Ab externo technique eases implantation of Xen gel stent
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In this column, Dr. Radcliffe describes his ab externo technique for implanting the Xen gel stent, thus eliminating the need for the prism and for changing the microscope and patient’s head positions. However, continued long-term studies can help further establish the safety and efficacy of this procedure.
Thomas “TJ” John, MD
OSN Surgical Maneuvers Editor
The Xen gel stent (Allergan) has been in our hands for several years and is a familiar procedure on the short list of stand-alone MIGS options.
The device has been valuable for helping my patients achieve target pressures, and it is particularly helpful for patients who are pseudophakic or do not need cataract surgery.
In the traditional ab interno approach to implanting Xen, we make a 1.8-mm inferotemporal corneal incision as well as an additional 1-mm side-port incision, fill the anterior chamber with viscoelastic, use a gonioscopic lens to see the trabecular meshwork, and finally inject Xen via the anterior chamber through the sclera and into the subconjunctival space. The drawbacks are that a corneal incision is necessary, we can only do ab interno placement at a superonasal location, and we have incomplete control over the final Xen positioning, often requiring a primary needling during the procedure. In one to three cases, we need to do bleb needling after the procedure is finished to prevent fibrosis.
Over time, surgeons have developed an ab externo approach to implanting Xen that eliminates corneal incisions during surgery while allowing us to choose the best location and even remove the stent and place it a second time if necessary. Because we open up the conjunctiva, we can apply mitomycin C directly to the sclera, cauterize vessels, perform a tenonectomy if needed and directly reposition the stent when required. Postoperative bleb needling is less common.
Ab externo technique
These are the basic steps to ab externo implantation.
1. Make a subconjunctival peritomy, disrupting the insertion of Tenon’s 1 mm to 2 mm behind the limbus to prevent it from ensnaring the stent and obstructing posterior flow (Figure 1).
Source: Nathan M. Radcliffe, MD
2. Create a conjunctival pocket. Consider scleral cautery if beneficial. Consider tenonectomy if the Tenon’s fascia seems exceptionally thick. Place two mitomycin-soaked sponges in the pocket and remove them after 1 to 2 minutes. Sponges placed directly on the sclera are well suited to stop fibrosis near the stent and incision site (Figure 2).
3. Use balanced salt solution to perform hydrodissection of the conjunctival pocket while rinsing the MMC. This encourages large bleb formation.
4. Using the Xen insertion system (the same one used for ab interno placement), inject the stent about 2 mm to 3 mm behind the limbus, entering the anterior chamber (Figure 3). Be sure to place the stent close to the iris and far enough from the cornea to avert any concerns about corneal decompensation. Once you place the stent, you can adjust it easily with forceps.
5. Close the incision with two interrupted 10-0 nylon wing sutures, making sure to get watertight closure. The flow is directed posteriorly, and a bleb leak is possible but unlikely.
6. Inject an antibiotic and a steroid.
The ab externo approach eliminates the need for a gonioscope and corneal incision. We could make a paracentesis incision to rehydrate the eye or reform the anterior chamber, but I have found that is not necessary because the anterior chamber is formed, and the Xen provides adequate internal resistance.
Potential for a transconjunctival technique
A second ab externo approach to Xen is taking shape: transconjunctival injection. It takes just a few moments, and the injector is the only tool. With this technique, MMC is injected, and the stent is placed through the conjunctiva (entering the conjunctiva 8 mm posterior to the limbus) and then enters the sclera 2 mm to 3 mm posterior to the limbus, as with the previous ab externo technique. If surgeons develop data showing this is a viable technique, then the speed and simplicity could make it a good option for emergencies or for patients less likely to scar, particularly if it can be performed in the office.
- References:
- Fea AM, et al. Clin Ophthalmol. 2020;doi:10.2147/OPTH.S178348.
- Tan NE, et al. Clin Ophthalmol. 2021;doi:10.2147/OPTH.S292007.
- For more information:
- Nathan M. Radcliffe, MD, can be reached at New York Eye Surgery Center, 1101 Pelham Parkway North, Bronx, NY 10469; email: drradcliffe@gmail.com.
- Edited by Thomas “TJ” John, MD, a clinical associate professor at Loyola University Chicago and in private practice in Oak Brook, Tinley Park and Oak Lawn, Illinois. He can be reached at email: tjcornea@gmail.com.
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