SLT Laser and iStent for Glaucoma: A Comparison of Minimally Invasive Drop-Free Treatment Options

For patients newly diagnosed with open angle glaucoma or ocular hypertension, the traditional first step has been topical pressure-lowering drops. Drops are effective and remain an important part of glaucoma management, but they are not without significant drawbacks. The most important is their chronic effect on the ocular surface. The evidence increasingly supports laser and minimally invasive surgical options as alternatives to drops, either as true first-line treatments or as ways to reduce or eliminate drop dependence. This post provides an honest, evidence-based comparison of the two most accessible minimally invasive options: selective laser trabeculoplasty (SLT) and trabecular micro-bypass stents (iStent and iStent inject). It covers how each works, what the published literature shows about IOP reduction and success rates, how they compare with each other and with cataract surgery alone, and how to think about choosing between them. For more background on cataract surgery in patients with glaucoma, including IOL selection and the effect of long-term drops on biometry, see the companion guide at corneaeyedoctor.com.

The problem with long-term glaucoma drops

Most glaucoma patients develop dry eye disease from their drops, and the principal cause is preservative toxicity. Understanding this explains why reducing or eliminating drop dependence is a clinically important goal, not a matter of patient convenience.

Most topical glaucoma medications contain the preservative benzalkonium chloride (BAK). BAK is a quaternary ammonium salt used to maintain sterility of the drop formulation. It is also a detergent that disrupts the lipid bilayer of cell membranes. Chronic exposure to BAK at the concentrations present in glaucoma drops causes dose-dependent toxicity to the corneal epithelium, conjunctival goblet cells, and the meibomian glands. The goblet cells produce mucin MUC5AC, which is an essential component of the inner tear film layer. Loss of goblet cell density from BAK toxicity destabilises the tear film and produces dry eye disease (DED).

A 2023 review published in Acta Ophthalmologica confirmed that most glaucoma patients have dry eye disease, which is largely caused by their eye drops, and that BAK preservation is a significant contributor. The same review noted that DED in turn reduces patient adherence to their glaucoma medication, creating a vicious cycle in which the treatment itself undermines compliance. A 2024 publication in Ophthalmology Management confirmed that the toxic effects of BAK include disruption of the corneal epithelial barrier, loss of goblet cells, and conjunctival inflammatory cell infiltration, and that limited evidence suggests BAK may even have adverse effects on trabecular meshwork cells, potentially worsening the glaucomatous outflow obstruction it is meant to be treating.

The clinical implication is that every month a patient uses preserved glaucoma drops, their ocular surface health is potentially declining. For patients who will require treatment for decades, the cumulative effect of BAK toxicity is significant. Reducing or eliminating drop dependence through laser or surgical treatment is not merely about convenience; it is about protecting the ocular surface for the long term, preserving the accuracy of any future cataract surgery biometry, and maintaining surgical options if more aggressive glaucoma treatment becomes necessary in future. The connection between glaucoma drop toxicity and biometry accuracy is explored in detail in our guide to dry eye from glaucoma drops and its effect on biometry.

Selective laser trabeculoplasty (SLT): mechanism, success rates, and evidence

SLT is a clinic-based laser treatment that lowers intraocular pressure by stimulating macrophage activity in the trabecular meshwork, improving aqueous outflow. It requires no incision, takes approximately five to ten minutes per eye, and can be repeated if the initial effect diminishes over time.

Selective laser trabeculoplasty uses a frequency-doubled Nd:YAG laser delivering 532nm green light in short pulses of three nanoseconds to selectively target melanin-containing trabecular meshwork cells. Unlike the original argon laser trabeculoplasty (ALT), which thermally scarred the trabecular meshwork, SLT causes selective photothermolysis of pigmented cells without coagulative damage to adjacent non-pigmented cells. This means the trabecular meshwork architecture is preserved and the treatment can theoretically be repeated.

The mechanism by which SLT lowers IOP is not fully understood but is thought to involve stimulation of macrophage activity at the treated meshwork, release of cytokines that promote remodelling of the extracellular matrix in the meshwork, and improvement of aqueous outflow through enhanced trabecular cell function.

