Trabeculectomy1

Trabeculectomy is the most widely used surgical option for glaucoma at a global level. The reasons for its success are its efficacy, relatively low cost and vast experience. Since this surgery has been performed for over 40 years, and therefore there is immense accumulated knowledge of its results, complications and resolution of these complications. The need to avoid the complications and the development of better surgical equipment have led to several changes and improvements to the original technique, described by Cairns in 1968.¹

The most common clinical indication for trabeculectomy is disease progression despite maximum medical and laser therapies tolerated by the patient³ (Table 1.2). Such surgery is often chosen when it is anticipated that it will not be possible to manage the clinical case using the usual medical alternatives. This is especially the case for very young patients, a very high IOP or a very advanced disease stage on diagnosis.² Trabeculectomy can also be selected to end a refractory hypertensive acute event or as an initial approach for congenital glaucoma (as an addition to trabeculotomy).

Furthermore, when the risk of vision loss (for surgical complication) is high and may significantly affect quality of life, as in single eye cases or potential loss of professional activity, a trabeculectomy might not be the best option.² In these cases, a usually safer (although less effective) operation could be performed. Other cases of relative contraindication for trabeculectomy are those where failure or severe complication risks are predictably very high, such as in neovascular or uveitic glaucoma. Here, a drainage device can be implanted, which is usually more effective (and safer, when valved).

Trust in their ophthalmologist or surgical team is the most reassuring aspect for the patient. Trust is gained when patients receive a good explanation of the state of their disease, surgical therapy proposed and surgery goal (preserving vision), not to mention the potential risks, including vision or even eye loss. A careful, thorough ophthalmological and medical clinical history must be taken. A full ophthalmological examination is also needed.

The ab externo trabeculectomy described here is a variant of the Cairns trabeculectomy as modified by Watson. This is the first-line surgery for glaucoma, and the operation which we perform the most in our glaucoma clinic. The goal is to obtain a protected fistula between the anterior chamber (AC) and the subconjunctival space.

In most cases, surgery can be performed with retrobulbar or peribulbar locoregional block by injecting 0.5% bupivacaine (which can be combined with 2% lidocaine). In these cases, the patient generally receives prior sedation and topical anesthesia from an anesthetist. In addition, general anesthesia is often used, especially in pediatric patients, young adults or noncollaborating patients.

Before this step, traction of the superior rectus muscle is performed by passing a suture (silk 4/0) below the muscle and tying a knot with the loose suture ends around a Kocher's forceps. The knot will be hanging and ensure muscle traction during surgery, thereby facilitating a better exposure of the upper perilimbic area. Conjunctival opening is performed with Wescott scissors at approximately 7 mm from the limbus, corresponding to a limbus-based conjunctival flap (Fig. 1.1).

Upper location, around 12 o'clock, is for the filtration bleb to remain under the upper lid. This reduces infection rate as well as leaving an adjacent space for a further trabeculectomy or placing a drainage device tube in case of failure. Scleromalacia areas or large blood vessels are avoided. After “cleaning” the Tenon capsule, hemostasis of the cut sclera was obtained by vessel cauterization with a bipolar device. The flap is achieved by quadrangular incision (4 × 4 mm) with limbic hinge using a Beaver mini-blade, to obtain a flap thickness of about 1/3 to 1/2 of the scleral thickness and to expose the gray area of the corneal-scleral transition (Fig. 1.2).

This is performed at the beginning of the operation to slowly decompress the AC, thus avoiding sudden decompression at the time of trabeculectomy. It is performed using a 15° blade in the cornea, next to the temporal limbus.

Using the same blade, a rectangular incision (1.5 × 3 mm) is made perpendicular to the ocular surface, which includes the corneal-scleral transition area. The forceps clamping the rectus muscle are removed to prevent the corresponding pressure over the ocular globe. After entering the AC, the sclerectomy is completed with Vannas scissors, maintaining a balanced salt solution (BSS) drip over the area to help hemostasis (Fig. 1.3).

Peripheral iridectomy is performed clamping the iris with a colibri forceps and cutting with scissors (Fig. 1.4). The AC is replenished with BSS. The scleral flap is sutured (Nylon 10 or 9/0) with a stitch in each corner of the flap. The suture should be tightened so that it can coapt the flap and allow it to drain humor from the opening.

A continuous, tight closure suture (with Vicryl or Nylon) is applied to ensure leakproofness of the filtration bleb (watertight suture, Fig. 1.5).

The most important factor in long-term IOP control is surgical wound scarring. As inhibitors of the scarring process, antimetabolites considerably increase the efficacy of classic trabeculectomy.⁴ However, this comes at a price: a higher incidence of complications such as infection, hypotony, and thin, cystic blebs with fluid leaks³ (Table 1.4).

