Wound modulation in glaucoma surgery: The role of anti-scarring agents

Abstract

Filtration surgery is one of the most frequently performed surgeries in the management of glaucoma, and trabeculectomy is considered the gold standard surgical technique for the same. Though trabeculectomy has been reported to have an excellent initial success rate, about 30% of them fail in 3 years, and nearly 50% of them fail in 5 years. The most significant risk of failure still seems to be wound scarring, especially episcleral fibrosis, leading to bleb failure. As a result, it is essential to explore the role of anti-scarring agents, including mitomycin C, and 5-fluorouracil in wound modulation and improving the bleb survival rate. Since these agents are widely used in trabeculectomy, it is crucial to understand the various modes of application, advantages, and adverse effects of these agents. On an evidence-based approach, all these points have been highlighted in this review article. In addition, the newer agents available for wound modulation and their scope for practical application are discussed.

Glaucoma is the leading cause of irreversible blindness in the world. The only modifiable risk factor in the management of glaucoma is the intraocular pressure (IOP) which is often achieved with medical therapy. Surgical treatment for glaucoma is usually considered when desired IOP control is not achieved with medical therapy or as a combined surgery along with cataract. Despite recent innovations in surgical treatment like glaucoma drainage devices or minimally invasive glaucoma surgeries, trabeculectomy is still the gold standard surgical treatment for glaucoma. Here, an alternate pathway is created for the exit of aqueous from the eye. However, this pathway is subject to various wound-healing processes. Though trabeculectomy has been reported to have an excellent initial success rate, about 30% of them fail in 3 years, and nearly 50% fail in 5 years.[1] The most significant risk of failure still seems to be wound scarring, especially episcleral fibrosis, leading to bleb failure. The success of the filtering surgery often depends upon the wound healing response. Surgical trauma results in the activation of the inflammatory cascade. Wound healing occurs through three important overlapping phases: inflammatory, proliferative, and remodeling. The fibroblast is the effector cell and a key determinant of sub-conjunctival or episcleral fibrosis,[2] one of the major causes of surgical failure and inadequate long-term IOP control following filtering surgery. This wound-healing response can be modified by using various anti-scarring agents, both in the intraoperative and postoperative periods. The most commonly employed agents include mitomycin C (MMC), 5-fluorouracil (5-FU), and anti-vascular endothelial growth factors (anti-VEGF) such as bevacizumab. Each of these agents has its own risks and benefits. Careful selection of anti-scarring agents is critical in achieving the delicate balance between excess filtration and scarring that allows for appropriate IOP control.

Methods

A detailed search of databases, including PubMed, Cochrane Library, and Embase was performed. The search was conducted with the following keywords: anti-metabolites, glaucoma surgery, anti-scarring agents, wound modulation, mitomycin C, 5-fluorouracil, bevacizumab, and anti-VEGFs.

Mitomycin C

Mitomycin C is an antibiotic with anti-proliferative and anti-tumor properties obtained from the fungus Streptomyces caespitosus.[3] It is an alkylating agent which is activated by enzymes like cytochrome P450 reductase. It causes cross-linking of DNA molecules between adenine and guanine, thereby arresting deoxyribonucleic acid (DNA) synthesis, inhibiting cell mitosis, and arresting the cell cycle.[4] Mitomycin is cell cycle non-specific but can exert its maximum effect in both the late G₁ as well as the early S-phases.[5]

MMC has been shown to improve the success rates of filtering surgery by inhibiting wound-healing response through its cytotoxic effects on fibroblasts and endothelial cells.[6] MMC was introduced in 1981 by Chen et al.[7] for trabeculectomy in eyes with poor prognosis, but in the early 1990s, its routine intraoperative use increased.[8]

