If there is no internal flow block and intraocular pressure (IOP) remains too high despite maximally tolerated medical therapy, surgery to relieve outflow block is needed. Such procedures are designed to increase the flow of aqueous out of the eye, thus reducing IOP.
Laser trabeculoplasty to relieve outflow block is described in detail in Chapter 31 . Laser trabeculoplasty is generally attempted before incisional surgery unless the IOP is very high or the optic nerve is severely damaged. Incisional surgery is sometimes the only viable intervention: when subnormal IOPs may be required, as in progressive disease; or when conditions are not amenable to trabecular laser response, such as inflammatory, traumatic, or developmental glaucomas; when the angle is damaged or covered by synechiae; or the cornea is clouded. Several studies have supported using filtration surgery as the initial therapy in routine open-angle glaucoma, citing better medium- and long-term visual outcome as one of the major benefits. Although this remains an area of active debate, there is widespread agreement that the individual circumstances of the patient must be evaluated before the appropriate initial therapy can be chosen.
Incisional surgery to relieve outflow block may create external filtration (e.g., trabeculectomy or full-thickness filtering procedures) or internal filtration (e.g., cyclodialysis), or it may essentially disrupt the trabecular meshwork from the outflow pathway (e.g., trabeculotomy ab externo and goniotomy). Regardless of the procedure used, the goal is to reduce the IOP to a level that will prevent further damage to the optic nerve but not reduce it so much as to cause problems from hypotony. The lowest IOP that can be tolerated by the eye is generally above 5 mmHg, although this may depend on the patient’s age: older patients (over 55 years old) can often retain 20/20 acuity with IOPs under 4 mmHg, whereas younger patients (with presumably more elastic and deformable sclera) may develop vision-altering hypotonous maculopathy at similar tensions. Low single-digit IOPs can predispose to cataracts, choroidal effusion, optic nerve swelling, or refractive instability. (Of course the central corneal thickness (CCT) correction for applanation readings needs be considered: some ‘low’ IOPs are, when adjusted for CCT, actually above the ‘hypotony’ range.)
There is no evidence to support the notion that a specific protective effect is conferred on glaucoma patients whose pressure is simply reduced to 20 mmHg or lower, as though this were a magical number. Some long-term studies indicate that more severely damaged nerves may require pressures in the low teens if damage is to be stopped; with a significant advantage conferred by reducing IOP fluctuations. Although it is felt that external filtration of aqueous does in fact dampen IOP fluctuations, the role of fluctuating pressures per se in contributing to progressive glaucomatous loss remains controversial. Some authors assert that fluctuating IOPs contribute neither to the conversion of ocular hypertensives into glaucoma nor to destabilization of visual field function. Long-terms results of the Advanced Glaucoma Intervention Study (AGIS), however, suggest that IOP fluctuation is a ‘risk factor’ for continued visual field and optic nerve deterioration.
There is ample prospective evidence to substantiate that lowering the IOP in glaucoma patients slows the rate of visual field loss, even in normal-tension glaucoma. Rather than choosing a specific target pressure, most multicenter studies prospectively select an end-point percentage for pressure reduction as the research target goal: e.g., a 30% reduction in the Normal-Tension Glaucoma Study, 25% reduction in the Early Manifest Glaucoma Trial, or 20% reduction in the Ocular Hypertensive Treatment Study.
Full-thickness procedures generally provide lower pressures for a longer time than did guarded filtration procedures viz trabeculectomies before the era of antimetabolite usage. However, such full-thickness procedures also had a higher complication rate in most surgeons’ hands. Efforts continue to achieve better pressure control with fewer complications using modifications of trabeculectomy technique, pharmacologic modifications of wound healing, and manipulations of flap closure using releasable sutures or laser suture lysis.
EXTERNAL FILTRATION SURGERY
The goal of external filtration is to create a new drainage pathway that allows aqueous to pass from the anterior chamber into the subconjunctival space. There the fluid either is absorbed into the conjunctival blood vessels or lymphatic equivalents or, if the bleb is thin walled, passes directly across the conjunctiva into the tear layer.
