Historical background

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Filtration surgery creates a fistula between the anterior chamber and the subconjunctival space (Fig. 35.1). The earliest fistulizing procedures were described in the nineteenth century. MacKenzie (1830) was the first to suggest the surgical relief of raised intraocular pressure (IOP) by a sclerotomy. In 1857 Crichet attempted to make a permanent paracentesis by including the iris in a limbal wound. De Wecker devised an anterior sclerotomy with a view to increasing the drainage of the aqueous by the formation of a filtering cicatrix. The results remained unsatisfactory for the wound tended to close1.

Fig. 35.1 Successful trabeculectomy with an intraocular pressure of 9 mmHg.

In 1906 LaGrange performed a sclerecto-iridectomy and in 1909 Elliot described limbal trephination that provided a permanent fistula to the subconjunctival space1. To decrease the rate of flow through the filtering ostium during the early postoperative period, iris was used as a wick to act as a partial plug, or iridencleisis2. Thermal cautery of the scleral wound edges with entry into the anterior chamber (Preziosi, 1934)1, Scheie’s modification with peripheral iridectomy, a thermal sclerostomy (1958)3, and posterior lip sclerectomy¹ (Iliff and Haas, 1962) were the most widely used operations until guarded filtration procedures were developed1.

In 1968 Cairns reported good results with ‘trabeculectomy’ in a series of glaucoma patients4. This procedure was supposed to remove a portion of trabecular meshwork to allow flow into the cut ends of the Schlemm’s canal, and used a partial-thickness scleral flap to cover the sclerostomy. Interestingly, it had been described in 1961 by Sugar, who should be acknowledged as the first to use the term ‘trabeculectomy’, but the flap was sutured tightly, with no subconjunctival filtration, and those cases were unsuccessful5. The ‘trabeculectomy’, more appropriately described as a guarded filtering procedure, became the most commonly used filtration procedure because of the reduction in the frequency of postoperative complications associated with over-filtration.

Mechanisms of IOP lowering

Filtration surgery lowers the IOP by creating a fistula between the anterior chamber and the subconjunctival space (see Fig. 35.1). The intraocular fluid beneath the conjunctiva and/or Tenon’s capsule form a filtering bleb and can either be absorbed through veins, conjunctival lymphatics or, in some cases where the conjunctiva is thin, pass directly into the tear film6,7.

Epidemiology

The prevalence of glaucoma in the developed world has been calculated to be between 1.1 and 2.1%, increasing with age. Even though it is usually a slowly progressive disease and the majority of people do not become blind, glaucoma represents a leading cause of blindness worldwide, and the commonest cause of preventable blindness. There are over 5 million blind from this disease, and more than 67 million people affected with glaucoma worldwide8.

Glaucoma care in the developed world has changed with the introduction of new medications and diagnostic technologies. Overall, there is a large increase in the volume of prescribing, accompanied by a large reduction in the number of guarded filtration operations. For example, in Scotland the number of guarded filtration procedures per 100 000 population was 61% less in 2004 than in 1994. This downward trend in Scotland has also been found in the UK, Australia, the USA, Canada, and France. In the USA there has been a substantial increase in laser trabeculoplasty procedures9–14.

Although the decrease in the guarded filtration surgery rate appears to be due to prescribing of new drugs, there may be other explanations for fewer filtering operations. An increase in the use of anti-metabolites in glaucoma surgery has improved the surgical outcome; there is no information about the number of repeated guarded filtration surgeries among the total number of operations. Although primary surgical intervention has been reported to have success rates of up to 90%, the current trend is to perform surgery if medical treatment fails.

Indications for glaucoma filtration surgery

Glaucoma treatment aims to prevent visual field loss progression and maintain patients’ quality of life. Factors that affect glaucoma patients’ quality of life include functional loss, inconvenience and side effects caused by medication(s), treatment expense, anxiety associated with the diagnosis of a chronic, and potentially blinding disease and the possibility of surgery15,16. Currently, IOP is the only major glaucoma risk factor that can be treated, and lowering IOP has been shown to reduce visual field loss progression17,18.

Guarded filtration surgery is the surgical procedure of choice for patients with glaucoma; it is typically indicated when neither medical nor laser therapy sufficiently controls IOP and disease progression is likely to diminish a patient’s quality of life. Medical therapy may be considered insufficient if it cannot maintain IOP within a range that is low enough to prevent further damage, if it is not tolerated due to side effects or if adherence to treatment is a problem.

Because patients’ visual needs and vision-related quality of life differ, patients should be assessed individually before physicians decide to perform surgery. Physicians should consider the likelihood of success and risk of complications from surgery. Glaucoma filtration surgery may be considered as initial treatment if there are contraindications to topical medication or when adherence to treatment is unlikely.

