As a general rule, although all commercially available aqueous shunts share a common design consisting of a silicone tube and an episcleral end-plate;
11 we can divide the existing shunts into two broad categories: valved (Ahmed Glaucoma Valve, New World Medical Inc, Rancho Cucamonga, CA) and non-valved (Baerveldt; Molteno, Molteno Ophthalmic LTD, Dunedin, New Zealand; Krupin, E. Benson Hood Lab Inc, Pembroke, MA; among others).
11,18,19 After implantation, the flow rate and absorption of aqueous humor depend mainly on the area of the end-plate and the thickness of the fibrous encapsulation that forms around it.
19–21 Thinner and large surface areas are usually associated with lower postoperative IOP.
7,11 Therefore, before the development of the fibrous capsule (3–6 weeks after the surgery), the aqueous flows almost unrestrictedly through the shunt, favoring ocular hypotony unless additional measures are taken. These include modification to the tubes with additional sutures near the plate or stents placed inside the tube's lumen by the surgeons at the time of the surgery.
18,21 These maneuvers must be done meticulously because, if complete occlusion of the tube is not achieved, the risk of postoperative hypotony is high.
18 These surgical techniques are time-consuming; the results are directly linked to the surgeon's expertise and they can potentially need additional invasive procedures in order to eliminate the obstruction (laser or surgery).
18 In some cases, these obstructions can cause ocular hypertension severe enough to require immediate reoperation.
19 Additional venting stab incisions may be done on the anterior side of the tube, in order to reduce this tendency; however, this increases the incidence of postoperative hypotony as well.
19 All non-valved shunts must typically have one or all of these modifications.