Trabeculectomy

Before the development of the trabeculectomy, the available surgical methods of intraocular pressure (IOP) reduction had a high rate of serious complications such as hypotony, flat anterior chamber, and endophthalmitis. Thus, in an effort to improve the safety of IOP-lowering surgery, the guarded filtering procedure was developed. Cairns was the first to report success using the trabeculectomy in 1968. ^{[1, 2]} Cairns' trabeculectomy, with some improvements and modifications at various steps, still is being performed today.

Trabeculectomy involves the creation of a fistula connecting the anterior chamber and the subconjunctival space. ^[1] This provides an alternative method of aqueous humor filtration when the natural trabecular outflow pathway is blocked or poorly functioning in cases of glaucoma. The goal is to create the right amount of flow without causing overfiltration. Its success relies on the continued patency of the fistula and the continued ability of the filtering bleb created out of the conjunctiva to absorb aqueous. Thus, the success of the procedure lies not only on the surgical technique but also in the intraoperative and postoperative measures to modulate wound healing. See the image below.

The indications for trabeculectomy are as follows ^[1] :

• Primary open-angle glaucoma

• Primary angle-closure glaucoma not responsive to iridotomy and/or iridoplasty

• Secondary open-angle glaucoma

• Secondary angle-closure glaucoma

• Childhood glaucomas

Trabeculectomy can be performed for all types of glaucoma. However, in many types and stages of glaucoma the ideal initial treatment is something other than trabeculectomy (eg, topical medication often is the first-line treatment for primary open-angle glaucoma). The challenge lies in deciding when to choose trabeculectomy over other treatment methods that are appropriate for a particular case. The risks and potential benefits of trabeculectomy must be weighed against the risks and potential benefits of the patient's current therapy (if any). The type and stage of the glaucoma and the presence of comorbid conditions (eg, cataract) also must be considered. Alternative glaucoma treatments include medication, laser procedures, and other incisional surgical procedures such as glaucoma drainage device implantation.

The relative contraindications to trabeculectomy are situations where the chance of failure is high, the risks outweigh the potential benefits, or the primary cause of the glaucoma is treatable. These contraindications include the following:

• Cases that are likely to respond well to less invasive treatments (eg, medications, laser iridotomy)

• Eyes with previous failed trabeculectomy

• Eyes with severely scarred conjunctiva (eg, chemical burns, Stevens-Johnson syndrome)

• Neovascular glaucoma with active neovascularization

• Uveitic glaucoma

• Blind eyes

Trabeculectomy can be performed under local anesthesia (retrobulbar, peribulbar, subconjunctival, or topical) or general anesthesia depending on patient and surgeon preference, on the patient's general health status, and on the estimated duration of the procedure for that particular case.

The following are needed to perform a trabeculectomy:

• Magnification: An operating microscope is recommended, but a powerful binocular loupe can be used if a microscope is not available.

• Surgical instruments: The basic ophthalmic microsurgical instrument set is needed.

• Hemostasis: Electrocautery or diathermy is recommended. A heated metal cautery method is not recommended because of the difficulty controlling the amount of heat delivered. Excessive tissue contraction often is induced by this method.

• Anterior chamber opening/sclerostomy: A trabeculectomy punch (eg, Kelly) is recommended for more precise and controlled excision of the corneal block (sometimes inaccurately called the trabecular block) but this step also can be performed using forceps, a blade, and Vannas scissors.

The patient is supine and the surgeon is seated superior to the patient's head if the procedure is performed at the superior or superonasal limbus. If the procedure is done at the superotemporal limbus, the surgeon may opt to sit superotemporal to the patient's head.

Preoperative Management

Surgical planning

• Determine target IOP. Lower IOP needs more intensive anti-inflammatory therapy and looser flap sutures.

• Consider type of glaucoma. This influences the target IOP and the risk for aqueous misdirection.

• Review risk factors for failure. More risk factors needs more anti-inflammatory/antifibrotic therapy.

• Consider using antifibrotic agents (for lower target IOPs and higher risk for failure).

• Determine route, dose, and duration of corticosteroid treatment.

• Consider need for postoperative cycloplegia (especially for shallow anterior chambers such as in angle-closure glaucoma).

• Plan tightness of scleral flap sutures (target IOP, risk for hypotony).

