Safe Simple Success

Ten years ago, when I was finishing up my glaucoma fellowship, our choices for glaucoma surgery included trabeculectomies and a variety of tube shunts. The Ex-Press shunt was one of the original attempts at making invasive trabeculectomy surgery safer. Although the device did not offer much of a safety improvement, in terms of preventing postoperative hypotony, it did make the surgery a bit easier, which was a benefit to the surgeon. However, the immediate postoperative and long-term risks of filtering surgery remained. The first commercial alternative to trabeculectomy surgery, within the subconjunctival space, was the Xen Gel Stent (internal lumen diameter of 45 µm). The original iteration used an ab interno delivery device – which remains the case, according to its FDA-approved labeling. This microinvasive glaucoma surgery stent was designed to be introduced into the subconjunctival space via the anterior chamber.

As it eliminates the need for a large conjunctival incision, Xen greatly reduces risk of bleb and conjunctival leaks as well as poor wound healing (1, 2). Compared with the Ex-Press shunt, the implant’s lumen size is designed to throttle IOP to prevent hypotony. In this way, the Xen establishes an IOP floor, adding greatly to the safety profile. The InnFocus MicroShunt, which is in FDA investigations, is another device that looks to minimize hypotony risk but still provides the pressure-lowering benefits of the subconjunctival space.

Since its FDA approval, the procedure to implant the Xen Gel Stent has gone through a variety of iterations.

Indications for use
 

The Xen gel stent was approved by the FDA in 2016 for use in primary open angle glaucoma and pseudoexfoliative glaucoma with open angles that are unresponsive to maximum tolerated medical therapy, as well as refractory open angle glaucoma that has failed previous surgical treatment. In clinical studies, patients generally achieve IOPs in the 12–18 mm Hg range. Candidates for Xen should have a healthy conjunctiva; the tissue should not be excessively scarred or have had previous surgical manipulations. Although not a requirement, it is better if patients can be supplemented with additional medication to achieve their IOP goal. I would not opt for Xen in a patient who is still progressing at a pressure of 11 mm Hg, for example. That is someone who likely needs a trabeculectomy to get them into the single digits.

I like to use the Xen in patients for whom I would have traditionally used a tube shunt because the pressure lowering effect is similar – in the mid-teens. My Xen candidates include those for whom a trabeculectomy is a little too aggressive or those with other risk factors, such as advanced age, systemic blood thinner use, thin conjunctiva, and an inability to keep the required close follow up post trabeculectomy. Another reason for choosing Xen in these patients is its superonasal positioning, which leaves a quadrant of tissue in case a future tube shunt is needed; this sequence of surgeries is typically relatively simple. In other words, using the Xen does not “burn any bridges.” The exception is in a patient who may ultimately need a trabeculectomy, which can be more difficult after Xen because the conjunctiva has already been manipulated and exposed to mitomycin-C.

Faster recovery
 

Compared with our traditional surgeries, which typically involve stitches that cause ocular discomfort while healing, the postoperative course is much easier with Xen. With traditional surgery, patients are on strict activity restrictions for several weeks – sometimes a month or more, if they’re slow to heal, or if they have an atypical or rocky postoperative course. In Colorado, where I practice, telling patients they cannot participate in their outdoor activities for a month is often a tough pill for them to swallow. Xen patients are usually on restrictions for only about a week as a result of the greatly reduced risk of hypotony.

Revisions over time
 

In the original FDA Xen trials, the surgery was a hybrid model involving an ab interno placement, but with a conjunctival peritomy to take the conjunctiva down so as to accommodate the mitomycin-soaked sponges that were used. In clinical practice however, many surgeons inject mitomycin-C instead of using sponges. Additionally, it was determined early on that the implant can get tangled in Tenon’s capsule, causing fibrosis at the distal tip, which requires needling. Subsequent implantation iterations aimed to solve this issue. Some surgeons have converted to a transconjunctival ab externo approach, keeping the needle as superficial as possible when placing the implant to avoid Tenon’s capsule. Some surgeons, like myself, choose to take the conjunctiva down for an ab externo approach. Those who like the original ab interno delivery of the Xen may perform a primary needling at the time of implantation or use special devices to try to sweep away Tenon’s as described by Lauren Dahr and colleagues (3). The goal is to create space to ensure that there is no tissue around the distal end of the implant.

Certainly, implanting the Xen is significantly less invasive and easier than using a tube shunt or having to create a watertight closure for a trabeculectomy. Still, we must properly position the Xen to obtain optimal outcomes. No matter the technique used by the surgeon, ensuring a clean distal tip at the completion of the case is important, as it should deliver results that are comparable to what has been seen in the published data.

The future of subconjunctival procedures
 

The biggest advantage of Xen is its safety profile. The focus now is how we can best optimize the subconjunctival space. It may be through a combination of a different or larger bore devices – alongside a better understanding of wound healing and wound modulation. One big question hanging over outcomes is how the eye responds to aqueous filtering into the subconjunctival space. There will always be interest in iterations that can make this type of surgery better – whether that’s a change to the implant, further finessing of the technique, or a modification that has an impact on wound modulation to make it an even more favorable choice.

^{Disclosures
Zachary Vest provides research support for Alcon, and is a consultant and speaker for Allergan.}