A gel stent allows for less invasive subconjunctival procedures and an easier recovery course than traditional forms of minimally invasive glaucoma surgery.
The marvel of minimally invasive glaucoma surgeries (MIGS)—and the reason we push them earlier in the disease course—is their safety and predictable postoperative recovery. In the past, we had fewer options, most of which were more invasive.
As a result, we had to delay surgery for years while patients suffered the discomfort, inconvenience and expense of multiple medications, until finally those medications were no longer enough and surgery was unavoidable. Now we can skip years of suffering and better manage glaucoma by making MIGS part of the treatment equation.
Nowhere is the divergence between traditional surgeries and more modern procedures more apparent than in subconjunctival surgery. Historically, we were hesitant to go into the subconjunctival pathway with trabeculectomy or tube shunt because of the often slow and variable nature of visual recovery and the complex postoperative management. Instead, we waited until glaucoma progressed and the target pressure was aggressively low, often near the 10–12 mm Hg range.
Thankfully, with the options available today, we no longer need to wait so long to perform subconjunctival surgery for patients who need it, and with this earlier intervention we have a better chance of achieving target IOPs because they are not so aggressively low.
With conventional pathway MIGS, such as stenting (iStent; Glaukos, Hydrus; Ivantis), viscodilating (iTrack; Nova Eye Medical, OMNI; Sight Sciences), stripping (Kahook Dual Blade goniotomy; New World Medical), cutting/GATT (OMNI) and others, visual recovery is generally not a major concern. Pressurising the eye intraoperatively usually prevents intraoperative and postoperative hyphaema, although if this does still occur, it usually clears up in a short period of time.
The mechanisms that make visual recovery so seamless for conventional pathway MIGS are also the reason they are not always the best option: the episcleral venous pressure is a floor we cannot cross, so it is hard to reach target pressures below the low to mid-teens. This floor, however, also means we do not get hypotony or flat chambers, which are often the cause of postoperative vision issues.
Unfortunately, some of our patients need those lower target pressures. We also do not know where the resistance to outflow is located in our glaucoma patients; in some, the distal channels are also collapsed or atrophied. In these cases, we are not able to achieve the IOP control we need through the conventional outflow system. Therefore, we need to turn to a subconjunctival procedure to bypass the conventional pathway.
The most common causes of postoperative visual acuity changes are hypotony, choroidals, maculopathy and ocular surface problems. With trabeculectomy and tube shunt, all these problems are possible, so it is difficult to gauge how long visual recovery will take with each patient.
Depending on how flat the chamber becomes and the position of the iris lens diaphragm complex, a return to baseline vision may take 1 week to 1 month or even longer. A non-valve tube might open up in a month and a half or even earlier when the suture dissolves, and that sudden change in IOP can cause a sudden shift in chamber depth and thus a change in vision. In addition, postoperative drops can prolong recovery by destabilising the ocular surface.1
Patients who endure weeks or months of recovery are generally not happy with their vision. IOP can be unstable, so they require multiple clinic visits, and they often call the clinic with questions about vision problems and irritation from eye drop medications. The recovery process may still be going on when it is time for the patient to schedule surgery on the fellow eye, which can make them reluctant to do so.
As an experienced glaucoma specialist, I still marvel at the high potential for variability of each step for a trabeculectomy. I use various techniques to manage outcomes by controlling the size, thickness and shape of the flap; titrating the flow by suture placement and tension; and trying to aggressively bring down the IOP yet prevent chambers from flattening postoperatively.
It takes time to build that comfort and skill set, and not all patients have access to surgeons who have completed 1,000-plus trabeculectomy procedures.2 For these and a variety of additional reasons, a less invasive and more predictable subconjunctival procedure is needed to allow access for more patients than traditional surgeries.
When we want to reach a lower pressure, avoid being beholden to conventional pathways where levels and locations of resistance can be uncertain and follow a more predictable visual recovery than we achieve with traditional subconjunctival procedures, subconjunctival MIGS can deliver the benefits with few limitations. The Xen Gel Stent (Allergan) uses the same bypass mechanism as trabeculectomy, but the only outflow created is a 45-μm lumen, rather than a larger, less consistent hole in the sclera with a variable flap.
With less structural deformation of the sclera, there is less chance of flat chambers postoperatively. Even if numerical hypotony occurs, the chambers do not flatten quite as much because they retain greater structural integrity. There is less concern that deformation of the eye will shift the iris lens diaphragm and cause a myopic shift or, in a pseudophakic patient, move or de-centre a toric lens.
Based on the approval trial, Xen has demonstrated a high safety profile with rare complications.3 The less invasive nature of the device means that the eye remains stable, with less inflammation in the anterior chamber and conjunctiva, often because there is no peripheral iridotomy. In addition, if implanting from an external approach, intraoperative flattening of the anterior chamber is rare.
Since flow occurs through a single point around 5 mm posterior to the limbus, rather than in multiple directions closer to the limbus as in a trabeculectomy, the bleb forms more posteriorly and often with a lower profile, resulting in a quieter bleb and shorter-duration corticosteroid use after surgery. With fewer corticosteroids and thus fewer topical preservatives, the ocular surface tends to experience less disruption and resulting postoperative vision fluctuation. Xen has also been shown to cause less endothelial cell loss than trabeculectomy, which could be an issue as patients get older and have other corneal risk factors.4
Although postoperative bleb needling is sometimes needed, my postoperative bleb needling rates have dropped significantly (now less than 10%) with changes in technique, intraoperative confirmation of a freely mobile stent and use of an intraoperative needling if resistance is found. If needling is required, it is a different type of intervention than a trabeculectomy needling. We do not see a sudden drop in IOP, so there is less change in chamber depth that could affect visual recovery.
In the end, Xen reaches similarly low pressures as traditional subconjunctival surgeries, with a predictable postoperative course, resulting in fewer visits (1 week and 1 month) and calls to my practice.5 In my experience, visual acuity is often similar to baseline on postoperative Day 1, and if not, it usually reaches baseline by the 1-week visit. Patients are happy and show no reluctance to undergo surgery on the second eye — all because their postoperative course is rather benign and visual acuity is maintained.
Owing to the high safety profile and rapid visual recovery, I have now offered this device to patients who are refractory to drops despite their level of glaucoma severity. For instance, a patient with mild or moderate open-angle glaucoma who is taking four classes of medication, has a history of a previous MIGS, has stable IOP and visual folds but is not compliant with drops would not typically have been a subconjunctival bypass candidate in the past, but this patient is now a perfect candidate for a Xen.
Welcome to the interventional glaucoma mindset.
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