Argon Laser Trabeculoplasty

The goal of treatment in glaucoma is to lower intraocular pressure (IOP) in order to slow down retinal ganglion cell loss. 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 ^[1] based on observations by Ticho and Zuberman, ^[2]  who demonstrated that argon laser treatment of angle structures could lower 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 result in a different composition of extracellular matrix with improved outflow facility properties (biological theory). ^[3]

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 considered to be ideal candidates for the procedure are older and phakic with sufficient TM pigmentation. However, newer reports have confirmed only pre-treatment IOP as a significant predictor of a successful response. ^[4] In patients younger than 50 years, ALT is not usually performed 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. ^{[5, 6]} ALT can be repeated, targeting the remainder 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. ^[3]

The Glaucoma Laser Trial performed over 3 decades ago has demonstrated equivalency between ALT and medical treatment as initial treatment modalities in patients with open angle-glaucoma. In the Glaucoma Laser Trial, 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. ^[7] 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.

In 1998, selective laser trabeculoplasty (SLT) was introduced by Latina et al. ^[8] SLT is a quick, painless, outpatient procedure with quick patient recovery and a good safety profile. It uses a non-thermal 532nm Q switched, frequency doubled Nd:YAG laser delivering a short laser pulse (of 3nsec duration) targeting the pigmented cells of the trabecular meshwork while achieving similar results, ^{[9, 10, 11, 12, 13, 14]}  without causing visible thermal or mechanical damage to the TM structures. ^{[15, 16]}  Other similar approaches that have been 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. ^{[3, 17]}

A few studies have compared SLT to modern medical treatment as primary and adjunctive therapy, ^{[12, 13, 18, 19]}  whereas some have also modelled the cost-efficacy of SLT compared to prostaglandin analogues, mainly in healthcare systems where medication cost is relatively high. ^{[20, 21]}  

In 2019, the Laser in Glaucoma and Ocular Hypertension (LiGHT) randomised controlled clinical trial reported that SLT allows 78% of treated eyes to maintain IOP control without the use of medications or the need for surgery 3 years after treatment, while also leading to a reduced number of glaucoma surgeries and very low rates of adverse events when compared with the use of eye drops. ^{[22, 23]}  SLT also was found to be more cost-effective than initial treatment with IOP-lowering drops for the UK’s fixed-budget national healthcare system. These findings led to the update of the guidelines issued by the American Academy of Ophthalmology ^[24]  and the European Glaucoma Society. ^[25]  In 2022, the LiGHT trial  reported that 69.8% of the eyes treated with SLT maintained IOP control for 6 years without the need for medical or surgical treatment, with fewer eyes showing disease progression and fewer eyes requiring trabeculectomy, when compared with medical treatment. ^[26]

Indications for ALT include the following:

• Open-angle glaucoma not adequately controlled on maximal tolerated medical therapy
• Open-angle glaucoma in cases in which compliance with medical therapy is less than optimal due to practical, social, or economic reasons, or if the medical adverse effect profile is considered unfavorable for the patient
• Open-angle glaucoma in patients not willing to receive medical treatment as initial therapy

Contraindications for ALT include the following:

• Any type of angle-closure glaucoma
• A relatively high preoperative IOP (>30 mm Hg) in conjunction with advanced optic nerve damage, in which cases a possible postoperative transient IOP elevation is thought to pose a significant risk for the patient’s vision
• Glaucoma associated with uveitis, trauma, juvenile open-angle glaucoma, or angle dysgenesis
• A history of previous ALT failure in the same or the fellow eye
• Little or no trabecular pigmentation (a negative predictor of success)
• Low-tension glaucoma (relative contraindication with respect to the possibility of achieving a clinically significant IOP reduction)
• Pseudophakia or aphakia (relative contraindication in view of a potentially higher failure rate; however, this may be debatable in cases of prior uncomplicated phacoemulsification)
• Young age (relative contraindication in view of a comparatively higher failure rate) ^[27]

Topical anesthesia is required (0.5% of proparacaine hydrochloride) to perform ALT without any significant discomfort on behalf of the patient.