SLT is performed in the clinic using a slit-lamp-mounted laser system. It takes approximately five to ten minutes per eye, requires only topical anaesthetic drops, and does not involve incisions, anaesthesia, or an operating theatre. The patient can drive home the same day in most cases and there is minimal postoperative recovery. A transient IOP spike in the first few hours after the procedure is managed with prophylactic anti-inflammatory drops. Mild anterior chamber inflammation for one to two days is expected and managed with topical NSAIDs or steroids.

The evidence for SLT as a first-line treatment is now substantial and is anchored by the LiGHT trial, the largest and most rigorous randomised controlled trial comparing SLT with topical drops as first-line treatment for open angle glaucoma and ocular hypertension.

The LiGHT trial randomised 718 patients to either SLT or topical drops as first-line therapy. At three years, 74.2 percent of SLT-treated patients were medication-free and surgery-free while maintaining their target IOP. At six years, 69.8 percent remained medication-free and surgery-free, with 90 percent of those having undergone only a single SLT treatment in the six-year period (Gazzard et al., Ophthalmology 2023;130(2):139-151). This is a remarkable result: nearly 70 percent of patients with newly diagnosed glaucoma or ocular hypertension were successfully managed without any drops for six years after a single SLT treatment.

The magnitude of IOP reduction from SLT in published studies ranges from 15 to 35 percent from baseline, with larger reductions in eyes with higher preoperative IOP. A meta-analysis published in a PMC review found that SLT reduces mean IOP by 20 to 30 percent from baseline, with IOP reduction maintained in approximately 80 percent of patients at two years. The overall success rate (defined as IOP reduction of at least 20 percent) ranges from 65 to 100 percent across published series depending on patient selection, baseline IOP, and follow-up duration.

For normal tension glaucoma, where baseline IOP is already in the statistically normal range, SLT produces a mean IOP reduction of approximately 16 percent at 12 months (from 16.1 to 13.3 mmHg) with a success rate of approximately 61 to 74 percent at one to two years (PMC11029453).

SLT can be repeated if the initial treatment effect diminishes over time. Studies show that repeat SLT achieves similar IOP reduction to the initial procedure in the majority of patients, making it a renewable treatment option.

iStent and iStent inject: mechanism, success rates, and evidence

The iStent inject creates direct channels through the trabecular meshwork into Schlemm's canal, bypassing the primary site of aqueous outflow resistance in open angle glaucoma. It is implanted at the time of cataract surgery and produces substantially greater IOP reduction in that setting than when used without concurrent cataract removal.

The iStent (Glaukos) is a trabecular micro-bypass stent, a tiny L-shaped titanium device measuring 1.0 mm in length that is implanted through the trabecular meshwork into Schlemm's canal using an ab interno approach through a clear corneal incision at the time of cataract surgery. The iStent inject and iStent inject W are second and third generation devices that deliver two stents simultaneously and are currently the most widely used version. These devices create direct channels through the trabecular meshwork into Schlemm's canal, bypassing the primary site of outflow resistance in open angle glaucoma.

Because the iStent devices are approved for combined implantation with cataract surgery, the published evidence is divided into two distinct categories: combined cataract plus iStent, and standalone iStent without concurrent cataract surgery. The IOP-lowering effect differs substantially between these two scenarios because cataract surgery itself lowers IOP through a separate mechanism.

Combined cataract and iStent inject:

A meta-analysis of 37 studies comprising 2,495 patients found that phacoemulsification alone produced a 4 percent IOP reduction from baseline, while one iStent with phacoemulsification produced a 9 percent IOP reduction, and two iStents with phacoemulsification produced a 27 percent IOP reduction (PMC4492499). In terms of medication reduction, phacoemulsification alone reduced medications by a mean of 1.01 medications per patient, compared with 1.33 medications after one iStent and 1.1 medications after two iStents.

A prospective controlled study of 240 eyes randomised to combined cataract plus iStent versus cataract alone found that 72 percent of iStent eyes versus 50 percent of control eyes achieved unmedicated IOP of 21 mmHg or less at one year, with a mean medication reduction of 1.4 for the iStent group versus 1.0 for the cataract-only group.

