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

Figure 38.9 Selected steps in performing trabeculectomy. A: Cauterization of area intended for margins of scleral flap. B: Margins of scleral flap outlined by partial-thickness incisions. C: Triangular scleral flap as an alternative technique. D: Anterior chamber entered just behind the hinge of the scleral flap. E: Completion of anterior and lateral margins of deep limbal incision with scissors. F,G: Flap of deep limbal tissue excised using Kelly punch. (Portions modified from Shields MB. Trabeculectomy vs. fullthickness filtering operation for control of glaucoma. Ophthalmic Surg. 1980;11:498, with permission.) After making a peripheral iridectomy, the scleral flap is approximated with 10-0 nylon sutures. Some surgeons prefer to approximate the scleral flap loosely with two sutures at the posterior corners to promote filtration around the margins of the flap. Others prefer tighter closure at the posterior corners and sometimes use additional sutures to avoid the complications of hypotony and a flat anterior chamber. However, loose and tight closures did not differ significantly within 3 months postoperatively in one study (70). Closure that achieves mild-tomoderate resistance to aqueous flow, thus maintaining anterior chamber depth, is optimal. It is especially important when using adjunctive antifibrosis agents,

because these eyes are P.494

much more susceptible to excessive filtration and hypotony. Most surgeons prefer to achieve tighter scleral wound closure, with the plan to lyse sutures postoperatively with an argon or diode laser, if necessary, using specially designed lenses (71, 72 73 and 74). An alternative to laser suture lysis is the use of releasable sutures, which can be removed, as required, at the slitlamp. Several effective techniques for releasable sutures have been described (75, 76, 77 and 78). As noted earlier, the scleral flap can be tested for adequate flow resistance before closing the conjunctival flap by injecting balanced salt solution into the anterior chamber via a paracentesis.

Modifications in Technique

The numerous variations of the guarded filtering procedure that have been reported primarily involve modifications in the scleral flap or in the fistulizing technique.

Variations in the Scleral Flap

Rather than making a square flap, some surgeons prefer a triangular (79), semicircular (80), or trapezoid shape. There is no apparent advantage of one shape over another with regard to long-term success. Some surgeons attempt to influence the degree of postoperative filtration by modifying the scleral flap. It has been suggested, for example, that the thickness of the flap correlates with the final IOP, in that thinner flaps provide greater filtration and lower pressures (81). Other variations in surgical technique have included attempts to enhance filtration around the flap by applying light cautery to the lateral margins (67), omitting all sutures for the scleral flap (82), or excising the distal 2 mm of the flap (82). These techniques predate the era of antifibrosis agents, however, and should be avoided when such adjunctive therapy is used. Placement of amniotic membrane under the scleral flap and suturing it with 10-0 nylon has been suggested for prevention of postoperative adhesion of conjunctiva and sclera in patients with whom the risk for failure is high (83). Another variation in creating the scleral flap involves the scleral tunnel technique, as has been used with phacoemulsification (84). The sides of the tunnel are then incised with scissors to create the flap.

Variations in the Fistulizing Technique

Watson (85, 86) modified Cairns's basic technique by starting the dissection of the tissue block posteriorly over ciliary body, separating it from the underlying structure, and excising it at Schwalbe line. Other techniques to create the fistula beneath a scleral flap include trephinations, sclerectomies, thermal sclerostomies, and sclerostomies with a carbon dioxide laser (80, 87, 88, 89, 90 and 91). Most surgeons use a Kelly Descemet membrane punch or Crozafon-De Laage punch to excise limbal tissue sections from the posterior lip of the initial incision beneath the scleral flap (92, 93).

Modifications for Neovascular Glaucoma

As discussed in Chapter 19, intraocular surgery in eyes with neovascular glaucoma is often complicated by intraoperative hyphema. One trabeculectomy variation to minimize this risk includes excision of a large trabecular segment, partial nonpenetrating cyclodiathermy in the scleral bed, and partial ablation of abnormal iris vessels with a wide sector iridectomy (94). Glaucoma drainage-device surgery and diode laser cyclophotocoagulation are increasingly considered to be the treatments of choice for this disorder (95), and they are discussed in other chapters.