Dosage and routes of administration

MMC is available as a lyophilized powder, which is diluted with a balanced salt solution (BSS) to obtain a concentration of 0.1–0.5 mg/mL. In vitro experiments have shown that MMC’s anti-proliferative effect is maintained for 18 months if refrigerated at 4°C.[9] Intraoperatively, one or more MMC-soaked weck-cel sponges or medical grade polyvinyl alcohol sponges (PVA) are placed on the intact episclera after conjunctival dissection, on the intended surgical area or under the scleral flap (intrascleral), on the scleral bed, before entering the anterior chamber. It is commonly used in the concentration of 0.2–0.4 mg/mL for 1–5 min. The application should be followed by careful removal of the sponges. While placing and removing the sponges, it will be good to maintain a count. Also, care should be taken to avoid the conjunctival edges to enhance conjunctival healing and prevent postoperative wound leaks. The treatment area should be irrigated with 10–40 mL of balanced salt solution (BSS) to avoid toxic complications of MMC. With this intraoperative sponge technique, the amount of MMC delivered at the surgical site can vary widely depending upon the nature, size, and number of the surgical sponges and also on the concentration and duration of MMC.

MMC can also be administered as a subtenon injection in a much lower concentration of 0.05–0.1 mg/mL. Such low concentrations are obtained by diluting 0.2 or 0.4 mg/mL MMC solution with 2% lidocaine. The injection is usually given 8–9 mm posterior to the limbus, a little off to the side to avoid the superior rectus muscle, using a 30-gauge needle. The blister raised is then spread gently around the superior conjunctiva using a muscle hook or surgical sponge.[10,11] Whatever might be the technique of application, one should aim for a broader treatment area to achieve a diffuse bleb.

Clinical evaluation

Trabeculectomy

Intraoperative MMC use has been shown to improve trabeculectomy outcomes. In a 9-year follow-up study of 114 patients with primary open-angle glaucoma (POAG) who underwent trabeculectomy with 0.2 mg/mL MMC or a placebo with BSS, Reibaldi et al.[12] reported improved outcomes with a mean IOP of 13.3 mmHg, a lower rate of further glaucoma surgery, lower rate of visual field progression in the MMC group as compared to the BSS group. Also, there was no difference in the incidence of complications including leakage, cataract, blebitis, and endophthalmitis in the low-dose MMC group compared to the placebo group with BSS.[12] The penetration of MMC in the ocular tissues will increase with either increased concentration or exposure time, especially during the first 3 min. However, prolonged exposure or high concentrations may not be necessary for surgical success. Several studies have shown that a lower MMC concentration of 0.1 or 0.2 mg/mL and a shorter exposure of 1 or 2 min may provide sufficient IOP control with lower complication rates, including hypotony, choroidal effusion, and progression of cataracts.[13,14]

While comparing the episcleral vs. intrascleral application of MMC, El Sayyad et al.[15] reported similar success rates, while Prata et al.[16] reported greater success rates with intrascleral application.

Randomized control trials have been performed to compare the safety and efficacy of injection mitomycin with the traditional technique of applying the sponges soaked in MMC.[17,18,19] The multicentre randomized comparative trial by Pakravan et al.[17] involved 80 patients of POAG where group 1 received a subtenon injection of 0.1 mL of 0.01% (0.1 mg/mL) MMC, while group 2 received 0.02% (0.2 mg/mL) MMC-soaked sponges; 6 months outcome showed that the injection MMC was equally safe and effective as compared to sponge application in terms of IOP control, the incidence of postoperative complications, and endothelial cell count. The 1-year prospective single-center randomized comparative trial by Kandarakis et al.[18] involved a total of 56 eyes of 49 patients with open-angle glaucoma, where the injection group received 0.15 mL of 0.01% (0.1 mg/mL) MMC diluted with preservative-free lidocaine 2% and the sponge group received sponges soaked in 0.02% (0.2 mg/mL) MMC applied under the tenon’s capsule and the scleral flap for 2 min found similar results as that of Pakravan et al. The notable difference was in the bleb morphology, where the bleb with injectable MMC appeared to be low, diffuse, and less vascular compared to MMC sponge application based on the Indiana bleb appearance grading scale. These features are crucial in the long-term functioning of bleb and, in turn, the success of trabeculectomy. Another prospective, randomized study by Maheshwari et al.[19] looked at 1-year results of MMC injection vs. sponges and found that there was an increased incidence of postoperative complications in the MMC sponge group, including choroidal detachment and malignant glaucoma in around 11% of participants and 33% had encapsulated bleb. The bleb morphology was reported to be better in the injection group.

