Laser Trabeculoplasty

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Article initiated by:
JoAnn A. Giaconi, MD
All contributors:
Sarwat Salim MD, FACS, Ahmad A. Aref, MD, MBA, Oscar D. Albis-Donado, MD, Daniel B. Moore, MD, Tiago Morais-Sarmento, M.D., JoAnn A. Giaconi, MD, Leonard K. Seibold, MD, Raheem Remtulla, Pedro Cardoso Teixeira
Assigned editor:
Arsham Sheybani, MD
Review:
Assigned status Update Pending
 by Ahmad A. Aref, MD, MBA on December 23, 2023.


Summary

Argon laser trabeculoplasty (ALT) was introduced by Wise and Witter in 1979 for the treatment of medically uncontrolled glaucoma. Soon after its introduction, the efficacy and safety of this new technique was studied in a large multicenter prospective clinical trial funded by NEI, Glaucoma Laser Trial (GLT), in which eyes receiving ALT 360 degrees were compared with timolol monotherapy. From 2.5 to 5.5 years of follow-up, GLT demonstrated that trabeculoplasty was as efficacious as medical therapy in treating early POAG. Despite these favorable results, laser therapy did not replace medications as primary therapy in patients with POAG. This was partly due to attrition seen in efficacy over time and introduction of more effective glaucoma medications, namely prostaglandin analogues. The role of laser trabeculoplasty was limited and it was used either as an adjunctive therapy or as an intermediate step between failed medical therapy and surgical intervention. Interest in laser trabeculoplasty has been re-ignited in the past few years with the introduction of selective laser trabeculoplasty (SLT). A number of studies comparing ALT and SLT have shown similar IOP reduction with the two lasers. Because SLT appears to be less destructive histopathologically, a potential benefit of repeatability has been advocated.

Disease Entity

Laser trabeculoplasty, both argon laser trabeculoplasty(ALT) and selective laser trabeculoplasty (SLT) types, is used to increase aqueous outflow facility through the trabecular meshwork (TM) in order to lower intraocular pressure (IOP) in cases of ocular hypertension and glaucoma.[1]

Disease

Both ALT and SLT are indicated for the treatment of ocular hypertension, primary open angle and secondary open angle glaucomas, such as pseudoexfoliation and pigment dispersion glaucoma. Steroid induced glaucoma is another possible candidate for the procedure. Narrow angle glaucoma, where the trabecular meshwork is not obstructed by iris apposition or synechiae, may also benefit. If there is synechial closure, trabeculoplasty is not advised. Contraindications are inflammatory, iridocorneal endothelial (ICE) syndrome, developmental, and neovascular glaucoma. Laser trabeculoplasty is also not effective in angle recession glaucoma due to distortion of the angle anatomy and TM scarring. If there is a lack of effect in one eye, then it is relatively contraindicated in the fellow eye.

General Pathology

Elevated intraocular pressure is caused by resistance to aqueous outflow at the trabecular meshwork and Schlemm’s canal (SC) junction. The purpose of both ALT and SLT is to increase outflow facility through the trabecular meshwork in order to lower IOP.

Pathophysiology

The exact mechanism of action of laser trabeculoplasty is not well established. Various theories have been proposed as explanations for the increased aqueous outflow facility seen following successful trabeculoplasty,[2][3] including mechanical, cellular, and biochemical theories.

The mechanical theory for ALT suggests that the laser electromagnetic energy is converted to thermal energy when it contracts the TM. Tissue contraction and scar formation result in mechanical stretching of the surrounding untreated regions of the meshwork , facilitating flow into SC with subsequent reduction in IOP.[4] However, there is some evidence that the mechanical theory may be flawed [5]

The celluar theory for ALT is based on stimulation and increased cell division and repopulation of the trabecular meshwork.[6]An increase in DNA replication and cell division following argon laser treatment have been demonstrated.[7][8]

The biochemical theory for both ALT and SLT suggests a release of chemical mediators after laser treatment that increase aqueous outflow facility. ALT has been shown to increase macrophage recruitment at the site of treatment, resulting in remodeling of the extracellular matrix and increased outflow facility.[9][10] ALT has also been shown to increase the release of interleukin-1 and tumor necrosis factor gene expression, which upregulate matrix metalloproteinase expression and remodeling of the extracellular matrix.[11][12] It has been demonstrated that in cultured human trabecular meshwork irradiated with the SLT laser, interleukins 8, 1-alpha, 1-beta, and tumor necrosis factor alpha are upregulated. When the trabecular meshwork medium was added to Schlemm’s canal endothelial cells, the Schlemm’s canal endothelium underwent a 4-fold increase in fluid permeability. [13]

In a study of patients that underwent ALT followed by trabeculectomy 3 hours to 1 year later, trabecular meshwork samples scanned with electron microscopy revealed heat-related disruption of trabecular beams, deposition of fibrinous material, cellular necrosis, and shrinkage of trabecular collagenous beams.[14] Studies conducted by Kramer and Noecker in eyes treated by ALT or SLT using scanning and transmission electron microscopy have shown coagulative damage, trabecular beam disruption, endothelial membrane formation on TM collapsed SC lumen, and intertrabecular debris in ALT-treated eyes, in contrast to eyes treated with SLT, in which minimal change was apparent on imaging.[15] In post SLT eyes, general structure of TM was intact with no endothelial membrane formation and SC lumen was not collapsed. Less histopathological destruction observed with SLT has promoted repeatability of SLT over ALT. It is important to note that some earlier studies of ALT did not show the same crater-like damage seen in the previously mentioned study, but only mild coagulative damage.[16][5] Less destruction seen with SLT laser system is secondary to its ability to selectively photolyse pigmented TM cells without inducing photocoagulation and collateral damage to non-pigmented cells or structures since its pulse duration is shorter (3 nsec) than the thermal relaxation time of melanin (1msec).

History

A detailed medical and ocular history is recommended prior to either form of trabeculoplasty.