A Goldmann 3-mirror lens is used to visualize the angle structures and direct the laser energy toward the TM. A coupling agent such as 2 or 2.5% hydroxypropyl methylcellulose (Gonak, Goniosoft, or Goniosol) is necessary since the radius of curvature of the Goldmann 3-mirror lens is smaller than that of the cornea. Alternatively, a Ritch lens can be used, which allows angle structure visualization without much rotation. The Ritch goniolens provides a 1.4-times magnified view of the angle structures, which, theoretically, should reduce the spot size accordingly. A continuous-wave argon green laser with a wavelength of λ=455-529 nm attached to a slit lamp and equipped with an aiming beam is used to generate the energy necessary to perform the procedure.

The Goldmann 3-mirror lens.

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ALT is performed with the patient in a sitting position in front of a slit lamp equipped with an argon laser.

Patients usually are pretreated 30-60 minutes before the procedure with (1) a drop of pilocarpine (1-2%) to put the iris on stretch, creating more space and therefore preventing inadvertent peripheral anterior synechiae formation and (2) a drop of apraclonidine 0.5% or Brimonidine 0.1-0.2% to decrease the incidence and the severity of a possible postoperative transient IOP elevation. The appropriate laser settings are a spot size of 50 μm, a duration of 0.1 sec, and a power of 500-1200 mW to achieve blanching depending on the level of TM pigmentation. In general, the more pigmented the TM, the less energy necessary.

The Goldmann 3-mirror lens is placed on the cornea, and adequate visualization of the angle structures is ensured. The authors of this article usually start with treatment of the inferior angle, because it is wider, which makes the procedure technically easier.

The laser beam is aimed at the junction of pigmented/nonpigmented TM, and the laser energy is adjusted until a mild blanching effect is observed. Vapor bubble formation is a sign of excessive energy and should be avoided. To ensure maximal delivery of the energy, the aiming beam should be kept in the center of the mirror.

Fifty laser spots are delivered equally spaced to the inferior 180° of the angle (see the image below). Treating all 360° is not more effective and is associated with a higher incidence of postoperative IOP spikes. The superior angle can be treated in a separate session in the future, if necessary. The procedure is completed without any significant pain for the patient.

Gonioscopic view of the angle indicating the correct placement of argon laser spots.

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The anticipated response can be delayed up to 6 weeks after the procedure.

Patients are instructed to use a topical corticosteroid eyedrop such as prednisolone acetate 1% 4 times daily for 4-7 days after the procedure, along with all their previous glaucoma medications.

Patients typically are seen in follow-up 1 week after the procedure.

The chance of success with re-treatment using ALT is considerably smaller compared with the initial procedure.

A transient IOP elevation may occur. The incidence and severity of IOP elevations are significantly reduced if patients are pretreated with apraclonidine or brimonidine. The energy power should be reduced in eyes with Pigmentary Glaucoma or Pseudoexfoliative Glaucoma. Most of the IOP spikes manifest within 1-2 hours after the procedure. Therefore, checking the IOP 1 hour after ALT is recommended to determine if additional actions are necessary. The incidence of IOP elevation has been reported as high as 12% (>10 mm Hg) on the first postoperative days. ^[28]

A mild iritis is not unusual after ALT and peaks the second day after the procedure, causing occasionally photophobia. Treating the patient with a short course of topical corticosteroids (1% prednisolone acetate 4 times a day) for 4-7 days after the procedure is recommended.

Peripheral anterior synechiae (PAS) that can compromise outflow may form if the laser energy is directed too posterior toward the iris and are more common in narrow angles. In most cases, PAS are small and not clinically significant.

Disruption of endothelial cells with polymorphism and an increase in cell size, ^[29] indicative of a mild reduction in numbers, also has been reported. Transient epithelial burns can be observed and are more likely with high power settings.

Finally, an increase in the incidence of bleb encapsulation after trabeculectomy, ^[27] which may represent the next step in the treatment of medically uncontrolled glaucoma, has been reported.