The optimum approach to filtering surgery in eyes with neovascular glaucoma, however, is to precede it with an intravitreal injection of anti-vascular endothelial growth factor (anti-VEGF) agent or panretinal photocoagulation, when possible, which often reduces neovascularization, decreasing the likelihood or degree of postoperative hyphema regardless of the surgical treatment used.

Trabeculectomy for neovascular glaucoma often fails because of scarring of the filtering bleb; therefore, the use of mitomycin C (MMC) and 5-fluorouracil (5-FU) has been advocated in those eyes. One study classified two thirds of eyes after panretinal photocoagulation followed by trabeculectomy with MMC as surgical success after 2 years of follow-up (96). Another study did not find that differences in the MMC concentration or application time affected postoperative IOP or complication rates (97). The number of successful surgeries was 71% at 1 month and decreased to 29% after 1 year. Postoperative needling, in

conjunction with intraoperative MMC use and postoperative intrableb 5-FU injection, was found efficacious and saved further surgery in some intractable cases (98). Regression of rubeosis, sometimes seen after trabeculectomy with MMC, has been suggested to be a pharmacologic side effect of MMC and not necessarily the effect of IOP lowering alone (99). In a preliminary study, trabeculectomy 1 month after use of intravitreal bevacizumab resulted in a decrease in anterior segment neovascularization and fewer complications (100). Even if an anti-VEGF agent is used, the effects of these agents are temporary, and further injections or panretinal photocoagulation may be needed for long-term control of neovascularization.

Modifications for Previous Intraocular Surgery

The fornix-based conjunctival flap, as previously discussed (16, 17 and 18, 26), is particularly useful in eyes that have had previous intraocular surgery involving the conjunctiva—for example, when a fornixbased conjunctival flap has been used during a cataract procedure. The conjunctiva in these eyes is usually tightly scarred down to episclera near the limbus, making preparation of a limbus-based flap difficult. When using a fornix-based flap, it is probably best to suture the lateral margins of the scleral flap to promote drainage posteriorly. An anterior vitrectomy may also be required if loose vitreous is in the anterior chamber or presents at the iridectomy site. Nonpenetrating trabeculectomy was once advocated for glaucoma in aphakia (101). However, early results from the Tube versus Trabeculectomy study indicate that implantation of glaucoma drainage devices may be a better alternative for these cases (102).

Use of antimetabolites for trabeculectomy in pseudophakic eyes may improve the success rate. However, in aphakic and pseudophakic eyes of children, after congenital cataract

P.495

surgery, trabeculectomy was successful in only one third of patients, regardless of whether MMC was used during the surgery (103). Implantation of glaucoma drainage devices has been gaining broader acceptance in these challenging cases.

Figure 38.10 R-50 model of Ex-PRESS mini glaucoma shunt (actual length, approximately 400 microns. Adjunctive Devices

The Ex-PRESS mini glaucoma shunt (Fig. 38.10) is a stainless steel device that was originally developed to be implanted subconjunctivally through the limbus, providing direct communication between the anterior chamber and subconjunctival space, resembling a full-thickness procedure, relying upon the intrinsic resistance of the device. Although successful in lowering IOP, procedures using this device had high complication rates, and as a result, surgeons began implanting the shunt device beneath a partial-thickness scleral flap (104, 105). The technique of implantation and indications of use resemble those of trabeculectomy (i.e., a guarded filtration procedure), aside from the absence of an iridectomy. In a retrospective comparison with trabeculectomies, implantation with Ex-PRESS mini glaucoma shunts achieved similar IOP control but had a lower rate of early postoperative hypotony (106).

Wound Healing

The most common cause of failure in glaucoma filtering surgery is scarring of the filtering bleb (107). The increased amount of collagen in the failed blebs suggests that proliferation of fibroblasts with associated production of collagen and glycosaminoglycans is important in the response to filtering surgery (3). However, as discussed in Chapter 37, wound healing is a complex process with several phases, and it is likely that bleb failure in filtering surgery involves many of these factors and certain unique characteristics of the glaucomatous eye. Newer antifibrotic agents and drug delivery systems are under development in an effort to improve efficacy and safety.