Physical Examination

The preoperative examination must include gonioscopic evaluation of the angle. This is routinely done at the slit lamp with a Zeiss, Posner, or Sussman lens, or with a standard single or triple mirror Goldmann type lens. Take note of whether or not the trabecular meshwork is visible without indentation since this is the structure that must be treated by trabeculoplasty. If the iris approach is somewhat steep, but the trabecular meshwork is revealed by rotating the eye towards the mirror, then there is probably sufficient angle area  for treatment. The presence or absence of synechiae should be looked for, as synechiae may be a contraindication to the procedure. The degree of trabecular meshwork pigmentation should be noted, as this may influence the initial energy level chosen for trabeculoplasty.

Diagnostic Procedures

Complete glaucoma evaluation should be done prior to recommending trabeculoplasty. This evaluation should include gonioscopy, intraocular pressure measurement, central corneal pachymetry, optic nerve examination and evaluation, and visual field testing.

General treatment

Laser trabeculoplasty can be used as a primary treatment or as an adjunctive treatment to medications. In the U.S., ALT is seldom chosen as the first-line treatment for IOP reduction, while SLT is increasingly gaining popularity as a first-line treatment.

Surgery

Approximately 30-60 minutes prior to either ALT or SLT, the eye should receive alpha adrenergic agonist, either apraclonidine or brimonidine, to decrease the risk of an immediate IOP spike. Topical anesthetic is used immediately prior to the procedure to anesthetize the eye for the laser contact lens.

In ALT, the argon green laser is typically set at a 50-micron spot size, 0.1-second duration, while the power setting can vary between 300-1000 mW, depending on response. The desired endpoint is blanching of the trabecular meshwork or production of a tiny bubble. If a large bubble appears, the energy should be titrated downward. The laser beam is focused through a goniolens at the junction of the anterior non-pigmented and the posterior pigmented edge of trabecular meshwork. Very posterior application of the laser beam tends to produce more inflammation, pigment dispersion, prolonged elevation of IOP and peripheral anterior synechiae (PAS). Many patients have satisfactory IOP reductions with treatment of 180º of the trabecular meshwork (approximately 40-50 applications). Treating 360º is associated with a higher incidence of pressure spikes, but additional 180 degrees of treatment can be performed later if treatment response is appreciated with initial treatment. The ALT procedure can also be performed with a diode laser. In this case, typical settings are 75-micron spot size, 0.1-second duration, and 600-1000 mW power.

In SLT, the laser is a frequency-doubled (532-nm) Q-switched Nd:YAG laser (Selecta 7000, Coherent Medical Group, Santa Clara, CA). The laser settings are fixed except for the power. Spot size is 400-microns and pulse duration is 3 ns. The large spot size results in low fluences (mJ/cm2). In more lightly pigmented angles, initial energy can be set at 0.8-1.0 mJ. In more heavily pigmented angles, the initial power can start off lower at 0.3-0.6 mJ. The aiming beam is centered over the trabecular meshwork and straddles the entire TM. Because precise placement of the laser beam is not necessary as it is in ALT, SLT is considered technically easier to do. The aiming beam will not be in sharp focus when the surgeon focuses on the trabecular meshwork to deliver treatment. The treatment endpoint is the appearance of small cavitation bubbles adjacent to the TM. Generally, 180 or 360 degrees are treated in a session. Laser spots can be placed contiguously or several spot sizes apart.

Surgical follow up

Similar to other laser procedures, it is routine to place a drop of apraclonidine or brimonidine in the eye after ALT or SLT to decrease the risk of a IOP spike.

Approximately 1 hour after both ALT and SLT, an intraocular pressure check is recommended. If the IOP is elevated beyond what is reasonable for the eye at one hour, the IOP must be treated and the patient should be seen the next day. The treatment required may be mild (i.e.—one hypotensive eyedrop) or aggressive (i.e.—systemic carbonic anhydrase inhibitors) depending on the eye’s circumstances. The follow-up interval will also depend on the severity of IOP spike. If the 1-hour postoperative IOP check is not elevated, the patient can be seen back in 1-2 weeks. The follow-up thereafter will depend on the patient and doctor, but a commonly followed routine is 4-6 weeks later and then every 3-4 months.

After ALT, a topical steroid is prescribed four to six times per day for 4-7 days, as the procedure is inflammatory. In SLT, it is more common not to prescribe any anti-inflammatory medications postoperatively, as it is felt that these agents may blunt the biological effects of the laser. Many surgeons will give a script for a non-steroidal anti-inflammatory to be used as needed if patient suffers ocular discomfort. Of note, a small prospective observer-masked study found that a 1-week course of topical prednisolone acetate 1% did not affect the IOP-lowering effect of SLT at 3 months.[17]  Patients are instructed to resume their usual antihypotensive drops immediately after the laser. A decision to discontinue drops can be made based on IOP response after 6-8 weeks. 

In 2018, the Steroids after Laser Trabeculoplasty (SALT) trial intended to assess the impact of topical anti-inflammatory drugs after a SLT procedure in inflammation, pain and IOP reduction in 96 eyes. Three subgroup of patients submitted to SLT were studied: a first subgroup with only artificial tears, a second subgroup submitted to steroid therapy (prednisolone 4id 5 days) and a third subgroup submitted to NSAID therapy (ketorolac 0.5% 4id 5 days). [18]

Regarding IOP reduction and inflammatory outcomes, there was no significant difference between subgroups at 1 week post-SLT. At 6 weeks post-SLT, there was a trend for greater reduction in NSAID (-6.37 mmHg) and steroid (-5.85 mmHg) subgroups than in saline tears (-4.26 mmHg), albeit statistically non-significant (p=0.14). However, at 12 weeks post-SLT, there was a statistically significant greater reduction in IOP in the NSAID (-6.22 vs. -2.96 mmHg, p=0.002) and steroid (-5.21 vs. -2.96 mmHg, p=0.02) subgroups than in the saline tears subgroup. When comparing NSAID and steroid subgroup, there was a trend for greater reduction of IOP in the NSAID, although statistically non-significant (p=0.18).[18]