While guarded filtration surgery is generally performed prior to glaucoma drainage devices for most cases of glaucoma, it is not recommended for some patients, including patients with active neovascular glaucoma, active uveitic glaucoma, severe limbal conjunctival scarring, epithelial downgrowth, aphakic eyes, especially after congenital cataract, and refractory infantile glaucoma. If guarded filtration surgery supplemented with anti-metabolites fails or is not an appropriate surgical option, implantation of a glaucoma drainage device could be considered. Cyclodestructive procedures are usually the last line of intervention in recalcitrant glaucoma or for eyes with poor visual function.

Preoperative assessment and anesthesia

The surgeon, patient, and anesthesia staff need to work together and be involved in the selection and execution of anesthesia during the surgery. Involving the patient in this decision by describing the patient experience prior to and during surgery is critical. Fear and anxiety are exaggerated when things are unknown or unexpected. If patients are prepared, they are better equipped to cope with the sensations they may feel during and after surgery.

The preoperative assessment and preparation of patients undergoing guarded filtration surgery under local anesthesia varies worldwide19. The standard preoperative assessment includes specific enquiry about bleeding disorders and drugs. There is an increased risk of hemorrhage in patients receiving anti-coagulants and a clotting profile assessment is required prior to injection techniques. Patients receiving anticoagulants are usually advised to continue medication. Clotting results should be within the recommended therapeutic range. Currently there is no recommendation (lack of data) for patient receiving anti-platelet agents. SubTenon’s block and/or topical anesthetic are favored in these patients.

There are numerous modes of anesthesia from which a surgeon can choose. Overall, there is not one type of anesthesia right for all cases, and the best choice varies from surgeon to surgeon, and patient to patient. The use of general anesthesia or regional (i.e. retrobulbar or peribulbar) block has declined with the availability of other safer and equally effective means of local anesthesia including subTenon’s, subconjunctival, and topical anesthesia associated with intracameral lidocaine20,21.

Topical ocular anesthesia has been demonstrated to be a safe and effective alternative to retro bulbar or peribulbar anesthesia for guarded filtration surgery. Local anesthetic eye drops are instilled three or four times, separated by a few minutes just prior to surgery. Another popular practice is the administration of topical anesthesia using viscous lidocaine gel instead of or in addition to drops (Fig. 35.2). Topical anesthesia does not provide ocular akinesia and may provide inadequate sensory blockade for the iris and ciliary body, thus intracameral injection of local anesthetics (preservative-free 1% lidocaine injected in doses of 1–5 ml) is also used22.

Fig. 35.2 Viscous lidocaine jelly is placed in the eye in the preop area and washed out prior to the start of surgery to provide analgesia.

SubTenon’s anesthesia (block) is a simple, safe, effective and versatile alternative to a sharp needle block for orbital anesthesia. Access to the space by the inferonasal quadrant is the most commonly described approach because the placement of the cannula in this quadrant allows good fluid distribution superiorly while avoiding the area of surgery and reducing the risk of damage to the vortex veins. After instillation of local anesthetic eye drops the patient is asked to look upwards and outwards, to expose the inferonasal quadrant. The conjunctiva and Tenon capsule are gripped with non-toothed forceps 5–10 mm away from the limbus. A small incision is made through these layers with scissors and sclera is exposed. A blunt curved metal subTenon’s cannula, (19 G, 25 mm long, curved, with a flat profile with end hole) securely mounted on to a 5 ml syringe containing the local anesthetic solution, is inserted through the hole along the curvature of the sclera. The local anesthetic agent of choice is injected slowly and the cannula is removed. With the above technique adequate anesthesia is achieved for the majority of ocular surgeries. Akinesia is volume dependent and, if 4–5 ml of local anesthetic agent is injected, most patients develop akinesia23,24.

Sight and life-threatening complications have been reported but are very rare. The rise in IOP after administration of a subTenon’s block is small or even non-significant25,26.

However, pulsatile ocular blood flow may be affected by subTenon’s block, in a similar way to retrobulbar and peribulbar injections. Therefore, caution is required in glaucoma patients with advanced optic nerve damage who may be at risk of visual ‘wipe-out’ (see below).

The current retrobulbar technique used by most ophthalmologists today was described by Atkinson in 1934, and until recently served as the most commonly used technique for intraocular surgery27.

Davis and Mandel are credited with introducing the peribulbar block in 1986 as a safer alternative to retrobulbar anesthesia28.

The decision between retrobulbar anesthesia and peribulbar anesthesia presents the surgeon with a choice between speed and safety. With a retrobulbar block a surgeon can ensure that adequate akinesia and anesthesia will result for glaucoma surgery; however, a blind injection into the orbit poses several potential complications including, but not limited to, retrobulbar hemorrhage, globe perforation, optic nerve damage, and brainstem anesthesia. Peribulbar anesthesia, involving the injection(s) of local anesthetic external to the muscle cone, decreases the likelihood of optic nerve and globe perforation while maintaining the desirable qualities of excellent akinesia and anesthesia (Fig. 35.3).

Fig. 35.3 Peribulbar block avoiding the muscle cone.

