July 2017




MIGS roundup
Comparing glaucoma procedures

by Ellen Stodola EyeWorld Senior Staff Writer


Slit lamp picture demonstrating the XEN Gel Stent. The black arrow shows the implant in the subconjunctival space. There is a classic low diffuse bleb.
Source: Davinder Grover, MD

Implantation of the XEN Gel Stent; side photo demonstrates the external hand positioning during the implantation
Source (all): Davinder Grover, MD

Experts discuss use of trabeculectomy, tube shunts, and XEN Gel Stent

Although trabeculectomy, tube shunts, and the XEN Gel Stent (Allergan, Dublin, Ireland) work by creating a pathway for aqueous humor to drain into the subconjunctival space, these are all very different options for different types of patients. Patient factors, such as disease severity, and more can play into which procedure a glaucoma surgeon chooses. Kateki Vinod, MD, New York Eye and Ear Infirmary of Mount Sinai, New York, and Davinder Grover, MD, Glaucoma Associates of Texas, Dallas, discussed aqueous drainage into the subconjunctival space and shared their thoughts on when to use each of these options and the advantages and disadvantages they offer.

Aqueous drainage into the subconjunctival space

The subconjunctival space is the site of aqueous drainage in traditional filtering glaucoma surgery, including trabeculectomy and tube shunt surgery, Dr. Vinod said. “Procedures targeting the subconjunctival space offer the advantage of achieving lower intraocular pressures than trabecular meshwork and Schlemm’s canal-based surgeries can, as the latter are limited by downstream resistance to outflow and episcleral venous pressure,” she added.
However, bleb-related problems, such as leaks and infections, may occur with operations that involve aqueous drainage to the subconjunctival space. “Such complications have become less common as a result of refinements in our trabeculectomy technique, including more diffuse application of antifibrotic- soaked sponges and, more recently, subconjunctival injection of antifibrotics,” Dr. Vinod said. “The risk of these and other complications associated with trabs and tubes has led to the development of MIGS procedures that seek to optimize safety.”
The eye is like a faucet and drain, Dr. Grover said, in that it makes water and drains water. The majority of open-angle glaucoma is thought to be caused by drainage problems through the trabecular meshwork. Dr. Grover said his approach is to tell patients that he will try to open their own drain, but if that’s not possible, a new drain will need to be created, which can be done with a tube shunt, trabeculectomy, or the XEN Gel Stent. The problem is that the aqueous is not supposed to be in the subconjunctival space, so when it is, it creates an aggressive scarring response. To address this, Dr. Grover said that mitomycin-C (MMC) or a drainage implant can be used. All innovations with trabs and tubes are designed to trick the eye to not act as aggressively to aqueous in the subconjunctival space, he said. Dr. Grover thinks that the XEN could help create a more predictable outflow pathway. “It’s the best of both worlds between [trabeculectomy] and tube,” he said.

Ideal patients for each option

Dr. Grover said the ideal procedure often depends on the patient, and he noted that certain cases may be more clear-cut than others. In particular, he considers patients’ activity and where they live. For example, if the patient works in a dirty environment, a trabeculectomy would not be ideal because this puts the patient at risk for infection. Tubes, he said, have the least risk of infection. Another example Dr. Grover gave was patients who scuba dive. In these patients, a tube shunt would be better. Dr. Grover added that in patients who are on blood thinners and can’t be taken off this medication, it’s risky to do a tube or trabeculectomy, and a XEN may be a better choice because of the lower risk of a sudden drop in pressure.
The XEN creates a bleb, so this could put the patient at risk for bleb-related complications. But Dr. Grover noted that these problems have not been seen in Europe where the XEN has been approved for a longer period of time.
Patients who are more prone to scar tissue formation tend to do better with glaucoma tubes, he added, and would not do well with a trabeculectomy or the XEN.
If the patient has mild/moderate glaucoma to early advanced glaucoma, Dr. Grover noted that his general approach is to try to open up their own drain, but if that doesn’t work, then he tells patients he has to create a new drain. Right now, the safest, most predictable, and least invasive way of creating a new drain is the XEN, he said.
According to Dr. Vinod, patients who are ideal candidates for subconjunctival filtering procedures tend to be those with moderate to advanced glaucoma whose intraocular pressures are uncontrolled with medical therapy and/or have demonstrated progression or are likely to progress. “Patients must not have extensive conjunctival scarring that would limit the ‘real estate’ available for a subconjunctival procedure whose success relies on formation of a bleb,” she said, referring to trabeculectomy and the XEN Gel Stent.
Dr. Vinod prefers trabeculectomy as her “go-to” surgery for patients requiring low target IOPs, such as those with normal tension glaucoma, as well as for patients without prior intraocular surgery and phakic patients with chronic angle closure whose anterior chambers are too shallow to accommodate a tube. Trabeculectomy remains the only available glaucoma procedure that allows titration of aqueous flow after surgery to produce a desired degree of IOP lowering, she added. She said that she will perform tube shunt surgery in patients with prior failed trabeculectomy or in patients who are likely to fail primary trabeculectomy surgery (such as those with neovascular glaucoma or uveitic glaucoma), as well as in patients with extensive conjunctival scarring from previous surgery or trauma. “Of course, these are basic guidelines for patient selection in my practice, and one must always consider specific patient factors in order to choose the optimal surgery for a given individual,” she said.
Dr. Vinod said that the XEN Gel Stent is one of the newer subconjunctival procedures, FDA approved last year, and offers the distinct advantage over trabs and tubes of being performed via an ab interno approach. “It is a gelatinous tube with a fixed length (6 mm) and inner lumen diameter (45 microns) designed to restrict aqueous outflow and limit hypotony, which is among the more frequent complications we encounter with trabs and tubes,” she said. “The XEN may be particularly helpful in patients with cataract and glaucoma who require a combined procedure, with potentially lower risks than those associated with combining cataract with either a trab or a tube.”
Dr. Vinod finds the XEN to be a reasonable primary procedure in many of the patients who are candidates for traditional incisional surgery, and she added that the absence of a conjunctival flap and superonasal placement would not preclude future incisional surgery if needed. “Like trabeculectomy, the XEN Gel Stent may require more in-office postoperative interventions, such as needling and antifibrotic injections, than a tube shunt and would therefore not be my procedure of choice in patients who cannot come in for frequent follow-up,” Dr. Vinod said.
When asked about using an adjunctive antifibrotic agent with each of these procedures, Dr. Vinod said that antifibrotic agents are routinely required with trabeculectomy and the XEN Gel Stent, but she does not use antifibrotics with tube shunts.
Dr. Grover also uses antifibrotics with the XEN and trabeculectomy, but not with tubes. “I think you have to use MMC with a XEN,” he said, “or it will fail.”

