Ординатура / Офтальмология / Английские материалы / Shields Textbook of Glaucoma, 6th edition_Allingham, Damji, Freedman_2010

scissors. Radial relaxing incisions on one or both sides of the conjunctival flap can improve surgical exposure. A muscle hook is then used to isolate the two rectus muscles on either side of the surgical site.

Figure 39.5 A: Creation of subconjunctival space in the superotemporal quadrant. B: Insertion of plate into subconjunctival space. C: Cutting the tube to the appropriate length.

Whenever possible, the superonasal quadrant should be avoided, especially with the larger plate designs, to reduce the risk for strabismus (discussed later) (58). The Ahmed drainage device, when placed in the superonasal quadrant, has also been shown to come within 1 mm of the optic nerve (59).

With Ahmed valved implants, balanced salt solution must be irrigated through the tube using 27-gauge cannula, before the insertion into the anterior chamber, to ensure that the valve opens properly.

The external plate is then tucked posteriorly into the sub-Tenon space (Fig. 39.5B) and is sutured to sclera with nonabsorbable 9-0 Prolene or nylon sutures through the anterior positional holes of the plate, with the anterior border at 8 to 10 mm posterior to the limbus. Variations of this technique are required for different plate designs. Implants with plates of larger circumferential dimensions, such as the Baerveldt, should be tucked under adjacent rectus muscles, whereas Schocket-type designs require dissection of one or more additional quadrants, depending on the extent of the encircling band. In the case of the Ahmed device, which has larger anteroposterior dimensions, extending the anterior border of the plate more than 8 mm behind the limbus is not advisable. However, it is advisable to have the reservoir plates at least 5 to 6 mm posterior to the limbus to prevent conjunctival erosion over the plate (Fig. 39.6).

With nonvalved devices, restriction of aqueous flow to avoid severe early postoperative hypotony can be achieved by using a two-stage implantation technique, in which the external plate is placed in the subconjunctival space without inserting the tube into the anterior chamber. The tube is inserted 6 to 8 weeks later, after the fibrous capsule has formed around the external plate (60, 61 and 62). A more popular technique is to occlude the tube by a ligature of 6-0, 7-0, or 8-0 Vicryl before inserting it into the anterior chamber. Injection of a balanced salt solution with a 30-gauge cannula into the tube helps to confirm that the tube is totally occluded. This procedure prevents any drainage of aqueous until 4 to 6 weeks after the operation when the Vicryl suture dissolves, allowing aqueous to drain into the preformed capsule. This technique provides the advantage over the two-stage technique of avoiding a second operation (63). Various tube ligatures and stents have been used to minimize postoperative hypotony (as discussed later under “Complications: Prevention an d Management”).

Figure 39.6 Exposed plate of an Ahmed glaucoma drainage device that was placed too close to the limbus.

Figure 39.7 Entering the anterior chamber through the limbal area with a 23-gauge or a 22-gauge needle parallel to the iris plane.

The tube is then cut, bevel up, to permit its extension 2 to 3 mm into the anterior chamber (Fig. 39.5C). Before the tube is inserted into the anterior chamber, a limbal area is cauterized to prevent bleeding from the insertion. A paracentesis can be made inferotemporally to allow placement of a small amount of viscoelastic in the anterior chamber. It is best to maintain the anterior chamber at a normal depth and avoid displacing iris posteriorly in order to assess the true position of the implant tube in the anterior chamber.

The anterior chamber is then entered through the cauterized limbal area with a 23-gauge or a 22-gauge needle, parallel to the iris plane (Fig. 39.7). The needle creates a watertight seal, preventing leakage around the tube and thus reducing the risk for postoperative hypotony (64). The angle at which the needle enters the anterior chamber is critical, because it is important that the tube, which will pass through this needle track, is positioned between cornea and iris, without touching the cornea.

The tube is then inserted into the anterior chamber via the needle track; there are specially designed tube-insertion forceps, but these generally are not necessary (Fig. 39.8). The tube can be secured to the sclera by using a nonabsorbable suture, such as 9-0 Prolene or nylon, but this step is also optional. Contact of the tube with the iris does not seem to cause any clinically noticeable problems, although tube occlusion by the iris and a distortion of the pupil have been reported (65, 66). The anterior chamber may need to be deepened with balanced salt solution, or viscoelastic, via the paracentesis, and the tube is checked for proper position in the anterior chamber.

The tube may occasionally erode through both sclera and overlying conjunctiva at the limbus. To avoid this potential complication, most surgeons suture a rectangle of preserved

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donor tissue of approximately 5 mm × 7 mm over the tube at the limbus (67) (Fig. 39.9). Processed pericardium (Tutoplast), donor sclera, dura, and fascia lata are available commercially for this purpose. It is also possible to use autologous sclera or to place the tube under a partial-thickness scleral flap, similar to a trabeculectomy procedure.

