The Changing Conceptual Basis of Trabeculectomy: A Review of Past and Current Surgical Techniques

Article Outline

• Abstract
• I. Introduction
• II. Aqueous Filtration Through Schlemm’s Canal (No Bleb)
  • A. Cairns’s Trabeculectomy
  • B. Watson and Barnett’s Trabeculectomy (Sclerokeratectomy)
  • C. Viscocanalostomy
  • D. Canaloplasty
  • E. Ab Interno Trabecular Surgery
    • 1. Trabeculotomy
    • 2. Goniocurettage
    • 3. Trabectome
    • 4. Laser Trabeculopuncture
    • 5. iStent
• III. Aqueous Flow Through the Sclera (Bleb Formation)
  • A. Guarded Filtering Surgery
    • 1. Conjunctival Flap
    • 2. Scleral Flap
      • a. Scleral Flap Shape
      • b. Laser Sclerostomy
      • c. Clear Cornea Flap
      • d. Small-incision Trabeculectomy Avoiding Tenon’s Capsule
    • 3. Modifying Wound Healing
      • a. Beta Radiation
      • b. Corticosteroids
      • c. Antimetabolites
      • d. Vascular Endothelial Growth Factor Inhibitors
      • e. Amniotic Membrane
      • f. OloGen Implant
    • 4. Modifying Filtration by Suturing Technique
      • a. Laser Suture Lysis
      • b. Releasable Sutures
      • c. Adjustable Sutures
    • 5. Bleb-associated Complications
      • a. Bleb Leakage
      • b. Bleb Dysesthesia
      • c. Encapsulated Bleb
      • d. Blebitis and Bleb-associated Endophthalmitis
      • e. Cystic Bleb
      • f. Hypotony
  • B. Transscleral Filtration
    • 1. Surgical Techniques
      • a. Sinusotomy
      • b. Ab Externo Trabeculectomy
      • c. Deep Sclerectomy
    • 2. Complications of Non-penetrating Glaucoma Surgery
    • 3. Nd: YAG Goniopuncture in Non-penetrating Glaucoma Surgery
• IV. Summary and Conclusion
• V. Method of Literature Search
• VI. Disclosure
• Outline
• References
• Copyright

Abstract 

The original intent of glaucoma surgery was to allow aqueous humor to exit more easily either through the sclera or into the suprachoroidal space. The former came to be called, generically, a glaucoma filtering procedure. As this surgery evolved, some explored the concept of lowering pressure without producing a hole in the sclera, with its resultant “filtering bleb.” For example, Cairns hoped that cutting open the edges of Schlemm’s canal would allow aqueous to leave without producing a filtering bleb; however, it became apparent that Cairns’s “trabeculectomy” only worked when a filtering bleb developed. The goal of today’s trabeculectomy is the creation of a longlasting transscleral fistula. In fact, trabeculectomy is a misnomer as excision of trabecular meshwork is unimportant. Frequently, the tissue excised to create a trans-scleral fistula is sclera, cornea, or both. The current trabeculectomy is really a guarded sclerokeratectomy. Newer techniques hope to increase aqueous outflow through Schlemm’s canal to avoid complications associated with subconjunctival filtering blebs. Non-penetrating glaucoma surgeries (deep sclerectomy, viscocanalostomy) and ab interno trabecular surgery attempt to lower intraocular pressure with bleb-less procedures. We describe the recent evolution of glaucoma surgery, particularly the idea that intraocular pressure may be lowered satisfactorily without creating a filtering bleb.

Key words: trabeculectomy, nonpenetration of the anterior chamber, antimetabolites, bleb complications, filtration surgery, complications of trabeculectomy, trabecular meshwork, Schlemm’s canal

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I. Introduction 

When introduced almost a half-century ago, trabeculectomy gained widespread acceptance in surgical management of glaucoma because low intraocular pressure (IOP) was achieved with fewer complications than those associated with competing procedures. The search for better results continues today as trabeculectomy itself has been modified to improve success rates and reduce complications. Interestingly, the original intention of pioneers in trabeculectomy was to lower IOP without transscleral filtration and the development of a filtering subconjunctival bleb. However, their efforts yielded the guarded filtration procedure that remains the mainstay of modern glaucoma surgery. Today, surgical innovation has returned to the concept of bleb-less glaucoma surgery, with procedures such as deep sclerectomy and viscocanalostomy.

In his seminal paper, Cairns described the goal of trabeculectomy as excising a short length of the Canal of Schlemm, with its trabecular adnexae, thus leaving two cut ends opening directly into aqueous humor, with no trabecular tissue remaining as a barrier at that point, and restoring the integrity of the corneoscleral coat over the area of the excision.³⁴ Although naming this procedure trabeculectomy was appropriate (because trabeculum was removed to open Schlemm’s canal), the procedure might also have been accurately called canalostomy. Importantly, the objective of trabeculectomy was to lower IOP without inducing external filtration, but in Cairns’s initial cases, inadvertent blebs occurred in one-third of patients. Given the complication rates of full-thickness filtering procedures used at that time, Cairns viewed this result as undesirable. Pathology later showed fibrotic closure of the cut ends of Schlemm’s canal. Additionally, the presence of Schlemm’s canal in the trabeculectomy specimen did not correlate with outcomes.²²⁶ A filtering bleb was present in the majority of successful cases, and the amount of fluorescein-stained aqueous in the bleb correlated with success rates.²²⁶ Therefore, the procedure that started as a “trabeculectomy” actually worked as a guarded filtration procedure, but the nomenclature remained unchanged.

We will not use the word trabeculectomy in the rest of this review unless the procedure being discussed is, in fact, a trabeculectomy. When commenting on the operation typically performed today and commonly called “trabeculectomy” we will use more accurate terminology, such as “sclerokeratectomy,” or whatever actually describes the operative technique.

The aqueous humor leaves the eye through two pathways. The conventional pathway consists of aqueous humor passing through the trabecular meshwork (TM), across the inner wall of Schlemm’s canal into its lumen, and then into draining collector channels, aqueous veins, and finally the episcleral venous system, rejoining the bloodstream from whence it came.¹⁰, ⁸⁹ The nonconventional or uveoscleral pathway may be traced through the uveal meshwork and anterior face of the ciliary muscle, the suprachoroidal space, and out through the sclera. The uveoscleral pathway carries less than 10% of total aqueous outflow.²⁴ Studies on non-glaucomatous eyes have shown that total outflow facility decreases with age. Although Becker¹⁹ showed that in primary open-angle glaucoma patients the magnitude of decrease in outflow facility was similar to that reported for non-glaucomatous patients, Larsson et al^142a concluded that the absolute value of outflow facility in open-angle glaucoma patients was significantly less than in age-matched controls.

The major site of resistance within the TM has not yet been well characterized; in normal eyes, however, the majority of outflow resistance occurs in the juxtacanalicular trabecular meshwork.⁵, ⁷¹, ¹¹⁶, ¹⁵⁵, ¹⁸³ In normal human eyes 75% of the resistance to the aqueous humor outflow is localized in the TM and 25% occurs beyond Schlemm’s canal.⁷⁰, ⁹², ²⁶⁸ Importantly, this is not the case with well-developed glaucoma.⁶⁶, ⁹⁸, ⁹⁹

To this point in this review we have considered the thinking that led those who developed trabeculectomy and the many modifications of trabeculectomy that followed. The goal was to develop a procedure for glaucoma that would be both most effective and least likely to cause complications. Because glaucoma was a condition that was defined by intraocular pressure above a certain level (usually 21 mm Hg), it seemed reasonable to set as a goal for surgery lowering the pressure below that.²⁵⁰ What has since become apparent is that the level of pressure tolerated by different eyes varies widely, some not having progressive optic nerve damage with high intraocular pressures and others rapidly losing vision with low pressures.

Additionally, there are suggestions that fluctuation, as well as the absolute level, of intraocular pressure may play a role in progressive damage to neural tissue.²³⁵, ²⁵¹ It is entirely possible that there are some eyes in which such fluctuation is of importance, and others in which the absolute level is more important.

A changed understanding of glaucoma has led to a changed conceptual framework for glaucoma surgery. With a better knowledge of the precise level of intraocular pressure tolerated by a particular eye, whether or not fluctuation of intraocular pressure is important, and where the site of resistance to aqueous outflow lies for that particular eye, it should become possible to select which of a variety of glaucoma surgical procedures is most appropriate for that particular eye. We detail differences in concept and technique of transscleral or transcanal filtration techniques.

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II. Aqueous Filtration Through Schlemm’s Canal (No Bleb) 

The idea of performing trabeculectomy was based on the concept that outflow resistance resides mainly in the TM and inner portion of Schlemm’s canal.⁹² Sugar²⁴⁸ was the first to perform experimental trabeculectomy with a lamellar scleral flap on eye-bank eyes and then in a living human eye. When performed on a woman with pigmentary glaucoma, the procedure was not satisfactory in controlling IOP, though gonioscopy showed that a portion of the trabeculum had been excised. Sugar²⁴⁸ coined the term trabeculectomy. In his procedure the flap was sutured tightly, with no intentional subconjunctival filtration.

