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.