Selective Laser Trabeculoplasty (SLT)

Selective laser trabeculoplasty (SLT) is a non-incisional, office-based laser procedure that targets the trabecular meshwork to enhance aqueous humor outflow and reduce intraocular pressure (IOP) in patients with open-angle glaucoma and ocular hypertension. SLT uses a frequency-doubled Q-switched Nd:YAG laser operating at 532 nm wavelength with very short pulse duration (3 nanoseconds) and low energy (0.4-1.0 mJ per spot, typically starting at 0.8 mJ and titrated). The 400 micrometer spot size covers approximately one trabecular meshwork beam. The 'selective' name refers to the laser's ability to selectively target melanin-containing pigmented cells (pigmented trabecular meshwork cells, predominantly endothelial cells) without thermal damage to non-pigmented surrounding cells, structures, and trabecular meshwork beams (cell-selective photothermolysis). This contrasts with older argon laser trabeculoplasty (ALT, 1979 Wise and Witter), which uses higher-energy continuous wave argon laser (50-150 micrometer spot, 1000 mW power) producing thermal coagulation and scarring of trabecular meshwork — limited to 1-2 lifetime treatments due to cumulative damage.

Mechanism of action: SLT does not directly create permanent structural changes. Instead, the selective laser energy targeted at pigmented trabecular meshwork cells initiates a cascade of biological responses: (1) released cytokines (IL-1, TNF-alpha, IL-6, IL-8) within hours, (2) recruitment of monocytes and macrophages within days, (3) phagocytosis of debris and remodeling of trabecular meshwork extracellular matrix over weeks, (4) increased aqueous humor outflow facility within 1-3 months, (5) net IOP reduction of 20-30% from baseline (similar to topical prostaglandin analog monotherapy). The biological response is non-destructive (no scarring), allowing repeated treatments. Indications: (1) Primary open-angle glaucoma (POAG) — first-line treatment per LiGHT trial. (2) Ocular hypertension — IOP greater than 21 mmHg with risk factors. (3) Pseudoexfoliative glaucoma. (4) Pigmentary glaucoma (although increased risk of post-laser IOP spike due to liberation of pigment). (5) Patient with poor adherence to topical medications. (6) Patient who prefers laser over drops. (7) Adjunctive therapy in patients on maximal tolerated medical therapy. Contraindications: closed-angle glaucoma (must be open-angle on gonioscopy), uveitic glaucoma with active inflammation, neovascular glaucoma, advanced glaucoma needing immediate surgical intervention, pregnancy (relative), and inability to cooperate with slit lamp procedure.

Procedure: pre-operative IOP measurement, gonioscopy to confirm open angle. Topical anesthetic drops. Pre-treatment with apraclonidine or brimonidine to reduce post-laser IOP spike risk. Patient seated at slit lamp with goniolens (Latina SLT lens, Goldmann 3-mirror, or Magna View) coupled to cornea with viscous coupling agent. Laser is set: 532 nm wavelength, 3 ns pulse duration, 400 micrometer spot, starting energy 0.8 mJ (titrate up by 0.1 mJ increments based on bubble formation — desired endpoint is small champagne bubbles at trabecular meshwork without excessive cavitation). Approximately 50-100 non-overlapping shots delivered to 180 degrees (hemi-SLT) or 360 degrees (full SLT) of trabecular meshwork. Procedure duration 5-10 minutes. Post-procedure IOP measured at 30-60 minutes and at 1 week. Topical NSAID or steroid drops for 1 week (some practitioners avoid steroids as may blunt SLT response). Effect develops over 4-6 weeks. Patients should continue glaucoma medications until follow-up confirms IOP reduction. Outcomes: 80% of patients achieve clinically significant IOP reduction (greater than 20%) at 1 year, with mean IOP reduction of 20-30% (5-7 mmHg from baseline 22-25 mmHg). Effect lasts 2-5 years on average. Repeat SLT achievable with similar success rate. LiGHT trial outcomes at 6 years: 78% of SLT-first patients achieved IOP target without medication, fewer surgical interventions (0% vs 2.8% in eye drop group), fewer cataract progression. Side effects: transient IOP spike (3-5%, mostly within 24 hours, treated with topical pressure-lowering drops or oral acetazolamide), mild transient anterior chamber inflammation (cells, flare for 1-3 days), conjunctival injection, mild ocular discomfort, transient blurring, headache. Rare complications: persistent IOP elevation requiring more intensive medical or surgical therapy, prolonged anterior uveitis, peripheral anterior synechiae, hyphema (rare), corneal endothelial damage (rare with proper technique), and macular edema (very rare). Repeat SLT considered when IOP rises after initial response, typically 2-5 years after first treatment.