By Lorraine (Lori) Provencher, MD
Every surgeon performs traditional glaucoma surgery a little differently. Techniques vary widely, supporting the notion that there is no
right way or wrong way to perform a trabeculectomy or a tube shunt surgery. There is, however, a method that works best for each surgeon, and nuanced, variable techniques are developed over time based on our training, our experiences, our network, and new data.
Minimally invasive glaucoma surgeries (MIGS) are not as variable, probably because they involve less manipulation of ocular tissue, are generally less complex, and have not been in use as long. An exception is the XEN Gel Stent (Allergan), a procedure that borders on MIGS by accessing the subconjunctival space like more traditional incisional glaucoma surgery. XEN was originally approved as an ab interno procedure, but techniques evolved to include ab externo (“ab-ex”) placement as well.
As a result of this evolution that takes place in the operating room, we can find the technique that works best in terms of surgeon skill set and patient outcomes.
We can see this evolution in action with ab externo XEN implantation with both open conjunctival and trans-conjunctival approaches. I learned the ab interno approach in fellowship, but I learned ab externo with open conjunctiva on my own by talking to colleagues, watching videos, and observing another surgeon. If you have experience with trabs and tubes, it’s very intuitive to do ab externo open conjunctiva XEN, which draws on analogous concepts. Ab externo placement is now my preferred technique. Here’s what I’ve learned as I transitioned from ab interno to ab externo:
- Ab interno XEN implantation is effective and important to know. So far, published data suggest similar success rates for both techniques, but needling rates are lower with ab externo. My ab-ex results have been similar, and I rarely needle. Less needling is meaningful, as needling comes with the risk of pain, infection and bleeding, and potential need for additional anti-metabolite, and it often requires patients to reset their steroid taper.
- Ab externo allows for more flexibility in which quadrant or clock hour the stent is placed, particularly straight superior or even superotemporal. Ab externo superior XEN placement may be impossible in patients with a very deep-set eye or a prominent brow. In this situation, ab interno works well.
- Opening conjunctiva also provides more control over the final placement of the external XEN tip. Where ab interno may result in variable distal tip placement and an uncertain early post-operative course (eg, Is the stent occluded? Is it wrapped in Tenon’s? Is there resistance in the bleb?), open conjunctiva is the great equalizer. At the end of every ab-ex case, I know exactly where it rests, how it’s flowing, and what the surrounding tissue looks like.
- In my experience with the ab interno technique, episcleral bleeding upon scleral penetration can lead to poor early flow, early encapsulation around the stent, and failure, and more needling. An open conjunctival technique allows the surgeon to easily cauterize any bleeding and/or excise any Tenon’s that may obstruct the distal end of the stent.
- I have changed my technique to inject mitomycin at the end of surgery, after my conjunctival closure, rather than the beginning. During surgery, I evaluate the health of the conjunctiva and thickness of Tenon’s capsule, and I tailor the mitomycin C dose accordingly based on how these tissues behave (usually 40 – 60 mcg).
- Patients who undergo ab interno placement with no incision of the conjunctiva and no sutures tend to recover faster visually with less ocular surface pain. With ab externo open conjunctiva XEN, patients seem to report more dry eye and thus have a slower visual recovery with more irritation. Frequent steroids, less controlled mitomycin C exposure, conjunctival manipulation, and cautery likely play a role. To compensate, I treat dry eye aggressively preoperatively and maintain this throughout the postoperative course.
One of the most rewarding aspects of surgery is the ability to innovate and constantly improve upon existing techniques, and I’ve found that glaucoma specialists are happy to have multiple options when it comes to XEN implantation. Ophthalmologists that do not perform trabs and tubes may be less comfortable working with the conjunctiva and less likely to adopt an ab externo technique. Nevertheless, they can still feel confident that they will achieve great results with ab interno implantation. No matter the method, XEN surgeons should strive to determine the technique that works best in their hands.
Lorraine (Lori) Provencher, MD, is a glaucoma specialist at the Cincinnati Eye Institute.
Disclosure: Dr Provencher reports she is a speaker and consultant for Allergan; and a speaker for MST.
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