A. Cairns’s Trabeculectomy 

In 1968, Cairns³⁴ reported 17 “trabeculectomy” procedures on eyes with uncontrolled glaucoma. After creating a flap starting in the cornea 2–3 mm anterior to the limbus and hinged on the sclera, a deep-scleral lamella containing Schlemm’s canal and trabeculum was excised and the flap was sutured firmly. A peripheral iridectomy was performed in seven patients. Seven of 17 patients developed a bleb, and histologic evaluation of the excised tissue in 8 patients showed trabecular tissue. Cairns stated that control of IOP for 10–14 weeks without subconjunctival drainage of aqueous humor occurred in about two-thirds of the patients. His explanation for the reduction of IOP was flow of the aqueous through the open ends of Schlemm’s canal. Further studies by Cairns and other investigators led to the conclusion that Cairns’s trabeculectomy was in fact a “guarded filtering procedure”: 1) bleb formation occurred in most successful cases, 2) histological studies on the excised block failed to show TM, and 3) patency of cut ends of Schlemm’s canal was observed in only a minority.³², ³³, ³⁷, ²¹⁹, ²⁴¹, ²⁶⁰

There were some cases in which the cut edges of Schlemm’s canal did remain open. In these cases, compression of superficial scleral veins years after the surgery caused passage of blood through the cut edges of Schlemm’s canal and into the anterior chamber. This finding indicated that at least some of Cairns’s trabeculectomies functioned as he intended.³⁴, ²²⁷

B. Watson and Barnett’s Trabeculectomy (Sclerokeratectomy) 

Watson and Barnett²⁷³ reported the result of their modification of Cairns’s trabeculectomy on 90 eyes of 60 patients who were followed from one to 6 years postoperatively. In this procedure, a 5 × 5 mm scleral flap with two-thirds of scleral thickness hinged anteriorly was made following limbal peritomy. Then, a second full-thickness corneoscleral flap containing the scleral spur and TM was excised under the superficial flap. Peripheral iridectomy was performed in those patients whose iris bulged into the wound. The scleral flap was replaced and sutured securely. Finally, the superficial flap of conjunctiva was closed. In 87% of eyes IOP was controlled without further medications or surgery. In 82 eyes (91%) a bleb formed, but in 8 (8.8%) the IOP was controlled with no evidence of bleb formation.

Since 1975 the Watson and Barnett²⁷³ procedure has been modified in many ways. Despite the evolution of new laser and surgical-filtering procedures, sclerokeratectomy (SK) remains the most commonly performed incisional glaucoma surgical procedure.²²⁰

C. Viscocanalostomy 

In this procedure a deep scleral flap is excised under the fashioned superficial scleral flap, providing access to Schlemm’s canal, and a viscoelastic substance—high-viscosity sodium hyaluronate (Healon GV)—is injected into the two cut ends of Schlemm’s canal, dilating it.¹⁶⁶ Histologic studies show that the dimensions of Schlemm’s canal are enlarged as far as 16 mm downstream from the insertion point of the cannula and visoelastic material injection and that the dilation decreases linearly with the distance from the injection site.²³⁷ Viscoelastic is also injected into the region of excised deep scleral flap, or “scleral lake,” with the intent of preventing healing. No subconjunctival filtering bleb is intentionally produced because the superficial scleral flap is tightly closed. Stegmann et al²⁴⁶ believed that aqueous humor filters through the trabeculo-Descemet’s membrane to the scleral space where it reaches the opened Schlemm’s canal ostia and finally flows into the episcleral veins.²⁴⁶

It is likely that this viscoelastic injection ruptures the inner and outer endothelial walls.²³⁷ The viscoelastic will not remain in the canal long enough to prevent healing of the cut ends of the canal. Hence, the operation may function by allowing the aqueous to bypass the juxtacanalicular trabeculum, the site of main resistance, and to enter Schlemm’s canal through these broken sites.

Stegmann et al reported the results of viscocanalostomy in 214 eyes from 157 black African patients with an average follow-up of 35 months.²⁴⁶ IOP of 22 mm Hg without medication was achieved in 82.7% of eyes. The average pressure reduction was 64%, and blebs formed in only 5%. Sunaric-Megevand et al, in two studies on 67 and 20 eyes, achieved complete success of 59%²⁵² and 55%,²⁴⁴ respectively, at a follow-up period (36 months), comparable with Stegmann et al’s study.²⁴⁶ Recently, Chai et al, in a meta-analysis of 458 eyes in 397 patients, evaluated the efficacy and safety of viscocanalostomy versus SK.³⁸ SK was found to have a lower mean IOP at postoperative months 6, 12, and 24. At postoperative month 24, SK surpassed viscocanalostomy in efficacy, with a mean IOP difference of 3.42 mm Hg (p < 0.0001). Subgroup analysis included mean IOP difference at postoperative 6 months, race, diagnosis, mitomycin C (MMC) exposure intraoperatively, and whether there was further postoperative intervention such as antimetabolite treatment or Nd:YAG laser goniopuncture. Although SK was found to be consistently superior in efficacy in all subgroups, it had significantly more postoperative adverse events (p < 0.008). Relative risk of hypotony, hyphema, shallow anterior chamber, and cataract formation were significantly lower in the viscocanalostomy-treated group, with a relative risk of 0.29 (p = 0.0005), 0.50 (p = 0.008), 0.19 (p = 0.0002), and 0.31 (p = 0.002), respectively.

D. Canaloplasty 

Canaloplasty can be considered a viscocanalostomy procedure with the addition of a 10.0 Prolene suture passed circumferentially through Schlemm’s canal and tied with enough tension to stretch the TM. The intent of canaloplasty is to increase outflow facility (Fig. 1). A microcatheter and ultrasound are required to perform this procedure. The iUltrasound (iScience Interventional, Menlo Park, CA, USA) is an 80-MHz probe that enables high-resolution scanning of the anterior chamber angle to localize Schlemm’s canal before surgery and assess its patency after surgery.¹⁰⁰, ¹²³, ¹⁴⁵ The 250-μm microcatheter, with an illuminated tip (iTrack, iScience Interventional), is passed through Schlemm’s canal.¹²³, ¹⁸¹ The procedure is contraindicated in eyes with angle recession, neovascular glaucoma, chronic angle closure, narrow-angle glaucoma, narrow inlets with plateau iris, and with previous surgery that would prevent 360° catheterization of Schlemm’s canal.⁸⁸

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• Fig. 1 

  A: iTrack passed through Schlemm’s canal 360° with a 10-0 Prolene suture knotted to one end of the probe. B: Tightening a 10-0 Prolene suture passed through Schlemm’s canal in canaloplasty surgery.

Lewis et al¹⁴⁴ published the largest series on canaloplasty. Their multicenter, international, prospective study evaluated canaloplasty’s 2-year postsurgical safety and efficacy. Ninety-four eyes with open-angle glaucoma were treated by canaloplasty or canaloplasty combined with cataract surgery. The mean preoperative IOP was 24.7 ± 4.8 mm Hg and mean number of medications used for treatment was 1.9 ± 1.0. Eyes with canaloplasty alone had a mean postoperative IOP of 16.3 ± 3.7 mm Hg and 0.6 ± 0.8 medications. Eyes with combined canaloplasty–cataract surgery had a mean postoperative IOP of 13.4 ± 4.0 mm Hg and 0.2 ± 0.4 medications. Successful catheterization was achieved in 83 (88%) eyes and a tension suture was placed in 74 (78.7%). In other words, in about 21% the suture could not be placed. The reported complications were hyphema (3 eyes), elevated IOP greater than 30 mm Hg (3), Descemet’s tear (1), hypotony (1), choroidal effusion (1), and exposed closure suture with epiphora and eyelid edema and erythema (1). In four cases the surgery was converted to SK. Although the major aim of this procedure is increasing filtration without a bleb, in 12% a bleb developed at some point during the 12-month follow-up period. Recently, Palmiero et al reported a case of bilateral Descemet’s membrane detachment in a 70-year-old man with primary open-angle glaucoma immediately following canaloplasty surgery in both eyes performed one week apart.¹⁹⁵ Despite the detachment, the cornea remained clear and Descemet’s membrane spontaneously reattached in both eyes within 3 months. The surgery is technically challenging. Although successful canaloplasty results in IOPs in the mid teens, SK has been shown to decrease IOP even lower.¹²³

E. Ab Interno Trabecular Surgery 

Microsurgery on the conventional outflow system for controlling IOP in glaucoma has been evolving over the past few decades. Grant found that the main resistance to outflow is located within the TM.⁹² Further, incising or bypassing the TM and entering Schlemm’s canal may eliminate outflow resistance from the TM. However, Grant’s work was done on normal eyes and whether his findings apply to eyes with glaucoma is controversial.⁹¹ These surgeries increase filtration without transscleral drainage of aqueous into the subconjunctival space. The adequacy of IOP-lowering is uncertain, and data to support these procedures are lacking.