Figure 39.8 Tube insertion into the anterior chamber via the needle track, using nontoothed or specially designed tubensertion forceps.

The conjunctiva is then sutured back to its original position using Vicryl sutures. Subconjunctival steroids and antibiotics are injected at the completion of the procedure in a quadrant away from the surgical site. The basic postoperative management is the same as that described in Chapter 38 for filtering surgery, using topical steroid-antibiotic and mydriatic-cycloplegic preparations for the first several weeks.

Modifications of Basic Technique

Sometimes the conjunctiva is scarred at the limbus, making conjunctival dissection impossible without destroying much of the conjunctival tissue. In this case, the initial conjunctival incision may be made approximately 8 mm from the limbus to create a limbal-based conjunctival flap. Another option is to use

the inferotemporal or a superonasal quadrant.

Various occlusion ligatures include a posteriorly placed suture, a releasable suture, and an anterior chamber tube ligature. A 5-0 nylon suture can be threaded into the tube at the plate end and secured with one or two absorbable sutures around the tube (68, 69). The exposed end of the nylon suture is positioned subconjunctivally near the limbus for subsequent removal. Biodegradable stents, such as collagen lacrimal plugs or 4-0 chromic suture, have also been evaluated, but they have been less satisfactory because they do not always dissolve (70, 71). The internal and external occlusion techniques may be combined. For example, a 5-0 nylon or 3-0 Supramid internal occlusion suture is placed, along with an external Vicryl ligature around the tube. The internal stent is then pulled without difficulty in the office treatment room (68).

Figure 39.9 Suturing of donor sclera or other patch material over the tube area at the limbus.

Using a clear corneal graft, tied with 8-0 nylon, instead of the pericardium or scleral graft to cover the outer portion of the tube provides the view of the tube with the suture for postoperative laser suture lysis (72).

As an alternative to the use of a preserved tissue, an autologous, partial-thickness scleral patch graft crafted from the sclera adjacent to the tube has been described. No complications were reported in the study, but the risk for perforation of the globe exists during the dissection of the flap, and this may not

be a good choice in the eyes with high myopia or with scleritis (73).

Some surgeons combine stent occlusion (Fig. 39.10) with longitudinal slits in the tube to provide early IOP control (74), and a laboratory study indicated that a slit valve of 2.0 mm appears to provide an

opening pressure of approximately 10 mm Hg (75). Using the 350-mm2 Baerveldt implant, the role of fenestrations in the tube and antimetabolites was studied in controlling IOP in the early postoperative period. An occlusive 7-0 Vicryl suture was placed just anterior to the plate, followed by a through-and- through penetration of the tube with a standard 15-degree blade in longitudinal orientation just anterior to the ligature. The IOP was elevated at day 21 because of fibrotic blockage of the fenestrations before the ligature dissolved, but the pressure was well controlled by antiglaucoma medication use or laser suture lysis of the 7-0 Vicryl occlusion suture. The use of antimetabolites did not improve the outcome (76).

Another potential complication, as with all filtration procedures, is failure due to excessive fibrosis. In a rabbit study that compared intraoperative use of mitomycin C, 0.5 mg/mL for 5 minutes, with

implantation of 200-mm2 Baerveldt implants, the mitomycin C-treated eyes had lower IOPs and higher perfusion rates at 2, 4, and 6 weeks (77). In one retrospective study, implantation of Ahmed glaucoma drainage devices combined with the use of mitomycin C achieved lower postoperative IOP, with fewer glaucoma medications and similar complication rates compared with Ahmed devices implanted without antimetabolites (78). However, three retrospective studies of Baerveldt and Molteno implants and a randomized trial with Baerveldt implants did not show any benefit to using intraoperative mitomycin C (76, 79, 80 and 81). At the present time, the preponderance of evidence indicates that use of antimetabolites in conjunction with glaucoma drainage-device surgery incurs little benefit.

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Figure 39.10 Example of a Baerveldt 350-mm2 implant in which a 5-0 nylon stent suture is placed and a 7-0 polyglactin suture is tied around the silicone tube, constricting it occlusively around the stent suture. The needle of the 5-0 nylon suture allows for easy externalization and placement under the bulbar conjunctiva.