Esfandiari et al.[20] published the 3-year results of a prospective randomized comparative trial comparing the MMC sponge vs. the injection group. However, the success rate and IOP reduction were comparable with both techniques, and bleb morphologic parameters were found to be more favorable after an intra-tenon injection of 0.1 mL of 0.01% (0.1 mg/mL) MMC.

Mitomycin C in other glaucoma surgeries

Intraoperative MMC during tube-shunt surgery has shown mixed results, with some authors reporting beneficial effects[21,22] while others reported no benefits.[23] A randomized trial by Costa et al.[23] evaluated the safety and efficacy of intraoperative MMC (0.5 mg/mL for 5 min) in eyes undergoing Ahmed glaucoma valve (AGV) implantation. Sixty eyes were randomized to receive either MMC or balanced salt solution (BSS). At the end of 18 months, the success rates were not different between the two groups: 62% for MMC and 67% for BSS (P = 0.75). They concluded that MMC does not increase the short or intermediate-term success rates of AGV implantation. Randomized clinical trials have proven the beneficial effects of MMC in combined phacotrabeculectomies.[24,25] The beneficial effects of MMC-augmented deep sclerectomy have been established well through a meta-analysis by Cheng et al.[26]

MMC in minimally invasive glaucoma surgeries (MIGS)

MIGS involves treating the angle from an internal approach and has the benefit of faster postoperative recovery time. The XEN implant (AqueSys, CA, USA) is one of the MIGS devices derived from cross-linked porcine collagen. A one-year prospective interventional study on 13 patients used the XEN-45 implant of 45µ diameter and a length of 6 mm that would provide aqueous filtration of around 2–2.5 mL/min.[27] Intraoperative 0.1 mL MMC 0.01% (0.1 mg/mL) was injected subconjunctivally using a 27-gauge needle under tenon in the superior nasal quadrant where the implant insertion was planned, and it remained for 10 min before the implant was injected. 41.7% of eyes achieved complete success and 66.7% achieved qualified success. Three eyes required needling; two patients had hypotony with choroidal detachment which responded to medical management.[27] This procedure involves an ab interno approach to place the implant, thereby leaving the conjunctiva intact and MMC seems to improve the chances of success and bleb survival rate.

In a prospective study over a period of 2 years, in 81 POAG patients, Beckers et al.[28] compared the success rates of PRESERFLO microshunt implant, between two different concentrations of intraoperative MMC: 0.4 and 0.2 mg/mL MMC, both for 2–3 min. Though the success rates were comparable between the two groups (78% in 0.2 mg/mL and 74% in 0.4 mg/mL, P = 0.7), the reduction of number of antiglaucoma medications was significantly higher in the 0.4 mg/mL group (90% vs. 51% in 0.2 mg/mL, P = 0.001). However, we need to wait for further prospective randomized control trials to understand the role of anti-metabolites in various MIGS.

Complications

Three-year follow-up results of tube vs. trab study found that MMC usage was associated with increased incidence of choroidal effusion, shallow or flat anterior chamber, wound leak, hyphema, aqueous misdirection, and suprachoroidal hemorrhage in the early postoperative period following trabeculectomy with MMC (0.4 mg/mL, for 4 min).[29] Late postoperative complications included persistent corneal edema, encapsulated bleb, delayed bleb leak, choroidal effusion, and hypotony maculopathy.[29] The risk for prolonged hypotony after MMC-augmented trabeculectomy is particularly high in cases with a young age, myopia, chronic use of systemic carbonic anhydrase blockers, and primary trabeculectomy. Thus careful use of anti-metabolites is crucial in primary trabeculectomy in young myopic patients. Moreover, the incidence of blebitis and bleb-related endophthalmitis was found to be much higher following trab with MMC.[29]

