XEN Gel Stent in situ Image: © 2017 Allergan. Used with permission
Dr Nathan Kerr
MBChB MD FRANZCO
Royal Victorian Eye and Ear Hospital, Centre for Eye Research Australia
Eye Surgery Associates, Melbourne
The XEN Glaucoma Treatment System is a new option for patients with refractory glaucoma. This minimally-invasive device drains aqueous into the subconjunctival space, a procedure long-considered the gold standard for glaucoma surgery.
Previously, this could be achieved only with invasive operations such as trabeculectomy or insertion of a glaucoma drainage device, which carry not-insignificant risks. The XEN aims to provide a safer and more predictable way of achieving subconjunctival filtration. In doing so, the XEN expands the role of minimally-invasive glaucoma surgery (MIGS) across the spectrum of glaucoma and has the potential to revolutionise the way refractory glaucoma is treated.
The XEN Glaucoma Treatment System consists of the XEN Gel Stent and the XEN Injector. The XEN Gel Stent is a soft, flexible, hydrophilic tube composed of porcine-derived collagen that has been shown to have good biocompatibility.1
Cross-linked with glutaraldehyde to prevent degradation, the XEN creates a permanent outflow pathway between the anterior chamber and the subconjunctival space. The XEN is 6 mm long and has an inner lumen diameter of 45 µm. These dimensions have been selected based on the Poisseuille equation (which predicts flow based on tube length, internal diameter and fluid viscosity) to prevent hypotony.
Figure 1. The XEN injector is passed through a clear corneal incision, across the anterior chamber, and advanced through the sclera into the subconjunctival space Image: © 2017 Allergan. Used with permission
Figure 2. Magnified view showing advancement of the injector into the subconjunctival space Image: © 2017 Allergan. Used with permission
Figure 3. Magnified view showing rotation of the bevel in the subconjunctival space Image: © 2017 Allergan. Used with permission
The XEN is performed under local anaesthetic in a 15-minute operation. Typically performed as a stand-alone procedure, the XEN can also be combined with cataract surgery for patients with both refractory glaucoma and visually-significant cataract. The implant is inserted using a preloaded injector via an ab interno approach through a 1.2 mm clear corneal incision, exiting the sclera 3 mm from the limbus in the superonasal quadrant. There is no conjunctival incision or suturing required, leading to a safer procedure and faster post-operative recovery.
Like other filtration surgery, anti-metabolites are administered prior to insertion to prevent subconjunctival scarring. While the procedure looks simple, it is technically demanding and must be well-executed to achieve optimum results and prevent complications. However, once mastered, the XEN represents the safest, fastest and least-invasive method of creating a new outflow pathway to the subconjunctival space.
Figure 4. Deployment of the XEN Gel Stent into the subconjunctival space Image: © 2017 Allergan. Used with permission
Figure 5. The XEN Gel Stent in situ draining aqueous from the anterior chamber to the subconjunctival space Image: © 2017 Allergan. Used with permission
Figure 6. Globe cross section with XEN Gel Stent in place Image: © 2017 Allergan. Used with permission
The XEN Glaucoma Treatment System is indicated for the management of refractory glaucoma, including primary open-angle glaucoma, pseudoexfoliative glaucoma and pigmentary glaucoma, which is unresponsive to maximum tolerated medical therapy. The XEN can be performed as a primary procedure2 or in eyes where previous surgical treatments, such as other MIGS devices or trabeculectomy, have failed.
As the XEN is minimally-invasive and has a good safety profile, it can be considered earlier in the disease process than trabeculectomy. Additionally, because the XEN drains to the subconjunctival space it has greater IOP-lowering efficacy than other MIGS procedures and is therefore also suitable for cases with significant IOP elevation and more advanced disease.
The XEN is contraindicated in active neovascular glaucoma or if there is intraocular silicone oil, vitreous in the anterior chamber, or an anterior chamber intraocular lens. The procedure should not be performed if there is significant conjunctival scarring or an existing glaucoma drainage device in the target quadrant. The safety and effectiveness of the XEN has not been established in congenital or infantile glaucoma.
