Ординатура / Офтальмология / Английские материалы / Glaucoma Surgery_Trope_2005
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avascularity can also develop. Although it is associated with low IOPs, thin avascular blebs are associated with bleb leak and late endophthalmitis. There is some evidence that late endophthalmitis is more likely after antimetabolite use although the literature is not entirely consistent (92 103). Conjunctival wound position influences the incidence of thin-wall cystic blebs as well as endophthalmitis in MMC trabeculectomies. Inferior MMC blebs have been reported to have an eightfold increased risk of endophthalmitis compared with superior blebs (104). In younger patients, limbus-based trabeculectomies flaps more commonly developed cystic blebs, late hypotony, and endophthalmitis (73).
The presence of blood is one of the fundamental drivers of the inflammatory and wound-healing responses. Although it is logical that hemorrhage would increase inflammation and wound-healing intensity, there is very little published data to support this. Sacu (90) did not find a relationship between the presence of subconjunctival hemorrhage and IOP in his series of MMC trabeculectomies. However, human serum has been reported to have an anti-apoptotic effect that can overcome the cytotoxicity of MMC on human tenon’s fibroblasts (105).
8.2.2.Bleb Area and Height
The area of bleb drainage is a critical determinant of bleb outflow and thus IOP. In a retrospective case review of nonantimetabolite trabeculectomies, eyes with more diffuse blebs had lower IOPs than those with blebs the same size as the scleral flap when examined between 1 and 5 years postoperatively (84). Although localized or focal blebs are relatively easy to discern, the full extent of a very diffuse bleb can be very difficult to estimate. Measurement of bleb area by high frequency ultrasound biomicroscopy also shows an inverse relationship between bleb area and IOP (106).
Bleb height is a measure of bleb pressure rather than flow. It reflects equilibrium between outward forces and resistance in the bleb wall. Shingleton (85) described bleb height in relation to IOP in an effort to aid diagnosis of the causes of bleb failure. Although high flow and a lax conjunctiva may cause a high bleb and hypotony from over-filtration, a high bleb is also seen with low flow across the bleb wall and high IOP in bleb encapsulation (85). Picht and Grehn (87,89) reported both encapsulation and higher blebs were associated with poor bleb outcome. Both bleb area and height are assessed in the grading systems in Figs. 18.1 and 18.2.
8.2.3.Bleb Wall Thickness and Conjunctival Transparency
Assessment of bleb wall thickness and conjunctival transparency is an attempt to gage the resistance to flow out of the bleb. Maumenee (86) hypothesized in 1960 that condensation and compression of collagen in the bleb wall reduced its permeability to aqueous. Molteno et al. (107) have reported decreased permeability of the capsule surrounding his glaucoma drainage device in response to high intrableb pressure. A histological study reported thick dense collagenous tissue in the wall of an encapsulated bleb (26). Sacu et al. (90) examined conjunctival transparency in MMC trabeculectomies but did not find an association with IOP.
8.2.4.Presence of Tenon’s Cysts or Bleb Encapsulation
Encapsulation of the bleb, also known as a cyst of Tenon’s capsule (108) occurs during the fibroblastic phase of wound-healing. It is usually localized to the scleral flap, is high and has a thick wall. IOP is usually close to preoperative levels. The cysts are in direct communication with the anterior chamber. Its wall markedly thickened and composed
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of collagen, fibroblasts, and vascular tissue and the inner surface of the wall is acellular (26,108).
The management of bleb encapsulation is dealt with in Chapter 19. Although IOP often falls with time, bleb encapsulation does represent failure of bleb function and is associated with more interventions and higher IOPs in the longer term (32,89).
8.2.5.Bleb Leak
Bleb leaks are discussed in detail in Chapter 23. Early bleb leak is a result of incomplete wound closure at the time of surgery, wound contraction, or buttonholing of the conjunctiva and should be differentiated from late-onset bleb leaks which develop with progressive bleb thinning and microtrauma. Profuse leak can cause hypotony with anterior chamber shallowing and choroidal effusions. This not only may increase inflammation but also restrain the surgeon from giving otherwise necessary anti-inflammatory and antimetabolite treatments. The FFSS identified early wound leak as a risk factor for long-term bleb failure in limbus-based trabeculectomies (109). The effect of wound leak in fornix-based trabeculectomies and with MMC is not clear.
8.2.6.Presence of Microcysts
Of all the signs associated with blebs, the presence of microcysts in the conjunctiva is the most commonly associated with a good outcome (Fig. 18.3). Rather than reflecting wound healing, microcysts appear to be areas of local conjunctival aqueous flow (26). As such their presence indicates not only bleb structure but also function. The relationship to bleb function and IOP appears consistent across the very small literature and does not appear to be influenced by MMC (85,87,89,90).
Figure 18.3 Conjunctival microcysts overlying a functioning bleb.
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Microcysts are particularly useful to demarcate the functional area of a very low bleb such as that found after phacotrabeculectomy with high-dose MMC. They may persist for many years, although they are not always present in all quadrants of blebs with good function. With time, the appearance of microcysts changes from an early more visible form to a mature form, which is not as easily seen.
8.3.Late Failure
Increasing time from surgery has been widely reported as a risk factor for bleb failure. However, little is known about the precise mechanisms of late failure. Histology of late failing blebs shows dense collagen scars and relatively few inflammatory cells (26,27). The process appears to be a slow continuation of the fibroblast activity seen in the earlier postoperative period.