• Discontinue anticoagulant medications, if possible.

Preoperative IOP lowering

• Higher IOP leads to higher risk for intraoperative suprachoroidal hemorrhage

• Oral and topical IOP-lowering medications

• Osmotic agents such as mannitol and glycerol, if needed

• Surgical methods

  • Paracentesis (very slow and controlled release of aqueous)

  • Vitreous tap

Preoperative anti-inflammatory treatment

• Postoperative scarring increases the risk for failure of the trabeculectomy.

• Topical corticosteroids started a few days before surgery are often sufficient.

• Consider oral steroids or peribulbar injection of steroids for high-risk cases.

  • Uveitic glaucoma

  • Young patients

  • Pigmented races

  • Scarred conjunctiva

  • Neovascular glaucoma

Miosis

• Topical pilocarpine, if needed

Intraoperative Technique

Anesthesia

If subconjunctival anesthesia is used, take care to avoid inducing a subconjunctival hemorrhage. If peribulbar or retrobulbar anesthesia is used, excessive amounts of anesthetic could cause a tight orbit and elevated IOP during the procedure. General anesthesia can markedly lower IOP, so the dose and type of preoperative IOP-lowering treatment should be adjusted for such cases.

Globe fixation

To improve the exposure of the conjunctiva and limbus at the surgical site, the eye needs to be rotated in the direction opposite from the surgical site. An extremely cooperative patient under subconjunctival or topical anesthesia may be able to do this voluntarily, but in most cases a traction suture is needed.

A corneal traction suture does not puncture the conjunctiva, can be placed at various locations and can pull the eye in different directions depending on where it's anchored. However, corneal perforation is a possibility. To place a corneal traction suture, a 7-0 or 8-0 nonslippery suture (eg, silk or polyglactin) on a spatulated needle is passed through one half to two thirds of the corneal thickness parallel to the surgical site approximately 1-2 mm away from the limbus. The eye then can be pulled in the direction desired, and the suture is anchored to the drapes using sterile tape or a clamp. Alternatively, the corneal traction suture can be placed near the inferior limbus and the suture looped through the inferior arm of a wire speculum lid retractor, which acts like a pulley.

The placement of a superior rectus traction suture is familiar to those who perform extracapsular cataract extraction. However, it is a blind procedure and could cause subconjunctival bleeding. Although the conjunctiva is perforated, a leak from the superior rectus traction suture site is extremely unlikely. To place a superior rectus traction suture, the superior rectus muscle (and the overlying conjunctiva) is grasped near its insertion and pulled away from the sclera using large-toothed forceps. A 4-0 or 5-0 nonslippery (eg, cotton or silk) suture on a round needle then is passed under the muscle and anchored superior to the eye to induce downward rotation of the globe.

Anterior chamber access

Anterior chamber (AC) access must be established prior to entering the AC at the surgical site. This allows the surgeon to manipulate the AC depth and the IOP by injecting or removing fluid. The paracentesis site or side port can be made using a needle or blade. The wound track should be directed away from the lens and parallel to the iris to avoid inadvertent lens injury. Some surgeons prefer to perform the paracentesis only after the antifibrotic agent has been applied because of fear of the antifibrotic agent entering the AC. See the image below.

Conjunctival incision and dissection

Inflammation causes scarring and closure of the newly created fistula. Gentle handling of the conjunctiva is essential to reduce inflammation. Sharp scissors for cutting the conjunctiva and nontoothed forceps for grasping the conjunctiva are needed. The Tenon capsule, and not the conjunctiva, should be grasped whenever possible.

The conjunctival incision can be made at the limbus (fornix-based conjunctival flap) or away from the limbus (limbus-based conjunctival flap). There is no clear evidence for the superiority of one type of conjunctival flap over the other, ^[3] but the resulting bleb tends to be more localized and elevated in limbus-based flaps. ^[4]

The advantages of a fornix-based flap are the relative ease of achieving good scleral exposure and the absence of a scar that potentially could limit the posterior flow of aqueous. The disadvantage of a fornix-based flap is the relative difficulty of ensuring a watertight closure at the limbus.

