Chapter 240 – Trabeculectomy
RONALD L. FELLMAN
• Trabeculectomy, a guarded filtration procedure, remains the “gold standard” for long-lasting intraocular pressure reduction in uncontrolled primary glaucoma.
• Patients expect a “quick fix” but glaucoma surgery is high in risk and requires exhaustive preoperative counseling and informed consent.
• Determine preoperative risk factors that lead to filtration complications and failure.
• Twenty-point preoperative trabeculectomy checklist.
• Step-by-step explanation of surgical procedure.
• Determine appropriate antimetabolite and correct dosage for filtration surgery.
• Factors in deciding on limbal-versus fornix-based conjunctival flaps.
• Determine best method of fashioning a scleral flap.
Trabeculectomy is the most popular form of glaucoma filtration surgery and remains the “gold standard” for surgical reduction of intraocular pressure (IOP) in uncontrolled, primary open-angle glaucoma. This partial-thickness filtration operation decreases eye pressure via the establishment of a limbal fistula through which aqueous humor drains into the subconjunctival space, establishing a filtering bleb. Successful filtration surgery significantly enhances quality of life. The patient’s outlook on life is improved, fear of blindness is reduced, and side effects and cost of medical therapy are abated. The outcome of filtration surgery is highly dependent on the type of glaucoma, severity of disease, pharmacological wound modulation, and surgical skill level.
The major advance of trabeculectomy over prior full-thickness filtration procedures (iridencleisis, trephination, sclerectomy, and thermal sclerostomy) was the concept of guarded filtration.  The ability to impede and guard outflow with a scleral flap significantly reduced the complications associated with overfiltration. During the 1980s, trabeculectomy improved with the development of laser scleral flap suture lysis, modified fornix-based conjunctival flaps, superior wound closure, and inhibition of fibrosis using the cytostatic antimetabolite 5-fluorouracil. The 1990s brought new cytocidal antimetabolites, which include mitomycin C.
Indications for Trabeculectomy Surgery*
FAILED MAXIMAL TOLERATED MEDICAL THERAPY AND FAILED LASER SURGERY OR POOR LASER CANDIDATE WITH ANY OF THE FOLLOWING:
Progressive glaucomatous optic nerve head cupping
Glaucomatous visual field progression
Anticipated optic nerve head damage as a result of excessive intraocular pressure
Anticipated visual field damage from glaucoma
Intolerable side effects that arise from medical therapy of glaucoma
Lack of compliance with anticipated or progressive glaucoma damage
* The indications vary considerably for individual patients and are a general guideline only for filtration surgery.
PREOPERATIVE EVALUATION AND DIAGNOSTIC APPROACH
The management goal in the glaucomas is to preserve vision and maintain a reasonable quality of life. The major indication for surgical intervention is progressive or anticipated glaucomatous disease that is likely to result in functional impairment during the patient’s lifetime. Functional visual impairment varies considerably, depending on the patient’s overall health, age, rate of visual field and optic nerve damage, and level of IOP. The majority of patients receiving maximal tolerated medical (see Chapter 233 ) and laser (see Chapter 235 ) therapy who experience progressive or anticipated glaucomatous damage are candidates for glaucoma filtration surgery ( Box 240-1 ). Trabeculectomy is highly effective and is the procedure of choice for most cases of uncontrolled primary open- and closed-angle glaucoma, exfoliation syndrome, pigmentary glaucoma, and pseudophakic glaucoma with a posterior chamber intraocular lens (PCIOL). The secondary glaucomas (e.g., neovascular, epithelial downgrowth, developmental, traumatic, aphakic, congenital) have a poorer prognosis with glaucoma filtration surgery and may require other treatment modalities.
Vision-Damaging Pressure Level
The IOP level that causes glaucomatous optic nerve or visual field damage is the vision-damaging pressure level. The surgeon establishes a vision-damaging pressure level and decides by how much to lower the IOP ( Box 240-2 ). The goal of trabeculectomy is to normalize the IOP, but normality for each patient may vary considerably.
A patient with low-tension glaucoma may need a postoperative IOP of 9 or 10?mmHg (1.2 or 1.3?kPa), whereas a patient with traumatic glaucoma may need only a postoperative IOP of 20–25?mmHg (2.7–3.3?kPa) to maintain vision. In general, surgeons strive for a 30% IOP reduction for mild to moderate disease and up to a 50% reduction in IOP for advanced disease or normal-tension glaucoma. Smaller percentage reductions in
Guidelines for Determination of Postoperative Intraocular Pressure*
Severe optic nerve damage or normal-tension glaucoma may require intraocular pressure levels of 9–13?mmHg (1.2–1.7?kPa); the risk of significant pressure reduction is warranted
For moderate nerve damage aim for intraocular pressure in the range 14–16?mmHg (1.9–2.1?kPa)
For minimal nerve damage, aim for intraocular pressure in the range 16–19?mmHg (2.1–2.5?kPa)
Normal-tension glaucoma requires a 40–50% reduction in intraocular pressure
Family history of blindness as a result of glaucoma, may indicate that a lower intraocular pressure is required
Inability to tolerate or comply with topical or systemic antiglaucoma medications requires adjustment of intraocular pressure downward
Poor overall cardiovascular health of the patient requires adjustment of intraocular pressure downward
Episcleral venous pressure, known tendency for choroidal effusion and high myopia require adjustment of intraocular pressure upward
For monocular visual status, be cautious of overaggressive intraocular
History of suprachoroidal hemorrhage in the fellow eye may require adjustment of the approach taken
*The formula used to establish a reasonable postoperative intraocular pressure level is complex. Several factors must be considered and integrated into the decision. Eyes that have severe nerve damage require lower intraocular pressures. The short- and long-term risks associated with the attainment of subnormal intraocular pressures are probably not justified for patients who have minimal disease.
