Glaucoma drainage devices: state of the art

[Glaucoma drainage devices: Indications, intraoperative management and postoperative follow-up]

Glaucoma drainage devices (GDD) are used for patients with secondary glaucoma, such as uveitic or neovascular glaucoma, which is uncontrolled under local therapy. They are also used in patients with conjunctival scarring, for example after a previous vitrectomy or after unsuccessful previous glaucoma surgery, such as trabeculectomy. They are also a treatment option for congenital glaucoma, aphakic glaucoma or for the treatment of iridocorneoendothelial syndromes. The conventional GDD were the Baerveldt, Molteno or Ahmed glaucoma implant, whereby the first two were valveless and the latter had a valve. Newer GDD include the PAUL glaucoma implant, the Ahmed Clear Path and the EyeWatch system. Hypotony is a feared complication after GDD surgery, and there are various options for avoiding it: external ligation of the tube or intraluminal suture obstruction. However, low IOP may still occur postoperatively. If early postoperative hypotension occurs in combination with a shallow anterior chamber, the injection of a viscoelastic can be helpful. Late hypotension is usually treated with permanent occlusion or removal of the tube. Furthermore, GDD erosion and migration can occur postoperatively, in such cases a surgical revision is required. Corneal decompensation can also occur after GDD; Descemet's membrane endothelial keratoplasty (DMEK) is the treatment of choice in many centers and is performed more frequently than a penetrating keratoplasty. Finally, double vision is a possible complication after GDI. Most patients have short-term, self-limited diplopia or can be adequately treated with prism glasses; a few require strabologic surgery.

The efficacy of adjunctive mitomycin C and/or anti-VEGF agents on glaucoma tube shunt drainage device surgeries: a systematic review

PURPOSE

The effectiveness of mitomycin C (MMC) in trabeculectomy has long been established. The aim of this review is to evaluate the efficacy and safety of adjunctive agents in tube shunt drainage device surgery for glaucoma or ocular hypertension, since controversy still exists regarding their benefit.

METHODS

We searched CENTRAL, PubMed, Embase, Web of Science, Scopus, and BASE for RCTs, which have used adjuvant antimetabolites-either MMC or 5-Fluorouracil (5-FU)-and/or anti-vascular endothelial growth factors (anti-VEGF) agents. The main outcome was IOP reduction at 12 months.

RESULTS

Ten studies met our inclusion criteria. Nine used the Ahmed Glaucoma Valve (AGV) implant, while the double-plate Molteno implant was used in one study. Four studies used MMC. The remaining six studies used an anti-VEGF drug - either bevacizumab, ranibizumab or conbercept. Only one MMC-study reported a significant difference in the IOP reduction between groups at 12 months, favouring the MMC group (55% and 51%; p < 0.01). A significant difference was also reported by two out of five bevacizumab-studies, both favouring the bevacizumab group (55% and 51%, p < 0.05; 58% and 27%, p < 0.05), with the highest benefit seen in neovascular glaucoma cases, especially when panretinal photocoagulation (PRP) was also used. Neither ranibizumab nor conbercept were found to produce significant differences between groups regarding IOP reduction.

CONCLUSION

There is no high-quality evidence to support the use of MMC in tube shunt surgery. As for anti-VEGF agents, specifically bevacizumab, significant benefit seems to exist in neovascular glaucoma patients, especially if combined with PRP.

Wound Modulations in Glaucoma Surgery: A Systematic Review

Excessive fibrosis and resultant poor control of intraocular pressure (IOP) reduce the efficacy of glaucoma surgeries. Historically, corticosteroids and anti-fibrotic agents, such as mitomycin C (MMC) and 5-fluorouracil (5-FU), have been used to mitigate post-surgical fibrosis, but these have unpredictable outcomes. Therefore, there is a need to develop novel treatments which provide increased effectiveness and specificity. This review aims to provide insight into the pathophysiology behind wound healing in glaucoma surgery, as well as the current and promising future wound healing agents that are less toxic and may provide better IOP control.

[Clinical results of the eyeWatch system: 1-year outcomes]

BACKGROUND

The eyeWatch System (EWS) (Rheon Medical, Lausanne, Switzerland) was developed as a glaucoma drainage device (GDD) to precisely control intraocular pressure (IOP) and thus prevent the development of postoperative hypotension and its complications.

OBJECTIVE

To report 1‑year outcomes from a single-center cohort undergoing EWS surgery.

MATERIAL AND METHODS

Retrospective review of patients undergoing EWS surgery at the University Eye Hospital Bonn, Germany from May 2021 to September 2022.

RESULTS

A total of 5 eyes of the first 5 consecutive patients treated with the EWS were included in this study. The mean preoperative IOP was 34.6 mm Hg (23-45 mm Hg) which was reduced to 12.2 mm Hg (9-18 mm Hg) with a mean reduction of 60%. Local pressure-lowering therapy was reduced from 2.8 agents preoperatively (1-4) to 0.6 agents after 1 year. All patients achieved the target IOP of below 21 mm Hg including 4 patients without any medication. The complication rate was low. One patient had to undergo two surgical revisions as the IOP was elevated to over 30 mm Hg after surgery and tube exposure became apparent during the course of the follow-up so that a second donor patch was sutured on. The same patient had an elevated IOP >30 mm Hg after a cMRI examination, so that a reduction of IOP by adjusting the EWS using the eyeWatch pen (EWP) was performed. IOP adjustments with the eyeWatch pen (EWP) were performed in 4 of 5 patients in the first year after surgery; the mean IOP before adjustment was 21.33 mm Hg (15-37 mm Hg) which was reduced to a mean of 8.58 mm Hg (4-16 mm Hg).

CONCLUSION

The EWS effectively lowers IOP and the amount of necessary pressure-lowering therapy. With the EWP postoperative adjustments of IOP are possible without further invasive measures.

Early Postoperative Aqueous Suppression Therapy and Surgical Outcomes of Ahmed Tube Shunts in Refractory Glaucoma

PURPOSE

To compare early vs. delayed use of aqueous suppressants on Ahmed glaucoma valve (AGV) outcomes.

DESIGN

Single-center retrospective comparative case series.

PARTICIPANTS

Patients who underwent AGV surgery at Wills Eye Hospital in the period between 2016 and 2021.

