Theodore Leng, MD
Contributing Editor
Glaucoma drainage devices are medical implants that serve to aid filtration by physically shunting aqueous humor to the subtenons and, more recently, the supraciliary space. The devices in current use consist of a tube that is placed into the anterior chamber or through the pars plana that drains aqueous to an external reservoir located at the equator of the globe underneath the rectus muscles (most commonly the superior and lateral). Benefits of shunting devices over trabeculectomy include a lack of a conjunctival bleb and its associated complications, less postoperative hypotony, and easier postoperative management. Aqueous shunts can be classified as valved and non-valved.
Indications
Traditionally, glaucoma drainage devices have been reserved for difficult or complicated glaucoma cases in which conventional trabeculectomy surgery has failed or is likely to fail. Other indications for shunt placement are active uveitis, neovascular glaucoma, inadequate conjunctiva, and aphakia.
In light of the recent Tube Versus Trabeculectomy (TVT) study (n = 212 patients), the indications for implantation of shunting devices has changed in favor of implanting more aqueous shunts. In fact, Medicare data shows that the number of shunting devices implanted has steadily increased while the number of trabeculectomies performed has steadily declined.
The TVT study sought to compare the clinical outcomes for trabeculectomy versus aqueous shunt implantation in eyes that had previous trabeculectomy or cataract surgery. The investigators found that there was no difference in IOP at 6 and 12 months between groups (mean 12 mmHg). There was a significantly higher failure rate (14%) in the shunt group (Baerveldt 350) and significantly more medications were used with shunts (1.3) when compared to the trabeculectomy (0.5) group, but trabeculectomies required more reoperations. Intraoperative complications (< 10%) did not differ between the two arms. The total number of postoperative complications was higher in trabeculectomies than in shunting devices (p = 0.001) with the main causes being higher rates of hyphemia, wound leak, and dysesthesia. Given these results, one would be justified in performing a shunting surgery in eyes that have had either previous cataract surgery or previous trabeculectomy. A primary TVT study is ongoing, which is examining the outcomes of trabeculectomy versus aqueous shunting in eyes that have not had any prior surgery.
Valved Devices
The rationale of a valve device is to provide a minimal amount of flow resistance, preventing hypotony by creating a “cut-off switch” to stop flow when a certain IOP is reached. The drainage devices with valves are the Kuprin (silicone plate material, 194 mm2 surface area), the single plate (184 mm2), and the double plate (364 mm2) Ahmed (polypropylene or silicone). Despite the design features of these valved devices, hypotony can still occur postoperatively and the presence of the valve lends itself to clogging and malfunction.
Non-Valved Devices
The non-valved glaucoma drainage implants consist of the single plate (135 mm2) and double plate (270 mm2) Molteno (polypropylene), as well as the Baerveldt (silicone, either 250 mm2 or 350 mm2). A common practice is to restrict the flow of these devices in the postoperative period with an external vicryl suture or an internal a rip-cord suture. Additionally, many surgeons choose to fenestrate the tube with a suture needle to allow some flow out before the vicryl suture dissolves or the rip-cord suture is removed.
A newer non-valved device is the Optonol Ex-PRESS shunt, a stainless steel shunt that was originally designed to be placed underneath the conjunctiva rather than a scleral flap (potentially making glaucoma surgery an in-office procedure), but subconjuctival implantation has resulted in variable outcomes. Today the Ex-PRESS shunt is usually placed under a trabeculectomy-style scleral flap with similar postoperative management (timed laser suture lysis, etc.). Data from a 15 month study showed no significant difference in IOP when compared to a traditional trabeculectomy with the Ex-PRESS shunt resulting in less risk of hypotony, fewer choroidal effusions, and less postoperative inflammation.
Another novel shunting device is the Solx DeepLight Gold Micro-Shunt, a 24-karat gold flat plate measuring 5.2 mm by 2 mm by 60 µm. Initially, about half of its micro-channels are open and the rest can be opened after implantation, using a titanium-sapphire laser. The Micro-Shunt creates a bridge between the supraciliary space and the anterior chamber with insertion accomplished by a single 2.5 mm incision and the use of a preloaded insertion device. The device’s potential advantage is avoidance of problems associated with conjunctival entry and filtering bleb formation, such as hypotony, a shallow anterior chamber, hyphema, late closure, and pressure spikes. However, supraciliary surgery may be complicated by bleeding, choroidal effusion, and hypotony. Studies are currently ongoing to determine the outcomes for this device.
Complications
The complications of traditional aqueous shunting devices are valve malfunction, ocular motility disturbances (from plate insertion under the rectus muscles), plate erosion, early overfiltration with resulting hypotony, flat anterior chambers and iris-corneal touch (prevented by valves or suture closure of the tube), corneal decompensation over area of tube (due to corneal endothelial cell loss), tube obstruction, tube migration (which may be prevented by securing the tube to the scleral surface with a nylon or prolene suture), and tube erosion (which may be prevented by placing a scleral, corneal, or pericardial patch over the tube).
In summary, aqueous shunting devices provide for an increasingly popular alternative to traditional trabeculectomy for the surgical management of glaucoma.
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