A New Look at Glaucoma Shunts
Improved designs, new data, make shunts an interesting option.
BY ANDREW RABINOWITZ, M.D.
Ten years ago, I was laughed off the stages and auditoriums at glaucoma meetings for suggesting that shunts, when evaluated overall, were a superior treatment than trabeculectomy. I believe that was because many glaucoma surgeons who attended those meetings were reluctant to switch from trabeculectomy, which is technically easier for surgeons, to aqueous shunts, which are technically more demanding and require a higher level of surgical skills.
Today, I feel vindicated in my beliefs as more and more recent data point to the relative advantages of shunts over trabeculectomy.
The shunts vs. trabeculectomy comparison actually begins in the late 1960s and early 1970s. These were critical years in the development of surgical treatments for glaucoma. It was in those years that trabeculectomy emerged as the preferred surgical treatment for the disease. It was also in those years that South African ophthalmologist Anthony Molteno, M.D., who was living in New Zealand, and American Theodore Krupin, M.D., were pioneering the use of glaucoma shunts.
The Ahmed Glaucoma Valve
Almost 40 years later, trabeculectomy still remains the "gold standard" surgical option for glaucoma, with shunts traditionally used in complicated cases where trabeculectomy has failed or where trabeculectomy is not a viable option. This second group would include patients with neovascular glaucoma, severe inflammatory glaucoma and those who have had previous eye surgery with scarring of the conjunctiva on the surface of the eye so that a standard trabeculectomy would not work.
But, as this article will explain, the second-fiddle status of glaucoma shunts may be about to change.
New Data Favor Shunts
Over the years, advances in shunt technology, especially in the use of more biocompatible materials, have led to improvements in the original Molteno design (IOP Inc., Costa Mesa, Calif.), along with the development in the 1990s of the widely used Baerveldt shunt (Abbott Medical Optics, Santa Ana, Calif.) and the popular Ahmed Glaucoma Valve (New World Medical, Rancho Cucamonga, Calif.), so named because it employs a valve to control the flow of aqueous fluid through the bleb.
The improved designs available today have led to shunts now proving effective in an estimated 85% to 90% of the cases in which they are used. This compares to the 30% to 40% effectiveness of the original shunts used in the 1970s and 1980s.
Supporting the improved effectiveness of shunts is newly presented 3-year data from the landmark Tube Versus Trabeculectomy (TVT) study, which strongly favors the performance of the shunt (in this study the shunt was the 350 mm Baerveldt implant) in such key areas as IOP control, reduced early complications, avoidance of persistent hypotony and duration of effectiveness. In addition, the risk of endophthalmitis has historically been less with shunts than with trabeculectomy.
Among the key 3-year TVT data were a comparison of failure rates and complications. The cumulative probability of failure was 15.1% in tube patients vs. 30.7% in trabeculectomy patients. The rate of complications was 34% for tube vs. 57% for trab.
Based on results of the TVT study, some glaucoma specialists say they are now expanding their use of shunts, either as a first-line surgical option or as an alternative to repeat trabeculectomies.
Highly respected glaucoma specialist Thomas Samuelson, M.D., of Minnesota Eye Consultants, is one thought leader who has been impressed by the encouraging results of the TVT study. He has recently stated that he is now performing fewer trabeculectomies and implanting more aqueous shunts, especially in high myopes, younger patients and contact lens wearers.
Spurred by the TVT data, shunt manufacturers now report an increased level of interest in their products, with more ophthalmologists taking the training to become proficient in implantation procedures. In addition, the demographics for glaucoma treatment are quite favorable as the huge baby boomer cohort enters its senior years.
"The increase in the use of glaucoma shunts is gradual. It's not happening overnight, but the trend is definitely there," says A. Mateen Ahmed, inventor of the Ahmed valve and president and CEO of New World Medical.