The patient undergoing ophthalmic surgical procedures irrespective of type of regional anesthesia employed should be fully conscious, responsive, minimally anxious, and free from discomfort and pain. The aim of sedation is to minimize anxiety while providing the maximum degree of safety. Short acting benzodiazepines, opioids, or intravenous anesthetic agents in minimum dosages are used. However, there is an increased risk of an intraoperative event with sedation. A means of providing supplementation air/oxygen must be available when sedation is administered, and blood oxygenation, monitored specifically.

General anesthesia provides the most controlled environment for surgery. However, it is associated with an increased risk of systemic complications (e.g. malignant hyperthermia, hemodynamic fluctuation, myocardial infarction, postoperative nausea and vomiting), and requires more medication, equipment, and personnel. As a result, it is the most costly form of anesthesia. General anesthesia remains the technique of choice for children, intellectually handicapped individuals, and demented or psychologically unstable patients. Patients may feel that they will not be able to cooperate during surgery and insist on general anesthesia29.

There are patients in whom general anesthesia is contraindicated or should be undertaken with caution. Patients with Marfan syndrome are at increased risk of cardiac and pulmonary complications under general anesthesia. Thorough review of glaucoma medications is necessary, since some ocular medications may interfere with general anesthesia. Topical epinephrine (rarely used today to treat glaucoma) may interact with halogenated hydrocarbon anesthetics leading to ventricular fibrillation. Used more often in the past to treat glaucoma, echothiophate inhibits plasma pseudo-cholinesterase, which also metabolizes anesthetics including succinylcholine, thus leading to overdosing and a prolonged dependency on mechanical ventilation.

Surgical technique

Choosing the surgical site

Most authors prefer to perform the first surgery at 12 o’clock as the filtration bleb is more likely to be covered by the superior eyelid. This location would allow, if needed, two subsequent filtration surgeries in the superior conjunctiva (i.e. supero-nasal and supero-temporal). As the risk for late endophthalmitis markedly increases when the surgical area is not covered by the upper eyelid, inferior, lateral, or nasal bleb positions are contraindicated. In repeated interventions or in cases with previous conjunctival surgery it is important to confirm the mobility of the conjunctiva. This can be done preoperatively with a moistened cotton-tip, and/or intraoperatively by injecting BSS under the conjunctiva. If there is extensive conjunctival scarring (i.e. there is no conjunctival mobility) a guarded filtering procedure is contraindicated.

A fixation or traction suture keeps the eye in a downward position to give a good area of exposure superiorly. A corneal traction suture in the quadrant of the planned surgery (7-0 or 8-0 black silk or nylon, or 7-0 or 8-0 Vicryl on a spatula needle) is preferred by the authors (Fig. 35.4). The needle is passed through clear, mid-stromal cornea approximately 2 mm from the limbus for approximately 3–4 mm. Alternatively a superior rectus traction suture (4-0 or 5-0 black silk on a tapered needle) can be used to rotate the globe inferiorly and bring the superior bulbar conjunctival into view. Using a muscle hook to rotate the globe downwards, the conjunctiva and superior rectus are grasped with toothed forceps and the threaded needle is passed through the tissue bundle.

Fig. 35.4 A&B, A corneal traction suture with an 8-0 Vicryl suture affords excellent visibility during anterior segment surgery.

Conjunctival dissection

A limbus- or fornix–based conjunctival flap is made with Wescott scissors and a non-toothed utility forceps. For some physicians, a fornix-based flap may be easier for combined cataract-filtration procedures.

When forming limbal-based flaps, the conjunctival incision is placed 8–10 mm posterior to the limbus. Once the conjunctiva is incised, the Tenon’s capsule is grasped, elevated, and cut with Wescott scissors. The conjunctival and Tenon’s wound should be lengthened to approximately 10–15 mm cord length with the Tenon’s incision longer than the conjunctival incision due to the relative elasticity of the conjunctiva. During anterior dissection, the surgeon should grasp the Tenon’s tissue rather than the conjunctiva to avoid an inadvertent buttonhole. The dissection is carried forward until the insertion of the Tenon’s is reached, to clearly expose the corneo-scleral limbus. In cases with extensive scarring from prior surgery, elevation of tissues with an injection of saline prior to the dissection of the conjunctiva is useful to delineate scarring and to determine a favorable site for surgery.

When making fornix-based flaps, the conjunctiva and Tenon’s are disinserted with a blade or Wescott scissors. The conjunctiva is incised as close to the limbus as possible. An oblique relaxing incision at one or both corners can be considered to improve scleral exposure, especially if the conjunctiva is very thin. Approximately a 2 o’clock limbal peritomy (5–6 mm) is sufficient. Blunt dissection is carried posteriorly using blunt-tipped Wescott scissors, avoiding the superior rectus muscle, but extending over the whole superior bulbar conjunctiva as widely and posteriorly as possible.

The advantages of the fornix-based conjunctival flap include improved exposure and access, reduced risk of conjunctival button-hole formation, and the formation of a more posterior and diffuse bleb, which may be associated with decreased incidence of late endophthalmitis (Fig. 35.5). However, with a fornix-based flap there is an increased risk of conjunctival wound leak in the early postoperative period.