Influence of Aqueous Humor on Wound Healing

Aqueous humor normally slows or fails to support the growth of conjunctival fibroblasts in tissue culture (108, 109 and 110). A possible explanation is that aqueous contains one or more inhibitory factors for fibroblast proliferation. Cell culture studies have shown that the high concentration of ascorbic acid, normally present in aqueous humor, is cytotoxic to dividing human Tenon capsule

fibroblasts, which may contribute to the development of a successful filtering bleb (111). Aqueous humor also contains a wide variety of growth factors, which maintain the normal function of ocular tissues in health and have a significant role in abnormal states and wound healing (112). Transforming growth factor-ß (TGF-ß), a potent modulator of tissue repair, is in human aqueous and plays a role in the healing process after glaucoma filtering surgery (113). Contrary to the influence of primary aqueous humor, aqueous obtained shortly after intraocular surgery or mixed with 20% desiccated embryo extract does promote proliferation of fibroblasts (110, 114). Secondary aqueous humor has also been shown to stimulate the proliferation of cultured corneal endothelial cells (115). In addition, aqueous humor has chemoattractant activity for ocular fibroblasts, and this activity is significantly greater in eyes with previously failed glaucoma surgery (116). Therefore, components of normal aqueous humor and alterations in some glaucoma patients are likely to influence both success and failure of the filtering bleb.

P.496

Other Factors Affecting Wound Healing

Numerous studies have suggested that young age and African heritage adversely influence the outcomes of glaucoma filtering surgery. The explanation for these observations is not clear. Histologic studies of conjunctival specimens obtained before trabeculectomy in patients with chronic open-angle glaucoma (COAG) showed no significant influence of age or ethnicity on conjunctival factors that might relate to surgical outcome (117, 118). A more significant influence may be chronic topical glaucoma medical therapy before trabeculectomy. Some studies have identified long-term topical combination therapy as a risk factor for failure of trabeculectomy (119, 120), although one study that compared success rates before and after the introduction of topical ß-bloc kers did not indicate that the preoperative use of topical medication influenced the outcome of surgery (121). Histologic studies of conjunctiva from patients after long-term topical glaucoma medical therapy revealed a significant degree of subclinical inflammation (122, 123), although one study could only correlate the number of goblet cells with successful outcomes (124). Conjunctival impression cytology correlated significant degrees of metaplasia with the number of glaucoma medications used (125). One clinical study, however, indicated that stopping topical adrenergic therapy and adding use of topical corticosteroids 1 month before surgery was associated with a decrease in the number of conjunctival fibroblasts and inflammatory cells and an improvement in the success rate of trabeculectomy (126). Another study suggested that a high number of conjunctival goblet cells may be a predictor of lower IOP after trabeculectomy without use of antimetabolites (127). The number of conjunctival fibroblasts and inflammatory cells increases after previous ocular surgery involving the conjunctiva, possibly causing an increased risk of trabeculectomy failure (128).

Antifibrotic Agents Corticosteroids

Considerable attention has been given to measures—p rimarily medication use—that may prevent bleb failure by modulating the wound healing process. The first of these to be used clinically were the corticosteroids. Tissue culture studies of human Tenon capsule fibroblasts have shown that both corticosteroids and nonsteroidal anti-inflammatory drugs inhibit cell attachment and proliferation (129, 130). Clinical investigations have confirmed the efficacy of topical corticosteroids, although no additional benefit was achieved with systemic steroids (131, 132). It has also been suggested that subconjunctival triamcinolone before filtering surgery may improve the success rate (133). Despite the benefit of corticosteroids, the incidence of bleb failure remains high with certain types of glaucoma (e.g., glaucoma in aphakia and pseudophakia and neovascular glaucoma), which has prompted the search for additional agents to modify wound healing.