"There are also economic and compliance aspects to using shunts earlier in the disease process," says Jason Malecka of IOP Inc., which is the U.S. distributor for Molteno. "Implanting a shunt is a one-time event that reduces the use of glaucoma medications. Having a shunt implanted is also a way to ensure at least a certain level of compliance. So from the standpoint of a cost-effective treatment that also doesn't totally depend on active patient compliance, shunts are a good solution."
Where Shunts Stand
Today, the most widely used glaucoma shunts are the Ahmed, the Baerveldt, the Molteno3 and the more recently approved Ex-Press Mini-Shunt (Optonol). Earlier shunts such as the Krupin and the Schocket played an important role in the development of shunt technology but have largely been surpassed in recent years by the newer designs.
There is general agreement that there is no single "best" glaucoma shunt, with the choice of shunt determined by surgeon preference and the patient's individual needs.
Each of the popular shunts has its own advantages. Surgeons hail the Ahmed's ease of implantation in a one-stage procedure and many like the valve feature that is designed to lower IOP quickly and reduce the potential of hypotony. The Baerveldt and Molteno shunts are both larger, capable of draining away more fluid and have solid reputations for long-lasting effectiveness.
Insights Into Shunt Use
As the glaucoma specialist for the large Barnet Dulaney Perkins practice in Arizona, I have had broad experience in choosing appropriate patients for shunt implantation, implanting the shunts and in the long-term management of these patients.
Over the past decade, the debate has been ongoing to determine the ideal role for aqueous shunts. The greatest challenge with implantation of aqueous shunts is limiting the early postoperative complications.
A Baerveldt shunt prior to implantation
The most common and potentially catastrophic early postop complication is hypotony. Excessive hypotony can be devastating to the success of the surgery and to the health of the eye.
As mentioned above, the commercially available shunts can be separated into two categories — valved and non-valved devices. Non-valved shunts consist of a plate connected to a silicone tube with an unobstructed lumen. The tube reaches from the base of the plate to its endpoint, which can be placed in the anterior or posterior chamber. Non-valved devices have straw-like lumens with no flow-restricting capabilities. With these devices, aqueous leaves the intraocular environment and is then expressed onto the anterior surface of the plate, which is sutured to the sclera.
The valved shunts have flow-restriction leaflets similar to cardiac valves. These valves are designed to shut when the intraocular pressure falls to a specific level. Closure of the valves prevents aqueous from leaving the eye when the IOP falls to a level beyond which the eye could become hypotonous. The theoretical advantage of the valved system is that there is a lower likelihood of the IOP dropping too low at any point postoperatively.
|New Concepts in Glaucoma Treatment Devices|
|There are four glaucoma shunts currently in wide use (Ahmed, Baerveldt, Molteno3 and Express). Over recent years, new therapeutic categories have emerged to facilitate outflow. Following is a brief description of three of the more promising devices:|
■ The iStent from Glaukos (Laguna Hills, Calif). The iStent, which is not yet commercially available in the United States, is the first ab-interno micro-bypass stent. The iStent ab-interno implant is a minimally invasive procedure that spares conjunctival tissue, thereby safely preserving future treatment options.
In current studies, the iStent is implanted in Schlemm's canal in combination with cataract surgery. The micro-bypass iStent is inserted through a clear-corneal incision under gonioscopic control while directly visualizing the angle and trabecular meshwork. The procedure can be performed under topical anesthesia, is blebless and studies indicate a that it is safe. Glaukos says studies show that the iStent has a potent effect on restoring natural physiologic outflow by allowing aqueous humor to flow directly into Schelmm's canal toward the episcleral drainage system while bypassing the trabecular meshwork.
