Loay Nawaz Rahman

Imperial College Healthcare NHS Trust, London, United Kingdom

Definition

Glaucoma is a common eye condition which involves optic nerve damage, resulting in progressive peripheral vision loss. It is most commonly caused by raised intraocular pressure (IOP), therefore most medical and surgical treatments aim to lower the IOP by either reducing production or increasing drainage of aqueous humour in the anterior chamber of the eye (1). It is important to note that glaucoma management is centred around prevention of further vision loss rather than restoration of vision that has already been lost, and thus early diagnosis and management are paramount.

Trabeculectomy is a form of glaucoma surgery which aims to increase outflow of aqueous humour by creating an alternative drainage pathway. The fluid collects in the subconjunctival space to create a small blister of fluid called a bleb which is then absorbed by blood vessels. The bleb is typically underneath the upper eyelid (2).

Indications

In glaucoma, surgical options are considered when medical management is insufficient in controlling the progression of disease. Trabeculectomy is usually an operative option in cases where there is rapid glaucoma progression, moderate to advanced glaucoma or previous failed laser glaucoma surgery.

Procedural Steps

Trabeculectomy is a multistep procedure which involves creating a small opening through the sclera into the anterior chamber called a sclerostomy, and removing a portion of the iris to increase aqueous humour outflow, termed peripheral iridectomy (3).

Anaesthesia

Although the surgery can be performed under general anaesthesia, it is more commonly undertaken using local anaesthetic injections. These include use of a sub-Tenon block, where the anaesthetic agent is injected between the sclera and Tenon capsule, and retrobulbar block, where the needle is inserted perpendicular to the inferior eyelid, avoiding penetration of the globe, and the agent is injected towards the orbital floor. The injections have the added benefit of achieving extraocular muscle paralysis alongside anaesthesia. The eyelids are then retracted using a speculum to allow access to the eye for the operation (4).

Traction suture

A partial thickness suture is placed in the peripheral cornea at the 12 o’clock position with the ends clipped to the drape overlying the patient’s cheek. This ensures the eye is fixed in the downward position to expose the superior aspect of the sclera.

Conjunctival peritomy

An incision is made at the limbus, the border between the cornea and sclera, using Vanna’s scissors at the 12 o’clock position. These scissors are placed under the incision to bluntly dissect the conjunctiva and Tenon capsule away to allow access to the sclera. The space between the Tenon capsule and sclera is referred to as the wound bed.

Cauterisation

A bipolar diathermy is used to cauterise bleeding vessels on the scleral bed. Excessive cautery should be avoided to prevent contracture of the scleral bed.

Cytotoxic agent

The wound bed is treated with swabs soaked in antimetabolites such as mitomycin C (MMC) or 5-fluorouracil (5-FU) for three minutes. These agents prevent scleral scarring and subsequent bleb failure through different mechanisms; MMC works by inhibiting fibroblast proliferation, whereas 5-FU blocks DNA synthesis. The eye is then irrigated thoroughly with saline.

Scleral flap dissection

A partial thickness flap hinged at the corneal limbus is created in the superior sclera using a crescent blade. It is commonly rectangular in shape and 4mm wide. A suture is inserted lateral to the scleral flap 2mm from the limbus and passed through to the cornea at partial thickness. This suture is then passed back through to the scleral flap from the cornea, and then into the scleral bed at the corner of the flap. This creates a releasable suture. A second releasable suture is created on the opposite side of the flap.

Paracentesis

Paracentesis involves passing a 30-degree blade through the part of the sclera near the limbus into the anterior chamber, usually on the temporal side at the 9 o’clock position. The paracentesis allows for irrigation of the anterior chamber with saline to maintain its structure.

Sclerostomy

An incision is made under the scleral flap hinge into the anterior chamber, usually half the diameter of the hinge. A Kelly’s punch is then passed through the incision and punched to create the sclerostomy.

Peripheral iridectomy

In most cases, a peripheral iridectomy is created, where a portion of the iris is removed to increase aqueous humour outflow. This is performed by pulling the iris through the sclerostomy using forceps and removing a portion of it using Vanna’s scissors. The iris is then released from the forceps hold back into the anterior chamber.

