View source for Laser Trabeculoplasty: ALT vs SLT

{{Article
|Authors=JoAnn.A.Giaconi.CMT
|Additional contributors=Tiago.MoraisSarmento
|Category=Articles, Glaucoma
|Assigned editor=Ahmad.A.Aref
|Reviewer=Ahmad.A.Aref
|Date reviewed=September 20, 2019
|Article status=Up to Date
}}
= 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&nbsp;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. However, additional studies are needed to confirm this advantage. Currently, the SLT/MED Study is being conducted to determine how SLT compares to available medications as a primary therapy in patients with POAG. This brief review will discuss proposed mechanisms of action for trabeculoplasty, describe the surgical technique and postoperative management, and review recent literature comparing these two modalities in terms of efficacy and safety profile. 

= 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)&nbsp;in order to lower intraocular pressure (IOP) in cases of ocular hypertension and glaucoma.<ref name=":1">Goyal S, Beltran-Agullo L, Rashid S, et al. Effect of primary selective laser trabeculoplasty on tonographic outflow facility: a randomized clinical trial. Br J Ophthalmol May 2010.</ref>

== 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&nbsp;laser trabeculoplasty&nbsp;is not well established. Various&nbsp;theories have been&nbsp;proposed as explanations for the increased aqueous outflow facility seen following successful trabeculoplasty,<ref name=":2">Thomas JV, Simmons RJ, Belcher CD. Argon laser trabeculoplasty in the presurgical glaucoma patient. Ophthalmol 1982; 89: 187-97.</ref><ref name=":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.</ref> 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.<ref name=":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.</ref> However, there is some evidence that&nbsp;the mechanical theory&nbsp;may be flawed <ref name="vanbuskirk5">van Buskirk EM, Pond V, Rosenquist RC. Argon laser trabeculoplasty. Studies of mechanism of action. Ophthalmol 1984; 91:1005-1010.</ref>

The&nbsp;celluar theory for ALT&nbsp;is based on&nbsp;stimulation and increased cell division and repopulation of the trabecular meshwork.<ref name=":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.</ref>An increase in DNA replication and cell division following argon laser treatment have been demonstrated.<ref name=":7">Bylsma SS, Samples JR, Acott TS, Van Buskirk EM. Trabecular cell division after argon laser trabeculoplasty. Arch Ophthalmol 1988; 106: 544-547. </ref><ref name=":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.</ref>

The&nbsp;biochemical theory for both ALT and SLT&nbsp;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.<ref name=":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.</ref><ref name=":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.</ref> 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.<ref name=":11">Melamed S, Pei J, Epstein DL. Short term effect of argon laser trabeculoplasty in monkeys. Arch Ophthalmol 1985; 103: 1546-1552.</ref><ref name=":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.</ref> 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. <ref name=":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.</ref>

Studies conducted by Kramer and Noecker in eyes treated&nbsp;by ALT&nbsp;or SLT&nbsp;using scanning and transmission electron microscopy&nbsp;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&nbsp;minimal change was apparent on imaging.<ref name=":14">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.</ref>&nbsp;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&nbsp;has promoted repeatability of SLT over ALT.&nbsp;It is important to&nbsp;note that some&nbsp;earlier studies of ALT did not show the same crater-like damage seen in the previously mentioned study, but only mild coagulative damage.<ref name=":15">van Buskirk EM. Pathophysiology of laser trabeculoplasty. Surv Ophthlamol 1989; 33: 264-272.</ref><ref name="vanbuskirk5"/> 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&nbsp;sufficient&nbsp;angle area &nbsp;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&nbsp;is increasingly gaining&nbsp;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 0.3 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&nbsp;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&nbsp;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&nbsp;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&nbsp;ocular&nbsp;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.<ref name=":16">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.</ref>&nbsp; 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&nbsp;6-8 weeks.&nbsp;

<span lang="EN-GB">In 2018, the </span>Steroids after Laser Trabeculoplasty<span lang="EN-GB">  (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). </span><ref name=":0">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.</ref>

<span lang="EN-GB">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).</span><ref name=":0" />

<span lang="EN-GB">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.</span><ref name=":0" />