postoperative IOP are less likely to stabilize the disease ; however, aggressive pressure reduction is associated with higher complication rates. Data from the Advanced Glaucoma Intervention Study (AGIS) demonstrate that eyes with 100% of visits with an IOP of less than 18?mmHg over 6 years have stable visual fields compared with 50% of visits with an IOP of less than 18?mmHg. The AGIS made it clear that low IOP (less than 14?mmHg) is associated with reduced progression of visual field defects. If the disease progresses, the ophthalmologist must determine whether the pressure reduction was great enough and look for other factors that may be worsening the disease. 
Informed Consent and Patient’s Attitude to Glaucoma
In today’s society, patients expect a “quick fix” for their medical problems. Ophthalmologists have promulgated this theme through the miracles of “high tech, see better the same day” cataract and refractive surgery. It is incumbent upon the physician to emphasize, until completely understood, that this is not the case for glaucoma filtration surgery. Unfortunately, optic nerve function cannot be restored using present levels of technology. At best, the glaucoma goal is visual field stability but not improved visual acuity.
Patients need to know that no quick fix exists for this treatable, but incurable, disease. Glaucoma surgery is high in risk compared with many other eye procedures. Proper informed consent means that the patient understands the nature and reason for the procedure along with explanations of the risks, benefits, and alternatives. The patient’s record chart must always substantiate these factors. From a medicolegal viewpoint, “if it is not documented, it did not happen.” Patients must understand that visual acuity may worsen temporarily after filtration surgery. A transient loss of at least one line of best corrected visual acuity is common after trabeculectomy, typically because of corneal topographic changes. Resolution occurs by 12 weeks.  If a cataract is present, surgery is likely to hasten its development, especially if a shallow or flat anterior chamber is found postoperatively. Possible complications must be discussed thoroughly with the patient; this includes permanent loss of vision as a result of fixation loss or suprachoroidal hemorrhage. Occasionally, acuity improves because of the removal of pilocarpine and subsequent pupillary enlargement. Additional reasons for visual loss include hypotony maculopathy, choroidal effusion or hemorrhage, cystoid macular edema, shallow anterior chamber, and other factors. The prognosis and complication rate of trabeculectomy are highly diagnosis dependent. An experienced glaucoma surgeon uniformly strives to minimize the surgical risks by modification of the procedure and technique as necessary.
Assessment of Filtration Risk Factors
Many nonsurgical factors exist that significantly influence the outcome of trabeculectomy. A thorough slit-lamp evaluation, gonioscopic examination, and record review can dramatically alter the surgical strategy and eventual outcome. The operative report from any prior glaucoma surgery must be reviewed. Slit-lamp gonioscopy is easier when carried out preoperatively rather than in the operating room; preoperative evaluation also ensures the correct diagnosis of glaucoma and enables the best site for filtration to be determined on the basis of peripheral anterior synechiae, IOL and haptic orientation, aberrant vessels, and wound dehiscence. The proposed location of the filter varies, depending on lid anatomy, conjunctival exposure, planned cataract surgery, prior filtration surgery, and limbal scarring. If vitreous prolapse is present, the appropriate method of removal must be determined in conjunction with the trabeculectomy.
Type of Glaucoma, Race, and Age
Open-angle glaucoma has the best risk-benefit profile. Well-documented risk factors for filtration failure include neovascular glaucoma, African race, aphakia, prior failed filtration, and uveitis.     One joy of longevity is a higher success rate for filtration surgery.
Prior Use of Antiglaucoma Medications
The prolonged use of antiglaucoma medications may have an adverse effect on the conjunctiva and result in a proliferation of lymphocytes and fibroblasts, which decreases the likelihood of successful filtration surgery. In these cases, antimetabolites may help to counter this effect in favor of long-term filtration. Adrenergic drugs seem to have the greatest tendency for drug allergy and may lead to postoperative Tenon’s capsule cyst formation. The offending drug is discontinued at least 2 weeks prior to filtration and IOP is controlled with the administration of a short-term course of oral carbonic anhydrase inhibitors. Low-dose topical corticosteroids prior to surgery may decrease conjunctival inflammation significantly.