MMC administered in trabeculectomy can rarely reach the anterior chamber resulting in serious corneal complications.[30,31] Corneal endothelial cell density can significantly decrease after MMC trabeculectomy in a dose-dependent manner resulting in decompensation of corneas.[30] In addition, corneal thinning, necrotizing keratitis, sclerectasia, and scleromalacia cases have also been reported as complications of MMC trabeculectomy.[31]

5-Fluorouracil

5-Fluorouracil is a fluorated pyrimidine analog. It inhibits the enzyme thymidylate synthetase, and consequently DNA synthesis by selectively acting on the synthesis(S) phase of the cell cycle. It also has disruptive effects on ribonucleic acid (RNA) synthesis. It acts by inhibiting fibroblast growth, as shown in in vitro experiments on human proliferating tenon capsule fibroblasts.[32]

Dosage and routes of administration

5-FU is available in ampoules, in the concentration of 50 mg/mL. It can be applied intraoperatively in the same way as MMC using weck-cel or PVA sponges on the episcleral surface, in a dose of 25–50 mg/mL for 5 min.[33] It is often used in the form of repeated subconjunctival injections in the postoperative period, especially when the bleb shows signs of failure. It is used as 5 mg in 0.1 mL and is injected 180° away from the bleb, with a maximum dosage of 25–50 mg.[33]

Clinical evaluation

The Singapore 5-fluorouracil trial, a retrospective review of a randomized control trial involving 170 patients with primary glaucoma, did not find a significant difference in IOP, failure rates, and the need for antiglaucoma medications between intraoperative 5-FU and placebo groups at the end of 8 years.[34] However, a retrospective analysis of eyes with uncomplicated glaucoma undergoing trabeculectomy as first incisional surgery, receiving postoperative subconjunctival injections of 5-FU up to 14 days post-surgery, found a significantly high success rate of 77.8% at 5 years in these low-risk eyes as compared to no anti-metabolites group which had a success rate of 62.2%.[35] In another study involving high-risk eyes, which had previous incisional surgeries, in the 5-FU group, which received subconjunctival injections, 5 mg twice daily from postoperative day 1 to day 7, followed by once daily injections for the next 7 days, the success rates were 49% at 5 years. This was significantly higher compared to the group randomized to no anti-metabolites, whose success rate was 21%.[36] However, long-term follow-up studies reported continual loss of IOP control over time.[37] Though there is evidence showing that early postoperative injections of 5-FU are beneficial, the exact timing and dosage of the injections are unclear.

Complications

5-FU is considered to be toxic to conjunctival and corneal epithelium. A prospective case–control study by Franks et al.[38] looked at the surgical outcomes, immediate and delayed complications of 5-FU injections in 49 eyes with a mean follow-up of 10 months. The most common early complication encountered in this study was corneal epithelial erosions as invariably there is a leak from the injection tract into the tear film leading to a corneal epithelial deficit. The side effects were found to be more serious in eyes with pre-existing corneal surface disorders.[38] Bleb leak with hypotony was encountered due to inhibition of fibroblast activity and scarring along the conjunctival incision line. The incidence of conjunctival thinning and bleb rupture has been found to be associated with 5-FU as it is toxic to conjunctival epithelium.[38] Delayed complications most commonly include the occurrence of thin-walled cystic bleb and hypotony.[38,39]

Nanotechnology in delivering anti-metabolites

Nanotechnology is revolutionizing the drug delivery system, especially in ocular conditions requiring long-term topical medications. Studies have explored the glaucoma drainage devices that are impregnated with anti-metabolites in in vitro models to reduce postop scarring and chances of failure.

