Ординатура / Офтальмология / Английские материалы / Glaucoma Surgery_Bettin, Khaw_2012

of the device to the sclera. The end plate is then positioned under the conjunctiva, approximately 8–10 mm posterior to the surgical limbus. Some GDI types allow for placement between the recti muscles. The implant is then sutured to the sclera with 8-0 or 9-0 nonabsorbable sutures on a spatulated needle. Attention is then paid to the GDI tube, which is trimmed with an anterior bevel if placed in the anterior chamber or through pars plana, or posterior bevel if placed into the ciliary sulcus, to prevent iris from occluding the tube tip.

Nonvalved GDIs require an additional step to restrict aqueous flow before fibrous encapsulation of the plate. This may be achieved by ligating the proximal portion of the tube with absorbable suture and/or by placing a suture within the tube lumen. Nonabsorbable suture may also be used to ligate the GDI tube, but this will require postoperative laser suture-lysis in order to establish aqueous flow. It is important for the surgeon to test for adequate ligation of the GDI tube by injecting BSS into the tube lumen and noting an absence of flow through the tube to the end plate. For temporary aqueous drainage before fibrous encapsulation of the nonvalved GDI plate, one or more perforations may be placed across both walls of the tube using a spatula needle.

A 23or 25-gauge needle is then used to enter the anterior chamber immediately posterior to the surgical limbus and parallel to the iris plane. A standard paracentesis incision may be performed prior to this maneuver for intraoperative control of the anterior chamber. The GDI tube is then grasped with nontoothed forceps and inserted through the needle track and into the anterior chamber. Ideally, the tube is placed just in front of the iris. The tube may be secured to the sclera with one or two 9-0 or 10-0 nylon sutures for additional fixation if desired.

In order to decrease the risk of postoperative exposure of the GDI tube, a tissue patch graft is placed over the tube at the anterior chamber entry site. The tissue graft is secured to the sclera with absorbable or nonabsorbable suture. Alternatively, the tube may be placed under a partial-thickness scleral flap before anterior chamber entry. Conjunctiva is then reapproximated and closed using an interrupted and/or continuous suturing technique. At the conclusion of the case, a subconjunctival injection of an antibiotic and corticosteroid is performed in the quadrant opposite to that of the surgical site.

Quadrant Placement

GDI surgery is typically performed in the superotemporal conjunctival quadrant. This quadrant usually affords more space for the GDI and is easily accessible from a superior or temporal surgical approach. Moreover, oblique muscle fibers are not present in this quadrant and the risk of diplopia is therefore minimized. Indeed, placement of the GDI in the superonasal quadrant may be associated with a higher incidence of diplopia due to restriction of the superior oblique, resulting in pseudo-Brown’s

syndrome [6]. Furthermore, in histopathological studies of enucleated eyes that underwent implantation of the Ahmed Glaucoma Valve (AGV; New World Medical, Ranchos Cucamonga, Calif., USA) in the superonasal quadrant, the posterior aspect of the explant plate was found to encroach on the optic nerve [7, 8].

However, superotemporal placement of the GDI may be precluded in eyes with a history of prior conjunctival incisional surgery (including prior GDI surgery) in this quadrant or the presence of silicone oil in the posterior segment. Inferonasal placement of the GDI is an effective alternative in these cases, particularly because the inferior oblique muscle does not interfere with placement of the plate at this location. Harbick et al. [9] retrospectively investigated the outcomes of inferonasal GDI surgery with the BGI in 182 eyes of 182 patients after 19 ± 15 months of followup. In this study, IOP was reduced from 28.6 ± 11.5 mmHg preoperatively to 12.4

± 5.7 mmHg at last follow-up. Only 3 eyes in this study experienced diplopia, one of which was corrected with spectacle prism therapy. Other complications included corneal decompensation (19 eyes), hyphema (14 eyes), choroidal effusion (12 eyes), and endophthalmitis (1 eye). A subset of the eyes underwent ultrasonic visualization of the relationship between the plate and optic nerve and there was no encroachment seen in this quadrant with this implant. The authors conclude that inferonasal placement of a GDI is a safe and effective option for glaucomatous eyes that cannot undergo superotemporal GDI surgery.

