Trabeculectomy and Tube Shunts

Trabeculectomy surgery is the most common operative procedure for managing glaucoma. Although the techniques have evolved over the decades, the basic procedure remains the same since filtering surgery was first performed a century ago: a scleral fistula enables aqueous fluid to drain into the subconjunctival space and create a bleb. Despite the long history of trabeculectomy surgery, the mechanisms of how aqueous fluid ultimately exits the eye after trabeculectomy are still not well characterized. Aqueous tube shunts are becoming more popular among glaucoma surgeons as an alternative to trabeculectomy in eyes with previously failed blebs or in eyes at high risk for bleb failure. Although the mechanism of aqueous outflow following tube shunt implantation may be similar to the mechanism after trabeculectomy, key differences exist that may be important for future developments in glaucoma surgery. Multiple potential routes of aqueous drainage exist for both trabeculectomy and tube shunt implants. Thus, aqueous drainage can be thought of as a complex system of pathways arranged in series and in parallel. In general, fluid flowing through any system will be directed according to the resistance along each potential pathway. After glaucoma surgery, the arrangement and resistance through these pathways will depend on the wound healing response (see Chapter 3), and thus the mechanism of aqueous drainage will alter as the wound healing response evolves. However, it is likely that one or 2 main routes of drainage will dominate as the tissue reaction stabilizes. When Cairns first described the trabeculectomy procedure, he argued that aqueous humor would flow out through the cut ends of Schlemm’s canal without subconjunctival drainage. However, as the procedure is currently performed, a filtering bleb is almost always created, and a block of trabecular meshwork is not consistently excised. Thus, modern trabeculectomy is a filtering procedure in which aqueous passes through the sclera into the subconjunctival space overlying the scleral flap. The existence of a filtering bleb does not eliminate the possibility of aqueous outflow through Schlemm’s canal into the aqueous veins. Indeed, tracer studies suggest that some aqueous enters the aqueous veins.