Trabeculotomy is an operation on Schlemm’s canal used mainly for congenital glaucoma. Trabeculotomy, first described by Smith, attempts to remove the resistance to aqueous flow across the TM.²³⁸ In the ab externo technique, a triangular or rectangular scleral flap is fashioned. Underneath the scleral flap, the outer wall of Schlemm’s canal is incised, and a U-shaped probe is inserted into both open ends of Shlemm’s canal and rotated 90° into the anterior chamber. The ab interno technique utilizes the Trabectome, which is detailed in a subsequent section. The success of trabeculotomy in adult glaucoma has been limited, but it can be of distinct benefit in congenital glaucoma as it likely removes resistance at the level of TM and gives aqueous access to the collector channels.⁹⁶ The reported success of this surgery in patients with congenital glaucoma is between 80% and 95%.¹⁶⁰, ²⁰²

In 439 eyes with primary open-angle or pseudoexfoliation glaucoma who underwent trabeculotomy, the success of surgery was greater in the pesudoexfoliative group (73.5%) than in the primary open-angle glaucoma group (58%). Adult glaucomatous patients with mild-to-moderate disk damage who are not good candidates for filtering surgery and who do not need an IOP below the mid teens seem to be reasonable candidates for a trabeculotomy.²⁵⁵ Gillies reported that although trabeculotomy controlled IOP in 11 participants with pseudoexfoliation glaucoma who underwent trabeculotomy, in 5 patients a bleb formed.⁸⁷ Those with both cataract and glaucoma are also reasonable candidates for phacotrabeculotomy.⁸⁸ Postoperative IOP control with phacotrabeculotomy is superior to phacoemulsification alone, but the possibility of early postoperative IOP spike is higher in the phacotrabeculotomy because of hyphema.²⁵⁶ Although these studies describe relatively good outcomes in adult patients, other studies have reported success rates less than 50%.¹⁵², ²¹⁹

This procedure is conceptually similar to goniotomy except that the trabecular tissue is scraped away from the scleral sulcus using a spoon-like instrument.¹¹², ¹¹³ Clear visualization of the chamber angle structures by gonioscopy and a deep and stable anterior chamber are prerequisites. Studies on the donor eyes clearly indicate the potential ability of this procedure to remove trabecular meshwork,¹¹³ but it may also cause damage to intracanalicular septae and splitting along the posterior wall of Schlemm’s canal.

Jacobi et al¹¹¹ performed goniocurattage on 25 eyes of 25 patients. The average preoperative IOP was 34.7 mm Hg with 2.2 medications. Follow-up averaged 32.6 months. Overall success, defined as postoperative intraocular pressure of 19 mm Hg or less with one pressure-reducing agent, was attained in 15 eyes (60%), 5 (20%) of which were controlled without medication and 10 (40%) that still required a single medication. In 10 eyes (40%) the procedure did not meet the criteria of surgical success. Complications included localized Descemet’s membrane detachment in 5 eyes (20%), moderate anterior chamber bleeding in 4 (16%), and sustained anterior chamber angle bleeding in 2 (8%). The average IOP in the 60% of cases that achieved “success” was 17.7 mm Hg. In contrast to the SK, a low teen target IOP cannot be reliably achieved with goniocurettage. Ferrari et al⁷⁶ performed this surgery using a vitreoretinal forceps to excise a quadrant of trabecular meshwork with a peeling-like approach (three clock hours). He called this method ab interno trabeculectomy. These 11 patients with primary open-angle glaucoma or pseudoexfoliation glaucoma underwent phacoemulsification and ab interno trabeculectomy and were followed for 3 years. The average preoperative IOP of 25 mm Hg with 2.4 medications was reduced to 15.3 mm Hg with 0.8 medications. No major complications occurred during the follow-up period; it is not known what percentage of this IOP reduction was from the cataract extraction, however.

Ab interno trabeculectomy has not yet been sufficiently studied to warrant a role in the treatment algorithm for glaucoma. Longer-term follow-up and a greater number of cases are needed before this procedure can be considered for eyes that would be expected to do well with conventional surgery. A variant using the Trabectome has recently gained popularity and is undergoing clinical testing.

There is limited data on goniotomy in adult glaucoma. In a randomized clinical trial on 32 eyes of 32 patients, 16 underwent goniotomy and 16 had MMC SK.²⁰¹ At 24 months, 14 of 16 eyes (87.5%) of the goniotomy group and 13 of the 16 eyes (81.25%) of the SK group showed an IOP ≤ 14 mm Hg. Although goniotomy was reported as a safe and effective procedure, more extended follow-up and larger series are needed to ascertain its role in adult glaucoma.

The Trabectome system (NeoMedix Inc., Tustin, CA, USA) consists of a disposable handpiece connected to a console with irrigation and aspiration as well as an electrocautery generator. The handpiece is advanced nasally across the anterior chamber while a surgical gonioscopy lens is used to visualize the target TM. The tip of the footplate is then inserted through the TM into Schlemm’s canal, and an electro-surgical element ablates and removes the strip of TM and inner wall of Schlemm’s canal (Fig. 2). In typical cases, about 90°–120° of tissue is removed in both directions from one entry site.⁸⁰, ¹⁸⁶ The aim of this procedure is to increase aqueous outflow by removing the area of greatest resistance. In histologic studies no evidence of thermal damage was seen deep in the TM or the surrounding tissues. Transmission electron microscopy of a human cadaver eye demonstrated an intact Schlemm’s canal, collector channel, and intact cells after the TM was ablated by the Trabectome.¹⁸⁶

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• Fig. 2 

  Tip of the Trabectome inside Schlemm’s canal ablating a strip of trabecular meshwork and inner wall of Schlemm’s canal.

Francis et al performed phacoemulsification and the Trabectome operation on 304 eyes.⁷⁹ Thirty-four patients completed 1 year of follow-up. The mean preoperative IOP decreased from 20.0 ± 6.3 to 15.5 ± 2.9 and medications required declined from 2.65 ± 1.13 to 1.44 ± 1.29. The main complication reported was transient hyphema (79–100%).⁷⁹, ¹⁸¹ Mosaed et al found that the IOPs stabilize after several months and remain in the mid teens through 5 years of follow-up of 10 patients.¹⁸¹

In the trabeculopuncture or goniopuncture procedure, a Q-switched neodymium (Nd:YAG) laser is used to create full-thickness holes in the TM. In theory, 20 holes of 10 μm in diameter are adequate to restore normal outflow facility in open-angle glaucoma.⁹⁰ Dietlein et al performed this procedure on 11 eyes of 11 patients who had IOP of 36 mm Hg with 2.8 medications and followed them for 1 year.⁵⁹ The average IOP was 22 mm Hg with 1.5 medications. In another study reported by the same authors, the average IOP dropped from 36.1 mm Hg to 21.3 mm Hg with a limited follow-up of 3 months.⁵⁸ Over time, the relatively small holes close, and the procedure fails.

In excimer laser trabeculostomy (ELT) laser spots are applied in the anterior chamber angle via an endoscopic camera or a goniolens. On average 8–10 laser spots are equally distributed at a distance of 500 μm from one another over the anterior trabeculum. In a study of 26 patients who underwent phaco-ELT, the IOP reduced by 8.79 ± 5.28 mm Hg (–34.70%).²⁶¹ The IOP was reduced from 25.0 ± 1.9 to 17.6 ± 2.2 mm Hg (–29.6%) in a study by Babighian et al on 15 patients with 2 years of follow-up.¹⁵ Although better results were observed in patients with moderately elevated IOP and cataract undergoing the combined procedure, additional data and longer follow-up are needed.

The iStent is an approximately 1.0-mm long L-shaped lightweight titanium stent (Glaukos, Laguna Hills, CA, USA) implanted by an ab interno approach that bypasses the trabecular meshwork to reroute the aqueous from the anterior chamber directly into Schlemm’s canal. The outer surface has three retention arches to ensure secure placement (Fig. 3). In an uncontrolled, multicenter evaluation of 58 patients with open-angle glaucoma (including pseudoexfoliative and pigmentary glaucoma) and cataract, the iStent demonstrated statistically and clinically significant results in reducing IOP and medication use versus cataract surgery alone. The number of medications was reduced from a mean of 1.6 ± 0.8 medicationa to 0.4 ± 0.62 (p < 0.001). The most commonly reported device-related adverse events were the appearance of stent lumen obstruction (7 eyes, 12%) and stent malposition (6 eyes, 10.3%).²⁴³

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• Fig. 3 

  A: The iStent trabecular micro-bypass stent. B: The stent in Schlemm’s canal bypassing the trabeculum.

Fea reported the results of a prospective, double-blind randomized clinical trial on eyes that had phacoemulsification alone (control group) or phacoemulsification with iStent implantation (combined group).⁷⁴ He followed the patients for 15 months and concluded that phacoemulsification with stent implantation was more effective in controlling IOP than phacoemulsification alone. The average preoperative IOP of 17.9 mm Hg in the combined group was reduced to 14.8 mm Hg. In the control group these figures were 17.3 mm Hg and 15.7 mm Hg, respectively. The combined group needed fewer anti-glaucoma medications (p = 0.007).

In both studies of the iStent, the outcomes were compared with a control group who had cataract surgery. The results could have been more valuable if they were compared to SK. Before a realistic appraisal of the efficacy and safety of this device may be determined, additional data are needed.

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III. Aqueous Flow Through the Sclera (Bleb Formation) 

A. Guarded Filtering Surgery 

After it became apparent that the Cairns’s trabeculectomy functioned best in the presence of a subconjunctival filtering bleb, efforts were made to embrace the formation of blebs while minimizing complications. Fronimopoulos et al were the first to introduce the concept of “guarded filtration” by a scleral flap.⁸² This important modification of full-thickness fistulizing operations, which were the dominant procedures before the introduction of trabeculectomy, significantly reduced the risk and complications of postoperative hypotony. Success rates of the SK procedure increased with the introduction of antifibrotic agents, albeit with an increased complication rate. Over the roughly 50 years of experience garnered with the Cairns SK procedure, many modifications have been tried to improve the success rate and to reduce tissue trauma and complications.

One modification is the Moorfields Safe Surgery System.¹¹⁷ The aim of this system is to improve the consistency of the surgery and its outcome, particularly from the patient’s point of view. Features of the Safe Surgery System include using: a corneal traction suture, a fornix-based conjunctival flap, the largest scleral flap possible, a large area of antimetabolite treatment, a small sclerostomy punch, adjustable sutures for the scleral flap, a corneal groove conjunctival closure technique, and continuous intraoperative infusion to prevent intraoperative hypotony and achieve accurate pressure titration.

