought different ways to restrict outflow by standardizing wound size and slowing flow by channeling it through a cylindrical implant. The Xen and InnFocus implants as well as the many new MIGS devices being studied make this an exciting time in the fight against glaucoma-related blindness.
Steven R. Sarkisian Jr, MD, is the glaucoma fellowship director at the Dean McGee Eye Institute, and he is a clinical associate professor at the University of Oklahoma College of Medicine in Oklahoma City. He is on the advisory board of InnFocus. Dr. Sarkisian may be reached at (405) 271-1093; firstname.lastname@example.org .
- Gedde SJ, Schiffman JC, Feuer WJ, Herndon LW, Brandt JD, Budenz DL; Tube versus Trabeculectomy Study Group. Treatment outcomes in the Tube Versus Trabeculectomy (TVT) study after five years of follow-up. Am J Ophthalmol. 2012;153(5):789-803.e2.
- Bailey AK, Sarkisian SR Jr. Complications of tube implants and their management. Curr Opin Ophthalmol. 2014;25(2):148-153.
- Grabner G. Two-year results after implantation of minimally invasive ab interno subconjunctival implant in refractory open-angle glaucoma patients. Paper Presented at: ASCRS Symposium; 2013; San Francisco, CA.
- Vold S. Implantation of a minimally invasive ab interno subconjunctival implant in combination with cataract surgery for the treatment of glaucoma. Poster Presented at: World Glaucoma Congress; 2013; Vancouver, Canada.
- Palmberg P. Two-year follow-up data for a soft and durable minimally invasive ab interno trans-scleral implant in open-angle glaucoma subjects. Poster Presented at: World Glaucoma Congress; 2013; Vancouver, Canada.
- Pinchuk L, Wilson GJ, Barry JJ, et al. Medical applications of poly(styrene-block-isobutylene-block-styrene) (“SIBS”), Biomaterials. 2008;29(4):448-460.
- Costa VP, Azuara-Blanco A, Netland PA, Lesk MR, Arcieri ES. Efficacy and safety of adjunctive mitomycin C during Ahmed Glaucoma Valve implantation: a prospective randomized clinical trial. Ophthalmology. 2004;111(6):1071-1076.