Then, the SALT trial dispels the theory of an “inflammatory” mechanism of action in SLT with underlying reduction of efficacy when prescribing topical anti-inflammatories after SLT procedures. Furthermore, the SALT trial suggests the use of NSAIDs or steroids post-SLT procedures as auxiliary agents in SLT short-term efficacy.[18]

Complications

A transient rise in IOP after laser trabeculoplasty is the complication of greatest significance to glaucoma patients undergoing this treatment. With 180° of ALT in the Glaucoma Laser Treatment Trial, a rise of > 5 mmHg was reported in 34% and a rise >10 mmHg was seen in 12% of patients. Of note, there was no perioperative alpha-adrenergic prophylaxis used in this trial.[19] The frequency of IOP spikes is reduced by two-thirds with the use of prophylactic alpha-adrenergics.[20] Postoperative IOP rise is more severe and frequent with higher energy levels, 360° treatments, posterior placement, heavy angle pigmentation, and a low preoperative outflow facility.[21] Spikes are usually transient, occur within the first hour although they may be delayed,[22] and most resolve with medical treatment by the next day.

In SLT prophylactically treated for a pressure spike, the reported rate of an IOP rise > 5 mmHg is around 10% or less and the rate of an IOP rise > 10 mmHg is around 3%.[23] There are rare cases requiring trabeculectomy for sustained IOP increases after both SLT and ALT,[24] and this possibility should be included in the informed consent process for either procedure.

Other complications seen with either form of trabeculoplasty, but which are popularly believed to occur more often with ALT although the literature does not show this to be true,[25] are low-grade iritis and the formation of PAS. Corneal edema attributable to HSV reactivation has been reported following SLT. The thought is that the inflammatory cascade following laser contributes to virus reactivation.[26] Hyphemas have also been reported.[27][28]

There is only one randomized, clinical trial comparing SLT (n=89 eyes) and ALT (n=87 eyes) with one year of follow-up. See table for comparative results.[25]

Complication ALT SLT
IOP Spike 3.4% 4.5%
PAS Formation 1.2% 1.1%
ALT treatment within 1 year 5.7% 3.4%
SLT treatment within 1 year 4.6% 6.7%
Trabeculectomy within 1 year 8.0% 9.0%

Efficacy of ALT and SLT

Selected Studies ALT

Author Number of Eyes/Dx Follow Up (Years) IOP Reduction(%)
Amon et al. 1990

Ophthalmologica

61 POAG 4.4 32
Lotti et al. 1995

Ophthalmic Surg

237 POAG 11 19
Sharma et al. 1997

Indian J Ophthal.

36 POAG 2 29
Odberg et al. 1999

Acta Oph. Scand.

168 POAG and PXF 8 32
Agarwal et al. 2002 40 POAG 5 30
BJO 39 POAG (on glaucoma medications) 5 13

Selected Studies SLT

Author Number of Eyes Follow Up Mean IOP Reduction % IOP Reduction
(mm Hg)
Gracner 2001 50 OAG 6 months 5.1 22.5
Ophthalmologica
Melamed et al. 2003 45 OAG 6 -18 months 7.7 30
 Arch Oph.
Lai et al. 2004 58 OAG/OHT 5 years 8.7 32
Clin Exp Oph.  
Cvenkel 2004 44 OAG 1 year 7.1 27.6
Ophthalmologica
McIlraith et al. 2006 74 OAG/OHT 1 year 8.3 31
J Glaucoma
Weinand et al. 2006 52 OAG 1 year 6 24.3
 Eur J Glauc. 4 years 6.3 29.3
Khawaja et al. 2019[29] 831 6 months 70%
12 months 45%
24 months 27%
Abe et al. 2022 835 916.8 ± 563.0 days 4.0

Selected Studies Comparing ALT and SLT

Author Number of Eyes Mean Follow Up Mean IOP Reduction P Value
(mm Hg)
SLT      ALT
Damji et al. 1999 36 6 Months 4.8       4.7 0.97
 BJO
Damji et al. 2006 176 12 Months 5.9      6.04 0.84
BJO
Popiela et al. 2000  27 3 Months 2.85     2.63 0.84
 Klin Oczna
Juzych et al. 2004 154   ALT 5 Years  % IOP Reduction ___
Ophthalmology 41    SLT 32%     31%

Prognosis

The effect of either form of laser trabeculoplasty diminishes over time. In a retrospective analysis of longer term outcomes of SLT (n=41) compared to ALT (n=154), success was defined as an IOP decrease of at least 3 mmHg without additional medication or surgery. Success rate in the SLT group at 1, 3, and 5 year follow-up time points was 68%, 46%, and 32%, respectively, while in the ALT group it was 54%, 30%, and 31%. There was no statistically significant difference at any time point.[30]

A prospective study that randomized patients to 180° of SLT versus ALT, found no statistically significant difference in IOP reduction between the two procedures. At 6 months, IOP decreased by 4.8±3.4 mmHg in the SLT group, and 4.7±3.3 mmHg in the ALT group.[31] An extension of the previously mentioned study to 12 months showed no difference between IOP results, and this extension allowed additional medications, laser and surgery, as would occur in clinical practice.[25]

ALT has been studied in a NEI-sponsored randomized multi-center trial, called the Glaucoma Laser Trial (GLT), which was published in the 1990s. The study compared 360° ALT to medical therapy with timolol 0.5% in newly diagnosed patients with primary open angle glaucoma. The GLT found that ALT lowered IOP by 9 mmHg compared to 7 mmHg with timolol alone. At two years, no further intervention was required in 44% of ALT eyes and in 30% of medication eyes. After seven years of follow-up, the ALT eyes had lower IOPs and less subjective field loss than medication eyes.[32][33][34]