The Ex-Press shunt (Optonol Inc., Kansas City, KS, USA) is a non-valved stainless steel device implanted through an entry tract into the anterior chamber, parallel to the iris plane under a scleral flap. The placement of an Ex-Press shunt without a scleral flap resulted in frequent extrusions and has been abandoned. When covered by a scleral flap, the procedure is essentially the same as an SK in which the opening into the anterior chamber is made with the Ex-Press shunt rather than a knife.¹⁴², ²¹⁴ The shunt avoids the need to excise sclera, but limits the surgeon’s options as to how much tissue is excised.

Incisions through the conjunctiva made posterior to the limbus are called limbus-based conjunctival flaps (LBCFs) and those where the conjunctiva is incised at the limbus are called fornix-based conjunctival flaps (FBCFs). Initially, the standard conjunctival flap for filtration surgeries was LBCF. In 1977 Mclntyre reported guarded filtration surgery with an FBCF on four patients. The conjunctival incisions were made 2–3 mm superior to the limbus, and dissection was carried directly to the scleral surface. Little posterior episcleral dissection was made beyond that necessary to expose the site of the scleral flap. Mclntyre reported no conjunctival wound leakage and a resulting bleb that was large and diffuse.¹⁵⁹

Further studies showed that FBCF is an easier technique to perform and teach, has a lower incidence of buttonholing, and may reduce the length of the procedure.²³¹, ²⁶², ²⁷⁸ Intraoperative disadvantages of a SK with an FBCF include difficulty cauterizing posterior sources of bleeding and achieving a water-tight closure at the limbus. Prospective studies comparing LBCF versus FBCF SK procedures with or without antimetabolite have shown no statistically significant difference in terms of success rate or IOP reduction.²⁵, ⁶⁷, ²⁰⁵, ²⁶², ²⁷⁵

Comparative studies of early postoperative complications in LBCF and FBCF SK procedures showed early wound leak rates of 11–30% for the FBCF group and 0–11% for the LBCF.²⁵, ⁶⁷, ²³¹, ²⁶² With regard to late postoperative complications, studies on SK without antimetabolite reported no difference in complication rates, but El-Sayaad⁶⁷ et al and Wells et al²⁷⁵ found rates of late-onset bleb leakage and bleb-related infections to be greater in patients with LBCF. Two studies reported a higher risk of cystic bleb formation (39–50% vs 11–20%) and a lower rate of diffuse blebs (22–50% vs 56–80%) with LBCF compared with FBCF trabeculectomy.², ²⁵

Relative advantages and disadvantages of both methods are shown in Table 1. We believe that the type of conjunctival flap used in a procedure should be individualized. For example, the risk of wound leak in patients who are poorly cooperative and who may be more likely to rub their eye postoperatively favors a LBCF. Alternatively, safely performing an LBCF may be precluded by a small interpalpebral fissure.

Table 1. Relative Advantages and Disadvantages of Limbus-based vs. Fornix-based Conjunctival Flaps

a. Scleral Flap Shape 

A number of clinical studies on SK suggest that variations in the shape and size of the scleral flap and internal sclerostomy do not influence the amount of flow through the fistula or long-term IOP control.⁶⁵, ¹²⁷, ²⁴⁵, ²⁷⁰ The use of a triangular or rectangular flap has no clear influence on the final outcome, nor does the size of scleral flap and trabecular block.¹²⁷, ²⁴⁵ SK using a small scleral flap, or “microtrabeculectomy”, is preferred by some surgeons and seems to provide medium- to long-term IOP control comparable to larger-flap techniques.²⁴⁵, ²⁶⁹, ²⁷⁰

b. Laser Sclerostomy 

In an attempt to reduce the amount of tissue dissection and manipulation as well as the amount of inflammation and scarring in the postoperative period, different types of lasers have been used to create the sclerostomy. They are grouped into two broad categories: ab interno and ab externo.

The ab interno laser sclerostomies were performed with a fiberoptic probe passed across the anterior chamber or using a reflected laser beam through a gonioprism. In ab externo sclerostomy a probe was inserted through a 1–2 mm conjunctival incision, and the laser energy was then delivered through the sclera until the anterior chamber was reached. Although the amount of inflammation and duration of surgery was lower than in the guarded filtering surgery, these procedures caused an unacceptably high rate of complications, including the early postoperative complications of full-thickness operations, a sclerostomy site closure by iris, inadvertent conjunctival buttonhole, ciliary body hemorrhage, cataract, and high astigmatism.⁶¹, ¹¹⁰, ²⁴²

c. Clear Cornea Flap 

This procedure was originally described by Philips¹⁹⁸ and Cairns.³¹ Major modification to this technique was introduced by Van Buskirk.²⁶⁷ A trapezoidal flap is dissected posteriorly at approximately one-third corneal thickness in the peripheral cornea. Then the flap is advanced posterior to the limbus using a spatula, and the posterior end is opened into the subconjunctival space. One bite of the anterior lip of the limbal wound is taken by a Kelly punch. Then, two bites of the posterior lip of a 2-mm incision wound created at the junction of the blue limbus and the transparent corneas are taken, thus creating the inner part of the fistula. As the iris spontaneously prolapses, a peripheral iridectomy is performed. The corneal flap is closed by 10.0 nylon sutures. The results for 20 eyes of 18 patients with a minimum follow-up of 3 months showed 85% success (IOP < 18 mm Hg on no medication), with 88% success (7 of 8 eyes) at 1-year follow-up. Eleven eyes received postoperative subconjunctival 5-fluorouracil.²⁶⁷ These results have not yet been duplicated by others.

d. Small-incision Trabeculectomy Avoiding Tenon’s Capsule 

In this procedure, after creating a 2-mm conjunctival peritomy, a half-thickness incision is made between the location where conjunctiva and Tenon’s capsule attach to the sclera. The intrascleral pocket is then dissected posteriorly by a spatula and opened into the sub-Tenon’s space. The initial limbal incision is deepened with a sharp blade in order to enter the anterior chamber, and a block at the floor of the scleral pocket is excised. After performing peripheral iridectomy the scleral incision and conjunctiva are closed.

In a prospective study Das et al⁵³ compared small-incision trabeculectomy avoiding Tenon’s capsule (SIT) with SK in 80 patients with open-angle glaucoma; 40 patients underwent SIT and 40 received SK. Success (IOP < 22 mm Hg without medication plus a reduction of 30% of the initial IOP) was achieved in 80% of the patients undergoing SK and in 90% of the patients with SIT with a minimum 12-month follow-up. No antimetabolites were used in these patients. These authors compared SIT with SK with MMC in another study. Sixty chronic angle-closure glaucoma patients underwent either SIT (n = 30) or SK with MMC (n = 30). Success criteria were the same as their previous report. Success was obtained in 93.3% of the SIT group and in 90% of the SK group at 24 months.⁵² There were no significant differences in the complications between study groups.⁵², ⁵³ Despite the high success rate, there is no other report on the efficacy and safety of this procedure.

Successful filtering surgery is generally characterized by the formation of a functioning filtering bleb—a subconjunctival accumulation of aqueous.

The success rate of glaucoma-filtering surgery has been limited by postoperative scarring. Scarring most commonly occurs at the level of the episclera, leading to flap fibrosis and eventual bleb failure.¹⁴ This scarring and obstruction is most commonly due to the wound-healing process in response to the injury of surgery.¹⁴, ¹⁰⁵ Many attempts have been made to regulate wound healing and improve the success of glaucoma surgery.

a. Beta Radiation 

The earliest reports of radiation as an adjuvant in filtration surgery date from the 1940s.⁴⁶, ¹⁰⁹ Iliff initially reported that beta radiation increased success in 8 of 11 black patients, but later concluded that beta radiation did not substantially alter postoperative fibrosis.¹⁰⁹ In a double-blind, randomized, controlled trial on 320 black patients with primary open-angle glaucoma it was shown that, 1 year after surgery, the risk of surgical failure was 30% in the placebo group, compared with 5% in the radiation group;¹²⁹ however, at the 2-year follow-up, the rate of cataract formation was greater in the radiation group (p = 0.01).

