Residency · Residency · Ophthalmology

Trabeculectomy and Tube Shunt Surgery

Trabeculectomy

Surgical Principles

Trabeculectomy creates a guarded fistula from the anterior chamber to the subconjunctival space, allowing aqueous humor to drain and form a filtering bleb beneath the conjunctiva and Tenon capsule. The standard technique involves creating a partial-thickness scleral flap -- usually rectangular or triangular -- that serves as a valve to modulate flow. Beneath this flap, a sclerectomy or punch creates the full-thickness fistula into the anterior chamber. A peripheral iridectomy is performed to prevent iris tissue from plugging the internal opening of the fistula.

Surgical Technique

The conjunctival flap is either fornix-based (the approach preferred by most surgeons) or limbus-based. A scleral flap measuring 3 to 4 mm in width at approximately 50% scleral thickness is dissected. An antimetabolite -- either mitomycin C or 5-fluorouracil -- is applied to the scleral bed to inhibit postoperative scarring. The sclerectomy is created with a Kelly punch or blade excision, and a peripheral iridectomy is performed. The scleral flap is closed with two releasable or adjustable sutures of 10-0 nylon, which allows the surgeon to titrate flow postoperatively by removing sutures. Conjunctival closure must be watertight, which is especially critical for fornix-based flaps.

Antimetabolites

Mitomycin C (MMC) is an alkylating agent that inhibits fibroblast proliferation, preventing the scarring that would otherwise close the fistula. It is applied intraoperatively by placing soaked sponges on the scleral bed at a concentration of 0.2 to 0.4 mg/mL for one to three minutes. MMC is more potent and longer-lasting than 5-FU and is the standard of care in most trabeculectomies today. However, its potency carries a higher risk of producing thin, avascular, cystic blebs that are prone to late leaks.

5-Fluorouracil (5-FU) is a pyrimidine analog antimetabolite that can be used intraoperatively at concentrations of 25 to 50 mg/mL or administered as postoperative subconjunctival injections of 5 mg (0.1 mL of 50 mg/mL) at the bleb margin. While less potent than MMC, 5-FU is valuable for postoperative bleb rescue when used in conjunction with needling procedures. Its side effects include corneal epithelial toxicity and wound leak.

Postoperative Management

Postoperative care centers on bleb assessment and IOP management. A well-functioning bleb is diffuse, elevated, and relatively avascular. A flat bleb suggests either surgical failure or overfiltration. If IOP remains too high in the early postoperative period, laser suture lysis can be performed to cut releasable sutures and increase aqueous flow through the fistula. Early hypotony is managed with cycloplegia, discontinuation of aqueous suppressants, and compression sutures if needed. Bleb encapsulation (Tenon cyst) typically develops between weeks two and six and presents as a domed, tense bleb with elevated IOP; it is treated with needling, aqueous suppressants, or 5-FU injection. Late bleb leaks are confirmed by a positive Seidel test and pose a significant risk for blebitis and endophthalmitis, potentially requiring surgical revision.

Complications

Early complications include hypotony, shallow or flat anterior chamber, choroidal effusions, suprachoroidal hemorrhage, and hyphema. Late complications include bleb leak, bleb dysesthesia (irritation from the elevated bleb), blebitis, bleb-related endophthalmitis (which carries a lifelong risk of approximately 1% per year), cataract progression, and hypotony maculopathy. Malignant glaucoma (aqueous misdirection) is a rare but serious complication presenting as a flat anterior chamber with elevated IOP; it is treated with cycloplegics, aqueous suppressants, and vitrectomy if medical management fails.

<image>Trabeculectomy surgical anatomy showing scleral flap, sclerectomy site, peripheral iridectomy, and formation of the filtering bleb under conjunctiva</image>

Tube Shunt Surgery (Glaucoma Drainage Devices)

Types of Devices

Glaucoma drainage devices consist of a silicone tube connected to an end plate that is sutured to the sclera. The two main categories are valved and non-valved devices.

The Ahmed Glaucoma Valve (AGV) is a valved device containing a Venturi-based flow-restricting mechanism that opens at approximately 8 mmHg. The advantage of the valve is immediate IOP reduction with lower risk of postoperative hypotony. The principal disadvantage is the hypertensive phase -- a paradoxical rise in IOP occurring in 40 to 80% of patients during weeks one to three postoperatively, caused by encapsulation of the plate.

The Baerveldt Glaucoma Implant (BGI) is a non-valved device with no flow restriction. Because unrestricted flow would cause immediate hypotony, the tube must be ligated at the time of surgery, typically with a 7-0 Vicryl suture that dissolves at four to six weeks, allowing flow to begin once a fibrous capsule has formed around the plate. Stent sutures through the tube lumen or venting slits may provide some interim flow. The Baerveldt is available in larger plate sizes (250 and 350 mm squared) and generally achieves lower long-term IOP than the Ahmed.