METHODS

Retrospective review of AGV surgery at Wills Eye Hospital 2016 to 2021 for refractory glaucoma. Two groups were created: group 1 or those who received early aqueous suppressant therapy in the first 2 weeks postoperatively whenever the intraocular pressure (IOP) was > 10 mmHg, and group 2 or those who received delayed treatment after 2 weeks whenever the IOP exceeded the target pressure. Aqueous suppressant therapy included topical beta blockers, carbonic anhydrase inhibitors, and/or alpha agonists.

MAIN OUTCOME MEASURES

The primary outcome measures were the frequency of hypertensive phase (HP) defined as IOP > 21 mmHg in first 3 months after an initial reduction to 21 mmHg or less in the first postoperative week not caused by tube obstruction or retraction, and month-12 surgical failure defined as 5 ≥ IOP > 21 mmHg on 2 consecutive visits, vision decline to no light perception (NLP), or glaucoma reoperation. Secondary outcome measures included changes in visual acuity (VA), IOP, and glaucoma medications at 12 months. Predictive factors for surgical failure were also identified.

RESULTS

A total of 407 eyes of 391 patients (260 in group 1, 147 in group 2) with similar baseline characteristics were included. Hypertensive phase was more common in group 2 than 1 (41.5% vs. 18.5%; P < 0.001). At month 12 (N = 303 eyes), group 1 was less likely to fail than group 2 (21.2% vs. 36.8%, P = 0.003). Multivariate regression analysis showed that HP (odds ratio [OR] = 10.47, P < 0.001), delayed aqueous suppression use (OR = 2.17, P = 0.003), and lower baseline VA (OR = 1.56, P = 0.015) were the strongest predictors of month-12 failure.

CONCLUSIONS

Early use of aqueous suppressants may lower the risk of HP and improve AGV outcomes.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Clinical outcomes of the PAUL® glaucoma implant: One-year results

BACKGROUND

To report one-year outcomes from a single-centre cohort undergoing PAUL® Glaucoma Implant (PGI) surgery.

METHODS

Retrospective review of patients undergoing PGI surgery at the University Eye Hospital Bonn, Germany, from April 2021 to September 2021.

RESULTS

Forty-five eyes of 41 patients were included. Qualified and complete success rates (95% CI) were 95.6% (88.9%-100%) and 73.3% (60%-86.7%) for Criterion A (IOP ≤ 21 mmHg), 84.4% (73.3%-93.3%) and 74.4% (51.1%-80.0%) for Criterion B (IOP ≤ 18 mmHg), 62.2% (48.9%-75.6%) and 46.7% (31.2%-62.2%) for Criterion C (IOP ≤ 15 mmHg) and 26.7% (13.3%-40.0%) and 22.2% (11.1%-33.3%) for Criterion D (IOP ≤ 12 mmHg), respectively. Mean IOP decreased from 26.1 mmHg (7-48 mmHg) to 12.0 mmHg (3-24 mmHg) (reduction of 48.83%) after 12 months with a reduction of IOP-lowering agents from 0.5 (0-3). One eye (2.2%) needed an injection of viscoelastic due to significant hypotony with AC shallowing, and four eyes (8.9%) developed choroidal detachments due to hypotony which resolved without further interventions after 6 weeks. Three patients (6.7%) developed tube exposure which required conjunctival revision with an additional pericardial patch graft. An intraluminal prolene stent was removed in 19 eyes (42.2%) after a mean time period of 8.4 months (2-12 m). Mean IOP before the removal was 21.9 mmHg (12-38 mmHg) and decreased to 11.3 mmHg (6-16 mmHg).

CONCLUSIONS

PGI surgery is an effective procedure for reducing IOP and pressure-lowering therapy. An intraluminal prolene stent impedes hypotony in the early postoperative phase and enables further IOP lowering without additional interventions during the postoperative course.

Anterior Chamber Washout During Ahmed Valve Glaucoma Surgery Reduces the Incidence of Hypertensive Phase

PRCIS

Intraoperative anterior chamber (AC) washout reduces the odds of hypertensive phase (HP) by 95% compared with those not receiving washout during Ahmed glaucoma implant (AGI), suggesting AC washout is an effective prophylaxis for postoperative HP.

PURPOSE

To characterize the incidence of postoperative HP in patients receiving AC washout at the time of AGI.

PATIENTS AND METHODS

A total of 24 patients with medically refractory glaucoma who underwent AGI surgery at a tertiary academic medical center in Southern California from December 2018 through March 2021 were included in this retrospective comparative case series. Patients who received a pediatric implant and underwent concurrent intraocular surgery or did not complete a minimum of 6 months of follow-up were excluded. Nine patient's eyes received AC washout and 15 controls that were analyzed through 6 months postprocedure. HP was defined as intraocular pressure (IOP) above 21 mm Hg within 6 months postprocedure with maximum tolerated medical therapy. AC washout was performed by irrigating the AC with a 5 mL balanced salt solution before placing the tube of the AGI into the AC. The rate of postoperative HP, defined as peak IOP >21 mm Hg, at 6 months follow-up, was observed as the primary outcome measure.

RESULTS

Patients included in the study had a high mean preoperative IOP ( X̅ = 44.11, SD = 13.85). There were no baseline differences between the washout and control groups. The odds of HP were significantly reduced (odds ratio: 0.050; 95% CI: 0.004, 0.706; P = 0.027) for patients who underwent intraoperative AC washout compared with those who did not. Bivariate analysis of subject baseline characteristics revealed that only washout status was significantly different in subjects with HP compared with subjects without HP ( P = 0.015). A multivariate logistic regression model using washout status and autoimmune conditions as covariates was significant in predicting HP ( X2 (2) = 12.337, P = 0.002), with washout as a significant predictor when controlling for autoimmune comorbidities ( P = 0.027).

CONCLUSIONS

AC washout significantly decreases the odds of HP after AGI surgery. AC washout with balanced salt solution adds minimal time and risks to surgery and therefore may be an effective adjunct during AGI placement.

[Comparison of glaucoma drainage implants]

Glaucoma drainage devices (tubes, GDI or GDD) are nowadays an important surgical option in the treatment of refractory glaucoma. They are frequently used in cases where previous glaucoma surgery has failed or in patients with a history of conjunctival scarring where other procedures are contraindicated or simply not possible. This article looks at the evolution of glaucoma drainage implants from the very beginning to the multiple designs, experiences and research studies that have made tubes an essential part of the armamentarium of modern glaucoma surgeons. The article describes the first concepts and then proceeds to the first commercialized devices which led to the widespread use of tubes such as Molteno®, Baerveldt® and Ahmed®. Finally, it looks at the innovations that have been carried out, particularly over the last decade with the advent of new tubes, such as Paul®, eyeWatch® and Ahmed ClearPath®. The factors associated with the success and failure of GDD surgery, including the indications, are different from those for trabeculectomy and increasing experience and larger amounts of data have helped glaucoma surgeons to become more comfortable with selecting the most appropriate procedure for the individual patients.