== Complications  ==

A transient rise in&nbsp;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 &gt; 5 mmHg was reported in 34% and a rise &gt;10 mmHg was seen in 12% of patients. Of note, there was no perioperative alpha-adrenergic prophylaxis used in this trial.<ref name=":17">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.</ref> The frequency of&nbsp;IOP spikes&nbsp;is reduced by two-thirds with the use of prophylactic alpha-adrenergics.<ref name=":18">Robin AL, Pollack IP, House B, Enber C. Effects of ALO 2145 on intraocular pressure following argon laser trabeculoplasty. Arch 1987; 105: 646-50.</ref> 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.<ref name=":19">Keightley SJ, Khaw PT, Elkington AR. The prediction of IOP rise following argon laser trabeculoplasty. Eye 1987; 1:577-80.</ref> Spikes are usually transient, occur within the first hour although they may be delayed,<ref name=":20">Weinreb RN, Ruderman J, Juster R, Zweig K. Immediate IOP response to argon laser trabeculoplasty. Am J Ophthalmol 1983; 95: 279-86.</ref> 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 &gt; 5 mmHg is around 10% or less and the rate&nbsp;of an IOP&nbsp;rise &gt; 10 mmHg is around 3%.<ref name=":21">Barkana Y, Belkin M. Selective Laser Trabeculoplasty. Surv Ophthalmol 2007; 52: 634-654.</ref> There are rare cases requiring trabeculectomy for sustained IOP increases after both SLT and ALT,<ref name=":22">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.</ref> 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,<ref name="damji23">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</ref> 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.<ref name=":24">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.</ref> Hyphemas have also been reported.<ref name=":25">Rhee DJ, Krad O, Pasquale LR. Hyphema following selective laser trabeculoplasty. Oph Surg Lasers Imaging 2009; 40: 493-4.</ref><ref name=":26">Shihadeh WA, Ritch R, Liebmann JM. Hyphema occurring during selective laser trabeculoplasty. Ophthalmic Surg Lasers Imaging 2006; 37(5): 432-3. </ref>

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.<ref name="damji23"/>

{| class="wikitable"
|-
! 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  ===

{| class="wikitable"
|-
! 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  ===

{| class="wikitable"
|-
| style="background: #f0f0f0" align="center" | '''Author''' 
| style="background: #f0f0f0" align="center" | '''Number of Eyes''' 
| style="background: #f0f0f0" align="center" | '''Follow Up''' 
| style="background: #f0f0f0" align="center" | '''Mean IOP Reduction''' 
| style="background: #f0f0f0" align="center" | '''% 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
|-
| &nbsp;Arch Oph. 
| 
| 
| 
| 
|-
| Lai et al. 2004 
| 58 OAG/OHT 
| 5 years 
| 8.7 
| 32
|-
| Clin Exp Oph. 
| &nbsp; 
| 
| 
| 
|-
| 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
|-
| &nbsp;Eur J Glauc. 
| 
| 4 years 
| 6.3 
| 29.3
|}

=== Selected Studies Comparing ALT and SLT  ===

{| class="wikitable"
|-
| style="background: #f0f0f0" align="center" | '''Author''' 
| style="background: #f0f0f0" align="center" | '''Number of Eyes''' 
| style="background: #f0f0f0" align="center" | '''Mean Follow Up''' 
| style="background: #f0f0f0" align="center" | '''Mean IOP Reduction''' 
| style="background: #f0f0f0" align="center" | '''P Value'''
|-
| 
| 
| 
| (mm Hg) 
| 
|-
| 
| 
| 
| SLT&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;ALT 
| 
|-
| Damji et al. 1999 
| 36 
| 6 Months 
| 4.8&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 4.7 
| 0.97
|-
| &nbsp;BJO 
| 
| 
| 
| 
|-
| Damji et al. 2006 
| 176 
| 12 Months 
| 5.9&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 6.04 
| 0.84
|-
| BJO 
| 
| 
| 
| 
|-
| Popiela et al. 2000&nbsp; 
| 27 
| 3 Months 
| 2.85&nbsp;&nbsp;&nbsp;&nbsp; 2.63 
| 0.84
|-
| &nbsp;Klin Oczna 
| 
| 
| 
| 
|-
| Juzych et al. 2004 
| 154&nbsp;&nbsp; ALT 
| 5 Years 
| &nbsp;% IOP Reduction 
| ___
|-
| Ophthalmology 
| 41&nbsp;&nbsp;&nbsp;&nbsp;SLT 
| 
| 32%&nbsp;&nbsp;&nbsp;&nbsp; 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.<ref name=":27">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.</ref> 