Filtering surgery requires an opening through the scleral wall at the limbus. The surgeon makes this opening much larger than the 15-μm diameter hole that (theoretically) is adequate for total aqueous flow out of the eye, because the healing process works to reduce the ultimate or effective size of the opening. Indeed, the healing process often obliterates the opening entirely. A larger initial opening, however, does not ensure success and may in fact lead to higher failure and complication rates. These rates increase because initial hypotony causes production of secondary aqueous, which apparently contains factors which accelerate wound healing, and may eventually reduce the flow of aqueous humor through the sclerostomy. This causes the episcleral surface to scar down around the sclerostomy and close it.
Early postoperative hypotony is to be avoided when possible. The ideal procedure would lower IOP to 8–10 mmHg immediately and keep it there. Ideally the collagenolytic activity of pure aqueous passing through the sclerostomy can modify the conjunctiva, converting it to an acellular matrix that is porous to aqueous percolation. If the aqueous contains protein and serum as a result of hypotony, healing is accelerated rather than retarded, and the outcome may be poor.
There are two basic types of external filtration procedures: guarded and full thickness.
When the filtering sclerostomy is protected from excessive flow either by partially closing it with a scleral flap or by suturing techniques, it is described in terms such as guarded , protected , subscleral , or partial-thickness filtration surgery . The advantage of such techniques is that the initial egress of aqueous from the anterior chamber is retarded, which reduces the incidence of postoperative flat chambers. Additionally, such maneuvers may reduce the incidence of hypotony and suprachoroidal hemorrhage.
Decreasing the incidence of postoperative hypotony and flat chamber appears to reduce inflammation, peripheral anterior synechiae, and cataract formation as well. Guarded filtration procedures may also reduce the long-term success rate of the surgery and prevent attainment of the very low pressures that seem desirable in advanced glaucoma or normal-tension glaucoma.
Guarded procedures with or without antimetabolites are generally preferred except under unusual circumstances. There are a number of guarded filtering techniques, of which trabeculectomy and its variations are the most popular.
Procedures such as thermal sclerostomy, posterior or anterior lip sclerectomy, or Elliott’s trephination have no guard over the external surface of the sclerostomy other than the conjunctiva and Tenon’s capsule. These procedures are referred to as the Scheie procedure or full-thickness filtration surgery . Prior to the use of antimetabolites in guarded filtration surgery, such full-thickness procedures were deemed appropriate if either very low pressures were desired (e.g., in normal-tension glaucoma) or if guarded filtration surgery had failed. They usually require a limbal-based conjunctival flap because of high aqueous outflow in the early postoperative period. Such procedures are associated with a high incidence of complications, including shallow (or flat) anterior chambers, premature cataract formation, and late infections.
RESULTS OF EXTERNAL FILTRATION SURGERY
External filtration surgery achieves reasonable IOP lowering in 65–85% of adults, depending on the condition of the eye, the use of antimetabolites, postoperative healing, duration of follow-up, and the skill with which the surgery is performed. This success rate may be increased to over 90% if resumption of IOP-lowering medications is included.
It is difficult to compare surgical results because of variations in techniques and definitions of success. In a prospective, randomized study of the differences between thermal sclerostomy and trabeculectomy, Blondeau and Phelps reported IOPs less than 22 mmHg in 65% of thermal sclerostomies and 76% of trabeculectomies without anti-metabolites followed up for 5 years. When medications were added, the success rates rose to 91% for the eyes treated with thermal sclerostomy and 94% for those treated with trabeculectomy. Pressures tended to be somewhat lower in eyes undergoing thermal sclerostomy, but visually significant cataracts occurred three times more often and hypotony twice as often with thermal sclerostomy. Thinner blebs were also more frequent with thermal sclerostomy. Even eyes with no detectable bleb at 5 years after either procedure (approximately one-third of the total), however, had average IOPs of 17 mmHg.
In a retrospective comparison of full-thickness filtration versus trabeculectomy, Lamping and co-workers found that the former offered much better long-term pressure control. They and others have noted an equal frequency of problems with hypotony with guarded and full-thickness procedures. The frequency of full-thickness procedures temporarily increased with the advent of holmium laser sclerostomy in the early 1990s, but convincing long-term success rates were elusive. The incidence of complications with full-thickness procedures has historically been high enough to dissuade the occasional glaucoma surgeon.