Anterior Segment Inflammation
Healthy aqueous humor nourishes the filtering bleb ( Fig. 240-1 ) and is the basis of long-term filtration. Intraocular inflammation manifested by anterior chamber flare and cells is the sign of a sick eye, a definite risk factor for filtration failure. The blood-aqueous barrier must be stabilized prior to and after surgery using topical or oral corticosteroids or topical nonsteroidal anti-inflammatory agents. Strong, indirectly acting miotics that break down the blood-aqueous barrier are discontinued 2 weeks prior to surgery. In the correct setting, mydriatic cycloplegics are also useful for stabilization of the blood-aqueous barrier.
Scarring of the Conjunctiva and Limbal Anatomy
Prior conjunctival incisions may significantly alter the surgical plan. Scar tissue may be delineated at the slit lamp by identification of immobile conjunctiva using a Q-Tip under topical anesthetic. For example, patients who have undergone prior
Figure 240-1 Filtering blebs. The behavior of filtering blebs is unpredictable after filtration surgery. Even with the best of efforts, undesirable bleb traits appear. A, Excessive inflammation and consequent vascularity lead to a host of factors that result in bleb fibrosis and ultimate failure. B, Desirable long-term characteristics include a moderate, diffuse bleb elevation, overall frosty pallor with a few small vessels, no demarcation zones, and conjunctival microcysts. C, Undesirable characteristics include excessive wall thickening with cyst formation, total avascularity, demarcation zones, necrosis with leaks, and extremely thin tissue. Totally avascular blebs are not desirable and usually indicate excessive antimetabolite dosage.
scleral buckle surgery or who have pseudophakia or aphakia may have severe disruption of the conjunctiva, especially at the limbus. If possible, whether the incision is to be limbus based or fornix based is decided prior to surgery.
Ocular Surface Disease
Any ocular surface disease that causes chronic conjunctival inflammation is a risk factor for filtration failure. Ocular rosacea and blepharitis are treated aggressively for several weeks prior to filtration with the administration of oral doxycycline 100?mg a day for 3 weeks, a topical antibiotic ointment, and lid hygiene. These eyes require long-term topical corticosteroids and oral antibiotics, even after surgery, to salvage the bleb.
Conjunctival Scarring, Wound Healing, and Bleb Failure
Trabeculectomy without antimetabolite is usually successful for most cases of low-risk, uncomplicated primary glaucomas. Postoperatively, if fibrosis is a problem, antifibrotic agents may be injected to salvage the bleb. However, more complex cases that have a greater potential for failure initially require antifibrotic agents to achieve useful pressure reduction. The majority of filters fail because of episcleral fibrosis. Administration of antiproliferative agents specific to the cell cycle increases the success rate of trabeculectomy in high-risk eyes by limitation of fibroblast proliferation at the filtration site. The two most common antimetabolites are 5-fluorouracil and mitomycin C. At current doses, 5-fluorouracil is mainly cytostatic and mitomycin C is cytocidal. Mitomycin C is approximately 100 times more potent than 5-fluorouracil. 5-Fluorouracil is much safer in virgin eyes that have good prognoses; mitomycin C is reserved for eyes that have a high likelihood of failure and/or prior failed conventional filtration. Before these powerful drugs are administered, the surgeon must understand the philosophy behind the advantages and disadvantages of antimetabolite trabeculectomy.
The use of 5-fluorouracil has improved the outcome of filtration surgery, which as a consequence is now performed more often.  The drug is a halogenated pyrimidine analog that inhibits DNA synthesis because it acts as a competitive inhibitor of thymidylate synthetase. 5-Fluorouracil is incorporated into both RNA and DNA, which results in defective protein synthesis and cell toxicity. 5-Fluorouracil is much more lethal to cells that grow logarithmically than to stationary cells and typically is administered as consecutive subconjunctival injections or as an intraoperative sponge application of 50?mg/ml.
Mitomycin C is an antitumor antibiotic isolate of fermentation filtrate from Streptomyces caespitosus. It is an alkylating agent that binds to DNA and, thereby, interrupts cell synthesis; it inhibits DNA, RNA, and protein synthesis and is highly toxic to vascular endothelial cells. Mitomycin C is one of the 10 most carcinogenic substances known to humans—the Occupational Safety and Health Administration work-practice guidelines for personnel who administer cytotoxic drugs must be followed strictly.
Mitomycin C increases the success rate of glaucoma filtration surgery  at variable application times and doses and is more effective than 5-fluorouracil. Mitomycin C is administered only as a single topical administration over the intended scleral flap during surgery using sponges of various sizes and doses in the range 0.1–0.5?mg/ml and application times of 1–5 minutes, all depending on the inherent risk factors for filtration failure. One obvious problem is the lack of standardization of application of the drug, which leads to unclear study results. Currently, mitomycin C is overused in virgin eyes that have primary glaucoma.
ALTERNATIVES TO SURGERY
Alternatives to surgery include medical and laser therapy and are discussed in Chapters 233–236 . In complicated, high-risk cases, alternatives include the use of drainage implants (see Chapter 242 ) and a cyclodestructive procedure (see Chapter 237 ).