In addition to mixed results on efficacy, beta radiation was reported to induce severe complications including scleral atrophy, ocular irritation, neovascularization, persistent scleral ulcer, ptosis, symblepharon, and iris atrophy.¹²², ¹⁸⁰, ²⁵⁷ This treatment modality is currently not in favor.

b. Corticosteroids 

Corticosteroids have a wide range of effects on different tissues. Their immunomodulatory and anti-inflammatory effects are related to leukocyte concentration, distribution, and vascular permeability. Corticosteroids reduce the leakage of serum and clotting factors leading in decreased formation of clots and fibrin. Corticosteroids also restrict the activity of mitogens and growth factors. Ultimately, use of corticosteroids diminishes of wound healing by downregulation of fibroblastic activity.¹⁴⁰

Corticosteroids have been shown in in vivo models to suppress the local inflammatory response in glaucoma filtration surgery.¹⁷⁷ A randomized clinical trial with 10-year follow-up on 46 eyes of 35 patients compared the effect of corticosteroid and nonsteroidal anti-inflammatory agents after SK and proved the superiority of steroids. The non-steroid-treated eyes needed more glaucoma surgeries and antiglaucoma medicines and had higher IOPs. Glaucoma was stabilized in 82.8% of the steroid-treated versus only 67% of the non-steroid-treated groups.⁸ Corticosteroids however, do not always prevent the scar tissue that causes surgical failure.

c. Antimetabolites 

i. 5-Fluorouracil 

5-Fluorouracil (5-FU), a pyrimidine base analog, has antimetabolic activity. Experimentally, 5-FU has been demonstrated to be an effective inhibitor of fibroblast growth. Khaw et al¹²⁴ showed in vitro and in vivo that a 5-minute exposure to 5-FU results in growth arrest and may have a long-lasting effect on cultured human Tenon’s fibroblasts. 5-FU was the first antiproliferative agent used after glaucoma surgery. The prospective, randomized, multicenter Fluorouracil Filtering Surgery Study demonstrated lower IOP, longer bleb survival, and decreased need for reoperation in recipients of subconjunctival 5-FU after SK at high risk for filtration surgery failure (previous cataract extraction or failed filtering surgery).⁷⁷, ⁷⁸ 5-FU is given as once- or twice-daily injections for 5–14 days postoperatively.¹³², ¹⁴³ When used in higher doses, it may cause corneal epithelial toxicity that may be severe enough to require discontinuance of the drug. This treatment is now less frequently used, especially in children who need sedation at the time of injection. Additionally, subconjunctival hemorrhage, pain, and patient anxiety are significant disadvantages.⁶¹

ii. Mitomycin-C 

MMC is an antineoplastic antibiotic that can interfere with any phase in the cell cycle. MMC is approximately 100 times more potent than 5-FU on a weight-for-weight basis.²⁸⁰ A single application of MMC may be superior to postoperative injections of 5-FU and may be a more effective agent in eyes at high risk for the failure of glaucoma filtration surgery. MMC is usually administered at the time of surgery using a sponge soaked in 0.2–0.5 mg/ml of this agent and placed between the sclera and conjunctival flap for 1–5 minutes.³⁹ After removing the sponge, the exposed area is rinsed with balanced salt solution. A prospective randomized trial comparing intraoperative 5-FU (50 mg/ml for 5 minutes) to MMC (0.4 mg/ml for 2 minutes) in 108 eyes undergoing primary SK found no difference in the success rates, number of medications, visual acuity, and complications at 1 year.²³⁴

Several studies have measured the intraocular concentration of MMC in animals and humans who received MMC during filtering operations.¹²¹, ¹⁷⁶, ²¹⁶ Seah et al determined the concentration in aqueous 2–7 minutes after MMC application in glaucomatous patients who underwent SK with MMC (0.5 mg/ml) where the soaked sponges were placed either on the intact sclera or beneath the scleral flap.²²² Mean concentrations of aqueous were 5 ng/ml in the former and 36 ng/ ml in the latter group. They showed that MMC penetrates into the eye shortly after application and that the intraocular level can be increased by positioning the sponge under the scleral flap. In contrast, Kirchhof et al¹²⁸ placed a sponge containing 20 μg of MMC on the intact sclera for 3 to 5 minutes and found that the concentrations in aqueous withdrawn after 10 minutes were below the limit of detection (10 ng/ml). One study suggested that when the sponge was placed under the dissected scleral flap, the success of the SK was higher than when applyed to the intact sclera.²⁰⁰ The difference between the results of these studies may be a reflection of the amount of MMC applied.

Although MMC is a powerful antiproliferative, no best technique for its application or an optimal dosage has yet been defined. Comparison of results of the studies about the efficacy of MMC in glaucoma surgery are difficult because postoperative hypotony is defined variably, the follow-up is often short, and surgical failures refer mostly to uncontrolled IOP, not visual loss. Additionally, the details of the cases and the total amount of MMC used is not always specified³⁹, ⁴⁷, ⁴⁸, ¹³⁰, ¹⁶¹, ¹⁶⁹, ¹⁷⁴, ¹⁷⁵, ¹⁹⁴, ²³⁶, ²⁸² In the Moorfields Safe Surgery System, Dhingra and Khaw recommended treating a large surface area with MMC at a concentration of 0.2 or 0.5 mg/ml for 3 minutes, or alternatively 5-FU 50 mg/ml.⁵⁷ The antimetabolite was washed out thoroughly with at least 20 ml of balanced salt solution. Some suggest using polyvinyl alcohol instead of methylcellulose sponges because the latter tend to fragment, risking leaving small pieces in the wound.²⁸⁵ The use of cottonoids with attached tails eliminates this problem and also allows safe placement and removal of soaked sponges well under Tenon’s capsule.¹⁶²

iii. Corneal complications of antimetabolites 

One study found greater endothelial cell loss in SK with versus without antiproliferative agents.¹⁹⁷ Another noted no significant difference in endothelial cell loss between 5-FU and MMC trabeculectomies.⁶² Shin et al showed that the percentage of endothelial loss was reduced in cases that underwent SK with MMC and received viscoelastic material in the anterior chamber before exposure to MMC (2.2%) compared with controls who received only balanced salt solution (7.7%).²³⁰

Sauder and Jonas injected 0.1–0.2 mg/ml of MMC subconjunctivally in seven eyes undergoing standard SK at the start of surgery before opening the conjunctiva.²¹⁷ One patient developed corneal thinning, and scleral melting occurred in two others. They concluded that subconjunctival injection of MMC should be avoided.

Superficial punctate keratopathy and conjunctival epithelial defect are the most common early complications of 5-FU use and can be minimized using a long injection track.²⁵⁸ Manche et al reported late-onset non-healing superior corneal epithelial defect 30 months after SK with MMC and six 5-mg injections of subconjunctival 5-FU adjacent to the bleb, in a patient who was finally treated with an autologous limbal stem cell transplantation with favorable outcome.¹⁵⁶

Intracorneal dissection of a drainage bleb, or overhanging or large blebs may cause tear film abnormalities with dellen formation. In the case of overhanging blebs the visual acuity can be affected by bleb-induced astigmatism.¹⁰¹ Additionally, alterations in the visual fields, visual loss, and even monocular diplopia have been described.⁹³

d. Vascular Endothelial Growth Factor Inhibitors 

Wound modulators such as MMC and 5-FU improved postoperative IOP, but increased complications such as bleb leaks and endophthalmitis.²¹, ⁸¹, ⁹⁴, ²³⁹ Alternative antifibrotic agents have thus been sought.

Angiogenesis, the process of new blood-vessel formation, is a key element in the proliferative phase of wound healing, supplying oxygen and nutrients to support the rapid growth of cell-mediated repair. A pathological study demonstrated that the neutralization of vascular endothelial growth factor (VEGF) reduced vascularity and decreased scar formation during wound healing.²⁷⁷ Inflammatory cytokines released after surgical trauma, including interleukin (IL)-1α, IL-1β, IL-6, tumor necrosis factor- α, and prostaglandin E2, and growth factors such as transforming growth factor (TGF)-α, TGF-β, platelet-derived growth factor, epidermal growth factor, fibroblast growth factor 4, or keratinocyte growth factor, have all been reported to induce VEGF expression.²², ²⁸³ The biological effect of VEGF is mediated by VEGF receptors 1 and 2.²⁰⁸ The binding of VEGF to its receptors on the surface of endothelial cells activates intracellular tyrosine kinases, triggering multiple downstream signals that promote angiogenesis. Hence, the wound-healing process is potentiated through both fibroblast activity and angiogenesis.

Bevacizumab is a humanized anti-VEGFA monoclonal antibody that binds to both of the VEGF receptors and downregulates the mitogenic, angiogenic, and permeability-enhancing effects of VEGF-A. Ranibizumab is a Fab (fragment antigen binding) portion of a recombinant humanized IgG1 kappa isotype murine monoclonal antibody that blocks VEGF’s physiologic activities.¹⁰⁶ An anti-VEGF agent could theoretically decrease new vascular growth and potentially lead to a healthier bleb.

A recent study by Memarzadeh et al showed that postoperative subconjunctival injection of bevacizumab was associated with improved SK bleb survival in the rabbit model. They noted that bevacizumab prolonged bleb survival and led to more favorable bleb morphology compared with 5-FU and control groups. Bleb survival was 16 days for the bevacizumab group versus 6.9 and 7.4 days for the 5-FU and control groups, respectively.¹⁶³

Several human series evaluated the efficacy of bevacizumab on SK. Except for one on the effect of bevacizumab on eyes with primary open-angle or angle-closure glaucoma undergoing first-time SK, the others studied neovascular glaucoma.⁷², ¹³¹, ²¹¹, ²⁵³, ²⁸⁴ A nonrandomized, open-label, prospective, interventional case series of 12 patients receiving subconjunctival injection of bevacizumab (1.25 mg/0.05 ml) adjacent to the bleb at time of SK reported encouraging results.⁹⁷ The mean preoperative IOP of 24.4 mm Hg decreased to 11.6 mm Hg at 6 months. The authors concluded that subconjunctival bevacizumab is a potential adjunctive treatment for reducing the incidence of bleb failure after SK. Further studies are needed to better understand how anti-VEGF agents might benefit patients undergoing glaucoma filtration surgery and to determine the proper dose and route of injection as well as the side-effect profile of bevacizumab on the corneal endothelium and TM.

e. Amniotic Membrane 

Amniotic membrane was initially used in the treatment of ocular surface disorders as it promotes epithelialization and acts as an inhibitor of fibrosis.¹²⁶, ²²⁹ Amniotic membrane has been shown to downregulate TGF-β signaling in cultured normal conjunctival and pterygium fibroblasts.²⁶³ Furthermore, such a membrane can exert potent anti-inflammatory effects, including the facilitation of macrophage apoptosis.⁷² These properties suggest the use of amniotic membrane as an anti-inflammatory, anti-scarring, and anti-angiogenic agent.