The advantage of a limbus-based flap is the relative ease of achieving a watertight closure. The disadvantages are the potential for scarring at the incision site, which would limit aqueous absorption to the area anterior to the incision. It also creates difficulty exposing the sclera during the procedure. This difficulty can be reduced by using a Tenon traction suture. ^[5]

When performing a fornix-based flap, the conjunctiva needs to be cut close to the limbus but not so close as to injure the limbal stem cells. Avoid leaving large, irregular conjunctival tags that could interfere with scleral flap dissection. A small radial relaxing incision can be made at one or both ends of the limbal incision, if needed. Without dissecting the conjunctiva excessively, the Tenon capsule insertion, which is located approximately 2 mm away from the limbus, is exposed and the Tenon capsule is released from its insertion. The dissection then is carried out in the sub-Tenon plane. There is no need to separate the conjunctiva from the Tenon capsule.

An alternative to a pure fornix-based conjunctival flap is to leave a narrow conjunctival skirt approximately 1 mm wide attached to the limbus. This allows for a more watertight conjunctiva-to-conjunctiva closure and is less harmful to the limbal stem cells.

When performing a limbus-based flap, the conjunctiva and Tenon capsule are incised as far away from the limbus as possible. Ideally, this is done approximately 8-10 mm from the limbus. Closer than that could cause scarring too close to the scleral flap. Further than that makes exposure of the limbus very difficult. Some surgeons prefer to incise the conjunctiva and Tenon simultaneously while others prefer to do it in layers.

The conjunctiva and Tenon capsule are dissected away from the episclera and sclera. Separating the episclera from the sclera should be avoided to prevent persistent bleeding and scarring. Westcott scissors are used to bluntly dissect large sub-Tenon pockets in the preferred direction of aqueous flow (posterior).

Light cautery is used to achieve hemostasis. Excessive cautery should be avoided to prevent contraction or shrinkage of the scleral flap.

Antifibrotic agents

The use of antifibrotic agents is one of the major modern improvements on Cairns' original trabeculectomy. Antifibrotic agents, specifically 5-fluorouracil (5-FU), were first used postoperatively as subconjunctival injections by Heuer and colleagues in 1984 ^[6] to suppress scar formation and preserve the patency of the filtering fistula. Later, Chen introduced the intraoperative application of mitomycin-C (MMC). ^[7] 5-FU can be used both for intraoperative application and postoperative injection. ^{[8, 9]} The use of MMC as a postoperative injection has been reported ^[10] but is not as widely accepted as 5-FU.

Initially, antifibrotic agents were reserved only for high-risk cases ^[11] with preexisting conjunctival scarring, but they now are being used even for primary cases. Their use increases the success of trabeculectomy but also increases the risk for certain complications such as hypotony, bleb leak, shallow anterior chamber, and thin bleb. ^{[12, 13, 14]} The consensus of the World Glaucoma Association states that the risk benefit ratio of using antifibrotic agents should be assessed for each individual patient prior to use. ^[15]

Although both 5-FU and MMC are effective at improving trabeculectomy success, ^{[12, 13, 14]}  some evidence shows that MMC-enhanced surgery achieves lower pressures. ^[16]  However, it appears to be associated with a greater risk for complications. In February 2012, the FDA approved a specific mitomycin product (Mitosol) indicated for adjunct to ab externo glaucoma surgery. It is available as a kit that contains the lyophilized drug (0.2 mg/vial), as well as the diluent, sterile sponges, tuberculin syringes, disposal bag, and other needed equipment for the procedure.

The antifibrotic agent can be applied to the scleral bed before or after the scleral flap is cut. Any absorbent material or sponge that does not fragment when soaked (eg, instrument wipe or cellulose sponge) can be used to apply the antifibrotic agent. Contact of the antifibrotic agent with the wound edges should be avoided. The antifibrotic-soaked sponges should be applied to the area where aqueous flow is desired and should not be placed too close to the limbus in the case of fornix-based conjunctival flaps. After all the sponges have been removed, the site is irrigated with copious amounts of saline solution to remove any excess antifibrotic agent.

The concentration of 5-FU used intraoperatively is 50 mg/mL applied for up to 5 minutes. The concentration of MMC used intraoperatively ranges from 0.1-0.5 mg/mL applied for 2-5 minutes. The concentration and duration depend on the risk for failure in a particular case. Some surgeons prefer to keep the concentration constant while adjusting the duration whereas others prefer the opposite.