[First clinical results with the PAUL® Glaucoma Implant at the University Eye Hospital Bonn]

BACKGROUND

Glaucoma drainage devices (GDD) are an invasive procedure for the treatment of glaucoma. The PAUL® Glaucoma Implant (PGI) has been developed as a new, innovative therapeutic procedure. The PGI differs from previous GDD with regard to the smaller size of the drainage tube.

OBJECTIVE

This study analyses 6‑months results of the PGI in terms of effectiveness and safety.

METHODS

A database of patients treated with the PGI at the University Eye Hospital Bonn was created and continuously updated based on follow-up controls. Statistical analysis was performed using SPSS Statistics for Windows (IBM Corp., Armonk, NY, USA).

RESULTS

A total of 53 eyes of the first 51 consecutive patients treated with the PGI were included in this study. Mean intraocular pressure was 26.62 mmHg (7-48 mmHg) preoperatively and reduced to 12.20 mmHg (3-22 mmHg) after 6 months. Local pressure-lowering therapy was reduced from 3.37 agents preoperatively to 0.30 agents after 6 months. The complication rate was low; only 3 patients (5.8%) had persistent hypotony. In 16 patients, the intraluminal prolene stent was removed in the postoperative course after an average of 2.9 months. Thereafter, these patients experienced a reduction of intraocular pressure from 22.21 to 11.07 mmHg.

CONCLUSION

The PAUL® Glaucoma Implant is a safe treatment modality that can successfully reduce intraocular pressure to a low level and reduce pressure-lowering local therapy. It has a low complication rate, particularly regarding postoperative hypotony.

The effect of the hypertensive phase on the long-term outcomes of Ahmed glaucoma valve (AGV) implantation

BACKGROUND

To investigate the long-term effect of hypertensive phase (HP) on the clinical outcomes of Ahmed glaucoma valve (AGV) implantation.

METHOD

The records of patients with different etiologies of glaucoma who underwent AGV implantation with at least 3 years of follow-up were retrospectively reviewed. HP was defined as the IOP > 21 mm Hg during the first three months after surgery. The main outcome measure was cumulative success defined as 5 < IOP ≤ 21 mmHg and 20% reduction from the baseline with or without IOP lowering medications. Results that do not achieve cumulative success or undergo glaucoma reoperation during the follow-up period are considered failures. The secondary outcome measures were intraocular pressure (IOP) and the number of glaucoma medications.

RESULTS

A total of 120 patients (28 patients of HP, 92 patients without HP) with an average age (± SD) of 48.9 ± 19.6 years and a mean follow-up of 4.5 ± 1.4 years were enrolled. The mean duration of survival was 5.3 ± 0.5 years in HP which was significantly shorter than 6.4 ± 0.2 years in non-HP (log rank = 4.2, P = 0.04). Mean IOP and number of IOP lowering agents were higher in postoperative visits at 1,2, 3, and 4 years in HP patients compared with non-HP (all Ps < 0.01). Higher baseline IOP was significantly associated with higher rates of surgical failure.

CONCLUSION

In the long-term follow-up, the duration of survival was significantly longer in the non-HP group. In the non-HP group, the failure rate was significantly lower than the HP group.

Three-Year Outcomes of Pars Plana Ahmed and Baerveldt Glaucoma Implantations for Neovascular Glaucoma in Japanese Eyes

PRCIS

Tube shunt implantation through the pars plana was effective for neovascular glaucoma (NVG) for at least 3 years, with few serious postoperative complications observed.

PURPOSE

The aim was to report 3-year outcomes of pars plana Ahmed and Baerveldt glaucoma implantation for NVG in Japanese eyes.

PATIENTS AND METHODS

This study examined 41 eyes of 39 patients who underwent tube shunt implantation through the pars plana with the Baerveldt glaucoma implant (BGI group, 26 eyes) or Ahmed glaucoma valve (AGV group, 15 eyes) for NVG and who were followed up for over 3 years at Osaka Medical College between January 2009 and April 2016. Outcome measures were intraocular pressure (IOP, mm Hg) at presurgery and at 6 months and 1, 2, and 3 years postoperative. Postoperative failure was defined as an IOP of >21 mm Hg or <5 mm Hg, further glaucoma surgery, or no light perception.

RESULTS

Mean IOPs at presurgery and at 3 years postoperative were 34.8±9.1 and 15.6±4.6 in the AGV group, and 36.9±9.2 and 12.8±5.5 in the BGI group. Mean antiglaucoma medication scores at 3 years postoperative were 1.3±1.4 in the AGV group and 0.4±0.8 in the BGI group (P=0.05). The number of eyes with a probability of failure at 6 months and at 2 and 3 years postoperative was 2, 3, and 4, respectively, in the BGI group, and 0, 1, and 2, respectively, in the AGV group.

CONCLUSION

Findings for NVG cases showed tube shunt implantation through the pars plana was effective. Equivalent good IOP reductions were noted in both groups, with the BGI group requiring fewer postoperative antiglaucoma medications compared with the AGV group. Furthermore, both groups exhibited few serious postoperative complications.

Posterior segment complications of Ahmed valve implantation

BACKGROUND

To assess the time, incidence, and outcome of posterior segment complications of Ahmed valve implantation (AVI).

METHODS

248 eyes that underwent AVI were reviewed retrospectively. Visual acuity, preoperative characteristics, and postoperative posterior segment complications were assessed.

RESULTS

The incidence of posterior segment complications of AVI was 31.4% (78/248). The mean follow-up period was 97.4 ± 53.5 months. The mean time to occur posterior segment complication was 1.5 months. The most common posterior segment complication was choroidal detachment (17.7%) and others included ocular decompression retinopathy (3.2%), hypotonic maculopathy (2.8%), vitreous hemorrhage (2.0%), retinal detachment (1.2%), endophthalmitis (1.2%), suprachoroidal hemorrhage (1.2%), epiretinal membrane (0.8%), cystoid macular edema (0.8%), and proliferative vitreoretinopathy (0.4%). Older age, hypertension, and postoperative hypotony had an increased risk of posterior segment complications of AVI.

