Trabeculectomy: Principles, Technique, and Management

Historical Context and Development of Filtration Surgery

• The Discovery of the Bleb:
  • The concept of the filtering bleb originated with von Graefe, who developed the surgical iridectomy.
  • He observed that patients who achieved successful intraocular pressure ($IOP$) control often developed an inadvertent bleb (a blister-like formation) on the surface of the eye.
• Full-Thickness Filtration Procedures:
  • These were the original intentional filtration surgeries where a full-thickness hole was created through the sclera and covered by conjunctiva.
  • Mechanism: Fluid exited the anterior chamber through the hole, gathered under the conjunctiva to form a blister (bleb), and was subsequently absorbed by the body.
  • Clinical Efficacy vs. Safety: While effective at lowering $IOP$—exemplified by a case study of a patient (the "iridencleisis" procedure) whose pressure remained controlled $54$ years post-operatively without medication—these procedures were fraught with high complication rates.
  • Common Complications of Full-Thickness Surgery:
    • Cataract formation (nearly universal).
    • Flat anterior chambers.
    • Suprachoroidal hemorrhage.
• Evolution to Trabeculectomy:
  • Trabeculectomy was designed as a more controlled alternative to full-thickness procedures, similar to how modern minimally invasive glaucoma surgeries ($MIGS$) aim to avoid the complications associated with traditional filtering blebs.

Concepts and Indications for Trabeculectomy

• Definition and Misnomer:
  • The term "trabeculectomy" is technically a misnomer. While originally intended to open the end of Schlemm's canal, the procedure actually involves creating a controlled egress of fluid through a partial-thickness scleral flap.
  • Modern incisions typically move further forward into the clear cornea rather than focusing specifically on the trabecular meshwork.
• General Indications:
  • Indicated for poorly controlled glaucoma where medical therapy and laser trabeculoplasty have failed.
  • Reserved for cases requiring an $IOP$ lower than episcleral venous pressure (approximately $8$ to $10 \text{ mmHg}$).
  • Normal Tension Glaucoma (NTG): A primary indication where damage occurs at pressures like $12 \text{ mmHg}$ and a target pressure of $8 \text{ mmHg}$ is required; medications and trabeculoplasty are generally unable to reach these low levels.
  • Used for patients with poor compliance regarding topical medical management.
• Contraindications:
  • Blind eyes: Surgery is not performed on eyes with no light perception.
  • Lack of mobile conjunctiva: Essential for successful bleb formation; contraindicated in eyes with extensive scarring from prior surgeries, scleral buckles, or chemical burns.
• Risk Factors for Surgical Failure:
  • Prior cataract surgery (specifically old-style large scleral incisions with conjunctival involvement).
  • History of failed filtering surgery.
  • Age and Pigmentation: Younger patients (due to more aggressive healing) and darkly pigmented patients (e.g., African American patients) are at higher risk of failure.
  • Active intraocular inflammation.
  • Neovascular glaucoma.
  • Iridocorneal Endothelial ($ICE$) syndromes.

Comparative Advantages and Research

• Pressure Reduction: Trabeculectomy is capable of achieving lower intraocular pressures than any other surgical procedure. One patient case demonstrated stable $IOP$ of $4 \text{ mmHg}$ in both eyes for over $11$ years with no visual field progression.
• Preservation of Future Options: Unlike tube shunts, which involve large conjunctival incisions and can limit future surgical options, trabeculectomy leaves more doors open for subsequent interventions if failure occurs.
• The TVT (Tube Versus Trabeculectomy) Study:
  • Looked at patients with prior cataract surgery or failed filtering surgery.
  • Randomized them to trabeculectomy with Mitomycin-C ($MMC$) or a Barveldt-$350$ tube shunt.
  • Both groups showed similar results at the $5$-year mark (this is covered extensively in Lecture $45$).

Pre-operative Preparation and Anesthesia

• Clinical Evaluation: Surgeons must confirm the presence of healthy, mobile conjunctiva and examine the eye for any existing scarring from injury or previous buckles.
• Medication Management:
  • Anticoagulants: Aspirin or other antiplatelets may be stopped if not medically essential (e.g., if used only for general hypertension prophylaxis in patients over $55$).
  • High Pre-op Pressure: Extremely high $IOP$ should be lowered using IV mannitol or very early intraoperative paracentesis to prevent a massive, sudden pressure drop when the eye is opened.
• Anesthetic Options:
  • Retrobulbar Block: Traditional approach but carries risks of retrobulbar hemorrhage.
  • Subconjunctival/"Topical" Anesthesia: Injection of a combination of Marcaine and Lidocaine. This is the speaker's preferred method because it is faster, allows the patient to look down during the case (often eliminating the need for traction sutures), and requires no post-operative patch.
  • General Anesthesia: Typically considered too aggressive for this procedure.

