47 EyeWorld Asia-Pacific | December 2024 GLAUCOMA The ab externo approach’s safety and efficacy has been compared to the on-label, ab interno approach many times. In these studies the ab externo procedure was found to have at least the same efficacy and safety as the ab interno procedure—in some cases even resulting in a higher success rate.1–4 “Some of the challenges of the ab interno approach are avoided when doing ab externo,” Dr. Yohannan said. These include the skill required for working in the angle with a mirrored gonioprism and finding the right plane for the stent to be placed in the subconjunctival space; a higher rate of needling or the need for primary needling; a higher rate of hypotony in the postop period; and an anterior subconjunctival bleb that may be very thin and avascular and prone to leakage, XEN exposure, or bleb dysesthesia. Dr. Yohannan said he converted to open conjunctiva, ab externo for the XEN 2–3 years ago, placing the stent subTenon’s. “I dissect into the sub-Tenon’s space. I apply high-dose mitomycin-C posteriorly in the sub-Tenon’s space (two sponges soaked with 0.4 mg/cc of mitomycin-C placed in the sub-Tenon’s space for 2 minutes and another 0.2 ml of the same concentration injected on the sponges),” he said. “I enter the anterior chamber with a 30-gauge needle. … Then I insert the stent through that 30-gauge path. There is very little flow around the stent. It’s all coming through the lumen in most cases, which I think promotes the formation of that posterior bleb. I’ll then pull Tenon’s forward, close it, and pull the conjunctiva forward and close it. The blebs that I get look much better, and the needling rate is much lower than with the ab interno approach.” While the operative time is more for the ab externo approach, Dr. Yohannan said over the postop course, it’s better for the surgeon’s time and the patient experience than ab interno, as he believes it avoids the hypotony and blurry vision. “You don’t have this anterior bleb as well, which can be more irritating to patients,” Dr. Yohannan said. “I find most patients tolerate the ab externo approach well. The surgery is more involved, but over the course of the patient’s lifetime, I would say it’s less involved. This is why I made the switch.” Dr. Li said there are still scenarios where she thinks ab interno is warranted. These include when she is trying to conserve conjunctiva or if she’s worried about postop healing. Obstacle #2: Needling While the needling rate is lower with the ab externo approach, according to Dr. Li and Dr. Yohannan (in some cases, it’s difficult to even see the stent to perform needling with the sub-Tenon’s placement), lessons have been learned over the years regarding needling with the XEN. “Initially we thought 30% of patients after the XEN needed an in-office needling, and that was a big hurdle in terms of the success and maintenance,” Dr. Li said. “More people are gravitating toward primary needling at the time of surgery prophylactically, to push back the Tenon’s and prevent them from coming near the XEN. That has decreased the postoperative needling rate.” Obstacle #3: Where to insert the XEN Dr. Li said there is a learning curve in deciding where to insert the XEN and how to ensure it gets into the right place. “We were initially thinking it had to be subconjunctival but supra-Tenon’s, and now we’re realizing that as long as it’s not intra-Tenon’s, it’s OK. It could be subTenon’s. There are various ways you can ensure that the XEN stent is in the right space. You can use an air bubble, balanced salt solution, viscoelastic, or even mitomycin-C to create a potential space for XEN insertion when doing it ab internally,” Dr. Li said. When placing sub-Tenon’s, Dr. Yohannan said it’s important to be liberal with mitomycin- C and to ensure you don’t kink the stent when you’re closing Tenon’s. Obstacle #4: Mitomycin-C Since XEN’s commercial launch, Dr. Li said how the use of mitomycin-C fits into the procedure—and at what concentration—has evolved as well. “There is still debate on that, but I think based on the patient profile, based on their surface tolerance, their Tenon’s tissue thickness, we can do a little bit more or less. There is more personalization as to how much mitomycin-C to give, and we also learned how it can affect the ocular surface under different concentrations and how to manage that postop,” she said.
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