Laser treatment of the trabecular meshwork (laser trabeculoplasty) Laser trabeculoplasty, laser burns of the trabecular meshwork under

microscopic control, is not usually successful in controlling open angle glaucoma permanently by itself (Figures 12, 13) although it is frequently performed. The Glaucoma Laser Treatment Trial (GLT) compared topical timolol (β-blocker) and argon laser trabeculoplasty (ALT) as initial therapy, and found similar results after 7 years⁷⁸. A full treatment of selective laser trabeculoplasty (SLT), (360°) compared with topical latanoprost as initial treatment showed no difference in IOP control at 1 year of follow-up⁷⁹. However, IOP spikes, hemorrhage, uveitis, scarring of the trabecular meshwork, corneal and lens damage can occur as a result of laser treatment of the trabecular meshwork.

Figure 12. Argon laser trabeculoplasty (from Yanoff & Duker, Ophthalmology, 3^rd edition, Elsevier 2009).

Figure 13. Argon versus selective laser trabeculoplasty. Note the larger SLT spots (from Yanoff & Duker, Ophthalmology, 3^rd edition, Elsevier 2009).

C. Surgery

1. Trabeculectomy

Trabeculectomy (Figure 14) is a surgical procedure to create a fistula from the anterior chamber to the potential subconjunctival space; the fistula is partially protected by a partial thickness scleral flap which offers some resistance to flow in order to try to prevent overfiltration. The goal is for the fistula to form a filtering bleb or conjunctival bulla from which aqueous humour is absorbed into the surrounding ocular tissues thereby reducing IOP. Modalities to alter wound-healing fibrosis in the subconjunctival space, such as antimetabolite (5-fluorouracil and mitomycin C) usage^80-86 and laser suture-lysis⁸⁷ have been used to try to improve postoperative success. However, while trabeculectomy is considered the “gold standard” for glaucoma surgeries there is a very significant complication and failure rate^80-86

Figure 14. Limbal based trabeculectomy (from Yanoff & Duker, Ophthalmology, 3rd ed, Elsevier 2009)

The risks and complications of trabeculectomy include:

 Intraoperative and early postoperative: suprachoroidal haemorrhage, infection, hypotony, shallow chamber, choroidal effusions, bleb leaks, aqueous

misdirection, cataract, discomfort, cystoid macular oedema, astigmatism, hypotony, maculopathy, vision loss, and commonly failure with need for additional surgery.

 Late postoperative: discomfort, aqueous leaks, intraocular infection, failure, cataract development or progression, ptosis, permanent vision decrease or loss, need for additional surgery.

2. Non-penetrating glaucoma surgery - NPGS (deep sclerectomy and viscocanalostomy)

In order to try to reduce complications associated with trabeculectomy surgery, non-penetrating filtering procedures have been developed to reduce intraocular pressure by enhancing the natural aqueous outflow channels reducing outflow resistance located in the inner wall of the Schlemm's canal and the juxtacanalicular trabecular meshwork⁸⁸. In the last few years viscocanalostomy and deep sclerectomy have become the most popular non-penetrating filtering procedures. Both involve removal of a deep scleral flap, the external wall of Schlemm's canal and corneal stroma behind the anterior trabeculum and Descemet's membrane, thus creating an intrascleral space (Figure 15). After aqueous humour passage through the intact trabeculo-Descemet‟s membrane, four mechanisms of aqueous resorption may occur; a subconjunctival filtering bleb, an intrascleral filtering bleb, a suprachoroidal filtration, and an episcleral vein outflow via Schlemm‟s canal⁸⁸.

Figure 15. Creation of a superficial and deep scleral flap (left). Peeling of the inner wall of Sclemm's canal and of juxtacanalicular trabeculum using fine forceps (right)⁸⁸.

Nonpenetrating glaucoma surgery is contraindicated in the presence of narrow angles because of complications of iris incarceration or anterior synechia, thus limiting the potential candidates for these procedures. Contraindications for NPGS not only include narrow-angles, but status post laser trabeculoplasty, post-traumatic angle-recession glaucoma and neovascular glaucoma. The technique is associated with a long learning curve. Published clinical trials comparing nonpenetrating

glaucoma surgery to full-thickness trabeculectomy have a consensus on the superior safety profile of non-penetrating glaucoma surgery but are not in agreement when it comes to efficacy⁸⁸.

3. Drainage implants (tubes and valves)

Drainage implants (Figure 16) are devices that produce a fistula from the acterior chamber via a tube with a pressure regulating valve or a tube with a resevoir. Such implants provide a good surgical option for many eyes in which there is a high probability of trabeculectomy failure from extensive conjunctival scarring and or previous failed procedures or the type of underlying glaucoma (neovascular, uveitic, iridocorneal endothelial (ICE) syndrome, etc). A role for these implants may exist at an earlier stage for some patients, particularly with the

increased concern about postoperative late bleb leaks and risks of late endophthalmitis following the use of antimetabolites; however the surgery is frequently more complex, involves use of a permanent foreign material, and has a significant rate of complications.

A recent trabeculectomy vs tube (TVT) study⁸⁹ reports IOP levels as low as trabeculectomy with tubes but with more postoperative medication needed. Tube surgery had a higher success rate compared to trabeculectomy with mitomycine (MMC) during the first 3 years of follow-up. Both procedures were associated with similar IOP reduction and use of supplemental medical therapy at 3 years. While the incidence of postoperative complications was higher following trabeculectomy with MMC relative to tube shunt surgery, most complications were transient and self-limited.

Figure 16. Silicone tube is inserted through 22- or 23-gauge needle opening at limbus. Note distal end of Supramid suture that protrudes forward beneath anterior cut edge of the

conjunctiva and the tube of the previously placed implant. (from Yanoff & Duker, Ophthalmology, 3rd ed, Elsevier, 2009).

Intraoperative complications include bleeding, misdirection of silicone tube, and loss of anterior chamber. Early postoperative complications include hypotony, with or without associated choroidal effusions. Hemorrhagic effusions are often

associated with pain, even though the eye may remain hypotonic. Cataract and infection may occur. Tube-corneal contact may occur leading to corneal

decompensation. Occlusion of the tube by vitreous may occur in aphakic eyes.

Late complications include the development of a thick capsule around the plate, which results in an elevation of IOP, erosion of the silicone tube through the sclera or scleral patch and conjunctiva, plate migration, limitation of eye movement, infection, retinal detachment, sterile hypopyon, and infection.