Surgical Technique: Step-by-Step

• Traction Sutures: Optional. A Vicryl suture can be placed in the clear cornea (usually at $12 \text{ o'clock}$) to orient the eye. This is unnecessary if the patient can look down under subconjunctival anesthesia.
• Conjunctival Incisions:
  • Limbus-based: The incision is made high in the fornix, with the conjunctiva hinged at the limbus.
  • Fornix-based: The incision is made at the limbal junction, with the conjunctiva hinged in the fornix. The speaker recommends leaving a small "skirt" of conjunctiva at the limbus to facilitate easier closure.
• Flap Location and Construction:
  • Position: Ideally started at $11 \text{ o'clock}$ or $1 \text{ o'clock}$. This leaves the $12 \text{ o'clock}$ position available for a future second trabeculectomy if the first fails.
  • Shape: Trapezoidal or rectangular flaps are preferred over triangles because the corner sutures can be tied tightly without causing the side of the flap to gape.
  • Dimensions: Roughly $3 \text{ mm}$ at the limbus (matching the length of a standard $75$ blade).
  • Technical Detail: The surgeon should aim for deep, perpendicular walls. Shallow "scratches" or bowl-shaped incisions reduce control.
• The Sclerostomy (Ostomy):
  • A block of tissue (transitioning into clear cornea) is removed using a Kelly punch or scissors.
  • The surgeon must leave "shoulders" of sclera around the ostomy to maintain control over fluid egress.
• Alternative: The Ex-PRESS Shunt:
  • A metal shunt is inserted under the scleral flap into the anterior chamber, replacing the need for a Kelly punch or scalpel ostomy.
• Iridectomy:
  • Performed to prevent the iris from plugging the sclerostomy.
  • Typically performed in hyperopes and phakic patients.
  • Rarely performed in myopes or pseudophakes by the speaker to reduce the risk of bleeding.
• Wound Closure:
  • Scleral Flap Closure: Two primary corner sutures (tight) and two interior "safety sutures" (looser). This allows for subsequent laser suture lysis without a sudden drop to zero pressure.
  • Conjunctival Closure: The speaker prefers a running horizontal mattress suture ($8$-$0$ Vicryl). This "welds" the tissue together, supporting the entire length of the incision, which is especially beneficial for patients with thin conjunctiva.

Intraoperative Pharmacotherapy (Anti-metabolites)

• Mitomycin-C ($MMC$):
  • Concentration: $0.1$ to $0.5 \text{ mg/mL}$.
  • Duration: Up to $5$ minutes (typically $2$ to $3.5$ minutes depending on failure risk).
  • Administration: Can be applied via soaked sponges or injected subconjunctivally (combined with lidocaine, marcaine, and epinephrine) prior to the surgical prep.
• $5$-Fluorouracil ($5\text{-FU}$):
  • Historically used as a series of $21$ injections over $14$ days (per the original study).
  • Extremely labor-intensive and harsh on the cornea; largely replaced by intraoperative $MMC$.

Post-operative Management

• Medication Regimen:
  • Corticosteroids: Prednisolone every $2$ hours for the first $3$ weeks, followed by a slow taper.
  • Atropine: Used in the operating room; continued post-operatively for hyperopes or patients with prior angle closure to prevent aqueous misdirection.
  • Antibiodics: Combination steroid-antibiotic at bedtime.
• Follow-up Schedule: Weekly visits for the first $6$ weeks.
• Interventions:
  • Digital Massage: Applying pressure through the upper eyelid to encourage fluid flow through the flap if pressure is elevated.
  • Laser Suture Lysis: Typically initiated around week $3$ to lower pressure. A Hoskins lens, Ritch lens, or the corner of a Zeiss lens is used with Argon green or Krypton red lasers (red is preferred if blood is present). The distal end of the suture is cut so the long piece lays flat.

Complications

• Early Complications (High Pressure):
  • Causes: Tight sutures, iris or fibrin blocking the ostomy, or pupillary block.
  • Aqueous Misdirection (Malignant Glaucoma): Characterized by a uniformly flat anterior chamber and high pressure. Common in patients with previous angle closure.
  • Suprachoroidal Hemorrhage: Differentiated by sudden, severe pain.
• Early Complications (Low Pressure/Hypotony):
  • Causes: Over-filtration, wound leaks, or choroidal effusions.
  • Snuff Syndrome: Rare loss of central visual acuity post-operatively, usually in patients with pre-existing fixation-splitting field loss.
• Late Complications:
  • Failure and Tenon's Cysts: A fibrous capsule forms over the bleb, trapping fluid. Treated with aqueous suppressants, time, or needling.
  • Bleb Infection:
    • Blebitis: Infection confined to the bleb; treated with topical fortified antibiotics.
    • Endophthalmitis: Infection moves into the globe; requires vitrectomy and carries a poor prognosis.
  • Dysesthesia: Painful blebs caused by the bleb migrating nasally/temporally or trapping air bubbles on a steep anterior surface (Dellen formation).

The $5\text{-FU}$ Filtering Surgery Study

• Design: A randomized clinical trial comparing subconjunctival $5\text{-FU}$ injections to standard trabeculectomy.
• Target Population: High-risk patients (aphakic, pseudophakic with large limbal incisions, or prior failed filters).
• Findings: $5\text{-FU}$ significantly increased the success rate of the surgery but also increased the risk of post-operative bleb leaks.
• Legacy: This study paved the way for the routine use of anti-metabolites (like $MMC$) in glaucoma surgery.