25.3 Glaucoma Surgical Procedures: Trabeculectomy, Aqueous Shunts & MIGS Devices

Glaucoma Surgical Procedures: Trabeculectomy, Aqueous Shunts & MIGS Devices

Core Clinical Mandate: When progressive glaucomatous optic neuropathy cannot be arrested by maximally tolerated medical therapy and laser trabeculoplasty, incisional surgical intervention is required. The ophthalmic medical technologist must understand the microsurgical steps, fluidic resistances, and post-surgical surveillance protocols for trabeculectomies, tube shunts, and micro-invasive glaucoma surgeries (MIGS).

Trabeculectomy: Surgical Architecture, Flap Mechanics & Antimetabolites

Originally developed by Cairns in 1968, trabeculectomy remains the historical reference standard for incisional glaucoma filtration. It creates a guarded, partial-thickness fistula from the anterior chamber into the sub-Tenon's / subconjunctival space, bypassing the obstructed trabecular meshwork to form a subconjunctival filtering bleb.

Step-by-Step Surgical Sequence

1. Conjunctival Peritomy:
   • Fornix-Based: Incision made at the corneal limbus, reflecting the conjunctiva and Tenon's capsule backward toward the fornix. Provides superior surgical exposure and facilitates diffuse posterior antimetabolite placement. Requires meticulous, watertight limbal closure.
   • Limbus-Based: Incision made 8 mm to 10 mm posterior to the limbus in the fornix, dissecting forward to the limbus. Eliminates risk of limbal edge leak but increases technical difficulty and carries a higher risk of surgical scarring near the filtration site.
2. Scleral Flap Creation: A partial-thickness scleral flap (one-third to one-half scleral thickness, typically 3.0 mm × 3.0 mm rectangular, triangular, or trapezoidal) is dissected anteriorly into clear cornea. The flap provides guarded physical resistance against unchecked aqueous egress, preventing immediate post-surgical hypotony.
3. Antimetabolite Application: Sponges soaked in antifibrotic agents are applied under Tenon's capsule and the scleral flap, followed by vigorous irrigation with 20 to 30 mL of balanced salt solution (BSS).
4. Sclerectomy (Ostium) & Peripheral Iridectomy (PI): A deep block of trabecular meshwork and inner sclera (approximately 1.0 mm × 1.5 mm) is excised beneath the flap using a Kelly punch. A surgical peripheral iridectomy is performed through the ostium using Vannas scissors and Bonn forceps. The PI prevents the peripheral iris root from prolapsing forward and blocking the internal ostium.
5. Scleral Flap Suturing: The flap is secured with interrupted 10-0 nylon sutures. Sutures may be placed as releasable sutures (externalized loops that can be removed at the slit lamp) or titrated postoperatively using an argon laser with a Hoskins or Blumenthal lens (laser suture lysis).
6. Conjunctival Closure & Seidel Testing: Conjunctiva is closed with 10-0 nylon or 8-0 Vicryl. Watertight integrity is verified by placing 2% sodium fluorescein over the suture line under cobalt blue illumination (Seidel test).

Antimetabolites in Filtration Surgery: Mitomycin C vs. 5-Fluorouracil

Postoperative wound healing and subconjunctival fibroblastic scarring represent the primary cause of trabeculectomy failure. Antimetabolites prevent this healing response:

Bleb Evaluation, Morphological Grading & Postoperative Triage

Clinical Bleb Morphology: Standardized Grading Scales

Postoperative bleb surveillance relies on standardized classification systems, such as the Moorfields Bleb Grading System (MBGS) and the Indiana Bleb Appearance Grading Scale (IBAGS), which evaluate four primary parameters:

1. Bleb Height: Classified as low (flat, scarred), moderate, or high (cystic).
2. Bleb Extent (Area): Measured in clock hours or horizontal millimeters of subconjunctival spread.
3. Vascularity: Graded from 0 (completely avascular, porcelain white) to 3 or 4 (severely hyperemic/erythematous). Moderate, diffuse, microcystic vascularity is ideal.
4. Seidel Leakage: Graded as negative, pinpoint leak, or streaming leak.

