Tuesday, February 7, 2023
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Argon Laser Trabeculoplasty

Overview

The goal of glaucoma treatment is to lower intraocular pressure in order to slow retinal ganglion cell loss. Currently, this can be achieved with medications, which typically constitute the first line of treatment, laser treatment, or surgery.

Contemporary argon laser trabeculoplasty (ALT) was introduced as a treatment modality for open-angle glaucoma by Wise and Witter
based on observations by Ticho and Zuberman,
who demonstrated that argon laser treatment of angle structures could lower intraocular pressure (IOP) without causing full-thickness openings through the trabecular meshwork (TM). The mechanism of action is widely debated. It is thought that (1) thermal energy directed towards the TM causes focal scarring of trabecular beams that constitute the TM, thereby opening up space in adjacent structures (mechanical theory), or that (2) inflammatory cytokines induce structural changes and allow repopulation of the TM with dividing trabecular epithelial cells from untreated areas, which are more effective in phagocytosis and produce a different composition of extracellular matrix with improved outflow facility properties (biological theory).

In 1998, selective laser trabeculoplasty (SLT) was introduced by Latina et al.
It uses a nonthermal laser to achieve similar results,
without causing visible damage to the TM structures.
Other similar approaches that are investigated include diode laser trabeculoplasty, titanium sapphire laser trabeculoplasty (TLT), pattern scan laser trabeculoplasty (PLT), and micropulse laser trabeculoplasty (MDLT) with different laser wavelength characteristics, spot size, and laser pulse length.

With appropriate patient selection, IOP is reduced by 25-30% 1 year after ALT. However, the effect fades over time, with an attrition rate of about 10% per year. Traditionally, patients believed to be ideal candidates for the procedure are older and phakic with sufficient TM pigmentation. However, newer reports have confirmed only pretreatment intraocular pressure as a significant predictor of successful response.
In patients younger than 50 years, ALT is not recommended unless significant TM pigmentation is present due to exfoliation or pigment dispersion syndrome.

Long-term studies have shown that ALT maintains IOP control in 67-80% of eyes 1 year after the procedure and in 30%-50% of eyes 5 years after the procedure.
ALT can be repeated, targeting the reminder of the TM (typically half of the angle is treated initially), but the effect to be expected is modest, especially if the first treatment session had poor outcomes. Repeat ALT success rates after initial favorable response range between 21% and 70%, with a fading effect over time.

The Glaucoma Laser Trial performed over 2 decades ago has demonstrated equivalency between ALT and medical treatment as initial treatment modalities in patients with glaucoma. In the Glaucoma Laser Trial study, 11% of the ALT-treated eyes required repeat ALT or glaucoma filtration surgery at the end of follow-up. In comparison, 34% of the eyes in the medical arm required ALT or glaucoma filtration surgery at the respective time-point.
However, the results of the study may not be directly applicable to current glaucoma practice as the newer prostaglandin analogues became available after study initiation.

A small prospective study
is comparing SLT with modern medical treatment and has demonstrated similar outcomes at 12 months. Moreover, several studies support the claim that SLT is at least as effective as and more cost-efficient than prostaglandins, at least in healthcare systems where the medication cost burden is relatively high.

 

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