THE CONJUNCTIVAL FLAP
A conjunctival flap is required for all filtration procedures. Both limbus- and fornix-based conjunctival flaps are used for guarded filters, whereas a limbus-based flap is preferred by most surgeons for full-thickness filtration surgery.
A limbus-based conjunctival flap allows tight wound closure, which may be important if early postoperative massage, suture cutting, or pharmacologic inhibition of wound healing is anticipated. On the other hand, there is a marked tendency over months to years for limbal-based blebs to ‘migrate’ towards the limbus over time: the long suture track of the superior conjunctiva can cicatricially contract and circumscribe the posterior flow of aqueous. This frequently results in elevated, thin-walled, ‘mulberry-shaped’ cystic blebs hugging the superior limbus, which can cause symptomatic irritation, or at worst, can be prone to leak or infection ( Fig. 34-1A ).
Technically the limbal-based filter is relatively easy to master, facilitating rapid surgery. The conjunctiva can first be elevated by injecting balanced salt solution (BSS) in the area of the flap to ease dissection. This injection should be made with a 30-gauge needle that penetrates the conjunctiva well away from the sclerostomy site. An incision is made at least 8 mm from the limbus and away from the insertion of the rectus muscles to avoid bleeding ( Fig. 34-2 ). The incision should be carried through Tenon’s capsule to the sclera. The capsule is then undermined or disinserted to elevate it from the sclera, and the incision is enlarged circumferentially to allow exposure of 4–5 mm of the limbal area. Blunt scissors should be viewed through the outer surface of the conjunctiva during this dissection to prevent button-holing. The incision can be elliptic to come in front of the rectus muscle insertion if necessary for adequate exposure.
Closure of the conjunctival incision varies markedly from surgeon to surgeon. Most prefer a water-tight closure achieved with a running suture that incorporates both Tenon’s capsule and the conjunctiva. Some prefer a meticulous two-layered closure, which combines an interrupted closure of Tenon’s capsule with a running closure of the conjunctiva.
Suture material also varies. Many surgeons prefer non-absorbable sutures such as 9-0 or 10-0 nylon. Others prefer 8-0 silk, which can be removed in 5–7 days; whereas 8-0, 9-0, or 10-0 Vicryl is preferred by some because it will be absorbed. Fine-tapered BV (blood vessel) vascular needles, such as those available on 10-0 Prolene, 9-0 or 10-0 Vicryl sutures, are preferred by some surgeons because they leave smaller needle tracts, which may prevent leaks, especially if wound-modulating agents are used. Non-absorbable nylon or Prolene sutures have been recommended for cases in which antimetabolites are used, because full healing may take several weeks and absorbable sutures can occasionally lose their strength before that time; however, long-term suture persistence may result in symptomatic ocular irritation.
The fornix-based conjunctival flap provides easier exposure of the surgical site and reduces handling of the conjunctival flap, but may require longer operative time. Unless they are closed carefully, such flaps may leak in the postoperative period and fail to retain aqueous, so that the bleb flattens. Such leakage may be problematic under several common postoperative circumstances: if massage is needed to elevate the bleb; if an anti-wound-healing agent such as 5-fluorouracil (5-FU) or mitomycin-C is used; or if postoperative lasering of scleral flap sutures is performed.
A fornix-based flap is created by cutting the conjunctiva and Tenon’s capsule together or separately ( Fig. 34-3 ) flush with the limbus over a circumference of about 6–8 mm. Pre-incisional ballooning of the subconjunctival space with epinephrine containing xylocaine may be helpful, using a 30-guage needle introduced at the 12 o’clock limbus. The conjunctival flap can then be undermined posteriorly with blunt dissection ( Fig. 34-4 ), exposing the area for the scleral incision. A short radial relaxing incision can be made at one or both ends of the flap if exposure is restricted or if a particularly wide exposure is needed to insert a seton or valve.
