Trabeculectomy and drainage implants

Guarded Filtration Surgery: Trabeculectomy (Cairns)

Guarded filtration surgery (trabeculectomy), introduced by Cairns in 1968, remains the reference standard surgical procedure for achieving profound, durable IOP reduction in progressive open-angle and secondary glaucomas.

Surgical Technique

1. Conjunctival Peritomy: A fornix-based or limbus-based peritomy is performed, and Tenon's capsule is dissected from the underlying episclera.
2. Antifibrotic Antimetabolite Application: To prevent postoperative subconjunctival fibrosis (the primary cause of surgical failure), an appropriate antimetabolite protocol is selected according to scarring risk and tissue. Mitomycin C cross-links DNA and inhibits proliferating cells. Concentration, exposure, placement, sponge retrieval and irrigation require a validated surgical protocol; excessive treatment increases leak, hypotony and infection risks.
3. Partial-Thickness Scleral Flap: A rectangular, triangular, or trapezoidal scleral flap (approximately 3.5 $\times$ 3.5 mm) is dissected at one-third to one-half scleral thickness into clear cornea.
4. Paracentesis & Sclerostomy: A side-port paracentesis is created to control anterior chamber depth. A deep block of trabecular tissue (sclerostomy) is excised using a Kelly punch or surgical blade.
5. Peripheral Iridectomy (PI): A surgical peripheral iridectomy is performed to prevent the peripheral iris from incarcerating into and obstructing the internal sclerostomy.
6. Flap & Conjunctival Closure: The scleral flap is re-approximated with interrupted 10-0 nylon sutures (titrated using releasable sutures or sutures amenable to postoperative argon laser suture lysis). Tenon's capsule and the conjunctiva are closed in a watertight fashion with 9-0 or 10-0 nylon/polyglactin sutures, confirmed by a negative fluorescein Seidel test.

Bleb Evaluation & The Moorfields Bleb Grading System (MBGS)

Postoperative bleb morphology dictates long-term filtration success:

• Favourable Bleb: Broad, diffuse, moderately elevated, pale or mildly vascular, with fine microcystic epithelial changes extending posteriorly.
• Encapsulated Bleb (Tenon's Cyst): A localized, dome-shaped, thick-walled, hypervascular bleb that typically develops 2 to 6 weeks post-operatively, accompanied by a sharp elevation in IOP. Managed medically with aqueous suppressants, topical steroids, or needle revision with 5-FU.
• Scarred / Flat Bleb: Dense subconjunctival fibroblastic scarring resulting in loss of filtration and elevated IOP.
• Avascular / Cystic Bleb: Thin, transparent, friable, localized ischemic bleb (often associated with high-dose MMC), carrying a lifelong risk of spontaneous leak, hypotony, and infection.

Postoperative Complications & Management

• Early Bleb Leak: Fluorescein shows bright green dilution (positive Seidel test). Leads to hypotony (IOP $<6$ mmHg), shallow anterior chamber, choroidal detachments, and increased risk of endophthalmitis. Management: Large-diameter bandage contact lens, pressure patching, autologous blood injection, or surgical revision with conjunctival advancement.
• Hypotony Maculopathy: Sustained low IOP ($<6$ mmHg) causes posterior scleral wall collapse, chorioretinal folds radiating from the fovea, optic disc edema, and profound visual decline. Management: Transconjunctival scleral flap resuturing, autologous blood patch, or bleb compression sutures.
• Blebitis vs. Bleb-Associated Endophthalmitis:
  • Blebitis: A painful inflamed or purulent filtering bleb without established vitreous infection needs urgent specialist assessment and intensive antimicrobial treatment guided by local protocol and sampling. Reassess frequently for progression to endophthalmitis; do not teach one unusually high compounded concentration or routine systemic antibiotic as mandatory for every case.
  • Bleb-Associated Endophthalmitis: Spread of infection into the posterior segment. Characterized by severe pain, marked visual loss, dense vitritis, and hypopyon. Common organisms include virulent pathogens: Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus. EMERGENCY MANAGEMENT: Immediate vitreous tap and intravitreal injection of Vancomycin (1.0 mg/0.1 mL) and Ceftazidime (2.25 mg/0.1 mL), combined with urgent pars plana vitrectomy for severe vitritis.

Glaucoma Drainage Devices (GDD / Tube Shunts)

Glaucoma drainage devices consist of a flexible silicone tube inserted into the anterior chamber (or ciliary sulcus/pars plana), connected to an equatorial silicone or polypropylene plate sutured to the sclera beneath Tenon's capsule.

Ahmed Glaucoma Valve vs. Baerveldt Glaucoma Implant

Clinical Indications & Surgical Technique Pearls

• Indications: GDDs are indicated in refractory glaucomas where trabeculectomy carries a high failure rate: neovascular glaucoma, uveitic glaucoma, failed prior trabeculectomies with heavily scarred conjunctiva, epithelial ingrowth, post-penetrating keratoplasty glaucoma, and extensive conjunctival scarring from previous retinal detachment surgery.
• Surgical Pearls:
  • Quadrant selection: Superotemporal placement is commonly used, but previous surgery, scarring and anatomy may require another quadrant. Plan muscle, optic-nerve and exposure risks for the actual device; inferonasal placement is not universally prohibited.
  • Plate Fixation: Sutured to the sclera 8 to 10 mm posterior to the limbus using non-absorbable 8-0 nylon sutures.
  • Tube Entry: The tube is trimmed bevel-up at a $30^\circ$ to $45^\circ$ angle and inserted into the anterior chamber parallel to the iris plane through a 23-gauge needle tract 1 mm posterior to the limbus.
  • Patch Graft: A donor patch graft (processed pericardium [Tutoplast], donor sclera, or cornea) is sutured over the extraocular tube to prevent conjunctival erosion and late extrusion.

Drug Safety, Adherence and Surgical Boundaries

Beta-blockers retain cardiac and respiratory risks, including betaxolol; night-time pressure effects may be smaller but are not necessarily zero. Brimonidine is contraindicated in children under two and can cause profound CNS depression; older small children also require caution. Sulfonamide-antibiotic allergy does not automatically establish cross-allergy to every CAI; assess the actual reaction and drug label. Netarsudil may cause reticular epithelial corneal oedema as well as hyperaemia and verticillata, especially in compromised corneas. Confirm local availability and product information.

Teach drop technique, spacing, adherence and gentle nasolacrimal occlusion to limit systemic absorption. Discuss ocular-surface toxicity and simplified regimens. Trabecular MIGS and bleb-forming procedures such as XEN/PreserFlo have different mechanisms and risks: the latter require conjunctival/bleb care and are not all conjunctiva-sparing. A small lumen or Ahmed valve does not eliminate hypotony. Non-valved tubes require temporary flow restriction using an appropriate ligature or stent strategy until resistance develops. TVT compared tube with trabeculectomy; Ahmed versus Baerveldt comparisons come from different trials. Cyclodestruction can be considered in selected seeing eyes as well as painful poor-vision eyes. Advanced glaucoma needing very low pressure may warrant filtration rather than a compulsory sequence through MIGS.