Fig. 35.5 A fornix-based flap affords excellent visualization of the limbus, especially in eyes with prior scarring. The scleral flap is half the thickness of the sclera.

Internal block excision and peripheral iridectomy

A block of tissue at the corneo-scleral junction is excised either with a sharp blade (e.g. a 15° or 30° blade) and Vannas scissors or with a punch, such as a Kelly’s punch. With the former, two radial incisions are made with the blade starting in clear cornea, at the most anterior point adjacent to the scleral flap, and extending posteriorly approximately 1–1.5 mm. The radial incisions are made approximately 2 mm apart. The blade, or the Vannas scissors, is used to connect the radial incisions, allowing the removal of a rectangular piece of tissue. Alternatively, the fistula can be created with a punch. An anterior corneal incision, parallel to the limbus, is made to enter into the anterior chamber, and a scleral punch is used to excise the limbal tissue (Fig. 35.6).

Fig. 35.6 The block has been removed with a punch, allowing for good aqueous flow. A peripheral iridectomy follows.

A peripheral iridectomy is performed to prevent obstruction of the sclerectomy by the iris and to prevent postoperative pupillary block. It is essential in patients with angle closure, and in those cases in which the iris is pushed towards the fistula. The iris is grasped near its root with toothed forceps, retracted through the sclerostomy, and a broad peripheral iridectomy performed with Vannas or De Wecker scissors. The iridectomy should not extend too posteriorly, to avoid iris root/ciliary body damage and bleeding. Pre- or intraoperative use of miotics facilitates a smaller iridectomy. In eyes with a widely open angle and deep anterior chamber (e.g. pseudophakia) a peripheral iridectomy may not be necessary. The disadvantage of a peripheral iridectomy is that it is associated with postoperative inflammation and hyphema. However, absence of a peripheral iridectomy may result in iris plugging the internal sclerostomy with subsequent failure of the procedure. The surgeon must individualize this decision.

Closure of scleral flap

The scleral flap can be sutured with two interrupted 10-0 nylon sutures in rectangular flaps, and with an interrupted suture in triangular flaps. Slipknots are useful to adjust the tightness of the scleral flap and the rate of aqueous outflow. Additional sutures can be used to control the outflow.

Usually the surgeon can identify the ‘key’ sutures that appear to have more influence restricting aqueous outflow than others. These key sutures can be identified during surgery when the anterior chamber is filled through the paracentesis and the flow around the scleral flap, observed. These key sutures should be identified in the patient’s records for ready access postoperatively. Greater caution is advised when releasing these ‘key’ sutures to increase outflow.

Many surgeons prefer to reduce the risk of postoperative hypotony and anterior chamber shallowing by suturing the scleral flap to limit the flow of aqueous into the subconjunctival space to a barely visible ooze. Postoperatively, if the IOP is higher than desired, laser suture lysis can lower the IOP in steps. A tight suture closure may be more important in patients in whom the preoperative IOP is markedly elevated, if mitomycin C has been used, in aphakic glaucomas, and patients with high risk of malignant glaucoma (chronic angle closure) and suprachoroidal hemorrhage (e.g. Sturge–Weber syndrome). In eyes with postoperative subconjunctival blood, a krypton red or a diode laser can be used instead of an argon.

The use of releasable sutures offers another strategy to close the scleral flap so that the flow can be increased postoperatively (Figs 35.7 and 35.8). Externalized releasable sutures may be removed without lasers. They offer advantages over planned laser suture lysis when the conjunctiva is inflamed or hemorrhagic, or the Tenon’s tissue thickened so that suture lysis is difficult. Several techniques have been described by Wilson (mattress-type suture with an externalized knot on the cornea)30, Shin (removable knot passed through the conjunctival bleb)31, Cohen and Osher (loop-knot suture externalized through the cornea)32, Hsu and Yarng (an externalized hemi-bow tie in the center of the filtering bleb)33, Maberley et al. (a two-arm ‘U’ suture that leaves no exposed suture end until one arm of the suture is removed)34, and Johnstone et al. (releasable ‘tamponade suture’)35.

Fig. 35.7 Internal view of the releasable suture. The suture can be tied tightly to control the aqueous flow in the postoperative period.

Fig. 35.8 External view of the releasable suture that can be removed at the slit lamp.

Releasable sutures are as effective as laser suture lysis. In theory, the disadvantages of releasable sutures include the need for additional intraoperative manipulation, possible postoperative discomfort from the externalized suture, corneal epithelial defects, and, rarely, intraocular infection.

Suture lysis or releasing sutures should be done when the function of the filtering bleb needs to be improved, ideally within the first 2 weeks after surgery when anti-metabolites have not been used; if done later, fibrosis of the scleral flap may halt any beneficial effect. However, the window of opportunity is enlarged when anti-metabolites have been used. Before suture lysis, gonioscopy is performed to confirm the presence of an open internal sclerostomy with no tissue or clot occlusion.

