SLT lowers IOP with favorable safety, says clinician

Key Points

Sydney, Australia-Selective laser trabeculoplasty (SLT) offers effective treatment for primary open-angle glaucoma (POAG) as well as some more difficult glaucomas, and importantly, it achieves its IOP-lowering benefit while fulfilling the dictum "first, do no harm," said Ivan Goldberg, FRANZCO.

SLT is performed with the Q-switched, frequency-doubled, 532-nm Nd:YAG laser using a 3-ns pulse. It was developed as an alternative to argon laser trabeculoplasty (ALT) and designed to target only pigment-containing cells in the trabecular meshwork. The two laser procedures are performed with the same number of spots (50), although the SLT spot is larger than the ALT spot (400 versus 50 µm). The selectivity of SLT is achieved using a shorter pulse duration (3 versus 100 million ns), less energy (0.4 to 1.4 mJ versus 300 to 600 mW), and a lower fluence (6 versus 40,000 mJ/mm² ).

"With these parameters, SLT selectively targets the melanin-rich cells of the trabecular meshwork, and, unlike ALT, it causes no apparent thermal tissue damage because its short pulse duration is below the thermal relaxation time of the trabecular meshwork tissue," said Dr. Goldberg, clinical associate professor of ophthalmology, University of Sydney, Australia. "Therefore, SLT should be repeatable without causing harm or further complications whereas ALT has limited and decreasing efficacy on re-treatment."

SLT may be considered a secondary intervention for IOP-lowering in patients for whom medical therapy failed and in patients who cannot tolerate it or have contraindications, but based on its efficacy and safety profile, SLT also might be a reasonable first-line intervention in some situations, he said. Further, Dr. Goldberg added, SLT can avoid compliance-related issues accompanying medications.

"There are no large-scale studies comparing SLT and medication as primary therapy. However, the results of the Glaucoma Laser Trial, which established the potential usefulness of ALT as first-line treatment for open-angle glaucoma, combined with the potential safety advantages of SLT, might provide support for first-line treatment with SLT," Dr. Goldberg said.

Any patient with visible trabecular meshwork on gonioscopy is a potential candidate for SLT whether the patient has POAG, so-called "normal-tension glaucoma," pseudoexfoliative, or pigmentary glaucoma. Dr. Goldberg said he advocates certain pre-treatment measures and treatment techniques for enhancing safety and maximizing efficacy. Administering topical pilocarpine and an alpha-2 agonist 20 to 30 minutes prior to the procedure can help to tighten the peripheral iris and thereby make the treatment technically easier and minimize the risk of a post-treatment IOP spike, he said.

The approximately 50 spots should be delivered over 180° of the meshwork so that no free space exists between them. Treatment of 180° can be completed in less than 5 minutes, and if IOP reduction a month later is less than desired, the second 180° of meshwork can be treated.

"This staged approach helps to reduce the risk of an IOP spike and avoids treating the entire meshwork in those patients in whom 180° of therapy achieve target IOP levels," Dr. Goldberg said.

Treatment endpoint

The treatment endpoint should be appearance of fine "champagne bubbles" with each laser application. Often, obtaining this appearance requires titration of laser energy not only for each patient but also between eyes and even within the same eye.

"More heavily pigmented meshwork contains more target chromophore and requires less energy. The laser settings might range from 0.3 mJ for an eye with heavily pigmented meshwork to 1.6 mJ for one that has an amorphous appearance," said Dr. Goldberg. Patients should be advised that they might develop some redness, discomfort, and even light sensitivity after the treatment; it usually resolves within a few days.

"Patients should be counseled to call if they are having a severe reaction or discomfort, but topical steroids and non-steroidal anti-inflammatory drugs probably should not be used routinely," he said. "The mechanism of action of SLT is thought to involve cytokine release by the trabecular endothelial cells, and these anti-inflammatory medications might inhibit that response."

It may take up to 4 weeks after a treatment session before the full IOP-lowering effect is achieved, Dr. Goldberg said. Therefore, patients already taking IOP-lowering medications should continue on their treatment until the effect of SLT can be evaluated. Patients who benefit from SLT but subsequently experience rising IOP may be re-treated, he added. "If there is no response to the initial treatment, there is no rationale to repeat SLT," Dr. Goldberg concluded.