The alternative technique of intraoperative injection of MMC into the sub tenon space before the start of conjunctival dissection ^[17] has become popular. This technique avoids the use of sponges, saves time, and allows precise control of the dose of antifibrotic (usually 10-20 mcg).

Antivascular endothelial growth factor (anti-VEGF) agents have been used on filtering blebs, but the evidence of benefit or harm still is lacking. ^[18]  Beta-radiation also has been shown to reduce scarring in trabeculectomy. ^[19]

Scleral flap dissection

A partial thickness limbus-based scleral flap is outlined using the cautery tip or by indenting the sclera. The edges of the flap are cut with a sharp blade angled perpendicular to the globe. The flap then is lifted and dissected between scleral lamellae toward the limbus. Keeping the dissection in the same plane all the way to the limbus produces a flap of even thickness throughout. As the flap dissection approaches the cornea, the blade should be angled slightly anteriorly, following the curvature of the peripheral cornea, to avoid premature entry into the AC. Alternatively, a crescent knife can be used to cut a flap pocket before cutting the sides using scissors. The flap must be handled gently and grasped only at its edges. See the image below.

The flap should be approximately one third to one half of the scleral thickness. A flap that is too thick could hamper aqueous flow and a flap that is too thin could become very fragile. The flap can be rectangular, triangular, square, or parabolic. There is no evidence of superiority of one flap shape over any other. The size of the flap varies, but, ideally, it should be around 2-3 mm radially by 3-4 mm wide. The minimum size of the scleral flap is such that there is at least a 1 sclerostomy wide space between the edge of the sclerostomy and the outer edge of the flap. See the image below.

Anterior chamber entry

The internal scleral flap dissection should extend all the way to the peripheral cornea to ensure that the sclerostomy will be made well anterior to the iris plane. This is especially important in eyes with shallow ACs to prevent the iris or ciliary body from becoming incarcerated in the sclerostomy. Thus, the term sclerostomy has become a misnomer because the tissue excised from under the anterior limit of the scleral flap actually is peripheral cornea. See the image below.

The AC is entered at the anterior limit of the scleral flap using a blade. The blade should be angled more posteriorly than the plane of dissection of the flap. Avoid causing the wound to gape during this step to prevent excessive AC shallowing. See the image below.

Sclerostomy (sclerectomy) and peripheral iridectomy

The posterior edge of the incision into the AC is then excised using a punch (eg, Kelly or Gass), blade, or scissors. When using a punch, ensure that the full thickness of the tissue is caught between the punch blades to avoid creating a lamellar or imperforate sclerostomy. The size of the sclerostomy depends on the amount of flow desired and the size of the scleral flap (refer to the Scleral flap dissection section above).

The iris then is grasped through the newly created opening using forceps, and a peripheral iridectomy is cut using small, sharp scissors. The base of the iridectomy should be wider than the sclerostomy to prevent incarceration of the iris into the wound. The iridectomy should be small and superior enough so that it is hidden by the upper lid to avoid causing diplopia or glare. See the images below.

Scleral flap closure

The scleral flap is closed using simple interrupted, releasable, or adjustable 10-0 nylon sutures, or a combination of these. The sutures are placed at the flap edges taking care to avoid puncturing the part of the flap overlying the sclerostomy. Areas where the flap is too thin should be avoided. The suture knots should be buried to avoid the suture "whiskers" from poking through the bleb.

Various releasable ^{[20, 21, 22, 23]} and adjustable ^{[24, 25]} suture techniques have been described. Both releasable and adjustable sutures are more difficult to place than simple interrupted sutures. Releasable sutures allow the surgeon to remove sutures at the slit lamp without the need for an argon laser (laser suture lysis). This is used to lower the postoperative IOP if it is higher than desired. Adjustable sutures, on the other hand, allow the loosening (or even tightening ^[25] ) of the suture to titrate the flow through the flap. See the images below.

After the first few flap sutures have been placed, the aqueous flow through the flap is assessed. First, the AC is reformed by injecting fluid through the paracentesis track. Then, using a small sponge, the flap edges are dried and the amount of aqueous leakage and the stability of the AC are observed. If the AC shallows quickly or there is excessive flow of aqueous, sutures can be added or loose ones replaced. If there is no flow through the flap, the AC will remain formed and the globe may be tense. In this case, the existing sutures can be loosened by gently stretching them or tight sutures replaced with looser ones.

