Added benefits of SLT are that it may be associated with fewer side effects and less expense for patients and third-party payers, and patient adherence is not an issue, Dr. Katz said.
"SLT performed with a frequency-doubled Nd: YAG laser has been demonstrated to target only pigmented cells in the trabecular meshwork. Unlike argon laser trabeculoplasty (ALT) and diode laser trabeculoplasty, there is no thermal injury to the angle structures with SLT," said Dr. Katz, a professor of ophthalmology, Thomas Jefferson University, and director of glaucoma, Wills Eye Institute, Philadelphia.
In addition, he noted, ALT and SLT have produced equivalent effects in lowering IOP in clinical trials.
One limitation of ALT is the poor response to repeat treatment, which has been attributed to the structural alterations resulting from the coagulative injury. "Theoretically, since there is no thermal injury when performing SLT, the procedure possibly may be repeated to reproduce the initial IOP reduction," Dr. Katz said.
With a chronic disease such as glaucoma, patient adherence to medical therapy is a challenge, he said. Because the early to moderate stages of glaucoma generally are asymptomatic, therapy nonadherence does not result in any negative effects that are apparent to the patient. In fact, Dr. Katz said, the patient may feel better due to the elimination of any ocular or systemic side effects from the drug that was discontinued. Therapy adherence is not an issue with SLT.
A previously conducted multicenter trial, the Glaucoma Laser Trial, showed the comparable effectiveness of initial ALT with medical treatment. The limitations of that study included a lack of laser repeatability and an antiquated medical stepping regimen that used eyes instead of patients for randomization, he said.
The study under discussion, which was performed at 17 sites, was a prospective, randomized, controlled clinical trial that compared the two treatment options, SLT and medical therapy, in patients with untreated primary OAG or high-risk ocular hypertension.
To participate in the study, patients had to be aged 25 to 82 years, have bilateral visual acuity of 20/70 or higher, and have IOP ranging from ≥24 mm Hg to less than 31 mm Hg in the more severely affected eye, and an IOP ≥20 mm Hg in the less severely affected eye.
Also, patients had to have received a diagnosis of primary OAG, pseudoexfoliation, or a mixed mechanism or ocular hypertension, with a central corneal thickness of less than 600 µm. The angle structures had to be adequately visualized.
Patients could not have undergone a previous ocular surgery in one or both eyes, with the exception of laser iridotomy.
In addition, patients could not have been taking any other medication known to increase IOP, such as steroids.
The target IOP was established using a formula from the Collaborative Initial Glaucoma Treatment Study based on starting IOP and an objective visual aid score.
The primary aim of the study was to determine whether SLT is equivalent to medical therapy in its initial IOP-lowering effect based on target IOP levels. Patients were followed up to 1 year, according to Dr. Katz.
Participants were randomly assigned to either SLT and repeat SLT or addition of topical glaucoma medications, both in four steps. For SLT, the first step consisted of SLT, 100 applications over 360°; the second step was 50 applications nasally over 180°; and the third step was 50 applications temporally over 180°. In the fourth step, the clinician chose the next therapy in the event of treatment failure.
For medical therapy, the first step consisted of a prostaglandin analog, latanoprost (Xalatan, Pfizer), bimatoprost (Lumigan, Allergan), or travoprost (Travatan, Alcon Laboratories). In the second step, a beta blocker, timolol maleate (Timoptic, Merck) or betaxolol (Betoptic S, Alcon), was added.
In the third step, brimonidine tartrate (Alphagan, Allergan) was added or substituted, and in the fourth step, dorzolamide (Trusopt, Merck), brinzolamide (Azopt, Alcon), or a fixed-combination was added or substituted.