Scleral sutures

The releasable sutures are tied to ensure closure of the scleral flap. They can also be easily removed in order to reduce IOP post-operatively if needed. An interrupted suture is then placed on the lateral side of the flap using a 10-0 nylon suture which is then rotated to ensure that the knot is hidden, reducing the risk of inflammation and conjunctival leak. The flap function is tested by placing a gauze at the edge of the flap to assess aqueous humour flow out of the anterior chamber. Following this, the traction suture is removed.

Conjunctival sutures

The conjunctiva above the flap is secured together using mattress sutures.

Subconjunctival injection

Prior to completing the surgery, antibiotic and steroid injections are inserted inferiorly avoiding the trabeculectomy site to reduce inflammation and risk of infection.

Post-operative care and long-term outcomes

Trabeculectomy is usually a day case procedure with the patient returning home on the same day. Patients are usually examined in the outpatient clinic the next day. They are then reviewed weekly for the first four weeks, but more frequently if the IOP is too low or high. The releasable sutures may be adjusted or removed to control the IOP and additional injections of steroids or 5-FU may be considered to counteract scarring of the bleb.

Patients are prescribed topical steroid (e.g. dexamethasone) and antibiotic (e.g. chloramphenicol) drops for the first month. Glaucoma medications may be discontinued to improve flow of aqueous humour to the bleb. Patients are advised to wear eye shield overnight for the first two weeks to prevent accidental damage to the operative site as well as avoiding strenuous activity and bending in the initial post-operative stage. Full vision may take up to three months to return, so patients are advised to avoid changing spectacles in the first three months post-operatively (5).

Trabeculectomy has a very high success rate where one study showed 90% efficacy after 20 years, with two-thirds of these patients no longer requiring glaucoma medication and only 10% needing a revision of their trabeculectomy (6).

Complications

Peri-operative complications

Intraoperative bleeding

Patients with hypertension, diabetes mellitus or on oral anticoagulants are at higher risk of conjunctival bleeding which typically resolves spontaneously. Scleral bleeding may also occur and can be terminated through direct pressure for example with cotton tip applicators (7). There is a risk of developing a retrobulbar haemorrhage when inserting the needle during anaesthesia. It can present with proptosis, subconjunctival haemorrhage and discolouration of the lids, and warrants immediate management by administering intravenous mannitol and conducting a lateral canthotomy. The trabeculectomy should be delayed until there is complete absorption of the blood.

Conjunctival hole

These may occur during the operation through use of tooth forceps on or poor handling of the conjunctiva. It is important to identify this intraoperatively and to treat immediately by closing the hole with sutures to prevent hypotony or scarring of the bleb.

Early Post-operative complications

High intraocular pressure with a deep anterior chamber

This occurs when there is inadequate aqueous outflow to the bleb. It is frequently due to a tight scleral flap, but it is important to ensure gonioscopy is performed to rule out other causes of obstruction to the sclerostomy site. To resolve this, the edges of the scleral flap are separated with digital pressure or by using a cotton tip applicator. If the IOP remains high, then releasable sutures can be removed to control the pressure (8).

Another important cause of high IOP with a deep anterior chamber is bleb encapsulation, also known as a Tenon’s cyst, which commonly presents two to four weeks post-operatively with a tight bleb. Bleb needling, where a needle is used to breakdown the bleb wall, can be performed followed by insertion of antimetabolites such as 5-FU or MMC to prevent further scar formation. If unsuccessful, then a bleb revision or repeat trabeculectomy may be considered.

High intraocular pressure with a shallow anterior chamber

The three main causes of this include pupillary block, suprachoroidal haemorrhage and aqueous misdirection.

Pupillary blocks can be treated with laser or surgical iridectomy to increase aqueous outflow.

A suprachoroidal haemorrhage occurs due to rupture of the long posterior ciliary artery. Typically, patients will complain of acute pain, nausea and loss of visual acuity following a recent trabeculectomy. Slit lamp examination would reveal a loss of red reflex, a flat anterior chamber and brown choroidal elevations, and ultrasound examination may reveal blood in the suprachoroidal space. Management revolves around the use of topical or systemic steroids for small bleeds, and drainage for large ones (9).

Aqueous misdirection refers to the diversion of aqueous humour to the posterior chamber which pools in the vitreous cavity. It is usually due to conjunctival leaks or overfiltration after suture removal. The IOP may be normal or high, and the anterior chamber flattens as the aqueous humour is diverted away from it. Medical management using aqueous suppressant and cycloplegic drops can be considered at first to help reduce the IOP. If still persistent then a laser may be applied to disrupt the anterior hyaloid (anterior border of the vitreous cavity) and shrink the ciliary body. An alternative, more invasive procedure that may be considered is pars plana vitrectomy with disruption of the anterior hyaloid. Both procedures ultimately aim to reverse the flow of aqueous humour back from the posterior chamber to the anterior chamber (9).