General anesthetic is avoided whenever possible because of the increased systemic risks and postoperative suprachoroidal effusion and hemorrhage associated with coughing. General anesthetic is necessary for pediatric patients and adults unable to cooperate as a result of altered mental status, anxiety, or severe claustrophobia. Care is required because prolonged postoperative apnea may be caused by the interaction of intraoperative succinylcholine with preoperative, indirect-acting parasympathomimetic drugs. Local anesthetic in conjunction with monitored anesthetic care is preferable for most patients and is conducive for outpatient surgery. Short-acting peribulbar anesthetics reduce the risk of respiratory arrest and allow rapid recovery of vision, which is especially helpful in monocular patients. Topical anesthetic may be useful in some cases. Excessive prolonged preoperative ocular compression is avoided in patients who have a history of ischemic optic neuropathy or fragile optic nerves.
The filtration checklist is reviewed prior to surgery ( Box 240-3 ). Patients who have virgin conjunctiva (free of ocular surface disease) and uncontrolled primary glaucomas and who have undergone
Preoperative Trabeculectomy Checklist*
Chart documentation that substantiates indications, risks, and benefits, and that alternatives to surgery were reviewed and understood by the patient
Ensure patient understands the challenge of the approach to glaucoma treatment
Ensure patient, family, or friend able to administer drugs postoperatively
Establish desirable postoperative target intraocular pressure level (see Box 240-2 ).
Review prior allergies, anesthetic complications, ocular surgeries, and history of choroidal effusion or hemorrhage
Obtain informed consent specific for glaucoma filtration surgery
Discontinue anticoagulants 4 days prior to surgery—check clotting time if indicated
Suppress intraocular flare and cells with corticosteroids, stabilize the blood–aqueous barrier, discontinue strong miotics if possible
Eliminate any topical antiglaucoma drugs that cause an allergic reaction (palpebral or bulbar conjunctival follicles, typically a result of adrenergic drugs)
Aggressively treat ocular surface disease prior to and after surgery (ocular rosacea, conjunctival hyperemia, blepharitis, and dry eye)
Perform gonioscopy—search for peripheral anterior synechiae, intraocular lens haptics; this helps in the decision on trabeculectomy site
Locate any prior filtration site or cataract wound—gonioscopy is helpful
Check for conjunctival scarring—shown by mobility when a Q-Tip is moved on anesthetized conjunctiva
If possible, avoid staphyloma, wound dehiscence, conjunctival scarring, and old suture-tract sites
Vitreous in anterior chamber—decide on method of vitrectomy
Determine location of proposed filtration site
Decide on limbus- or fornix-based conjunctival flap
If tight scleral flap indicated, determine if oral carbonic anhydrase inhibitors can be used in the immediate postoperative period
If antimetabolite indicated, decide on agent, dose, and duration of application
Decide on anesthetic (short-acting anesthetic for monocular patients and no significant external digital pressure for badly damaged nerves)
*There are many factors to consider prior to surgery. If any of these factors are not considered, complications are likely to increase.
no prior ocular surgery are the best candidates for successful glaucoma filtration surgery. If possible, areas of conjunctival scarring are avoided. If significant scarring is present at the limbus, a fornix-based conjunctival flap is preferable. Whenever possible, inferior filters are avoided as a result of the high (8%) rate of endophthalmitis, and complications must be anticipated on the basis of type of glaucoma and risk factors. For example, acute IOP reduction, aphakia, and concurrent vitrectomy all increase the incidence of suprachoroidal hemorrhage. Armed with this knowledge, the surgeon must try to avoid hypotony in these eyes by closure of the flap using additional sutures, avoiding sudden intraoperative decompression and prolonging postoperative scleral-flap suture lysis. In virgin eyes, it is preferable to dissect a limbus-based conjunctival flap because the incision is distanced from the filtration site, wound leaks are less likely to occur and easier to repair, and sutures are less symptomatic. Described in the following is one of many techniques used for trabeculectomy. It is important for the surgeon to maintain a good skill level for the construction of both limbal- and fornix-based flaps. A good surgical assistant is essential during glaucoma surgery—retraction of various tissue planes by the assistant is critical to the success of the procedure.
Obtain Adequate Conjunctival Exposure
By far the most common error during limbus-based glaucoma filtration surgery is anterior placement of the conjunctival incision, for which the major reason is poor exposure and visibility of the superior tissues. The corneal traction suture ( Fig. 240-2 ) allows excellent infraduction of the globe with visibility of the
Figure 240-2 Corneal traction suture. The dashed line represents the typical position of the globe after adequate akinesia. In this straight-ahead position, it is difficult to place the conjunctival incision accurately. Infraduction using a 9-0 Vicryl suture on a cutting needle passed tangentially through superior cornea allows excellent visibility of the superior tissues.
superior conjunctiva and is preferable to the superior rectus bridle suture for several reasons:
• No holes in the conjunctiva created by the traction suture;
• Rotation of the globe into any inferior quadrant to gain better exposure of the superior tissues;
• Better exposure during wound closure; and
• Impossible to perforate the sclera and retina.
