Go With the Flow: Nonpenetrating Procedures Evolve

The surge in new filtering surgery techniques gives glaucoma specialists more options than ever before.

By E. Randy Craven, MD

Resistance to aqueous outflow occurs mostly due to anatomical changes to the trabecular meshwork. Morton Grant showed this over 50 years ago. Since that era, we have seen surgical procedures come and go that burn, cut, remove or change the trabecular tissue. The trabeculectomy procedure was developed with the idea that getting aqueous through the blocked tissue could be done by re moving a piece of the area of the blockage.

The “trab” has been king of the glaucoma filtration procedures for several decades. Trabeculectomy has not been perfect, however. We have learned that aqueous leaking through the sclera creates imperfect results at times; sometimes the pressure is too low and problems occur.

Even one of the fathers of trabeculectomy, J.E. Cairns, voiced a desire to control transscleral flow from the trabeculectomy to reduce complications. With transscleral flow, you get the potential for hypotony, bleeding, flat chambers, choroidal effusion and blebs. His modification of the original scleral incision moved to a clear-corneal incision with the hopes of avoiding a bleb and re-establishing physiologic outflow (Cairns JE. Clear Cornea Trabeculectomy. Trans Ophthalmol Soc UK 1975; 104: 142-145).

Somewhat before and for sure after Cairns's search for achieving outflow through the trabecular meshwork (and subsequently the canal of Schlemm and the collector channels) we see various procedures appear that hope to establish the flow of aqueous through the physiologic outflow pathway. This is the “nonpenetrating” group of procedures. They are a group of surgical procedures that hopefully leave the eye with a layer of tissue intact to resist outflow somewhat. It may be that it is only a thin “window” of tissue, but it nevertheless provides some resistance to outflow.

The idea behind nonpenetrating procedures is that they get the aqueous to flow through tissue rather than a hole, and then the fluid hopefully will take more of the natural outflow route rather than go transscleral as we see with the high-flow trabeculectomy. Some nonpenetrating procedures that glaucoma surgical procedure developed include:

● Sinusotomy (Krasnov, 1968)
● Deep sclerectomy (Fyodorov, 1984)
● Nonpenetrating trabeculectomy (Zimmerman, 1984)
● Nonpenetrating deep sclerectomy with collagen (Kozlov, 1990)
● Viscocanalostomy (Stegmann, 1999)
● Canaloplasty (Lewis, 2007)

Currently, we glaucoma specialists are excited about the nonpenetrating procedures — sparked for the most part by the canaloplasty procedure (iScience, Menlo Park, Calif.). The canaloplasty was developed with the input of Robert Stegmann and John R. Kearney, and quickly took off in Germany. Now, many are performed across the globe and more surgeons are trying to understand the steps going into the procedure.

If you want to be a canaloplasty surgeon, in this article I will describe the concepts to keep in mind and the techniques that will lead to success. The first step is to learning how to do the deep sclerectomy procedure.

The principles of all the nonpenetrating procedures are fairly similar. First, you create an external scleral flap that will cover the deeper sclerectomy (Figure 1, previous page). Since you want the collector channels and aqueous veins to drain the aqueous, avoid excessive cautery. The shape of the scleral flap is probably best determined by what you are comfortable with and whether you think you can control the flow best by having tight closure of the flap.

Figure 1. Scleral flap created for deep sclerectomy.

I personally like to make the flap wide enough that I know I will have good closure over the inner flap. In Figure 1, the flap is about 4.5 mm wide. This outer flap is about half the thickness of the sclera.

Then the inner (seep sclerectomy) flap is created with the thickness approaching 95% of the sclera. You may see the ciliary body through the thin scleral bed. Don't be alarmed — it's normal to have an occasional show-through of the sclera.

The inner flap is carried forward to the cornea. This may raise concerns about perforating the Descemet's window, which happens about 15% to 25% of the time for surgeons who are just starting to learn this procedure. Simply convert to a trabeculectomy if you do perforate. Rhein Medical (St. Petersburg, Fla.) makes a canaloplasty manipulator that is great for dissecting forward into the cornea (Figure 2).

Figure 2. Dissection forward into the cornea, using the Rhein canaloplasty manipulator.

Assuming the creation of the inner flap was successful, you will end up with a Descemet's “window.” Remove the inner scleral flap, and then peel off the outer wall of Schlemm's canal. Use a fine-tipped forceps, such as the Utrata or Mermod forceps. Frequently, you will see a “trough” of the canal showing you clearly where you are.

At this point we can go one of several paths: (1) deep sclerectomy with collagen implant or wick, (2) viscocanalostomy or (3) canaloplasty. The text and images below will review the techniques for each.

1. Deep Sclerectomy with Collagen Implant or Wick

Once the window is created and you have peeled away the outer wall of Schlemm's canal, you sew in a collagen implant, such as the Aquaflow from Staar Surgical shown in Figure 3 below.

Figure 3. Aquaflow collagen implant sutured in place.

Next, Figure 4 below shows the “scleral lake” where the collagen wick has expanded. This does afford the ability to open the scleral flap or open the Descemet's window for pressure reduction.

Figure 4. This ultrasound image shows the scleral lake created by expansion of the collagen wick.