Indications and technique of intraoperative application of anti-metabolites

To reduce postoperative subconjunctival fibrosis and preserve bleb function postoperative topical steroids are frequently used36. The use of anti-fibrotic agents in filtering procedures is associated with a higher success but also with a higher complication rate (hypotony due to over-filtration, bleb leak, and ocular infection). For this reason an individualized consideration of the risk/benefit ratio is recommended. A survey of glaucoma specialists in the American Glaucoma Society indicated that anti-fibrotic agents are used in the majority of operations37, but in the UK they are used less frequently38. The agent 5-fluorouracil (5FU) is usually administered postoperatively but some surgeons use it intraoperatively (50 mg/ml) for 5 minutes. Mitomycin C (0.1–0.5 mg/ml solution) (Fig. 35.9) is more potent than 5FU, and increases the chances of success and the risks of complications39–42.

Fig. 35.9 Mitomycin C is used to prevent fibrosis and scarring. It is kept frozen until it is needed in the operating room.

The concentration and time of exposure (1–5 minutes) vary according to the expected risk for fibrosis and the target IOP. Anti-fibrotic agents are delivered using multiple shaped, soaked cellulose sponges or fragments of filter paper over the episclera, either before or after dissecting the scleral flap. Some surgeons apply the anti-metabolite under the scleral flap. It is important to treat a wide surface, posterior and to both sides of the surgical area, rather than treating only the conjunctiva and episclera immediately over and adjacent to the scleral flap.

The conjunctival-Tenon’s layer is draped over the sponge, avoiding contact of the wound edge with the anti-fibrotic agent. After the application, the sponge is removed and the entire area is irrigated thoroughly with balanced salt solution (20–100 ml). The plastic devices that collect the liquid run-off are disposed of according to local toxic waste regulations.

Postoperative care and prevention of failure of the filtering bleb

Failed blebs are those associated with inadequate IOP control and impending or established obstruction of aqueous outflow. Early failure of filtering blebs is characterized by a high IOP, deep anterior chamber, and low and hyperemic bleb. Failing blebs should be recognized promptly because if obstruction is not relieved permanent adhesions between conjunctiva and episclera can lead to closure of the fistula (see below). Failure of filtering operations is most commonly due to subconjunctival scarring (Fig. 35.10). Internal obstruction of the fistula by blood clot, vitreous, iris, or incompletely excised Descemet’s membrane is also possible.

Fig. 35.10 Inflamed scarred bleb with subsequent increase in intraocular pressure.

Digital ocular compression and focal compression can be used to improve the function of a temporarily non- or poorly functioning filtering bleb43. Digital ocular compression (DOC) can be applied to the inferior sclera or cornea through the inferior eyelid, or to the sclera posterior to the scleral flap through the superior eyelid. Focal compression can be applied with a moistened cotton tip at the edge of the scleral flap. In the early postoperative period laser suture lysis or removal of an externalized releasable suture can enhance the filtration. Gonioscopy performed prior to the laser should confirm an open sclerostomy with no tissue or clot occluding its entrance (Fig. 35.11). After the suture is cut, if the bleb and IOP are unchanged, ocular massage or focal pressure can be applied. Usually only one suture is cut at a time to avoid the possible complications of over-filtration, hypotony, and flat anterior chamber. The timing of suture release is critical. Suture lysis is effective within the first 2 weeks after surgery without anti-metabolites. If anti-metabolites have been used at the time of surgery, suture lysis can be effective several weeks or even months after surgery.

Fig. 35.11 The internal osteum is open, assuring that there is no iris blocking the filtration site.

‘Warning signs’ of failing filtration are increased bleb vascularization, bleb inflammation, and/or bleb thickening. The recognition of these signs is important because treatment with additional topical and even subconjunctival steroids and subconjunctival injections of 5-fluorouracil (5 mg in 0.1 ml of a 50 mg/ml solution) may control the healing response. In cases of established subconjunctival–episcleral fibrosis an external revision or ‘bleb needling’ can be tried44–47.

A 27- or 25-G needle is used to cut the edge of the scleral flap and restore aqueous outflow. Entry of the needle tip into the anterior chamber beneath the flap is important but should be undertaken with extreme caution in phakic eyes (Fig. 35.12). The technique can be repeated as needed. The outcome may be more favorable if there was a previously well-established filtration bleb before the fistula became occluded. Repeated subconjunctival injections of 5FU after revision may increase the probability of success. Mitomycin C before or after needling has also been proposed. The most common complications are associated with postoperative hypotony (see above).

Fig. 35.12 A&B, Needling a bleb with a 27-gauge needle can break the fibrous capsule surrounding the bleb and maximize aqueous flow. Often it is necessary to lift the flap in order to establish a low, diffuse bleb.

