Longitudinal bleb morphology in anterior segment OCT after minimally invasive transscleral ab interno Glaucoma Gel Microstent implantation

Abstract

Purpose: Like the classic trabeculectomy, the minimally invasive, ab interno XEN Glaucoma Gel Microstent (XEN-GGM) creates a filtration bleb in the conjunctiva. The goal of this study was to investigate internal bleb morphology over time with anterior segment optical coherence tomography (AS-OCT) after XEN-GGM implantation.

Methods: In a prospective, single-centre, single-armed cohort study, blebs were characterized using AS-OCT in 78 eyes of 60 patients at day 1, at weeks 1 and 2 and at months 1, 3, 6, 9 and 12 after XEN-GGM implantation in patients with open-angle glaucoma. Morphological bleb characteristics were correlated with IOP and surgical success.

Results: Anterior segment optical coherence tomography data indicate early and late bleb changes in the course of 12 months. Uniform blebs in AS-OCTs showed higher IOPs at all examinations between week 1 (17.7 ± 4.8 mmHg versus 11.3 ± 7.1 mmHg, p = 0.001) and month 3 (16.4 ± 6.1 versus 13.4 ± 6.1, p = 0.04). Subconjunctival tissue separation bleb morphology was associated with lower mean IOPs during the course of 12 months (r = -0.75, p = 0.031). Predictors for surgical failure at month 12 were microcystic multiform bleb morphology in AS-OCT at month 3 (60% versus 15%, relative risk 4.0, p = 0.043) and uniform bleb morphology at month 9 (33% versus 23%, relative risk 1.4, p = 0.015).

Conclusion: Bleb appearance after XEN surgery seems to be different to classic trabeculectomy literature. The present data suggest correlation of IOP and surgical long-term success with bleb morphology in AS-OCT. Prevalence of small diffuse cysts is directly associated with lower IOPs, while cystic encapsulation at 3 months predicts higher surgical failure.

Keywords: XEN; glaucoma; minimally invasive glaucoma surgery; subconjunctival implant; surgical success.

Figure 1

Location of anterior segment optical coherence tomography (AS‐OCT) scans for assessment of bleb morphology after XEN Glaucoma Gel Microstent (XEN‐GGM) implantation. Slit lamp photograph of a bleb 1 week after XEN‐GGM implantation. The XEN‐GGM is highlighted in blue. One scan was obtained radially to the limbus and through the outer part of the XEN‐GGM (a). A second scan perpendicular to the first one was taken to image the subconjunctival part of the XEN‐GGM close to its exit site (b). A third scan was taken parallel and 1 mm posterior to the outer XEN‐GGM lumen through the site of maximal bleb elevation (c).

Figure 2

Bleb morphology in anterior segment optical coherence tomography (AS‐OCT) after XEN Glaucoma Gel Microstent implantation. Bleb morphology in AS‐OCT was classified into four groups: 1. uniform (no fluid‐filled hyporeflective spaces in subconjunctival space, top left and top right), 2. subconjunctival separation (multiple small spaces in more superficial layers, middle left and middle right), 3. microcystic multiform (multiple cystic hyporeflective areas in deep layer separated by thin septae, which are highlighted with “↓”; their thicker bleb wall makes the bleb appear encapsulated in AS‐OCT, Bottom left) and 4. multiple internal layer (hyporeflective spaces in deep and superficial layers of conjunctiva with channels of fluid parallel to the surface of the sclera; fluid‐filled spaces highlighted with “►”; bottom right).

Figure 3

Course of intraocular pressure and number of glaucoma medications after XEN Glaucoma Gel Microstent implantation. Mean intraocular pressure (IOP, Top) ranged from 9.2 at day 1 to 15.9 mmHg at month 9 within the first year postoperatively. A significant mean IOP reduction could be achieved after XEN Glaucoma Gel Microstent implantation at each postoperative visit (each p < 0.001). A significant reduction in number of IOP lowering medications was also detected postoperatively (each p < 0.001, Bottom). Whiskers indicate 95% confidence intervals for means.

Figure 4

Stacked histograms of bleb morphology in anterior segment optical coherence tomography (AS‐OCT) after XEN Glaucoma Gel Microstent implantation over time. Uniform bleb morphology showed at week 1 (p < 0.004) higher mean intraocular pressure (IOP) compared to the other three morphologies in AS‐OCT separately. Comparing uniform bleb morphology against all other morphologies combined, a statistically significant higher mean intraocular pressure (IOP) was detected in uniform blebs at week 1 (p < 0.001), week 2 (p < 0.001), month 1 (p = 0.005) and month 3 (p = 0.04). A strong negative correlation between the timelines of subconjunctival separation morphology prevalence at all visits in the AS‐OCT and the mean IOP could be shown (r = −0.75, p = 0.031). Significant changes of bleb morphology in AS‐OCT between visits are shown in Table 2.