A study of prospectively enrolled SLT treated eyes (n=29) has been reported in a study from Hong Kong that had a similar design to the GLT in that one eye was randomized to 360° SLT while the other eye was given topical medication. The patients were recently diagnosed with POAG or OHTN with no previous treatment and were followed for 5 years after treatment intervention. After 5 years, 27.6% of the SLT eyes required additional treatment. No difference in IOP reduction was found between treatments. Mean IOP reduction was 32.1% in SLT eyes and 33.2% in medically treated eyes.[35] Both this study and the GLT can be criticized for overestimating the effect of trabeculoplasty as a result of cross-over effect of the medications in the contralateral eye. In another study comparing 180° degrees of  SLT treatment to latanoprost as initial treatment for newly diagnosed OAG and OHTN, in which the treatment was chosen by the patient, IOP percent decrease (~30%) was similar between groups with average starting pressures in the mid-20s. [36]

The LiGHT trial compared SLT to eye drops as first-line treatment for ocular hypertension and glaucoma and enrolled 718 patients. At 36 months, eyes randomized to SLT were within target IOP at a higher percentage of visits compared to the eye drops group (93% vs. 91.3%, respectively) and were less likely to require glaucoma surgery. SLT was found to be more cost-effective than an eye-drop first approach.[37] Follow-up analyses from the LiGHT trial found that patients treated with medications first were more likely to experience rapid visual field deterioration.[38] A post hoc analysis of the LiGHT trial extension evaluated secondary SLT in patients initially treated with eye drops for at least 3 years. In stable eyes switched from drops to SLT, medication burden decreased from 1.38 to 0.59 active ingredients, and 60.5% of eyes required no medical or surgical treatment at 72 months. In medically uncontrolled eyes, SLT escalation reduced IOP by 4.6 mmHg, or 21.8%, with 62.5% reaching target IOP without surgery, although 18.7% still required trabeculectomy.[39] Six-year follow-up from the LiGHT trial showed durable benefit of primary SLT compared with initial eye-drop therapy. At 6 years, 69.8% of eyes initially treated with SLT remained at or below target IOP without medical or surgical treatment. Eyes initially treated with drops had higher rates of disease progression than SLT-treated eyes, 26.8% versus 19.6%, and were more likely to require trabeculectomy, 32 eyes versus 13 eyes. No serious laser-related adverse events were reported.[40] Another six-year LiGHT trial visual field analysis found slower mean deviation progression in eyes treated with SLT first compared with drops first. The mean visual field MD rate was −0.26 dB/year in the SLT-first arm compared with −0.37 dB/year in the drops-first arm, with the difference most evident in mild open-angle glaucoma. These findings further support first-line SLT as a treatment strategy that may improve long-term disease control and visual field preservation.[41]

A large real-world UK study evaluated 831 eyes undergoing first-recorded SLT across 5 ophthalmology teaching centers. SLT significantly reduced IOP by 4.2 mmHg at 12 to 18 months and 3.4 mmHg at 24 to 36 months, although medication use increased slightly over time. Treatment success declined from 70% at 6 months to 45% at 12 months and 27% at 24 months.[29]

A multicenter real-world Brazilian study of 835 eyes found that primary SLT significantly reduced mean IOP from 18.4 mmHg to 14.8 mmHg and reduced mean glaucoma medication burden from 1.8 to 1.4 medications. Kaplan-Meier analysis showed estimated treatment success rates of 88% at 12 months, 70% at 24 months, and 54% at 36 months. Denser angle pigmentation and corticosteroid use after SLT were associated with a lower risk of treatment failure, and no sight-threatening complications were reported.[42] Furthermore, a randomized controlled trial from Tanzania compared SLT with twice-daily timolol 0.5% in patients with open-angle glaucoma and IOP greater than 21 mmHg. At 12 months, treatment success was achieved in 61% of SLT-treated eyes compared with 31% of timolol-treated eyes, with similar adverse event rates between groups. This study suggests that SLT may be especially valuable in settings where medication adherence, access, and long-term drop use are major barriers to glaucoma care.[43]

After 360° of angle is treated by ALT, it is recommended that no further ALT is performed. Repeat ALT 1-year success rates vary from 21% to 73%.[44] [45] [46] [47] With SLT it has been suggested that since there is minimal tissue alteration that the procedure can be repeated with good efficacy. In a study of 360° SLT after prior 360° SLT that was successful for at least 6 months (n=44 eyes), IOP reduction was seen with the second SLT treatment although the magnitude of IOP decrease was smaller, average decrease 5 mmHg after first SLT and 2.9 mmHg after second SLT. [48]A post hoc analysis of the LiGHT trial evaluated repeat 360-degree SLT in medication-naive OAG and OHT eyes that required retreatment within 18 months. Repeat SLT maintained drop-free IOP control in 67% of 115 eyes at 18 months, with no clinically relevant adverse events. After adjustment for pretreatment IOP, repeat SLT produced at least comparable IOP-lowering efficacy and duration of effect to the initial SLT treatment.[49]

SLT performed in eyes with previous ALT is comparatively effective.[25][50] [51] [52] In one study, IOP was reduced by 5 mmHg or more in 40% of eyes without prior ALT and in 57% of eyes with prior ALT.[53] The effect of trabeculoplasty on diurnal curve has been studied. Both ALT and SLT have been shown to decrease diurnal IOP fluctuation.[54] [55] [56] In different subgroups of patients, both ALT and SLT have been found efficacious when compared to treatment for POAG. In pseudoexfoliation[57][58][59] SLT has not been found to be less efficacious in pseudophakic eyes compared to phakic ones.[60] Whereas it is generally thought that ALT is better performed while an eye is phakic. [61]

Laser trabeculoplasty may be effective after failed glaucoma filtering surgery.[62]

One study compared the cost-effectiveness of generic topical prostaglandin analogues (PGAs) versus laser trabeculoplasty in patients with newly diagnosed mild POAG and found that PGAs provide marginally better value compared to laser trabeculoplasty when utilizing a 25 year time horizon. However, when assuming more realistic levels of medication adherence (25% less effective), laser trabeculoplasty became a more cost effective alternative.[63]