Complications of guarded filtration surgery

Overall, severe visual loss is uncommon but the incidence of transient complications is high. For example, in the Collaborative Initial Glaucoma Treatment Study (CIGTS) early complications occurred in 50% of 465 trabeculectomies. The most frequently complications were shallow or flat anterior chamber (13%), encapsulated bleb (12%), ptosis (12%), serous choroidal detachment (11%), and hyphema (10%). Suprachoroidal hemorrhage occurred in 0.7% of cases; there were no cases of endophthalmitis67. In the UK, a national survey of trabeculectomy was conducted and of 1240 reported cases, early complications were reported in 46%, and late complications in 42% of cases68. The most common early complications were hyphema (24%), shallow anterior chamber (23%), hypotony (24%), wound leak (17%), and choroidal detachment (14%) (Fig. 35.13). The most frequent late complications were cataract (20%), visual loss (18%) and encapsulated bleb (3%). Recently, the Tube Versus Trabeculectomy (TVT) study in 212 patients found intraoperative complications in 10% and 7% of cases during trabeculectomy and tube surgery respectively, and postoperative complications in 57% and 34% of patients after trabeculectomy and tube surgery respectively. Most complications were self-limited69.

Fig. 35.13 Hyphema and shallow chamber following a guarded filtration procedure.

Intraoperative complications

Suprachoroidal hemorrhage (Fig. 35.14) is a serious complication that can be seen during or after any intraocular surgery70–74. If it occurs intraoperatively and cannot be controlled (i.e., expulsive hemorrhage), it can lead to loss of vision. The incidence of suprachoroidal hemorrhage in glaucoma patients undergoing various types of intraocular surgery has been reported to be 0.73%. Ocular risk factors for suprachoroidal hemorrhage include glaucoma, aphakia, pseudophakia, previous vitrectomy, vitrectomy at the time of glaucoma surgery, myopia, and postoperative hypotony. Systemic risk factors are arteriosclerosis, high blood pressure, tachycardia, and bleeding disorders. The source of the hemorrhage is usually one of the posterior ciliary arteries, particularly at the point of entrance of the short posterior ciliary vessels into the suprachoroidal space.

Fig. 35.14 Suprachoroidal hemorrhage with the retina and fibrin seen behind the implant.

Intraoperative suprachoroidal hemorrhage can be associated with sudden collapse of the anterior chamber. The patient may complain of sudden pain breaking through the local anesthesia. If the process is gradual, a dark mass can be observed through the pupil to evolve slowly obscuring the red reflex; however, if the process is abrupt, the hemorrhage is more expulsive. Once a suprachoroidal hemorrhage has been identified, prompt and secure closure of the incision is the first strategy, with gentle reposition of prolapsed uvea. The surgeon’s finger can tamponade the incision site temporarily while sutures are placed. Meanwhile, intravenous acetazolamide (500 mg) and mannitol 20% (1–1.5 g/kg) are administered. Once the eye has been closed, the anterior chamber can be reformed through the incision or a paracentesis tract. After this point, a conservative approach is probably appropriate. Some authors have advised immediate drainage of the hemorrhage through posterior sclerostomies (usually not possible because it rapidly clots). Prognosis for recovery of vision is good as long as the eye can be closed without loss of uvea.

Several steps can be taken in ‘high risk’ eyes to prevent this serious event: before surgery, correction of bleeding problems and preoperative intravenous mannitol at the time of surgery may be helpful. The patient is urged to restrict activities (bending, weight lifting) and to avoid Valsalva-positive conditions (constipation, vigorous coughing, sneezing, or nose blowing) during the early postoperative period.

Conjunctival buttonholes and tears

These can lead to failure of bleb formation and flat anterior chamber. The usual cause of conjunctival buttonholes is penetration of the tissue by the tip of a sharp instrument (needle, scissors, and blade) or the teeth of a forceps. Buttonholes and tears are more likely to occur in cases with extensive conjunctival scarring. To diagnose a buttonhole intraoperatively, the conjunctiva should be carefully examined at the end of the procedure by filling the anterior chamber with saline and raising the filtering bleb. If recognized, the buttonhole should be closed at the time of surgery. If it is located in the center of the conjunctival flap, a ‘purse-string’ closure is attempted either internally on the undersurface of the conjunctiva or externally if the flap has already been re-approximated; 10-0 or 11-0 nylon on a tapered (‘vascular’) needle should be used. When the conjunctival buttonhole or tear occurs at the limbus, it can be sutured directly to the cornea, which should be de-epithelialized. A mattress suture or, if large, a running suture with 10-0 nylon can be used. When the buttonhole or tear occurs near the incised edge of a limbal-based conjunctival flap, the sutures used to close the conjunctival incision can be placed anterior to the tear to close it.

Scleral flap disinsertion

A thin scleral flap can be torn or amputated from its base during the surgical procedure. If a sclerostomy has not yet been performed, a new scleral flap should be dissected in a different area. If a sclerostomy has already been created, re-approximation of the scleral flap can be attempted with 10-0 or 11-0 nylon sutures. If unsuccessful, additional tissue is usually necessary to cover the sclerostomy. This can be obtained by transferring a piece of partial-thickness sclera from the area adjacent to the defect. Alternatively, donor sclera, fascia lata, or pericardium can be used.