An 8-0 Vicryl traction suture is placed to two thirds of the corneal thickness, approximately 0.5?mm from the limbus, at the 11, 12, or 1 o’clock position, depending on the quadrant needed for exposure. The globe is rotated inferiorly to expose at least 12?mm of superior conjunctiva.
For maximum superior quadrant exposure, the corneal traction suture is clipped to the drape at the 6 o’clock position—the position may be varied slightly. Insufflation of the conjunctiva is carried out using 2–3?ml of balanced salt solution administered through an angled 30 gauge needle 10–12?mm posterior to the limbus ( Fig. 240-3 ). This elevation and separation of the conjunctiva and Tenon’s capsule from the episclera aids in the dissection of the conjunctival flap and is especially helpful in the identification of areas of scarring. The tip of blunt-nosed, sharp Westcott scissors is placed into the conjunctival needle tract, and the conjunctiva and underlying Tenon’s fascia are incised parallel to the fornix for a total chord length of 8–10?mm.
A fornix-based conjunctival flap ( Fig. 240-4 ) is preferred for eyes that have undergone prior surgery or have difficult limbal exposure or scarring. With this technique, poor-quality limbal tissue may be excised and healthier conjunctival tissue advanced forward to the limbus. A corneal traction suture is still useful for visualization during construction of the scleral flap but may not be necessary, depending on exposure. The conjunctiva is incised at the limbus for at least 6?mm using sharp Westcott scissors and leftover conjunctival epithelial remnants are excised from the limbus to ensure adequate postoperative
Figure 240-3 Conjunctival insufflation and incision. Insufflation of the conjunctiva is carried out with balanced salt solution through a 30gauge needle at least 10?mm from the limbus; the conjunctive is incised with sharp Westcott scissors through the needle tract for a chord length of 8–10?mm.
healing. Blunt-tipped Westcott scissors are used to create a subconjunctival pocket by dissection of the conjunctiva and Tenon’s capsule off the episclera toward the fornix. The superior rectus muscle, a site of heavy bleeding, must be avoided. Occasionally, a radial extension to the conjunctival incision is required to gain adequate exposure of the filtration site. This radial extension is angled slightly toward the canthus. The conjunctiva must not be touched with toothed instruments. If buttonholes occur during dissection, the defect is repaired by incorporation of Tenon’s fascia into the hole, which is closed using a tapered 10-0 microvascular needle. The success rate of limbal-based and fornix-based conjunctival flaps is similar, with cystic blebs more common with a limbal-based approach.
Conjunctival Flap and Tenon’s Capsule Dissection
Management of Tenon’s capsule is accomplished alongside conjunctival dissection. In this era of antimetabolites, just enough Tenon’s capsule is removed to visualize the underlying scleral flap sutures. Adequate Tenon’s capsule decreases long-term bleb breakdown, hypotony, and endophthalmitis, but an excess of Tenon’s capsule may lead to fibrosis and filtration failure.
For limbus-based surgery, blunt forceps are used to capture the edge of Tenon’s capsule directly under the conjunctival wound edge. The capsule is incised down to the episclera, with care taken to avoid the underlying rectus muscle and to leave a cuff at the wound margin ( Fig. 240-5 ). As the flap is dissected toward the limbus, Tenon’s capsule is thinned slightly over the filtration area. During the entire conjunctival dissection, the tips of the blunt Westcott scissors are visualized through the semitransparent conjunctiva. Excess capsule is excised from the episclera. A Weck-cell sponge is useful to separate mechanically Tenon’s capsule from the limbal junction. Older patients who have thinner Tenon’s capsules require minimal manipulation to this layer. Younger patients who have thicker Tenon’s capsules require excision of just enough tissue to visualize the scleral flap sutures. Tenonectomy is difficult in younger patients who have redundant tissue. The best technique is to visualize the tip of
Figure 240-4 Fornix-based conjunctival flap. A fornix-based flap is preferable in eyes that have undergone prior surgery, or have limbal scarring, or for which limbal visibility is difficult. A subconjunctival pocket is dissected toward the fornix and, occasionally, a radial incision (dashed line) is required for adequate exposure of the proposed filtration site.
the scissors at all times through the semitransparent conjunctiva as the limbus is approached; thus, the conjunctiva is not broken through.
For limbus-based flaps, once dissection has been carried out to the limbus, the assistant must retract the conjunctiva up against the superior cornea using a blunt Weck-cell sponge and thereby expose the surgical limbus. If an excess of Tenon’s capsule or fibrous adhesions are found at this corneal-sclera junction, the sharp Westcott scissors may be used in a tangential fashion to remove carefully the excess tissue; alternatively, blunt dissection is carried out using a blade. Toothed instruments must not be used to retract the tissues.
Excess Tenon’s capsule may also be removed from a fornix-based flap by retraction of Tenon’s capsule down toward the episclera, approximately 1?mm from the wound edge, with simultaneous elevation of the anterior edge of the conjunctiva. This creates a tissue plane for excision using the blunt-tipped Westcott scissors. With visualization through the semitransparent conjunctiva, excision is carried out only over the area where scleral flap suture lysis may be necessary; this leaves an anterior Tenon’s gasket for added wound-closure protection. Because antimetabolites are toxic to fibroblasts, excessive removal of Tenon’s capsule along with an overdose of antimetabolite results in chronic hypotony.