Based on the evidence outlined above, we recommend superotemporal GDI placement as a first preferred location, with inferonasal placement as a second option. In cases that preclude superotemporal or inferonasal GDI placement, we recommend inferotemporal placement to reduce the risk of encroachment on the optic nerve. However, caution must be exercised with this approach as there may be an increased risk of diplopia due to interference with the inferior oblique muscle. Furthermore, this location is more likely to be cosmetically unappealing due to visibility of the tissue patch graft and distortion of the lower eyelid by the filtering bleb. We recommend the use of cornea as the patch graft material with inferior GDI placement because of its superior cosmetic result.

Pars Plana and Ciliary Sulcus Placement

GDI tubes are most often placed in the anterior chamber as far posteriorly as possible. However, the risk of some of the complications of GDI surgery, such as corneal decompensation, hyphema, and iris incarceration of the tube may be minimized by inserting the tube through the pars plana or ciliary sulcus and into the posterior segment or posterior chamber, respectively.

The outcomes of patients that underwent GDI placement in the posterior segment were reported by de Guzman et al. [10] in a retrospective chart review of 33 cases with a mean follow-up period of 30.2 months. Twenty-two of these cases underwent GDI

surgery with the Molteno double-plate implant (Molteno Ophthalmic Ltd, Dunedin, New Zealand), and 11 underwent surgery with the Baerveldt 350 mm2 implant. Simultaneous pars plana vitrectomy (PPV) was performed in 28 eyes; 5 eyes had undergone previous PPV. In this series, IOP was reduced from a preoperative level of 33.06 ± 8.47 to 13.4 ± 4.4 mmHg postoperatively. Complications included corneal decompensation (5 cases), tube blockage (3 cases), conjunctival wound dehiscence (1 case), large choroidal effusion (1 case), and epiretinal membrane (1 case). Two cases of vitreous incarceration of the tube were observed in this study and necessitated further surgical intervention. This underscores the importance of complete vitrectomy (including vitreous removal at the vitreous base) prior to pars plana GDI tube insertion.

Weiner et al. [11] investigated the outcomes of GDI surgery with ciliary sulcus tube insertion in 36 pseudophakic eyes of 32 patients with preoperative shallow anterior chambers, corneal transplants, corneal guttata, or corneal edema in a retrospective case series. The authors employed intraoperative viscoelastic devices to assist in ciliary sulcus tube insertion. After a follow-up period of 21.8 ± 16.6 months, only 1 case of corneal decompensation (in an eye with prior corneal transplant) was observed. IOP was reduced from a preoperative level of 27.9 ± 11.8 to 10.1 ± 3.9 mmHg postoperatively. Ten eyes in this series suffered early postoperative hyphema that cleared spontaneously, except for one eye with neovascular glaucoma which required an anterior chamber washout. The authors attribute the relatively high incidence of postoperative hyphema to tube insertion through ciliary sulcus, which is more vascularized than the anterior chamber angle.

Tube Coverage

GDI tubes may be covered using various material and techniques to protect the tube from erosion through the overlying conjunctiva [12]. A partial thickness scleral flap at the time of device implantation may allow for tube coverage [13]. Alternatively, a donor tissue graft may be used to cover and protect the tube. Donor sclera [14], dehydrated human dura mater [15], clear cornea [16], fascia lata [17], and human pericardium [18] have all been reported as effective tube patch grafts. Smith et al. [12] performed a retrospective case series of 64 patients that had undergone GDI surgery with different patch graft materials in order to determine whether one material may be more prone to melt and subsequent conjunctival erosion. After 65.7 ± 13 months of follow-up, 3 cases of tube erosion in 2 eyes were identified. One of these erosions involved melting of dura patch graft while the other involved a sclera patch graft. A similar incidence of graft thinning, defined as visibility of the underlying 10-0 nylon sutures, was observed in eyes with sclera patch grafts (26.1%), dura patch grafts (22.2%), and pericardium patch grafts (26.1%). The authors conclude that no graft material appears more prone to melting and subsequent tube erosion than another.