Postoperative Bleb Complications

• Early Hypotony with Shallow Chamber: Occurs within days post-op due to overfiltration or wound leak. If the anterior chamber is flat (corneo-lenticular touch), the lens capsule will opacify and endothelial cells will rapidly perish. Managed with cycloplegics, pressure patch, oversized bandage contact lens, OVD injection into the AC, or suture revision.
• Failing / Encapsulated Bleb (Tenon's Cyst): Characterized by high IOP, a tense, localized, elevated, thick-walled bleb with engorged episcleral vessels occurring at 2 to 6 weeks. Managed with aqueous suppressants, digital ocular massage, or slit-lamp bleb needling combined with 5-FU or MMC injection.
• Blebitis vs. Bleb-Related Endophthalmitis (BRE):
  • Blebitis: Localized infection confined strictly to the filtering bleb without vitreous involvement. Presents with pain, conjunctival injection, and a milky-white, purulent bleb ("white bleb with red eye"). Anterior chamber contains mild cells without hypopyon or vitritis. Treated emergently with intensive topical fortified antibiotics (vancomycin 25–50 mg/mL and ceftazidime 50 mg/mL or tobramycin 14 mg/mL alternating every 30–60 minutes) and oral fluoroquinolones.
  • Bleb-Related Endophthalmitis: Extension of bacterial infection into the intraocular cavities. Characterized by severe aching pain, profound visual loss, hypopyon, and dense vitritis. Virulent pathogens include Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae. Requires emergent vitreous tap and intravitreal antibiotic injection (vancomycin 1 mg/0.1 mL + ceftazidime 2.25 mg/0.1 mL), or immediate pars plana vitrectomy (PPV).

Aqueous Drainage Implants (Glaucoma Tube Shunts)

Aqueous drainage implants divert aqueous humor through a flexible silicone tube into an equatorial reservoir plate anchored 8 mm to 10 mm posterior to the limbus. Fluid passes across the surrounding semi-permeable fibrous capsule into orbital capillaries and lymphatics.

Valved vs. Non-Valved Aqueous Shunts

Surgical Details & Complication Profiles

• Plate Fixation: Sutured to sclera in the equatorial space between adjacent rectus muscles (superotemporal quadrant is preferred to avoid the superior oblique tendon and avoid inferior strabismus complications).
• Tube Entry & Donor Patch Grafts: The tube enters the anterior chamber through a 23-gauge needle tract angled parallel to the iris plane. In pseudophakic eyes with compromised anterior chambers, the tube can be inserted into the ciliary sulcus or pars plana (the latter requiring complete pars plana vitrectomy). The extraocular subconjunctival portion of the tube must be covered with a patch graft (donor sclera, processed pericardium [Tutoplast], or donor cornea) to prevent tube erosion through the conjunctiva.
• Tube-Cornea Touch: If the tube is placed too anteriorly, it strikes the corneal endothelium, inducing localized corneal edema and progressive endothelial cell loss requiring tube repositioning or endothelial keratoplasty.

Minimally Invasive Glaucoma Surgery (MIGS)

Minimally Invasive Glaucoma Surgery (MIGS) refers to a class of micro-incisional surgical procedures designed to lower intraocular pressure with an exceptional safety profile, rapid recovery, and minimal anatomical disruption compared to filtering surgery. Most MIGS procedures are performed ab interno through a clear corneal incision under intraoperative direct gonioscopic visualization.

