Artificial aqueous drainage devices are being used more frequently in the surgical management of complicated glaucomas. Increased experience with drainage implants has expanded their use as alternatives to conventional filtering surgery when the latter has been unsuccessful at adequately lowering intraocular pressure (IOP), is unlikely to succeed, or is technically difficult to perform.

Modern drainage devices all conform to a common design, comprising a long silicone rubber tube attached at one end to a synthetic plate or band of variable shape and surface area. The proximal end of the tube is inserted into the anterior or posterior chamber through a scleral fistula at the level of the limbus or pars plana. The plate or band is sutured to the episcleral surface posterior to the insertions of the rectus muscles. The synthetic explant material serves to inhibit tissue adhesion as the plate becomes encapsulated. The fibrous capsule surroundilng the plate serves as a subconjunctival reservoir into which aqueous humor is shunted via the silicone tube. Both the inherent resistance of the capsular wall to passive transmural fluid diffusion and the total capsular surface area help to determine the steady-state IOP.

The earliest modern implants were those developed by Molteno and by Krupin in the early 1970s. The basic design has remained essentially unchanged. Newer implants (Baerveldt, Schocket, Ahmed, Joseph, White, OptiMed) have been introduced with modifications designed to enhance IOP control or limit early postoperative complications. The devices in common use differ primarily on the basis of whether or not the tube contains a pressure-sensitive valve, and the shape and surface area of the scleral explant.

When a drainage device with non-valved tube is used, it is necessary to create some restriction to aqueous flow through the tube in the early postoperative period, prior to fibrous encapsulation of the scleral plate. This generally takes the form of one of a variety of suture ligatures with or without a releasable stent. Profound hypotony will generally result if a drainage tube is inserted without complete restriction to aqueous flow for the initial one to two postoperative weeks.

Currently there are two drainage implants in which the tube has been modified to provide some restriction to aqueous flow - the Ahmed Glaucoma Valve and the Krupin Eye Disk. The need for ligatures or stents at the time of surgery is obviated by a valve mechanism which is theoretically designed to maintain the IOP within a specific physiologic range (i.e., approximately 8 to 12 mmHg). However, recent clinical and in vitro studies have shown that the “valves” in these devices perform more like flow restrictors than true valves. Their ability to consistently maintain the IOP within strictly defined limits is unreliable at best. The concept of avoiding both excessively high IOPs (as might occur with a non-valved, ligatured tube) or hypotony and its attendant complications (as might result from a non-ligatured, non-valved tube) is not achieved in a significant proportion of cases.

IOPs above the stated opening pressure as well as below the stated closing pressure of the valve occur with either device. Hypotony can be difficult to manage, and may lead to significant complications such as serous or hemorrhagic choroidal detachment, flat anterior chamber, and cataract formation, particularly in eyes with recalcitrant glaucoma that have undergone multiple prior intraocular surgeries and may have had very high preoperative IOPs. The unidirectional valve mechanisms are also limited by their inability to prevent episodic hypotony associated with mechanical compression of the globe or sudden Valsalva (i.e., during extubation, emesis, or vigorous crying).

High pressures secondary to valve obstruction can be equally problematic. Manipulations such as digital pressure and anterior chamber injection of balanced salt solution or tissue plasminogen activator may be performed at the slit-lamp and are sometimes effective in alleviating valvular blockage. Occasionally, fibrous capping of the distal portion of the tube in the vicinity of the valve occurs. Surgical revision with amputation or disassembly of the valve mechanism may be required.

The above concerns regarding the use of drainage devices with flow-restriction modifications, must be assessed in the context of their potential functional advantage. Their primary benefit is the ability to provide immediate IOP reduction. Moreover, there is no need for temporary tube ligatures or stents. This reduces operating time and eliminates the potential need for a second operation (albeit sometimes a minor office procedure) to release a ligature or remove a stent should this become necessary prior to spontaneous release.

It should be remembered, however, that fibrous encapsulation of the scleral plate may occur as early as one week postoperatively. Once this has occurred, it is the bleb capsule which provides the resistance to aqueous outflow and determines the steady-state IOP. The valve mechanism no longer serves any useful purpose, but does remain a potential source of complications, as obstruction from particulate debris, fibrin, blood, or fibrous tissue is more likely to occur at the point of flow restriction. Additionally, the currently available valved devices both contain a relatively small surface area scleral plate (184 mm2). To the extent that a large surface area for aqueous drainage is beneficial in maintaining pressure control, this is a further disadvantage. In a patient with very high preoperative IOP in whom immediate pressure reduction is desired, consideration might be given to performing an “orphan” trabeculectomy (without antimetabolite) and using a non-valved drainage tube with an occlusive ligature. The trabeculectomy is designed to fail over a period of several weeks, providing IOP control prior to spontaneous tube ligature release.

In addition to protection from IOP extremes in the immediate postoperative period, valved drainage devices may provide an added margin of safety for an extended period in situations where abnormal ciliary body function is anticipated. Under conditions of aqueous hyposecretion, as may occur with severe ocular ischemia and chronic/recurrent uveitis, the IOP should be maintained above the stated closing pressure of the valve thereby avoiding hypotony.

In summary, careful preoperative assessment and an understanding of the physiology of glaucoma drainage devices will allow the surgeon to select the appropriate implant. The potential difficulties associated with valved drainage devices and the unpredictable function of the valve mechanisms limit their usefulness. In eyes at high risk for postoperative complications (e.g., aphakic, vitrectomized, multiple prior intraocular surgeries) a non-valved tube with a temporary ligature provides the most reliable protection against early postoperative hypotony. In situations in which the preoperative IOP is extremely elevated on maximal medications (especially in the presence of advanced optic nerve head damage) or in which aqueous hyposecetion is anticipated postoperatively, use of a drainage device containing a pressure-sensitive valve should be considered.

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