Fujishima et al⁸³ incorporated a layer of amnion under the scleral flap to try to prevent adhesion between the SK flap and the underlying scleral bed. They carried out surgery on 13 high-risk eyes. All had limbal-based conjunctival flaps with 0.4 mg/ml of MMC applied for 2 minutes, in addition to a 2 × 7-mm flap of amniotic membrane sutured between the scleral flap and sclera. At 24 months follow-up, all had an IOP of <20 mm Hg.

The lack of a control group in this case series makes it difficult to determine if the amniotic membrane had any beneficial effect. After this study, many investigators used amniotic membrane transplantation during SK in both experimental and clinical studies with promising results. Zheng et al²⁸⁶ and Eliezer et al⁶⁹ showed that the success rate of SK with an amniotic membrane transplant is similar to that of SK with MMC and trabeculectomy without antifibrotics, respectively, whereas others noted that IOP is better controlled with SK when combined with amniotic membrane and MMC.²⁶, ⁵⁴, ⁵⁵, ⁶³, ¹⁴⁸, ²⁸¹, ²⁸⁷ The favorable outcomes could be attributed to a synergistic beneficial effect of MMC and an amniotic membrane on controlling fibrosis at the episcleral surface of the SK site, but it remains unclear whether SK with an amniotic membrane transplant and MMC is more effective than SK with MMC or SK without antifibrotics.

f. OloGen Implant 

Ologen, a bioengineered, biodegradable, porous collagen-glycoaminoglycan matrix implant (OloGen; OculusGen Biomedical Inc., Taipei, Taiwan) has been developed as an augmentat to glaucoma surgery. Ologen attempts to limit postoperative scaring.⁴⁰ Ologen reduces conjunctival contraction and promotes formation of an almost normal subconjunctival stroma.¹⁰⁸ In a prospective randomized trial, Rosentreter et al²¹⁰ performed SK with an Ologen implant positioned over the scleral flap in 10 patients with uncontrolled open-angle glaucoma and compared the results with10 controls who underwent SK augmented with MMC. Complete success was defined as an IOP of ≤18 mm Hg and a relative decrease in IOP of ≥20%, without any additional glaucoma surgery or medication. Qualified success was defined as an IOP of ≤18 mm Hg and an additional reduction of ≥20% in IOP with one topical medication. One year after surgery, the complete success rate was 100% in the MMC group and 50% in the Ologen group (p = 0.01). No patient in the MMC group required anti-glaucomatous medication to achieve the target IOP, but five patients in the Ologen group needed a topical therapy (mean number of medications was 0.8 ± 1.1; p = 0.07.). At the last follow-up, filtering blebs developed significantly more avascular areas in the MMC group than in the Ologen group (p < 0.01).

In a study by Papaconstantinou et al 40 eyes underwent SK either with or without an Ologen implant.¹⁹⁶ Six months after surgery, 90% in both groups achieved complete successes, which was defined as IOP less than 21 mm Hg without medications, (p > 0.5). All eyes in the Ologen group and 95% in the control group were considered qualified successes, which was defined as IOP less than 21 mm Hg with medications (p > 0.6). Although complication rates were not statistically significant different between the groups, one eye from the Ologen group developed endophthalmitis, and two eyes required surgery to repair bleb leaks. Based on the available literature, there is no convincing evidence indicating that Ologen offers any significant advantages compared with SK alone or SK augmented with MMC.

Management of the filtering bleb after surgery, especially during the first 3 weeks, is as important as the procedure. Adjustment of aqueous flow through the flap and into the bleb must be balanced to avoid, on one hand, hypotony and wound leaks due to over filtration, and on the other hand, scarring and bleb failure if flow is insufficient. Flap suture manipulation is most effective in the first six postoperative weeks. Timing of the suture removal is predicated on the IOP, the appearance of the bleb, and the degree of filtration desired for the individual patient. Generally, suture manipulation is avoided during the first postoperative week if possible. The ideal time to alter flap sutures is 2–3 weeks after surgery. Flap suture manipulation after the fourth postoperative week is often less effective, though response in antifibrotic-augmented trabeculectomies may be more vigorous and possible later in the postoperative course.⁸⁵, ¹⁵⁴, ²⁷²

a. Laser Suture Lysis 

Graded suture lysis following SK has become an established to titrate filtration. The technique of suture lysis was first performed with a Goldmann goniolens.¹⁴⁷ Multiple instruments have been used for this procedure: the Zeiss four-mirror gonioscopy lens; the especially designed Hoskins, Ritch, Mandelkorn, and Blumenthal lenses; and even test tubes and micropipettes.¹⁴⁷, ¹⁵⁴, ¹⁵⁷, ²⁰⁷ All compress the conjunctiva to bring the suture as close to the surface as possible to facilitate a clear view of the suture and to increase the laser effect. The argon laser settings typically are for a spot size of 100 μm, power of approximately 200 mW, and time of 0.1 second or longer.¹⁵⁴ Settings may vary among physicians and for different lasers. The diode laser can be used for laser suture lysis, especially when hemorrhage is present, to avoid flap perforation by hemoglobin absorption of the argon laser beam.¹⁴⁶ Laser suture lysis complications include excessive filtration, hypotony, and a shallow anterior chamber, aqueous humor misdirection, conjunctival hole, and suprachoroidal hemorrhage.⁸⁵, ¹⁰⁷

b. Releasable Sutures 

Closure of the scleral flap suture using sutures that may be released later with inexpensive and readily available forceps has advantages. The suture may be adjusted in locations with no access to a laser.²⁷² Contrary to laser suture lysis, good visualization through the bleb is not needed. Additionally, the likelihood of creating a conjunctival buttonhole is eliminated.²⁵⁹ Additionally, removal of releasable sutures is often quicker.

A disadvantage is that the externalized suture may act as a wick, seeding the eye with bacteria and causing endophthalmitis.²⁸, ²⁰⁹ Following suture removal all of the complications associated with excessive filtration may occur, in addition to aqueous humor leaking along the suture tract, suture breakage, failure to release the suture, and subconjunctival hemorrhage.¹³³, ²⁵⁹, ²⁶⁵ Releasable sutures also require more surgical time to place than interrupted sutures.

c. Adjustable Sutures 

Pressure on the eye, suture release, or laser suture lysis are the most commonly practiced methods of manipulating IOP in the early days after SK. If the sutures are not loosened the IOP may return to pre-surgery pressure levels within hours. One method of using adjustable sutures involves placing two fixed, 10.0 nylon sutures at the posterior corners of the scleral flap and then 10.0 nylon adjustable sutures that are tightened with a slip knot which is buried under the conjunctiva. Postoperatively, these sutures can be loosened by using a blunt-tipped suture-adjustment forceps.¹², ²⁷⁴

a. Bleb Leakage 

Bleb leaks may occur early in the postoperative period or months to years later. Early postoperative leaks can be secondary to a buttonhole in the conjunctiva during a filtering procedure or a wound leak through the conjunctival incision. Antimetabolites may increase the possibility of early bleb leaks, which were found in 33% of cases supplemented with postoperative 5-FU compared with 20% in a control group.⁷⁸ Late bleb leaks are more commonly seen in avascular and thin blebs. The frequency of the late bleb leak after sclerokeratectomy supplemented with antifibrotics ranges from 1.8% to 10%.²¹, ²³³ Lamping et al¹⁴¹ reported an overall frequency of late bleb leaks after filtering procedures not supplemented with antifibrotic agents of 2.3%.

Histologicaly, blebs after SK with MMC have changes which predispose the bleb to leakage: irregularities in the conjunctival epithelium, breaks in the basement membrane, and conjunctival and subconjunctival hypocellularity.¹⁷³, ²²⁸ Overall, trabeculectomies performed with MMC have higher rates of late-onset bleb leaks than those performed with 5-FU.²¹, ⁹⁴ This may not be due to a difference between MMC and 5-FU, but rather to how they are used.

b. Bleb Dysesthesia 

Filtering blebs usually are asymptomatic or reasonably well tolerated, but some may have late postoperative ocular burning, foreign-body sensation, tearing, and pain. These are the manifestations of “bleb dysesthesia”.²⁷ The amount of discomfort seems to be directly related to the area of the bleb not covered by the lid and to the height of the bleb adjacent to the cornea. Factors that predispose to bleb dysesthesia include young age (which is likely a confounding aspect), superonasal bleb location, poor lid coverage of the bleb, and the distribution of the tear film in the area of the bleb that result in “bubble formation” upon blinking.¹⁶, ²⁷ Modifications that may reduce the likelihood of this complication include using several flat pieces of a surgical sponge when applying the antifibrotic agent; placing the antifibrotic agent diffusely over the sclera, especially well away from the area of the scleral flap (such as along the edges of the superior rectus muscle); performing hydrodissection of the conjunctiva nasally and laterally while rinsing away the antifibrotic agent; doing the SK at 12 o’clock rather in the superonasal or superotemporal quadrant; and, possibly, using a fornix-based rather than limbus-based conjunctival flap.¹⁶

c. Encapsulated Bleb 

Encapsulated blebs are localized, dome-shaped, elevated, tense, with vascular engorgement of the overlying conjunctiva, and a thick connective tissue that tend to occur 2–8 weeks after SK. Such cysts are associated with an increase in IOP after an initial period of pressure control. Risk factors for bleb encapsulation include prior argon laser trabeculoplasty, prior surgery involving the conjunctiva, male sex, glove powder, topical steroids, and prior treatment with sympathomimetics.⁷⁵, ¹⁸⁹, ²⁰⁶, ²²⁵, ²⁶⁶