5-Fluorouracil

5-FU was the first drug to be studied extensively as an adjunct to corticosteroids in the control of wound healing following trabeculectomy. This pyrimidine analog antimetabolite, which blocks DNA synthesis through the inhibition of thymidylate synthesis, has been shown to inhibit fibroblast proliferation in cell

cultures (134, 135). The subconjunctival injection of 5-FU after filtering surgery significantly improved bleb formation in monkeys and improved the rate of success in difficult clinical cases (136, 137 and 138). A subsequent multicenter, randomized clinical trial of 213 patients with glaucoma in aphakia or pseudophakia or a previous failed filter in a phakic eye confirmed the ability of 5-FU to improve the success rate of filtering surgery in these high-risk cases (139, 140). However, the protocol required twice-daily subconjunctival injections of 5-mg 5-FU for 7 days and then once daily for 7 more days. In addition, serious complications included conjunctival wound leaks and corneal epithelial defects in the early postoperative course, plus an increased risk for late-onset bleb leakage (139, 140). Therefore, lower effective doses, alternative delivery systems, and alternative agents have been sought.

Success has been reported with daily injections of 5-mg 5-FU for 7 to 14 days (141, 142 and 143), which probably represents the most common range of dosages in current use. In one study of patients with COAG and “secondary” glaucoma or refractory gl aucoma, the average total dose of 5-FU was 36.5, 36.0, and 49.5 mg, respectively, and the probability of IOP control below 16 mm Hg with 5-year followup was 77.9%, 66.8%, and 26.9%, respectively (143). Adjunctive use of 5-FU increases the success rate of trabeculectomy in eyes undergoing initial filtering surgery, patients younger than 40 years, infants, and patients requiring extremely low IOPs (144, 145, 146, 147 and 148). However, the complication rate is higher than in trabeculectomy without 5-FU, and caution is advised, especially with initial surgery (140). There is a high risk of failure in patients with neovascular glaucoma (149). The treatment is believed to be most effective if started prophylactically on the first postoperative day, although success has been reported with starting 3 to 15 days postoperatively when signs of impending bleb failure are noted (150, 151). Several clinical trials have shown that 5-FU is also beneficial when used intraoperatively, usually on a surgical sponge soaked in 25 to 50 mg/mL of the drug and applied to the surgical site for 5 minutes (152, 153, 154, 155 and 156). The type of sponge can also affect the intraocular tissue levels of 5-FU delivered (157).

Mitomycin C

Use of MMC was reported by Chen (158) in 1983 to enhance the IOP-lowering efficacy of trabeculectomy when applied intraoperatively in eyes at high risk for surgical failure. MMC is an antineoplastic antibiotic isolated from Streptomyces caespitosus. Tissue culture studies of human Tenon capsule fibroblasts revealed almost complete inhibition of fibroblast proliferation (159), the degree of which correlated with the outcome of filtering surgery (160). When compared with 5-FU, the effect on rabbit fibroblast proliferation was much more prolonged with MMC (161). 5-FU was toxic to cultured mouse fibroblasts while sparing bovine vascular endothelial cells, whereas MMC was cytotoxic for both cell types (162). Intraoperative application of

P.497

MMC in rabbits significantly prolonged bleb duration after glaucoma filtering surgery (163). Subsequent clinical trials supported the benefit of MMC use as an adjunct to trabeculectomy (164), and randomized comparisons with postoperative use of subconjunctival 5-FU generally showed intraoperative use of MMC to have superior IOP-lowering efficacy after trabeculectomy (165, 166, 167, 168 and 169). MMC has been shown to enhance the success rate of trabeculectomy for refractory glaucoma in black patients, in glaucoma associated with uveitis, in congenital and developmental glaucoma, in normal-tension glaucoma, and in primary, uncomplicated trabeculectomies (170, 171, 172, 173, 174, 175 and 176). A retrospective study has shown that primary trabeculectomy with MMC maintained an IOP level of 15 mm Hg or less in more than 80% of patients after 1 year and in 60% after 6 years, suggesting that the use of MMC may be justified in primary trabeculectomies in patients with severe glaucoma (177). However, care must be exercised with the use of adjunctive MMC, especially in the primary, uncomplicated cases, because of the significant incidence of serious complications. Although adjunctive use of MMC is less likely to cause the postoperative complications that are typically associated with 5-FU, such as corneal epithelial toxicity and wound leaks (165, 166, 167 and 168), it is associated with other complications that can be even more serious. The most significant of these is hypotony maculopathy, in which prolonged IOP reduction is associated with disc edema,