A five-year real-world study of iStent inject with or without phacoemulsification published in PubMed in 2025 showed significant and sustained reductions in IOP and medication burden at five years in eyes with mild and moderate open angle glaucoma (PubMed 40193042).

Standalone iStent inject (without cataract surgery):

The standalone iStent data shows more modest results than combined procedures, which is expected because the IOP-lowering synergy from cataract surgery is absent. A real-world community-based study published in PMC in 2025 comparing combined cataract plus iStent inject W versus cataract surgery alone found statistically significant IOP reduction at 3, 6, and 12 months and significant medication reduction at 12 months in the combined group versus the cataract-only control (PMC12675655).

In the five-year study, the standalone subgroup (iStent without cataract surgery) achieved meaningful IOP reductions that were sustained over the follow-up period, though the absolute reductions were smaller than in the combined group.

Cataract surgery alone as a pressure-lowering intervention

Phacoemulsification without any concurrent glaucoma procedure lowers IOP in open angle glaucoma patients by a mean of approximately 2 to 4 mmHg, primarily because replacing the thick cataractous lens with a thin IOL widens the anterior chamber angle. This effect is real but modest compared with SLT or combined cataract plus iStent.

Cataract surgery performed in isolation, without any concurrent glaucoma procedure, has a well-documented IOP-lowering effect in patients with open angle glaucoma. The mechanism involves widening of the anterior chamber angle when the relatively thick cataractous lens is replaced with a thin IOL, changes in zonular tension affecting aqueous dynamics, and the short-term anti-inflammatory effects of surgery.

A systematic review and meta-analysis published in the Journal of Glaucoma in August 2024 confirmed that cataract surgery meaningfully lowers IOP in patients with primary open angle glaucoma and ocular hypertension at 12 months after surgery. A prospective multicentre Australian study found that cataract surgery reduced mean IOP by 2.22 mmHg in POAG patients over a mean of 4.8 years, with 34 percent of eyes achieving a reduction of at least 3 mmHg (PubMed 32031310).

The IOP reduction from cataract surgery alone is therefore real and clinically meaningful for some patients, but it is modest (approximately 2 to 4 mmHg or 4 percent from baseline in most series) compared with SLT (15 to 35 percent) or combined cataract plus iStent inject (9 to 27 percent). For patients with mild to moderate glaucoma whose IOP is well-controlled on one medication, cataract surgery alone may reduce or eliminate that medication. For patients requiring more substantial pressure reduction, cataract surgery alone is unlikely to be sufficient.

Cataract surgery followed by SLT versus combined cataract and iStent: an honest comparison

For patients with both a visually significant cataract and open angle glaucoma, the two main minimally invasive options are combined cataract plus iStent inject in a single operation, or cataract surgery now with SLT postoperatively if additional pressure reduction is needed. There is no head-to-head randomised trial directly comparing these strategies.

This is the clinical decision point that is most relevant to patients who have both a visually significant cataract and glaucoma requiring better IOP control. The two main options are:

Option A: Cataract surgery now, followed by SLT postoperatively if additional pressure reduction is needed.

Option B: Combined cataract surgery with iStent inject implantation in a single operation.

There is no head-to-head randomised trial directly comparing these two strategies. The published evidence allows a comparison of expected outcomes based on the individual procedure data, which has limitations but is the best available basis for decision-making.

Expected IOP reduction from Option A (phacoemulsification then SLT): Cataract surgery alone produces approximately 2 to 4 mmHg reduction (4 percent from baseline). Subsequent SLT produces approximately 3 to 6 mmHg additional reduction (15 to 30 percent from new baseline after cataract surgery). Combined expected reduction is approximately 5 to 10 mmHg from preoperative baseline, with the caveat that the SLT is a separate procedure performed weeks to months after cataract surgery and that not all patients will require it.

Expected IOP reduction from Option B (combined phacoemulsification plus iStent inject): Published data suggest 9 to 27 percent IOP reduction from baseline in the combined procedure, with 72 percent of patients achieving unmedicated IOP control at one year in the pivotal trial.