The frequency of encapsulated blebs in trabeculectomies without antimetabolites ranges from 8.3% to 28%.⁷⁵, ¹⁹⁰, ¹⁹², ²²¹, ²²⁵ The higher and lower rates have been reported in trabeculectomies with 5-FU.¹⁸⁹, ¹⁹⁰, ¹⁹¹, ²³⁶ Azuara-Blanco et al reported a low incidence of encapsulated blebs (2.47%) in a large series (283 patients) of sclerokeratectomies supplemented with MMC.¹³ In contrast, Campagna et al found a higher incidence of 29% in 100 eyes after SK with MMC.³⁵ The causes of encapsulation are not clearly identified, but inflammatory mediators and collagen-producing fibroblasts are probably involved.¹⁴, ¹⁸⁹

d. Blebitis and Bleb-associated Endophthalmitis 

A bleb-related infection is classified as blebitis when the infection is limited to the bleb and there is no anterior chamber reaction. When there is an associated anterior chamber reaction, the eye may be in the early stages of endophthalmitis. When there is posterior segment inflammation, then it is clear that endophthalmitis is present.²⁴⁰

Risk factors include inferior and nasal blebs, thin-walled blebs (especially those seen after trabeculectomies with antifibrotics), trauma, contact lens wear, nasolacrimal duct obstruction, releasable sutures, young age, and bleb leakage.¹⁷, ²⁰, ²⁸, ⁸⁶, ¹¹⁴, ²³² In a case-control study Soltau et al reported that eyes with bleb-related infections were 26 times as likely to have a bleb leak detected at the time of infection.²⁴⁰ Late-onset bleb leaks are more common after SK associated with 5-FU use than without it.¹⁰¹ MMC use is associated with even more frequent late-onset focal bleb leaks than the use of 5-FU.⁹⁴ The incidence of bleb-related endophthalmitis in non-augmented SK ranges from 0.2% to 1.5%.⁸¹, ¹²⁰, ¹⁷⁸, ¹⁷⁹, ¹⁹³, ²³⁹ With intraoperative 5-FU and MMC, this incidence increases to 1.0–5.7% and 0.3–4.9%, respectively.¹⁸, ⁸⁴, ⁸⁶, ⁹⁵, ¹⁰⁴, ¹⁷⁹, ²³⁹

The overall incidence of late endophthalmitis after SK depends on the time of follow-up. The reported prevalence of acute postoperative endophthalmitis after any type of intraocular surgery is 0.093%, whereas the reported incidence of late-postoperative bleb-related infections after SK ranges from 0.4% to 6.9%.¹, ¹⁹⁹

e. Cystic Bleb 

The desired morphology after SK is a diffuse, posteriorly extending bleb with no increased vascularity. This bleb is in contrast to the thin-walled, cystic bleb, which is often located anteriorly. The thin avascular bleb type is thought to be more susceptible to trauma secondary to blinking, bleb leaks, hypotony, and bleb-associated infection.¹¹, ¹⁰³, ¹²⁰ In those patients who have localized MMC application the possibility of a thin avascular bleb with a line of scar surrounding the bleb, “ring of steel”, is higher. Treating the largest possible area with MMC at a concentration of 0.2 or 0.5 mg/ml, or alternatively 5-FU 50 mg/ml, lessens the formation of this scar line.⁵⁷

Previous investigators have studied the effect on the morphology of the drainage bleb of varying the conjunctival flap type and the antimetabolite treated area.⁶⁷, ²⁴⁷ Some have concluded that little difference is present between limbus- and fornix-based flaps and some that limbus-based flaps are more likely to become cystic and avascular.²⁵, ²⁷⁵ It has not been shown that this is related to the type of conjunctival flap rather than the method of applying MMC, however. The incidence of cystic bleb-related complications after SK with MMC has been found to be as high as 25%.³, ¹⁸, ²⁵, ⁶⁰, ¹⁵⁸, ²³², ²⁴⁷

f. Hypotony 

The two possible causes for hypotony after SK are decreased aqueous production and overfiltration. Aqueous undersecretion may be caused by ciliochoroidal detachment, toxic effect of antifibrotics on the ciliary body, or inflammation. Overfiltration occurs when too much aqueous passes through the sclera or into the suprachoroidal space.¹⁴, ²⁹, ²¹⁸

It is believed that the use of antifibrotic agents leads to a greater rate of late-onset bleb leaks.⁹⁴ Chronic hypotony, that which persists for at least 3 months, may be associated with hypotony maculopathy and loss of visual acuity. Furthermore, a late-onset bleb leak is a significant risk factor for bleb infection.¹⁵³ With the use of 5-FU and MMC, the incidence of hypotony maculopathy is reported to be 3–24% in primary filtering surgeries and 5% in complex filtering surgeries.²⁰³ Predisposing factors for the development of hypotony maculopathy are young age, white race, and myopia.¹⁴

Hypotony and its complications may be decreased by good scleral flap closure. Hypotony is also related to dose, positioning, and duration of antimetabolite exposure. Excessive filtration at any point in time may cause hypotony, so caution regarding cutting or releasing sutures is proper. Additionally, the scleral flap should be sufficiently large to cover the sclerostomy, because the main function of the scleral flap is to provide resistance to aqueous outflow to prevent hypotony.²⁸⁵

B. Transscleral Filtration 

Non-penetrating glaucoma surgery (NPGS) is a generic term for a group of guarded filtration techniques that have a common feature, not penetrating the globe and leaving in situ at least the internal TM. These surgeries revive the concept of the aqueous exiting through the cut edge or wall of Schlemm’s canal (the hypothesis of Cairns’s trabeculectomy³⁴). In contrast to SK, these surgeries are contraindicated in angle closure, and angle recession, and in patients who have undergone prior argon laser trabeculoplasty.¹⁶⁶ When filtration through the trabeculo-Descemet’s membrane is considered to be insufficient because of elevated IOP, Nd:Yag goniopuncture can be performed. Laser goniopuncture is required postoperatively in 10–71% of cases,¹⁰², ¹⁶⁶ in which case the procedure cannot be regarded as non-penetrating surgery.

In a study by Mielke et al¹⁷² laser goniopucture was not performed, and the success obtained was 24% in deep sclerectomy without MMC and 13% in deep sclerectomy plus MMC. Deep sclerectomy did not control IOP in the majority of these patients, despite use of MMC.

a. Sinusotomy 

Kraznov believed that aqueous outflow resistance in patients with glaucoma was located at the level of aqueous drainage veins. In order to bypass the resistance area, he removed a lamellar band of sclera and opened Schlemm’s canal over 120° (10 to 2 o’clock), a procedure he called sinusotomy.¹³⁶, ¹³⁷ Schlemm’s canal is unroofed with no superficial scleral flap to cover the sclerectomy, whereas the inner wall of Schlemm’s canal is untouched. Although Kraznov reported a success rate of 83%, he provided no data regarding criteria, duration of follow-up, or the number of patients.¹³⁶, ¹³⁸, ¹⁸⁴ Sinusotomy never became popular.¹⁶⁷

b. Ab Externo Trabeculectomy 

This surgical technique is similar to sinusotomy except for the presence of a superficial scleral flap and excision of the inner wall of Schlemm’s canal and the juxtacanalicular trabeculum that are regarded as the main sites of outflow resistance at least in normal eyes.¹⁶⁷ Zimmerman et al reported good results with this procedure, but they abandoned it because of surgical difficulties.²⁸⁸, ²⁸⁹

c. Deep Sclerectomy 

In this procedure, after fashioning the superficial scleral flap a second one is dissected to the clear cornea. Schlemm’s canal is unroofed and the anterior trabecular meshwork and Descemet’s membrane are exposed. The juxtacanalicular trabeculum and Schlemm’s canal endothelium are then removed using a small blunt forceps. Deep sclerokeratectomy seems a more accurate name for this procedure (Fig. 4). To avoid the secondary collapse of the superficial flap over the trabeculo-Descemet’s membrane and the remaining scleral layer, a space-maintainer implant is sometimes placed in the scleral bed.¹⁶⁶, ¹⁶⁷, ¹⁷⁰

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• Fig. 4 

  A: Excised narrower deep scleral flap under the superficial scleral flap in deep sclerectomy. B: Ultrasound biomicroscopic image showing intrascleral aqueous lake in a patient with deep sclerectomy

(images courtesy of Dr. Shaarawy).

