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Non-penetrating trabecular surgery: experts provide practical surgical advice
Practitioners provide insight on variations of this surgical technique, including dissection tools and the proper use of adjuncts and implants.
Ocular Surgery News convened a round table of experts to discuss non-penetrating trabecular surgery for glaucoma at the International Glaucoma Symposium in Prague. Surgeons discussed everything from the incision they use to their opinions about the use of lasers as an adjunct to the procedure.
The moderator of the round table session was Philippe Sourdille, MD, of Nantes, France. The participants were Giorgio Marchini, MD, Manfred Tetz, MD, Roberto Carassa, MD, and Tatiana V. Kozlova, MD.
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Terminology
Philippe Sourdille, MD: Let’s first discuss terminology, because there is some confusion, especially for non-specialists, about this surgery.
Giorgio Marchini, MD: I would like to stress the difference between viscocanalostomy and deep sclerectomy. With viscocanalostomy, we try to achieve outflow in the normal way. With deep sclerectomy, we try to obtain a more refined route for filtration.
Manfred Tetz, MD: There are many variations in terminology. I would like to divide the surgical procedures into two groups: those targeting deep sclerectomy and those targeting the tissue in Schlemm’s canal.
Roberto Carassa, MD: I agree. There are some similarities between the procedures. Deep sclerectomy took some things from viscocanalostomy, and viscocanalostomy took some things from deep sclerectomy. Most people who are doing deep sclerectomy now are enhancing the surgical technique by injecting something into Schlemm’s canal or at least are paying attention to Schlemm’s canal. That was not really done at the beginning.
On the other hand, people are doing viscocanalostomy using some of the small tricks used in deep sclerectomy, such as the stripping of the juxta-canalicular trabeculum.
However, there is still a big difference between the two groups, because deep sclerectomy is a surgical procedure that is aimed at increasing the outflow in the subconjunctival space, whereas viscocanalostomy is a technique that is aimed at restoring the natural outflow pathway.
Dr. Tetz: The common denominator of all these procedures is that they all contain elements of non-penetrating external micro-trabeculectomy. The variations are how much tissue we take away and how deep we dissect.
Tatiana V. Kozlova, MD: The main principle of outflow in the modern generation of non-penetrating operations (both deep slerectomy and viscocanalostomy) is the application of a trabecula-Descemetic plane for outflow, which was described and substantiated by my father Valentin Kozlov in 1987.
Dr. Sourdille: This is why we use the term “non-penetrating” trabecular surgery.
Dissection
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(FIGURE COURTESY OF ROBERTO CARASSA, MD.) |
Dr. Sourdille: What are the significant surgical factors? Do we have to dissect anterior to Schwalbe’s line, and how much?
Dr. Kozlova: Yes, I think that we have to go 2 mm anterior from the scleral spur and probably 1.1 mm in front of Schwalbe’s line, because only this zone has morphological features that may promote filtration. This zone includes the trabecula, the anterior portion of the trabecula and the limbal edge of Descemet’s membrane after removing the overlying corneal-scleral tissue. The morphological structure of the transitional zone is very similar to the structure of trabecula and promotes stable filtration.
Dr. Marchini: My experience is only with deep sclerectomy, and I think that we have to go 2 mm anterior in clear cornea.
Dr. Carassa: From the few basic studies we have, it seems that Descemet’s membrane itself is not playing much of a role in the percolation of the fluid.
Advancing the internal flap is done to make clear that we are really opening the anterior trabecular meshwork and that we are getting enough space in order to have an easy cut of the inner flap. Advancing too much will not change the amount of fluid coming out.
Dr. Tetz: Is there a limit for how far in front of Schwalbe’s line we can go? If we go very far, we’re making a larger window area. This may act like a trampoline. This means that we are also at a higher risk that the trampoline will bulge from the inside to the outside.
Dr. Sourdille: Somewhere around 1 mm is the maximum. By going further, we prolong the operation, and we are at a much higher risk of complications.
Do we have to remove the juxtacanalicular meshwork to create optimal outflow conditions?
