Selective Laser Trabeculoplasty (SLT)
Master selective laser trabeculoplasty for boards: how SLT lowers IOP, its 532 nm mechanism, indications and why it beats ALT for repeatability.
Selective laser trabeculoplasty is applied to the:
Selective Laser Trabeculoplasty in 30 Seconds
Selective laser trabeculoplasty uses a Q-switched 532 nm Nd:YAG laser to target pigmented trabecular meshwork cells and enhance aqueous outflow. It lowers intraocular pressure in open-angle glaucoma and ocular hypertension and provides a repeatable, tissue-sparing alternative or adjunct to topical therapy.
How SLT Works
A frequency-doubled Q-switched 532 nm Nd:YAG laser delivers short high-energy pulses absorbed selectively by melanin in pigmented trabecular meshwork cells, sparing non-pigmented structures. This triggers a biological response including macrophage recruitment, cytokine release and trabecular meshwork remodeling that raises outflow facility. The exact mechanism is not fully settled, but selective photothermolysis is the central concept. What makes SLT "selective" is the contrast with argon laser trabeculoplasty: ALT creates coagulative thermal burns and permanent trabecular scarring, while SLT largely preserves trabecular architecture with minimal collateral thermal damage and can be repeated when clinically indicated.
Indications and Patient Selection
- Open-angle glaucoma or ocular hypertension as primary therapy or adjunct to drops.
- Patients with medication intolerance, allergy or poor adherence.
- Pseudoexfoliation glaucoma, which tends to respond well due to increased trabecular pigmentation.
- Pigmentary glaucoma, with caution: heavily pigmented angles absorb more energy and carry higher risk of IOP spikes. Use lower energy settings.
- SLT is generally ineffective while the angle is closed. It may be considered after successful laser peripheral iridotomy or cataract surgery if sufficient trabecular meshwork is visible and open.
- Avoid in active uveitic glaucoma or uncontrolled intraocular inflammation.
- Previous ALT is not an absolute contraindication, although SLT response may be diminished in previously treated areas due to existing trabecular damage.
4 Facts Exams Always Ask
- SLT uses a frequency-doubled Q-switched 532 nm Nd:YAG laser.
- SLT targets pigmented trabecular meshwork cells via selective photothermolysis, sparing non-pigmented tissue.
- Typical IOP reduction is roughly 20 to 30 percent from baseline in appropriate candidates.
- SLT can be repeated when clinically indicated. Repeat ALT, by contrast, generally has substantially lower efficacy and is rarely performed.
- Transient IOP spike is the most important early complication of SLT. IOP is commonly checked 30 to 60 minutes after treatment, particularly in heavily pigmented angles at higher risk of pressure elevation.
The Classic Trap
The classic error is treating SLT and ALT as interchangeable, or claiming SLT cannot be repeated. They differ fundamentally in mechanism and consequence. ALT relies on thermal burns that scar the trabecular meshwork and limit retreatment. SLT uses selective photothermolysis of pigmented cells with minimal thermal damage, largely preserving trabecular architecture and allowing repeat treatment. If a question asks which laser procedure can be safely repeated, the answer is SLT.
Clinical Pearl
The LiGHT trial demonstrated that first-line SLT achieved IOP control comparable to initial topical therapy, with approximately 70 percent of eyes remaining drop-free at 6 years and fewer glaucoma surgeries in the SLT group. That evidence moved SLT from a late add-on toward a legitimate primary option. When a patient wants to avoid daily drops or struggles with adherence, primary SLT is now defensible and board-examinable.
Checkpoint 1
What best describes the mechanism of selective laser trabeculoplasty?
Checkpoint 2
Which feature is a practical advantage of selective laser trabeculoplasty compared with argon laser trabeculoplasty?
A 68-year-old with pseudoexfoliation glaucoma is on two topical agents but reports frequent missed doses and stinging. IOP is 24 mmHg, the angle is open and there is no active inflammation. Gonioscopy shows heavy trabecular pigmentation and no prior laser treatment to the angle. She wants to reduce her drop burden.
Selective laser trabeculoplasty preferentially targets pigmented trabecular meshwork cells while sparing adjacent nonpigmented structures. What is the proposed biological mechanism by which this laser lowers IOP?
Sources & Reviewer Info
Diagram created with AI image generation, clinically reviewed for accuracy.
References
- American Academy of Ophthalmology. Basic and Clinical Science Course, Section 10: Glaucoma.
- Shields'' Textbook of Glaucoma.
- American Academy of Ophthalmology. EyeWiki: Selective Laser Trabeculoplasty.
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