63

Aqueous Shunting Procedures

Leonard K. Seibold, MD; Malik Y. Kahook, MD; and Sarwat Salim, MD, FACS

Aqueous shunting procedures were introduced by Molteno and colleagues^1–4 in 1968 to treat refractory glaucomas. In the initial design, an 8.5-mm² acrylic plate was attached to an acrylic tube to allow the formation of a bleb, with free communication to the anterior chamber that could not shrink to an area less than that of the plate. This device successfully controlled intraocular pressure (IOP) in many patients who were otherwise poor surgical candidates. However, problems related to early postoperative hypotony from overfiltration and late IOP increases resulting from fibrous encapsulation of the filtering bleb limited the usefulness of this procedure.

During the past 40 years, design modifications and improvements in surgical techniques have led to greater success and lower complication rates with the Molteno implant (Nova Eye Medical). In addition, other glaucoma drainage devices (GDDs) have been introduced and offer unique features designed to facilitate implantation, improve IOP control, and reduce acute postoperative hypotony.

TRADITIONAL GLAUCOMA DRAINAGE DEVICES

Available GDDs differ in size, shape, and composition material. One fundamental feature distinguishing various types is the presence or absence of a valve in the implant. The valved or flow-restrictive devices allow only unidirectional flow from the anterior chamber to the subconjunctival space with a minimum opening pressure. Examples of valved devices include the Ahmed Glaucoma Valve (AGV; New World Medical) and Krupin valve (production discontinued). The nonvalved or open-tube or nonrestrictive devices provide passive flow in both anterograde and retrograde direction. Examples of nonvalved devices include the Molteno, Baerveldt (Johnson & Johnson Vision), Schocket, and Eagle Vision (product no longer commercially available) implants. Because nonvalved devices offer no flow restriction, additional measures, to be discussed later, are undertaken intraoperatively and postoperatively to prevent hypotony. Table 63-1 provides a summary of commercially available devices with their respective characteristics.

Molteno Implant

The Molteno implant is a nonvalved device consisting of a silicone tube with an internal diameter of 0.33 mm and an outer diameter of 0.63 mm and is connected to either a rigid polypropylene or a flexible silicone plate. Single-plate (area: 137 mm²) and double-plate implants (area: 274 mm²) are available.

A dual-chamber, single-plate implant that incorporates a pressure ridge on the upper surface of the episcleral plate has been introduced in an effort to reduce immediate postoperative hypotony and related complications. The double-plate implants are available in right and left models and consist of 2 plates connected by a 10-mm silicone tube. The double-plate implant has been shown to provide improved IOP control because of its larger surface area but is also associated with a higher rate of postoperative hypotony.⁵

Recently, the third-generation Molteno 3 implants have been introduced in both single-plate and double-plate models (Figures 63-1 and 63-2). These newer devices have a lower profile as the height of the original ridge has been reduced and its shape modified from a triangular to elliptical appearance. These modifications are expected to decrease the incidence of postoperative hypotony and improve IOP long-term control.

The anterior chamber tube to encircling band implant consists of a silastic tube connected to a No. 20 band 360 degrees in length (surface area: 350 to 450 mm²). The band, placed under 2 or more rectus muscles, creates a reservoir for aqueous drainage. Surgical outcomes with the Schocket tube shunts have been compared to double-plate Molteno implants. Although the encircling band provides a larger surface area for aqueous drainage, the final IOP was reported to be lower than with the Molteno implants.6,7

Krupin Valve With Disc

The Krupin valve with disc is no longer being manufactured, but is mentioned for historical purposes. It was the first device to incorporate a unidirectional and pressure-sensitive slit valve (opening pressure 11 mm Hg, closing pressure 9 mm Hg) that served to maintain anterior chamber depth and IOP in the immediate postoperative period. The Krupin valve with disc consisted of a silastic tube with an internal diameter of 0.38 mm connected to an oval silicone plate measuring 13 mm × 18 mm and was 1.75 mm thick.

Ahmed Glaucoma Valve

The AGV is a valved implant consists of a silastic tube with an outer diameter of 0.63 mm and an inner diameter of 0.3 mm that is connected to a silicone sheet valve and is held in an elliptical polypropylene or silicone plate (Figure 63-3). The pressure-sensitive valve consists of 2 opposed silastic sheets that separate to allow aqueous flow at an IOP between 8 and 12 mm Hg. One of the earlier studies⁸ assessed the pressure-flow characteristics of various GDDs in vitro and in vivo in rabbits. Both the Ahmed and Krupin valves functioned as flow-restricting devices, but a true valve function for these was not elucidated. Subsequent studies,^9,10 however, demonstrated a consistent valve behavior for the AGV, and IOP was regulated within a desired range by decreasing or increasing resistance as a function of flow.

