Residency · Residency · Ophthalmology

Glaucoma Laser Therapy: SLT, ALT, and Cyclophotocoagulation

Selective Laser Trabeculoplasty (SLT)

Mechanism of Action

SLT uses a frequency-doubled Q-switched Nd:YAG laser operating at 532 nm to selectively target melanin-containing cells within the trabecular meshwork. Unlike argon laser trabeculoplasty, SLT does not cause coagulative thermal damage or permanent structural alteration of the trabecular meshwork tissue. Instead, it triggers a biological response in which macrophages are recruited to the meshwork and cytokine-mediated remodeling of the outflow pathways enhances aqueous drainage. The IOP-lowering effect is therefore achieved through biological mechanisms rather than mechanical tissue destruction.

Technique

The procedure is performed at the slit lamp with a gonioscopy lens, such as the Latina SLT lens, applied to the cornea with a coupling agent. The spot size is 50 micrometers with a pulse duration of 0.3 nanoseconds. Energy is titrated from 0.3 to 1.4 mJ until small cavitation ("champagne") bubbles are just visible at the treatment site. Treatment can be applied over 360 degrees (approximately 100 spots) or 180 degrees; most studies have found no significant difference in efficacy between the two approaches.

Efficacy and Evidence

SLT produces an average IOP reduction of 20 to 30% from baseline. The landmark LiGHT trial, published in Lancet in 2019, compared SLT as first-line therapy against topical eye drops for newly diagnosed POAG and ocular hypertension. SLT was non-inferior to drops at 36 months, with 74% of SLT-treated eyes requiring no drops at all. SLT was also more cost-effective than topical therapy and offered a better adherence profile by eliminating the need for daily drops. The IOP-lowering effect of SLT typically lasts one to five years, and the procedure can be repeated when the effect wanes. While success rates may diminish with subsequent treatments, retreatment remains a viable option.

Indications

Based on the LiGHT trial evidence, SLT is now considered appropriate as first-line therapy for POAG and ocular hypertension. It is also used as adjunctive therapy when medications are insufficient, in patients with adherence difficulties, and in patients intolerant of topical medications due to BAK sensitivity or ocular surface disease. Eyes with pseudoexfoliation and pigmentary glaucoma often show an excellent response to SLT because of the heavy pigmentation in their trabecular meshwork.

Complications

The most common complication is a transient IOP spike, which is why IOP should be checked approximately one hour after the procedure. Mild anterior chamber inflammation is expected and self-limited. Rare complications include hyphema, corneal edema, and peripheral anterior synechiae formation.

<image>SLT laser application to the trabecular meshwork via goniolens, showing 360-degree treatment pattern with champagne bubble formation</image>

Argon Laser Trabeculoplasty (ALT)

Mechanism and Technique

ALT uses an argon laser at 488 to 514 nm wavelength applied to the anterior portion of the trabecular meshwork. Settings include a 50-micrometer spot size, 0.1-second duration, and power of 400 to 800 mW. Typically, 50 spots are applied over 180 degrees in the initial session. Unlike SLT, ALT creates thermal coagulative damage to the trabecular meshwork tissue. The proposed mechanisms of IOP lowering include mechanical opening of adjacent intertrabecular spaces due to tissue shrinkage and biological remodeling of the outflow pathways.

Efficacy

ALT reduces IOP by approximately 20 to 25% from baseline. The Glaucoma Laser Trial (GLT) demonstrated that ALT was as effective as timolol as initial therapy for open-angle glaucoma. However, the effect wanes significantly over time: approximately 50% of treated eyes fail by five years, and up to 90% fail by ten years. Retreatment with ALT is generally not recommended because of cumulative scarring of the trabecular meshwork.

Limitations Compared to SLT

ALT causes permanent structural damage to the trabecular meshwork, making it a one-time treatment that cannot be safely repeated. It carries a higher risk of PAS formation than SLT. For these reasons, ALT has been largely supplanted by SLT in modern clinical practice.

<image>Comparison of ALT and SLT trabecular meshwork effects: ALT showing coagulative thermal damage versus SLT showing selective melanocyte targeting without structural damage</image>

Cyclophotocoagulation

Transscleral Cyclophotocoagulation (TSCPC)

Transscleral cyclophotocoagulation uses a diode laser at 810 nm applied through the sclera to destroy the ciliary epithelium, reducing aqueous production. The G-probe is positioned 1.2 mm posterior to the limbus to target the ciliary body. Typical settings are 1500 to 2500 mW power with 2 to 4 seconds duration per spot, applying 15 to 20 spots while deliberately sparing the 3 and 9 o'clock positions to protect the long posterior ciliary arteries and their blood supply to the anterior segment. Traditional continuous-wave TSCPC is a destructive procedure and has historically been reserved for refractory or end-stage glaucoma with poor visual potential.