Various techniques have been devised for a rapid but leak-proof closure of the fornix-flap. A careful running suture in two layers, first closing Tenon’s and then its overlying conjunctiva at the limbus, though tedious, is reliably water tight. ( Figs 34-5 and 34-6 ). A complex single-suture closure has been described by Wise as a running mattress closure ( Fig. 34-7A,B ). Slightly quicker to learn and perform is Khaw’s use of lateral #9-0 or #10-0 nylon or Vicryl sutures to initially close the distal ends of the conjunctiva phimotically to the limbal tissue; and then bury the knots of three to five separate #10-0 Vicryl mattress sutures in shallow corneal ‘scratch’ incisions, parallel to the limbus, thus firmly attaching conjunctiva along the length of the superior cornea ( Fig. 34-7C,D ).
Comparisons between the fornix-flap and limbus-flap conjunctival closures, before the widespread adoption of antimetabolites, recognized no difference in success between them. Another study of difficult glaucoma cases, without the benefit of antimetabolites, showed a higher failure rate with fornix-based flaps compared with limbus-based flaps. On the other hand, fornix-based flaps lessen the likelihood of developing elevated, thin-walled cystic blebs at the limbus (as commonly seen with limbus-based flaps), thus reducing the risk of serious complications such as leaks or endophthalmitis. The advantage of diffuse, low blebs with fornix-flap closure, when combined with mitomycin usage and releasable (or adjustable) flap sutures, makes it a technique of increasing and compelling popularity.
EXCISION OF TENON’S CAPSULE
Some studies have suggested that excision of Tenon’s capsule in young people, in African-Americans, or in people who require reoperations may enhance filtration success. Evidence for this is not conclusive, and the use of antimetabolites has made much of the issue moot. If excision of Tenon’s capsule is desired, the dissection can be facilitated by injecting xylocaine or saline between the capsule and conjunctiva; care must be taken to avoid conjunctival buttonholes.
GUARDED FILTRATION PROCEDURE
Trabeculectomy, with its many modifications, is the most commonly used guarded filtration procedure. Cairns introduced the modern-day trabeculectomy in the 1960s. It was initially believed that aqueous escaped through the cut ends of Schlemm’s canal, but it subsequently became obvious that the major effect of the surgery occurred via filtration of aqueous into the subconjunctival space. The reduced incidence of hypotony and flat anterior chambers made trabeculectomy attractive to glaucoma surgeons.
Trabeculectomy has become the standard glaucoma procedure, with excellent results for most forms of open-angle and chronic angle-closure glaucoma. Aphakic, inflammatory, traumatic, and other secondary forms of uncontrolled glaucoma also are treated by trabeculectomy; success rates are good when wound-healing retardants are used, although success rates tend to be lower than in uncomplicated cases. So long as mobile conjunctiva is available superiorly, despite a history of prior surgeries, the predictability of the trabeculectomy and its long-term efficacy at maximally lowering IOP make it the procedure of choice for the majority of uncontrolled glaucoma eyes.
As discussed more extensively in Chapter 37 , trabeculectomy can be successfully combined with cataract extraction under a variety of circumstances. The most important advance allowing combined surgeries at one sitting has been the advent of the small-incision cataract/intraocular lens procedure. Thus modern techniques have broadened the indications for combining these procedures, and many surgeons report excellent results.
There is a wide variety of surgical preferences and techniques developed in the last half-century of the trabeculectomy’s development, and we begin with a generalized approach, and conclude with the specifics of a successful fornix-flap technique from the Moorfield’s Eye Hospital in London.
The initial trabeculectomy procedure is usually performed at a site superiorly and slightly nasal. (This preserves the superotemporal area for repeat trabeculectomy or tube surgery if needed.) A corneal traction suture (e.g. 7-0 or 8-0 Vicryl) is preferable to a superior rectus suture, so as to minimize conjunctival perforation superiorly, an area of potential bleb formation. In aphakic or pseudophakic glaucoma, the surgical area selected should have minimal conjunctival scarring. This can be determined by attempting to move the anesthetized conjunctiva with an instrument or by injecting xylocaine with epinephrine under the conjunctiva at the time of surgery. If the conjunctiva is tightly adherent to the globe, another site should be selected, based on the response to subconjunctival fluid dissection. In the face of inoperable superior conjunctival scarring, an alternative procedure, such as an inferonasal glaucoma shunt, can be selected.