Therefore, glaucoma cases in which the risk-benefit ratio favors the use of antimetabolites (Table 1.5) are the most difficult and more prone to failure: uveitic, neovascular, traumatic, congenital, and infantile-juvenile glaucoma, as well as previous surgery. Currently, 5-fluorouracil and mitomycin C (MMC) are the most commonly used antimetabolites. The author uses MMC (0.2 mg/mL) on a small sponge, which he applies for 1–2 minutes over the sclera and scleral bed of the flap before sclerectomy. Caution is essential when handling MMC and the area must be flushed with plenty of BSS after application.

Implants made of reabsorbable materials (such as OculusGen and Ologen) are available. These can be easily applied next to the scleral flap to delay scarring and facilitate formation of a good filtration bleb.

Neovascular glaucoma is a condition with a poor surgical prognosis due to vascularization, high associated pressures and fast postoperative scarring. Anti-vascular-endothelial growth factor (VEGFs) inhibitors injected intraocularly 1 week before trabeculectomy cause marked neovascularization regression, which gives surgeons a time frame for higher surgical safety, especially preventing perioperative bleeding.

Hyphema is very frequent, especially if the ciliary body is damaged during the trabeculectomy itself. In general, intraoperative bleeding can be prevented if antiaggregation or anticoagulation medication is suspended approximately 10 days beforehand. Another cause of intraocular bleeding is sudden ocular decompression, which can also be minimized by paracentesis. This mostly happens in very high preoperative IOP cases, which can be treated with oral acetazolamide or by injecting a hyperosmotic agent immediately before surgery (125 cc mannitol 20%, IV). If the eye is inflamed, a topical vasoconstrictor drug (e.g. brimonidine) can also be applied immediately before surgery. Balanced salt solution should be constantly dripped during and immediately after sclerectomy in that area to facilitate hemostasis. The hyphema must be suctioned and the AC replenished with BSS, or preferably air, since this is more effective in anterior segment hemostasis. Most of these considerations also apply to the prevention of hemorrhagic choroidal detachment.

Another common complication is intraoperative athalamia, which also occurs after sudden decompression or when larger-scale trabeculectomy is performed, or even due to careless pulling of eye structures or unnecessary pressure on the eye. The AC must be restored as often as needed throughout the surgery. If there is a risk of iridocorneal or corneal-lenticular contact at the end of the operation, and it is impossible to restore the AC with BSS or air, a viscoelastic material can be injected to maintain the chamber and/or the scleral flap can be further sutured, taking care to hydrate the edges of the paracentesis entry port.

Although relatively rare, vitreous loss is a complication which may compromise the success of this surgical technique. It happens most often when vitreous is already present in the anterior segment following trauma or complicated cataract surgery. However, vitreous loss may occur without previous vitreous identification, especially in a pseudophakic eye. A careful, open vitrectomy should be performed using Wecker scissors.

Although expulsive hemorrhage is one of the rarest complications, it is certainly the most feared of all. It is a form of rapid-onset suprachoroidal hemorrhage in the intraoperative period,⁵ and it is most common in aphakic and pseudophakic eyes. Any ocular incision must be closed as soon as possible and a posterior sclerotomy must be performed at once to drain blood and avoid extrusion of intraocular contents.

The success of a trabeculectomy does not depend merely on an uneventful procedure, the immediate postoperative follow-up is also important,⁶ particularly for the detection and resolution of common complications⁷ (Table 1.6).

This usually results from hyperfiltration and upon rest only, the normal height in the AC chamber is restored and IOP rises within two weeks after surgery. In case of athalamia with iridocorneal apposition the chamber must be restored as soon as possible. This can be achieved through the surgical paracentesis entry port to avoid corneal decompensation. When hypothalamia is caused by fluid loss through a conjunctival gap or suture dehiscence, the safest approach is to suture the lesion.

Anterior segment bleeding may occur in the first 3–5 days after surgery and it usually resolves with no particular intervention rather than rest. Large hyphemas more often cause clots, which are usually drained by flushing the AC. This can be performed through a 2 mm paracentesis.

This complication, which is frequently associated with hypotony and hypothalamia,⁸ may be exudative or hemorrhagic. Spontaneous resolution is common when resolving hypothalamia, with rest, cycloplegia and corticoid therapy. However, when it is persistent and significant, it may require surgical drainage by sclerotomy.

This complication accompanies persistent hypotony and causes vision loss. Choroidal folds can generally be observed with fundoscopy. Therefore, this may happen soon after trabeculectomy, or later. First of all, the cause of hypotony must be known so that the right treatment is chosen. It may be necessary to resuture the scleral flap to raise the IOP as quickly as possible.

