SLT safer than ALT for open-angle glaucoma

Reading, MA-Selective laser trabeculoplasty (SLT) is safer than argon laser trabeculoplasty (ALT) as a treatment for primary open-angle glaucoma (POAG) and offers the considerable advantage of not causing structural damage to the trabecular meshwork, said Mark A. Latina, MD.

Dr. Latina, an anterior segment surgeon in private practice at the Reading Health Center, Reading, MA, also noted that SLT, which can be used as either primary therapy or secondary therapy following failure of ALT, is easy to perform and less dependent on trabecular pigmentation.

"I think SLT can be used aggressively as first-line therapy, and the studies thus far suggest that it is extremely effective at reducing IOP in that group of patients. It can be used following ALT with twice as great effectiveness compared with repeat ALT, and it allows us to treat patients earlier and with lower risk of adverse effects," said Dr. Latina, who is also on the staff of the Massachusetts Eye and Ear Infirmary.

"The idea is to produce a biological effect without coagulation damage," Dr. Latina explained. "We can selectively target pigmented cells, leaving adjacent nonpigmented cells or structures unaffected. This is a very different morphologic effect compared with an argon laser."

SLT is performed with a Q-switched, frequency-doubled Nd:YAG laser, while ALT is performed with a continuous-wave argon laser. With an argon laser, all of the pigmented and nonpigmented calls are affected due to thermal diffusion. SLT is less dependent on trabecular meshwork pigmentation than ALT.

"Because the laser is so efficient at targeting melanin, you can effectively treat lightly pigmented trabecular meshworks," Dr. Latina said.

Trabecular meshwork affected

Another difference between the two procedures is that ALT produces a thermal burn or blanching, while SLT may not produce any effect on the trabecular meshwork. The argon laser also produces a coagulation burn to the meshwork, and there will be no flow through the affected area, although it may increase flow in the adjacent regions.

The two treatment approaches are very different, Dr. Latina said, although the lasers have some similarities. The blue-green argon laser produces light at about 514 nm, while the green Nd:YAG laser has a wavelength of 532 nm. The pulse duration of SLT is 3 nanoseconds, compared with 0.1 seconds in ALT.

Spot size for SLT is 400 µm, which provides full coverage at the trabecular meshwork, while the spot size of the argon laser is 50 µm. In SLT, the spots are adjacent and confluent to each other, while in ALT, a space is left between spots because of thermal damage and scarring to the meshwork.

Dr. Latina cited several studies of SLT and ALT.

In a randomized, prospective study of the two techniques in 119 eyes, IOP showed similar responses in both groups after 36 months of follow-up. The average IOP reduction was 6.5 mm Hg in the SLT group and 6 mm Hg in the ALT group. The difference was not statistically significant.

The most important advantage of SLT over ALT may be its use as primary therapy, Dr. Latina said, and he described results of a relevant study by Melamed and coworkers in which 88% of patients showed at least a 5 mm Hg reduction in IOP; 5% of patients had no response. In another study of SLT as primary therapy, patients with ocular hypertension showed an average IOP reduction of approximately 32% after 2 years of follow-up. Patients with POAG had a 35% reduction in IOP.

Studies have also been performed of SLT as therapy following ALT. The probability of patients doing well and experiencing a reduction in IOP after receiving two ALT treatments is about 30%. When SLT follows an ALT treatment, patients showed a response in 66% of eyes, twice that of ALT, with a reduction in IOP of about 24%, Dr. Latina said.

Describing the safety of the two laser procedures, Dr. Latina said it is generally safe to treat 360° of the angle with SLT.

"This is usually not recommended for ALT due to the inflammatory response and potential for pressure spikes," he explained.