Laser Trabeculoplasty

ALT vs. SLT: Mechanism

Argon laser trabeculoplasty (ALT) uses a continuous-wave argon (blue-green) laser to produce discrete thermal burns on the trabecular meshwork. The resulting coagulative shrinkage and scarring mechanically widen the adjacent, untreated trabecular spaces, improving outflow.

Selective laser trabeculoplasty (SLT) works on a different principle. A frequency-doubled, Q-switched Nd:YAG laser (532 nm) selectively targets pigmented trabecular meshwork cells through photothermolysis, sparing the adjacent nonpigmented structures and producing no coagulative scarring. The photothermal effect itself is only the first step: SLT is thought to also trigger a biologic cascade of macrophage recruitment that remodels the meshwork over time, adding a delayed component to the outflow improvement. Because the tissue isn't scarred the way it is with ALT, SLT can be repeated more readily, a real practical advantage when disease progresses. ALT's larger, more destructive burns carry a correspondingly higher risk of peripheral anterior synechiae.

Technique & Indications

Both procedures need direct visualization of the angle through a goniolens, and both require an open, visible trabecular meshwork. Extensive peripheral anterior synechiae is a contraindication for either, since the meshwork itself becomes inaccessible to the laser once the angle is closed off. SLT typically uses a 400-micron spot size, matching the width of the meshwork, with roughly 100 spots for a full 360° treatment. The most common complication is a transient IOP elevation in the hours to days afterward, thought to result from released pigment and debris temporarily obstructing outflow.

The LiGHT Trial

The Laser in Glaucoma and ocular Hypertension Trial (LiGHT) tested SLT as initial therapy, not as a fallback reached for once medications had already failed. Patients started on SLT had better disease control, fewer subsequent surgical interventions, and lower overall treatment costs than patients started on topical medication. That result is why SLT is now discussed as a legitimate first-line alternative to eye drops for many patients with open-angle glaucoma or ocular hypertension.