This rare complication occurs especially in chronic closed-angle glaucoma cases treated surgically, as well as in microphthalmia. It is generally due to an increase in volume and pressure behind the iridolenticular diaphragm, which in turn may have several causes: pupillary block due to incomplete iridectomy, choroid or suprachoroidal space expansion (by exudate or blood), and ciliary block.

Fortunately, this is a rare event. However, it happens more frequently in some types of eyes: traumatized, vitrectomized, aphakic, highly myopic (>26 mm) or congenital glaucoma. Risk factors are also anticoagulant therapy or preoperative IOP (>35 mm Hg). Clinically, it occurs within 4–5 days after surgery and patients complain of painful vision loss, with high IOP. Resolution is often achieved by drainage through posterior sclerotomy.

This is another rare complication which may occur in the immediate postoperative period or many years after trabeculectomy. The risk factors are: antimetabolite use,¹⁰ very thin or leaky blebs and blebs outside the upper fornix. When the infection is confined to the bleb with minimal AC reaction, this is usually called blebitis and generally responds to antibiotics.¹¹ In cases of posterior segment invasion and a classical picture of endophthalmitis, aspirative vitreous biopsy and intraocular injection of antibiotics are required.

Filtration block may occur anytime during the first weeks after surgery. This may result from surgical complications (usually occurring within the first week) or simply from vigorous scarring of operated tissues. If the AC is well formed, a gonioscopy should be performed to check if anything is blocking the trabeculectomy exit (such as a clot or part of the iris). If blockage is caused by the iris, treatment with pilocarpine is initiated; if this fails, argon laser is applied in the iris to the surgical wound. A clot may be released by applying pressure with a spatula in the posterior area of the sclerotomy. When nothing seems to be blocking the exit, there may be an overtight suture in the scleral flap, which can be resolved by laser suture lysis. Flap scarring may also be present. In the early postoperative period, the filtration process may be rescued by applying what is known as ocular massage.¹² The patient is shown how to perform anteroposterior digital pressure on the upper lid, over the bleb, and to repeat this several times a day.

This complication occurs in approximately 10% of cases, usually in the 2nd and 5th week postoperatively. The bleb is well-formed, inflamed and tense, with a well-formed AC and gradually increasing intraocular tension. Such bleb encysting is more common with a limbus-based conjunctival flap and is often resolved with drugs which decrease the production of aqueous humor and topical corticoid therapy.¹³ When encysting cannot be resolved medically, an intervention known as needling is performed.¹⁴

This intervention is performed in the surgical block, in an outpatient setting and under topical anesthesia with prior injection of 2% lidocaine in the conjunctiva next to the bleb. A 27-gauge needle (in a 2 mL syringe) is inserted in the conjunctiva, posterior to the bleb. The needle is advanced with lateral movements in the edge of the bleb. If the needle point and scleral flap can be seen clearly, patency of the trabeculectomy can be confirmed and a point can be cut from the flap, if indicated. Finally, 5-fluorouracil¹⁵ or MMC can be injected (0.1 mL injection of 0.02 mg/mL MMC), laterally into the bleb, taking care to avoid the drug entering the AC.

This complication is usually associated with a hyperfiltrating, thin-walled, cystic bleb which can be easily perforated, resulting in leakage of aqueous humor and higher susceptibility to infection¹⁶ and hypotonic maculopathy (Table 1.6). These blebs are more frequent when antimetabolites are applied.¹⁷ Clinically, the patient may complain of vague discomfort, watering eyes especially at night, and oscillopsia. Sometimes it is not easy to detect the leak, but a Seidel test is generally positive. Small orifices can be closed by applying eye drops to diminish the production of aqueous humor, topical antibiotic therapy and a therapeutic contact lens, and the patient should be advised to return as soon as signs of infection appear, since infection is the main risk of this condition. For large leak orifices or when the previous therapy has failed, several other therapies have been described: autologous blood injection in the bleb,¹⁸ compressive suture, application of trichloroacetic acid or of the so-called “biological glue”. These measures often fail or are only temporarily successful, requiring surgical revision with conjunctivoplasty after excision of most of the cystic bleb.

The incidence of cataract formation is higher after trabeculectomy than after other glaucoma surgical procedures, such as nonpenetrating glaucoma surgery.³ This seems to be due to a higher complication rate involving the anterior segment.

Two-year success rates for trabeculectomy are approximately 90% in surgery-naive eyes operated on by experienced surgeons.¹⁹ However, the ophthalmologist must weigh the risks and benefits expected from the trabeculectomy for each particular case. The use of antimetabolites has significantly increased trabeculectomy efficacy, but it must be borne in mind that complications are also more frequent.