Pathophysiology

Once GDI surgery has been performed, an active tissue response occurs over the end plate, culminating in the formation of a fibrous capsule. Aqueous humor passively diffuses through this capsule wall and into periocular spaces, capillaries, and lymphatics for IOP lowering [19]. The fibrous capsule consists of a multi-layered wall of dense fibrous connective tissue and, once formed, provides for the primary resistance to aqueous flow in the postoperative period. Gradual congestion and edema of the capsule wall may develop 3–6 weeks postoperatively. This may lead to a transient increase in aqueous flow resistance and associated IOP rise (hypertensive phase). This period may last for up to 4–6 months, and occurs more commonly with implantation of valved GDIs [20].

Glaucoma Drainage Implant Types

Nonvalved Glaucoma Drainage Implants

The BGI consists of a silicone tube connected to a barium impregnated silicone end plate. Three models of this implant are available for clinical use. The BG 103– 250 and BG 101–350 models consist of end plates with surface areas of 250 and 350 mm2, respectively. The BG 102–350 model consists of a 350 mm2 plate connected to a smaller, angled plate (Hoffman Elbow) by a 7 mm silicone tube. The Hoffman Elbow is designed for insertion into the pars plana. Fenestrations in the end plate of all models allow for ingrowth of the fibrous capsule and reduced bleb profile. The BGI is often placed beneath two adjacent rectus muscles and then fixated to the sclera with nonabsorbable sutures placed through fixation holes in the end plate [21].

Seven models of the Molteno implant are available for clinical use. Two of these models are double-plate implants (L2/R2 and DL2/DR2) and consist of two separate polypropylene end plates connected by a 10 mm silicone tube. The double-plate models are available in right (R2 and DR2) and left (L2 and DL2) eye configurations, require placement in two conjunctival quadrants, and have a total surface area of 265 mm2. The newest single-plate model is the Molteno3, which is available in 175 and 230 mm2 surface areas. Each of the Molteno3 models consists of primary and secondary drainage areas within a polypropylene plate. The primary drainage area lies in the anterior portion of the end plate in a subsidiary ridge and is designed to limit aqueous flow when adhered firmly to Tenon’s capsule. Once IOP rises sufficiently, Tenon’s capsule tissue is stretched, allowing for additional filtration into the posterior secondary drainage area. This design allows for a ‘biological’ valve which may prevent postoperative hypotony while providing for adequate long-term IOP control [22].

Valved Glaucoma Drainage Implants

The AGV (New World Medical, Inc., Rancho Cucamonga, Calif., USA) is available in 12 different designs. Six of these models (FP7, FP8, FX1, FX4, PC7, and PC8) are composed of a flexible silicone end plate and another six models (S2, S3, B1, B4, PS2, and PS3) consist of a rigid polypropylene plate. Two of these models (FX1 and B1) are available in a dual-plate design for an increased surface area of 364 mm2. The single plate models have surface areas ranging from 96 (FP8 and S3) to 184 mm2 (FP7 and S2). Four of these models (PC7, PC8, PS2, and PS3) are fitted with a Pars Plana Clip designed for insertion into the posterior segment. The Pars Plana Clip is also available as a separate accessory for attachment to any of the valve models. The end plate of all models, except the FX4 and B4, is designed with a valve system which theoretically restricts aqueous outflow at an IOP below 8–10 mmHg. This valve system consists of two silicone membranes connected to a silicone tube and housed within the larger end plate structure. The end plate exerts tension on the silicone membranes to keep them well apposed at lower IOPs, therefore restricting flow. An IOP above 8–10 mmHg forces apart the silicone membranes to reestablish aqueous flow [23].