vascular tortuosity, and chorioretinal folds in the macular area, potentially producing marked reduction in visual acuity (178). The main cause of the hypotony is excessive filtration, and histologic studies of excised overfiltering blebs have revealed an irregular epithelium and a largely acellular subepithelium of loosely arranged connective tissue (179, 180 and 181). However, another mechanism of hypotony may be aqueous hyposecretion, in that one enucleated human eye revealed disruption of the ciliary body epithelium beneath the site of MMC application (181). Other potential complications, as suggested by animal studies, include anterior chamber reaction and corneal endothelial toxicity if the MMC gains entry into the eye (182, 183). (The management of these complications is discussed later in the chapter.) The following modifications in technique may minimize the complications.

In early protocols, a sponge soaked in MMC, 0.5 mg/mL, was applied to the subconjunctival tissues for 5 minutes. Subsequent attempts to reduce the risk of hypotony have included reduced concentrations and exposure times (179, 184). It has also been suggested that adjustment of exposure time according to each patient's risk of excessive fibrosis may enhance the balance between successful IOP control and incidence of complications (179). Some retrospective studies suggested that MMC, 0.2 mg/mL, applied for 2 minutes may be as effective as higher doses but may be associated with few complications (184, 185). However, the optimum protocol has yet to be established.

Various sponges have been advocated as vehicles for MMC, including Merocel and various microsurgical sponges, and it may be that manipulating the size or shape of the sponge can influence the effect of the MMC (186, 187 and 188). One study suggested that placing the sponge beneath the scleral flap rather than over intact episclera may improve the success rate without increasing complications (177, 189). In rabbits, irrigating the ocular tissues with balanced salt solution after removal of the sponge substantially reduced intraocular diffusion of MMC (190).

In an experimental model, irrigation reduced the MMC concentration only in the external half of the sclera, leaving the deep intrascleral concentrations unchanged (191), suggesting that a lower-dose MMC application without irrigation may be a rational approach (192). Intraoperative application of MMC without touching the conjunctiva or Tenon capsule was ineffective in inhibiting the development of thin, avascular blebs in eyes undergoing primary trabeculectomy (193).

Other Antifibrotic Agents

Alternative agents that have been evaluated as antiproliferative drugs include cytosine arabinoside, bleomycin, rapamycin, doxorubicin, daunorubicin, 5-fluorouridine, 5'-monophosphate, 5-fluoroorotate, heparin, Taxol, cytochalasin-B, colchicine, immunotoxins, and interferon-a-2b (134, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206 and 207). Suramin, a substance that inhibits the action of growth factors, was evaluated in rabbits and in humans in a prospective study (208, 209). The use of suramin had fewer complications, compared with use of MMC, with similar success rates, suggesting that it may become an alternative to antimetabolite therapy in glaucoma surgery (209). Beta irradiation also inhibited fibroblast proliferation in tissue culture and delayed wound healing in rabbits (210, 211 and 212). In a preliminary clinical trial, beta irradiation did not improve the results of trabeculectomies (213), although a study of patients aged 18 years or younger with congenital glaucoma suggested a beneficial effect on the prognosis of trabeculectomy (214). Amniotic membrane transplantation or subconjunctival perfluoropropane (C3F8) gas has been suggested as a safer alternative to MMC (215, 216 and 217).

In addition to drugs that influence fibroblast proliferation, agents have also been evaluated that will alter other phases of the wound healing process. For example, tissue plasminogen activator, which causes localized fibrinolysis, ?-interferon and calcium ionophores, which inhibit collagen biosynthesis, and ß- aminopropionitrile and d-penicillamine, which inhibit cross-linking of collagen, have shown promise in in vitro and in vivo studies (218, 219, 220, 221, 222, 223, 224, 225, 226 and 227). Perioperative injection of bevacizumab has also shown promise to decrease conjunctival scarring and vascularity (228, 229).

TGF-ß is a potent stimulant of scarring, and it has been identified as an important component of wound healing, particularly in the conjunctival scarring response (230). It has been found in human aqueous and appears to play a role in the healing process after glaucoma filtering surgery (113). Inhibition of

TGF-ß appears to be a more physiologic approach to wound healing modulation (231, 232).

