• Glaucoma drainage devices (GDDs) are designed to lower intraocular pressure (IOP). They are also known as aqueous shunts, aqueous shunting devices, or tube shunts.

• Traditionally, they have been used to lower IOP in cases of failed filtration surgery and recalcitrant and complex glaucomas, such as inflammatory, neovascular, and traumatic glaucoma. Increasingly, since the publication of the Tube versus Trabeculectomy Study that compared Baerveldt 350-mm² implants to trabeculectomy with mitomycin-C,¹ some centers are using GDDs as a primary glaucoma surgery.

• All GDDs consist of a silicone tube connected to an episcleral plate (or explant), which is made of various materials in different shapes and surface area sizes depending on the specific model (Figs. 25-1 and 25-2). The plate is placed against the sclera at the globe’s equator, while the tube is inserted into the anterior chamber (or, more rarely, through the pars plana). Despite nonreactive materials, a collagenous and fibrovascular capsule (or bleb) develops around the episcleral plate. Once within the bleb, aqueous flows passively through the wall of the capsule and is reabsorbed by venous capillaries and lymphatics.

• GDDs can be divided into restrictive and nonrestrictive devices.

• Thus far, no one GDD model is superior to the others. The Ahmed Baerveldt Comparison Trial was a multicenter, randomized clinical trial comparing the safety and efficacy of the Ahmed FP-7 and Baerveldt 350-mm² implants. At 5-year follow-up, mean IOP was lower in the Baerveldt group on a similar number of
glaucoma medications, but that there were also a greater number of early and serious complications in the Baerveldt group compared to the Ahmed group.² The Ahmed versus Baerveldt study also compared these two devices. After 5-year follow-up, the Baerveldt group had a significantly lower cumulative probability of failure and required fewer medications than the Ahmed group, but it also experienced more hypotony-related complications.³

• Local anesthesia is recommended because some of the surgical steps can be painful without adequate anesthesia. A retrobulbar, peribulbar, or sub-Tenon’s injection can be utilized.

• The superotemporal quadrant (Fig. 25-3) is the preferred location for implantation of the first GDD (additional shunts can be implanted in other quadrants). For better exposure of the surgical site, a corneal traction (Fig. 25-4) or superior rectus bridle suture is helpful. The tube should be flushed with balanced salt solution to ensure patency using a 30-gauge cannula.

• Generally, a fornix-based peritomy is used. A 90- to 110-degree conjunctival incision is adequate for single-plate implants. Stevens tenotomy scissors are used to bluntly dissect posteriorly in the quadrant to make room for the plate. The episcleral plate is placed between adjacent rectus muscles with its anterior edge at least 8 mm posterior to the limbus (Fig. 25-5). Nonabsorbable sutures (6-0 to 8-0 nylon, prolene, mersilene) are passed through the fixation holes of the episcleral plate and sutured to the sclera.

• The optimal length of tubing is estimated by laying the tube across the cornea. The tube is then trimmed, bevel up, to extend 2 to 3 mm into the anterior chamber (Fig. 25-6). A corneal paracentesis is made to provide access to the anterior chamber in case of collapse (Fig. 25-7). A 22- or 23-gauge needle is used to create a track into the anterior chamber, parallel to the plane of the iris, starting approximately 1 to 2 mm posterior to the corneoscleral limbus (Fig. 25-8). The tube is then inserted through this track into the anterior chamber with smooth forceps (Fig. 25-9).

• Proper positioning of the tube in the anterior chamber is essential, ensuring that it does not touch the iris, lens, or cornea. The tube may or may not be fixed to the sclera with sutures of 10-0 nylon or prolene (Fig. 25-10). This anterior suture is wrapped tightly around the tube to prevent movement into or out of the anterior chamber, but not so tightly that it occludes the tube lumen. To avoid conjunctival erosion by the tube, processed pericardium, donor sclera or cornea, and, less commonly, fascia lata or dura, are used to cover the anterior scleral portion of the tube (Figs. 25-11 and 25-12). The patch graft is sutured in place using interrupted sutures. Alternatively, a partial-thickness limbal-based scleral flap can be fashioned, and the tube entry is made underneath this flap, so that the sutured flap covers the tube (Figs. 25-13 and 25-14).

• The tube can also be placed through the pars plana (Figs. 25-8, 25-9, 25-10, 25-11, 25-12, 25-13, 25-14, 25-15, 25-16, 25-17, 25-18, 25-19, 25-20, 25-21, 25-22, 25-23, 25-24, 25-25, 25-26, 25-27, 25-28, 25-29, 25-30 and 25-31) in cases where placement into the anterior chamber is difficult or undesirable. This approach requires a pars plana vitrectomy by a retina surgeon with careful attention to remove the vitreous skirt in the quadrant where the tube will be inserted.