The episcleral surface planned for the scleral flap is lightly cauterized ( Fig. 34-8 ) to reduce bleeding. Excessive cauterization should be avoided, however. Cauterization can be done with wetfield cautery or with a microdiathermy instrument. Microdiathermy offers the advantage of pinpoint cauterization, which is useful when cauterizing individual vessels during the early parts of the procedure, and later in the operation for persistent microhemorrhage from the iris, ciliary body or deep sclera after excising the trabeculectomy specimen.
The scleral flap is usually one-third to one-half the scleral thickness, rectangular or triangular in shape, and dissected anteriorly towards the limbus ( Fig. 34-9 ). Antimetabolites may be administered before or after the scleral flap is developed, but usually before any opening is made into the anterior chamber ( Fig. 34-10 ). It is important to place a paracentesis at the peripheral limbus using a super-sharp blade after preparing the scleral flap but before otherwise entering the globe ( Fig. 34-11 ). The paracentesis site is used to fill the chamber in the course of the procedure (e.g., with intracameral miotic or saline), or to re-form a flat anterior chamber with saline or viscoelastic during the first postoperative weeks. After the scleral flap is extended past the limbus into the cornea and the paracentesis site has been made, the anterior chamber is entered under the flap ( Fig. 34-12 ) and a block of tissue approximately 1.5–2.5 mm wide is removed with a Descemet’s punch just anterior to the scleral spur. Removal of the trabeculectomy block too posterior to the scleral spur offers no advantage and increases the risk of hemorrhage.
The surgeon may excise the block with Vannas scissors, a trephine, a scleral punch, or thermal cautery ( Fig. 34-13 ). The success rate of these approaches is similar. A peripheral iridectomy should be performed in all phakic eyes, with care taken to avoid the iris base and ciliary body to prevent hemorrhage. (In an effort to avoid encountering vitreous prolapse through the trabeculectomy site, some surgeons omit an iridectomy in aphakic or pseudophakic eyes if another iridectomy is already present, or if laser iridotomy is readily available in the postoperative setting.)
The scleral flap is reapproximated with 9-0 or 10-0 nylon sutures placed so that the anterior chamber is maintained after injection of saline, with a slow leak of fluid through the scleral wound indicating adequate filtration flow ( Figs 34-14 and 34-15 ). In a simulation of the patient’s natural blinking effect on filtration, flow adequacy at the site can be checked by gently ‘burping’ or depressing posterior to the scleral flap with a surgical instrument. Based on clinical experience, the surgeon tightens the flap, anticipating future adjustment with either releasable sutures or with laser suture lysis during the follow-up period. Since the surgeon’s careful assessment of flow is critical before closing the eye, intracameral viscoelastic – which temporarily interferes with fluid flow – is rarely helpful during uncomplicated filtering surgery.
The conjunctival flap is closed as described above, depending on the use of the fornix-based or limbal-based approach. The conjunctival flap should be water tight, as assessed by intracameral filling and inspection of the bleb ( Fig. 34-16 ). Especially if an antimetabolite has been used, intraoperative leaks should be scrupulously identified (with either high-magnification inspection or fluorescein drops) and then closed; a #10-0 nylon or a #10-0 Vicryl suture on a tapered BV vascular needle works well for this. Most filters show quiet tissue after healing has completed ( Fig. 34-17 ).
Moorfields Safer Surgery System technique
The technique popularized as the ‘Moorfields Safer Surgery System’ has been meticulously developed for consistentresults in a wide range of complicated glaucomas. The hallmark features of this technique as originally described are a fornix-basedconjunctival flap, an anterior chamber maintainer, a standardized punch technique, and a combination of adjustable and releasable sutures. It is also compatible with a single-site combined phacoemulsification/intraocular lens procedure, with minimal modification.
