Glaucoma Filtering Surgeries

Trabeculectomy remains the gold-standard glaucoma filtration surgery. Although it achieves lower target IOP in eyes with advanced glaucoma, it is associated with a high risk of intraoperative and postoperative complications. Some of these complications may require additional interventions and have a major impact on patients’ quality of life. In recent years, MIGS options have emerged to improve safety and provide faster visual recovery. The Xen Gel Stent (Allergan) is a subconjunctival device that results in a bleb formation. Since its approval by the FDA in 2016, Xen has become widely available and frequently used. In this article, Leonard K. Seibold, MD, reviews the current literature and compares trabeculectomy and Xen in terms of efficacy, safety, and quality of life.

—Sarwat Salim, MD, FACS; Section Editor

Initially described more than 50 years ago, trabeculectomy has long been considered the gold standard for glaucoma surgical intervention due to its robust IOP-lowering efficacy. However, unpredictable wound healing, extensive postoperative care, and safety concerns with the filtering procedure have plagued patients and surgeons alike.

The Xen Gel Stent (Allergan) was approved by the FDA in 2016 to provide a less invasive approach to glaucoma filtering surgery that allows for a more predictable recovery with an improved safety profile compared to trabeculectomy. Although both trabeculectomy and Xen implantation are bleb-forming procedures, the Xen was designed to limit flow, thereby minimizing the potential for overfiltration, hypotony, and their associated complications. The stent was also designed to be implanted in an ab interno fashion, with no need for a conjunctival incision or suturing in most cases.

Despite Xen’s potential advantages over trabeculectomy, a closer look at comparative studies is crucial to compare the clinical outcomes of these two filtering procedures.

EFFICACY

In the largest comparative study to date, Schlenker et al analyzed 185 eyes that received a Xen stent and 169 eyes that underwent trabeculectomy.¹ The hazard ratio for failure of the Xen relative to trabeculectomy was not significant for both complete and qualified success in the international multicenter study. In another study, Marcos Parra and colleagues compared the outcomes of 121 eyes undergoing Xen implantation or trabeculectomy either as standalone procedures or in combination with cataract surgery.² They found that mean IOP was significantly lower at every timepoint in the trabeculectomy group and that the mean IOP reduction was significantly greater in the trabeculectomy group. The success rate (IOP of 6–16 mm Hg) was similar between groups at 67.7% in the Xen group and 76.8% in the trabeculectomy group.

In another study of 82 eyes that received the Xen stent and 89 eyes treated with trabeculectomy, Wagner and colleagues found that there was no difference in success rate (IOP < 19 mm Hg) at 1 year between the two groups.³ IOP reduction, however, was significantly greater in the trabeculectomy group (10.5 mm Hg) compared to the Xen group (7.2 mm Hg). Theilig and colleagues analyzed the outcomes of 100 eyes treated with the Xen and 100 eyes treated with trabeculectomy in a single-site, single-surgeon setting.⁴ They found that mean IOP and medication burden were significantly reduced in both groups to a similar degree at 1 year. Additionally, surgical success rates were similar between groups for both qualified and complete success.

Finally, in a study of 179 eyes undergoing trabeculectomy or Xen implantation, Sharpe and colleagues found that the trabeculectomy group achieved a lower IOP at 6 months than the Xen group (13.5 vs 10.8 mm Hg), even though the trabeculectomy group started with a significantly higher baseline IOP than the Xen group (20.4 vs 17.8 mm Hg).⁵ Success rate, defined as at least a 20% IOP reduction from baseline, was 56% in the Xen group and 78% in the trabeculectomy group. Although the success rate was similar between the procedures for achieving an IOP in the middle teens or less, trabeculectomy appeared to provide lower IOP levels and may therefore be the better alternative when trying to achieve low IOP targets (< 10–12 mm Hg).