CONCLUSIONS

The overall incidence of posterior segment complications of AVI was 31.4%. Older age, hypertension, and postoperative hypotony were significantly associated with posterior segment complications of AVI.

Conventional glaucoma implants and the new MIGS devices: a comprehensive review of current options and future directions

Glaucoma is a progressive optic neuropathy that is the second leading cause of preventable blindness worldwide, after cataract formation. A rise in the intraocular pressure (IOP) is considered to be a major risk factor for glaucoma and is associated with an abnormal increase of resistance to aqueous humour outflow from the anterior chamber. Glaucoma drainage devices have been developed to provide an alternative pathway through which aqueous humour can effectively exit the anterior chamber, thereby reducing IOP. These devices include the traditional aqueous shunts with tube-plate design, as well as more recent implants, such as the trabeculectomy-modifying EX-PRESS^® implant and the new minimally invasive glaucoma surgery (MIGS) devices. In this review, we will describe each implant in detail, focusing on their efficacy in reducing IOP and safety profile. Additionally, a critical and evidence-based comparison between these implants will be provided. Finally, we will propose potential developments that may help to improve the performance of current devices.

Preparation of 5-fluorouracil loaded chitosan microtube via in situ precipitation for glaucoma drainage device application: in vitro and in vivo investigation

The main reason for the failure of glaucoma drainage devices is the chronic inflammatory reaction caused by the poor biocompatibility of biomaterials, which leads to the migration and proliferation of scleral fibroblasts surrounding the devices thus forming the encapsulation that will block the aqueous humor drainage channel. In order to inhibit the reaction of scleral fibroblasts after glaucoma drainage device implantation, this study designed and fabricated a 5-fluorouracil (5-FU) loaded chitosan microtube (CMT) for glaucoma aqueous humor drainage. The CMTs were made by template based adsorption-precipitation method using chitosan with excellent biocompatibility as raw material due to its characteristic of pH dependent solubility. The physical properties of CMTs were investigated. The in vitro and in vivo biocompatibilities were studied as well. The developed drainage device is expected to possess the dual function of reducing intraocular pressure and inhibiting excessive fibrosis after glaucoma drainage device implantation, thereby providing a novel way for the research of glaucoma drainage devices.

Efficacy and Safety of the Susanna Glaucoma Drainage Device After 1 Year of Follow-up

PRECIS

This study found a rate of failure of 13.6% of the new Susanna Glaucoma Drainage Device (SGDD) after 1 year of follow-up. Severe complications occurred in 1 patient (4.5%).

PURPOSE

The purpose of this study was to determine the success and complications rates of the SGDD implant in refractory glaucoma.

PATIENTS AND METHODS

This is a retrospective study. Medical records from all consecutive refractory glaucoma patients (failed trabeculectomies with the use of adjunctive mitomycin) followed in a private clinical practice were included in the study. All patients had undergone SGDD implant surgery from September 2016 to July 2019 were included. All surgeries were done by the same surgeon (R.S.). Primary outcome was surgical failure, defined as intraocular pressure (IOP) >18 mm Hg and/or <20% IOP reduction from baseline, IOP<5 mm Hg, reoperation for glaucoma, need of implant removal or loss of light perception vision. Secondary outcomes included mean IOP, use of supplemental medical therapy, and complications.

RESULTS

In all, 22 eyes were analyzed. Mean patient age was 63±15 years (range: 27 to 87 y). Mean IOP decreased from 23±7 to 11±8 mm Hg (P<0.001, paired t test) at an average of 18±9 months after the tube implant. The mean SD number of glaucoma medications was reduced from 3.3±1 to 1.5±1.2 at the last postoperative visit (P<0.01, paired t test). No early postoperative complications occurred. There was 1 case of late persistent hypotony in a patient with previous endocyclophotocoagulation that was solved with tube ligature with silk 8-0. Failure because of high IOP occurred in 2 cases. There was no extrusion or erosion of the tube and the plate.

CONCLUSION

The SGDD presented a 13.6% failure rate (86.4% success rate) with very few complications, being an efficient and safe alternative for refractory glaucoma.

Comparison of complications after Ahmed versus Baerveldt implant in glaucoma patients: one year follow-up

AIM

To compare surgical results of the Ahmed and Baerveldt implant procedures in glaucoma patients at 1y follow-up at Jakarta Eye Center (JEC) Eye Hospitals.

METHODS

This cohort retrospective study was conducted on glaucoma patients aged ≥18y who had undergone Ahmed and Baerveldt implant surgery. Intraocular pressure (IOP), visual acuity, glaucoma medication, success rate, early and late postoperative complications, and the number of resurgeries were analyzed.

RESULTS

A total of 351 eyes in the Ahmed group and 94 eyes in the Baerveldt group were included in this study. At 1y follow-up, the mean IOP was found to be significantly lower in the Baerveldt group (13±4.47 mm Hg) compared to the Ahmed group (15.02±5.73 mm Hg; P=0.025). Glaucoma medication was required in both the Ahmed and Baerveldt groups (58.92% vs 71.67%). Comparable success rate was found in both groups. The Ahmed group revealed a complete and qualified success of 86.82%, and failure of 13.17%. Similarly, the Baerveldt group showed complete and qualified success in 87.75% and failure in 12.25% cases. In the Ahmed group, 11.97% early complications, 26.06% late complications and 9.97% resurgeries were observed. In comparison, in the Baerveldt group, 23.40% early complications, 30.95% late complications and 11.70% resurgeries were observed.

CONCLUSION

Both groups of glaucoma implants show significant IOP reduction, however, the Baerveldt implant group demonstrates greater IOP reduction with more failure rates and complications than the Ahmed implant group.