Postoperative complications

Hypotony

Hypotony can be caused by excessive aqueous outflow, most frequently due to excessive filtration, wound leak, or cyclodialysis cleft. Reduced aqueous production should also be considered (ciliochoroidal detachment, inflammation, inadvertent use of aqueous suppressants). Transient hypotony is very common after glaucoma surgery, but it may lead to other possible complications including flat anterior chamber, Descemet’s membrane folds, choroidal effusions, suprachoroidal hemorrhage, cataract, macular and optic disc edema, and chorio-retinal folds (predominantly in young myopic patients)75–89 (Fig. 35.15).

Fig. 35.15 Hypotony following glaucoma surgery can be either transient or longstanding.

The initial management of early postoperative hypotony with a formed anterior chamber is conservative with topical steroids and cycloplegics. Intervention is indicated in cases when hypotony is associated with other complications such as persistent low IOP with loss of visual acuity and hypotony maculopathy (see below). Treatment should be aimed at correcting the specific cause of hypotony. When there is a flat anterior chamber with lens–corneal touch, immediate surgical intervention is necessary to prevent endothelial damage and cataract formation. Reformation of the anterior chamber with viscoelastic can be done at the slit lamp or under the operating microscope through the paracentesis made intraoperatively. When there are large appositional choroidal effusions, drainage of the fluid is recommended (Fig. 35.16).

Fig. 35.16 Choroidal effusions will usually spontaneously resorb; however, if the bleb is failing, and the chamber is shallow, they should be drained.

Most commonly hypotony results from over-filtration of a filtering bleb. Treatment options are available. Several methods have been reported to induce an inflammatory or healing reaction in the filtering bleb, which modifies the morphology of the filtering bleb and increases IOP75–89. Surgical revision is the most efficacious option. Resuturing the scleral flap (occasionally with scleral patch grafting when resuturing is not possible) is our favored option. Resuturing the scleral flap can be done through the conjunctiva.

Some patients with intraocular hypotony develop loss of central vision secondary to marked irregular folding of the choroid and retina. The incidence of hypotony maculopathy after glaucoma surgery has increased with the use of anti-fibrotic agents, specifically mitomycin C. The maculopathy is most likely to occur in young myopic patients (especially males), who may have a sclera more susceptible to swelling and contraction. The retina often shows a series of stellate folds around the center of the fovea. The retinal vessels are tortuous and sometimes engorged. There may be swelling of the peripapillary choroid simulating papilledema. Visual loss is due to the marked folding of the central retina. Early detection of this condition is key, since prompt correction of the cause will usually result in visual improvement.

‘Malignant’ glaucoma

Malignant glaucoma (also called aqueous misdirection or ciliary block glaucoma) is characterized by increased IOP, and shallowing or flattening of the anterior chamber without pupillary block (i.e., in presence of a patent iridectomy) or chorio-retinal pathology such as suprachoroidal hemorrhage. It is more common after surgery for angle closure, in phakic, hyperopic (small) eyes. In this condition increased pressure within the vitreous cavity displaces the lens forward, and shallows the anterior chamber90 (Fig. 35.17).

Fig. 35.17 Aqueous misdirection or malignant glaucoma presents with a flat chamber in the presence of a patent iridectomy and an intraocular pressure higher than expected in the presence of a shallow anterior chamber.

Malignant glaucoma usually occurs in the early postoperative period, and is initially managed with mydriatic–cycloplegic drops, aqueous suppressants, and hyperosmotic agents. If the condition is relieved (e.g. the anterior chamber has deepened), the hyperosmotic agents are discontinued first, then the aqueous suppressants are reduced or even stopped over several days. Cycloplegic drops should be continued for weeks to months.

In pseudophakic eyes a peripheral hyaloidotomy with the Nd:YAG laser may be efficient and can often be accomplished through an existing peripheral iridectomy91,92. If not successful, zonulo-hyaloido-vitrectomy via the anterior segment has been used successfully in a series of pseudophakic patients93. Pars plana vitrectomy should be considered when other therapies fail94,95, removing the anterior vitreous and part of the anterior hyaloid. Pars plana tube-shunt insertion with vitrectomy has been recommended to treat patients with aqueous misdirection, especially in cases with angle closure glaucoma95.

To prevent malignant glaucoma, intra- and postoperative shallowing of the anterior chamber should be minimized. Over-filtration should be avoided; judicious suture lysis or cutting/pulling releasable sutures and slow tapering of cycloplegics are recommended. A postoperative shallow anterior chamber due to over-filtration should be vigorously treated.

Wipe-out

The phenomenon of severe visual loss after surgery, with no obvious cause, is known as ‘wipe-out’ or ‘snuff syndrome’ (Fig. 35.18). Wipe-out may affect patients who have very severe glaucomatous damage; while it is uncommon, it remains an important concern among glaucoma surgeons96–99.

Fig. 35.18 Wipe-out of the central vision and visual field is a rare occurrence noted after guarded filtering surgery in eyes with very advanced optic nerve changes.

Possible mechanisms for this wipe-out include: direct damage to the optic nerve from the anesthetic technique (local pressure from a hematoma or simply from the volume of anesthetic injected), an IOP spike, decreased blood flow to the optic nerve, and postoperative hypotony100–103.