The Krupin Valve Implant consists of a silicone end plate with a surface area of 184 mm2. The distal portion of the tube contains horizontal and vertical silicone slits which are tightly adhered at an IOP less than 9 mmHg. At IOP above 11 mmHg, the slits are forced open in order to allow for unidirectional aqueous flow [24].

Comparison of Glaucoma Drainage Implant Types

In a histopathological animal experiment comparing two GDIs of differing surface areas, Minckler et al. [19] observed greater aqueous flow through the larger filtration bleb that formed over the larger end plate. In order to further investigate the clinical impact of GDI size on IOP lowering, Heuer et al. [25] performed a randomized clinical trial comparing single-plate and double-plate Molteno implants in 132 aphakic or pseudophakic eyes. The group defined clinical success as eyes achieving an IOP ≥6 mmHg and ≤21 mmHg, not requiring additional glaucoma surgery, and not experiencing a devastating complication by last follow-up. Using a 2-year lifetable analysis, a higher success rate was found for eyes that underwent double-plate Molteno implant compared to single-plate implant surgery (71 vs. 46%, respectively; p = 0.0035). Results of this study suggest that the degree of IOP lowering after GDI surgery is indeed dependent on the total surface area of the end plate. However, in a randomized clinical trial comparing two BGI models in 107 eyes, and using a similar definition for success, a higher success rate was found for the smaller 350 mm2 model compared to the 500 mm2 model (5-year life-table success rates of 87 vs. 70%, respectively; p = 0.05) [26]. There may be an upper limit to GDI end plate surface area, beyond which additional IOP lowering cannot be expected [27].

Barton et al. [28] and Budenz et al. [29] performed a multicenter, randomized, controlled clinical trial comparing the efficacy and safety of the AGV and the BGI in 276 eyes of 276 patients in the Ahmed Baerveldt Comparison (ABC) Study. Prior studies comparing the surgical outcomes of different GDI types were performed in retrospective fashion. The primary outcome in the ABC Study was surgical failure, defined as IOP >21 mmHg or not reduced by 20% from baseline, IOP <5 mmHg, reoperation for glaucoma or removal of implant, or loss of light perception vision. Secondary outcomes included mean IOP, visual acuity, and complications. Preoperative IOPs were similar in eyes randomized to the AGV and BGI (31.2 ± 11.2 and 31.8 ± 12.5 mmHg, respectively; p = 0.71). After 1 year of follow-up, no difference in the cumulative probability of failure was observed between eyes randomized to the AGV and the BGI (16.4 vs. 14.0%, respectively; p = 0.52). A greater decrease in IOP was observed for eyes randomized to the BGI compared to the AGV after 1-year follow-up (IOP 13.2 ± 6.8 vs. 15.4 ± 5.5 mmHg, respectively; p = 0.007). A greater number of patients randomized to surgery with the BGI experienced a serious complication (defined as a complication requiring a return to the operating room, or associated with loss of 2 lines or more of Snellen vision, or both) compared to eyes that underwent surgery with the AGV (45 vs. 29 eyes, respectively; p = 0.014). Both treatment groups experienced a significant decrease in Snellen VA after 1 year of follow-up, but there was no difference in this outcome between treatment groups (p = 0.74). The ABC Study provides valuable data regarding the efficacy and safety of two GDI types. The study investigators conclude that neither implant demonstrated clear superiority over the other. The ABC study is ongoing and is designed to continue follow-up of participants to 5 years.