Prognostic Factors

Prognostic factors associated with SLT success have been examined extensively, with elevated baseline IOP consistently demonstrating an association with treatment success. In contrast, factors such as age, glaucoma type, glaucoma severity and visual field mean deviation have not consistently demonstrated a correlation with SLT success. The role of trabecular meshwork pigmentation remains controversial, with some studies suggesting an association with treatment response and others finding no significant relationship. A large retrospective study of 997 eyes evaluated predictors of SLT success, defined as at least 20% IOP reduction from baseline. SLT success was associated with higher baseline IOP and greater angle pigmentation before treatment, while age, glaucoma severity, total SLT power, glaucoma type, visual field mean defect, retinal nerve fiber layer thickness, medication use, and prior glaucoma treatments were not associated with success.[64] A LiGHT trial substudy confirmed that higher baseline IOP predicts greater IOP lowering after both SLT and prostaglandin analog therapy. In treatment-naive eyes, SLT was more likely than prostaglandin analog drops to achieve at least 20% IOP reduction when baseline IOP was greater than 22.5 mmHg, while both treatments were less effective at lower baseline IOP.[65] In a UK study, higher baseline IOP was strongly associated with treatment success, while age, baseline visual field mean deviation, and concurrent use of IOP-lowering medication were not significantly associated with SLT success.[29] Another retrospective study of 252 eyes with open-angle glaucoma found that higher baseline IOP was the main predictor of SLT success. Eyes with baseline IOP greater than 18 mmHg had greater IOP reduction at 2 and 6 months, while age, glaucoma type, glaucoma severity, trabecular meshwork pigmentation, and total laser energy were not significantly associated with success or postoperative IOP spikes.[66]

Direct selective laser trabeculoplasty (DSLT)

DSLT is a newer automated, noncontact, image-guided form of laser trabeculoplasty that delivers frequency-doubled Q-switched Nd:YAG laser energy (532 nm) in ~3 ns pulses trans-limbally to the trabecular meshwork without requiring a gonioscopy lens or coupling medium.[67] [68]Treatment is delivered in an automated, circumferential, consecutive pattern of 100–120 laser spots over 360 degrees, guided by integrated eye-tracking to ensure accurate alignment and reduce operator dependency compared with conventional SLT. [67] Two commercial platforms have been developed. The Belkin Vision Eagle™ device, the system used in the pivotal clinical trials, received FDA clearance in 2023. Following Alcon’s acquisition of Belkin Vision in 2024, the technology was relaunched as the Alcon Voyager™, which received FDA clearance in 2025.[69][70]

The mechanism of IOP reduction is analogous to SLT, relying on selective photothermolysis of pigmented trabecular meshwork cells to stimulate aqueous outflow remodelling, while sparing adjacent non-pigmented tissue.[67] Because laser energy is delivered trans-limbally rather than across the iris, patients may experience fewer IOP spikes and less intraocular inflammation compared with conventional SLT. [71]

DSLT is indicated for open-angle glaucoma (OAG) and ocular hypertension (OHT) in eyes with adequate fixation and without active uveitis, neovascularisation of the angle, or implants that obstruct the limbally directed beam.[67][68]

Procedural advantages. The procedure requires only a lid speculum, without the need for a gonioscopy lens or coupling gel, which improves patient comfort. Automated targeting accommodates a variety of patient and practitioner body morphologies and reduces the learning curve associated with gonioscopy. Total treatment time is approximately 1–2 minutes. These features may improve access to laser trabeculoplasty, particularly in settings where gonioscopy expertise is limited.[67][68]

Clinical evidence. The first prospective human study (Goldenfeld et al., 2021) evaluated 15 eyes with OAG, OHT, and pseudoexfoliation glaucoma. It shown a sustained IOP reduction of 18–27% over 6 months alongside a significant decrease in hypotensive medication burden (from 1.6 to 0.4 drugs). No serious adverse events occurred. [71]

The GLAUrious trial (Gazzard et al., 2025) was a prospective, multicentre, randomised, evaluator-masked non-inferiority study conducted between 2021 and 2023, comparing DSLT (Eagle™ device) with conventional SLT in patients with OAG or OHT. The 6-month washout IOP decreased by 5.5 mmHg (20.6%) after DSLT and by 6.2 mmHg (23.6%) after SLT. Although the 6-month primary non-inferiority endpoint was narrowly missed, the trial demonstrated comparable non-washout IOP reduction of 3.2 mmHg in both groups at 12 months, with similar safety profiles.[67] Punctate subconjunctival haemorrhage was more frequent after DSLT, but no sight-threatening complications were reported in either arm.[67]

A retrospective 2025 real-world study from a UK tertiary centre compared 16 eyes treated with DSLT and 16 eyes treated with SLT over 12 months, reporting comparable IOP reduction and treatment survival rates at all time points (38% vs 43% at 12 months, respectively), with no complications in either group. [72] A prospective Italian 2025 observational study of 54 eyes with primary OAG and primary angle-closure glaucoma (after prior iridotomy) reported sustained IOP reduction of 3.76 mmHg and a decrease in mean number of hypotensive medications of 0.79 at 12 months of follow-up.[73]

Repeatability. Unlike SLT, for which repeat treatment after a successful initial response is well established, the safety and efficacy of repeat DSLT have not yet been evaluated in clinical trials, this remains an area for future investigation.[67]

The comparative features of ALT, SLT, and DSLT are summarised in Table 5.

Feature ALT SLT DSLT
Delivery method Gonioscopy lens, direct contact Gonioscopy lens, direct contact Non-contact, trans-limbal, automated
Tissue effect Coagulative damage Selective photothermolysis
Typical IOP reduction 20–30%; effect wanes over years 20–30%; durable; repeatable ~3–6 mmHg at 6–12 months; non-inferior to SLT
Operator dependency High (gonioscopy required) Moderate (gonioscopy required) Low (automated targeting)
Repeatability Limited Well established Under investigation
Regulatory status Long established FDA-approved 2002 FDA-cleared 2025 (Alcon Voyager™)

Table 5. Comparative features of argon, selective, and direct selective laser trabeculoplasty.