Ponnusamy et al.[40] successfully manufactured a film of polylactic-co-glycolic acid (PLGA) with two layers which were loaded with 5-FU and MMC on the surface of AGV for drug release at a continuous rate. This device was tested for around a month in a cell culture medium. It was designed to release a small dose of MMC in an initial burst which limits the immune cell infiltration in the critical period immediately after surgery followed by a slower release of the less potent 5-FU over longer time intervals to inhibit the fibroblast proliferation.[40]

A biodegradable plug filter was engineered by Maleki et al.[41] to address immediate postoperative hypotony in non-valved glaucoma drainage devices like Molteno and Baerveldt implants. The plug is engineered with polylactic acid (PLA) or PLGA. The biodegradable nature of the implant negates the risk of blockage seen in valved devices. The plug-filter geometry simplifies its integration with commercial shunts. Aqueous humor outflow regulation can be achieved by controlling the diameter of a laser-drilled hole in the plug. In vitro testing was conducted using phosphate-buffered saline in a 37°C silicone oil bath to simulate the aqueous humor and body temperature.[41] All these technologies are still in the experimental phase and we need to wait for animal and human trials to explore their clinical applications.

Anti-vascular endothelial growth factor

VEGF has a vital role in wound healing by inducing angiogenesis and promoting the migration and proliferation of inflammatory cells and fibroblasts.[42,43] Ranibizumab, Pegaptanib, and Bevacizumab are the commonly used anti-VEGF agents. Among these, most studies evaluating the role of anti-VEGF have used bevacizumab, a humanized, monoclonal antibody against VEGF.

Dosage and routes of administration

Bevacizumab is administered intraoperatively either as subconjunctival injections or as intracameral injections. In both techniques, it is administered after the surgery after conjunctival closure and anterior chamber formation. The dose of bevacizumab commonly used is 1.25 mg (0.05 mL of 25 mg/mL solution). The subconjunctival injections are given over the area of the scleral flap with a 30-gauge needle. The needle entry is usually made 7–8 mm away from the flap to avoid persistent leakage in the postoperative period.

Clinical evaluation

In a double-blind, randomized control trial involving 87 eyes of 87 patients with POAG or pseudoexfoliation glaucoma, patients were randomized to receive either intraoperative 0.2 mg/mL of MMC or 1.25 mg of intracameral bevacizumab. Follow-up over a period of 1 year revealed that the results of intracameral bevacizumab were comparable to that of MMC. However, there was a significant early bleb leak in the bevacizumab group.[44] Another randomized study compared subconjunctival bevacizumab with that of MMC 0.2 mg/mL for 3 min in 36 eyes over 7 months and concluded that bevacizumab was effective in controlling IOP, but the effects were less prominent than MMC.[45] A more recent meta-analysis has shown that bevacizumab did not have any advantage over MMC, and it had a higher rate of bleb leaks and encysted blebs when compared to MMC.[46] The available evidence for anti-VEGF use is limited due to the small sample size and short follow-up period.

Beta irradiation as an anti-scarring agent

Beta radiation has been shown to inhibit wound healing and improve survival of glaucoma filtration surgery in vitro and in vivo.[47] Beta radiation causes growth‐arrest, primarily due to its effects on cytochrome P450 which controls the cell cycle.[48] A Cochrane review analyzed four trials that randomized 551 people to trabeculectomy with beta irradiation vs. trabeculectomy alone. It was found that people who had trabeculectomy with beta irradiation were less likely to have eye pressure that was too high, one year after surgery compared to people who had trabeculectomy alone.[49] However, people who had beta irradiation had an increased risk of cataract after trabeculectomy.[49]

Ologen implant

Ologen (OculusGen Biomedical Inc. Taipei, Taiwan) is a tissue engineering product that is a biodegradable, porous, porcine, disc-shaped collagen matrix implant. The efficacy of Ologen has been demonstrated in animal models where it was found to be beneficial in avoiding early scar formation and maintaining long-term IOP control.[50] It was designed to improve the long-term success of trabeculectomy by decreasing subconjunctival scarring.[50] It acts as a 3D collagen-glycosaminoglycan scaffold specifically designed to promote wound healing with minimal scarring.[51] It has been used to create a prominent and healthy vascular bleb following trabeculectomy. Ologen usually gets degraded within 90–180 days after its implantation.[51]

