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Trabecular bypass procedures gain ground in glaucoma surgery
In the sixth report from the OSN Section Editor Summit, Glaucoma Section Editor Thomas W. Samuelson, MD, discusses surgical techniques designed to improve aqueous outflow.
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A note from the editors:
Ocular Surgery News convened its annual Section Editor Summit in February. In this installment of reports, OSN Glaucoma Section Editor Thomas W. Samuelson, MD, gives an update on trabecular bypass procedures.
by Thomas W. Samuelson, MD
OSN Glaucoma Section Editor
Augmentation of aqueous outflow is the favored strategy to lower IOP in glaucoma surgery. Traditionally, outflow is directed through the sclera into the subconjunctival space with subsequent bleb formation. However, other outflow pathways or “reservoirs” are under construction in clincal trials as surgeons continue their quest to eliminate the need for a filtration bleb and the hazards that go along with blebs, such as late leaks and infection. Several surgical procedures are under investigation in glaucoma surgery to evaluate new routes of aqueous outflow to better regulate IOP.
Trabecular bypass is a term used to describe procedures that use an implantable device to communicate between the anterior chamber and the distal outflow system. This essentially bypasses the trabecular meshwork by directing fluid into Schlemm’s canal and then into the distal collector channels and the aqueous drainage vessels. One of the most significant advantages of such surgery is the elimination of a filtration bleb. The reservoir for outflow in such procedures is the conventional outflow system itself. That is, all of the aqueous remains subscleral, which is preferrred to transscleral flow.
The first question one should consider when accessing trabecular bypass is whether or not this is a scientifically valid strategy. In the normal eye, aqueous is produced at a rate of about 1% per minute. That is, every 100 minutes, the entire volume of aqueous is turned over. In such a dynamic system, it is essential that the outflow match the inflow, or pathologic increases of IOP may ensue. For aqueous to evacuate the eye, it must overcome outflow resistance. In the normal eye, roughly half of this resistance is derived from the trabecular meshwork and about half is from the distal system, ie, Schlemm’s canal, the collector channels and aqueous vessels.
Doug Johnson, MD, and others have confirmed that about half of the outflow resistance in the normal eye is in the trabecular meshwork, and that the bulk of pathologically increased resistance in glaucomatous eyes is also in the meshwork and in the inner wall of Schlemm’s canal. Thus, it would seem logical that bypassing the trabecular meshwork, either by removing it or stenting it, would be a scientifically valid approach to the surgical treatment of glaucoma. Of course, such a strategy presumes that the canal and distal system are functional. Not all investigators agree with this premise.
Surgical technique
 Thomas W. Samuelson |
One of the early approaches to bypass the meshwork and the inner wall was viscocanalostomy and nonpenetrating deep sclerectomy. In such procedures, a superficial scleral flap is created as in trabeculectomy and a second deeper scleral flap is created down to the level of Schlemm’s canal. The dissection is carried further anteriorly onto clear cornea to create the so-called trabeculo-Descemet window. This is a meticulous dissection that creates a permeable membrane, but without entering the eye. This creates communication between the anterior chamber, the deep scleral lake and Schlemm’s canal.
Nonpenetrating surgery relies on the ability of aqueous to permeate the thin membrane that is created. Yet procedures such as viscocanalostomy and nonpenetrating deep sclerectomy have not been widely adopted, in part because they are technically difficult to perform and in part because they have not been shown to be more effective than conventional glaucoma surgery. But it is widely believed that such procedures are likely to be safer than procedures that rely on transscleral flow and bleb formation.
I believe that the theory behind these surgeries is valid, but would prefer a more reliable method than the trabeculo-Descemet window to ensure that flow is maintained long term.
The iStent from Glaukos applies the same concept, in that it attempts to create communication between the anterior chamber and Schlemm’s canal but with a simple and elegant approach that uses an ab interno approach. The iStent is inserted through a clear corneal incision of 1 mm to 2 mm and placed in Schlemm’s canal.
There are advantages to implanting this through a clear corneal incision, namely that you do not have to do any kind of conjunctival or scleral dissection, so you reserve the tissue if you need to do a more traditional surgery later.
This procedure is more simplified than the ab external approach, and it enables you to accomplish the same objectives.
In Europe, data from patient series have shown a reduction in the number of medications patients used, as well as a modest reduction in IOP. Each of these reductions was statistically significant.
In the United States, my group, Minnesota Eye Consultants, is involved in the Food and Drug Administration clinical trials prospectively comparing cataract extraction alone vs. cataract extraction randomized to the iStent, but the data have not yet been released.
Multiple implants needed
One potential limitation with this strategy is that we are not gaining access to the entire circumference of Schlemm’s canal by communicating with the anterior chamber in a focal region. Most investigators agree that there is limited circumferential flow within the canal.
If Schlemm’s canal was laid out on a table, it would be 36 mm long with 30 or so collector channels that drain aqueous fluid. Yet you do not actually gain access to the entire canal with a single bypass. At most, you might get a quadrant’s worth. Therefore you might need multiple stents strategically placed around the canal.
You can see an immediate reflux of blood upon implanting the stent because Schlemm’s canal also communicates with the episcleral vasculature. At this point in the procedure, you do not have much pressure in the eye, and blood will reflux in the anterior chamber. This can affect your visualization, so you have to work quickly if you want to implant multiple stents in the same procedure.
Future considerations
Ultimately, the reservoir selected may depend on the patient. You will achieve the lowest pressures by choosing the reservoir with low inherent resistance, such as the suprachoroidal space. This is the strategy of the Solx Gold Shunt. Theoretically, given the capacity for the suprachoroidal space to absorb fluid, diverting aqueous to this reservoir may achieve lower IOP.
However, the challenge will be to create a resister that can effectively regulate flow. An analogy would be the subconjunctival space. The subconjunctival space has a low inherent resistance until the conjunctiva contracts and ultimately scars. Therefore, in trabeculectomy, the scleral flap provides the resistance to outflow early in the postoperative course until the conjunctiva contracts and adds its own resistance. The device itself must provide the resistance when the reservoir has low inherent resistance, as is the case with the suprachoroidal space.
Trabecular bypass procedures that communicate with Schlemm’s canal likely will not achieve pressures as low as those that target the suprachoroidal space because of the inherent resistance of the distal system. So you might still look to other reservoirs. When all of this information is evaluated, I think we will have two or three procedures to pick from. For example, although they are technically not within the trabecular bypass category discussed here because they do not utilize a bypass device, the iScience canaloplasty and the Trabectome should be mentioned due to the good work that is being described with these new procedures. These procedures also augment the flow of aqueous into the distal outflow system and have shown promise in clinical trials.
For more information:
- Thomas W. Samuelson, MD, can be reached at Minnesota Eye Consultants, 701 E. 24th St., Suite 106, Minneapolis, MN 55404; 612-813-3628; fax: 612-813-3656; e-mail: twsamuelson@mneye.com. Dr. Samuelson has no direct financial interest in the technologies mentioned in this article, however, he is an investigator for Glaukos, iScience and Solx.