Future Directions

One area of ongoing investigation is whether the durability of SLT can be improved by modifying both the energy settings and retreatment schedule. Realini et al. proposed that low-energy SLT, repeated annually regardless of measured IOP, may provide longer medication-free survival than standard-energy SLT repeated only after treatment effect wanes.[74] This approach is being evaluated in the Clarifying the Optimal Application of SLT Therapy (COAST) Trial, which will assess whether planned annual low-energy SLT can optimize laser-first treatment for patients with mild to moderate primary open-angle glaucoma or high-risk ocular hypertension.[74]

References

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  2. ↑ Thomas JV, Simmons RJ, Belcher CD. Argon laser trabeculoplasty in the presurgical glaucoma patient. Ophthalmol 1982; 89: 187-97.
  3. ↑ Brubaker RF, Liesegang TJ. Effect of trabecular photocoagulation on the aqueous humor dynamics of the human eye. Am J Ophthalmol 1983; 96: 139-47.
  4. ↑ van der Zypen E, Bebie H, Frankhauser F. Morphologic studies about the efficiency of laser beams upon the structure of the angle of the anterior chamber. Facts and concepts related to the treatment of the chronic simple glaucoma. Int Ophthalmol 1979; 1:109-22.
  5. ↑ 5.0 5.1 van Buskirk EM, Pond V, Rosenquist RC. Argon laser trabeculoplasty. Studies of mechanism of action. Ophthalmol 1984; 91:1005-1010.
  6. ↑ Acott TS, Samples JR, Bradley JM, et al. Trabecular repopulation by anterior trabecular meshwork cells after laser trabeculoplasty. Am J Ophthalmol 1989; 107: 1-6.
  7. ↑ Bylsma SS, Samples JR, Acott TS, Van Buskirk EM. Trabecular cell division after argon laser trabeculoplasty. Arch Ophthalmol 1988; 106: 544-547.
  8. ↑ Bylsma SS, Samples JR, Acott TS et al. DNA replication in the cat trabecular meshwork after argon laser trabeculoplasty in vivo. J Glaucoma 1994; 3:36-43.
  9. ↑ Parshley DE, Bradley JM, Samples JR, et al. Early changes in matrix metalloproteinases and inhibitors after in vitro laser treatment to the trabecular meshwork. Curr Eye Res 1995; 14: 537-544.
  10. ↑ Parshley DE, Bradley JM, Fisk A, et al. Laser trabeculoplasty induces stomyelsin expression by trabecular juxtacanalicular cells. Invest Ophthalmol Vis Sci 1996; 37:795-804.
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  12. ↑ Bradley JM, Andersson AM, Colvis CM, et al. Mediation of laser trabeculoplasty-induced matrix metalloproteinase expression by Il-1beta and TNF-alpha. Invest Ophthalmol Vis Sci 2000; 41: 422-430.
  13. ↑ Alvarado JA, Alvarado RG, Yeh RF et al. A new insight into the cellular regulation of aqueous outflow: how trabecular meshwork endothelial cells drive a mechanism that regulates the permeability of Schlemm’s canal endothelial cells. Br J Ophthalmol 2005; 89: 1500-1505.
  14. ↑ Rodrigues MM, Spaeth GL, Donohoo P. Electron microscopy of argon laser therapy in phakic open-angle glaucoma. Ophthalmology. 1982;89(3):198-210.
  15. ↑ Kramer TR, Noecker RJ. Comparison of the morphologic changes after selective laser trabeculoplasty and argon laser trabeculoplasty in human eye bank eyes. Ophthalmol 2001; 108: 773-79.
  16. ↑ van Buskirk EM. Pathophysiology of laser trabeculoplasty. Surv Ophthlamol 1989; 33: 264-272.
  17. ↑ Realini T, Charlton J, Hettlinger M. The impact of anti-inflammatory therapy on intraocular pressure reduction following selective laser trabeculoplasty. Ophthalmic Surg Lasers Imaging 2010; 41: 100-3.
  18. ↑ 18.0 18.1 18.2 Groth SL, Albeiruti E, Nunez M, Fajardo R, Sharpsten L, Loewen N, Schuman JS, Goldberg JL. SALT Trial: Steroids after Laser Trabeculoplasty: Impact of Short-Term Anti-inflammatory Treatment on Selective Laser Trabeculoplasty Efficacy. Ophthalmology. 2019 Jun 6. pii: S0161-6420(18)32934-8. doi: 10.1016/j.ophtha.2019.05.032.
  19. ↑ Glaucoma Laser Trial Research Group. The Glaucoma Laser Trial 1. Acute effects of argon laser trabeculoplasty on intraocular pressure. Arch Ophthalmol 1989; 107: 1135-42.
  20. ↑ Robin AL, Pollack IP, House B, Enber C. Effects of ALO 2145 on intraocular pressure following argon laser trabeculoplasty. Arch 1987; 105: 646-50.
  21. ↑ Keightley SJ, Khaw PT, Elkington AR. The prediction of IOP rise following argon laser trabeculoplasty. Eye 1987; 1:577-80.
  22. ↑ Weinreb RN, Ruderman J, Juster R, Zweig K. Immediate IOP response to argon laser trabeculoplasty. Am J Ophthalmol 1983; 95: 279-86.
  23. ↑ Barkana Y, Belkin M. Selective Laser Trabeculoplasty. Surv Ophthalmol 2007; 52: 634-654.
  24. ↑ Harasymowycz PJ, Papamatheakis DG, Latina M, et al. Selective Laser Trabeculoplasty complicated by IOP elevation in eyes with heavily pigmented trabecular meshworks. Am J Ophthalmol 2005; 139: 1110-3.