Single-plate (area: 184 mm²) and double-plate (area: 364 mm²) implants are available. A pars plana clip allows insertion of the tube into the pars plana in aphakic or pseudophakic eyes after complete vitrectomy (Figure 63-4). Smaller-size implants are also available for pediatric use (area: 102 mm²). However, most surgeons use a normal-size implant in children with the expected growth of a child’s eye and to achieve better long-term IOP control.

Baerveldt Implant

The Baerveldt implant consists of a silicone tube with an internal diameter of 0.3 mm and an external diameter of 0.64 mm and is connected to a 1-mm-thick barium-impregnated silicone plate that allows radiographic identification. The plates are available with surface areas of 250 mm² (22 × 14 mm) and 350 mm² (32 × 14 mm). The pars plana variant consists of a 350-mm² plate with the silicone tube attached to a small silicone episcleral plate with an angled cannula to be inserted through a sclerostomy into the pars plana. This special Hoffman elbow provides a watertight closure that may not be achieved with insertion of the standard tube. All Baerveldt plates consist of 4 fenestrations on the body of the implant to allow fibrous capsule growth between the anterior conjunctival surface and posterior scleral walls (Figure 63-5). These fenestrations are intended to reduce the height of the resulting bleb to lessen the mass effect on the adjacent extraocular muscles and to minimize ocular motility disturbances.

The Ahmed ClearPath implant (New World Medical) is a nonvalved device consisting of a tube with 0.3-mm inner diameter and 0.63-mm outer diameter connected to a 0.84-mm-thick silicone plate. There are 2 plate sizes available with surface areas of 250 mm² (14 × 22 mm: 250 model) or 350 mm² (15 × 32 mm: 350 model). The 250 model is oval shaped and designed to fit between the recti muscles, while the 350 model contains lateral wings that should be placed under the recti muscles. Both models contain anteriorly located suture holes to facilitate suture fixation as well as enhanced plate curvature to better conform to the globe (Figure 63-6).

GDDs are typically reserved for patients with severe uncontrolled glaucoma who have failed previous glaucoma surgery. In addition, the devices appear to be advantageous as a primary procedure in patients with a high likelihood of trabeculectomy failure, including neovascular and uveitic glaucomas.11–17 Their role in managing congenital and developmental glaucomas has increased exponentially.^18–24 Additional indications include traumatic glaucoma, aphakic and pseudophakic glaucoma, post-keratoplasty glaucoma, and other secondary glaucomas.^25–28 In eyes with useful remaining vision, these devices may be preferable to cyclodestructive procedures that are associated with a high rate of visual loss and phthisis bulbi. Recently, interest has increased in using these devices as a primary surgical procedure for uncontrolled primary open-angle glaucoma. Two randomized, prospective clinical trials comparing trabeculectomy to tube surgery in eyes with (study completed) and without (study ongoing) prior ocular surgery will be discussed later in this chapter. Table 63-2 provides a summary of indications for inserting GDDs.

The most important factor determining the type of implant selected is the target IOP, both in the short-term and long-term. Early IOP control is determined by the presence or absence of a valve in an implant as the tube itself offers no resistance to aqueous flow. The valved devices provide more immediate IOP control and a lower rate of hypotony. Because nonvalved devices are often occluded with a stent or ligature suture, the postoperative IOP is unchanged and requires continuation of all preoperative medications until a fibrous capsule forms and the ligature dissolves or stent removed. With all devices, long-term IOP control depends on the surface area of the implant, which determines bleb size, tissue response to the implant, and thickness of the fibrous capsule controlling percolation of aqueous humor through the bleb wall. There is some suggestion29 that earlier exposure of aqueous humor to the developing capsule may interfere with long-term IOP control. If this is indeed true, then 1-stage or 2-stage insertion of nonvalved devices may be preferable to valved devices to avoid immediate flow of aqueous humor to the plate.

