Most familiar glaucoma surgical approaches open up the flow of aqueous into the subconjunctival space via an ab-externo approach, creating a passage from the sclera through the angle (such as trabeculectomy, express shunt and glaucoma drainage devices). Outcomes depend on a variety of factors: rate of aqueous production, scarring of the passage and formation of a capsule in the conjunctiva. For a glaucoma surgeon, it is a fine balance between hypotony and high IOP, as well as risks of suprachoroidal hemorrhage, a leak or scarring at a later time.^1-6

Minimally invasive glaucoma surgeries (MIGS) use an ab-interno approach that requires less tissue manipulation in hopes of decreasing the risks of glaucoma surgery. Current FDA-approved MIGS include iStent (Glaukos), CyPass (Alcon) and XEN gel stent (Allergan). Some MIGS procedures rely on Schlemm’s canal as a final point of drainage. This route is dependent on the venous system and therefore cannot achieve IOP lower than the distal outflow system (episcleral venous pressure).

Other MIGS procedures use the suprachoroidal space as a final target, and the results are less predictable and could cause hypotony.

The XEN gel stent offers an alternative — drainage to the subconjunctival space with a possibility of keeping eye pressure on the lower side — while using the more desirable ab-interno approach.⁷

XEN OVERVIEW

The XEN gel stent, cleared by the FDA in November 2016, has a unique design. This 6-mm hydrophilic tube is composed of porcine gelatin crosslinked with glutaraldehyde. The dehydrated stent is a rigid tube that becomes soft and flexible upon hydration. The available XEN offers a 45-micron inner lumen (FDA approved); the 63- and 140-micron lumen XEN is not currently FDA approved. Due to its flexibility, the implant conforms to the tissue, so there is less chance of migration and erosion when compared to conventional drainage device implants.⁷

SURGICAL ADVICE

Surgical implantation of the XEN gel stent requires a corneal incision and injection of viscoelastic, which expands the anterior chamber (AC) for visualization and stability. Surgeons may choose to administer mitomycin-C (MMC) via a subconjunctival injection at a desired dose. The procedure is performed with or without a gonioprism since the stent can be placed anywhere in the angle. A preloaded single-use injector is inserted into the chamber, and the injector needle advances through the sclera into the subconjunctival space.

Once the bevel of the needle is seen through the conjunctiva, the stent can be released. Upon injection, the stent becomes hydrated and softens. After placement, the surgeon completes the procedure by removing the injector.

OUTCOMES

International and U.S. studies have established the safety and effectiveness of XEN gel stent in patients with primary open-angle, pigmentary and pseudoexfoliation glaucoma. The most recently released data for a multicenter, prospective, nonrandomized, open label international study followed 975 patients undergoing either primary or secondary surgery with either 140-, 63- or 45-micron size lumen for up to 36 months and studied their outcomes, which included mean IOP, IOP change, number of medications and adverse events.

The mean pre-operative IOP was 22.9 mm Hg on an average of 2.6 medications (no medication washout) and the mean postoperative IOP at 36 months was 13.2 mm Hg on 0.7 medications. Adverse events included less than a 3% chance of hypotony, shallow anterior chamber and choroidal effusion. There were no cases of a flat AC, endophthalmitis or suprachoroidal hemorrhage.⁷ Longer-term follow-up is ongoing in both the United States and Europe.⁸

Dr. Varma presented another multicenter nonrandomized prospective study at the 2016 AAO meeting in Chicago. The study included 68 patients implanted with 63-mm or 140-mm lumen diameter implants along with adjunctive MMC injections. Mean IOP was 23.3 mm Hg on 2.9 medications at baseline and fell to 13.5 mm Hg on 0.6 medications at 36 months. Postoperative adverse events included a 2.5% chance of hypotony, shallow AC and choroidal effusion. There were no cases of a flat AC, endophthalmitis or suprachoroidal hemorrhage.⁹

CONCLUSION

The XEN gel stent offers an ab-interno approach to creating a fistula to the subconjunctival space. It appears to have a similar efficacy and adverse event profile similar to trabeculectomy. OM

REFERENCES

1. Watson PG, Jakeman C, Ozturk M, et al. The Complications of trabeculectomy (a 20-year follow-up). Eye. 1990;4:425-438.
2. Nouri-Mahdavi K, Brigatti L, Weitzman M, Caprioli J. Outcomes of trabeculectomy for primary open-angle glaucoma. Ophthalmology. 1995;102:1760-1769.
3. Ticho U, Ophir A. Late complications after glaucoma filtering surgery with adjunctive 5-fluorouracil. Am J Ophthalmol. 1993;115:506-510.
4. Costa VP, Wilson RP, Moster M, et al. Hypotony maculopathy following the use of topical mitomycin C in glaucoma filtration surgery. Ophthalmic Surg. 1993;24:389-394.
5. Topouzis F, Coleman AL, Choplin N, et al. Follow-up of the original cohort with the Ahmed glaucoma valve implant. Am J Ophthalmol. 1999;128:198-204.
6. Smith SL, Starita RJ, Fellman RL, Lynn JR. Early clinical experience with the Baerveldt 350-mm glaucoma implant and associated extraocular muscle imbalance. Ophthalmology. 1993;100:914-918.
7. Reznik A, Varma R. Ab-Interno Subconjunctival Glaucoma Implant for Advanced Open-Angle Glaucoma. In: Aref A, Varma R, eds. Advanced Glaucoma Surgery. Springer; 2016:51–56.
8. Grover D. Presentation of Xen data. Presented at: AGS annual meeting; March 3-6, 2016; Fort Lauderdale, FL.
9. Varma R. Presentation of Xen data. Presented at: AAO annual meeting; October 15-18, 2016; Chicago, IL.

About the Authors

Dr. Reznik is an assistant professor of clinical ophthalmology at USC Roski Eye Institute, Glaucoma Service. Her glaucoma research focuses on the development of novel surgical techniques and new approaches to treat patients with early glaucoma.

Dr. Varma is dean, Keck School of Medicine of USC, May S. and John Hooval Dean’s chair in Medicine, Grace and Emery Beardsley chair in Ophthalmology, chair, USC Care board of directors and director of the USC Roski Eye Institute and Glaucoma Service. His primary research focuses on epidemiologic studies of eye disease in children and aging populations. He is the principal investigator of three large NIH/National Eye Institute-funded studies of eye disease in ethnic populations: Los Angeles Latino Eye study; the Chinese American Eye study and the African American Eye study. He was also the PI for the NIH/NEI-funded Multi-Ethnic Pediatric Eye Diseases study.

Financial Disclosures: Dr. Varma has financial relationships with Aerie Pharmaceutical, Allergan, Bausch + Lomb, Genentech and Isarna Therapeutics.