  25. ↑ 25.0 25.1 25.2 25.3 Damji KF, Bovell AM, Hodge WG, Rock W, Shah K, Buhrmann R, Pan YI. Selective laser trabeculoplasty versus argon laser trabeculoplasty: results from a 1-year randomized clinical trial. Br J Ophthalmol 2006; 90: 1490-1494
  26. ↑ Moubayed SP, Hamid M, Choremis J, Li G. An unusual finding of corneal edema complicating selective laser trabeculoplasty. Can J Ophthalmol 2009; 44: 337-38.
  27. ↑ Rhee DJ, Krad O, Pasquale LR. Hyphema following selective laser trabeculoplasty. Oph Surg Lasers Imaging 2009; 40: 493-4.
  28. ↑ Shihadeh WA, Ritch R, Liebmann JM. Hyphema occurring during selective laser trabeculoplasty. Ophthalmic Surg Lasers Imaging 2006; 37(5): 432-3.
  29. ↑ 29.0 29.1 29.2 Khawaja, Anthony P., et al. "Real-world outcomes of selective laser trabeculoplasty in the United Kingdom." Ophthalmology 127.6 (2020): 748-757.
  30. ↑ Juzych MS, Chopra V, Banitt MR, et al. Comparison of long-term outcomes of selective laser trabeculoplasty versus argon laser trabeculoplasty in open angle glaucoma. Ophthalmol 2004; 111:1853-1859.
  31. ↑ Damji KF, Shah KC, Rock WJ, et al. Selective laser trabeculoplasty versus argon laser trabeculoplasty: a prospective randomized clinical trial. Br J Ophthalmol 1999; 83: 718-22.
  32. ↑ The Glaucoma Laser Trial (GLT) 2 Results of argon laser trabeculoplasty versus topical medicines. The Glaucoma Laser Trial Group. Ophthalmol 1990; 97: 1403-13.
  33. ↑ The Glaucoma Laser Trial (GLT) and glaucoma laser trial follow-up study: 7 Results. Glaucoma Laser Trial Group. Am J Ophthalmol 1995; 120: 718-31.
  34. ↑ The Glaucoma Laser Trial (GLT) 3 Design and methods. Glaucoma Laser Trial Group. Control Clin Trials 1991; 12:504-24.
  35. ↑ Lai JS, Chua JK, Tham CC, Lam DS. Five-year follow up of selective laser trabeculoplasty in Chinese eyes. Clin Experiment Ophthalmol 2004; 91:361-5.
  36. ↑ McIlraith I, Strasfeld M, Colev G, Hutnik CM. Selective laser trabeculoplasty as initial and adjunctive treatment for open-angle glaucoma. J Glaucoma 2006; 15:124-30.
  37. ↑ Gazzard G, Konstantakopoulou E, Garway-Heath D, et al. Selective Laser Trabeculoplasty versus Eye Drops for First-Line Treatment of Ocular Hypertension and Glaucoma (LiGHT): a Multicentre Randomised Controlled Trial. Lancet 2019;393:1505-16.
  38. ↑ Wright DM, Konstantakopoulou E, Montesano G, et al. Visual Field Outcomes from the Multicenter, Randomized Controlled Laser in Glaucoma and Ocular Hypertension Trial (LiGHT). Ophthalmology 2020; 127:1313-1321.
  39. ↑ Konstantakopoulou, Evgenia, et al. "Selective laser trabeculoplasty after medical treatment for glaucoma or ocular hypertension." JAMA ophthalmology 143.4 (2025): 295-302.
  40. ↑ Gazzard, Gus, et al. "Laser in glaucoma and ocular hypertension (LiGHT) trial: six-year results of primary selective laser trabeculoplasty versus eye drops for the treatment of glaucoma and ocular hypertension." Ophthalmology 130.2 (2023): 139-151.
  41. ↑ Montesano, Giovanni, et al. "Six-year rate of visual field progression in the Laser in Glaucoma and Ocular Hypertension (LiGHT) Trial." Ophthalmology (2025).
  42. ↑ Abe, Ricardo Y., et al. "Real-world data from selective laser trabeculoplasty in Brazil." Scientific Reports 12.1 (2022): 1923.
  43. ↑ Philippin, Heiko, et al. "Selective laser trabeculoplasty versus 0· 5% timolol eye drops for the treatment of glaucoma in Tanzania: a randomised controlled trial." The Lancet Global health 9.11 (2021): e1589-e1599.
  44. ↑ Feldman RM, Katz LJ, Spaeth GL, et al. Long-term efficacy of repeat argon laser trabeculoplasty. Ophthalmol 1991; 98: 1061-5.
  45. ↑ Richter CU, Shingleton BJ, Bellows AR, et al. Retreatment with argon laser trabeculoplasty. Ophthalmol 1987; 94: 1085-9.
  46. ↑ Starita RJ, Fellman RL, Spaeth GL, et al. The effect of repeating full-circumference argon laser trabeculoplasty. Ophthalmic Surg 1984; 15:41-3.
  47. ↑ Weber PA, Burton GD, Epitropoulos AT. Laser trabeculoplasty retreatment. Ophthalmic Surg 1989; 20:702-6.
  48. ↑ Hong BK, Winer JC, Martone JF, et al. Repeat selective laser trabeculoplasty. J Glaucoma 2009; 18:180-3.
  49. ↑ Garg, Anurag, et al. "Efficacy of repeat selective laser trabeculoplasty in medication-naive open-angle glaucoma and ocular hypertension during the LiGHT trial." Ophthalmology 127.4 (2020): 467-476.
  50. ↑ Birt CM. Selective laser trabeculoplasty retreatment after prior argon laser trabeculoplasty: 1-year results. Can J Ophthalmol 2007; 42:715-9.
  51. ↑ Kano K, Kuwayama Y, Mizoue S, et al. Clinical results of selective laser trabeculoplasty. Nippon Ganka Gakkai Zasshi 1999; 103:612-6.
  52. ↑ Song J, Lee PP, Epstein DL, et al. High failure rate associated with 180° selective laser trabeculoplasty. J Glaucoma 2005; 14:400-8.