Early bleb leak

An inadvertent buttonhole in the conjunctiva during a filtering procedure or a wound leak, through the conjunctival incision, can lead to an early leaking bleb69–71,77. Bleb leaks are more likely with extensive conjunctival scarring. Early bleb leaks usually heal spontaneously, but if there are associated complications such as shallow or flat anterior chamber, then active intervention is recommended. Our preferred strategies to treat leaking blebs include bandage contact lens, fibrin tissue glue, and surgical revision (see above).

Late bleb leak

Spontaneous late bleb leaks are more frequent in avascular, thin blebs, which occur most often when anti-metabolites are used in the filtering procedure. Leakage of the filtering bleb can be associated with hypotony, and may increase the chances for bleb infection and subsequent endophthalmitis77 (Fig. 35.19).

Fig. 35.19 Pinpoint bleb leak is accompanied by a fluid wave seen under the edge of the bleb, made visible with fluorescein.

Bleb leaks can heal spontaneously and conservative management with prophylactic antibiotics is recommended if there are no associated problems. Multiple techniques have been proposed to resolve a non-healing bleb leak104–111. We recommend surgical revision, attempting to save the established initial filtration site. The ischemic and thin-walled bleb tissue is excised, or denuded of conjunctival epithelium by blade debridement or cautery to allow long-term adherence of grafted conjunctiva, or fresh conjunctiva adjacent to the bleb is mobilized and sutured to cover the de-epithelialized bleb by rotational, sliding, or free conjunctival grafts.

Hypotony associated with bleb leak presents difficult challenges. To plan a surgical intervention, it must be determined whether the hypotony is caused directly by the leak or not. In the latter situation, additional sutures to the scleral flap or scleral reinforcement with a patch graft would be necessary. The use of a donor patch graft (e.g. sclera, pericardium or cornea) is likely to compromise the function of the filtering bleb.

Encapsulated blebs

Encapsulated blebs are localized, elevated, and tense, with vascular engorgement of the overlying conjunctiva and thick connective tissue. Commonly appearing within 2–6 weeks after surgery, bleb encapsulation leads to a rise of IOP after a period of control112–116. Initial management of encapsulated blebs includes anti-glaucoma medications and topical steroids. When surgical revision is needed the simplest technique is to cut the fibrotic wall with a 27-gauge needle, a Ziegler knife, or an MVR blade and apply anti-fibrotic agents.

Symptomatic blebs

Filtering blebs are usually asymptomatic. Some patients have discomfort, which is most common with nasal, large blebs extending onto the cornea (Fig. 35.20). Tear film abnormalities with Dellen formation and superficial punctate keratopathy may occur. Artificial tears and ocular lubricants can be helpful, especially in cases with an abnormal tear film. Partial surgical excision, or trans-conjunctival compression sutures, is usually helpful although bleb failure is possible. Large blebs that extend onto the cornea can be freed by blunt dissection. The corneal extension can be excised with a cut parallel to the limbus, usually with excellent results, and without leakage.

Fig. 35.20 Large nasal dissecting bleb causing persistent ocular discomfort and dellen.

Bleb-related ocular infection

Ocular infections related to filtration procedures can occur months to years after the initial surgery116–126. The incidence of bleb-related ocular infections after filtration procedures ranges from 0.2 to 1.5% after mid- to long-term follow-up. Thin bleb wall and most importantly bleb leaks are associated with a higher risk of infection. A surprising risk factor is long-term use of topical antibiotics. Bleb-related ocular infections can affect the subconjunctival space, the anterior segment, and the vitreous cavity.

Patients with bleb-related ocular infection usually present with ocular pain, photophobia, blurring, tearing, redness, and purulent discharge. Examination often reveals conjunctival and ciliary injection, most intense around the bleb edge, a milky-white appearance of the bleb, purulent discharge, and variable intense periorbital chemosis. If the anterior chamber is involved there is corneal edema, cells and flare, keratic precipitates and, in some cases, hypopyon. The most common causative organisms are Streptococcus and Staphylococcus. A positive Seidel’s test is common.

Apply the general principles that guide management of ocular infections. In blebitis without anterior chamber reaction, frequent topical application of a commercially available broad-spectrum antibiotic can be used, with very close supervision. If there are some cells in the anterior chamber, treatment with fortified topical antibiotics around the clock is advisable. If there is severe cellular activity in the anterior chamber, hypopyon, or vitreous involvement a vitreous sample should be obtained, with injection of intravitreal antibiotics.

Prevention of bleb-related ocular infection is important. It seems reasonable to use antibiotics in some cases of leaking blebs, or recurrent bleb-related infections, but routine long-term use of topical antibiotics is not recommended. Conjunctivitis and blepharitis should be treated promptly, and soft contact lens wear should be avoided, or at the very least maintained meticulously. Patient education about early symptoms and signs of infection, with rapid access to appropriate care, is currently most important to minimize chances of severe visual loss.