The XEN creates a bleb that is typically low, diffuse and comfortable. Post-operative recovery is much faster than trabeculectomy with little blurring of vision, minimal restriction on physical activity and less need for intensive topical steroids. Fewer post-operative visits are required compared to trabeculectomy and there is no need for suture removal or lysis. While complications such as malposition, occlusion or rarely, erosion can occur, these can typically be prevented with good surgical technique. As the XEN requires bleb management and occasionally needling, the procedure is best performed by glaucoma surgeons who are experienced in bleb management.
The efficacy and safety of the XEN has been evaluated in the APEX study, a prospective, multi-centre, 24-month study of 215 patients in 14 countries with mild-to-moderate primary open angle glaucoma and no history of intraocular surgery. Patients underwent either the XEN procedure alone or in combination with cataract surgery.
In a preliminary report, there was an average reduction in IOP of 7.6 ± 4.8 mmHg to a mean of 13.8 mmHg at 12 months.3 This was associated with a reduction in IOP-lowering medications from 2.6 ± 1.1 at baseline to 0.6 ± 0.8 at 12 months. There were no cases of bleb leak and only one patient (0.5 per cent) developed symptomatic hypotony requiring injection of viscoelastic. There was no difference in efficacy between the XEN as a stand-alone procedure or in combination with cataract surgery. The 24-month results are due to be published this year. Other smaller studies have shown similar results.4–6
The success rates of XEN compare favourably with gold-standard trabeculectomy. In a multi-centre, retrospective interventional cohort study, there was no detectable difference in failure rate between stand-alone XEN insertion or trabeculectomy with mitomycin C.7
Future of glaucoma surgery
The XEN represents a major advance in the surgical management of glaucoma. It combines the efficacy of subconjunctival filtration with the safety and fast recovery of minimally-invasive glaucoma surgery. While there will always be a place for trabeculectomy in selected patients, particularly those with very advanced disease, for the majority of patients with refractory open-angle glaucoma the XEN provides an attractive alternative.
Dr Nathan Kerr
Dr Nathan Kerr specialises in minimally-invasive glaucoma surgery (MIGS) and completed a Glaucoma Fellowship at Moorfields Eye Hospital in London. He was the first Australian surgeon to be accredited in the use of the XEN Gel Implant and is co-editor of MIGS.org, a source of patient information on glaucoma surgery. Dr Kerr is a consultant ophthalmologist at the Royal Victorian Eye and Ear Hospital, and consults privately at Eye Surgery Associates.
- Shute TS, Dietrich UM, Baker JFM, et al. Biocompatibility of a novel microfistula implant in non-primate mammals for the surgical treatment of glaucoma. Invest Ophthalmol Vis Sci 2016; 57: 8: 3594-3600. doi:10.1167/iovs.16-19453.
- Kerr NM, Wang J, Barton K. Minimally invasive glaucoma surgery as primary stand-alone surgery for glaucoma. Clin Exper Ophthalmol 2016. doi:10.1111/ceo.12888.
- Barton KB, Chelvin S, Vera V. XEN45 Implantation for Primary Open-Angle Glaucoma: One-Year Results of a Multicenter Study. European Glaucoma Society Congress 2016.
- Galal A, Bilgic A, Eltanamly R, Osman A. XEN glaucoma implant with mitomycin C 1-year follow-up: result and complications. J Ophthalmol 2017; 2017: 5457246. doi:10.1155/2017/5457246.
- Sheybani A, Dick HB, Ahmed IIK. Early clinical results of a novel ab interno gel stent for the surgical treatment of open-angle glaucoma. J Glaucoma 2016; 25: 7: e691-6. doi:10.1097/IJG.0000000000000352.
- Sheybani A, Lenzhofer M, Hohensinn M, Reitsamer H, Ahmed IIK. Phacoemulsification combined with a new ab interno gel stent to treat open-angle glaucoma: Pilot study. J Cataract Refract Surg 2015; 41: 9: 1905-1909. doi:10.1016/j.jcrs.2015.01.019.
- Schlenker MB, Gulamhusein H, Conrad-Hengerer I, et al. Efficacy, safety, and risk factors for failure of standalone ab interno gelatin microstent implantation versus standalone trabeculectomy. Ophthalmology 2017. doi:10.1016/j.ophtha.2017.05.004.