Nano-structured glaucoma drainage implant safely and significantly reduces intraocular pressure in rabbits via post-operative outflow modulation

Glaucoma is a leading cause of irreversible vision loss predicted to affect more than 100 million people by 2040. Intraocular pressure (IOP) reduction prevents development of glaucoma and vision loss from glaucoma. Glaucoma surgeries reduce IOP by facilitating aqueous humor outflow through a vent fashioned from the wall of the eye (trabeculectomy) or a glaucoma drainage implant (GDI), but surgeries lose efficacy overtime, and the five-year failure rates for trabeculectomy and tube shunts are 25-45%. The majority of surgical failures occur due to fibrosis around the vent. Alternatively, surgical procedures can shunt aqueous humor too well, leading to hypotony. Electrospinning is an appealing manufacturing platform for GDIs, as it allows for incorporation of biocompatible polymers into nano- or micro-fibers that can be configured into devices of myriad combinations of dimensions and conformations. Here, small-lumen, nano-structured glaucoma shunts were manufactured with or without a degradable inner core designed to modulate aqueous humor outflow to provide immediate IOP reduction, prevent post-operative hypotony, and potentially offer significant, long-term IOP reduction. Nano-structured shunts were durable, leak-proof, and demonstrated biocompatibility and patency in rabbit eyes. Importantly, both designs prevented hypotony and significantly reduced IOP for 27 days in normotensive rabbits, demonstrating potential for clinical utility.

Nanotechnology for Medical and Surgical Glaucoma Therapy-A Review

Glaucoma is the second leading cause of blindness worldwide. Even though significant advances have been made in its management, currently available antiglaucoma therapies suffer from considerable drawbacks. Typically, the success and efficacy of glaucoma medications are undermined by their limited bioavailability to target tissues and the inadequate adherence demonstrated by patients with glaucoma. The latter is due to a gradual decrease in tolerability of lifelong topical therapies and the significant burden to patients of prescribed stepwise antiglaucoma regimens with frequent dosing which impact quality of life. On the other hand, glaucoma surgery is restricted by the inability of antifibrotic agents to efficiently control the wound healing process without causing severe collateral damage and long-term complications. Evolution of the treatment paradigm for patients with glaucoma will ideally include prevention of retinal ganglion cell degeneration by the successful delivery of neurotrophic factors, anti-inflammatory drugs, and gene therapies. Nanotechnology-based treatments may surpass the limitations of currently available glaucoma therapies through optimized targeted drug delivery, increased bioavailability, and controlled release. This review addresses the recent advances in glaucoma treatment strategies employing nanotechnology, including medical and surgical management, neuroregeneration, and neuroprotection.

Comparison of two prototypes of a magnetically adjustable glaucoma implant in rabbits

Glaucoma drainage devices are used in surgical glaucoma therapy. Success of controlling the intraocular pressure is limited due to fibrous implant encapsulation and fibrin coating on the implant which lead to drainage obstructions. An innovative implant with a magnetically adjustable valve was developed. The valve opening of the implant should eliminate inflammatory products from the outflow area and affect fibrous tissue formation to achieve a sufficient long-term aqueous humour outflow. Lifting of this valve should disturb cell adhesion by exerting mechanical forces. Before testing this hypothesis, the flow characteristics of glaucoma drainage devices, especially the outflow resistance by regular IOP, should be considered in a pilot study, as they are important in preventing too low postoperative intraocular pressure known as ocular hypotony. Therefore, two prototypes of the innovative implant differing in their valve area design were examined regarding their flow characteristics in a limited animal experiment lasting two weeks. Ten healthy New Zealand White rabbits were divided into two groups (A & B) with different implanted prototypes. Daily, tonometry and direct ophthalmoscopy were performed to assess the intraocular pressure and the inflammatory reaction of the eye. After two weeks, the rabbits were euthanised to evaluate the initially histological inflammatory reaction to the implant. In group A, one case of hypotony emerged. When considering the entire observation period, a highly statistically significant difference between the intraocular pressure in the operated eye and that in the control eye was detected in group A (p < 0.0001) in contrast to group B (p = 0.0063). The postoperative inflammatory signs decreased within two weeks. Histologically, a typical but low level foreign body reaction with macrophages and lymphocytes as well as mild to moderate fibrosis was seen after the short experimental period. Based on our tonometric results, prototype B seems to be the system of choice for further research assessing its long-term function and biocompatibility.

Towards smart self-clearing glaucoma drainage device

For patients who are unresponsive to pharmacological treatments of glaucoma, an implantable glaucoma drainage devices (GDD) are often used to manage the intraocular pressure. However, the microscale channel that removes excess aqueous humor from the anterior chamber often gets obstructed due to biofouling, which necessitates additional surgical intervention. Here we demonstrate the proof-of-concept for smart self-clearing GDD by integrating magnetic microactuators inside the drainage tube of GDD. The magnetic microactuators can be controlled using externally applied magnetic fields to mechanically clear biofouling-based obstruction, thereby eliminating the need for surgical intervention. In this work, our prototype magnetic microactuators were fabricated using low-cost maskless photolithography to expedite design iteration. The fabricated devices were evaluated for their static and dynamic mechanical responses. Using transient numerical analysis, the fluid-structure interaction of our microactuator inside a microtube was characterized to better understand the amount of shear force generated by the device motion. Finally, the anti-biofouling performance of our device was evaluated using fluorescein isothiocyanate labeled bovine serum albumin. The microactuators were effective in removing proteinaceous film deposited on device surface as well as on the inner surface of the microchannel, which supports our hypothesis that a smart self-clearing GDD may be possible by integrating microfabricated magnetic actuators in chronically implanted microtubes.

Factors related to hypertensive phase after glaucoma drainage device implantation

Background

The aim of the study was to evaluate factors related to hypertensive phase (HP) after glaucoma drainage device (GDD) implantation.

Patients and methods

Retrospective charts review of glaucoma patients who underwent GDD implantation and completed 12 months of follow-up at King Chulalongkorn Memorial Hospital since 2004 was performed. Patients were divided into two groups according to the presence or absence of HP. The data were analyzed for factors associated with HP. The surgical outcomes including surgical failure, post-operative visual acuity (VA), post-operative number of medications, post-operative intraocular pressure (IOP), and post-operative complication were compared between the two groups.

Results

Seventy-two patients were included. Most were diagnosed with secondary glaucoma (N=51, 70.8%), and Baerveldt was the most implanted GDD (N=49, 68.1%). Mean ± SD of IOP was significantly lower: 27.1±9.6 mmHg at pre-operation and 13.7±59 mmHg at 12-month follow-up (p<0.001). HP was identified in 38/72 patients (52.8%, 95% CI 40.7-64.7). Risk factor of HP was pre-operative VA equal or better than 20/70 (p=0.021, OR 7.5; 95% CI 1.4-41.4). Presence of underlying heart disease (patients in this group had taken anti-platelets) was found as a protective factor for HP (p=0.027, OR 0.06; 95% CI 0.00-0.72). It was speculated that the anti-inflammatory effect of the anti-platelets, particularly aspirin, may be responsible for controlling inflammation and retarding fibrosis formation around drainage implants. Failure rate at 12 months was 24/72 (33%). There was no difference in all surgical outcomes between the two groups. Survival analysis also showed no significant difference between HP and non-HP group.