Clinical evaluation

In a prospective randomized pilot study, Senthil et al.[51] included 39 eyes of 33 subjects with medically uncontrolled primary glaucoma, aged 18 years or above, who underwent trabeculectomy either with MMC 0.4 mg/mL for 2 min (20 eyes) or with an ologen implant (19 eyes). At 6 months, the mean IOP reduction was significantly lower in the MMC group compared to the Ologen group, however, at 1 and 2 years follow-up, the IOP reduction was similar in both groups and there was no difference in the incidence of complications. A retrospective analysis of 30 eyes over 7 years following trabeculectomy with Ologen showed 63% success in the first year, reducing to 55% after 5 years of follow-up.[52] A prospective, randomized, comparative study was carried out involving 31 eyes of POAG, which underwent trabeculectomy with the Ologen implant, and another 32 eyes post-trabeculectomy augmented with 0.2 mg/mL of mitomycin C for 2 min.[53] The 5-year follow-up results showed that the complete success rate was much higher in the Ologen group (61.29%) compared with the MMC group (31.25%). The overall success rate was 83.87 and 59.38% in the Ologen and MMC groups, respectively. Ologen has a pore size varying from 20 to 300 µm designed to permit loose, random, nonlinear organization of regenerating fibroblasts and extracellular matrix.[50] This tissue bio-engineering device could be considered a viable alternative for patients with contraindications to the use of anti-metabolites and also in young patients with a high risk of overfiltration, hypotony, and wound scarring.

Bleb needling/bleb revision

Episcleral scarring is the primary cause of filtration failure. Hence bleb needling, either external or internal approach, is often considered before a repeat filter or glaucoma drainage device. Since the risk of failure is high in these eyes with failed trabeculectomies, needling is usually augmented with either MMC or 5-FU. Bleb needling aims at re-establishing the aqueous flow by breaking the fibrous adhesions between the conjunctival and the episcleral tissue or between the scleral flap and its bed. It is preferably done in an operating room; however, it can be done in the slit lamp as well. Under topical anesthesia, a 26-gauge needle bent near the hub mounted on a 2-cc syringe is inserted into the subconjunctival space from a site away from the bleb with the bevel facing upwards. The needle is gently moved toward the prospective bleb area. If there is extensive scarring, the conjunctiva can be ballooned by injecting 2% lignocaine solution before inserting the needle. The tip of the needle is moved in a swiping motion against resistance. Usually, the bleb rises as the adhesions break.

Dosage and route of administration

The dose of 5-FU is 5 mg in 0.1 mL of saline. MMC is used in varying concentrations ranging from 0.02 mg in 0.1 mL of saline to 0.2 mg/mL. These are given as subconjunctival injections before establishing the aqueous humor flow, using a 29 or 30-gauge needle, 7–8 mm away from the bleb, usually in the superotemporal quadrant. It can be done in the operating room or as an outpatient procedure using a slit lamp. After the injection, generally, 10–30 min are allowed to pass before the needling procedure. Alternatively, it can be given after the needling procedure, 180° away from the bleb.

Clinical evaluation

In a study comparing the long-term outcomes for needle revision with 5-FU, 5 mg (0.1 mL of 50 mg/mL) or MMC 0.02 mg (0.1 mL of 0.2 mg/mL) in 98 failed trabeculectomies, MMC appeared more effective than 5-FU at 1 and 2 years without significant difference in the complication rates.[54] Eyes with 5-FU needling were more likely to fail when compared to MMC revision. Additionally, with 5-FU augmented needling, repeated interventions were needed.[54,55,56] Studies have shown that the results of the revisions with bevacizumab 1.0 mg (0.04 mL of 25 mg/mL) and 0.1 mL of MMC (0.4 mg/mL) mixed with 0.1 mL preservative-free lidocaine (1%) are comparable with that of MMC alone at the end of 6 months follow-up period.[57] Most of the trials advocate the injections of anti-fibrotic agents 5–10 min before the bleb needling.[54,55,57] There are no randomized controlled trials comparing the results of needling revisions augmented with various anti-scarring agents. The available evidence is mainly in the form of comparative case series.

