Subconjunctival Microinvasive Glaucoma Surgeries

An Update on the Xen Gel Stent and the PreserFlo Microshunt

Anna T. Do; Hardik Parikh; Joseph F. Panarelli

Disclosures

Curr Opin Ophthalmol. 2020;31(2):132-138. 

In This Article

Xen Gel Stent: Technique, Outcomes, and Complications

The Xen gel stent is a biocompatible, hydrophilic tube made from porcine gelatin cross-linked with glutaraldehyde. In contrast to silicone material used in tube shunts, it does not incite significant inflammation or a foreign body tissue reaction.[11] This reduces the risk of fibrous proliferation, progressive inhibition of aqueous flow, and bleb failure. The stent utilizes the principles of laminar fluid dynamics (Hagen–Poiseuille equation) to avoid postoperative hypotony.[12] After several models with various luminal diameters were tested, the Xen45 has become the most commonly used model worldwide and the only one that is currently available in the United States. It is 6 mm in length, has an external diameter of 150 μm, internal diameter of 45 μm, a flow rate of 1.2 μl/min (at 5 mmHg pressure gradient), and provides 6–8 mmHg flow resistance that reduces the risk of hypotony.[13] As the gel stent hydrates after contact with water within 1–2 min, the flexible stent conforms to tissue reducing the risk of erosion.[13,14] Unlike many other MIGS, the Xen is Food and Drug Administration (FDA) approved for use with cataract surgery and as a stand-alone procedure.

The Xen gel stent comes preloaded in an injector system with a 27-g sharp beveled needle tip and is designed for ab interno insertion. Many surgeons, however, opt to insert the stent via an ab-externo technique either with an open conjunctival peritomy or closed conjunctival approach.

For the ab-externo approach, the surgeon may decide to use a closed conjunctival technique. In this case, the needle of the inserter pierces the conjunctiva approximately 6–8 mm from the limbus. The inserter is then tunneled in the subconjunctival space to the scleral entry point and into the anterior chamber to deploy. The injector is then withdrawn, leaving the Xen in proper position. A cannula can be used to ensure the distal tip is freely mobile under the conjunctiva and not entangled in the Tenon's layer. This method can be technically challenging, particularly if the patient's conjunctiva is friable. The advantage of this technique is that it does not require any corneal incision or conjunctival suturing. An alternative to this approach is to use an open conjunctival technique. This is slightly more invasive in that it requires a 3–4-clock hour peritomy in the superior quadrant where the Xen is to be implanted. The Tenon's layer can be dissected off the sclera so that the stent can be inserted into the anterior chamber as one would insert a tube shunt. The advantages of this technique are that the positioning of the Xen can be easily and precisely manipulated after deploying. This allows the surgeon to leave the outer stent lumen tucked deeper in the sub-Tenon's space which has been shown to exhibit greater IOP reduction, lower risk of erosion, and lower risk of bleb failure.[15] The disadvantage of this method is that it requires conjunctival sutures which may lead to additional episcleral fibrosis and scarring.

When placing via an ab-interno 'closed conjunctiva' approach, the inserter is passed through an inferotemporal 1.2-mm clear corneal incision, across the anterior chamber to pierce the trabecular meshwork, and through the sclera approximately 2 mm from the limbus. A sliding mechanism on the inserter is advanced to deploy the stent into the subconjunctival space and retract the needle into the hub. Advantages of this technique include speed, simplicity, and a self-sealing corneal entry that obviates the need for sutures. This is an elegant approach when it works well. Challenges arise when the device is not deployed cleanly into the subconjunctival space. The distal end of the gel stent can curl if immediately obstructed by tissue within the Tenon's layer. Needling on the table to free the distal end of the stent is currently recommended when this happens. The technique appears simple, but the delivery of the Xen needs to be fairly precise (1 mm in anterior chamber, 3 mm intrascleral, 2 mm beneath the conjunctiva). If too much of the stent is left in the anterior chamber after it is deployed, it often requires reimplantation as the Xen becomes difficult to grasp and manipulate once it becomes hydrated. When more than 2 mm of the stent is left beneath the conjunctiva, there is a greater possibility that it could become kinked and project upward. An inadvertent perforation of the conjunctiva is also possible with this technique (piercing with the sharp inserter when trying to deliver the stent just beneath the conjunctiva). When this happens, there is a higher risk for postoperative bleb leak and subsequent failure. To reduce the incidence of these complications, an 'open conjunctiva' approach can be taken here as well. This is a more invasive procedure but may offer a more predictable way to achieve repeated success. Studies are currently underway to compare outcomes via these approaches (Table 1).

Xen gel stent surgery has been associated with a fast learning curve amongst both residents and experienced surgeons, with a recent study concluding that the IOP lowering efficacy and complication rates are comparable to those from high-volume surgical centers after six surgeries.[15] Although the main indications for the Xen gel stent in both Europe and the United States are for eyes with open-angle glaucomas that are unresponsive to maximally tolerated medical therapy, there have been recent published reports for the Xen stent successfully being used to treat intraocular hypertension in iridocorneal endothelial syndrome,[16] after dexamethasone-implant intravitreal injections,[17] after failed trabeculectomies,[18] and implanting a Xen stent inside a glaucoma drainage implant tube to narrow the lumen and treat chronic hypotony.[19]