Special Situations Pars Plana Insertion

In aphakic (or possibly pseudophakic) eyes in which a vitrectomy has been performed, the tube can be inserted through a pars plana incision into the vitreous cavity. The pars plana tube shunts are usually used when placing the tube into anterior chamber is impossible or undesirable or when a need for pars plana vitrectomy coexists. A Hoffman elbow has been designed for pars plana insertion, and excellent results were demonstrated with the Baerveldt implant following pars plana vitrectomy and fluid-gas exchange. A mean postoperative IOP of 14 mm Hg with an average of 0.6 glaucoma medications was reported in one study (82). The pars plana insertion has the advantage of keeping the tube away from the cornea (especially after penetrating keratoplasty [PKP]) and iris and of reducing the risk for epithelial downgrowth. It is especially important in eyes with corneal grafts. Repositioning of the glaucoma drainage device from the anterior chamber into the vitreous cavity after pars plana vitrectomy for anterior segment complications, such as corneal decompensation, or recurrent tube erosion, is another option (83).

Preexisting Scleral Buckle

The treatment of a retinal detachment may be associated with postoperative glaucoma. The presence of the scleral buckle pre sents a special challenge in cases in which IOP cannot be controlled medically. Conjunctival scarring caused by retinal surgery can significantly decrease the success of trabeculectomy, even with the use of antimetabolites. Cyclodestructive procedures can be used, but they are unpredictable and may cause significant complications. Glaucoma drainage devices are a useful option to control IOP in such eyes, although the presence of scleral buckle makes placement of the plate challenging.

When a scleral buckle has been placed in the eye for more than 3 months, a silicone tube can be inserted into the anterior chamber, with the distal end introduced into the fibrous capsule of the preexisting scleral buckle, which serves as an external reservoir for aqueous drainage. Because the buckle is already encapsulated, no ligation of the tube to restrict the flow is necessary. In one study, the IOP was successfully controlled in 85% of patients (36).

Long Krupin-Denver valved implants with a flow restrictor at the distal end of the tube can further decrease the chances of postoperative hypotony in eyes with scleral buckle. If the scleral buckle was placed more recently than 3 months and the fibrous capsule has not formed yet, a smaller Baerveldt implant can be used, and the “wings” of the device sometimes need to be trimmed to position the plate underneath the existing scleral band. The fibrous capsule then is expected to grow around the buckle and the Baerveldt implant (84).

Successful insertion of a Baerveldt drainage device behind or over a preexisting scleral buckle, or in the segment without retinal hardware, has been described. Excising the capsule overlying the band allows continuous encapsulation of the band and Baerveldt plate to achieve greater IOP reduction. After 1 year,

IOP control was achieved without medications in 78% of patients with 350-mm2 plates, but in only 29%

of patients with 250-mm2 plates (85). Preexisting Corneal Graft

Glaucoma after PKP remains a difficult management issue. PKP often causes additional damage to the angle, inducing peripheral anterior synechiae formation, with further impediment to aqueous outflow. Control of post-PKP glaucoma is complicated by the need to preserve graft clarity for visual function. When medical management fails, if the angle is open and grossly normal, argon laser trabeculoplasty may be an option. If further intervention is indicated, a glaucoma drainage device, in eyes with good visual potential, is recommended. For eyes with poor visual potential (or for patients who cannot undergo surgery), transscleral cyclophotocoagulation may be a better option (86).

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However, placement of glaucoma drainage devices in the anterior chamber may be complicated by tubecornea touch and endothelial decompensation, particularly after corneal transplantation. Only 70% and 55% of the corneal grafts survived at 2 and 3 years, respectively, after insertion of glaucoma drainage

devices into the anterior chamber (87). A retrospective review of simultaneous PKP and Ahmed glaucoma drainage-device implantation showed 92% and 50% graft success and 92% and 86% IOP control at 1 and 3 years, respectively (88).

Pars plana insertion is a reasonable option for patients who have undergone PKP or in whom PKP is anticipated, despite the need for a complete pars plana vitrectomy. The pars plana approach avoids complications related to limbal tube placement and offers better corneal graft survival, but the incidence of posterior segment complications may be higher for pars plana insertion (89).

Implantation of the tube through the ciliary sulcus is another alternative to anterior chamber angle placement in pseudophakic or aphakic eyes with refractory glaucoma and a high risk for corneal decompensation, or eyes with a shallow anterior chamber or extensive synechial angle closure. Positioning of the tube under the iris may be particularly advantageous in the presence of an anterior chamber intraocular lens, because the tube would not disturb the lens. This procedure is contraindicated in phakic eyes because of a possible injury to the crystalline lens (90).

COMPLICATIONS: PREVENTION AND MANAGEMENT Hypotony

Until the fibrous capsule has developed around the external plate to regulate aqueous flow, the open, nonvalved drainage devices, as noted earlier, provide very low resistance to flow, and hypotony in the early postoperative course with nonvalved implants is a serious complication. By far the best way to prevent this potential complication is by temporarily obstructing the tube lumen. Many techniques have been described to achieve this goal. Basic techniques include suture ligation of the tube, as previously described; temporary occlusion of the tube lumen with a stent; two-stage implantation; or use of a valved implant. Early postoperative hypotony was found in fewer than 10% of patients after Ahmed glaucoma drainagedevice surgery (9, 91). If early postoperative hypotony happens in combination with a flat anterior chamber, then injection of dense viscoelastic into the anterior chamber and close observation in the first 24 hours may be helpful. If the flat chamber and hypotony reoccur, then removal of the tube from the anterior chamber is recommended to prevent corneal decompensation with planning to reposition the tube into the anterior chamber within the next few days.