Surgical Technique

A conjunctival peritomy is performed in the chosen quadrant, with the superotemporal quadrant preferred. The plate is sutured to the sclera 8 to 10 mm posterior to the limbus using 8-0 nylon sutures. The tube is trimmed to the appropriate length and inserted bevel-up through a needle track created with a 23-gauge needle, extending 2 to 3 mm into the anterior chamber. The exposed portion of the tube is covered with a patch graft -- typically processed pericardium, sclera, or cornea -- to prevent erosion through the conjunctiva. The conjunctiva is then closed.

Tube vs. Trabeculectomy: Key Trials

TVT Study (Tube Versus Trabeculectomy)

The TVT study compared the Baerveldt 350 with trabeculectomy with MMC in eyes that had undergone prior incisional surgery. At five years, IOP lowering was comparable between the two procedures, but the tube had a higher cumulative success rate, required fewer reoperations, and the trabeculectomy group experienced more early complications. The conclusion was that a tube shunt is preferred for eyes with prior incisional surgery.

PTVT Study (Primary Tube Versus Trabeculectomy)

The PTVT study compared the same two procedures as primary (first-time) surgery. At three years, trabeculectomy achieved lower IOP but had a higher complication rate. Reoperation rates were similar between groups, though the tube group experienced more persistent diplopia. Follow-up is ongoing, and the results continue to inform the debate about which procedure should be performed first.

ABC/AVB Trials

The Ahmed Baerveldt Comparison (ABC) and Ahmed Versus Baerveldt (AVB) trials compared the two major tube shunt devices. At five years, the Baerveldt achieved lower IOP with fewer medications, while the Ahmed had fewer early hypotony-related complications. The Baerveldt had a higher rate of reoperation for complications. Both devices are valid options, and the choice between them depends on the clinical scenario -- the Ahmed is preferred when minimizing early hypotony risk is a priority, while the Baerveldt is favored when the lowest possible long-term IOP is the goal.

<image>Comparison of Ahmed Glaucoma Valve and Baerveldt Glaucoma Implant showing valve mechanism, plate design, and tube placement in the anterior chamber</image>

Trabeculectomy vs. Tube Shunt Comparison

Choosing Between Trabeculectomy and Tube Shunt

Factors Favoring Trabeculectomy

Trabeculectomy is generally preferred for first-time incisional glaucoma surgery because it may achieve the lowest IOP. It is most successful in eyes with healthy conjunctiva that has not been disturbed by prior surgery, when a very low target IOP is needed, and in the absence of significant risk factors for postoperative scarring.

Factors Favoring Tube Shunt

A tube shunt is preferred in eyes with a prior failed trabeculectomy, prior conjunctival surgery (such as scleral buckle placement or multiple prior procedures), neovascular glaucoma (after anti-VEGF therapy and panretinal photocoagulation), uveitic glaucoma, active conjunctival inflammation or scarring, and aphakia or complex anterior segment anatomy.

<image>Postoperative slit-lamp appearance of a well-functioning filtering bleb after trabeculectomy, and a tube shunt with patch graft visible superiorly</image>

Bleb Management and Rescue Procedures

Bleb Needling

Bleb needling is an office-based or operative procedure used to revive encapsulated or failing blebs. A 27-gauge needle is used to lyse subconjunctival adhesions and open the drainage pathway. An injection of 5-FU (5 mg) or MMC (0.01 to 0.02 mg) at the time of needling helps prevent re-scarring. Success rates range from 40 to 80%, depending on the timing of the intervention and the adjunctive antimetabolite used.

Bleb Revision

Surgical bleb revision is indicated for chronic leaks, overfiltration, or blebs that are cosmetically or functionally problematic. Techniques include conjunctival advancement, free conjunctival autograft, or amniotic membrane grafting.

Clinical Pearls

MMC concentration and exposure duration should be adjusted based on patient risk factors -- younger age, African descent, and active inflammation all demand more aggressive antimetabolite use to counteract the heightened fibrotic response. Patients must be educated about the lifelong risk of bleb-related endophthalmitis and taught the warning signs: new redness, pain, discharge, and vision loss. Blebitis, which is a bleb infection without vitritis, is treated aggressively with fortified topical antibiotics; if it progresses to endophthalmitis with vitritis, a vitreous tap and intravitreal antibiotic injection are required. The hypertensive phase after Ahmed valve implantation is common and expected -- it is managed with aqueous suppressants and patience. Non-valved tubes require careful ligation at surgery to prevent postoperative hypotony. In neovascular glaucoma complicated by vitreous hemorrhage, a combined tube shunt with pars plana vitrectomy should be considered.

References

• Gedde SJ, et al. Treatment outcomes in the Tube Versus Trabeculectomy (TVT) study after five years of follow-up. Am J Ophthalmol. 2012;153(5):789-803.
• Gedde SJ, et al. Primary Tube Versus Trabeculectomy (PTVT) study: three-year results. Ophthalmology. 2021;128(1):48-59.
• Budenz DL, et al. Five-year treatment outcomes in the Ahmed Baerveldt Comparison study. Ophthalmology. 2015;122(2):308-316.
• Christakis PG, et al. The Ahmed Versus Baerveldt study: five-year treatment outcomes. Ophthalmology. 2016;123(10):2093-2102.
• American Academy of Ophthalmology. Basic and Clinical Science Course, Section 10: Glaucoma.

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