Dr. Kozlova: In all cases, we remove juxtacanalicular tissue. This tissue is very often the main obstruction for occluded outflow.
Dr. Sourdille: Do you use forceps and a scraper?
Dr. Kozlova: Yes, we use a fine iris or capsule forceps.
Dr. Marchini: Removing the juxtacanalicular tissue is the essential goal and the characterizing aspect of this surgery.
Dr. Tetz: I don’t do it routinely. I only do it when I clinically have the impression that the percolation is not sufficient. If I have sufficient, easy percolation, I don’t remove it.
Dr. Carassa: I agree.
Dr. Sourdille: Have you found a cleavage plane? Most of the time, when cutting with a blunt knife, you can just push the trabecular membrane apart, and it will peel off with no traction, just pushing. What do you think is the pathophysiological significance of that cleavage plane?
Dr. Tetz: In my experience, the cases in which it doesn’t work so easily are cases that have had previous laser trabeculoplasties. You actually see very fine, white, little strands. I think laser trabeculoplasty is one of the worst things to be done to glaucomatous eyes, especially when we want to perform such a sophisticated microtrabeculectomy.
Dr. Sourdille: Another clinical condition under which you will not find the cleavage plane is congenital glaucoma.
Flap suturing
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Dr. Sourdille: Do you think that there is any significant difference in the number of sutures of the superficial flap, according to the necessity of external filtration, to have a higher success rate?
Dr. Kozlova: From 1984 to 1986, we used two-sutured external scleral flaps and a hydrogel device for maintaining the intrascleral lake. Our experience shows that subconjunctival bleb formation is also important for this type of surgery. Then we started to do only one loose suture on the superficial scleral flap, in cases when we used devices such as the collagen device.
Dr. Sourdille: What is your current thinking? How many sutures do you use?
Dr. Kozlova: One loose suture on the superficial scleral flap is quite enough.
Dr. Sourdille: Do you think that this is a significant factor to increase the external filtration?
Dr. Kozlova: Yes, it is a significant factor for adequate bleb formation.
Dr. Marchini: I think so too, because at the beginning of our experience we used more sutures. Then we used ultrasound biomicroscopy (UBM) to study the mechanism of outflow, and I think that we have to use only a few sutures. Now, we routinely use only two sutures in order to hold the superficial flap and to contain the insert.
Dr. Sourdille: Do you think that there is any significant difference in the number of sutures?
Dr. Tetz: Yes, even for viscocanalostomy. I started out using eight to 10 sutures to make it very watertight. I’m currently using approximately five 10-0 nylon sutures. I do not dislike the small filtration, but although it often disappears, only 30% of patients have a filtration bleb clinically discernible at 1 year postoperatively. So I reduced the number of sutures, but I didn’t change to one or two, because I will lose the decompression chamber, at least from the functional point of view. One suture will not keep the whole scleral lid in position, except if you use a triangular flap, and I’m not doing that.
Dr. Sourdille: Are you saying that the number of sutures will make a difference in the percentage of external filtration? If you use fewer sutures, there is more external filtration?
Dr. Tetz: Yes.
Dr. Carassa: I differentiate the number of sutures depending on whether I’m doing a viscocanalostomy or deep sclerectomy.
I mainly do viscocanalostomy in unoperated eyes, and I do deep sclerectomy whenever I feel that Schlemm’s canal has some sort of impairment due to previous surgery. When I do viscocanalostomy and I work with Schlemm’s canal, I usually tightly suture the external flap. The logic for this is to maintain the intrascleral chamber and to force the aqueous into Schlemm’s canal.
Whenever I do deep sclerectomy, I don’t feel that Schlemm’s canal is playing much of a role. I just give two stitches. But usually I work with antimetabolites when I do deep sclerectomy.
Dr. Sourdille: Did you notice that, even with the addition of antimetabolites, there is a significant difference in terms of external filtration between your two-suture deep sclerectomy and your five-suture viscocanalostomy? Is there more filtration with fewer sutures?
Dr. Carassa: Yes. With fewer sutures, there is more external filtration.
