Glaucoma Filtration Surgery

Introduction & Indications - IOP's Escape Plan

• Goal: Create a new aqueous outflow pathway to ↓ IOP, preventing optic nerve damage.
• Also known as trabeculectomy or glaucoma filtering surgery.
• Mechanism: Bypasses the trabecular meshwork, allowing aqueous humor to drain into subconjunctival space, forming a filtering bleb.

• Indications:
  • Medically uncontrolled glaucoma (max tolerated medical therapy fails).
  • Progressive visual field loss despite acceptable IOP.
  • Laser trabeculoplasty failure.
  • Specific glaucomas: e.g., neovascular, inflammatory (after control).
  • Patient non-compliance/intolerance to medications.

⭐ Failure to achieve target IOP despite maximally tolerated medical therapy is the most common indication for filtration surgery.

• Target IOP: Individualized, often < 21 mmHg, or 30% reduction from baseline, or lower in advanced disease (e.g., < 12-15 mmHg).

Trabeculectomy - Gold Standard Sluice

• Goal: Creates a guarded fistula from anterior chamber (AC) to subconjunctival space, forming a filtering bleb for aqueous outflow. Bypasses trabecular meshwork.
• Indications:
  • Medically uncontrolled Primary Open Angle Glaucoma (POAG) with progressive visual field (VF) loss.
  • Chronic Angle Closure Glaucoma (CACG) post-Laser Peripheral Iridotomy (LPI) with persistently high Intraocular Pressure (IOP).
• Antimetabolites (prevent bleb scarring, ↑ success):
  • Mitomycin C (MMC): 0.2-0.5 mg/mL applied for 1-5 min.
  • 5-Fluorouracil (5-FU): 50 mg/mL (intraoperative or post-op injections).
• Key Steps:

• Ideal Bleb: Diffuse, slightly elevated, microcystic surface, avascular centrally with good peripheral vascularity.
• Complications:
  • Early: Hypotony, shallow AC, bleb leak, choroidal detachment.
  • Late: Bleb failure (fibrosis), cataract, blebitis/endophthalmitis (📌BRI: Bleb-Related Infection), chronic hypotony.

⭐ The most common cause of late trabeculectomy failure is subconjunctival fibrosis leading to scarring of the filtration bleb.

Other Filtration Surgeries - Beyond The Trab

• Non-Penetrating Glaucoma Surgery (NPGS):
  • E.g., Deep Sclerectomy, Viscocanalostomy.
  • Spares trabeculo-Descemet membrane; ↓ risk of hypotony, endophthalmitis.
  • Aqueous percolates via scleral/Descemet's window. Less IOP lowering than trabeculectomy.
• Glaucoma Drainage Devices (GDDs) / Aqueous Shunts:
  • E.g., Ahmed Glaucoma Valve (AGV), Baerveldt, Molteno.
  • Tube diverts aqueous from anterior chamber to an equatorial plate → subconjunctival bleb.
  • Indications: Failed trabeculectomy, refractory glaucomas (e.g., neovascular, uveitic).
  • AGV: valved; Baerveldt: non-valved (larger plate).
  • Complications: Hypertensive phase, tube migration/erosion, diplopia.
• Minimally Invasive Glaucoma Surgery (MIGS):
  • E.g., iStent, Hydrus, XEN Gel Stent, PreserFlo.
  • Ab-interno or ab-externo approaches; target Schlemm's canal, suprachoroidal space, or subconjunctival space.
  • Safer profile, modest IOP reduction; often combined with phacoemulsification.

⭐ The "hypertensive phase" in GDDs, typically occurring 2-6 weeks post-op, is thought to be due to fibrous encapsulation around the plate, temporarily reducing outflow before channels form within the capsule an average of 3 months post-op for non-valved devices and sooner for valved devices like AGV due to its flow-restrictive mechanism from the start of surgery.

Post-Op & Complications - Blebs & Bumps

• Ideal Bleb: Diffuse, avascular, microcysts.
• Dysfunctional Blebs:
  • Encapsulated (Tenon's Cyst): Localized, thick-walled, ↑IOP.
  • Failing/Failed: Flat, vascularized, ↑IOP.
  • Over-filtering: Large, thin, ↓IOP (hypotony).
  • Leaking: Seidel test positive. Risk: hypotony, infection.

• Blebitis: Infected bleb: inflammation, discharge.
  • Early: Staph.
  • Late: Strep, H. flu.
  • Risk: endophthalmitis. Rx: Intensive topical antibiotics.
• Hypotony Maculopathy: ↓IOP → chorioretinal folds, ↓vision.

⭐ Encapsulated blebs (Tenon's cysts) typically develop 2-8 weeks post-op, presenting as a localized, tense, elevated bleb with ↑IOP.

High‑Yield Points - ⚡ Biggest Takeaways

• Trabeculectomy is the gold standard surgical procedure, creating a new aqueous outflow pathway.
• A filtering bleb under a partial-thickness scleral flap is the desired surgical outcome.
• Antifibrotic agents like Mitomycin C (MMC) or 5-Fluorouracil (5-FU) are crucial to prevent bleb scarring and failure.
• Key complications include hypotony, bleb leak, blebitis, endophthalmitis, and late cataract formation.
• Non-penetrating glaucoma surgeries (e.g., deep sclerectomy) aim for ↑safety but may have ↓efficacy.
• Aqueous shunts or Glaucoma Drainage Devices (GDDs) are reserved for refractory glaucoma or failed trabeculectomies.