A fornix flap is prepared, with careful posterior dissection lateral to the superior rectus muscle, in preparation for a dispersed application of mitomycin-C-soaked sponges for a diffuse, posterior bleb ( Fig. 34-18 ). Next a 4–6 mm wide half-thickness scleral tunnel is prepared 4 mm from and centered at the 12 o’clock limbus. Only partial (1–2 mm) lateral incisions of the tunnel flap are made, but not extended to the limbus itself; this inhibits any lateral aqueous accumulation at the limbus and instead encourages its posterior flow superiorly ( Fig. 34-19A ).
The recommended applicators for antimetabolite are bisected, 6-mm polyvinyl-alcohol round, corneal sponges (used in LASIK surgery), whose advantages include a predictable release of antimetabolite as well as resistance to shredding under the conjunctiva. (Alternative applicators can be cut free-hand as longitudinal strips of triangular-shaped cellulose sponges, which when cut thin enough will, with hydration by antimetabolite, become flat rectangular strips, easily insinuated and removed from the subconjunctival space ( Fig. 34-19B,C,D ). Either mitomycin-C (0.2 mg/cc) or 5-FU as 5 mg/0.1 cc is applied to the sponge strips. As many as six hemi-sponges are carefully insinuated posteriorly beneath the conjunctiva, adjacent to the superior rectus muscle and laterally ( Fig. 34-20 ). While the sponges are in place, the conjunctival edges are carefully suspended away from the mitomycin. A small sponge (or strip fragment) is placed beneath the trabeculectomy flap in the bed of the scleral tunnel ( Fig. 34-21 ). After 3 minutes the sponges are all removed, a sponge count is performed, and subconjunctival irrigation performed.
Next a Lewicke anterior chamber maintainer is placed through a microvitreoretinal (MVR)-blade incision at the 6 o’clock peripheral cornea. Adjusting the bottle height for transcorneal flow of fluid allows precise control of chamber depth and IOP ( Fig. 34-22A,B ). A relatively small trabeculectomy stoma (0.5–1 mm) is created with a scleral punch beneath the flap, and a peripheral iridectomy performed ( Fig. 34-23 ). The flap is closed with two or more ‘adjustable’ sutures, using a 4-loop slip-knot closure ( Figs 34-24 and 34-25 ). The conjunctival flap is then closed at the limbus using lateral phimotic stitches and corneal ‘scratch’ incisions to bury #10-0 Vicryl mattress sutures (see Fig. 34-7C,D ). The merit of the ‘adjustable’ sutures is that postoperatively at the slit lamp, a blunt forceps (e.g., fine needle driver) can transconjunctivally wiggle and loosen them, effecting an incremental drop in IOP ( Fig. 34-25 ).
The typical postoperative course following trabeculectomy surgery is characterized by little discomfort, several weeks of improving vision, and frequent office visits. Unless severe hypotony, a flat anterior chamber, or a hyphema is present, there is little reason to limit the patient’s activity beyond the routine restrictions common to outpatient eye surgery. Although hospitalization may be useful for the convenience or medical access of the patient, it is not warranted on the basis of the procedure’s healing course: trabeculectomy is considered an outpatient operation. Subconjunctival steroids and antibiotics are injected at the end of surgery, and topical steroids are usually instilled 4–8 times per day during the first postoperative weeks. Cycloplegics are sometimes used to reduce photophobia and prevent synechiae. Some surgeons though, to minimize any non-essential potential conjunctival irritant which might adversely affect the bleb, use neither mydriatic nor antibiotic, applying only steroid drops after surgery.
With remodeling over the first few months, low posterior diffuse blebs are less prone to complications, such as leakage, than are thin-walled, multicystic blebs. Several schemes for clinically classifying blebs have been proposed, which if widely adopted could significantly help standardize the surgical literature in distinguishing outcomes and problems with specific bleb morphology. One system from Moorfields ( Fig. 34-26A ) has been developed for non-ophthalmic graders assessing clinical postoperative photographs. A slightly simpler system, called the Indiana Bleb Appearance Grading System, is easily applied at the slit lamp for clinical notations ( Fig. 34-26B ). The value of consistent descriptions of blebs is relevant not only to research protocols, but to direct clinical care as well; for example, the not uncommon issue as to whether or not a contact lens is ‘safe to wear’ following trabeculectomy. Clinically the decision is based on the bleb morphology: common wisdom is that it can be worn more safely with low, diffuse blebs than with elevated thin blebs; but greater descriptive precision to correlate with the infection rates for specific types of blebs would be invaluable. And on the technological horizon, new advances with in-vivo confocal imaging of filtration blebs may yield even greater information as to structural and functional correlations.