In a prospective, open-labeled, 24-month study of patients with primary open-angle glaucoma and cataract who had the iStent implanted post-combined cataract surgery, the iStent demonstrated a clinically significant and safe effect on reducing IOP. At baseline, patients had a mean IOP of 21.5 mm Hg and were on an average of 1.7 ocular hypotensive medications. At month 24, 85% of patients achieved a target IOP of ≤18 mm Hg and half of the patients discontinued all ocular hypotensive medications. Reported complications were minimal, with no instances of hypotony, post-surgical flat chambers or choroidal effusion observed. Based on further clinical investigation, multiple iStents may provide the flexibility of titrating therapy to achieve target pressures across the range of disease-stage severity.
■ The Solx Gold from Solx, Inc. (Boston). The Solx Gold is designed to function without the formation of a bleb. The Solx Gold, currently in a phase 3 clinical trial, is described as the first biocompatible, 24-karat gold implant that uses the eye's natural pressure differential (uveal scleral outflow) to reduce IOP without a bleb. It is approximately 3 mm wide, 6 mm long and about the thickness of a human hair. The Gold Shunt is a flat plate designed for implantation through a single microincision. It contains numerous microtubular channels that bridge the anterior chamber and the suprachoroidal space, controlling aqueous outflow to reduce IOP.
■ The Aquashunt from Opko Health (Miami). Developed by highly respected glaucoma specialist Bruce Shields, M.D., of Yale University, the Aquashunt is now in a small clinical trial in the Dominican Republic. Opko has stated that the Aquashunt will require a relatively simple implantation procedure that will make it appealing to general ophthalmologists who do not normally perform a large number of glaucoma surgeries. But that remains to be proven and glaucoma shunts still primarily remain in the hands of experienced glaucoma specialists.
Probably the most commonly implanted shunt today is the Ahmed. This valved device allows for implantation without the need to ligate the tube. The non-valved shunts (Molteno, Baerveldt, Krupin) all require some type of flow restriction or obstruction at the time of surgery so as to prevent excessive early hypotony.
With the non-valved shunts, surgeons attempt to limit postop hypotony by using some method of tube obstruction. This tube obstruction generally lasts 4 to 6 weeks. During this time, a fibrovascular cocoon forms around the plate of the device. This cocoon serves as an extrascleral reservoir to collect aqueous humor. From this cocoon, the aqueous then is absorbed into the venous system and is removed from the extraocular environment.
If the tube is not ligated or obstructed in some fashion, the early aqueous egress tends to be excessive and prevents the eye from re-establishing a normotensive state.
Tube obstruction can involve one or two methods. One method of tube ligation involves using an absorbable suture (7-0 Vicryl) to completely occlude the lumen of the tube near the base of the plate. This is analogous to using a piece of dental floss to sew shut the lumen of a drinking straw. This prevents early hypotony by limiting the amount of aqueous egress for the first 4 to 6 weeks postoperatively. Another way to limit outflow is to use a larger gauge suture, such as 4-0 prolene, fed through the lumen of the tube and serving as an obturator or "rip-cord." This obturator can be manipulated postoperatively to enhance aqueous egress. Finally, some surgeons use both a prolene obturator as well as a Vicryl tie-off in order to avoid hypotony.
Proper Shunt Implantation
The majority of shunts are placed on the superior scleral hemifields, which provide a more pre dictable and safer area for implantation. Specifically, the supero-temporal quadrant provides the best balance between safety and efficacy. The superonasal quadrant is the second choice for implantation but can lead to a greater likelihood of strabismus. I believe that the inferior hemifields should be avoided due to an unacceptably high complication rate, including but not limited to excessive hypotony and globe-threatening endophthalmitis.
The Molteno3 glaucoma shunt
Implantation of an aqueous shunt involves suturing the plate of the device onto the sclera. Prior to this step, the device is usually "primed" or tested to ensure proper function. Once the plate is properly affixed to the sclera, the tube is then assessed for length and trimmed with a bevel that is oriented anteriorly for anterior chamber placement or posteriorly for posterior chamber placement.