Conclusion

HP commonly occurred after GDD implantation. Pre-operative VA was a risk factor for the development of HP, whereas presence of underlying heart disease was a protective factor. No association between HP and surgical outcomes was identified.

Comparison of Six Different Silicones In Vitro for Application as Glaucoma Drainage Device

Silicones are widely used in medical applications. In ophthalmology, glaucoma drainage devices are utilized if conservative therapies are not applicable or have failed. Long-term success of these devices is limited by failure to control intraocular pressure due to fibrous encapsulation. Therefore, different medical approved silicones were tested in vitro for cell adhesion, cell proliferation and viability of human Sclera (hSF) and human Tenon fibroblasts (hTF). The silicones were analysed also depending on the sample preparation according to the manufacturer's instructions. The surface quality was characterized with environmental scanning electron microscope (ESEM) and water contact angle measurements. All silicones showed homogeneous smooth and hydrophobic surfaces. Cell adhesion was significantly reduced on all silicones compared to the negative control. Proliferation index and cell viability were not influenced much. For development of a new glaucoma drainage device, the silicones Silbione LSR 4330 and Silbione LSR 4350, in this study, with low cell counts for hTF and low proliferation indices for hSF, and silicone Silastic MDX4-4210, with low cell counts for hSF and low proliferation indices for hTF, have shown the best results in vitro. Due to the high cell adhesion shown on Silicone LSR 40, 40,026, this material is unsuitable.

Use of subconjunctival injections of 5-fluorouracil to rescue and prolong intraocular pressure reduction for a failing Ahmed glaucoma implant

PURPOSE

5-Fluorouracil (5-FU) has been well described for a failing trabeculectomy bleb, but not for aqueous shunts. We sought to determine whether subconjunctival 5-FU prolongs the intraocular pressure (IOP) efficacy of Ahmed shunts.

METHODS

We included all patients with Ahmed FP-7 implantation by one surgeon at Yale University. Patients with <3 months follow-up were excluded. Injections were done on a case-by-case basis, usually for IOP > 21 on >2 medications. Five-milligram (0.1 cc) injections were made over the plate. The control group consisted of Ahmed FP-7 patients without injections. The main outcome measure was IOP. Secondary outcome was success (IOP <21 mmHg, 20% decrease from preoperative IOP, and no reoperation).

RESULTS

The average age of controls was 72.5 ± 16.6 years, and 63.7 ± 18.8 with 5-FU (p = 0.02). Forty-four patients received 5-FU and 45 did not. Mean preoperative IOP in controls was 31.5 ± 11 mmHg on 3.1 ± 1 medications, and 31.9 ± 9.0 mmHg (p = 0.86) on 3.3 ± 0.9 medications with 5-FU (p = 0.18). At a mean 137 days after surgery, mean pre-injection IOP was 25.3 ± 7.7 mmHg on 2.0 ± 1.3 medications. Five years following implantation, control IOPs averaged 12.9 ± 7.1 mmHg (53% decrease from preoperative IOP, p < 0.001) on 1.4 ± 1.1 medications versus 17.2 ± 4.9 mmHg (46% decrease from preoperative, 32% decrease from pre-5FU IOP, p < 0.001) on 2.7 ± 0.8 medications with 5-FU. The IOP at 5 years was statistically similar in both groups (p = 0.23). Five-year success rates trended higher with 5-FU (77 vs. 67%, p = 0.38).

CONCLUSIONS

Subconjunctival injection of 5-FU sustained a significant long-term decline in intraocular pressures in eyes with failing Ahmed shunts. Outcomes between eyes receiving injections and controls were statistically similar.

Surgical Outcomes of Additional Ahmed Glaucoma Valve Implantation in Refractory Glaucoma

PURPOSE

To evaluate the surgical outcomes of the implantation of an additional Ahmed glaucoma valve (AGV) into the eyes of patients with refractory glaucoma following previous AGV implantation.

METHODS

This study is a retrospective review of the clinical histories of 23 patients who had undergone a second AGV implantation after a failed initial implantation. Age, sex, prior surgery, glaucoma type, number of medications, intraocular pressure (IOP), visual acuity, and surgical complications were analyzed. Surgical success was defined as IOP maintained below 21 mm Hg, with at least a 20% overall reduction in IOP, regardless of the use of IOP-lowering medications.

RESULTS

Following the implantation of a second AGV, the mean IOP decreased from 39.3 to 18.5 mm Hg (52.9% reduction, P<0.001). The mean number of postoperative IOP-lowering medications administered decreased from 2.8 to 1.7 after the second AGV implantation (P<0.001). The cumulative probability of success for the procedure was 87% after 1 year and 52% after 3 years. Three patients (13.0%) experienced bullous keratopathy after the second AGV implantation. None of the patients showed any evidence of diplopia or ocular movement limitation as a result of the presence of 2 AGVs in the same eye. Prior trabeculectomy was found to be a significant risk factor for failure (P=0.027).

CONCLUSIONS

A second AGV implantation can be a good choice of surgical treatment when the first AGV has failed to control IOP.

Effect of Two Novel Sustained-Release Drug Delivery Systems on Bleb Fibrosis: An In Vivo Glaucoma Drainage Device Study in a Rabbit Model

PURPOSE

To evaluate two drug delivery systems, a nonbiodegradable poly(2-hydroxyethyl methacrylate) (P[HEMA]) system with mitomycin C (MMC) and a biodegradable poly(lactic-co-glycolic acid) (PLGA) system with 5-fluorouracil (5-FU) with and without MMC for their ability to reduce fibrosis when attached to an Ahmed glaucoma valve (AGV) and implanted in a rabbit model.

METHODS

New Zealand albino rabbits (48) were divided into six equal groups, and AGVs, modified as described below, were implanted in the right eye of each rabbit. The groups included (1) PLGA alone; (2) P(HEMA) plus MMC (6.5 μg); (3) PLGA plus 5-FU (0.45 mg); (4) PLGA plus 5-FU (1.35 mg); (5) PLGA plus 5-FU and MMC (0.45 mg and 0.65 μg, respectively); (6) PLGA plus 5-FU and MMC (1.35 mg and 0.65 μg, respectively). The rabbits were followed for 3 months prior to euthanasia.