Dr. Sourdille: Are you doing limbus- or fornix-based incisions?
Dr. Kozlova: I’d rather use a small limbal fornix-based incision to diminish the injury of vessels.
Dr. Marchini: When I don’t use antimetabolites, I perform a fornix-based incision. When I use antimetabolites, I perform a limbal-based incision.
Dr. Tetz: I use a fornix-based limbal incision, and I try not to cut on either from both sides. I basically just cut one side, so it’s like one lid to the conjunctiva that you de-flap. I used to start my incision too close on the right-hand side. Now, I try to keep at least a 3-mm to 4-mm distance from the scleral incision site, because from that initial incision I get some scarring growing toward the scleral lid.
Dr. Carassa: I always do a fornix-based cut in order to get better exposure.
Dr. Sourdille: My personal technique was more often fornix-based, but when I use antimetabolites, I usually do a limbal-based fornix incision, at a distance from the limbus, especially in the cases where I think I will need early massaging of the eye.
Size of the flap
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(FIGURE COURTESY OF TATIANA V. KOZLOVA, MD.) |
Dr. Sourdille: How large do you make the lake or chamber in the scleral space?
Dr. Kozlova: The width of the superficial scleral flap is approximately 4 mm. We think that the best volume is about 6 to 9 mL.
Dr. Marchini: The superficial scleral flap is 5 mm by 5 mm or 5 mm by 6 mm, and the deep scleral flap is 4 mm wide. It may be rectangular or triangular, depending on the inserter that I use.
Dr. Kozlova: In UBM investigations, we have observed that in eyes with controlled IOP intrascleral space is usually well detected but may vary in its dimensions. In cases of stable IOP elevation (after unsuccessful laser goniopuncture) a progressive reduction of the intrascleral space and increase of its acoustic density was observed. Further statistics showed negative correlation between the rate of IOP and the size of the intrascleral lake.
Dr. Marchini: We ran a very similar study, and we didn’t find any correlation. There was a change in the lake over time that was not constant, but it occurred mainly within the first 6 to 8 months with viscocanalostomy.
Dr. Sourdille: What is the longest follow-up with UBM studies as far as the anatomy of the lake is concerned?
Dr. Marchini: Three years for some cases. After 1 or 2 years, we have less reduction in the height of the lake, and more with the length of the lake. In the first 6 to 8 months, we have more sloping that occurs and then a sort of flattening of the curve.
Dr. Sourdille: So, after 2 years, you expect the reduction to continue?
Dr. Marchini: I think so.
Dr. Carassa: I don’t. There is certainly a bimodal curve — a steep curve at the very beginning and then it tends to level — but we are talking about rather small numbers.
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(FIGURE COURTESY OF TATIANA V. KOZLOVA, MD.) |
Dr. Sourdille: This is a very important point. Shallowing of the intrascleral space is related to IOP, at least after 2 years.
Dr. Tetz: There seems to be an earlier phase and a later phase. In the early phase, you said you didn’t find the correlation between the size of the chamber and the IOP. But could you find a correlation between the pressure reduction from pre- to postop? Does the pressure reduction correlate to the lake size?
Dr. Carassa: No. We looked at that, and, more or less, there is no correlation.
Dr. Sourdille: In non-penetrating surgery, without the use of antimetabolites, we never get a pressure below 10 mm Hg unless we have unexpected external filtrations. This may be in some way related to the episcleral venous pressure or to the conjunctival venous pressure.
Do we all agree that the decompression chamber should be kept as long as possible?
Dr. Tetz: Yes.
Knife or laser?
Dr. Sourdille: Do you dissect with a knife, excimer or Er:YAG?
Dr. Kozlova: I have experience with the diamond knife. This method is very good, but the learning curve is very long. In this connection to avoid negative consequences we tried to facilitate the main stage of the operation. First, after Schlemm’s canal opening, we dissect Descemet’s membrane from the residual corneal scleral tissue with a blunt spatula. Then we put our wide cutting board spatula in this pocket. On the surface of this same instrument, it’s possible to cut the little scleral layers with a metal blade. We also remove this tissue on the surface of the cutting board spatula with an excimer laser.
