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, [5, 6, 7, 8] 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. [3, 10]
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. [12, 13] 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. [16, 17]
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) 
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 (see image below) 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.
ALT is performed with the patient in a sitting position in front of a slit lamp equipped with an argon laser.
Patients are usually 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% 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, 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.
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 are typically 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. 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. 
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,  indicative of a mild reduction in numbers, has also been reported. Transient epithelial burns can be also observed and are more likely with high power settings.
Finally, an increase in the incidence of bleb encapsulation after trabeculectomy,  which may represent the next step in the treatment of medically uncontrolled glaucoma, has also been reported.