RESULTS

The bleb wall thickness was significantly less in groups 2, 5, and 6 compared to the rest. At 3 months, the PLGA polymer had completely disappeared, while the P(HEMA) polymer remained intact. There were no statistical differences in the degree of clinically graded conjunctival injection, histologic inflammation, or histologic fibrosis among the six groups.

CONCLUSIONS

We successfully created a sustained-release drug delivery system that decreased the postoperative fibrosis using both a nonbiodegradable P(HEMA) polymer and a biodegradable (PLGA) polymer. Both systems appear to work equally well with no side effects.

TRANSLATIONAL RELEVANCE

These results are supportive of the antifibrotic effect of the slow-release drug delivery system following glaucoma drainage device implantation, thus paving the way for human pilot studies.

Effect of early treatment with aqueous suppressants on Ahmed glaucoma valve implantation outcomes

OBJECTIVE

To evaluate the effect of early aqueous suppressant treatment on Ahmed glaucoma valve (AGV) surgery outcomes.

DESIGN

Randomized clinical trial.

PARTICIPANTS

Ninety-four eyes of 94 patients with refractory glaucoma.

METHODS

After AGV implantation, 47 cases (group 1) received topical timolol-dorzolamide fixed-combination drops twice daily when intraocular pressure (IOP) exceeded 10 mmHg, whereas 47 controls (group 2) received conventional stepwise treatment when IOP exceeded target pressure.

MAIN OUTCOME MEASURES

Main outcome measures included IOP and success rate (6 mmHg < IOP < 15 mmHg and IOP reduction of at least 30% from baseline). Other outcome measures included best-corrected visual acuity, complications, and hypertensive phase frequency.

RESULTS

Groups 1 and 2 were both followed up for a mean of 45±11.6 and 47.2±7.4 weeks, respectively (P = 0.74). Mixed model analysis revealed a significantly greater IOP reduction in group 1 at all intervals (P<0.001). At 1 year, the cases exhibited a significantly higher success rate (63.2% vs. 33.3%; P = 0.008) and reduced hypertensive phase frequency (23.4% vs. 66.0%; P<0.001).

CONCLUSIONS

Early aqueous suppressant treatment may improve AGV implantation outcomes in terms of IOP reduction, success rate, and hypertensive phase frequency.

Evaluation of tissue interactions with mechanical elements of a transscleral drug delivery device

The goal of this work was to evaluate tissue-device interactions due to implantation of a mechanically operated drug delivery system onto the posterior sclera. Two test devices were designed and fabricated to model elements of the drug delivery device-one containing a free-spinning ball bearing and the other encasing two articulating gears. Openings in the base of test devices modeled ports for drug passage from device to sclera. Porous poly(tetrafluoroethylene) (PTFE) membranes were attached to half of the gear devices to minimize tissue ingrowth through these ports. Test devices were sutured onto rabbit eyes for 10 weeks. Tissue-device interactions were evaluated histologically and mechanically after removal to determine effects on device function and changes in surrounding tissue. Test devices were generally well-tolerated during residence in the animal. All devices encouraged fibrous tissue formation between the sclera and the device, fibrous tissue encapsulation and invasion around the device, and inflammation of the conjunctiva. Gear devices encouraged significantly greater inflammation in all cases and a larger rate of tissue ingrowth. PTFE membranes prevented tissue invasion through the covered drug ports, though tissue migrated in through other smaller openings. The torque required to turn the mechanical elements increased over 1000 times for gear devices, but only on the order of 100 times for membrane-covered gear devices and less than 100 times for ball bearing devices. Maintaining a lower device profile, minimizing microscale motion on the eye surface and covering drug ports with a porous membrane may minimize inflammation, decreasing the risk of damage to surrounding tissues and minimizing disruption of device operation.

Corticosteroids versus NSAIDs on intraocular pressure and the hypertensive phase after Ahmed glaucoma valve surgery

PURPOSE

To compare the effect of topical steroids versus nonsteroidal anti-inflammatory drugs on intraocular pressure (IOP) and the hypertensive phase (HP) after Ahmed glaucoma valve surgery.

PATIENTS AND METHODS

Prospective, randomized, double-masked controlled trial. Twenty-eight consecutive consenting patients scheduled for Ahmed glaucoma valve surgery were randomized to receive either postoperative topical dexamethasone or ketorolac. The main outcome measure was IOP. Secondary outcomes included incidence of HP, visual acuity, number of glaucoma medications, postoperative complications, and subsequent procedures.

RESULTS

The mean postoperative IOP (in mm Hg) in the ketorolac versus dexamethasone arms respectively was as follows: 8.8±4.7 versus 10.0±4.5 at week 1 (P=0.500); 10.7±6.7 versus 17.5±10.4 at week 2 (P=0.053); 11.0±6.5 versus 18.0±7.3 at week 4 (P=0.013); 14.8±8.6 versus 17.5±5.2 at week 6 to 8 (P=0.323); and 14.8±9.6 versus 17.8±7.5 at week 10 to 12 (P=0.374). Four patients (31%) in the ketorolac arm versus 8 patients (53%) in the dexamethasone arm exhibited the HP (P=0.276). Wound leak was the most severe complication and there were 3 cases (23%) in the ketorolac group versus nil in the steroid group (P=0.087). Conjunctival retraction was observed in 8 patients (62%) in the ketorolac arm versus 2 patients (13%) in the dexamethasone arm (P=0.016).

CONCLUSIONS

Mean IOP was greater at all time points postoperatively in the steroid group with the difference between groups statistically significant at week 4. The nonsteroidal anti-inflammatory drug group showed greater wound-healing problems.

A new manner of reporting pressure results after glaucoma surgery

Purpose

To evaluate to what extent contemporary glaucoma abstracts offer complete information and to suggest a new manner of pressure results reporting.

Materials, methods, and results

Most of the 36 relevant surgical glaucoma abstracts found in one issue of International Glaucoma Review contain insufficient data-supported statements. Such abstracts cannot offer a clear picture of the study essence if economic, linguistic, or political barriers prevent access to the full text. In order to enrich abstract content and to avoid typographic space waste, a formula is suggested to provide, in one single line of symbols and figures, all the necessary data for statistical interpretation at two evolution moments: the first significative control (6 months) and the final one.

