Glaucoma Drainage Devices & MIGS

Tube Shunts: Valved vs. Non-Valved

In an eye without prior vitrectomy, the tube of an aqueous shunt is placed in the anterior chamber near the peripheral iris angle. Pars plana placement is reserved for eyes that have already had a vitrectomy or that have significant corneal endothelial compromise, where an anterior-chamber tube would sit too close to the endothelium.

Whether a device is valved is the distinction that matters most clinically. The Ahmed glaucoma valve has an internal flow-restricting mechanism that opens only once a threshold pressure gradient is reached, so it provides built-in protection against hypotony in the early postoperative period. The Baerveldt and Molteno implants have no such mechanism. Because nothing inside the device limits flow, they're placed with a ligature: the tube is tied off temporarily and only opens once a fibrous capsule has formed around the plate, which is what ultimately provides the resistance a valved device gets from its own mechanism.

A recognized late complication is tube exposure through the overlying conjunctiva. Repair typically involves a patch graft, donor sclera, pericardium, or cornea, laid over the exposed segment to reinforce the tissue and keep the eye from becoming a route for infection.

MIGS: A Different Risk-Benefit Tradeoff Entirely

Microinvasive glaucoma surgery (MIGS) isn't defined by any single technique so much as by a shared risk profile: modest pressure reduction traded for a much wider safety margin than trabeculectomy or tube shunts carry. The approach is ab interno and minimally traumatic, which is what buys that safety margin in the first place.

• iStent trabecular micro-bypass is a small titanium device implanted through the trabecular meshwork directly into Schlemm's canal, bypassing outflow resistance at the meshwork itself. It's typically combined with cataract surgery in mild-to-moderate open-angle glaucoma.
• Suprachoroidal microstents shunt aqueous from the anterior chamber into the suprachoroidal space instead, routing outflow through the uveoscleral pathway rather than the trabecular one.