XEN Gel Stent For Glaucoma From AqueSys

The XEN Gel Stent is a 6.0 millimeter long flexible tube made of chemically treated gelatin. It has a very small internal diameter designed to limit the rate of flow through the tube. This tube is placed in the eye with one end in the anterior chamber. This allows fluid to exit the eye and form a bleb at the other end of the tube.

[Dick B. XEN Gel Stent Implantation in Combination with Laser-assisted Cataract Surgery [Internet]. 2014 [cited 2015 May 15]. Available from: https://youtu.be/Rj1wM7iku6w]

As a simple tube, the XEN Gel Stent may not seem like much of an innovation. What makes this device unique is that it is implanted from the inside of the eye using what is called an “ab-interno” technique. Traditional “ab-externo” (from the outside) trabeculectomy requires making and incision in both the conjunctiva and sclera (white eye wall). This may result in bleeding and scarring, both of which could lead to bleb failure. Additionally, ab-externo glaucoma surgery techniques tend to take much longer to perform than ab-interno techniques (aka minimally invasive glaucoma surgeries, or “MIGS”).

In order to implant the XEN Gel Stent, an incision is made in the cornea. This incision is very small, on the order of a millimeter or so and is very similar to the incision used during cataract surgery. A specially-designed needle-tipped implantation device is inserted through this incision and passed across the anterior chamber into the angle. A small amount of pressure is then applied which guides the tip through the sclera into the space between the sclera and conjunctiva. The XEN Gel Stent is then injected into this space. One end of the implant remains in the anterior chamber which then allows fluid to pass in a controlled manner from the anterior chamber into the subconjunctival space. This results in a bleb and a lower intraocular pressure (IOP).

Whether a fistula in the wall of the eye is created from the inside (ab-interno) or from the outside (ab-externo), the resulting bleb is at significant risk of failure from scar formation. This, use of an antimetabolite such as Mitomycin-C (MMC) is thought to be necessary even when the XEN Gel Stent is implanted. For those who are wishing to avoid a thin-walled bleb and its associated life-style limitations, the XEN Gel Stent is not going to be an improvement over trabeculectomy.

How Well Does the XEN Gel Stent Appear to Work?

One Year Results (with Cataract Surgery)

Data presented at the 2015 American Society of Cataract and Refractive Surgeons (ASCRS) Annual Meeting^[1] suggests that implantation of the XEN Gel Stent at the time of cataract surgery can reduce IOP by approximately 30% (average drop from 20.8mmHg to 14.4mmHg). Additionally, glaucoma medication use was reduced by almost 60% (average decrease from 2.7 to 1.1). As this study only included patients who also needed cataract surgery, it is not possible to know how much IOP lowering was due to the XEN Gel Stent and how much was due to the cataract surgery.

Three Year Results (with and without Cataract Surgery)

Longer-term results of the XEN Gel Stent were also presented at the 2015 ASCRS Annual Meeting.^[2] This ongoing study included hundreds of participants, a small fraction of whom were three years out from implantation of the XEN Gel Stent. Approximately half of the participants had cataract surgery performed at the same time as placement of the XEN Gel Stent. Intraocular pressure (IOP) was reduced by approximately 30% at one year, 30% at two years, and 40% three years out from surgery. Additionally, glaucoma medication use was reduced by approximately 75% at all time points. Approximately 5% of participants required additional glaucoma surgery to achieve IOP control. According to the authors of the study, there were no significant differences in outcomes between those who had only glaucoma surgery with the XEN Gel Stent and those who had both cataract surgery and implantation of the XEN

[Polytech-Domilens GmbH. Aquesys XEN Animation 2014 mit Ton [Internet]. 2014 [cited 2015 Aug 15]. Available from: https://youtu.be/c_FoDZiZlu8]

Risks of the XEN Gel Stent

Implantation of the XEN Gel Stent is essentially a modified trabeculectomy that results in a bleb. As such, it shares the risks of trabeculectomy.  At least in theory, fewer patients should experience short- or long-term hypotony (IOP that is too low). Until a study is published that compares the results of the XEN Gel Stent to trabeculectomy we will not know if this theory plays out in practice. 

In Summary

The XEN Gel Stent appears to be an effective alternative to traditional ab-externo trabeculectomy. As with all trabecular bypass surgeries, however, it is dependent upon formation of a bleb with all of its associated risks and lifestyle limitations. Thus, although the XEN Gel Stent may be a “Minimally Invasive Glaucoma Surgery” (MIGS) in the sense that it is relatively fast and easy to perform, it is in most other respects a trabeculectomy.

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