Porosity of Bleb Capsule declines rapidly with Fluid Challenge

Efficacy of XEN gel stent implantation combined with mitomycin C injection in the treatment of open-angle glaucoma: a meta-analysis

OBJECTIVE

To assess the efficacy of XEN gel stent implantation combined with mitomycin C (MMC) injection in controlling intraocular pressure (IOP) and the dosage of postoperative medication for open-angle glaucoma (OAG) patients.

METHODS

The PubMed, Embase, Cochrane Library, and Science Direct databases were searched from inception to August 2022 without any language restriction. All studies that compared IOP and the dose of medication before and after XEN gel stent implantation and MMC injection for OAG patients were included. Data extraction and methodological quality evaluation were performed.

RESULTS

A total of 26 studies (2329 eyes) were involved in meta-analysis. It was revealed that IOP at 1 month after XEN implantation was significantly lower than preoperative IOP [group 1, IOP between 19.2 and 21.2 mmHg: mean difference (MD) = 7.60, 95% confidence interval (CI) [6.55, 8.66], I2 = 0%, P < 0.01; group 2, IOP between 21.6 and 22.8 mmHg: MD = 5.83, 95% CI [4.94, 6.71], I2 = 0%; group 3, IOP between 23.5 and 24.3 mmHg: MD = 9.22, 95% CI [8.33, 10.10], I2 = 0%; group 4, IOP between 24.4 and 26.2 mmHg: MD = 11.64, 95% CI [10.49, 12.80], I2 = 0%)]. Moreover, IOP at 3 months after XEN implantation was also significantly lower than preoperative IOP (MD = 8.31, 95% CI [2.54, 8.46], Z = 13.92, P < 0.00001). IOP at 6 months after XEN implantation was significantly lower than preoperative IOP, and no heterogeneity was found between two groups (group 1, IOP between 17.8 and 21.8 mmHg: MD = 5.71, 95% CI: [5.05, 6.36], I2 = 0%, P < 0.01); group 2, IOP between 22.1 and 23.9 mmHg: MD = 7.92, 95% CI [7.15, 8.70], I2 = 0%, P < 0.01). Low heterogeneity was noted in one group (group 3, IOP between 24.3 and 26.2 mmHg: MD = 9.32, 95% CI [8.66, 9.97], I2 = 30%). It was found that IOP at 12 months after XEN implantation was significantly lower than preoperative IOP (MD = 8.11, 95% CI [7.09, 9.12], Z = 15.68, P < 0.00001). There was high heterogeneity among the different baseline IOP groups at 24-month post-surgery (CHi2 = 41.74, df = 1, I2 = 97.6%, P < 0.01), while no heterogeneity was identified in two groups (group 1, IOP between 19.2 and 22.1 mmHg: MD = 6.30, 95% CI [5.76, 6.85], I2 = 0%, P < 0.01); group 2, IOP between 22.8 and 23.8 mmHg: MD = 9.11, 95% CI [8.45, 9.77], I2 = 0%). Subgroup analysis indicated that the dosage of medication was significantly reduced at 6 months after XEN implantation (group 1, the amount of medication was 2.3-2.96: MD = 1.90, 95% CI [1.78, 2.02], I2 = 0%, P < 0.01); group 2, the amount of medication was 3.2-3.3: MD = 2.59, 95% CI [2.43, 2.75], I2 = 0%). Furthermore, the dosage of medication at 12 months (MD = 1.96, 95% CI [1.72, 2.21], Z = 15.80, P < 0.01) and 24 months (group 1, the amount of medication was 1.89-2.7: MD = 6.30, 95% CI [5.76, 6.85], I2 = 0%, P < 0.01); group 2, the amount of medication was 2.72-3.07: MD = 9.11, 95% CI [8.45, 9.77], I2 = 0%) after XEN implantation was significantly lower compared with the preoperative level.

CONCLUSION

XEN implantation combined with MMC injection significantly decreased IOP in OAG patients at 1, 6, 9, 12, and 24 months after treatment. In addition, XEN implantation combined with MMC injection decreased the amount of medication used at 6, 12, and 24 months after treatment in OAG patients.