At this stage, a buttonhole-free conjunctival flap has been dissected along with removal of the desired amount of Tenon’s capsule. Adequate hemostasis is necessary to dissect a scleral flap. Light cautery is used to stop vessel bleeding and to outline the proposed scleral flap and immediate surrounding area. Cautery to the epithelial side of the conjunctiva is avoided, and excessive scleral cautery may result in severe astigmatism.
The Antimetabolite Decision
The necessary decisions regarding antimetabolite usage must be made prior to surgery. Typically, the greater the number and
Figure 240-5 Conjunctival flap and Tenon’s capsule dissection. After the conjunctival incision has been made, the cuff of Tenon’s capsule at the wound margin is grasped with utility forceps and the plane of the dissection is carried toward the limbus and angled slightly upward as the limbus is approached.
magnitude of risk factors for filtration failure, the more potent the antimetabolite must be. The first decision is whether the case carries a low, medium, or high risk for failure. For a low risk of failure, no antimetabolite is necessary. Postoperatively, if the bleb is injected, a series of 5?mg 5-fluorouracil injections is administered into the inferior cul-de-sac. For medium-risk cases, either an intraoperative sponge application of 5-fluorouracil (50?mg/ml) for 5 minutes or low-dose mitomycin C (0.2?mg/ml for 2–5 minutes) is indicated, and postoperative injections of 5-fluorouracil are titrated if necessary. If only one risk factor is present, 2–3 minutes of mitomycin C application is appropriate; for multiple risk factors, 3–5 minutes.
For very high-risk eyes, the administration of higher concentrations of mitomycin C may be necessary (0.4?mg/ml) for a period of 2–5 minutes. However, if these potent concentrations are used in low- or medium-risk eyes, long-term hypotony and bleb breakdown are likely. Prior to scleral flap dissection, a rectangular segment of Merocel instrument wipe or Weck-cell sponge soaked with either 5-fluorouracil 50?mg/ml or mitomycin C is placed over the area of intended scleral flap and the conjunctiva retracted over the sponge for the designated time. Immediately after the pledget has been placed, any pools of antimetabolite present are soaked up using a Weck-cell sponge, which is discarded. The size of the pledget varies from 5 to 10?mm and is not standardized—the surgeon uses best judgment depending on risk factors and anatomy.
The conjunctiva is retracted, the sponge is removed, and the area is irrigated well using at least 10?ml of balanced salt solution. The appropriate Occupational Safety and Health Administration work-practice precautions must be enforced when antimetabolites are handled.
Scleral Flap Dissection
Dissection of the scleral flap requires inferior rotation of the eye to allow good exposure of the proposed area. The technique of freehand dissection requires increased magnification to obtain a more detailed view. The globe is secured inferiorly using the
Figure 240-6 Outline and dimensions of scleral flap. A 3–4?mm2 scleral flap is dissected carefully and used to guard the trabeculectomy site. This size of flap ensures adequate coverage of the trabecular block. A scleral flap tunnel technique is useful in eyes that have undergone prior surgery at the limbus, especially prior cataract surgery.
corneal traction suture while the assistant retracts conjunctiva over the cornea using a blunted Weck-cell sponge. The flap may be located at the 11, 12, or 1 o’clock position, depending on the limbal anatomy. However, the flap must not be placed too far to the medial or lateral side because this results in symptomatic filtration blebs and dellen formation. A variety of instruments may be used to dissect the partial-thickness scleral flap, such as a No. 67 Beaver blade, razor blade, or Grieshaber blade. The choice of instrument is by surgeon’s preference and may vary slightly from case to case. Initially, the borders of the flap are outlined to two thirds of the scleral thickness ( Fig. 240-6 ). If the incision is too superficial, the flap has a tendency to be thin, which results in shredding and dissolution of the tissue. Special precautions are necessary when flaps are fashioned in eyes that have undergone previous limbal surgery, especially prior filtration or superior limbal cataract surgery. Such eyes pose many problems for the inexperienced surgeon.
In general, the shape of the flap does not matter as long as it is constructed properly, usually a 4 × 4?mm two-third thickness square ( Fig. 240-7 ). Flap size may vary according to the adjacent anatomy. Thin flaps tear or avulse from the bed and may tear during suture closure. The most common error is failure to dissect the flap into clear cornea—dissection into clear cornea ensures adequate room for removal of the corneoscleral block. Thick flaps are probably not detrimental because postoperatively the bulk of flow occurs through the wound margins of the scleral flap. The dissection is kept as close to the 12 o’clock position as possible because excessive medial or lateral flap placement leads to permanent symptomatic filtration blebs—the bane of glaucoma surgeons and patients. The flap dissection must be initiated at one of the posterior corners of the flap. Once an edge has been dissected, the freehand dissection is carried forward uniformly into clear cornea anterior to the trabecular meshwork. This ensures enough room for block removal without removal of scleral spur, which would create a cyclodialysis cleft. As the dissection reaches the limbus, the angle of dissection is altered anteriorly to follow the globe contour (otherwise, the anterior chamber may be entered prematurely).