Although subconjunctival administration of a TGF-ß 2 antibody in the postoperative period improved the

outcome of glaucoma surgery in an animal model and appeared more efficacious than 5-FU without some of the side effects (233), a randomized, controlled trial of CAT-152, a monoclonal antibody P.498

against TGF-ß 2, was no more effective than placebo and required frequent subconjunctival injections in

the postoperative period (234). Tranilast (N-3',4'-dimethoxycinnamoyl-anthranilic acid), a drug with antikeloid and antiscarring properties, inhibits TGF-ß 1 secretion and therefore could be a promising drug

to prevent scarring after glaucoma filtration surgery (235, 236 and 237).

Figure 38.11 Limboscleral trephination. A: Trephine button partially excised by tilting the trephine anteriorly. B: Completion of excision by cutting posterior attachment with scissors.

In the future, combinations of agents may be administered according to the various phases of the wound healing process to prevent bleb failure.

Earlier Full-Thickness Fistulas Sclerectomy

The original type of limbal fistula, which has been largely replaced by trabeculectomy with or without adjunctive use of antifibrosis agents, involves the creation of a direct opening through the full thickness of the limbal tissue. The fistula may be created by using various techniques.

In 1906, LaGrange (238) described a technique in which a full-thickness limbal incision was made, and a piece of tissue was then excised from the anterior lip of the wound to create a limbal fistula. Holth (239) modified this procedure 3 years later by performing the sclerectomy with a punch. However, the sclerectomy technique that became most popular in the mid-20th century was the posterior lip sclerectomy described by Iliff and Haas (240).

Trephination

In 1909, Elliot (241) and Fergus (242) described a glaucoma filtering procedure in which the fistula was created with a small trephine placed just behind the corneolimbal junction. Elliot (243) later modified the technique by splitting the peripheral cornea and placing the trephine more anteriorly (sclerocorneal trephining). However, this modification produced a thinner filtering bleb with an increased chance of late infection, and Sugar (244) advocated a return to the original, more posterior placement of the trephine, which he called limboscleral trephination (or trepanation) (245) (Fig. 38.11).

Thermal Sclerostomy (Scheie Procedure)

In 1924, Preziosi (246) described a filtering technique in which a limbal fistula was created by entering the anterior chamber angle with an electrocautery instrument. Scheie (247) later described a procedure that also used cautery but differed from the operation described by Preziosi in that a limbal scratch incision was first made, and the cautery was then used to retract the wound edges, thereby creating the fistula.

The thermal sclerostomy technique (Fig. 38.12) involves application of light cautery to the sclera in a 1 × 5-mm area behind the corneolimbal junction. A 5-m m limbal scratch incision is then made through the cauterized area, perpendicular to the scleral surface, and cautery is applied to the lips of the incision until the wound edges separate by at least 1 mm.

The escape of aqueous from the limbal incision may interfere with the application of cautery, which can be partially avoided by stopping the initial scratch incision just before it enters the anterior chamber, applying cautery, and then completing the incision (248). In addition, bipolar cautery can be effectively used in the wet field. Another modification is to place a temporary suture across the fistula to avoid an early flat anterior chamber (249).

Iridencleisis

This procedure differs from the other forms of full-thickness filtering surgery in that a wedge of iris is incarcerated into the limbal incision to maintain a patent channel for aqueous outflow (250). This was once a popular procedure, but it lost favor partly because of the suspicion that the associated incidence of

P.499

sympathetic ophthalmia was higher than with other filtering procedures. Although this fear was unsubstantiated, the operation never regained popularity.

Figure 38.12Thermal sclerostomy. A: Limbal incision created (may initially be partial or full thickness). B: Application of cautery to the lips of the incision to separate the wound edges. A partial-thickness incision is then extended into the anterior chamber and cautery is applied to the depths of the wound. Laser and Other Sclerostomy Techniques

Laser Sclerostomy Ab Externo

Laser energy has also been used to create the fistula, most of which are full thickness. This can be performed from an ab externo or ab interno approach. The argon laser has been studied for the former approach (251), although the holmium laser, also referred to as THC:YAG (thulium, holmium, and

chromium-doped yttrium aluminum garnet crystal) laser, has undergone the most extensive evaluation for laser sclerostomy ab externo (252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266 and 267). The laser operates in the near-infrared region, with a wavelength of 2100 nm. A rightangle exit of the laser beam from the tip of the fiberoptic probe allows subconjunctival advancement of the probe from a small conjunctival incision to the limbus where the fistula is created.

