The short pulse duration is below the thermal relaxation time of the tissue, and therefore no thermal damage occurs. Adjacent cells which do not contain melanin are unaffected by the treatment. SLT does not cause thermal or structural damage as observed with scanning electron microscopy. Conversely, extensive scarring, coagulative damage, and craters are seen in patients treated with argon laser trabeculoplasty.
The mechanism by which laser treatment to the trabecular meshwork results in lowered IOP is not clearly established. Mechanical, cellular, and biochemical mechanisms have been proposed. It may be that a combination of these three processes acts to achieve the IOP-lowering effect. The thermal burn may contract tissue, stretching open adjacent regions of the trabecular meshwork to increase outflow. This mechanism seems unlikely for SLT, as it does not cause thermal burning. The laser application may also stimulate trabecular endothelial cells to replicate, increasing outflow. Finally, selective stimulation of the trabeular meshwork cells may stimulate the release of cytokines, upregulate metalloproteinases, and recruit macrophages into the zone resulting in reorganization of the extracellular matrix and increased outflow.
In the FDA clinical trial, two groups of patients were examined. First, patients with open-angle glaucoma who were uncontrolled on maximum medical therapy were evaluated. In addition, patients who had prior argon laser trabeculoplasty (ALT) who also had poorly controlled IOP (intraocular pressure of 22mm Hg or greater) were evaluated to determine the efficacy of retreating patients with SLT. 120 patients were enrolled with a mean age of 67.7 years. 101 patients completed the study and were followed for 26 weeks with no change in glaucoma medications. All patients had an IOP reduction. Specifically, 70.3% had a minimum reduction of intraocular pressure of 3mm Hg. In patients who had not had previous laser, the average IOP reduction was 6.5mm Hg (25% reduction compared to preop). In patients with previous ALT, there was an average 5.7mm Hg IOP reduction (22% drop). There were minimal adverse events associated with SLT.
Two-year data from a prospective, randomized clinical trial comparing ALT and SLT showed equivalent 5-6mm Hg IOP reduction.