A prospective study that randomized patients to 180° of SLT versus ALT, found no statistically significant difference in IOP&nbsp;reduction between the two procedures. At 6 months, IOP decreased by 4.8±3.4 mmHg in the SLT group, and&nbsp;4.7±3.3 mmHg in the ALT group.<ref name=":28">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.</ref> 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.<ref name="damji23"/>

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.<ref name=":29">The Glaucoma Laser Trial (GLT) 2 Results of argon laser trabeculoplasty versus topical medicines. The Glaucoma Laser Trial Group. Ophthalmol 1990; 97: 1403-13.</ref><ref name=":30">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.</ref><ref name=":31">The Glaucoma Laser Trial (GLT) 3 Design and methods. Glaucoma Laser Trial Group. Control Clin Trials 1991; 12:504-24.</ref>

The longest follow-up 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&nbsp;diagnosed with POAG or OHTN&nbsp;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.<ref name=":32">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.</ref> 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 &nbsp;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.
<ref name=":33">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.</ref> 

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%.<ref name=":34">Feldman RM, Katz LJ, Spaeth GL, et al. Long-term efficacy of repeat argon laser trabeculoplasty. Ophthalmol 1991; 98: 1061-5.</ref> <ref name=":35">Richter CU, Shingleton BJ, Bellows AR, et al. Retreatment with argon laser trabeculoplasty. Ophthalmol 1987; 94: 1085-9.</ref> <ref name=":36">Starita RJ, Fellman RL, Spaeth GL, et al. The effect of repeating full-circumference argon laser trabeculoplasty. Ophthalmic Surg 1984; 15:41-3.</ref> <ref name=":37">Weber PA, Burton GD, Epitropoulos AT. Laser trabeculoplasty retreatment. Ophthalmic Surg 1989; 20:702-6.</ref> 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.  <ref name=":38">Hong BK, Winer JC, Martone JF, et al. Repeat selective laser trabeculoplasty. J Glaucoma 2009; 18:180-3.</ref>

SLT performed in eyes with previous ALT is comparatively effective.<ref name="damji23"/><ref name=":39">Birt CM. Selective laser trabeculoplasty retreatment after prior argon laser trabeculoplasty: 1-year results. Can J Ophthalmol 2007; 42:715-9.</ref> <ref name=":40">Kano K, Kuwayama Y, Mizoue S, et al. Clinical results of selective laser trabeculoplasty. Nippon Ganka Gakkai Zasshi 1999; 103:612-6.</ref> <ref name=":41">Song J, Lee PP, Epstein DL, et al. High failure rate associated with 180° selective laser trabeculoplasty. J Glaucoma 2005; 14:400-8.</ref> 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.<ref name=":42">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.</ref>

The effect of trabeculoplasty on diurnal curve has been studied. Both ALT and SLT have been shown to decrease diurnal IOP fluctuation.<ref name=":43">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.</ref> <ref name=":44">Greenidge KC, Spaeth GL, Fiol-Silva Z. Effect of argon laser trabeculoplasty on the glaucomatous diurnal curve. Ophthalmol 1983; 90:800-804.</ref> <ref name=":45">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.</ref> 

In different subgroups of patients, both ALT and SLT have been found efficacious when compared to treatment for POAG. In pseudoexfoliation<ref name=":46">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.</ref><ref name=":47">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.</ref><ref name=":48">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.</ref> SLT has not been found to be less efficacious in pseudophakic eyes compared to phakic ones.<ref name=":49">Werner M, Smith MF, Doyle JW. Selective laser trabeculoplasty in phakic and pseudophakic eyes. Ophthalmic Surg Lasers Imaging 2007; 38:182-8.</ref> Whereas it is generally thought that ALT is better performed while an eye is phakic.  <ref name=":50">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.</ref>

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.<ref name=":51">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.</ref>

= Additional Resources =
* American Academy of Ophthalmology. ''Glaucoma: Laser trabeculoplasty'' [http://store.aao.org/practicing-ophthalmologists-learning-system-2017-2019.html Practicing Ophthalmologists Learning System, 2017 - 2019] San Francisco: American Academy of Ophthalmology, 2017.

= References  =
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