The largest and only comparative study was done by Schlenker et al.[20] to compare the safety and efficacy of the Xen45 (185 eyes) to trabeculectomy (169 eyes). This retrospective cohort study evaluated the Xen and trabeculectomy as standalone primary procedures with MMC (0.05–0.2 mg/ml injected or soaked sponges).[20] Patients underwent surgery between January 1, 2011, and July 31, 2015, at four academic ophthalmology centers in Canada and Europe and were followed for a median of 15 months. The primary outcome was hazard ratio of failure which was defined as IOP outside of the range 6–17 mmHg on two consecutive readings despite needling interventions after the one-month postoperative period. Qualified success was defined as IOP within range allowing for medications, and secondary outcomes measured IOP ranges 6–14 and 6–21 mmHg, interventions, complications, and reoperations. The adjusted hazard ratio of failure of the gel stent relative to trabeculectomy was 1.2 [95% confidence interval (CI) 0.7–2.0] for complete success and 1.3 (95% CI, 0.6–2.8) for qualified success. None of the three defined thresholds for failure found a significant difference in hazard ratio between Xen and trabeculectomy. In the 6–17-mmHg threshold analysis, the hazard ratio was 1.00 (95% CI, 0.68–1.45) for complete success and 1.13 (95% CI, 0.61–2.09) for qualified success. White ethnicity was associated with lower risk of failure, and diabetes was associated with higher risk of failure. There were more needlings required in the Xen group compared with the trabeculectomy group (43.2 versus 30.8%). Anterior chamber reformation was more frequent in Xen eyes compared with trabeculectomy (11.9 versus 7.8%). Bleb repair or conjunctival suturing was more frequent in the trabeculectomy group (5.4 versus 1.2%). There was a trend toward higher rates of reoperation in the Xen group (10.3 compared with 5.9%). Overall, this study found that, as a standalone procedure, trabeculectomy performed better in phakic, white patients with lower preopoerative IOP, whereas the Xen performed better in nonwhite, pseudophakic patients with higher preoperative IOP.[20]

Another large study by Karimi et al.[21] retrospectively evaluated the IOP lowering effect of Xen alone (187 eyes) versus Xen combined with cataract (72 eyes) over 18 months. In the Xen alone group, the postoperative IOP was 14.3 (SD ± 4.7) mmHg and 13.8 (SD ± 2.6) mmHg in the Xen with cataract group at month 12 (P = 0.5367). Cataract surgery did not demonstrate any additional IOP lowering effect; however, this study is limited by the fact that only 91 of the 259 cases (34%) reached 12-month follow-up.[21]

Gillmann et al.[22] performed a prospective, interventional study to compare the safety and efficacy of the Xen as a standalone procedure and combined with cataract surgery in pseudoexfoliative glaucoma (PXFG) and primary open-angle glaucoma (POAG). Patients underwent surgery between January 2015 and June 2016 and were followed for two years; 57 eyes with POAG and 53 eyes with PXFG were included. Prior to Xen implantation, 0.1 ml of 0.02% MMC was injected, and an ab-interno approach was used. Seventy-two percent of POAG and 76% of PXFG eyes underwent combined Xen/Phacoemulsification, whereas the rest underwent standalone Xen gel stent placement. There was a 26.8% decrease in mean medicated IOP in the POAG group compared with 28.3% in the PXFG group at two years (P = 0.75). Needling was performed in 42.8% of POAG eyes and 43.2% of PXFG eyes at an average of 162.8 and 134.9 days, respectively. Additional glaucoma surgery was required in 14.3% of POAG and 15.9% of PXFG eyes. Complete success (unmedicated IOP < 16 mmHg) was achieved in 51.4% POAG eyes and 57.1% PXFG eyes. This study concluded that the Xen is as safe and effective in PXFG and POAG patients when used as a standalone procedure or combined with cataract surgery.

In a prospective observational study, Lenzhofer et al.[23] evaluated the surgical outcomes of ab-interno implantation of Xen in 66 eyes of 54 patients based on the position of the distal lumen. Anterior segment optical coherence tomography (AS-OCT) imaging was done to visualize the distal tip of the Xen in cross-section and tangentially (length-wise) through the lumen of the Xen to determine the position of the distal lumen of the stent: deeper (sub-Tenon or intra-Tenon) versus subconjunctival and correlate surgical success with distal lumen position. Complete surgical success was defined as unmedicated IOP lowering at least 20% compared with baseline, and qualified success was similarly defined with medication IOP. The intra and sub-Tenon group showed higher qualified surgical success (90%; 95% CI, 73–98) compared with the intraconjunctival group (61%; 95% CI, 44–77) at 1 year. Although the intra and sub-Tenon group had a lower percentage of patients who required needling (68 compared 80% in the intraconjunctival group), this was not statistically significant (P = 0.70). The intraconjunctival group underwent a higher number of needlings (1.9 ± 1.7 [95% CI, 1.3–2.5], compared with 1.2 ± 1.2 [95% CI, 0.7–1.6] in the intra or sub-Tenon's group (P = 0.03). This study demonstrates superior surgical outcomes when the stent is placed deeper in the Tenon layer and suggests that hydrodissection of these layers to form a fluid pocket in the sub-Tenon space prior to injection of the stent may allow easier ab-interno deployment of the stent in the deeper layers. This deep implantation may also lead to lower rates of bleb leaks and bleb-related endophthalmitis.

Ideal IOP on postoperative day 1 is between 3 and 10 mmHg. Midha et al.[24] found that in patients undergoing standalone Xen implantation who had a postoperative day 1 IOP of more than 20 mmHg, the probability of needling was as high as 80%. This number decreased to about 35% if day 1 IOP was less than 10 mmHg and to 20% if IOP was less than 5 mmHg. Higher postoperative day 1 IOP not only predisposed to a higher probability of needling but also to a higher number of needling interventions. Given that early Xen blebs tend to directed posteriorly and are low lying, it can be challenging to identify when flow is reduced. IOP trends needs to be monitored carefully and AS-OCT may be helping in objectively assessing the amount of flow/extent of bleb.

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