Dr. Sourdille: What are the advantages of excimer for you?
Dr. Kozlova: It is good for clinics that are equipped with an excimer laser, and it may shorten the learning curve. As for direct ablation, I think that it has many disadvantages, because the ablation is irregular in this case due to different rate of energy absorption by sclera and corneal tissue.
The Er:YAG laser also opens the possibility for future modifications of this technique.
Dr. Sourdille: Do you start the dissection of the deeper plane at the posterior part or at the anterior part?
Dr. Kozlova: I start at the posterior.
Dr. Marchini: I too go from posterior to anterior. I don’t have experience with excimer or Er:YAG lasers, but I think that we can use different knives for dissecting the different planes. It is essential to have a very sharp side incision of the deep scleral flap, mainly anteriorly. So I think a diamond knife may be of some help for this.
Dr. Tetz: I don’t think that the excimer is the right laser here, because it has a very wide destruction zone for this type of microsharp, wall incision. There is more potential with the erbium, but this is all from experiments we have done in the sclera. Right now, I think I can handle the technique very nicely with a knife, especially in terms of speed. As you all know, when you do the ablation with the laser, you add time and cost to the procedure.
Dr. Carassa: I start posteriorly doing a parabolic cut with a sharp diamond knife. Then I do the dissection with a blunt steel blade.
We started using the excimer laser, but we were not so enthusiastic about it.
Dr. Sourdille: The main problem to me is the horrible quality of the surgical microscopes that are now working with the excimer lasers. You get little magnification, nothing that can be called illumination, and you have to totally rely on the machine without any control.
Adjunct implants
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(FIGURE COURTESY OF TATIANA V. KOZLOVA, MD.) |
Dr. Sourdille: What adjunct implants do you use?
Dr. Kozlova: In our early experiences with non-penetrating surgery, non-resorbable mini-implants made of hydrogel were applied. But most acrylic compounds have a pronounced tendency to capsule formation, and adequate filtering bleb formation was not achieved. Later, in 1987, Valentin Kozlov proposed the first resorbable device made from biological polymer-collagen. From that time a strong preference is given to resorbable devices as an absolutely safe and effective means for intrascleral space maintaining. It has been applied in about 60% of non-penetrating surgeries at the Fyodorov Eye Microsurgery Institute in recent years. In cases of high fibrotic processes, biopolymer devices are not always sufficient and may be dissolved more quickly than necessary. That’s why interest in non-resorbable devices is still vital.
Dr. Marchini: The implant is important to maintain the intrascleral lake and the intrascleral space. I was not convinced of this, but we now have data from a study that prospectively compared the use of an implant with no implant. I don’t know if it is useful to improve the success of the operation, but I think that is important in order to maintain the space.
We use a reticular hyaluronic acid implant, and the duration of this implant is longer than we had thought. It is present after 1 year in some cases.
We don’t know if this insert modifies the scar tissue. This happened in the rabbit, but I don’t know if this happens in humans.
Mainly at the beginning of follow-up, it may be important in order to avoid hypotony.
Dr. Tetz: I used Healon GV (sodium hyaluronate, Pharmacia) in 150 eyes and Healon5 (sodium hyaluronate 2.3%, Pharmacia) in 300 eyes. However, any time that I had a failure, either early or late, which I attributed to a collapse of the chamber of the scleral pocket, I re-intervened and used the SKGel hyaluronic acid implant (Corneal). For these revisions, I have had good experience with the SKGel.
I’m now wondering, especially in soft or thin sclera, whether I should immediately go with a longer-lasting cross-linked hyaluronic acid or whether to continue with what I’m doing and use it only for revisions. Also, I continued to use the Healon5 injection into Schlemm’s canal.
Dr. Sourdille: Did you find clinical differences in terms of IOP between Healon GV and Healon5?
Dr. Tetz: No, not that would be attributed to the substances.
Dr. Carassa: I’m mainly doing viscocanalostomy, and I’m mainly using Healon GV, both for the canal and for the lake. This is related to the fact that there are no convincing data telling us which implant is better from a clinical standpoint. Considering the high cost of other implants, I’m staying with Healon now and waiting for new data.
