The success of subconjunctival glaucoma surgery depends on the formation of an adequate filtering bleb. Post-operatively, blebs can be categorized by macroscopic appearance or by internal morphology using anterior segment optical coherence tomography (ASOCT). Important parameters include bleb height, bleb wall thickness, tissue reflectivity and the presence of epithelial microcysts. This study compared bleb morphology in 15 eyes treated with trabeculectomy and ten eyes treated with the Xen Gel Stent.1
Trabeculectomies were performed using a fornix-based conjunctival flap, triangular scleral flap and 0.02% mitomycin C (MMC) for two minutes. Xen Gel Stents were implanted into the superonasal quadrant using an ab interno approach in eyes pre-treated (15 mins before surgery) with a 0.1 ml injection of 0.02% MMC. Bleb morphology was examined using Swept-Source OCT and using the Indiana Bleb Appearance Grading Scale (IBAGS) at least one year after surgery.
Using the IBAGS, Xen blebs tended to be flatter than trabeculectomy blebs, however both procedures produced blebs of similar horizontal extent. Twenty percent of eyes in each group had avascular cystic blebs. Using ASOCT, Xen blebs were significantly flatter than trabeculectomy blebs (417 ± 183 µm versus 618 ± 256 µm) but in both cases the conjunctiva was significantly thicker than in a control group (244 ± 45 µm). Although Xen blebs were flatter than those seen with trabeculectomy, both groups had a similar average reduction in intraocular pressure (IOP), suggesting effective conjunctival filtration.
Interestingly, the conjunctival epithelium was thicker in Xen than trabeculectomy blebs (65 ± 18.5 µm versus 60 ± 17.7 µm) but Xen blebs seemed to be associated with less subconjunctival fibrosis. None of the Xen eyes compared to six out of nine trabeculectomy eyes were noted to have subepithelial fibrosis on ASOCT.
There is likely to be wide variation in bleb morphology produced with both procedures and it is not clear whether any patients had bleb needling which will inevitably affect morphology
Bleb evaluation is important to determine functionality and this study demonstrates the different characteristics of blebs produced following Xen and trabeculectomy surgery. However, there is likely to be wide variation in bleb morphology produced with both procedures and it is not clear whether any patients had bleb needling which will inevitably affect morphology. The study is also limited by a small sample size and high risk of inclusion bias as the case selection process was not described. In addition, it is not clear whether the Xen Gel Stents were placed under the conjunctiva or Tenon's. The finding of an absence of fibrosis in eyes treated with Xen is also surprising given the high needling rate reported with this device.1