2. Viscocanalostomy

With viscocanalostomy, instead of sewing in the wick, you cannulate with a very fine (30g or smaller) cannula and then dilate the canal with a cohesive viscoelastic (Figure 5). The thought is that this allows stretching of the canal adjacent to the lake and creates a place for the aqueous to go from the leaking trabecular meshwork and Descemet window.

Figure 5. Viscocanalostomy with dilation of the canal using the Healon GV viscoelastic.

3. Canaloplasty

Once you have successfully done a few of the other nonpenetrating procedures, consider the canaloplasty. You can start straightaway with canaloplasty, but you may have better success in creating the Descemet window and understanding the anatomy without the need for the catheter.

As shown below in Figure 6, the canaloplasty involves finding the canal, then passing the catheter 360 degrees through it and tying a 10-0 or 9-0 Prolene suture to the catheter, then dilating the canal with Healon GV (from the catheter as you pull it out) and pulling the suture around the canal 360 degrees. The sutures are then tied tightly to stretch the trabecular meshwork. The amount the canal is opened does seem to correlate with success of the procedure.

Figure 6. Canaloplasty involves passing a catheter 360 degrees through Schlemm's canal, which is then dilated with viscoelastic.

Figure 7. Here, you can see the canal of Schlemm opened by the stretch of the 10-0 Prolene.

Figure 8. Here, the Prolene knots can be seen, as well as some blood between the Descemet's window and the scleral flap.

Conjunctival closure is similar to that of trabeculectomy. Many people are now using mitomycin-C on the scleral flap (and under the Tenon's and conjunctiva) to allow for the possibility of converting the procedure into what would in essence be a trabeculectomy — the thought being that the distinction between the nonpenetrating procedure (any of them) and the trabeculectomy is the tissue we leave intact; namely, Descemet's and the trabecular meshwork.

So if you opt to use the YAG laser to open either the Descemet's window or the trabecular meshwork, you will end up with a procedure that allowed you the safety of the window initially (and hopefully avoided hypotony) and then opening it gave you the opportunity for transscleral or increased flow into the canal of Schlemm.

Then, if your time isn't too far out from the day of the operation, you might be able to achieve some transscleral flow if you cut the sutures on the scleral flap, leading to a bleb or at least boggy conjunctiva with microcysts and evidence of transscleral flow (Figure 9).

Figure 9. Here, you can see the conjunctival tissue elevated (just a small amount) showing some transscleral filtration is present.

Personally, I have found that even the canaloplasties I did several years ago without antimetabolites and tight sutures to the scleral flap still had some evidence of transscleral flow.

Choosing the Procedure

Procedure choice and patient selection tend to be surgeonspecific, as all are viable options for a good number of patients. Below are my own indications for each.

Canaloplasty:

● No prior surgery.
● Done in conjunction with cataract surgery.
● Super-thin conjunctiva.
● Usually preferable that the patient has deep angles.
● Pseudoexfoliation syndrome.
● POAG patients.
● Sometimes useful in secondary OAG patients.
● Good success personally with Sturge-Weber syndrome.

Deep Sclerectomy with Wick or Viscocanalostomy:

● Prior surgery preventing catheter being able to go 360 degrees, such as prior ECCE.
● Orphan procedure with valve.
● Prior retinal surgery with scarred conjunctiva in which trabeculectomy may not be the best option.
● Want more of a posterior bleb — perhaps there has been trouble with an anterior bleb with trab in opposite eye.
● Oil in the eye.
● A history of choroidal effusion or hemorrhage in either eye.
● The patient is on anticoagulants and has trouble coming off this therapy.
● Monocular patient that “has” to see as fast as possible.

Complications and Concerns

With any new procedure, there will be a learning curve. Probably the biggest issue will be that of avoiding perforation of the Descemet's window — and dealing with it when it inevitably occurs. Note that there are different degrees of perforation and times when it may occur. Most commonly it happens when you are dissecting away the deep scleral flap from Descemet's. I have also had perforations occur while passing the catheter with the canaloplasty, when securing the suture with the wick in a deep sclerectomy, or when tying down the 10-0 Prolene with the canaloplasty.

Depending on the amount of the perforation, two things need to be considered: (1) Should I convert to a trab because the perforation is so large that the iris will probably incarcerate into the stoma? (2) Is the location so posterior that even a small perforation will allow for iris incarceration? I have elected to do iridectomies with micro-forceps and intraocular scissors in some of these cases and leave the deep sclerectomy as is. There are times that I really want what resistance I can get, even with the tight scleral flap.

Hyphemas do occur with any of these procedures. Bleb issues are less frequent than with trabs, but can occur. Over-filtration can also occur. Elevated IOP in the early postop period is usually either due to blood in the lake of the sclerectomy or because the window was not thin enough or large enough. I would hold off on using the YAG to open the window as long as you can, as the IOP does seem to drift down with time. These procedures do seem to have steroidresponsive IOP rise as we see with trabs, or even more so.

Conclusion

As you consider adding these newer procedures to your glaucoma surgical toolbox, remember they are anatomy intensive and require a steady hand and eye for detail work. With familiarity, new opportunities to use them will become apparent and you will be glad you now have the option. The future will undoubtedly offer improvements for the nonpenetrating procedures — so stay tuned and clued in to this rapidly-evolving option. OM