Although the main source of aqueous flow is through the trabeculo-Descemet’s membrane,⁷³ there are other proposed mechanisms of aqueous resorption following deep sclerectomy. Fluid may filter to the subconjunctival or subchoroidal space as well as through Schlemm’s canal. A subconjunctival filtering bleb appears in 50%. Subchoroidal outflow may occur by passage of aqueous through the scleral bed remaining after deep sclerectomy. Additionally, aqueous may pass through the Schlemm’s canal ostium from the scleral space, as evidenced by the formation of aqueous drainage veins in the scleral space months later.¹¹⁵, ¹⁶⁶, ¹⁶⁷, ¹⁷⁰

Although the implants used with some types of deep sclerokeratectomy add significantly to the cost of surgery, IOP control may be better. In one randomized, prospective study comparing deep sclerectomy with and without a collagen implant, complete success (IOP < 21 mm Hg without medication) was achieved in 38.5% at 48 months in the group without implants compared with 69.2% in the implant group. The average IOP at 48 months were 10.0 and 16.0 in the with and without implant groups, respectively (p < 0.005).²²⁴

Some prospective randomized studies comparing the deep sclerectomy and SK found a statistically lower IOP in the SK groups and significantly fewer complications in the deep sclerectomy groups.⁴³, ⁶⁸ Others have reported comparable success rates, and the small differences reported in other studies in IOP control between deep sclerectomy and SK have not always been confirmed.⁶, ⁴⁴, ⁴⁵, ¹⁷¹

Deep sclerectomy is indeed a filtering surgery, and it comes as no surprise that antiproliferative agents improve its success. One prospective randomized study evaluated the effectiveness of deep sclerectomy with and without MMC (0.2 mg/ml/2.5 min). After 36 months of follow-up, the MMC group had a greater reduction in IOP (11.7 mm Hg or 42.3% compared with 7.1 mm Hg or 27.6%, p < 0.05) and increased surgical success.¹³⁵Although others have reported similar results,⁷, ¹⁸⁵ in a randomized prospective study with 18 months of follow-up, success was low and MMC was not did not improve outcomes (24% success without MMC compared to 13% with MMC, p = 0.5).¹⁷²

In a meta-analysis of the tolerability and efficacy of MMC-augmented filtering surgeries, it was shown that deep sclerectomy with MMC was as effective as SK with MMC in lowering IOP. A significantly lower proportion of patients, however, reached the target IOP with deep sclerectomy augmented with MMC than with SK plus MMC. Deep sclerectomy plus MMC was better tolerated and was associated with a significantly lower frequency of shallow anterior chambers and cataracts than SK with MMC, with pooled relative risks of 0.31 (0.16–0.60) and 0.23 (0.11–0.47), respectively.⁴²

In a recent meta-analysis including 17 randomized controlled trials, deep sclerectomy and viscocanalostomy were significantly less effective than SK. The complete success rate risk differences were –0.16 (95% confidence interval, –0.30 to –0.02) and –0.10 (95% confidence interval, –0.19 to 0.00). NPGSs were associated with fewer complications compared to SK.⁴¹

NPGS offers a lower rate of complications when compared to conventional SK, with or without antimetabolites.⁶, ³⁰, ³⁶, ³⁸, ⁴³, ⁴⁴, ⁴⁹, ¹¹⁸, ¹³⁹, ¹⁵⁰, ¹⁷¹, ²¹⁵ Perforation of the trabeculo-Descemet’s membrane seems to be the most common intraoperative complication,¹¹⁹, ¹⁶⁶ occurring in about 30% of the first 10–20 cases. After the initial learning phase, the perforation rate drops to around 2–3%.¹¹⁹, ²¹²

The reported frequency of late hypotony in NPGS is 1.2%,²¹³ less than the SK plus MMC rate of 3–24%.²⁰³ Dahan et al reported transient macular edema related to hypotony in 3.5% of patients who had NPGS.⁵⁰ Mendrinos et al states that no case of flat anterior chamber after NPGS has been reported, but lack of reported case does not mean this cannot occur.¹⁶⁴ Further, IOP on the first postoperative day is usually around 5 mm Hg, lower than following SK.

Descemet’s membrane detachment is a rare complication after NPGS, occurring in about one out of 250 to 300 operated eyes.¹⁸⁸, ²⁰⁴, ²⁶⁴ In viscocanalostomy, this probably happens when the viscoelastic material is injected into the artificial ostia of Schlemm’s canal with the cannula slightly misdirected. In deep sclerectomy the reason may be the passage of aqueous humor from the scleral space to the sub-Descemet space secondary to an increased pressure in the bleb after trauma, encysted bleb, and vigorous ocular massage. Hemorrhagic Descemet’s membrane detachments,¹³⁴, ²⁷⁹ intracorneal hematoma,¹⁸⁷ and intracorneal inclusion of high molecular weight sodium hyaluronate¹⁴⁹ are other corneal complications of NPGS.

Implant exposure, though not common, may occur in those patients who undergo NPGS.⁵¹, ¹⁵¹, ²⁷⁶ Although the majority of reports showed significantly lower incidence of postoperative cataract formation after visocanalostomy when compared to SK,¹²⁶, ¹⁹⁷ Shaarawy et al²²³ reported the progression of existing senile cataracts in 21.5% of eyes of 105 patients over 64 months.

Bleb fibrosis is slightly more frequent after NPGS than after SK.¹⁶⁶ Shaarawy et al²²³ reported bleb fibrosis needing bleb needling and 5-FU injection in 25 out of the 105 patients (23%) who had deep sclerectomy with collagen implant, comparable to the study by Karlen et al.¹¹⁹

Although in NPGS the anterior chamber is not opened, endothelial cell loss occurs. A prospective comparative study between SK and deep sclerectomy showed lesser cell loss (2.6%) in the deep sclerectomy compared to 7% in the SK (p = 0.02). Endothelial cell loss increased significantly over the course of the study at 12 months, with more loss in the SK group (p = 0.04).⁹

No cases of endophthalmitis after NPGS have been reported so far. This may be a reflection of the relatively short follow-up and smaller body of data available. In addition, there have been no case series looking specifically at postoperative bleb-related infections. Only three isolated case reports of infections have been published: fungal keratitis,²⁵⁴ bacterial keratitis,¹⁶⁵, ²⁷¹ and blebitis.²⁷¹ It is suggested that the trabeculo-Descemet’s membrane provides a physical barrier to microbial agents, similar to that conferred by the posterior lens capsule.²³ There have been no studies evaluating the permeability of the trabeculo-Decemet’s membrane to pathogens, however. In order to determine if the frequency of infection after NPGS is less or more than after penetrating surgeries, randomized trials with sufficiently long follow-up periods are needed.

An insufficient surgical dissection or fibrosis of the trabeculo-Descemet’s membrane can be the reason for elevated IOP. Mermoud et al performed goniopuncture in 41 of 100 patients who underwent deep sclerectomy with collagen implants.¹⁶⁸ The long-term (2-year) success rate was 68%. Hamel et al performed goniopuncture in 15 (71.4%) of their 21 highly myopic patients.¹⁰² The average IOP before goniopuncture was 20.3 mm Hg; after goniopuncture, it was 11.5 mm Hg. Twelve (36%) of 33 eyes of 33 patients in a study by Alp et al⁴ needed goniopuncture. Success rates of laser-applied eyes at the last visit were 75% and 33%, with and without medication, respectively (p = 0.568). Intraoperative macro/microperforations had not occurred in any of those who needed laser goniopuncture, but did in one-third of those who did not require laser. Drusedau et al⁶⁴ reported the rate of perforations during viscocanalostomy as 30%, and the IOP decrease was greater in these eyes. However, O’Brart et al¹⁸⁷ reported that 48% had findings suspicious for microperforation, loss of convexity, and increased percolation, but that the success rate in these eyes was no different from the non-perforated ones. While long-term results are evaluated, laser goniopuncture appears to be required in about half of the cases after NPGS, and the success rate is reportedly higher in the lasered group.

The reported complications of goniopuncture include choroidal detachment, iris synechia, and iris prolapse.⁷, ⁵⁶, ¹²⁵, ¹⁶⁸, ¹⁸² Such complications raise the question whether these procedures are actually non-penetrating. By opening the trabeculo-Descemet’s membrane, goniopuncture converts a non-penetrating filtration procedure into a penetrating one. Nevertheless, the combined complication rates of NPGS and goniopuncture are significantly lower than the complication rates associated with certain types of SK.

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IV. Summary and Conclusion 

Sugar²⁴⁸, ²⁴⁹ and Cairns³⁴ devised a procedure to facilitate egress of aqueous through Schlemm’s canal by excising a fragment of the TM, determined experimentally to be the site of greatest outflow resistance. We now known that, in open-angle glaucoma, pathology may also exist more distal to the trabecular meshwork, especially in chronic forms.⁹⁸, ⁹⁹ The major reason for successful IOP reduction in Sugar’s and Cairns’s series was actually aqueous draining to the subconjunctival space. Modern trabeculectomy is a type of guarded filtration operation in which a sclerectomy, keratectomy, or sclerokeratectomy is created under a partial-thickness scleral flap. The procedure now almost never involves excising the TM.

We have attempted to clarify the nomenclature and concepts underlying a variety of surgical techniques currently used in treatment of glaucoma. At present, there is great interest in lowering IOP by increasing “filtration” without producing a bleb. Conceptually, this represents a return to Sugar’s and Cairns’s theory for the design of their trabeculectomy procedure. Although the longevity of trabeculectomy ultimately resulted from a mechanism unintended by its creators, new surgical techniques in glaucoma may demonstrate that their original concept remains worthwhile.

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V. Method of Literature Search 

In order to prepare this review we conducted a Medline and PubMed search of the literature for the period between 1960 and 2010 using the following key words as well as various combinations of them: trabeculectomy, guarded filtering surgery, mitomycin-C, 5-fluorouracil, amniotic membrane, anti-VEGF, beta radiation, non-penetrating glaucoma surgery, viscocanalostomy, deep sclerectomy, Trabectome, iStent, ab interno trabeculectomy, ab externo trabeculectomy, and canaloplasty. Reference lists from the selected articles were used to obtain further relevant articles not included in the electronic database.

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VI. Disclosure 

The authors reported no proprietary or commercial interest in any product mentioned or concept discussed in this article.

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Outline 

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