An indication of the ideal flow rate is when it takes several seconds for the bottom of the gutter around the flap edges to fill with fluid. However, the flow rate endpoint depends on the target postoperative IOP and the surgeon's preference. It is better to err on the side of too little flow because there are many effective and noninvasive remedies for postoperative high IOP (eg, IOP-lowering medication, laser suture lysis, releasable sutures, release of fluid through the paracentesis track) but there are hardly any for excessively low postoperative IOP.

Conjunctival closure

When there is a satisfactory amount of aqueous flow through the flap and the flap suture knots are buried, conjunctival closure can begin. As is the case during the initial dissection of the conjunctiva, the conjunctiva should be handled gently and only Tenon capsule grasped whenever possible.

Nylon 10-0 sutures with a round (vascular) needle are preferred. Absorbable sutures can be used but they may induce conjunctival inflammation. Spatulated (side-cutting) needles also can be used when suturing the conjunctiva to the limbus (fornix-based flap). However, using a spatulated needle to suture conjunctiva is more likely to cause a conjunctival buttonhole or tear than a round needle.

Limbus-based conjunctival flaps can be sutured using a simple running or an interlocked running suture. It can be closed in layers or the Tenon capsule and conjunctiva can be closed together. There should be no tags of Tenon capsule protruding from the wound as these could cause leakage or fistula formation.

Fornix-based conjunctival flaps are more difficult to close. The closure can be conjunctiva-to-cornea if the conjunctiva was cut flush with the limbus or conjunctiva-to-conjunctiva if a narrow skirt of conjunctiva was left attached to the limbus. First, the rolled up edges of the distal conjunctiva are gently unrolled. For conjunctiva-to-cornea closure, the conjunctiva then is stretched to slightly beyond its original insertion. Simple interrupted or purse-string sutures are used to anchor the nasal and temporal edges of the conjunctiva to slightly beyond their original position (called the "wing sutures"). The conjunctiva becomes stretched between the nasal and temporal wing sutures and this is confirmed if an indented line is seen running from the nasal to the temporal wing sutures. The conjunctiva should be watertight at this point, leaking only when pressure is applied to the bleb. See the image below.

For added safety, a few mattress sutures can be used to anchor the conjunctiva parallel to the limbus along the indented line created by the placement of the wing sutures. All sutures should be placed or tied in such a way that the knots are buried beneath the conjunctiva or within the limbal tissue. If burying the knots is not possible, the suture ends can be left slightly long to reduce the foreign body sensation that would be caused by protruding short suture whiskers.

For conjunctiva-to-conjunctiva closure, the conjunctival edges are apposed using a running suture or a few mattress sutures.

If aqueous was flowing through the scleral flap, a bleb would have formed during the closure of the conjunctiva. The bleb can be pressed gently using a sponge and the wound observed for leakage. Sterile fluorescein dye can be used as an adjunct for this step. If the scleral flap was closed tightly, a bleb can be induced by indenting the sclera beside the flap.

Postoperative Care

The proper postoperative care of a trabeculectomy is as important as good surgical technique. A technically perfect trabeculectomy surgery could still fail if necessary postoperative interventions are not performed in time. The challenge of postoperative trabeculectomy management lies in recognizing that there is a need for a postoperative intervention, choosing the type of intervention needed, and intervening at the right time. The bleb appearance, IOP, and anterior chamber status are routinely checked in the posttrabeculectomy period. Gonioscopy and dilated fundus examination are performed when indicated. See the image below.

Preoperative medications

All preoperative IOP-lowering medications are discontinued after the trabeculectomy.

Antibiotics

Topical antibiotic eye drops are the standard of care. Systemic antibiotics are needed only for special circumstances (eg, breach in sterile technique).

Corticosteroids

Topical corticosteroid eye drops are used postoperatively. The dose of the steroid depends on the degree of scarring expected. High-risk cases can be given subconjunctival or peribulbar depot injections or oral steroids. Topical steroids usually are used for at least 6 weeks postoperatively, but the duration depends on the level of inflammation in the bleb and on the presence of risk factors for failure.