Rabbit studies supported the feasibility of the holmium procedure (252, 253), and preliminary clinical trials provided encouraging short-term results (254, 255). Longer follow-up, however, revealed estimated probabilities of success of approximately 65% at 1 year, 57% at 30 months, 44% at 2 years, and 36% at 4 years (256, 257, 258 and 259).

However, higher incidences of hypotony (due to the fullthickness nature of the fistula), choroidal detachments, and iris incarcerations, along with a progressing rate of failure, make this procedure less effective than trabeculectomy for long-term pressure control (266, 267). Other lasers that have been evaluated for laser sclerostomy ab externo include a giant-pulsed (up to 200 W of peak power at pulses of 20 or 40 milliseconds) neodymium:yttrium-aluminum-garnet (Nd:YAG), a picosecond Nd:YLF (yttrium lithium fluoride) 1053-nm laser, a semiconductor diode laser, and a 193-nm excimer laser (268, 269, 270, 271, 272 and 273). The sclerostomies created with diode laser (260) have been associated with heat coagulation damage and disruption of scleral collagen. Pulsed laser has also been associated with thermal and mechanical damage (261). A continuous-wave, mid-infrared diode laser system appears to be superior to pulsed lasers (261). Excimer lasers have also been used in a modified trabeculectomy to precisely remove scleral tissue overlying the Schlemm canal, leaving the trabecular meshwork intact (274, 275 and 276), and in a modified nonpenetrating filtering surgery to perform trabeculodissection under a scleral flap through the Schlemm canal and the juxtacanalicular trabecular meshwork (277). The approach of using lasers for an ab externo approach for laser sclerostomy has been largely abandoned. Laser Sclerostomy Ab Interno

In addition to creating full-thickness fistulas with laser energy from the external approach, lasers and other instruments have been evaluated to create sclerostomies ab interno (i.e., from the anterior chamber to the subconjunctival space). The main theoretical advantage of this technique is that it requires no dissection of the conjunctiva, which is elevated before creating the sclerostomy with a fluid injection over the surgical site, thereby reducing the risk of scarring and bleb failure. The first attempt at laser sclerostomy ab interno was made with a Q-switched Nd:YAG laser, focused into the anterior chamber angle through a special gonioprism (278, 279 and 280). This was shown to be effective in creating a sclerostomy but required very high levels of energy. Subsequent modifications involved staining the sclera with methylene blue dye by iontophoresis and using a pulsed dye laser with a wavelength of 660 nm, which is maximally absorbed by methylene blue (281, 282 and 283). In a prospective study in rabbits, internal sclerostomy made by a pulsed dye laser appeared to have similar efficacy in lowering IOP as a posterior lip sclerectomy (284).

Other attempts at laser sclerostomy ab interno have used contact laser probes, the tip of which is introduced into the anterior chamber via a limbal incision 180 degrees from the sclerostomy site. The tip is passed across the anterior chamber to the trabecular meshwork, where the sclerostomy is created (Fig. 38.13). This technique has been evaluated with continuous-wave Nd:YAG lasers; high-energy argon blue-green laser;

P.500

and excimer, erbium, diode, and Nd:YLF lasers (285, 286, 287, 288, 289, 290, 291, 292, 293 and 294). An intraocular endoscope has been suggested for use with an erbium:YAG laser for precise location of the sclerostomy and to reduce scarring at the filtering site (295). Rabbit studies of Nd:YAG, holmium, and erbium lasers indicate that increasing wavelengths are associated with decreasing thermal damage around the sclerostomy, which would theoretically decrease subconjunctival scarring and filtration failure (296, 297). Excimer laser trabeculectomy ab interno performed on rabbit eyes produced permanent openings into the Schlemm canal through trabecular meshwork, reducing the outflow resistance (298). In human eyes with glaucoma, openings created in the trabecular meshwork with an