A glaucoma seton, such as the Ahmed Valve from New World Medical (by the way, it is not named or designed by Ike Ahmed MD as many novice surgeons think) allows us to bypass the trabecular meshwork and provide a path for egress of aqueous in order to lower intra-ocular pressure. At our teaching hospital we deal with many patients who have neo-vascular glaucoma from proliferative diabetic eye disease or from retinal vascular occlusive disease. The patient gets surgery and initially the Ahmed valve seton works beautifully, lowering the IOP to the low teens (13 to 15 mmHg is typical). The valve in this device consists of two pieces of silicone which form a one-way flap valve which opens when the IOP is in this low teens range. A few months after the surgery, the patient returns and the IOP is creeping up and then a few months later it is in the high twenties or even 30 mmHg. What happened? The valve still works but the footplate of the Ahmed seton has been encapsulated with scar tissue that the patient formed. And when you lift the eyelid, you see the distended conjunctiva which is under pressure. We need to lyse this encapsulation and then we will help return the eye to a normal IOP.
This video shows the technique to needle this encapsulation, drop the IOP, and restore outflow: