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

commonly referred to as a filtering procedure. Although several variations for this surgical procedure have been described, all filtering operations share the same basic mechanism of action and general surgical principles. We first consider these aspects and then discuss specific filtration techniques and potential complications.

MECHANISMS OF ACTION Drainage Fistula

The basic mechanism of all filtering procedures is the creation of an opening, or fistula, at the limbus, which allows a direct communication between the anterior chamber and subconjunctival space. This fistula bypasses the trabecular meshwork, Schlemm canal, and collecting channels. From the subconjunctival spaces, aqueous is absorbed by surrounding tissues or crosses the conjunctival epithelium and drains with tears through the nasolacrimal duct.

Filtering Bleb

Most, but not all, successful glaucoma filtering procedures are characterized by an elevation of the conjunctiva at the surgical site, which is commonly referred to as a filtering bleb. The clinical appearance and function of these blebs vary considerably with regard to extent, elevation, and vascularity (1, 2). The blebs that are most often associated with good intraocular pressure (IOP) control have decreased vascularity with numerous microcysts in the epithelium and are either low and diffuse or more circumscribed and elevated (3) (Fig. 38.1).

The histologic appearance of both functioning and failed filtering blebs consists of normal epithelium with no encircling-type junctions between the cells that would limit fluid flow (3). The subepithelial connective tissue may contain viable activated fibrocytes (4), and the histologic appearance at this level correlates better with bleb status than does that of the epithelium, in that functioning blebs have loosely arranged tissue with histologically clear spaces, whereas the failed blebs have dense collagenous connective tissue (3). Change in morphology and a decrease in the number of epithelial and goblet cells have been found in conjunctival epithelium overlying thin cystic blebs (5).

Figure 38.1 Types of functioning filtering blebs. A: Low, diffuse bleb. B: Discrete, elevated bleb. Note that both are avascular.

Routes of Aqueous Drainage

Studies have suggested that aqueous in the filtering bleb usually filters through the conjunctiva and mixes with the tear film or is absorbed by vascular or perivascular conjunctival tissue (6, 7, 8, 9 and 10). Less commonly, a filtering procedure may be associated with IOP control in the absence of an apparent filtering bleb. This is more common when the fistula is covered by a partialthickness scleral flap (trabeculectomy), and suggested mechanisms of aqueous drainage in these cases include flow through

(a) lymphatic vessels near the scarred margins of the surgical area; (b) atypical, newly incorporated aqueous veins; and (c) normal aqueous veins (6, 10). Preservation of the aqueous drainage route beneath the scleral flap, as seen on ultrasound biomicroscopy, appears to correlate with the development of a filtering bleb following trabeculectomy (11, 12).

BASIC TECHNIQUES OF FILTERING SURGERY

The various types of filtering surgery differ primarily according to the method used to create the

Limbal Stab Incision (Paracentesis Site)

Some surgeons create a paracentesis, which consists of a selfsealing, beveled incision into the anterior chamber at the limbus, usually temporally at the horizontal meridian, or in the inferior-temporal quadrant, as a route for injecting fluid at the end of the procedure. This can be done with a tapered, pointed knife, or number 75 blade before entering anterior chamber under the scleral flap. If antifibrosis agents (discussed later in this chapter) are to be used, however, it may be best to wait until after that step of the operation, to avoid a route for potential entry of the drug into the anterior chamber.

Preparation of the Conjunctival Flap

Preparation of the conjunctival flap is a critical step in all filtering procedures, in that the most common cause of failure is scarring of the filtering bleb. Although techniques differ among surgeons, meticulous detail with minimal tissue damage and bleeding is essential.

Position of the Flap

Some surgeons elect to make the flap at the 12-o'clock position to take advantage of the wider limbus in this area. Others prefer

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one of the superior quadrants, leaving the adjacent quadrant available for future surgery if required. The inferior quadrant was used in the past when previous ocular surgery resulted in scarring of conjunctiva in the superior quadrants (14). However, the latter technique is associated with an increased risk for endophthalmitis and should be avoided (15).

Figure 38.4 Incision through conjunctiva in preparation of a limbus-based conjunctival flap. Limbus-Based versus Fornix-Based Flap

Conjunctival flaps for glaucoma filtering surgery have traditionally been limbus based—that is, with t he initial incision in the fornix (Fig. 38.4). More recently, a fornix-based flap has gained favor (Fig. 38.5), particularly in association with a trabeculectomy (16, 17 and 18). Several studies have compared limbusand fornix-based conjunctival flaps in association with trabeculectomy and reported similar success rates, whether used in combination with cataract surgery or as a separate procedure (19, 20, 21, 22, 23, 24, 25, 26, 27 and 28). However, one investigative team found slightly better postoperative IOP control with the limbusbased flap (29), whereas others found better pressure control and more diffuse blebs with the fornix-based flaps (17, 30). One retrospective study found cystic leaking blebs only in eyes with limbus-based flaps (24).

Figure 38.5 Incision through conjunctiva in preparation of a fornix-based conjunctival flap.

Surgeons differ on this aspect of filtering surgery, with some preferring the relative ease and improved surgical exposure of the fornix-based flap and others preferring the tighter wound closure that may be achieved with the limbus-based flaps. One circumstance in which a fornix-based conjunctival flap is especially useful is when such a flap was used previously, as during an extracapsular cataract or scleral buckling surgery, leaving a band of scar tissue at the limbus. In these cases, it is difficult to dissect a limbus-based flap sufficiently anteriorly without creating holes in the conjunctiva. A preferable alternative is to excise the band of scar tissue and pull the new edge of the conjunctival flap down to peripheral cornea.

Management of Tenon Capsule

There is some controversy regarding the value of removing all or a portion of Tenon capsule, the main source of fibroblasts in the area of the conjunctival flap. Two studies revealed no difference in postoperative IOP control between eyes with excision of the capsular tissue and those in which it was left partially or totally intact (31, 32). For this reason, many surgeons routinely preserve Tenon capsule by dissecting between the capsule and episclera when preparing the conjunctival flap. This may be especially important when using adjunctive antifibrosis agents, to avoid excessively thin or leaking filtering blebs in the late postoperative course. Sub-Tenon space also appears to be the best cleavage plane for aqueous drainage because there is less scarring and subsequently less resistance to flow (33). Modified fornix-based techniques have been described, such as small incision trabeculectomy and microtrabeculectomy (34, 35 and 36), by using 2.5- to 3-mm conjunctival peritomy within 2 mm of the limbal area and avoiding Tenon capsule. Other surgeons excise variable amounts of Tenon capsule when it appears to be unusually thick, as in young patients. This can be accomplished by dissecting between the conjunctiva and Tenon capsule and then excising the capsule from the episclera. An alternative approach

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is to dissect Tenon capsule from underlying episclera, strip a portion of the capsule from the conjunctiva with gentle traction, and then excise the exposed portion of capsular tissue.

Figure 38.6 Retraction of conjunctival flap over the cornea.

With all techniques, blunt dissection is used when possible to avoid bleeding, and sharp dissection is used only when required. Gentle handling of the conjunctiva is essential at all times, and nontoothed conjunctival forceps are preferable to avoid tearing or crushing the conjunctiva. During the fistulizing part of the operation, it is important to keep the conjunctival flap moist and to minimize handling of the tissue. With a limbus-based flap, this can be conveniently accomplished by reflecting the flap over the cornea with a surgical sponge (Fig. 38.6) or nontoothed forceps. When manipulating the conjunctivaTenon capsule flap, it is best to grasp the capsule and avoid touching the conjunctiva.

Use of Viscoelastic Agents

The injection of a viscoelastic agent (e.g., sodium hyaluronate) into the anterior chamber at the completion of the filtering procedure did not reduce the incidence of postoperative flat anterior chambers in most studies (37, 38 and 39). However, injecting the agent through a paracentesis incision at the outset of a trabeculectomy procedure was associated with a lower incidence of this complication (40, 41), presumably by avoiding intraoperative hypotony and the subsequent suprachoroidal effusion that may initiate the cascade of events leading to a shallow anterior chamber. Others have supported this finding and noted that the technique also tends to minimize intraoperative bleeding (42) but not postoperative hyphema or postoperative corneal endothelial cell loss (43, 44). Complications of

intracameral viscoelastics include iris prolapse during surgery and a higher early postoperative IOP, for which the preoperative use of pilocarpine, 2%, and a slightly less tight closure of the scleral flap were recommended (41, 44). Injection of dense viscoelastics, such as Healon or Healon 5, into the anterior chamber may be used as a temporary solution for postoperative flat anterior chambers (45, 46 and 47). Peripheral Iridectomy

A peripheral iridectomy is a routine part of all standard filtering procedures and is usually made after the fistula has been prepared (48). However, if the iris prolapses into the limbal wound, it is generally best to make the iridectomy and then complete the fistula. The iridectomy should extend beyond the margins of sclerectomy to avoid obstruction of the fistula by the peripheral iris. The technique for the incisional peripheral iridectomy is discussed in Chapter 36.

Complications of the surgical iridectomy itself can include inflammation, hyphema, and iridodialysis. It is preferable not to make the iris incision too close to the iris root for concern of incising the ciliary body and inducing significant bleeding. Some surgeons omit the peripheral iridectomy in patients who have pseudophakia or undergo a combined trabeculectomy with cataract surgery using a small-clear corneal incision. This is especially true in cases where there is a deep anterior chamber and the risk of iris incarceration into the sclerectomy is low. In one study, patients with and without peripheral iridectomy had similar postoperative vision and IOP control (49, 50).

Closure of the Conjunctival Flap

Watertight closure of the conjunctival flap is also a critical aspect of any filtering procedure, because a leaking wound may lead to a persistently flat bleb or anterior chamber, or both. This can lead to failure of the filtering bleb to develop properly. A fine absorbable suture, such as 10-0 polyglycolic acid or polyglactin, on a tapered, vascular needle is desirable, because it minimizes leakage at the suture sites and excessive tissue reaction. For closure of a limbus-based flap, a running suture with close bites provides the tightest closure. When Tenon capsule has been preserved, a double running closure, first of Tenon tissue, and then conjunctiva, may increase the chances of tight wound closure (Fig. 38.7) (51). Alternatively, several interrupted sutures that close Tenon capsule may be used to approximate the wound edges before running closure. This is especially important when adjunctive antifibrosis agents are used.

A running suture can also be placed along the limbus for fornix-based flaps, especially when a small edge of conjunctiva is retained adjacent to the limbus (52). Various techniques have been described for placing a running mattress suture at the limbus, which provides tight wound closure and is especially useful when adjunctive antifibrosis agents are used (52, 53 and 54). In other situations, surgeons find it adequate to use a single interrupted suture at one or both ends of the conjunctival flap (18, 55) (Fig. 38.7), which stretches the conjunctiva tightly over peripheral cornea.

If a paracentesis is made at the outset, balanced salt solution, or a viscoelastic, by using a cannula on a syringe, may be injected into the anterior chamber via that incision at two stages during the completion of the procedure. The first of these is after suturing the scleral flap in a trabeculectomy to ensure appropriate flow around the flap. The anterior chamber should deepen and the eye should become slightly firm before

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fluid begins to flow around the flap edges. If the flow is too brisk and the chamber collapses, more sutures should be added. Conversely, sutures may need to be loosened if the eye remains too firm. The second stage of fluid injection is after closure of the conjunctival flap. This should deepen the anterior chamber and create a sustained elevation of the bleb, thereby demonstrating patency of the fistula and watertight closure of the conjunctival incision. Some surgeons examine the conjunctival closure for bleb leaks by coating the bleb surface with fluorescein at the end of the case.

Figure 38.7 A: Closure of conjunctival flap with running suture. (From Shields MB. Trabeculectomy vs. full-thickness filtering operation for control of glaucoma. Ophthalmic Surg. 1980;11:498, with permission.) B: Closure of fornix-based conjunctival flap. C: Closure of fornix-based conjunctival flap—Wise closure. (From Ng PW, Yeung BY, Yick DW, et al. Fornixbased trabeculectomy with Wise's suture technique in Chinese patients. Ophthalmology. 2000;107:2310-2313, with permission.) POSTOPERATIVE MANAGEMENT

Topical mydriatic-cycloplegics may be used for the first 2 to 3 weeks to help maintain the anterior chamber depth, particularly in patients with phakic eyes and those with postoperative hypotony. Some investigators feel that these agents may also reduce postoperative inflammation (56). Topical antibiotics are used routinely for 7 to 10 days. Use of topical corticosteroids decreases conjunctival scarring and is associated with higher success rates with trabeculectomy. They are typically used for 4 to 6 weeks, although some surgeons use lowdose topical corticosteroids indefinitely. (The effect of postoperative corticosteroid use is discussed in more detail later in the chapter.)

FISTULIZING TECHNIQUES

There are two basic types of fistulas: (a) those which extend through the full thickness of the limbal tissue and (b) those which are covered by a partial-thickness scleral flap. During the first half of the 20th century, the former technique was used exclusively. The concept of a guarded fistula (trabeculectomy) began to gain popularity in the 1970s. With the advent of adjunctive antifibrosis agents and laser suture lysis, the fullthickness procedures lost favor compared with conventional trabeculectomy surgery and

are now primarily of historical interest. Partial-Thickness Fistulas (Trabeculectomy)

The standard full-thickness filtering procedures were often complicated by excessive aqueous filtration, which led to a high incidence of prolonged flat anterior chambers, associated with corneal decompensation, synechiae formation, and cataracts. In addition, the filtering blebs often became thin and were susceptible to rupture, creating the danger of endophthalmitis. One way to minimize these complications is to place a partialthickness scleral flap over the fistula. This concept was suggested by Sugar (57) in 1961 but was popularized by the 1968 report of Cairns (58). Both authors referred to the technique as a trabeculectomy, and this remains the most commonly used technique for filtering surgery today.

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Figure 38.8Possible routes of aqueous humor flow associated with a trabeculectomy: 1, Aqueous flow into cut ends of Schlemm canal (rare); 2, cyclodialysis (if tissue is dissected posterior to scleral spur); 3, filtration through outlet channels in scleral flap; 4, filtration through connective tissue substance of scleral flap; 5, filtration around the margins of the scleral flap.

Theories of Mechanism

It was originally thought that aqueous might flow into the cut ends of Schlemm canal (58). Subsequent studies, however, showed fibrotic closure of the canal at its cut ends in monkey and human (59, 60) eyes, and the presence of Schlemm canal in the “tra beculectomy” specimen did not correlate with the outcome of the procedure (61, 62 and 63). Furthermore, it was noted that most successful cases had a filtering bleb, and the amount of fluorescein-stained aqueous in the filtration area correlated with the success of the procedure (64), suggesting that external filtration was the principal mode of IOP reduction. The outer layers of limbus and anterior sclera do not differ ultrastructurally from the inner layers in a way that might predispose to increased passage of aqueous (65). Perfusion studies of human autopsy eyes, in which a trabeculectomy was created and the margins of the scleral flap were sealed with

adhesive, did show a significant flow through the scleral flap (66). Fluorescein angiographic studies of eyes with successful trabeculectomies showed the primary route of external filtration to be around the margins of the scleral flap (67). It may be that external filtration occurs around or through the scleral flap, depending on how tightly the flap is sutured or the thickness of the scleral flap. Use of antimetabolites often leads to alterations of the scleral flap ranging from a complete melt to a minimal decrease of integrity. Other contributors to the outflow resistance are the surface area of the bleb available for diffusion and the quality of the conjunctiva overlying the bleb (i.e., thin and avascular, or thicker with only decreased vascularity) (Fig. 38.8). Other possible mechanisms of IOP reduction by trabeculectomy include cyclodialysis, if the fistula extends posterior to scleral spur (59), or aqueous outflow through newly developed aqueous veins, lymphatic vessels, or normal aqueous veins (68, 69). Basic Trabeculectomy Technique

With the trabeculectomy technique (Fig. 38.9), the margins of the scleral flap, adjacent to the corneolimbal junction, are outlined first with light cautery and then with partial-thickness scleral incisions. The original technique described by Cairns involved a 5 × 5-mm square, but numerous variations in size and shape of the scleral flap have been described, as discussed later in this section. A lamellar flap, hinged at the limbus, is then dissected forward until at least 1 mm of the bluish-gray zone of the peripheral cornea is exposed. It is difficult to precisely determine the relative thickness of the scleral flap, but in general, it should be one-half to two-thirds sclera thickness.

The fistula is begun by first entering the anterior chamber with a knife just behind the hinge of the scleral flap, and then widening the incision with the knife or scissors to within approximately 0.5 mm of the scleral flap margins. Radial incisions are then extended posteriorly on either end of the initial incision for 1 mm, and the resulting flap of deep limbal tissue is reflected until the angle structures can be visualized, and the tissue is excised with scissors along the scleral spur. Today most surgeons prefer using a scleral punch (described later).

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