  53. ↑ Latina MA, Sibayan SA, Shin DH, et al. Q-switched 532nm Nd-YAG laser trabeculoplasty (selective trabeculoplasty), a multi-center, pilot, clinical study. Ophthalmol 1998; 105:2082-90.
  54. ↑ Kóthy P, Tóthy M, Holló G. Influence of selective laser trabeculoplasty on 24-hour diurnal intraocular pressure fluctuation in primary open-angle glaucoma: a pilot study. Ophthalmic Surg Lasers Imaging 2010; 41:342-7.
  55. ↑ Greenidge KC, Spaeth GL, Fiol-Silva Z. Effect of argon laser trabeculoplasty on the glaucomatous diurnal curve. Ophthalmol 1983; 90:800-804.
  56. ↑ Lee AC, Mosaed S, Weinreb RN, Kripke DF, Liu JH. Effect of laser trabeculoplasty on nocturnal intraocular pressure in medically treated glaucoma patients. Ophthalmology 2007; 114:666-670.
  57. ↑ Odberg T, Sandvik L. The medium and long-term efficacy of primary argon laser trabeculoplasty in avoiding topical medication in open angle glaucoma. Acta Ophthalmol Scand 1999; 77:176-81.
  58. ↑ Threlkeld AB, Hertzmark E, Strurm RT, et al. Comparative study of the efficacy of argon laser trabeculoplasty for exfoliation and primary open-angle glaucoma. J Glaucoma 1996; 5:311-6.
  59. ↑ Gracner T. Intraocular pressure response of capsular glaucoma and primary open-angle glaucoma to selective Nd:YAG laser trabeculoplasty: a prospective, comparative clinical trial. Eur J Ophthalmol 2002; 12: 287-92.
  60. ↑ Werner M, Smith MF, Doyle JW. Selective laser trabeculoplasty in phakic and pseudophakic eyes. Ophthalmic Surg Lasers Imaging 2007; 38:182-8.
  61. ↑ Brown SV, Thomas JV, Budenz DL, Bellows AR, Simoon RJ. Effect of cataract surgery on intraocular pressure reduction obtained with laser trabeculoplasty. Am J Ophthalmol 1985; 100:373-6.
  62. ↑ Fellman RL, Starita RJ, Spaeth GL, Poryzees EM. Argon laser trabeculoplasty following failed trabeculectomy. Ophthalmic Surg. 1984;15:195-198.
  63. ↑ Stein JD, Kim DD, Peck, WW. Giannettia SM, Hutton DW. Cost effectiveness of medications compared with laser trabeculoplasty in patients with newly-diagnosed open-angle glaucoma. Arch Ophthalmol 2012;130:497-505.
  64. ↑ Ayala, Marcelo, and Enping Chen. "Predictive factors of success in selective laser trabeculoplasty (SLT) treatment." Clinical ophthalmology (2011): 573-576.
  65. ↑ Fahy, Eamonn T., et al. "The impact of baseline intraocular pressure on initial treatment response in the light trial: selective laser trabeculoplasty versus medication." Ophthalmology 131.12 (2024): 1366-1376.
  66. ↑ Hirabayashi, Matthew, Vikram Ponnusamy, and Jella An. "Predictive factors for outcomes of selective laser trabeculoplasty." Scientific reports 10.1 (2020): 9428.
  67. ↑ 67.0 67.1 67.2 67.3 67.4 67.5 67.6 67.7 Gazzard, Gus, et al. "Randomized noninferiority trial of direct selective laser trabeculoplasty in open-angle glaucoma and ocular hypertension: GLAUrious study." Ophthalmology (2025).
  68. ↑ 68.0 68.1 68.2 EyeNet Magazine. An early look at the new DSLT technology. American Academy of Ophthalmology. Published June 1, 2025. Accessed May 4, 2026. Available at: https://www.aao.org/eyenet/article/direct-selective-laser-trabeculoplasty
  69. ↑ BELKIN Vision. BELKIN Vision announces FDA clearance for the Eagle™ device [press release]. Yavne, Israel: BELKIN Vision; December 12, 2023. Available at: https://www.prnewswire.com/news-releases/belkin-vision-announces-fda-clearance-for-the-eagle-device-302012569.html. Accessed 17/05/2026.
  70. ↑ Alcon. Alcon announces US launch of Voyager DSLT, first-of-its-kind treatment for glaucoma and ocular hypertension [press release]. Fort Worth, TX: Alcon; February 19, 2025. Available at: https://www.alcon.com/media-release/alcon-announces-us-launch-voyager-dslt-first-its-kind-treatment-glaucoma-and-ocular/. Accessed 17/05/2026.
  71. ↑ 71.0 71.1 Goldenfeld M, Belkin A, Ofir S, et al. Direct selective laser trabeculoplasty (DSLT) for open-angle glaucoma and ocular hypertension: first human prospective clinical trial. Br J Ophthalmol. 2021;105(8):1073–1077.
  72. ↑ Zollet P, Macario F, Healy R, Manasses DT, Sebastian RT, Romano MR. Real world comparison of direct selective laser trabeculoplasty versus selective laser trabeculoplasty: 12-month retrospective study of a tertiary center in the UK. Biomedicines. 2026 Jan 12;14(1):156. doi: 10.3390/biomedicines14010156.
  73. ↑ Lanza M, Cantarella G, Alessio G, et al. Direct selective laser trabeculoplasty (DSLT) in open-angle and angle-closure glaucoma: 12-month outcomes from a prospective study. BMJ Open Ophthalmol. 2025;10(1):e002857.
  74. ↑ 74.0 74.1 Realini, Tony, et al. "Low-energy selective laser trabeculoplasty repeated annually: rationale for the COAST trial." Journal of glaucoma 30.7 (2021): 545-551.
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