Role of Antifibrotics

To date, 3 randomized, controlled clinical trials have been performed to assess the efficacy and safety of adjunctive antifibrotic agents in eyes undergoing GDI surgery [30–32]. One of these studies [32] is limited by several methodological flaws [33, 34], including significant losses to follow-up. Cantor et al. [30] found no difference in percent change from baseline IOP or incidence of complications in 25 eyes of 25 patients randomized to intraoperative MMC (0.4 mg/ml for 2 min) or control BSS at the time of double-plate Molteno implant surgery after 1 year of follow-up. Costa et al. [31] reached a similar conclusion in a study of 60 eyes of 60 patients randomized to MMC (0.5 mg/ml for 5 min) or BSS at the time of AGV implantation. In this trial, KaplanMeier survival analysis revealed a similar probability of success (defined as postoperative IOP between 6 and 21 mmHg, with or without antiglaucoma medications and IOP reduction of at least 30% from baseline values) in both treatment groups. The incidence of complications was similar in the two groups as well. Currently, there is a lack of evidence supporting the use of adjunctive antifibrotic therapy during GDI surgery.

Complications

Complications associated with GDI surgery may be classified as early or late. In the ABC study, early complications (defined as occurring <3 months postoperatively) included choroidal effusion, shallow anterior chamber, diplopia, tube-corneal touch, corneal edema, and hyphema [29]. Several of these early complications relate to immediate postoperative hypotony. Immediate hypotony is more likely to occur with a nonvalved device when flow is not restricted intraoperatively [35]. For this reason, we recommend complete restriction of flow using an intraluminal and/or ligating suture when inserting a nonvalved GDI. One or more tube fenestrations may be placed for temporary IOP control in these cases. Although valved devices are designed to limit aqueous outflow, immediate hypotony may also occur following insertion of these devices. Aqueous leakage around the needle tract created for anterior chamber entry or valve malfunction are mechanisms of immediate hypotony in these cases. The risk of immediate hypotony may be decreased by performing GDI surgery as a two-stage procedure [36]. The first stage consists of fixating the end plate to the sclera and tucking the tube aside. The second stage consists of intraocular insertion of the tube 4–6 weeks afterwards, once the fibrous capsule (the primary resistor to aqueous outflow) has formed.

Nine percent of patients randomized to GDI surgery in the TVT study experienced persistent corneal edema after 3 years of follow-up [5]. Corneal edema after GDI surgery may be secondary to hypotony or endothelial cell damage due to tube-cornea touch. The risk of corneal edema and/or corneal graft failure may be minimized by placing the tube portion of the GDI far from the corneal endothelium and in the iris plane. Pars plana or ciliary sulcus placement of the tube may be indicated in cases at high risk for corneal endothelial cell damage [11].

Gedde et al. [37] identified conjunctival erosion over the tube to be a major risk factor for the development of late-onset endophthalmitis after GDI surgery. Five percent of patients randomized to GDI surgery in the TVT study experienced tube erosion after 3 years of follow-up [5]. The earliest sign of impending tube erosion is loss of conjunctival capillaries over the tube [34]. Once a tube has eroded through conjunctival tissue, surgical intervention is definitively warranted to reduce the risk of endophthalmitis. The tube may be revised with a new patch graft or the GDI may be explanted with implantation of a new GDI in a separate conjunctival quadrant.

Motility disturbances associated with GDI surgery may be due to extraocular muscle restriction by the end plate, stretching of the extraocular muscles by the end plate bleb, surgical trauma, scarring between the extraocular muscles and the device, or a ‘dragging’ effect by the bleb [38]. Five percent of patients randomized to GDI surgery in the TVT study experienced persistent diplopia after 3 years of follow-up [5]. Avoidance of the superonasal quadrant for GDI placement may reduce the risk of an associated motility disturbance. Strabismus associated with GDI surgery may

be managed with spectacle prism therapy [38], extraocular muscle surgery [39], or explantation of the GDI.

Donald L. Budenz, MD, MPH Department of Ophthalmology

University of North Carolina at Chapel Hill School of Medicine 5151 Bioinformatics

CB 7040

Chapel Hill, NC 27599-7040 (USA)

Tel. +1 919 843 0297, E-Mail dbudenz@med.unc.edu