Comparison of complications of anti-scarring agents

The complications of 5-FU and MMC are similar, with the exception of epithelial toxicity. It is more common with 5-FU injections than with the intraoperative application of MMC.[58] However, MMC is associated with corneal endothelial cell loss, corneal thinning, necrotizing scleritis, sclerectasia, and scleromalacia.[31] Though the reported rate of complications varies widely, MMC can lead to more severe complications than 5-FU.[2] The complications reported with anti-VEGF include bleb leaks and tenon’s cyst formation.[40,42]

MMC vs. 5-FU vs. anti-VEGF

Though MMC is more effective at inhibiting wound healing and scar formation than 5-FU experimentally, clinical data reveal conflicting results. Most prior studies have reported no difference between MMC and 5-FU in the low-risk group.[59,60] However, in eyes with a high risk of failure, intraoperative MMC is more effective than postoperative 5-FU injections with lower overall IOPs, decreased dependence on postoperative ocular antihypertensive medications, and decreased corneal toxicity.[61,62] In a systematic review of 11 randomized trials involving trabeculectomy in a mixed population of both high and low risk, the authors reported that the risk of failure was slightly lower with MMC compared to 5-FU at 1 year.[63] The complications were found to be less frequent in the MMC group, and there were no differences in the visual outcomes between the groups. Overall, the review was in favor of MMC.[63] The data available on the use of anti-VEGF is limited to either a short follow-up period or small samples. Table 1 summarizes the results of various trials on anti-fibrotic agents.

Table 1.

Summary of major studies on antimetabolites

Reference	Study aim	Design	Study population	Major outcomes
Reibaldi et al.[12]	To evaluate the long-term efficacy and safety of trabeculectomy with low-dosage MMC or placebo in POAG	9 year Prospective Randomized case control study	114 POAG patients	In POAG low-dose MMC with intensified postoperative management improved the outcome of the trabeculectomy with a low incidence of complications
Esfandiari et al.[20]	To report the 3-year outcome of trabeculectomy with MMC soaked sponges versus intra-Tenon injection of MMC in eyes with uncontrolled POAG	Randomized clinical trial	82 patients	Success rate and IOP reduction were comparable with both techniques, bleb morphologic parameters were more favourable after intra-Tenon injection of 0.1 mL of 0.01% MMC
Wong et al.[34]	8-year outcomes of Asian subjects who underwent trabeculectomy augmented by intraoperative 5-FU or placebo	Retrospective review of a randomized controlled trial	243 patients	There was no significant difference in IOP between the 5-FU and the placebo group at 8 years
Singh et al.[52]	To report long-term safety and efficacy of trabeculectomy with collagen implant in Indian population	Retrospective analysis with a minimum 3-year follow-up	30 eyes	Ologen-augmented trabeculectomy is effective in controlling IOP over a long-term follow up from a minimal of 3 to a maximum of 7 years
List of comparative studies
Nilforushan et al.[45]	To compare the outcome of trabeculectomy with subconjunctival bevacizumab with that of trabeculectomy with MMC	Prospective, randomized, comparative study, 1-year outcomes	34 eyes	Adjunctive subconjunctival bevacizumab with trabeculectomy is effective in controlling the IOP profile; however, its effect is less prominent than that of MMC
Yuan et al.[53]	To evaluate the effectiveness and safety of the Ologen as an aid for trabeculectomy performed for primary open-angle glaucoma compared with mitomycin C	Prospective, randomized, parallel assignment, comparative study, 5-year outcomes	31 eyes in Ologen group and 32 eyes in MMC group	Ologen provides higher rates of surgical success compared with mitomycin C for patients with primary open-angle glaucoma undergoing trabeculectomy
Cabourne et al. (Cochrane Library)[63]	To assess the effects of MMC compared to 5-FU as an antimetabolite adjunct in trabeculectomy surgery	Intervention review	11 trials that enrolled 687 eyes of 679 participants, with a minimum 1-year follow-up	Found weak evidence that MMC may be more effective in achieving long-term lower IOP than 5-FU