With the rapid evolution of surgical techniques and preferences over the past three decades, there are few rigorous long-term randomized controlled trials comparing surgical versus medical treatment for primary open-angle glaucoma. Lower IOPs are usually achieved surgically, with better preservation of visual fields; acuity, however, is sometimes adversely affected by surgery due to cataract formation.
In the literature published before the wide use of antimetabolites with trabeculectomy, pressure levels of 21 mmHg or lower with or without medications were achieved in the first 2 years in about 80% of eyes with primary open-angle glaucoma secondary glaucomas generally responded less well. Success in aphakic eyes was usually less than 50% young patients also had a lower success rate. After 4–5 years, however, success rates for achieving IOPs between 16 and 21 mmHg diminish towards 50%, regardless of race.
There is substantial cumulative evidence that use of an antimetabolite at the time of trabeculectomy surgery (either 5-FU or mitomycin-C) provides lower long-term IOP control than not using such agents. However, the reported trials include different types of glaucoma at variable risk for surgical failure; a variety of drug concentrations, application times, and delivery methods; and different lengths of clinical follow-up. Trends suggest that in the first year following surgery, 5-FU and mitomycin results are comparable, with over 85% of eyes with ‘controlled’ IOPs; longer follow-up suggests a decay rate towards 65% at 5 years and towards 40% over 10 yrs. Despite long-term fall-off of surgical control, both the lower IOPs achieved and the dampening of IOP fluctuation are felt to significantly reduce the rate of visual field deterioration. Serious complications such as bleb leaks, hypotonous maculopathy, and endophthalmitis are seen with both drugs, and neither has rigorously demonstrated superiority in clinical efficacy or safety.
Some eyes undergoing trabeculectomy may develop a modest IOP elevation during the first several weeks after surgery. This pressure rise is transient and may not alter the success of the procedure. In one series evaluated before the advent of antimetabolites, releasable sutures or of laser suture lysis, up to 45% of eyes had pressures higher than 20 mmHg during the first month after surgery. At 1 year, however, less than 15% had pressures over 20 mmHg. It is important to realize that such a rise in IOP does not necessarily affect the ultimate outcome of the operation, and aqueous inhibitors should be avoided during the early post-operative period to maximize bleb dynamics. Although transient high post-operative IOP may be tolerated in some patients, it is not desirable. The ideal postoperative course begins with very low pressure (around 4–6 mmHg) and sees the pressure rise to roughly 10 or 12 mmHg in 6–8 weeks. Factors that contribute to surgical failure include intrinsic difficulties (e.g., eye with previous surgery or trauma, ocular-surface disease with conjunctival inflammation) and intraoperative factors (e.g., use of superior rectus suture, inexperience with trabeculectomy technique, etc.).
Surgical options and modifications
A number of intraoperative modifications of the original trabeculectomy procedure have been popularized. Although every surgeon has individual preferences, none of the variations below have demonstrable superiority over its alternatives.
Triangular versus rectangular flap
A triangular scleral flap is easier to dissect in a single plane towards the limbus than is a rectangular flap. Sometimes a single suture at the apex is sufficient for closure.
