Aqueous Humor Drainage After Trabeculectomy
After trabeculectomy, aqueous humor drains from the anterior chamber through a surgically created opening in the sclera (sclerostomy), passes beneath a partial-thickness scleral flap, and then filters into the subconjunctival space where it forms a filtering bleb and is absorbed by surrounding tissues. 1
Mechanism of Drainage
The drainage pathway consists of three key components:
- Anterior chamber to subscleral space: Aqueous exits through the sclerostomy (the opening created when tissue is excised during surgery) 2
- Beneath the scleral flap: Fluid flows through the aqueous drainage route under the partial-thickness scleral flap, with the thickness of this route correlating with successful bleb development 3
- Into subconjunctival space: Aqueous then enters the subconjunctival space where it creates the filtering bleb and is absorbed by conjunctival and episcleral vessels 1
The Filtering Bleb
The bleb is the visible elevation of conjunctiva where aqueous accumulates before absorption:
- Bleb morphology predicts success: The size of the filtering bleb correlates inversely with postoperative intraocular pressure (larger blebs = lower IOP) 3
- Diffuse vs. cystic blebs: Diffuse, low-lying blebs with healthy overlying conjunctiva are preferred over cystic blebs, which carry higher complication risks including infection, leakage, and astigmatism 4, 5
- Surgical technique matters: Fornix-based conjunctival flaps are more likely to produce diffuse blebs with normal vascular patterns compared to limbus-based approaches 4
Important Clinical Considerations
Resistance to Flow
The scleral flap provides controlled resistance to aqueous outflow, preventing excessive drainage and hypotony 1. This is why trabeculectomy is technically a "guarded" filtering procedure—the partial-thickness flap guards against uncontrolled flow 2.
Role of Antifibrotic Agents
Mitomycin-C (MMC) and 5-fluorouracil are used to reduce subconjunctival scarring that would otherwise close the drainage pathway and cause surgical failure 1. However, these agents increase the risk of hypotony, bleb leaks, and late-onset infections by creating thin, avascular blebs 1, 6.
Common Pitfall
The term "trabeculectomy" is actually a misnomer—excision of trabecular meshwork is unimportant to the procedure's success 2. What matters is creating a functional transscleral fistula with an overlying filtering bleb. The procedure works specifically because it creates a new outflow pathway to the subconjunctival space, not through enhancement of conventional outflow routes 2.
Contrast with Other Procedures
Unlike trabeculectomy, non-penetrating procedures (deep sclerectomy, viscocanalostomy) attempt to avoid creating a full-thickness opening and subconjunctival bleb, instead relying on filtration through residual tissue 1. However, trabeculectomy with its subconjunctival drainage pathway remains more effective at lowering IOP 1.