Figure 240-7 Dissection and position of scleral flap. The scleral flap is dissected uniformly at approximately one half to two thirds corneal thickness.
Scleral Tunnel Technique Using a Crescent Blade
The scleral tunnel technique is most helpful in eyes that have prior limbal wounds that tend to fall apart during freehand flap dissections. The scleral tunnel is constructed by incision of sclera tangentially, 3?mm posterior to the limbus, for a width of 3–4?mm. A crescent blade is used to tunnel into clear cornea in the same fashion as for cataract surgery. The crescent blade is slid from side to side until the sides are extended to about 4?mm width. Very fragile flaps are less likely to tear using this technique. If the scleral fibers are so tenuous as to prevent further flap dissection, a punch is slid into the pocket and a block of trabecular tissue removed. Otherwise, the flap is finished by insertion of Vannas scissors at the posterolateral wound margins and the incision connected to the limbus, which creates a three-sided flap.
A temporal paracentesis tract allows access for anterior chamber insufflation to gauge flow through the trabeculectomy site. A supersharp blade is used to create a temporal, beveled, self-sealing, penetrating corneal tract. Obviously, the lens must be avoided during this maneuver and the paracentesis must penetrate completely through the cornea.
Outline and Removal of Corneoscleral Block
While the assistant rotates the scleral flap over the cornea, a 15° or No. 75 blade is used to outline a 1 × 1?mm scleral block. The medial and lateral margins are incised in an anteroposterior direction using a 15° blade such that an adequate scleral ledge is left on both sides ( Fig. 240-8 ). The anterior incision starts in clear cornea and the posterior extension is the scleral spur. The anterior radial incisions are connected using Vannas scissors, with care taken to avoid the underlying iris and lens. The corneoscleral block is reflected posteriorly and amputated at its base. Light wet-field cautery is applied to the cut ends of Schlemm’s canal, which significantly decreases the chance that postoperative hyphema will spill from the canal. The size of the corneoscleral block in relation to the dimensions of the scleral bed determines
Figure 240-8 Removal of corneoscleral block. A, The medial and lateral margins are incised in an anteroposterior direction using a 15° blade such that an adequate scleral ledge is left on both sides. B, Typical appearance of trabeculectomy site after removal of corneoscleral block.
the amount of flow through the filter. A very large corneoscleral block leaves a small scleral ledge, which results in overfiltration and hypotony. If the corneoscleral block is too small, the ledge is too large and it is difficult to achieve flow through the flap, especially if it is secured tightly. Excessive posterior block removal into the ciliary body may cause an inadvertent cyclodialysis cleft and bleeding. Anterior block removal may miss the trabecular meshwork, but a fistula still forms. The relationship of the block to limbus is shown in Figure 240-9 . Another convenient method of block removal involves anterior chamber entry with a supersharp blade at the base of the scleral bed. A trabecular punch is slid into the tract and a block of tissue is removed.
Even though rarely is an iridectomy needed in modern-day cataract surgery, it is required for filtration procedures to relieve pupillary block and prevent obstruction of the internal filter opening. After removal of the corneoscleral block, the iris obstructs the opening. The iris is secured using 0.12 forceps in a basal position and retracted outside the posterior ledge of the opening in a tangential fashion, and a section of iris is removed using DeWecker scissors. Aqueous humor pours forth from the posterior chamber. If no fluid is seen, aqueous misdirection syndrome
Figure 240-9 Relationship of corneoscleral block to limbal anatomy. Typically, the block includes an anterior portion of the scleral spur, trabecular meshwork, Schwalbe’s line, and cornea. Postoperative gonioscopy allows the ophthalmologist to view the sclerostomy site.
must be suspected and appropriate action taken. The iridectomy size should be slightly larger than the size of the block. The patency is checked by direct observation of lens capsule or red reflex. Pigment epithelium is removed using a sponge if needed. In pseudophakic eyes that have large capsulectomies or in aphakic eyes, vitreous may appear. In rare instances, a peripheral iridectomy is unnecessary, especially if it might cause vitreous prolapse. Vitrectomy has its own set of complications and so must be avoided if possible.
Scleral Flap Closure
Typically, four to five 10-0 preplaced nylon sutures are positioned prior to block removal to facilitate rapid closure. After the preplaced sutures have been tied, immediate reinsufflation of the anterior chamber is carried out to regain global integrity.
Several sutures may be necessary to close the flap adequately ( Fig. 240-10 ). If the flap is very thin, a tapered, microvascular 10-0 nylon needle may be used to prevent cheesewiring of the tissues. If the flap is closed too tightly, no flow occurs and postoperative IOP is elevated. If the flap is closed too loosely, overfiltration occurs. To gauge the proper flow through the filter site is a difficult art but a critical step in successful filtration surgery. A bare ooze of aqueous through the scleral flap is a reasonable endpoint, which is attained by the adjustment and replacement of sutures as necessary. This additional operative time may save hours of postoperative work.