Plate material has been studied to determine its influence on final IOP, as it may affect tissue reaction and the degree of bleb encapsulation. Ayyala and colleagues^30,31 demonstrated more inflammation with the polypropylene plate (Molteno implant) than with the silicone plate (Krupin implant) when inserted subconjunctivally in rabbits. Two retrospective studies^32,33 compared AGV silicone (model FP7) and polypropylene (model S2) and reported similar results with both models in terms of IOP control, final visual acuity, and postoperative antiglaucoma medications. In one of these studies,^32,33 the silicone valve was associated with fewer serious complications. The AGV silicone and polypropylene material has also been investigated in a prospective, multicenter, comparative series³⁴ that reported improved final IOP control with the silicone model compared with the polypropylene model. The investigators observed more Tenon’s cysts in the polypropylene group.

Plate size of various implants has been investigated to determine its influence on the final IOP. Heuer and colleagues⁵ reported improved IOP control with the Molteno double plate when compared with the single plate in a prospective study assessing outcomes in aphakic and pseudophakic glaucoma. In a retrospective study,³⁵ the double-plate Molteno demonstrated lower mean IOP when compared with the single-plate AGV, 13.3 ± 5.1 mm Hg vs 19 ± 5.8 mm Hg (P = .009), respectively, at 24 months. In a prospective study comparing the 350-mm² and 500-mm² Baerveldt implants, Lloyd and colleagues³⁶ reported statistically comparable results with respect to IOP control, visual acuity, and complications. In another prospective study comparing 350-mm² and 500-mm² Baerveldt implants, Britt and colleagues³⁷ found better IOP control with the 350-mm² Baerveldt implant than with the 500-mm² model. Allan and colleagues38 compared long-term outcomes of the 250-mm² and 350-mm² Baerveldt implants and found no difference in terms of success rates, vision, IOP, medication requirement, and complication rates. These studies indicate that size of the implant does matter, but to a limited extent. Further studies are warranted to determine which one of these variables—size, shape, or composition—is most likely to affect the long-term success of GDDs.

Anesthetic Considerations

The choice of anesthesia for inserting a GDD depends on the presence of other medical comorbidities, the cooperation level of the patient, and the comfort of the surgeon. The most commonly used anesthesia is a peribulbar or retrobulbar block, which provides both akinesia and anesthesia. A sub-Tenon’s injection is also a good alternative. Topical or intracameral anesthesia is usually not sufficient because of manipulation of extraocular muscles with some implants. General anesthesia may be reserved for patients with claustrophobia, altered mental status, or history of poor cooperation with local anesthesia in previous surgeries.

SURGICAL TECHNIQUE

Implantation of a GDD requires careful attention to detail at every step of the procedure to improve results and minimize postoperative complications. For a 1-plate implant, a fornix-based or limbus-based conjunctival incision is created, extending for 90 degrees to 110 degrees centered between 2 rectus muscles. Implantation of the plate is easier with a fornix-based flap; however, the conjunctival closure is more involved. If a fornix-based conjunctival flap is created, 1 or 2 radial relaxing incisions are usually required to allow adequate exposure for insertion and fixation of the plate. A corneal or scleral traction suture can be placed to improve exposure in the working quadrant (Figure 63-7). With the conjunctiva and Tenon layers retracted away from the globe to expose bare sclera, the implant is positioned between 2 rectus muscles so that the anterior edge is approximately 8 to 10 mm posterior to the limbus (Figure 63-8). Larger implants (Baerveldt) are inserted with the long axis directed toward the apex of the orbit and then rotated horizontally so that the tube points directly toward the anterior chamber and the wings of the implant are under the rectus muscles. A muscle hook should be used to identify and mark muscle insertions for proper positioning of the plate between or under the muscles. If a 2-plate implant is used, 1 plate is positioned in each of 2 adjacent quadrants. The tube connecting the 2 plates may be passed under or over the intervening rectus muscle. With all valved implants, the tube should be primed with balanced salt solution with a 27- or 30-gauge cannula prior to the plate anchorage to ensure that the valve leaflets are not fused after sterilization techniques (Figure 63-9). The tube of the nonvalved implant should be irrigated as well to ensure its patency.

Once the implant has been appropriately positioned, each plate is secured to the globe with 2 nonabsorbable sutures (8-0 or 9-0 nylon sutures on a spatulated needle). The suture knots should be rotated into the fixation eyelets to prevent erosion through the conjunctiva. Secure attachment to the underlying sclera is essential to prevent anterior, posterior, or lateral migration of the implant during the postoperative period. Inadequate scleral fixation may also lead to retraction of the tube from the anterior chamber or expulsion of the entire plate from the subconjunctival space.