Cycloplegic

Atropine eye drops can prevent postoperative aqueous misdirection syndrome (malignant glaucoma). This is especially important in eyes at risk for aqueous misdirection such as angle-closure glaucoma cases or those with shallow anterior chambers, but some surgeons prefer to use this routinely. However, atropine eye drops cause mydriasis, which can be disturbing or uncomfortable for the patient. Atropine usually is given 3 times a day initially then slowly tapered over several weeks.

Antifibrotic injection

Severe bleb inflammation (severe congestion, presence of corkscrew vessels) can be managed using subconjunctival injection of 5-FU (5 mg in 0.1 mL) or MMC. The site of injection depends on surgeon preference but the trend now is to inject the antifibrotic adjacent to or into the bleb rather than away from the bleb. The injection can be repeated several times depending on the response of the bleb. This measure has the greatest potential for success if done in the first 2 weeks postoperatively (up to 4 weeks if intraoperative mitomycin-C was used). The potential complications of this injection are severe corneal epithelial toxicity and early or late bleb leak. The corneal toxicity can be minimized by irrigating the eye after injection to wash away antifibrotic agent that may have leaked from the injection site. See the image below.

Increasing aqueous flow

If the flow through the scleral flap seems inadequate in the first few days postoperatively (ie, high IOP) and obstruction of the internal opening has been ruled out by gonioscopy, measures to increase the flow through the flap can be used. These include digital pressure (ocular/digital massage) and release of sutures (laser suture lysis or releasable sutures).

Digital pressure is applied to the eye either at the limbus opposite the trabeculectomy site or adjacent to the scleral flap. Bleb elevation and lowering of IOP indicate that aqueous has flowed through the flap. Lack of response to this measure could indicate scleral flap adhesion to the scleral bed or conjunctival adhesion to the sclera. Digital pressure mostly is used as a diagnostic measure prior to performing another method of increasing flow but is sometimes used as a therapeutic measure (ie, the patient is taught to do it a few times a day). When digital pressure is used therapeutically, most surgeons prefer to use it only temporarily to help the bleb form during the maturation period, whereas others also use it as a long-term adjunctive IOP-lowering measure.

Laser suture lysis (LSL) is performed using an argon laser and a suture lysis lens. The conjunctiva is compressed with the lens until the suture becomes visible. A very small spot size (50 microns) of high power (300-800 mW) and short duration (0.02 sec) then is used to cut the flap suture through the conjunctiva. This method is not effective in eyes with severe conjunctival chemosis, congestion, or hemorrhage.

Release of sutures is simpler than LSL because it can be done at the slit lamp and it can be done even if the flap sutures are not visible under the bleb. All it requires is that the releasable suture was placed properly with a portion left outside the conjunctiva for easy access.

Both LSL and release of sutures should be done in the first 2 weeks after surgery (up to 4 wks if intraoperative mitomycin-C was used). Only one suture should be lysed or released at a time to reduce the risk for a sudden drop in IOP. Immediate elevation of the bleb and lowering of IOP indicate a successful suture release. Occasionally, gentle application of digital pressure is needed immediately after the suture release if adhesions have developed around the scleral flap.

Bleb needling

Bleb needling can be performed in the immediate postoperative period or months to years after the trabeculectomy. ^{[26, 27, 28, 29]} When subconjunctival and subflap adhesions have developed or are suspected to have developed, the usual measures to increase aqueous flow and reduce inflammation no longer may be effective. When this occurs, bleb needling may be needed to break the adhesions.

There are many techniques of bleb needling. One technique is to use a G25 or G27 needle on a tuberculin syringe loaded with the antifibrotic agent of choice (usually 5-FU). This is inserted into the conjunctiva at least 7-8 mm away from the scleral flap edge, avoiding blood vessels as much as possible. The needle then is brought toward the flap, sweeping the bevel from side to side as it approaches. The bleb may begin elevating already at this point. If necessary, the needle even can be inserted under the flap if the flap edges are visible. Before fully withdrawing the needle, the antifibrotic is injected into the peribleb area the same way it is done for a simple antifibrotic injection. The potential serious complications of this procedure include choroidal effusion, suprachoroidal hemorrhage, blebitis, hypotony, and bleb leak.