9.INTERVENING IN THE FAILING BLEB
The use of antimetabolites in trabeculectomy and interventions for specific postoperative complications are considered in other chapters in this book. The following is an overview of measures thought to be beneficial for bleb survival. Key interventions are summarized in Table 18.3.
Table 18.3 Interventions for the Failing Bleb
Intervention |
Optimal time of intervention |
|
|
Local corticosteroids |
Early |
5 Fluorouracil |
Early |
Increasing trans scleral flow |
Early (suture adjustment, needling) |
|
Late (needling) |
Oral Predisone/NSAID/Colchicine |
Early |
|
|
9.1.Preoperative/Perioperative
The most important intervention to prevent bleb failure is appropriate patient selection for trabeculectomy and preoperative selection of antimetabolite for higher-risk cases. The use of high-dose MMC in low-risk patients can lead to prolonged hypotony and vision loss. In contrast, in sufficiently high-risk patients, a high-dose MMC bleb can still fail rapidly. Cessation of sympathomimetic drops combined with topical fluoromethalone for 30 days preoperativly (50) and preoperative subconjunctival triamcinolone in the area of the future bleb (110,111) have been reported to improve postoperative outcomes in higher-risk patients.
During surgery, fornix-based conjunctival flaps, two-site surgery for phacotrabeculectomy and larger treatment areas when using antimetabolites may all reduce the risk of bleb failure.
9.2.Early Postoperative Period
9.2.1.Local Corticosteroids
The first 6 8 weeks after trabeculectomy are a critical time in the wound-healing process. Application of topical steroids is a standard clinical practice after trabeculectomy.
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Strength and frequency can be tailored to the degree of bleb inflammation or preoperative risk factors. A prospective trial of topical prednisolone acetate 1% in nonantimetabolite trabeculectomies reported better IOP and glaucoma control and fewer additional glaucoma interventions in the corticosteroids group at 5 and 10 years. No added benefit of systemic prednisolone was found (112 114). We use topical steroids for 6 months in higher-risk cases and longer where necessary in pseudophakic patients. Subconjunctival steroids are also regularly given intraoperatively at the completion of the trabeculectomy. Although Dexamethasone 4 mg/mL is common, longer-acting depots of betamethasone or triamcinolone can help to reduce the frequency of postoperative steroid drops.
9.2.2.Flap Manipulation
Trans-scleral aqueous flow maintains tissue space and retards bridging by scar tissue. Ocular massage, suturelysis and tension adjustment of adjustable trapdoor sutures are all advocated to improve function in response to impending bleb failure. The effect of these measures decreases as the wound heals. Unless wound-healing has been severely retarded (e.g., high-dose MMC), they have little effect after 6 weeks.
9.2.3.Fluorouracil
The FFSS and many other studies have shown the benefit of postoperative subconjunctival injections of 5FU in reducing the wound-healing response. Although the FFSS protocol demanded two injections per day, 1808 from the bleb, practice has now switched to weekly or bi-weekly injections of 5 10 mg behind the bleb. One trial reported very good results with five weekly injections following combined cataract and glaucoma surgery (115). Fluorouracil can also be used on an ad hoc basis with both the frequency and the dose of 5FU titrated to the bleb appearance (116). Concern about the potential severe complications of inadvertent intraocular administration of MMC (117) has prevented its widespread use as a subconjunctival injection although a number of studies have reported it to be of benefit (118 124).
9.2.4.Systemic Antifibrosis Therapy
Oral systemic therapy to prevent bleb failure has been advocated for many years by Molteno (82,125). The therapy consists of Prednisolone 2.5 10 mg tid, Diclofenac SR 100 mg daily, and Colchicine 0.2 0.3 mg tid. This can be combined with topical Atropine 1% tid and Adrenaline 1% tid to the affected eye. In a prospective noncomparative case series, 77 eyes were treated in this manner for threatened bleb failure. The therapy was commenced 1 4 weeks post surgery (mean 11 days) and continued for 1 11 weeks (mean 6). The authors reported that after 3 4 days there was a consistent and marked reduction in bleb vascularity often accompanied by bleb extension and reduction in IOP. Six blebs failed between weeks 4 11 but were restored with IOP control after needling while on the therapy. After 12 years, 11% of eyes had IOP .21 mmHg or required further glaucoma surgery. Systemic side effects were encountered in nine patients and described as minor. Seven required oral histamine type 2 receptor antagonists (82).
9.3.Late Postoperative Period
Traditionally, late bleb failure has not been treated itself. Rather, IOP-lowering medications were reintroduced or further glaucoma surgery performed. There are no reports
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suggesting a postoperative time period beyond which bleb failure does not occur. The little histopathological evidence available suggests late failure is associated with excessively dense and thick bleb walls (26) suggesting that some aspect of the wound-healing response continues indefinitely. The most widely reported intervention for bleb failure in the late postoperative period is needle revision. This is discussed in Chapter 20. If signs of bleb inflammation develop, reintroduction of topical corticosteroids may be beneficial. We use 5FU and intensive topical steroids for an extended period to prevent bleb failure after subsequent cataract surgery.
10.CONCLUSION
Bleb failure due to wound healing is the single most important challenge in modern glaucoma-filtering surgery. An understanding of the timing and phases of the woundhealing process, combined with thorough risk-factor assessment and careful postoperative examination of the bleb will greatly aid the glaucoma surgeon in timely and appropriate interventions to maximize function and longevity of the trabeculectomy bleb.
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