After the plate is attached to the globe, the tube is laid across the cornea and cut with sharp scissors to create a beveled edge with the opening toward the cornea for anterior chamber insertion. The tube should extend approximately 2.5 to 3 mm into the anterior chamber to minimize the risk of tube-cornea touch or retraction out of the anterior chamber. A 23-gauge needle is used to create a track through which the tube is inserted into the anterior chamber just anterior and parallel to the iris (Figure 63-10A). Occasionally, insertion of the tube through the scleral track is difficult. A well-beveled tube end (30 to 45 degrees, bevel up) and nontoothed forceps simplify this step. Alternatively, a tube inserter (New World Medical) is available to facilitate this maneuver (Figure 63-10B). After the tube has been inserted into the anterior chamber, its position is checked carefully to ensure that there is no tube-cornea touch or iris incarceration (Figure 63-10C). If the tube is malpositioned, a new entry track should be created to the side of the original track through which the tube should be reinserted. The original entry site should be sutured closed to maintain anterior chamber depth intraoperatively and to avoid postoperative overfiltration and hypotony. The tube may be secured to the sclera a few millimeters anterior to the plate with an 8-0 Vicryl (Ethicon, Inc) suture (some surgeons prefer an 8-0 or 9-0 nylon suture). This suture helps to stabilize the tube and should not be tight; otherwise, it will restrict flow in valved devices.

The tube is covered to prevent its erosion through the conjunctiva. Patch graft materials include processed pericardium, sclera, fascia lata, dura, or cornea. Usually, a 6-mm × 6-mm patch graft is used to provide good coverage. The graft is positioned to overlay the tube insertion site into the anterior chamber, and the limbal edge is thinned to avoid an overhanging bleb that can lead to Dellen formation. The patch graft should be secured to the globe with interrupted sutures at the anterior corners by using either 8-0 Vicryl or nylon sutures (Figure 63-11). Alternatively, fibrin glue has been shown to be an effective, although expensive, substitute for sutures and was reported to reduce surgical time and postoperative inflammation.³⁹ Additional studies are needed to understand its role further. All suture ends should be buried beneath the scleral graft to prevent them from later eroding through the conjunctiva. If the patch graft material is not available, a partial-thickness scleral flap can be constructed. The needle track and tube entry are done under this flap. The flap is then sutured with 10-0 nylon sutures.

After the patch graft has been placed, the conjunctiva and Tenon layers are pulled over the plate, tube, and patch graft and are secured into place with an 8-0 Vicryl suture (Figure 63-12). In some cases, the monofilament 9-0 Vicryl suture is preferred because of its higher tensile strength and finer vascular needle to prevent buttonholes when handling thin conjunctiva. Prior to conjunctival closure, some surgeons cauterize the limbal margin of the cornea to remove the epithelium and to provide a bed to facilitate the attachment of the conjunctiva during the healing process. Wing sutures at both ends of the peritomy are recommended to approximate the conjunctiva at the limbus. A locked running Vicryl suture is useful in closing the radial relaxing incision.

At the end of the operation, the eye should be inspected to ensure that the implant plate, patch graft, and intraocular portion of the tube are in a good position. Fluorescein drops or strips can be used to inspect the conjunctiva for leaks. Any buttonholes found in the conjunctiva should be closed with a 9-0 Vicryl suture. At the conclusion of the procedure, a subconjunctival injection of antibiotic and steroid is given.

MODIFICATIONS TO PREVENT HYPOTONY WITH NONVALVED IMPLANTS

Internal Tube Occlusion (Stent)

Aqueous drainage through a nonvalved device can be regulated in the early postoperative period by passing a 4-0 or 5-0 Prolene (Ethicon, Inc) or nylon suture through the lumen of the implant tube.⁴⁰ The stent suture is brought out through the tube, over the implant plate, and is placed with its free end in an adjacent subconjunctival quadrant (Figure 63-13). Once the fibrous capsule around the plate has formed, the stent suture is removed at the slit lamp under local anesthesia and the aqueous humor can flow freely into the fibrous capsule that has formed around the implant plate. Although this technique is effective in preventing early postoperative hypotony, immediate postoperative IOP elevations from tube occlusion may make early suture removal necessary, increasing the risk of hypotony, shallow chamber, and related complications.