Early trabeculectomy technique specified a rectilinear 4-mm X 6-mm long scleral flap overlying a 3-mm wide X 1.5-mm deep sclerostomy. A ‘short-flap’ modification reduces these dimensions to a 3-mm long X 3-mm wide rectangular flap. This was developed to more nearly approximate full-thickness filtration while keeping the chamber retention aspects of the guarded filter. This technique has advantages when used in conjunction with postoperative lasering of flap sutures because of the ease in visualizing subconjunctival sutures so close to the limbus. The scleral flap extends less than 1 mm from the sclerostomy on all sides. Sutures at each distal corner are tied securely to retain the chamber. Additional flap sutures may be used as desired. Because the flap is small, it leaks more freely; this could be too much if excessive cautery near the edges is employed. If enhanced filtration is desired, the scleral flap sutures can be lasered or released postoperatively. Theoretically this technique combines the advantages of guarded and full-thickness filtration.
Note that both the triangular and rectangular scleral flaps are cut down to the limbus itself. In contrast, either a scleral tunnel approach or the Moorfields Safer Surgery technique specifically avoid bringing the flap edges so far anteriorly, to facilitate posterior aqueous flow away from the limbus.
Postoperative lasering, adjustment, or release of sutures
If the scleral flap has been secured with 9-0 or 10-0 nylon sutures, the sutures may be cut in the postoperative period with the argon green, argon blue-green, diode, or krypton red laser. (The yttrium-aluminum-garnet (YAG) laser can also cut sutures but is capable of rupturing conjunctival and episcleral blood vessels, possibly leading to subconjunctival hemorrhage; hence it is rarely used.) Laser suture lysis is greatly facilitated by compressing the overlying conjunctiva to visualize the suture. This can be done without magnification with the edge of a four-mirror Zeiss gonioprism or with the Hoskins laser suture lens (see Figs 32-4 , Figs 32-5 ). High-magnification suture-lysis contact lenses are commercially available (e.g., Mandlekorn lens [ Fig. 34-27 ] or Blumenthal lens [ Fig. 34-28A ]), which, without coupling gel, both blanche the conjunctiva and intensify the power density of the laser beam ( Fig. 34-28B ). Such lenses are enormously helpful with argon, krypton, and diode slit-lamp delivery lasers.
Generally the sutures should usually be cut within the first three postoperative weeks to enhance filtration before irrevocable scarring occurs. The outside window may extend as far as 8 weeks if mitomycin-C is used intraoperatively and there is little conjunctival inflammation in the interim. Because mitomycin-C and other antimetabolites delay tight wound healing for many weeks, a late wound leak or hyperfiltration with hypotony can follow suture lysis. Often, however, small, pinpoint bleb leaks from the laser beam will spontaneously heal within a few days, in the temporary reduction or absence of steroids, and use of antibiotic drops. Gentle digital pressure on the globe through the lid or directly on the posterior edge of the scleral flap will often open the wound and elevate the bleb if it fails to do so spontaneously after the suture is cut. Laser suture lysis is also useful to release sutures that are inducing astigmatism and/or to enhance filtration after combined cataract and trabeculectomy surgery.
If the suture can be seen clearly, it can be cut with a single application of argon laser energy delivered at 400 mW for 0.1 second with a 50- μm spot size. Unfortunately, it can be very difficult to visualize and cut sutures through an inflamed, thickened and failing bleb. Careful focus with the laser maximizes energy delivery and minimizes collateral tissue damage. Blood overlying the suture may prevent suture visualization or cause excessive absorption of the laser energy, potentially leading to a conjunctival hole. Though such a small breach may leak aqueous, a reduction in topical steroids and patience usually effect spontaneous healing within a few days.
Several alternative methods have been described. Though not widely available in an office setting, an endolaser probe to compress the conjunctiva overlying the suture has been used in cutting sutures under a conjunctival flap. An ingenious adaptation of the standard scleral flap closure can allow visualization and laser lysis of these sutures with slit-lamp gonioscopy in the postoperative period, independent of the conjunctival appearance and dependent only on sufficient corneal clarity for gonioscopy. This technique is adaptable to any variation of trabeculectomy technique; it is illustrated here using a standard #10-0 nylon for closing the scleral flap ( Figs 34-29 through 34-34 ). So long as the anterior chamber remains deep and there is no internal obstruction of the trabeculectomy stoma (by iris, blood, debris, etc.), this technique reliably and elegantly bypasses difficulties of subconjunctival edema or hemorrhage which can obscure sutures which might require lysing.