Safety Profile of Minimally Invasive Glaucoma Surgery

Kateki Vinod; Steven J. Gedde


Curr Opin Ophthalmol. 2021;32(2):160-168. 

In This Article

Subconjunctival Minimally Invasive Glaucoma Surgery

The Xen gel stent (Allergan plc, Dublin, Ireland) is a 6-mm cylindrical gelatin device with a 45-μm lumen that is designed for ab interno insertion into the subconjunctival space. Once implanted through a scleral fistula, the hydrated gel stent conforms to surrounding tissues and forms a bleb. An optimally positioned gel stent should have 1 mm visible within the anterior chamber, 2 mm within the scleral fistula, and 3 mm within the subconjunctival space to direct aqueous flow posteriorly and prevent device migration.[52] Intraoperative use of antimetabolites enhances bleb function, as in trabeculectomy. The gel stent may be placed as a stand-alone procedure or at the time of cataract surgery. Some surgeons opt for an ab externo approach to gel-stent implantation, either through the closed conjunctiva or after creating a limited conjunctival peritomy.

Grover et al.[53] conducted a prospective one-year study of 65 patients with refractory glaucoma who underwent gel stent placement with mitomycin C via an open conjunctiva, ab interno approach. Transient hypotony without visual sequelae developed in 16 (24.6%) patients and IOP spikes occurred in 14 (21.5%) patients. Nearly one-third of patients (21; 32.3%) required postoperative needling. Eighteen (27.7%) patients lost two or more lines of BCVA, four of whom experienced persistent vision loss at 12 months. Less common complications included hyphema (3; 4.6%), choroidal effusion (2; 3.1%), stent migration (1; 1.5%), stent erosion (1; 1.5%), bleb leak (1; 1.5%), bleb dysesthesia (1; 1.5%), and corneal edema persisting for more than 30 days (1; 1.5%).

Various adverse events have since been described that were either not reported or occurred infrequently in the study by Grover et al..[53] Rezkallah et al.[54] described a total hyphema two days after uncomplicated gel-stent insertion in a patient anticoagulated with fluindione (a vitamin K antagonist), which resolved spontaneously by the first postoperative month without loss of IOP control. Suprachoroidal hemorrhage associated with gel stent placement has been reported both intraoperatively[55] and postoperatively.[56,57] Two cases of postoperative suprachoroidal hemorrhage have been described, both of which developed on the second postoperative day and were preceded by hypotony on day one.[56,57]

Stent-related adverse events include obstruction, migration, degradation, and erosion. The gel stent's small luminal diameter risks occlusion with Tenon's capsule at its distal end and requires intraoperative confirmation that the tip is straight and freely mobile. The proximal tip of the gel stent can become occluded by blood, fibrin, iris, or Descemet's membrane[57] within the anterior chamber and can lead to severe IOP elevation. Stent obstruction with red blood cells has been reported one month after spontaneous resolution of a postoperative hyphema.[58] A fibrin plug can also form within the stent lumen in the setting of uncontrolled postoperative inflammation.[59,60] Intraocular pressure control has been achieved via application of YAG laser to the proximal stent tip to dislodge occluding material,[60] mechanical removal of consolidated debris using microforceps,[61] and replacement of the gel stent or conversion to traditional glaucoma surgery.[58,59,62] Laser iridoplasty has also been used to retract iris from within the stent lumen.[63] Free-floating clumps of iris pigment epithelium obstructing the stent may be more difficult to visualize and extract.[62]

Migration of an improperly positioned Xen gel stent may result from mechanical factors such as blinking, eyelid rubbing, or postoperative needle revision. Partial inward migration may induce contact between the stent and iris, causing postoperative inflammation,[64] or between the stent and cornea, causing endothelial cell loss.[65] Complete inward migration may result in total dislocation into the anterior chamber,[66,67] whereas outward migration of the gel stent may result in retraction from within the anterior chamber,[57] leading to IOP elevation. Degradation of the intracameral and intrascleral portions of the gel stent with partial luminal closure and IOP elevation was described in one patient three years after implantation.[68]

Conjunctival erosion overlying the extraocular portion of the Xen gel stent has been described in several case reports.[67,69–78] Endophthalmitis complicating gel stent erosion has been reported 3.5 months to 11 months postoperatively,[72–74,77] and one month after bleb needling with 5-fluorouracil.[75] Conjunctival advancement,[69] conjunctival autografting,[70] and ab interno stent repositioning with direct conjunctival closure[78] have been used to repair eroded gel stents.

Subconjunctival and episcleral fibrosis can compromise bleb function, requiring needling or open revision in the operating room if the gel stent cannot be adequately visualized.[52] Of note, inadvertent gel stent amputation has been described during transconjunctival bleb needling.[79] Although blebs associated with the gel stent are typically low and diffuse,[53] bleb dysesthesia was reported by Sekaran et al.[80] in six of 215 (2.8%) patients who underwent superonasal gel stent placement. Bleb hypertrophy and encapsulation may require interventions such as aqueous suppression, needle decompression, or surgical revision to reduce bleb height.[80–82] Overfiltration following gel stent insertion can result in hypotony and its vision-threatening sequelae, and refractory cases may require reoperation.[83]

Olgun et al.[84] compared corneal endothelial cell density in 80 eyes undergoing trabeculectomy or gel stent placement in a prospective nonrandomized study. At three months, patients in the gel stent group experienced a statistically significant loss of endothelial cell density compared with baseline (2156.2 ± 559.7 cells/mm2 at baseline versus 2098.4 ± 556.2 cells/mm2 at 3 months; P = 0.002). The rate of change in endothelial cell density was significantly higher in the trabeculectomy group (−10.0 ± 9.7%) than the gel stent group (−2.1 ± 13.8%; P = 0.002). Gillmann et al.[65] reported an endothelial cell density loss of 2.1% in one patient over 30 days due to contact between the proximal gel stent tip and the corneal endothelium during eye movement. The endothelial cell density stabilized after gel stent removal.