MicroPulse Transscleral Cyclophotocoagulation (MP-TSCPC)

MicroPulse delivery modifies the cyclophotocoagulation technique by delivering laser energy in short bursts with intervening off periods that allow tissue cooling between pulses. This makes the treatment less destructive than continuous-wave TSCPC. Typical settings use a duty cycle of 31.3%, power of 2000 mW, and treatment duration of 80 to 160 seconds per hemisphere. The probe is applied using a "sweeping" technique across the inferior and superior hemispheres. The lower risk of phthisis, chronic hypotony, and vision loss compared with traditional TSCPC means that MP-TSCPC can be offered to eyes with better visual potential, expanding the role of cyclodestruction beyond end-stage disease. The procedure can also be repeated with a good safety profile.

Endoscopic Cyclophotocoagulation (ECP)

ECP delivers diode laser energy via an intraocular endoscope that provides direct visualization of the ciliary processes. Because the laser is applied directly to the target tissue under visualization rather than transsclerally, lower energy is required. ECP is typically combined with cataract surgery, making it useful for patients with moderate glaucoma undergoing phacoemulsification. The risk of phthisis is lower than with transscleral approaches.

<image>Transscleral cyclophotocoagulation probe placement showing positioning 1.2 mm posterior to the limbus with avoidance of 3 and 9 o'clock meridians</image>

Glaucoma Laser Procedures Comparison

Clinical Decision-Making

Selecting the Appropriate Laser

SLT is the preferred laser for first-line or adjunctive treatment of open-angle glaucoma -- it is repeatable, safe, and well-tolerated. ALT is now primarily a historical option, largely replaced by SLT. Continuous-wave TSCPC is reserved for refractory glaucoma, eyes with poor visual potential, and neovascular glaucoma. MP-TSCPC is appropriate for refractory glaucoma in eyes with better visual potential and is finding an emerging role in moderate disease. ECP is best used in combination with cataract surgery for mild-to-moderate glaucoma.

Post-Laser Management

After SLT or ALT, patients receive topical anti-inflammatory drops (an NSAID or corticosteroid) for five to seven days, and existing glaucoma medications are continued. After TSCPC, management includes a topical corticosteroid, cycloplegic agent, and continuation of systemic and topical glaucoma medications, with close monitoring for hypotony. IOP is checked at one hour after SLT or ALT, and at one day and one week after cyclophotocoagulation.

Clinical Pearls

SLT is now considered appropriate as first-line monotherapy for open-angle glaucoma based on the LiGHT trial evidence. Highly pigmented angles, such as those seen in pseudoexfoliation and pigment dispersion syndrome, respond well to SLT, but lower energy settings should be used to avoid IOP spikes from excessive trabecular meshwork inflammation. SLT can be repeated when its effect wanes, while ALT should not be repeated due to cumulative tissue damage. MP-TSCPC is expanding the indications for cyclodestruction beyond end-stage disease into moderate glaucoma with preserved vision. IOP should always be checked at 30 to 60 minutes after SLT to detect early pressure spikes, and prophylactic brimonidine or apraclonidine can be administered to prevent post-laser IOP elevation. In neovascular glaucoma, the underlying ischemic cause must be treated with panretinal photocoagulation and anti-VEGF therapy before or concurrent with cyclophotocoagulation.

<image>Micropulse transscleral cyclophotocoagulation sweeping technique diagram showing the probe movement pattern across superior and inferior hemispheres</image>

References

• Gazzard G, et al. Selective laser trabeculoplasty versus eye drops for first-line treatment of ocular hypertension and glaucoma (LiGHT): a multicentre randomised controlled trial. Lancet. 2019;393(10180):1505-1516.
• Glaucoma Laser Trial Research Group. The Glaucoma Laser Trial (GLT) and GLT Follow-up Study results. Am J Ophthalmol. 1995;120(6):718-731.
• Aquino MC, et al. Micropulse versus continuous wave transscleral diode cyclophotocoagulation in refractory glaucoma. J Glaucoma. 2015;24(9):e93-e99.
• American Academy of Ophthalmology. Basic and Clinical Science Course, Section 10: Glaucoma.
• Latina MA, et al. Selective laser trabeculoplasty: a new treatment option for open angle glaucoma. Curr Opin Ophthalmol. 2002;13(2):94-96.

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