Evaluate Scleral Flap Suture Visibility
It is always safer to err on the side of extra flap sutures. High IOP is easier to treat using scleral flap suture lysis than a return to surgery to place additional sutures. Suture visibility is checked by pulling Tenon’s capsule and conjunctiva over the scleral flap. If the sutures cannot be seen, Tenon’s capsule is excised until they are visible. As an alternative to laser suture lysis, some surgeons prefer the placement of releasable sutures when the scleral flap is closed. These have a slip knot, the loose end of which is superficially buried in the cornea and can be removed using forceps at the slit lamp at any stage postoperatively; no thinning of the Tenon’s capsule is required.
Figure 240-10 Scleral flap closure. Typically, the scleral flap is closed using multiple 10-0 nylon sutures to protect and guard the fistula from excessive outflow.
Tenon’s Capsule and Conjunctiva Wound Closure
LIMBUS-BASED TWO-LAYER CLOSURE.
Prior to wound closure, the globe is rotated inferiorly with the corneal traction suture for adequate visualization ( Fig. 240-11 ). The conjunctival flap is repositioned such that the flap sutures are visible; if this is not possible, excess Tenon’s capsule is removed carefully, as described earlier. A double-layer closure with fine, tapered needles protects against wound leaks (in this era of antimetabolites, the added protection is helpful). A running, locking 8-0 tapered Vicryl needle is used for Tenon’s capsule closure, followed by a
9-0 Vicryl monofilament running tapered needle for conjunctival closure. After closure, insufflation of the anterior chamber is carried out using balanced salt solution, the bleb is inflated, and the wound checked for leaks. Any leaks must be closed with a 10-0 tapered needle. Wound leaks are the cause of most failed trabeculectomies. A double-layer closure significantly decreases leaks and their litany of complications.
Figure 240-11 Double-layer wound closure and bleb test.
FORNIX-BASED CONJUNCTIVAL CLOSURE.
Several techniques are used to close a fornix-based conjunctival flap. Hooded techniques are the simplest but the most prone to leaks, especially with antimetabolite usage. A vertical mattress suture technique described by Wise  is the most time consuming and precise and yields the best results ( Fig. 240-12 ). A tapered 2850 9-0 nylon needle that has a tiny cutting tip is used. It is essential to keep the limbal suture bite longer than the distance between the corresponding conjunctival suture holes. When the suture is tightened, the intervening conjunctiva stretches tightly against the sclera. This is very effective in the prevention of wound leaks.
The best way to check for conjunctival wound leaks is to use the bleb test. Insufflation of the anterior chamber is carried out through the preplaced paracentesis tract using a 30gauge needle. Visualization of the needle all the way into the chamber is essential before the injection to avoid detachment of Descemet’s membrane. The bleb forms immediately after injection; if it collapses within a few seconds, a leak is probable. The bleb and wound margins must always be checked for leaks using a dry Weck-cell sponge. If in doubt, the conjunctiva and wound may be painted with fluorescein to better highlight any leak.
Medications and Postoperative Care
Intraoperative topical atropine may help to stabilize the blood-aqueous barrier and relax the ciliary body. Subconjunctival antibiotic and corticosteroid are injected, with great care taken to avoid globe penetration. Proper wound healing is critical for successful filtration, and topical corticosteroids are the mainstay for prevention of inflammation and scarring.  Administration may vary from every hour to every 6 hours in the immediate postoperative period and is tapered over 2 months depending on bleb appearance. Care in the first 2 weeks postoperatively is time consuming and laborious. The bleb is scrutinized for leaks and vascularity; the anterior chamber for flare, cells, and depth; and the posterior pole for choroidal effusion. Therapy is adjusted to
Figure 240-12 Fornix-based wound closure. A vertical mattress suture is placed—precise construction is required to keep the limbal suture bite longer than the distance between the corresponding conjunctival suture holes (see stars).
correct any abnormality of the bleb, chamber, or posterior pole. The timing of laser scleral flap suture lysis is critical to the long-term health of the bleb. Early lysis may result in hypotony and late lysis in bleb failure. The use of antimetabolites has extended the time window for laser suture lysis.
COMPLICATIONS OF GLAUCOMA FILTERING SURGERY
The complications are discussed in detail in Chapter 243 .
Glaucoma filtration surgery is an evolving science. Controversy arises over antimetabolite usage, type of conjunctival incision, candidates for surgery, and postoperative care. The outcome is highly dependent on the skill level of the surgeon, type of glaucoma, age and race of the patient, and prior ocular surgery that involves scarring of the conjunctiva.
The reader must remember that a perfectly executed limbal-based trabeculectomy in a virgin eye may be a work of art; however, the same technique in an eye with limbal scarring may lead to conjunctival buttonholes and other sight-threatening disasters.
The key to the art of trabeculectomy is the surgeon’s ability to modify and adapt technique to each and every patient’s ocular and systemic situation.
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