Late hypotony from glaucoma drainage-device implantation is usually treated with permanent occlusion of the proximal tube or removal of the tube from the anterior chamber, which permanently removes the effect of the entire implant. Permanent ligation of the tube to the distal plate of doubleplate Molteno implant has the advantage of reducing, but not completely eliminating, the effect of the implant (92). Elevated Intraocular Pressure

Glaucoma drainage-device procedures can also be complicated by elevated IOP in the early or the late postoperative period. Before the ligature around the tube dissolves, there may be a transient elevation of the IOP. It can be prevented by combining a trabeculectomy without mitomycin C with the drainage device, or it can be managed medically. Within the first 7 to 10 days after surgery, a hypotensive phase may present with low IOP, conjunctival and corneal edema, and congestion of conjunctival blood vessels in tissues covering the plate of the implant. This may be followed by a second, hypertensive phase, which is characterized by IOP elevation associated with the formation of the capsule. In this phase, the edema disappears and fibrous tissue develops in the deepest layers of the bleb. During the first 1 to 4 weeks of this phase, the bleb wall becomes congested, causing the IOP elevation. Congestion and inflammation subsequently subside, with IOP reduction and stabilization over the next 3 to 6 months. The hypertensive phase portends a poor prognosis for IOP control (9, 93).

Elevated IOP in the early postoperative period may be due to obstruction of the tube by fibrin, blood, iris, vitreous membranes, or silicone oil (Fig. 39.11). This was observed in 11% of eyes after implantation of an Ahmed glaucoma drainage device (91), with iris and fibrinous membranes being the most common tissues responsible for blockage (30.8% each), followed by a neovascular membrane, a fibrinous strand, and an iridocorneal endothelial membrane. Iridectomy at the site of the tube ostium has been recommended to prevent iris plugging the tube ostium (94) but requires a larger incision. Nd:YAG laser membranectomy was effective for reopening blocked glaucoma tube shunts and maintaining the patency over time in 84.6% of the

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eyes in one retrospective study, but recurrence of the blockage occurred in 53.8% of eyes within the first 11 weeks. Postlaser complications included moderate anterior chamber reaction, hyphema, corneal edema, pressure spike, and a shallow anterior chamber (95). Distal tube occlusion by fibrous tissue has been reported after placement of glaucoma drainage devices in the fibrous capsule around a preexisting scleral buckle (36, 56, 96).

Figure 39.11 Occlusion of a glaucoma drainage device by fibrin in a patient with neovascular glaucoma. (From Junk AK, Katz LJ. Tube shunts for refractory glaucomas. In: Tasman W, Jaeger EA, eds. Duane's Clinical Ophthalmology. Vol 6. Lippincott Williams & Wilkins; 2007:chap 17.)

Reported techniques to open the occluded tube include irrigation of the tube with balanced salt solution using a 30-gauge cannula through a paracentesis incision, the use of Nd:YAG or neodymium-doped yttrium lithium fluoride to open occluded tubes, and the intracameral injection of tissue plasminogen activator (0.1 cc of 5 to 13 µg) to dissolve a fibr in clot (97, 98, 99, 100 and 101).

Late IOP elevation, especially when the intraocular portion of the tube appears to be patent, is usually due to an excessively thick fibrous capsule. Needling revision can improve function of the encapsulated drainage implant. It is more successful when the drainage device has a larger surface area, although the risk for severe complications, including endophthalmitis, exists (102).

When needling is unsuccessful after a few attempts, removing a portion of the encapsulated bleb beneath the conjunctiva may be beneficial. In a retrospective study assessing 95 eyes (of 79 consecutive patients) that underwent a single-stage Molteno implantation, 14 eyes (of 12 patients) developed recurrence of the encapsulated bleb. With a mean follow-up of 30 months, the mean IOP after capsule excision was significantly lower than the preoperative IOP, achieving a 75% success rate (103).

Topical corticosteroid therapy can cause IOP elevation despite the presence of a functioning glaucoma drainage device (104).

Migration, Extrusion, and Erosion

Tube migration may occur after glaucoma drainage-device procedures (105). If the tube is not adequately secured to the sclera, it may migrate posteriorly out of the anterior chamber, which may require repositioning of the tube and securing it to the sclera with additional 9-0 Prolene sutures.