SAFETY

Safety outcomes were generally comparable in the aforementioned studies, although some complications were more common after trabeculectomy. In the study by Wagner et al, overall complications were said to be similar, but no details were given.³ Theilig et al also found comparable rates of complications, but those requiring surgical intervention (ie, bleb revision and suturing) occurred more frequently after trabeculectomy.⁴

Overall complication rates were also similar in the study by Schlenker and colleagues; however, more patients in the trabeculectomy group than in the Xen group sustained vision loss.¹ In two other studies, hyphema was more common after trabeculectomy, as were anterior chamber flattening and bleb leaks.^2,5 These results combined demonstrate a moderate safety advantage for the Xen stent compared to trabeculectomy, but both procedures can lead to significant adverse events, including hypotony and serous choroidal detachments.

RECOVERY AND POSTOPERATIVE INTERVENTIONS

Notable differences between the Xen procedure and trabeculectomy are the visual recovery, postoperative care, and interventions required after surgery. A postoperative needling procedure is more likely to be necessary in eyes receiving the Xen stent compared to eyes undergoing trabeculectomy (20%–43% for Xen vs 5.4%–31% for trabeculectomy).^1,2,5 This is not altogether surprising given the stent’s small diameter and the device’s susceptibility to occlusion by fibrosis.

In contrast, eyes undergoing trabeculectomy frequently required laser suture lysis (21%–50%) as a postoperative intervention.^1,4,5 This combined with more frequent complications can necessitate additional postoperative visits. In the Schlenker study, patients in the Xen group required one fewer postoperative visit than the trabeculectomy group, and the former had less surgically induced astigmatism.¹ With regard to visual recovery, one study found that patients’ BCVA was significantly reduced at all time points after trabeculectomy but was unchanged from baseline after Xen implantation.⁴

BLEB MORPHOLOGY

Using swept-source OCT, Teus et al analyzed 15 eyes after trabeculectomy and 10 eyes after Xen implantation to compare postoperative bleb morphology.⁶ Blebs were significantly flatter after the Xen procedure compared to trabeculectomy. The conjunctiva of Xen blebs was more similar to that seen in healthy controls than was the conjunctiva of trabeculectomy blebs. A similar IOP reduction was found between the two groups despite these distinctions.

QUALITY OF LIFE

A cross-sectional study by Basilio et al assessed quality of life measures in 34 patients with open-angle glaucoma who underwent Xen implantation or trabeculectomy.⁷ Using the Glaucoma Symptom Scale questionnaire, quality of life scores were found to be similar between groups and were negatively correlated to the need for glaucoma medications and IOP level in each group.

CORNEAL ENDOTHELIUM

One study looked specifically at corneal endothelial cell density and compared the effects of Xen implantation and trabeculectomy on this parameter. The investigators found that trabeculectomy resulted in a 10.0% decrease in mean corneal endothelial cell density at 3 months postoperatively compared to a 2.1% decrease in the Xen group.⁸ Although the rate of decrease was significantly greater in the trabeculectomy group, long-term monitoring is necessary to better compare the effects of each surgery on corneal endothelial health.

FUTURE DIRECTIONS

The studies described herein provide valuable comparison between trabeculectomy and Xen implantation, but their retrospective nature is a collective shortcoming. Prospective, randomized trials are needed to better understand how these procedures perform in a large, diverse population over time. One such study of 162 participants (sponsored by Allergan) was recently completed; data and a report are forthcoming.⁹ Additionally, studies comparing the ab externo approach to Xen insertion (both transconjunctival and open conjunctiva) with trabeculectomy are warranted as this technique becomes more popular among surgeons. It should be noted that all the aforementioned studies utilized the ab interno approach for Xen insertion.

CONCLUSION

The Xen Gel Stent has been shown to be an effective and safe option for subconjunctival glaucoma surgery. Head-to-head studies have demonstrated a benefit over trabeculectomy in terms of safety and visual recovery; however, most studies have found that trabeculectomy yielded a lower mean IOP and greater mean IOP reduction compared to the stent. Nevertheless, success rates (IOP < 16–18 mm Hg) were similar in most studies. Surgeons should consider these findings when determining the most appropriate procedure for each patient.