A model for designing intraocular pressure-regulating glaucoma implants

Glaucoma is a group of eye conditions that damage the optic nerve, the health of which is vital for vision. The key risk factor for the development and progression of this disease is increased intraocular pressure (IOP). Implantable glaucoma drainage devices have been developed to divert aqueous humor from the glaucomatous eye as a means of reducing IOP. The artificial drainage pathway created by these devices drives the fluid into a filtering bleb. The long-term success of filtration surgery is dictated by the proper functioning of the bleb and overlying Tenon’s and conjunctival tissue. To better understand the influence of the health condition of these tissues on IOP, we have developed a mathematical model of fluid production in the eye, its removal from the anterior chamber by a particular glaucoma implant–the PRESERFLO^® MicroShunt–, drainage into the bleb and absorption by the subconjunctival vasculature. The mathematical model was numerically solved by commercial FEM package COMSOL. Our numerical results of IOP for different postoperative conditions are consistent with the available evidence on IOP outcomes after the implantation of this device. To obtain insight into the adjustments in the implant’s hydrodynamic resistance that are required for IOP control when hypotony or bleb scarring due to tissue fibrosis take place, we have simulated the flow through a microshunt with an adjustable lumen diameter. Our findings show that increasing the hydrodynamic resistance of the microshunt by reducing the lumen diameter, can effectively help to prevent hypotony. However, decreasing the hydrodynamic resistance of the implant will not sufficiently decrease the IOP to acceptable levels when the bleb is encapsulated due to tissue fibrosis. Therefore, to effectively reduce IOP, the adjustable glaucoma implant should be combined with a means of reducing fibrosis. The results reported herein may provide guidelines to support the design of future glaucoma implants with adjustable hydrodynamic resistances.

Evaluation of the Ultrastructural and In Vitro Flow Properties of the PRESERFLO MicroShunt

Purpose

To measure the in vitro flow properties of the PRESERFLO implant for comparison with the theoretical resistance to flow.

Methods

The PRESERFLO was designed to control the flow of aqueous humor according to the Hagen-Poiseuille (HP) equation. Scanning electron microscopy (SEM) was performed to analyze the ultrastructure, and flow measurements were carried out using a gravity-flow setup.

Results

SEM images of the PRESERFLO showed luminal diameters of 67.73 × 65.95 µm and 63.66 × 70.54 µm. The total diameter was 337.2 µm, and the wall was 154 µm wide. The theoretical calculation of the resistance to flow (R) for an aqueous humor (AH) viscosity of 0.7185 centipoises (cP) was 1.3 mm Hg/(µL/min). Hence, assuming a constant AH flow of 2 µL/min, the pressure differential across the device (ΔP) was estimated to be 2.6 mm Hg. The gravity-flow experiment allowed us to measure the experimental resistance to flow, which was R_E = 1.301 mm Hg/(µL/min), in agreement with the theoretical resistance to flow R given by the HP equation.

Conclusions

The experimental and theoretical flow testing showed that the pressure drop across this device would not be large enough to avoid hypotony unless the resistance to outflow of the sub-Tenon space was sufficient to control the intraocular pressure in the early postoperative period.

Translational Relevance

The fluid properties of glaucoma subconjunctival drainage devices determine their specific bleb-forming capacity and ability to avoid hypotony and therefore their safety and efficacy profile.

An Ex-Press implant versus trabeculectomy in a fibrotic bleb with late failure after previous trabeculectomy

AIM

To compare the outcome of an Ex-Press implant and subscleral trabeculectomy (SST) in the management of glaucoma after previous trabeculectomy on a fibrotic bleb.

METHODS

This randomized prospective study included 28 eyes from 28 patients (age range: 42-55y) with primary open angle glaucoma (POAG) presented with elevated intraocular pressure (IOP) with fibrotic bleb despite previous SST for more than 4mo. The eyes enrolled in the study were divided into two groups: group I (subjected to Ex-Press implant surgery) and group II [subjected to SST with mitomycin C (MMC)]. The follow-up continued one year after surgery to evaluate IOP, visual acuity (VA), visual field (VF), and postoperative complications.

RESULTS

A significant decrease in IOP was found in both groups with a higher reduction in Ex-Press implant surgery with the mean IOP of 14.50 mm Hg (P=0.001), while the SST group recorded the mean IOP of 16.50 mm Hg (P=0.001) after one year. However, the difference between the two groups in terms of the decrease in IOP was insignificant. Fewer postoperative complications were recorded in the Ex-Press implant surgery and more cases requiring further anti-glaucomatous medications were seen in the SST group. Both groups showed stability in terms of VA and VF.

CONCLUSION

Ex-Press implant surgery and SST with MMC are two surgical alternatives for controlling IOP in late failure that occurs more than 4mo after previous SST with a fibrotic bleb. However, Ex-Press shunt is a safer surgery with fewer complications.

Experimental models of glaucoma filtration surgery

Glaucoma filtration surgery plays an important role in achieving intraocular pressure (IOP) reduction in patients who have high IOP despite maximum medical therapy. Preclinical experimental models of glaucoma filtration surgery contribute a great deal to our knowledge of the wound healing processes that predispose to scarring and may lead to poor outcomes. However, this research needs to be interpreted in the light of the specific study design, animal model and methods used. We review the existing literature addressing various models of experimental glaucoma filtration surgery, discuss the considerations in assessing these models and describe future steps in evaluating potential therapeutics and bleb characteristics that could impact translational research in this field.

