Baseline IOP predicts treatment response for SLT, PGA drops

Primary treatment with either selective laser trabeculoplasty (SLT) or prostaglandin analog (PGA) drops for ocular hypertension and open-angle glaucoma results in a decrease in intraocular pressure (IOP), and this benefit is greater with higher baseline IOP, a study has shown.

“In treatment-naïve eyes, at higher baseline IOP, SLT was more successful at achieving ≥20-percent IOP lowering than PGA drops,” according to the researchers. “At lower baseline IOP, a statistically greater percentage, but not absolute, IOP lowering was seen with PGA drops compared with SLT, although the clinical significance of this is uncertain.”

This post hoc analysis of randomized control trial data included 1,146 eyes from 662 patients: 559 eyes in the SLT group and 587 in the medication group. The research team assessed IOP reduction at different levels of baseline IOP at 8 weeks after treatment with either SLT or PGA drops. They then compared the results between the two treatment arms.

A linear mixed-effects model was used to test differences in absolute and percentage IOP lowering between SLT and PGA drops. Using a logistic mixed-effects model, the researchers estimated differences in the probability of achieving ≥20-percent IOP lowering between the two groups.

Mean IOP did not significantly differ between the SLT and PGA drops groups either at baseline or at 8 weeks after treatment initiation. Both SLT and drops demonstrated greater IOP reduction at higher baseline IOP and lesser IOP reduction at lower baseline IOP. [Ophthalmology 2024;131:1366-1376]

At higher baseline IOP, SLT achieved more IOP lowering than PGA drops, but PGA drops showed a better performance at lower baseline IOP. Compared with SLT, the difference in percentage IOP reduction was significant at baseline IOP of ≤17 mm Hg.

Of note, there was a marked difference in the association between baseline IOP and probability of ≥20-percent IOP lowering between the two treatments (p=0.01). Specifically, SLT was more successful than PGA at baseline IOP of >22.5 mm Hg.

“At higher baseline IOP, SLT performed better, and these findings were supported by large group sizes. At lower baseline IOP, drops performed better, but the clinical relevance of this finding was limited by lower numbers of eyes,” the researchers said.

“Although we seem to have demonstrated a larger initial IOP reduction with PGA drops at 8 weeks for patients with low baseline IOPs, this does not necessarily translate into a broad recommendation to initiate therapy with drops rather than SLT,” they added.

Despite similarly treated IOPs, other factors must be taken into account, including adherence, side effects, tolerability of treatment, and other additional findings of greater visual field preservation with SLT, according to the researchers. [Ophthalmology 2020;127:1313-1132]

Mechanism of action

It remains unclear why higher baseline IOP correlates with a greater reduction in IOP, both in absolute and relative terms, after SLT and PGA drops. An earlier study suggests that higher pressure gradient may allow better trabecular outflow following SLT, and this notion could be extended to medical therapy targeting aqueous outflow. [Acta Ophthalmol 2016;94:692-696]

Other studies have shown that by improving aqueous outflow through the trabecular meshwork (TM), the mechanism of action of SLT may induce improved response with higher baseline IOP. [Invest Ophthalmol Vis Sci 2017;58:1462-1468; Ophthalmology 2020;127:748-757]

“With higher baseline IOP in ocular hypertension and primary open-angle glaucoma, presumably greater resistance to outflow exists at the level of the TM and perhaps greater potential for IOP lowering with a treatment that targets that pathway does, also,” the researchers said.

“This also could explain the more reliable IOP reduction at higher baseline IOP observed with SLT compared with PGA drops reported in this study, with PGA targeting primarily the unconventional pathway and SLT targeting the TM directly, where outflow obstruction is highest,” they added.