Ординатура / Офтальмология / Английские материалы / Glaucoma Surgery_Trope_2005

2.CLINICAL SIGNS AND SYMPTOMS

Encapsulated blebs (also termed cysts of Tenon’s capsule, “cystic” blebs and exteriorization of the anterior chamber) have a characteristic clinical appearance: highly elevated and localized (dome-shaped) filtering bleb, firm to palpation, with vascular engorgement of the overlying conjunctiva, and absence of microcysts (Fig. 19.1). The sclerotomy is patent on gonioscopy and the IOP is usually elevated, but can also be normal (11 14). The patients may experience pain or discomfort, usually because of tear film disturbance caused by the large filtering bleb, which can even lead to dellen formation (11,15). Interference of the superior lid function caused by a large elevated bleb can result in ptosis (11,15).

3.HISTOLOGY

Successful trabeculectomy bleb walls have a subconjunctival extracellular connective tissue layer of low density containing microcystic spaces for the passage of aqueous humor (16). The histology of an encapsulated bleb has a thick-subconjunctival connective tissue membrane with areas of active fibroblast proliferation. These blebs are not considered true cysts because they do not have an epithelial wall lining (15,17,18).

4.PATHOPHYSIOLOGY

The pathophysiology of encapsulated blebs is still unknown. Scott and Quigley (12) theorized that during the first 2 weeks after surgery, there is cellular proliferation and

Figure 19.1 (See color insert) Slit lamp photograph of an encapsulated bleb, with its character istic dome shaped appearance and enlarged conjunctival vessels.

synthesis of extracellular material in the bleb wall and episcleral surface in contact with the bleb wall (19). The IOP remains at low normal levels due to decreased aqueous humor production. Between the second and the fourth postoperative week, aqueous flow returns to normal expanding the bleb cavity. In a bleb wall which is porous with microcysts, aqueous passes into conjunctival vessels or the tear film, maintaining IOP under control. Conversely, in a thick-walled nonporous bleb wall, increased aqueous flow causes bleb wall compression with increased wall tension, preventing further aqueous drainage. A vicious cycle of inadequate fluid movement occurs and high IOP develops. In addition, high IOP may slow and even stop blood vessel flow around the bleb cavity (conjunctival and episcleral), further impairing aqueous drainage (12).

Other important processes occur during wound healing. Fibroblast proliferation with collagen synthesis in the extracellular space is followed by wound contraction. During this process, two different groups of fibroblast have been observed. The first group, called myofibroblast, synthesizes contractile proteins. These proteins are identified as a major source of wound contraction and scarring seen during the healing process (20,21). The other group of fibroblast synthesizes collagen (20). It is theorized that the myofibroblasts are mainly involved in the formation of flat, scarred bleb, whereas in thick-walled, encapsulated blebs, the noncontractile collagen-producing fibroblasts play a major role (22). Inflammatory mediators present after filtering surgery are implicated as important triggers of the healing process and consequently bleb encapsulation (22,23). Interestingly, 5-fluorouracil, which inhibits fibroblasts proliferation (24) and increases the long-term trabeculectomy success (2,3,5), does not reduce the rate of encapsulated blebs (3,5,25 27). It is speculated that collagen-producing fibroblasts are less sensitive to 5-fluorouracil than the myofibroblasts (22,26,28).

5.INCIDENCE AND RISK FACTORS

The reported incidence of encapsulated filtering blebs following glaucoma filtering surgeries ranges from 2.5% to 29.0% (3,12,14,17,25,26,28 34). Differences in surgical technique likely contribute to the enormous variation of these reported incidence rates, such as full thickness vs. guarded procedures, tenonectomy vs. nontenonectomy, and limbusvs. fornix-based conjunctival flaps. Other factors that may influence the reported incidences are biased selection of patients and the diagnostic criteria used to define encapsulated blebs. As an example, Richter et al. (14) and Schwartz et al. (29) did not require elevated IOP for the diagnosis of encapsulation, whereas in the study performed by Ophir and Ticho (26) elevated IOP was a prerequisite for this diagnosis.

The commonly reported risk factors related to the occurrence of encapsulated blebs include presurgical argon laser trabeculoplasty, topical steroid use, postoperative inflammation, glove powder in the bleb, among others (see Table 19.1 for a full list). The diversity of risk factors indicates that there is no known single etiologic factor. It is speculated that most of these risk factors are associated with intraocular inflammation (17), conjunctival inflammation (22,23), or increased predisposition to wound healing (37).

The role of anti-fibroproliferative agents such as 5-fluorouracil and mitomycin-C on the bleb encapsulation process has not been fully elucidated. Despite inhibiting fibroblast proliferation (24,38,39), these agents, particularly 5-fluorouracil, appear not to decrease bleb encapsulation rates following trabeculectomy (3,5,30). The rate of encapsulation following trabeculectomies with mitomycin-C is more controversial, with some studies reporting low rates (6,28) and others reporting rates similar to surgeries without antifibroproliferative agents (25,27,33). Possible explanations from the discrepancy in the

Table 19.1 Risk Factors Implicated in the Development of

Encapsulated Blebs

Argon laser trabeculoplasty (14,34)

Prior conjunctival surgery (31)

History of Tenon’s cysts on the other eye (31,34)

Long term use of sympathomimetics (31)

Long term use of beta blockers (14)

Male gender (30,33,34)

Corticoisteroid therapy (35,36)

Unusual postoperative inflammation (18) and uveitis (23)

Congenital glaucoma (14)

Juvenile glaucoma (14)

Trabeculectomy on the fellow eye (37)

Glove powder (30)

findings could include different surgical technique, dose of mitomycin-C, definition of bleb encapsulation, and studied population.

In conclusion, it is still unclear whether anti-fibroproliferative agents inhibit the development of encapsulated blebs. Most studies support the idea that 5-fluorouracil does not decrease the incidence of encapsulated blebs. Mitomycin-C, however, may prevent or delay the development of encapsulated blebs.

6.TREATMENT

The management of encapsulated blebs is either medical or surgical. Medical treatment alone is successful in over 70% of cases, but IOP may stabilize in the high teens (Table 19.2).

6.1.Medical Treatment

Medical treatment involves the use of antiglaucoma drops, particularly aqueous humor suppressants, and digital massage (12,14,17,26,31 33,40). The most common medications used are beta-blockers, alpha-2 agonists and carbonic anhydrase inhibitors. Scott and

Table 19.2 Success Rate of Medical Treatment for Encapsulated Blebs

Figure 19.2 (See color insert) Needling procedure. Under sterile conditions, a lid retractor is placed and a 30 gage bent needle is advanced into the cyst through a remote conjunctival entry; side to side movements of the needle are used to tear the wall of the cyst (A). At the end of the procedure, topical fluorescein 2% is applied to test for leakage (B).

cataract extraction when compared with patients without bleb encapsulation. Our experience suggests that patients with bleb encapsulation usually have higher long-term IOP and require more glaucoma medications.

8.SUMMARY

Bleb encapsulation can occur after trabeculectomy with or without anti-proliferative agents, usually between the second and the fourth postoperative week. The characteristic dome-shaped appearance is associated with a varied number of risk factors. Proper recognition and treatment are fundamental to long-term pressure control. Most cases respond to medical management, but bleb needling or surgical excision of the cyst might be required in some cases.

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