Classification of MIGS by Anatomical Outflow Mechanism:
1. TRABECULAR OUTFLOW (Schlemm's Canal Bypass / Scaffold)
   ├── iStent / iStent inject (Titanium micro-bypass stents)
   ├── Hydrus Microstent (8-mm nitinol scaffold spanning 90°)
   ├── Kahook Dual Blade / Trabectome (Excisional goniotomy)
   └── OMNI / GATT (360° viscodilation + transluminal trabeculotomy)

2. SUPRACHOROIDAL OUTFLOW (Uveoscleral Pathway)
   ├── MINIject (Flexible biocompatible porous silicone)
   └── CyPass Micro-Stent (WITHDRAWN 2018 due to endothelial cell loss)

3. SUBCONJUNCTIVAL FILTRATION
   ├── XEN 45 Gel Stent (Ab interno porcine cross-linked gelatin)
   └── Preserflo MicroShunt (Ab externo SIBS biomaterial + MMC)

1. Trabecular Meshwork Bypass and Schlemm's Canal Scaffolds

Because the juxtacanalicular trabecular meshwork provides the primary resistance to aqueous humor outflow in primary open-angle glaucoma, bypassing this tissue restores physiological outflow into collector channels:

• iStent inject: Consists of two heparin-coated titanium micro-stents (each 360 µm long) implanted ab interno through the trabecular meshwork into Schlemm's canal, spaced 2 to 3 clock hours apart under gonioscopic view.
• Hydrus Microstent: An 8 mm curved, flexible scaffold made of nickel-titanium alloy (nitinol). It is inserted into Schlemm's canal, dilating and scaffolding approximately 90° (3 clock hours) of the canal to expose multiple collector channel ostia while maintaining a patent inlet directly into the anterior chamber.
• Goniotomy and Trabeculotomy (KDB & GATT):
  • Kahook Dual Blade (KDB): An engineered footplate and dual parallel blades excise a full-thickness strip of diseased trabecular meshwork over 90° to 120°, opening direct access to the canal without leaving an implant.
  • GATT (Gonioscopy-Assisted Transluminal Trabeculotomy): A 5-0 prolene suture or illuminated microcatheter (iTrack) is threaded 360° through Schlemm's canal and then pulled tight to cleave the entire trabecular meshwork circumferentially.

2. Suprachoroidal Shunts & The CyPass Withdrawal

Suprachoroidal devices channel aqueous from the anterior chamber into the suprachoroidal space, utilizing the physiological uveoscleral outflow pathway and the negative hydrostatic pressure gradient between the anterior chamber and the suprachoroidal space.

• The CyPass Clinical Lesson: The CyPass Micro-Stent was FDA approved in 2016 and voluntarily withdrawn from the global market in 2018 following the 5-year COMPASS-XT study. The trial revealed statistically significant, progressive corneal endothelial cell loss directly correlated with the number of retention rings protruding anteriorly into the anterior chamber. Physical proximity to the endothelium and micro-movement of the rigid polyimide tube induced mechanical endothelial injury.
• Modern Suprachoroidal Technology: The MINIject utilizes a soft, flexible, porous silicone material (Star BioMaterial) that conforms to suprachoroidal anatomy without rigid anterior chamber protrusion.

3. Subconjunctival Gelatin Stents: The XEN 45

• The XEN Gel Stent is a 6.0 mm flexible tube made of porcine-derived cross-linked gelatin, inserted ab interno from the anterior chamber into the subconjunctival space.
• Hagen-Poiseuille Physics: The internal lumen diameter is precisely 45 µm. According to the Hagen-Poiseuille equation (where fluid resistance is inversely proportional to the fourth power of the radius, $R \propto 1/r^4$), a 6 mm tube with a 45 µm lumen generates an intrinsic fluid resistance of 6 to 8 mmHg at physiological aqueous flow rates (2 to 3 µL/min). This built-in resistance physically prevents postoperative hypotony without requiring scleral flap suturing.
• Antifibrotic Requirement: Like trabeculectomy, subconjunctival gelatin stents require intraoperative subconjunctival Mitomycin C to prevent bleb encapsulation and failure.