Conclusion

The current manner of results reporting in surgical glaucoma abstracts is subject to too little standardization, allowing insufficiently data-supported statements. Abstracts, especially those printed in small-circulation language journals, should be conceived and standardized in such a manner that any abstract review reader is capable of grasping the essence of the study at first glance. The suggested manner of reporting results would bring satisfaction to all areas of the process. Publishers would save typographic space, readers would find all the necessary data for statistical analysis and comparison with other studies, and authors would be convinced that the essence of their work would penetrate in spite of any economic, linguistic, or political barriers.

Outcome of Ahmed valve implantation when preoperative IOP less than 21 mm Hg

PURPOSE

To examine the results of Ahmed glaucoma drainage device (GDD) implantation in patients with a preoperative intraocular pressure (IOP) of 20 mm Hg or less.

METHODS

Noncomparative retrospective case series.

RESULTS

Sixty-six surgeries in 63 patients were analyzed. Mean follow-up was 51 months. The mean drop in IOP was 3.8 mm Hg (23.3%, P<0.0001) at 12 months and 3.9 mm Hg (24.0%, P<0.0001) at final follow-up. The number of glaucoma medications used postoperatively was significantly less than preoperatively. Although the mean visual acuity was poorer postoperatively this did not reach statistical significance. Surgical success was defined as IOP > or =5 mm Hg and 20% lower than preoperatively with or without hypotensive therapy, visual acuity perception of light or better and no further glaucoma surgery. About 57.6% and 53.0% of patients were considered a success at 12 months and final follow-up, respectively. Fourteen patients (21.2%) underwent additional glaucoma surgery, 8 of which had a second Ahmed GDD inserted whereas 6 had cyclodestruction. All failures were due to inadequate IOP control or further glaucoma surgery, with no cases considered failures due to vision loss or hypotony. Perioperative complications developed in 6 cases (9.1%) whereas long-term problems related to surgery occurred in 8 cases (12.1%).

CONCLUSIONS

In patients with IOPs of 20 mm Hg or less who require IOPs in the low teens and below Ahmed GDD surgery does seem to be an effective option. The success rates of surgery, however, must be balanced against the risk of complications. In addition, the relative merits of Ahmed GDD implantation versus mitomycin trabeculectomy and/or nonvalved GDDs requires further investigation.

Replacement of Ahmed aqueous drainage devices in eyes with device-related complications

PURPOSE

To examine the results of removal and simultaneous replacement of Ahmed aqueous drainage devices (ADDs), which require removal due to complications.

METHODS

Noncomparative retrospective case series of 6 patients.

RESULTS

For the period from January 1999 to December 2007, 325 Ahmed ADD insertions were performed in 272 patients. From this, we identified 6 patients (1.8%) who underwent removal of an ADD for device-associated complications. All had replacement of the Ahmed ADD in a different quadrant at the same surgery. The mean time interval from the original valve insertion was 31.2 months (median, 32.5; range, 3 to 67 mo). The indication for tube removal was chronic uveitis in 3 patients, plate exposure in 2 patients, and tube exposure in 1 patient. We examined the results at 12 months postsimultaneous removal and replacement of the ADD and at final follow-up (median, 25 mo; range, 13 to 52 mo). The preoperative complications resolved in all cases, with the inflammation settling postoperatively in the patients with preoperative uveitis and no patients developing tube or plate exposure at last follow-up. The mean preoperative intraocular pressure (IOP) was 16.0 mm Hg (median, 15.0; range, 9 to 29 mm Hg). At 12 months and final review, the mean IOP was 10.8 mm Hg (median, 10.0; range, 1 to 24 mm Hg) and 11.0 mm Hg (median, 10.0; range, 3 to 24 mm Hg), respectively. The mean number of glaucoma medications preoperatively was 2.8 (median, 3.5; range, 0 to 5). This was reduced to 1.7 (median, 1.5; range, 0 to 4) at 12 months and 2.2 (median, 1.5; range, 0 to 5) at final follow-up. Of the 6 cases in this series, 4 (66.6%) were considered to have adequate IOP control postoperatively. One patient developed postoperative hypotony, with an IOP of 4 mm Hg and reduced vision due to hypotony maculopathy. This patient declined further surgical intervention. One patient had a preoperative IOP of 29 mm Hg on 4 drops and acetazolamide 250 mg b.i.d. At 14 months postsurgery, IOP was 24 mm Hg on 4 drops and the patient underwent insertion of a second ADD. At last follow-up 14 months later, IOP was 10 mm Hg on 4 drops. There were no other significant intraoperative or postoperative complications in this series.

CONCLUSIONS

In patients who require removal of Ahmed ADDs due to complications, removal of the offending ADD and replacement in another quadrant is effective in both resolving the complications and maintaining IOP control.

Second Ahmed valve insertion in the same eye

PURPOSE

To examine the results of same-eye second Ahmed glaucoma drainage device (GDD) insertion in eyes with refractory glaucoma despite previous Ahmed GDD insertion and maximal tolerated medical therapy.

METHODS

Noncomparative retrospective case series.

RESULTS

Twenty-one patients who had undergone 2 GDD surgeries in the same eye were identified. Following chart review, 19 patients had follow-up of at least 1 year and were included in the analysis. All surgeries involved Ahmed valves. The mean drop in intraocular pressure (IOP) at 12 months and at final follow-up was 8 (43%) and 7.9 mm Hg (42%), respectively. The mean number of glaucoma medications used postoperatively (2.4 at 12 mo and 2.6 at final follow-up) was significantly less than preoperatively (4.1). Although the mean visual acuity was poorer postoperatively, this did not reach statistical significance. Sixteen of the 19 patients (84.2%) were defined as complete or partial success, as they achieved IOP of >/=5 and </=21 mm Hg and >/=20% decrease on preoperative IOP with or without glaucoma drops. Three patients were considered complete failures at 12 months and final follow-up. There were no significant intraoperative complications. One patient (5.3%) required anterior chamber reformation on day 1 postoperatively. One patient (5.3%) suffered a decrease in vision from 20/70 preoperatively to counting fingers postoperatively owing to corneal decompensation.

CONCLUSIONS

Second Ahmed GDD surgery seems to be a safe and effective option when IOP remains uncontrolled despite previous GDD implantation.