A 22-gauge needle is then used to enter the eye to create an entrance tract for the tube. The tube is then placed through this tract and anchored to the sclera with 9-0 nylon sutures. The tube is then fenestrated anterior to the tie-off. A piece of sclera or pericardium is then sutured over the tube from the limbus to the base of the plate. A patch graft is laid over the tube to prevent erosion of this tubing through the sclera. Once the patch is properly placed, Tenon's layer and the conjunctiva are then closed in a watertight fashion. When the wounds are closed, the intraocular pressure can be adjusted by creating a temporal paracentesis and either adding or removing balanced saline or viscoelastic materials.
In cases that require immediate IOP control, the Ahmed device offers an opportunity to place the shunt without the use of an obturator or ligature. The flow-restrictor valve system prevents the intraocular pressure from dropping too low too early. The surface area of the single-plate Ahmed is roughly 180 mm. The device can be easily placed in any single quadrant. If the patient is deemed to need greater IOP control than that achieved with a single-plate Ahmed, the surgeon can opt to install a double plate Ahmed shunt. The double-plate device involves operating in two quadrants. This requires more extensive surgery but in general does not increase surgical risks.
If a surgeon wishes to achieve lower long-term intraocular pressure without having to perform two-quadrant surgery, the Baerveldt 350 mm non-valved device is an excellent choice. Because the device is placed under two muscles, the larger plate size can be deployed without having to violate two quadrants. Most surgeons ligate the tube with a 6-0 or 7-0 Vicryl suture. The ligation suture will dissolve in about 4 to 6 weeks, allowing full flow to the plate at that time.
During the first 4 to 6 weeks following implantation, a fibro-vascular cocoon forms around the plate. This cocoon will become the filtering "bleb." The ligation provides protection against excessive outflow while the cocoon is developing. Control of intraocular pressure during the time during which the ligature is dissolving is via outflow from a fenestration in the tube. This fenestration will ultimately be sealed as a vascular sheath envelops the tube, preventing any egress from the fenestration by week 6.
The Molteno3 and the Krupin Device function similarly to the non-valved Baerveldt device.
Use of the Express Mini-Shunt
Lately there has been increased interest in the use of the Express Mini-Shunt from Optonol Ltd., located in Neve Ilan, Israel.
This device is a relative hybrid of an aqueous shunt and a trabeculectomy. The Express is a metallic stent inserted into the anterior chamber under a standard scleral flap.
When the device was initially introduced, it was inserted without the creation of a scleral flap. Many surgeons noted thinning of the conjunctiva and Tenon's overlying the shunt. This thinning posed a major infection risk. Thus, surgeons evolved to first creating a scleral flap, and then inserting the shunt under the protection of a scleral flap. The longer-term efficacy of this type of implantation has been confirmed by recent clinical studies.
The scleral flap used in this procedure can be identical to the flap created for trabeculectomy. Unlike a trabeculectomy, which involves the removal of a block of trabecular tissue followed by an iridectomy, the Express is placed into the eye once the flap has been fashioned. The Express does not require removing any trabecular or scleral tissue. Additionally, it does not require, in most instances, the creation of a surgical iridectomy. This simplifies the surgery by removing these two steps. Following insertion of the Express, the scleral flap is closed with either absorbable or permanent suture. The conjunctiva and Tenon's are then closed above the flap.
This procedure may provide an excellent platform for less-experienced surgeons to transition from trabeculectomy to the addition of aqueous shunts to their surgical arsenal.
|Efficiencies with Glaucoma Surgery: Incorporating Fibrin Glue into Your Technique|
Our practice has been using a technique for 3 years that saves time and money and has become an important part of how we do many glaucoma surgeries and other procedures that require conjunctival closure. We have replaced sutures with fibrin glue in many incisional procedures. This technique allows us to perform surgeries in a quicker manner without sacrificing the safety of our patients' eyes.