Dr. Sourdille: What is the rationale of filling the canal with viscoelastic? Do you think that you create microperforations? Sometimes we see tiny bubbles coming into the anterior chamber after filling the canal. What is the purpose of injecting a viscoelastic substance into Schlemm’s canal?
Dr. Carassa: It’s a way to prevent scarring.
Dr. Tetz: The question has not been completely answered yet. Neither has the question of how much filtration you can get, on average, on any individual case. But it makes a lot of sense to me because the canal is 150 µm wide and 60 µm deep. It’s like an empty hole with a soft shell. It will collapse. During the operation, you widen it and give it a circular diameter, a circular cross-section.
If fluid enters the opening, it keeps a kind of continuous flow going. What it does in the individual case and how many microperforations or ruptures we really get is still to be evaluated.
Dr. Carassa: We still don’t know what role Schlemm’s canal is playing in viscocanalostomy. I am part of an ongoing experimental study on monkeys, done at the University of Wisconsin, which is aimed at defining the mechanism of action of viscocanalostomy. From the preliminary data, we found structural changes both in the inner and outer walls of Schlemm’s canal that seem related to aqueous outflow. If this will be confirmed, then a significant role of Schlemm’s canal and of viscoelastic injection during viscocanalostomy will be demonstrated.
Antimetabolite use
Dr. Sourdille: When do we consider treatment with antimetabolites or other drugs?
Dr. Carassa: I consider antimetabolites whenever I’m facing a high-risk case. Risk factors include age, race, secondary glaucomas and previous failed surgeries. I’m talking about deep sclerectomy. Usually I don’t use antimetabolites in viscocanalostomy. I use mitomycin C with deep sclerectomy.
Dr. Sourdille: Do you have experience with other antimetabolites?
Dr. Carassa: I have used 5 fluonouracil (5 FU), but I’m not getting as much as what I’m getting with mitomycin C.
Dr. Sourdille: Where do you apply mitomycin C?
Dr. Carassa: One randomized, prospective study considered whether it’s better to apply it over, under, or over and under.
If you apply it under the flap, there is a bigger risk of getting the mitomycin C inside the chamber, and the mechanism of action is toxic action on the ciliary body in aqueous production. If you apply it outside the flap, then it’s less risky, and you will reduce the fibroblast proliferation. I usually apply it over the flap.
From what we know, in terms of lowering IOP with a higher risk, it is best to apply both over and under. The most risky is applying under, and the most often used is applying over the flap.
Dr. Tetz: Whenever I use it, it is on the compromised eye and the previously operated eye — in other words, an eye that has failed at least two previous surgeries.
I use mitomycin intraoperatively, because to use 5-FU correctly according to its acting mechanism, you should do it for 2 or 3 weeks on a daily basis. That is a long time for patients.
Dr. Marchini: When I decide to use an antimetabolite in high-risk cases, during the operation, I use mitomycin C, 0.2% for 2 minutes, both over and under.
When I don’t use antimetabolites, but in the follow-up I see that scar formation may close the external filtration, I use 5-FU — between one and four injections every 48 hours.
Dr. Kozlova: In my practice antimetabolites are used mostly in repeated surgery or in special cases with high risk of fibrosis. We are used to applying mytomicin C under the flap.
Dr. Sourdille: We know that a single subconjunctival application can go to the ciliary body through a full-thickness sclera. We also know that we cannot wash out mitomycin C from the deep scleral layers.
Additionally, mitomycin C, unless you inject it inside the anterior chamber, is not toxic to the endothelial cells, because they do not multiply.
To me, there are two reasons to use mitomycin C. One is the risk of failure, and second, and probably most important, is the level of IOP we want to achieve. In all cases of normal-tension glaucoma I routinely apply mitomycin C to have an average IOP of 9 mm Hg or 10 mm Hg, with minimum IOP of 5 mm Hg. We all agree that using mitomycin C in non-penetrating trabecular surgery is reasonably safe and has several indications.