Potential complications

Dr. Vinod said that one complication with trabeculectomy and tubes is hypotony. Complications of hypotony can include shallow anterior chamber and choroidal effusion, more often as a result of overfiltration, but occasionally due to wound leaks. “Hypotony is usually transient and amenable to medical management but can sometimes be visually significant,” she said. “Infectious complications resulting from wound leaks or tube erosion are less common but can be devastating.”
Early studies of the XEN Gel Stent suggest that it is a fairly safe procedure with transient hypotony occurring in less than 10% of patients, Dr. Vinod said. “However, the initial XEN studies did not use antifibrotics, and long-term data regarding late complications relating to mitomycin-C use, bleb scarring and failure, and effects on endothelial cell counts are unavailable for this relatively novel device,” she said.
A tube is a foreign body in the eye, and it does put the eye at risk of erosion and double vision, Dr. Grover said. The tough part with a trabeculectomy is it’s a relatively invasive procedure, and there can be bleb-related problems, he said. Trabeculectomy is also not as predictable as a tube because it depends on a number of factors, including the size of the flap, how you close the conjunctiva, and how it heals.
With the XEN, there is a lower rate of erosion, and it does not put patients at risk for double vision. But it also has the predictability of a tube because it’s a controlled outflow. “The other beauty of it is it does not require a conjunctiva dissection,” Dr. Grover said. The XEN is placed ab interno, so you’re not taking the conjunctiva down, he said, and it doesn’t preclude you from doing a tube or trab later.


Potential future procedures

The success of a trabeculectomy or XEN Gel Stent relies on the availability of mobile conjunctiva/Tenon’s to facilitate bleb formation, Dr. Vinod said. In patients with a prior failed trabeculectomy, glaucoma surgeons may opt to perform a second trabeculectomy or a tube shunt, depending on the location and extent of conjunctival scarring. In the presence of a previously failed superotemporal trabeculectomy, she tends to place an inferonasal tube shunt.
The XEN Gel Stent, Dr. Vinod said, is easier to place in previously operated eyes, as it is implanted ab interno and therefore doesn’t require a conjunctival flap. However, sufficiently mobile conjunctiva is still needed for bleb formation. The superonasal quadrant is preferred as it is accessible via a temporal clear corneal incision and allows room for future incisional surgery if needed.
Typically, Dr. Grover puts the XEN in the superonasal quadrant and leaves the superior quadrant for a possible trab and superotemporal quadrant for a tube. If you do a trabeculectomy or tube and it fails, you can still do a XEN if the superonasal quadrant is untouched. The other thing that differentiates XEN, he said, is a faster visual recovery and faster recovery to activity.

Phakic influence on procedure selection

For phakic patients requiring very low intraocular pressure for disease stability, Dr. Vinod usually favors trabeculectomy as a primary surgery. “My decision to perform a trab, tube, or XEN in patients who are already pseudophakic depends on a variety of patient factors other than target IOP, including age, prior non-cataract ocular surgeries, ocular comorbidities likely to require surgical intervention in the future, ability to follow-up in the postoperative period, etc.,” she said. Like trabeculectomy and tube shunts, the XEN can be performed in conjunction with cataract surgery in patients with visually significant cataract and glaucoma, she added, with the potential for fewer complications in the postoperative period than combining cataract surgery with a trab or tube. “Ultimately, lens status is one of several factors that influence my decision-making process when selecting the procedure that offers the best chance for disease stability and quality of life in my patients,” Dr. Vinod said.

Editors’ note: Dr. Grover has financial interests with Allergan. Dr. Vinod has no financial interests related to her comments.

Contact information

: dgrover@glaucomaassociates.com
Vinod: kvinod@nyee.edu

Comparing glaucoma procedures Comparing glaucoma procedures
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