Decreasing aqueous flow

The postoperative IOP may be low owing to excessive flow of aqueous through the trabeculectomy site or due to ciliary body hyposecretion. Possible indicators of excessive flow are a very high and/or very diffuse bleb and a shallow anterior chamber. However, the bleb may be low if there is an associated bleb or wound leak and the anterior chamber may be normal in depth. Conservative management of excessive flow (whether the excessive flow is through the scleral flap to the subconjunctival space, through a bleb or wound leak to outside the eye, or both) includes reducing the anti-inflammatory treatment to allow some scarring and external tamponade using a large diameter contact lens, a Simmons shell, or patching. If conservative measures fail or are inadequate, flap and/or conjunctival sutures can be added or revised, but this additional surgery increases the risk for bleb failure.

Preoperative Management

The initial postoperative IOP measurements could be influenced by residual effects of the patient's previous topical IOP-lowering medications. If possible, oral medication (ie, acetazolamide) should be substituted for the topical medications several days to 2 weeks before the trabeculectomy. The rapid washout period of acetazolamide allows for a more accurate idea of the effect of surgery on the postoperative IOP.

Intraoperative Technique

Anesthesia

Use the smallest needle possible (eg, gauge 30) when injecting subconjunctival anesthesia to reduce the chance of bleeding and creating a buttonhole. Try to insert the needle in a spot where the conjunctiva eventually will be incised.

Corneal bridle suture

Use high magnification when placing the corneal bridle suture. This reduces the likelihood of corneal penetration or creating a suture track that is too shallow.

Paracentesis

Cataract surgeons are used to making a paracentesis incision directed toward the pupil to be able to manipulate the lens. But for a trabeculectomy where contact with the lens must be avoided, it is better to direct the paracentesis incision parallel to the limbus, especially when operating on a phakic eye. See the image below.

Conjunctival dissection

Some elderly individuals have developed spontaneous disinsertion of the Tenon capsule. If the Tenon capsule insertion cannot be found, do not aggressively manipulate the episclera in an attempt to find it.

When dissecting the posterior sub-Tenon pockets, orient the curvature of the scissors to match the curvature of the globe to make the dissection easier and smoother. When dissecting very far posteriorly keep in mind the location of the optic nerve and vortex veins.

Keep the conjunctiva moist throughout the procedure to keep the cut edges from rolling up and to reduce postoperative conjunctival inflammation.

Antifibrotic application

When inserting sponges (also referred to as "onlay" type of antifibrotic application), grasp the leading side of the antifibrotic-soaked sponge with the tip of the forceps and grasp the body of the sponge with the blades of the forceps. This prevents the sponge from curling or crumpling as it is inserted and allows it to lay flat under the conjunctiva.

It is safer and more efficient to use a few large sponges than many small ones.

Scleral flap dissection

Minimal resistance from the scleral tissue when performing the lamellar scleral flap dissection toward the limbus indicates that the dissection is continuing in the same plane. Increased resistance indicates that different planes are being dissected. This makes dissection more difficult and could produce a flap of varying thickness.

Whenever possible, the sides of the scleral flap should not be cut flush to the limbus. The dissection of the scleral flap is continued as a short tunnel 1-2 mm beyond the anterior limit of the sides of the flap. This encourages aqueous flow posteriorly rather than along the limbus and it helps prevent limbal leaks when using a fornix-based conjunctival flap. However, this is difficult for beginners because the limbal landmarks are not easily visible. It also is not compatible with the blade or scissors technique of creating the sclerostomy and it may make the insertion of the scleral punch difficult. See the image below.

Before inserting the punch under the flap and into the AC, apply pressure to the handle so that the cutting edges move closer together. This improves the steadiness of the punch as the cut is made.

Once the punch tip is inside the AC but before the cut is made, reorient the punch so that it is almost perpendicular to the limbus and tug gently. Resistance indicates that the lip of the internal incision is well engaged. This helps create a full-thickness punch with nonshelved edges. See the image below.

Peripheral iridectomy

To achieve an iridectomy with a wide base, hold the scissors parallel to the limbus, pull the iris away from the scissors while a first cut is made, and then push the iris toward the scissors while a second cut is made to complete the iridectomy.

When pulling the iris to make the iridectomy, seeing a peaked pupil indicates a good grasp on the iris. Be wary of cutting the iris if the entire pupillary border is not visible because this could lead to an inadvertent sector iridectomy.