A novel bilayered expanded polytetrafluoroethylene glaucoma implant creates a permeable thin capsule independent of aqueous humor exposure

The purpose of these studies was to evaluate clinical, functional, and histopathological features of glaucoma drainage implants (GDIs) fabricated from novel, custom-tailored expanded polytetrafluoroethylene (ePTFE). Implants of matching footprints were fabricated from silicone (Control) and novel, bilayered ePTFE. ePTFE implants included: (a) one that inflated with aqueous humor (AH) (High), (b) one that inflated with a lower profile (Low), (c) an uninflated implant not connected to the anterior chamber (Flat), and (d) one filled with material that did not allow AH flow (Filled). All implants were placed in adult New Zealand White rabbits and followed over 1-3 months with clinical exams and intraocular pressure. The permeability of tissue capsules surrounding GDIs was assessed using constant-flow perfusion with fluoresceinated saline at physiologic flow rates. After sacrifice, quantitative histopathological measures of capsule thickness were compared among devices, along with qualitative assessment of cellular infiltration and inflammation. Capsular thickness was significantly reduced in blebs over ePTFE (61.4 ± 53 μm) versus silicone implants (193.6 ± 53 μm, p = .0086). AH exposure did not significantly alter capsular thickness, as there was no significant difference between High and Filled (50.9 ± 29, p = .34) implants. Capsules around ePTFE implants demonstrated permeability with steady-state pressure: flow relationships at physiologic flow rates and rapid pressure decay with flow cessation, while pressure in control blebs increased even at low flow rates and showed little decay. Perfused fluorescein dye appeared beyond the plate border only in ePTFE implants. ePTFE implants are associated with thinner, more permeable capsules compared to silicone implants simulating presently used devices.

Long-term outcome of low-cost glaucoma drainage device (Aurolab aqueous drainage implant) compared with Ahmed glaucoma valve

Purpose

To compare the long-term outcome of Aurolab aqueous drainage device (AADI) and Ahmed glaucoma valve (AGV).

Method

Retrospective analysis of patients with refractory glaucoma who underwent AGV (AGV-FP7) and AADI (AADI Model 350) implantation. The outcome measures were intraocular pressure (IOP), requirement of antiglaucoma medications (AGMs) and re-surgery for IOP control. The postoperative complications were classified as early (≤3 months), intermediate (>3 months to ≤1 year) or late (>1 year).

Results

173 patients (189 eyes) underwent AGV implantation (AGV Group) while 201 patients (206 eyes) underwent AADI implantation (AADI group). The IOP in AADI group was significantly lower than AGV group at all time points till 2 years and comparable at 3 years. AADI group had significantly higher number of AGM in preoperative period and significantly lower number in postoperative period till 3 years compared with AGV group. AADI group had more hypotony-related complications but statistically insignificant (p = 0.07). The surgical interventions were significantly higher in AGV (n = 18) compared with AADI group (n = 5) in late postoperative period (p = 0.01). At 3 years, overall success was seen in 58.18% in AGV and 73.08% in AADI group (p = 0.15). Complete success was seen in 7.27% patients in AGV and 25.00% patients in AADI group (p = 0.02).

Conclusion

Both AADI and AGV implant had comparable mean IOP at 3 years with lesser requirement of AGM in the AADI group. Both procedures appear to be safe with slight preponderance of hypotony-related complications in AADI group.

Recommendations for the management of elevated intraocular pressure due to bleb fibrosis after XEN gel stent implantation

Surgical management of glaucoma offers a means of effective disease control. A gel stent that facilitates drainage to the subconjunctival space offers intraocular pressure (IOP) reduction similar to traditional glaucoma filtering surgeries in a less invasive manner. However, like all subconjunctival filtering procedures that result in a bleb, fibrosis can present as a cause of elevated IOP. The following proposed techniques and recommendations for managing elevated IOP due to bleb fibrosis after gel stent implantation are based on the clinical experience of the authors. The goal of this paper is to improve outcomes following gel stent surgery by providing guidance on assessment of bleb function and strategies for bleb enhancement.

Blebs, Barriers, and Bagpipes: Why is It so Hard?

Doug Johnson was a clinician-scientist who made great contributions to the understanding of outflow from the eye. This lecture is in honour of Doug and explores the author's understanding of outflow in the surgical context.

HOW TO CITE THIS ARTICLE

Coote M. Blebs, Barriers, and Bagpipes: Why is It so Hard? J Curr Glaucoma Pract 2016; 10(3):79-84.