Currently, there are very few incidences where we choose to use sutures over fibrin glue, yet the adoption rate has still been relatively low in the profession as a whole, perhaps because M.D.s are creatures of habit. If we are comfortable with sutures, we may not see any reason to change. While I too was comfortable with sutures and didn't have a high rate of complications from them, the benefits of fibrin glue were worth the change. Because we are able to finish a surgery faster, we see more patients (and gain more revenue) in the same time. We also now use our time more wisely — i.e., on the steps of the case that are most crucial rather than in the closure phase, which may not be the most important part.
The technique takes a little bit of time to learn. Those who do, however, find it to be quite easy and straightforward. For some preparations (Tisseel, Baxter) the two components of the product, glue and accelerant, need to be heated in a container that comes with the glue. This is typically done by a nurse in the OR. You then combine the components using two syringes or one syringe with two compartments, the substance becomes sticky and more viscous, and you apply it to the area needing closure. After 15 seconds, the glue will hold the tissue in place. Once you get the hang of it, this technique works very well. Also, using it does not preclude the option to use sutures in addition; probably the most watertight seal can be achieved with the combination of both glue and sutures. Using the glue is easily reversible and forgiving of error; if the position is not quite correct the tissues can be easily separated and then re-glued. Any excess can be trimmed away. Furthermore, because the fibrin glue helps to facilitate hemostasis, there tends to be less postoperative migration of blood and less sub-conjunctival scarring.
We are able to use fibrin glue in any kind of case from tube shunts to canaloplasty, primarily to close the conjunctiva, but it can also seal early bleb leaks after trabeculectomy for one to two days before the body naturally heals it. In trabeculectomy, we also have had success using the glue to create a space under the conjunctiva that's free of blood and scarring.
When implanting tube shunts, we found that fibrin glue is effective at holding a pericardial patch graft in place on top of the shunt, to prevent erosion from friction caused by the eyelid. While it's still sometimes necessary to place a suture as a safety measure, in my practice fibrin glue almost completely eliminates the need for sutures for this use. It's also effective for temporarily holding the plate of a shunt such as the Baerveldt in place at least for a while, which can be helpful in judging positioning.
In canaloplasty, it can secure the scleral flap above the lake as well. Because this is a low-flow situation, the glue will limit flow for the first few days. We haven't seen clinically significant bleb formation in the short term, and in the long term there is no evidence of leakage through the flap. Using the glue also obviates patient reports of postop irritation from the suture. Significantly, using fibrin glue in canaloplasty also shortens the procedure time by almost 30 minutes.
While I opt for fibrin glue most of the time, there are some cases where I don't think it's the best choice. If there is a lot of scarring, I prefer to augment the use of the glue with a limbal suture to prevent retraction. And if a patient has had a few previous surgeries, I don't use this technique alone because the glue isn't as strong as it might need to be in these more complex cases. However, if I have a younger eye with no previous surgeries, I find no reason not to use fibrin glue.
Robert J. Noecker is associate professor, vice chair of ophthalmology and director of glaucoma and anterior segment surgery service at the University of Pittsburgh School of Medicine.
An Increased Role for Shunts
Over nearly two decades, I have honed my skills as a glaucoma surgeon. I believe that in the hands of a well-trained surgeon, the success rate of an aqueous shunt provides an excellent option for achieving short, intermediate and long-term IOP control.
In my own experience, the use of aqueous shunts allows me to provide better long-term IOP control with less postoperative manipulation. I believe if early hypotony can be controlled, the long term IOP control is better than that which I achieve with trabeculectomy. Additionally, I can achieve good long-term IOP levels without the use of anti-metabolites and their associated infection risks. OM
|Andrew Rabinowitz, M.D., is the glaucoma specialist at Barnet, Dulaney Perkins Eye Center, Phoenix, Ariz. His surgical techniques have been used in educational films demonstrating the appropriate usage of glaucoma shunts. He has no financial interest in any of the products mentioned in this article. Dr. Rabinowitz can be reached via e-mail at firstname.lastname@example.org.|