Flap suturing

Some surgeons prefer to pre-place the first 1 or 2 flap sutures before AC entry. This allows rapid closure of the scleral flap and lowers the risk for transient severe hypotony, which is thought to lead to suprachoroidal hemorrhage and the phenomenon of loss of central vision postoperatively (ie, wipe out or snuff out) in eyes with advanced glaucomatous visual field loss. ^{[30, 31]} However, pre-placing sutures can be difficult due to the need for careful arrangement of the loose suture ends and the high likelihood of snagging or snarling the suture.

Prevention of intraoperative hypotony

This is especially important for eyes with advanced optic nerve damage and floppy eyes (eg, vitrectomized, highly myopic). An AC maintainer or viscoelastic can help stabilize the anterior chamber and prevent hypotony.

Postoperative Care

Injecting a small bolus of air behind the antifibrotic agent to tamponade the injection site may help minimize the incidence of corneal toxicity.

The complications of trabeculectomy can be classified into intraoperative, early postoperative, and late postoperative.

Intraoperative complications

Intraoperative complications include the following:

• Conjunctival buttonhole or tear

• Subconjunctival hemorrhage

• Scleral flap buttonhole, tear, or disinsertion

• Premature entry into the anterior chamber

• Crystalline lens injury

• Hyphema

• Imperforate sclerostomy

• Vitreous loss

• Intraoperative choroidal effusion/suprachoroidal hemorrhage

• Imperforate peripheral iridectomy

• Inadvertent sector iridectomy

• Cyclodialysis/iridodialysis

• Intraoperative aqueous misdirection syndrome (a.k.a. malignant glaucoma or ciliary block glaucoma)

Early postoperative complications

Early postoperative complications include the following:

• Filtering bleb-related complications including the following:

  • Wound leak or dehiscence

  • Early bleb leak

  • Early bleb failure

  • Encapsulated bleb (Tenon cyst)

• Hypotony

• Choroidal effusion/suprachoroidal hemorrhage

• Shallow or flat anterior chamber

• Over- or underfiltration

• Aqueous misdirection syndrome (ie, malignant glaucoma or ciliary block glaucoma)

• Infection

• Pupillary block (due to an imperforate peripheral iridectomy)

• Loss of central visual acuity (snuff out or wipe out) in eyes with advanced glaucomatous visual field loss

• Corneal or ciliary body toxicity due to the antifibrotic agent

Late postoperative complications

Late postoperative complications include the following:

• Blebitis and bleb-related endophthalmitis

• Late bleb failure

• Late bleb leak

• Hypotony

• Cataract formation or progression

• Cystic bleb

• Bleb dysesthesia (symptomatic bleb)

• Overhanging bleb

Trabeculectomy can be performed simultaneously with other procedures such as cataract surgery, keratoplasty, and vitrectomy if warranted by the clinical situation. The most common combination is trabeculectomy with cataract surgery because of the high incidence of cataracts in patients with glaucoma.

The decision to perform combined cataract surgery and trabeculectomy versus sequential surgery depends on many factors. These include the stage of the glaucoma, the visual impact of the cataract, the quality of IOP control, the impact of multiple trips to the operating room on the patient's general health and finances, and the target postoperative IOP. In addition, the following should be considered:

• The technique of cataract extraction to combine with trabeculectomy should be the least invasive as this incites less inflammation. Thus, phacoemulsification is preferred over extracapsular cataract extraction. ^{[32, 33, 34, 35]}

• There appears to be no difference in IOP control using a 1-site versus a 2-site phacotrabeculectomy. ^{[36, 37]}

• Combined cataract surgery and trabeculectomy produces less IOP-lowering than trabeculectomy alone. ^{[38, 39, 40, 41]}

• Cataract surgery performed in an eye with a functioning trabeculectomy can cause loss of IOP control or outright failure of the trabeculectomy. ^{[42, 43, 44, 45]} A longer interval between trabeculectomy and cataract surgery is associated with increased survival of the trabeculectomy. ^[46]

The success of trabeculectomy performed on an eye that has previously undergone uncomplicated cataract surgery does not appear to be diminished. ^[47]

Multiple devices have been used to enhance trabeculectomy success, including a mini-shunt, ^[48] a biodegradable collagen matrix implant (Ologen), ^{[49, 50]} and amniotic membrane. ^[51]  However, there is only low-quality evidence that these devices are beneficial. ^[52]