Low intraocular pressure with a shallow anterior chamber and flat bleb

The most common causes include conjunctival wound leak and serous choroidal detachment.

Conjunctival wound leaks need to be considered in cases of hypotony (low IOP) without a visible bleb. A small leak may resolve spontaneously or respond to conservative treatment that includes eye patching, use of topical aqueous suppressants to reduce leakage or antibiotic drops to induce scarring such as aminoglycosides. Larger leaks may require surgical closure.

Serous choroidal detachment (SCD) is the accumulation of serous fluid in the suprachoroidal space secondary to increased transmural pressure caused by hypotony. Once detected, it is initially managed conservatively with topical steroids and cycloplegics whilst identifying and treating the cause of hypotony. Surgical intervention is required if there is no resolution. This would involve choroidal drainage, where an incision is created through the sclera to the suprachoroidal space (10).

Low intraocular pressure with a shallow anterior chamber and elevated bleb

Typically, this is caused by excessive overfiltration and tends to resolve spontaneously. In cases where there is a very shallow anterior chamber or the presence of hypotony with choroidal effusions, cycloplegic and aqueous suppressant drops may be used.

Late post-operative complications

Chronic hypotony

This is defined as an IOP of less than 5mmHg that persists for longer than three months. It presents with reduced visual acuity and hypotony maculopathy, the latter presenting as choroidal folds and retinal striae on examination. The IOP may be increased through application of soft contact lenses, or by reducing the bleb size using cryotherapy or a laser. The patient may require further surgery to close the scleral flap if the previous method is unsuccessful (11).

Blebitis

Infection of the bleb, blebitis, presents with pain, blurred vision, a red eye, and milky-white appearance of the bleb. It commonly affects thin walled blebs and is more likely to occur in myopic and diabetic patients. Management revolves around early use of topical antibiotics as there is risk of progression to endophthalmitis which would require intravitreal antibiotics (12).

Bleb failure

Fibrosis can obstruct aqueous outflow leading to failure of the bleb. Bleb needling with antimetabolites can be performed to breakdown scar tissue formation and prevent further scar formation. If still persistent, the patient may need a revision of their trabeculectomy.

Symptomatic blebs

Most patients do not notice the bleb, but some complain of a foreign body sensation and visual disturbance. Artificial tears and ocular lubricants can help in smaller blebs but larger blebs may require compression sutures in order to reduce the height of the bleb (12).

Bleb leak

Thin-walled blebs are more prone to leaking causing inflammation. Conservative management with aqueous suppressants, broad spectrum antibiotics and soft contact lens application can help reduce both the leak and inflammation.

References

1. American Academy of Ophthalmology. Trabeculectomy [Internet]. 2021 [cited 28 December 2021]. Available from: https://eyewiki.aao.org/Trabeculectomy
2. John Hopkins Medicine. Trabeculectomy [Internet]. 2021 [cited 28 December 2021]. Available from: www.hopkinsmedicine.org/health/wellness-and-prevention/trabeculectomy
3. Murdoch I. How I approach trabeculectomy surgery. Community Eye Health. 2006;19(59):42-43.
4. Calenda E, Olle P, Muraine M, Brasseur G. Peribulbar anesthesia and sub-Tenon injection for vitreoretinal surgery: 300 cases. Acta Ophthalmologica Scandinavica. 2000;78(2):196-199
5. Keith Barton. Trabeculectomy. Glaucoma UK; 2021.
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7. Jampel HD, Musch DC, Gillespie BW, Lichter PR, Wright MM, Guire KE. Collaborative Initial Glaucoma treatment study group. Perioperative complications of trabeculectomy in the collaborative initial glaucoma treatment
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11. Costa VP, Wilson RP, Moster MR, Schmidt CM, Gandham S. Hypotony maculopathy following the use of topical topical mitomycin C in glaucoma filtration surgery Ophthalmic Surg. 1993;24:389–94
12. Azuara-Blanco A, Katz LJ. Dysfunctional filtering blebs Surv Ophthalmol. 1998;43:93–126