Drainage Implants

Historical Perspective

Modern glaucoma drainage implants (GDIs) have evolved from the eponymous device first described by Molteno in 1969. Refinements in implant design and surgical technique have led to the routine use of GDIs in the surgical management of glaucoma. Analysis of Medicare claims data demonstrated a 77% decline in trabeculectomy use in the United States between 1994 and 2012 with a concurrent 410% increase in the number of GDI surgeries. Surveys of the American Glaucoma Society membership have shown a similar shift in surgical practice patterns between 1996 and 2008, with a significant increase in the use of GDIs in various clinical settings.

Basic Concept

Drainage implants share a common design consisting of a silicone tube that is inserted into the eye through a scleral fistula and shunts aqueous humor to an episcleral endplate located in the equatorial region of the globe. Capsule formation around the endplate occurs over a period of 4–6 weeks and provides resistance to aqueous flow. Aqueous pools within the reservoir created by the capsule and diffuses across the bleb wall for eventual reabsorption into the systemic circulation, thereby lowering intraocular pressure (IOP). Thinner blebs with larger surface areas tend to produce lower IOP.

Commercially available GDIs include the Ahmed glaucoma drainage device (New World Medical, Inc., Rancho Cucamonga, CA, USA), Baerveldt glaucoma implant (Abbott Medical Optics, Inc., Santa Ana, CA, USA), and Molteno glaucoma drainage device (Molteno Ophthalmic, Ltd., Dunedin, New Zealand). These differ in surface area, endplate composition, implant profile, and presence or absence of a valve ( Table 10.29.1 ). Valved devices (i.e., Ahmed valves) house a mechanism that allows aqueous egress above a minimum opening pressure and provide immediate IOP control, whereas nonvalved devices (i.e., Baerveldt, Ahmed ClearPath, and Molteno implants) require ligation to provide temporary restriction to flow while awaiting formation of an adequate capsule.

Indications

Drainage implants were traditionally reserved for eyes with complex, refractory glaucomas with one or more prior failed filtering surgeries or considered at high risk for trabeculectomy failure. Eyes with neovascular glaucoma, uveitic glaucoma, certain childhood glaucomas, iridocorneal endothelial syndrome, or extensive scarring from prior surgery, trauma, or conjunctival cicatrizing disease were among those chosen for primary GDI surgery. The role of GDIs has expanded in recent years, in part because of evidence from randomized clinical trials supporting their use in eyes at lower risk for filtration failure (see “Evidence From Randomized Clinical Trials” section).

Glaucoma drainage implant surgery may be performed using a staged approach or in combination with cataract, corneal, and/or vitreoretinal surgery in patients with multiple ocular comorbidities. Drainage implants appear to be less prone to failure than trabeculectomy in eyes requiring subsequent cataract surgery, because postoperative inflammation can compromise the longevity of a filtering bleb. Patients who have suffered a bleb-related complication or trabeculectomy failure in one eye may be considered for a primary drainage implant if incisional glaucoma surgery is required in the fellow eye. Although all patients need careful postoperative follow-up after incisional glaucoma surgery, those who are unable or less likely to adhere to the more rigorous postoperative visit schedule required after trabeculectomy may be better candidates for GDI surgery.

Preoperative Considerations

Assessment of conjunctival mobility at the slit lamp in eyes with previous surgery can help guide quadrant selection for placement of a GDI. Preoperative gonioscopy allows identification of high peripheral anterior synechiae that may impede anterior chamber tube entry through certain portions of the angle. Glaucoma drainage implants are most commonly inserted into the anterior chamber, but a shallow anterior chamber or endothelial cell dysfunction may necessitate sulcus or pars plana placement in patients with pseudophakia or aphakia. Sulcus or pars plana placement may also be preferable in pseudophakic or aphakic eyes with a corneal graft or those in which future corneal surgery is likely, because drainage implants may contribute to graft failure.

Patients in whom pars plana tube placement is planned should undergo thorough pars plana vitrectomy with particular attention to shaving of the vitreous base in the quadrant of intended tube insertion. Repeat vitrectomy is usually necessary in patients who have been previously vitrectomized to ensure adequate vitreous removal. Combined pars plana vitrectomy and pars plana GDI insertion has been shown to lower IOP successfully in patients with aphakia and pseudophakia, with an efficacy comparable with that of anterior chamber tube insertion. If preoperative examination reveals vitreous in the anterior chamber because of prior posterior capsular rupture or zonular dehiscence, then a vitrectomy is indicated before or at the time of GDI surgery to minimize the risk of vitreous occlusion of the distal tube tip.

Patients with neovascular glaucoma should be treated preoperatively with panretinal cyclophotocoagulation or anti–vascular endothelial growth factor whenever possible to minimize the risk of intraoperative bleeding. Use of preoperative intravitreal bevacizumab has been associated with a decreased risk of postoperative hyphema but has not demonstrated an effect on postoperative IOP or success rates after GDI surgery.

Implant Selection

Valved devices are often selected in patients with severely elevated preoperative IOP who require immediate IOP lowering, such as those with neovascular glaucoma. Patients at high risk for hypotony, including those with uveitis or who have undergone prior cyclodestructive procedures, may also benefit from placement of a valved implant. Evidence from recent clinical trials may further guide the selection of a particular implant type, endplate size, or endplate composition (see “Evidence From Randomized Clinical Trials” section). Ultimately, the choice of implant depends on the surgeon’s comfort and preference, which are determined by his or her experience.