External Tube Occlusion (Ligature)

The flow of aqueous humor through a nonvalved device can also be restricted by placing a suture ligature around the external aspect of the tube.^41,42 The external occlusion may be accomplished using a nonabsorbable 7-0 suture with a releasable knot or a 7-0 or 8-0 absorbable Vicryl suture tied around the tube. Alternatively, a 9-0 nylon or 10-0 Prolene suture may be used to ligate the tube inside the anterior chamber to allow for later suture lysis with the argon laser. The remnant of this suture remains in the anterior chamber. Similar to an internal occluding suture, release of an external ligating suture is usually performed 4 to 6 weeks postoperatively, which allows time for a fibrous capsule to form around the plate to provide resistance to aqueous flow when the suture is released. Because of this prolonged delay before the IOP is reduced, this technique is poorly suited for patients with exceedingly high pressures during the preoperative period. Attempts to tie the ligature sutures at different tensions to create a titrated aqueous flow rate are unpredictable and may result in under- or overfiltration. Some surgeons create venting slits anterior to the ligature suture with either a needle or a knife for aqueous drainage, thereby allowing immediate IOP control in the early postoperative period.

To prevent postoperative hypotony, a shunt procedure may be performed in 2 stages.2 In the first stage, the plate is attached to the globe, and the tube is left in the subconjunctival space without entering the eye. Four to 6 weeks later, after a capsule has formed around the implant, the conjunctiva is opened, and the tube is inserted into the anterior chamber to complete the procedure. A major disadvantage of this procedure is that it does not reduce IOP until the second stage is complete, making it poorly suited for eyes with extremely high IOP in the preoperative period. It also requires a second operative visit. Combining the first stage of the operation with a trabeculectomy in an adjacent quadrant allows for immediate IOP control and provides time for capsule formation around the plate. In this procedure, the second stage of the procedure is performed when the trabeculectomy fails and IOP increases.

PARS PLANA INSERTION

The tube of the GDD is most commonly placed in the anterior chamber. However, the tube may also be placed in the sulcus in a pseudophakic eye or through the pars plana in a vitrectomized eye. This procedure can be accomplished using 1 of 2 options: a Pars Plana Clip (Model PC, New World Medical), which can be used with any drainage device, or Hoffman elbow, which is mounted on a Baerveldt 350-mm² implant. The plate is secured to the sclera as described. A 21-gauge needle is inserted through the sclera approximately 3 mm from the limbus in a pseudophakic eye and 3.5 mm from the limbus in a phakic eye. The needle should be angled parallel to the iris plane for proper positioning of the tube through this track. The tube should be of adequate length (4 to 5 mm) to allow visualization through the pupil. The tube can be secured and ligated, and the rest of the procedure can be completed as described above. The clip or the elbow portion should be secured to the sclera using nonabsorbable sutures. Pars plana models reduce the incidence of tube-related anterior segment complications, especially in the setting of a shallow anterior chamber, penetrating keratoplasty, or concomitant need for retina surgery; however, the eye must be completely vitrectomized to avoid occlusion of the tube with the vitreous.⁴³

SITE OF IMPLANTATION

With the exception of the 2-plate implants, most glaucoma implants are placed in a single quadrant. Whenever possible, single-plate implants should be placed in the superotemporal quadrant. This area provides the easiest access for the surgeon to implant the plate and is least likely to produce motility disturbances. Implantation of a large-plate aqueous shunt in the superonasal quadrant has been associated with Brown’s superior oblique tendon syndrome.⁴⁴ Substantial hypertropias and limitations of downgaze have been reported with inferior implantation of a 2-plate Molteno and the Krupin valve with disc.⁴⁵ If severe conjunctival scarring necessitates the placement of an inferior or superonasal shunt, the possibility of ocular motility disturbance and diplopia should be discussed with the patient. Using a smaller plate implant may be less likely to cause a muscle imbalance; however, the risk of strabismus must be weighed against the better IOP control, which is often achieved with a larger or multiple-plate implant. In eyes containing silicone oil, the implant is placed in the inferior quadrant to minimize loss of oil, which is lighter than aqueous and floats up (Figure 63-14).

ROLE OF ANTIFIBROSIS AGENTS

Molteno³ was the first to suggest that the success rate of aqueous shunting procedures could be improved by adding antifibrosis therapy. Molteno used a combination of epinephrine, atropine, topical steroids, oral steroids, and colchicine to reduce postoperative scarring and bleb fibrosis in an attempt to increase the success rate of his procedure. Because of the high frequency of systemic side effects associated with this regimen, it is rarely used today.

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