Choice of anti-fibrotic agent

A careful preoperative assessment of the risk factors for filtration failure is a must.[64] Patient-related factors including age, race, and evaluation of ocular features, especially the conjunctiva, subconjunctival tissues for vascularity, and signs of inflammation, will help us to decide on the choice of anti-fibrotic agent, based on risk factors for failure[64] and complications [Table 2].[65] Khaw et al.[65] categorized patients as low risk, intermediate risk, and high risk and recommended the choice and dose of anti-fibrotic agents based on the risk category [Table 3]. The available evidence and also the European Glaucoma Society (EGS) guidelines support the use of MMC in high-risk eyes.[66] In low or medium-risk eyes, one can consider 5-FU. Furthermore, examination of the fellow eye adds value, particularly if the fellow eye has undergone filtering surgery. The bleb morphology, IOP control, and postoperative complications in the fellow eye should always be borne in mind before deciding on the choice, dose, and duration of the anti-fibrotic agent.

Table 2.

Possible risk factors for anti-fibrotic-related complications[65]

Thin or hyperelastic scleral tissue prone to collapse e.g., Buphthalmos/Myopia/Ehlers-Danlos syndrome (significant risk) Bleb placed in the interpalpebral (10× risk) Inferiorly positioned bleb (10× risk) Elderly patient Primary surgery, Virgin conjunctiva No previous medications Thin conjunctiva or sclera

Table 3.

Risk categorization and appropriate antifibrotic agent for glaucoma filtration[65]

Lower-risk patients (none or intraoperative 5-FU 50 mg/mL)

No risk factors (listed under other categories)

Minimal topical medications

Elderly patients (Age >70)

Intermediate-risk patients (intraoperative 5-FU 50 mg/mL or MMC 0.2 mg/mL)

On long-term topical anti-glaucoma medications

Previous cataract surgery (conjunctival sparing)

Several low-risk factors

Previous surgeries involving conjunctival incision, e.g., squint surgery, retinal detachment, manual small incision cataract surgery

High-risk patients (intraoperative MMC 0.4/0.5 mg/mL)

Chronic persistent uveitis

Previous failed trabeculectomy/tubes

Neovascular glaucoma

Combined trabeculectomy and cataract surgery

Chronic conjunctival inflammation

Multiple risk factors

Aphakic glaucoma

Conclusion

Trabeculectomy largely depends on achieving the fine balance between the required IOP with a well-functioning bleb and the surgery/anti-metabolite-related complications. This balance often defines the success rates of the procedure in the long run.

It is crucial to analyze and select the right anti-scarring agent for each individual based on the associated risk factors. Moreover, wound healing is a dynamic process and the use of anti-metabolites at various stages of trabeculectomy including the peri-operative, immediate postoperative, and late postoperative period will define the outcome of trabeculectomy in terms of functionality and IOP control. It is also essential to keep in mind the associated risk factors of anti-metabolites. Hence it is crucial to analyze the available evidence based on the needs of the patient, assess the risk category under which our patients will fall, and decide on the type of anti-scarring agent, and the mode of application. The choice between MMC and 5-FU is often based on the relative risk of scarring, whether there is a high or medium risk for trabeculectomy failure. Surgeons often do not use antimetabolites for low-risk cases with a virgin conjunctiva. With a medium risk, either 5-FU or a low dose of MMC may be considered. A high dose of MMC is the preferred choice in high-risk category patients.

Financial support and sponsorship:

Nil.

Conflicts of interest:

There are no conflicts of interest.