On paper, the combined procedure appears to offer superior IOP reduction in a single operation. However the honest comparison requires acknowledging several nuances.

First, not every patient who has cataract surgery will need additional pressure lowering. For patients with mild glaucoma or ocular hypertension well-controlled on one medication, cataract surgery alone may achieve adequate control, avoiding the additional cost and procedural complexity of iStent implantation. Performing SLT postoperatively is then available as a low-risk, repeatable option if additional reduction is needed, without the permanence of an implanted device.

Second, SLT has a very favourable risk profile. It is reversible in the sense that it does not place a permanent implant in the eye and does not alter the anatomy in ways that might complicate future procedures. A stent implanted in Schlemm's canal cannot be removed. If the eye develops problems requiring filtration surgery in future, the presence of an iStent may complicate but does not preclude those procedures.

Third, iStent implantation requires the angle to be clearly visible and the anatomy to be favourable. In eyes with angle abnormalities, narrow angles, or challenging anatomy, stent implantation may be technically difficult or impossible. SLT works on the trabecular meshwork through an intact cornea and does not require angle surgery.

Fourth, for patients who do not have a visually significant cataract at the time of their glaucoma diagnosis, standalone SLT is the minimally invasive option and standalone iStent without cataract surgery produces more modest results. In this setting, SLT is the clearly favoured option.

SLT versus standalone iStent: which is more effective without concurrent cataract surgery?

For patients who do not yet need cataract surgery, SLT is the preferred minimally invasive option. The six-year LiGHT trial data showing 70 percent of patients medication-free after a single SLT treatment represents the strongest evidence base for any minimally invasive glaucoma intervention. Standalone iStent without concurrent cataract produces more modest results and is not repeatable in the same way.

For patients who do not require cataract surgery, the comparison is between SLT alone and standalone iStent inject alone. The published evidence favours SLT in this setting.

SLT as a standalone procedure has been extensively studied and the LiGHT trial data showing 70 percent of patients medication-free at six years after a single treatment represents the highest-quality evidence available for any minimally invasive glaucoma treatment. SLT is also renewable: if the effect diminishes after five or more years, the treatment can be repeated with comparable efficacy to the initial procedure.

Standalone iStent without concurrent cataract surgery produces meaningful IOP reduction but the effect size is smaller than combined cataract plus iStent, and it is not directly comparable to SLT in published head-to-head trials. The standalone iStent option is more invasive, requiring an incision and implantation into Schlemm's canal, and the device is permanent. It is also not repeatable in the same way as SLT.

For patients with a phakic eye who need glaucoma pressure reduction and do not yet have a visually significant cataract, SLT is generally the preferred minimally invasive first option. Combined cataract plus iStent inject becomes relevant when cataract surgery is also indicated.

Who is each treatment most suitable for?

SLT is best suited to patients without a visually significant cataract who want a drop-free or drop-reducing option, and is appropriate as a true first-line treatment in newly diagnosed patients. Combined cataract plus iStent is best suited to patients with coexisting cataract and mild to moderate glaucoma who want better pressure control and medication reduction in a single procedure.

SLT is most suitable for:

• Patients with newly diagnosed open angle glaucoma or ocular hypertension who have not yet started drops, as first-line treatment in place of drops
• Patients already on glaucoma drops who want to reduce or eliminate medication dependence
• Patients with open angle glaucoma who have not yet developed a visually significant cataract
• Patients for whom surgery carries higher risk and a repeatable, reversible option is preferred
• Patients with dry eye disease from glaucoma drops who need to reduce their drop burden
• Any patient who cannot reliably adhere to a daily drop regimen

Combined cataract plus iStent inject is most suitable for:

• Patients with both a visually significant cataract and mild to moderate open angle glaucoma requiring better IOP control
• Patients already on one to two glaucoma medications who want to reduce their drop dependence at the time of cataract surgery
• Patients in whom a single combined procedure is preferred over staging cataract surgery and then SLT

Cataract surgery alone followed by SLT if needed is most suitable for:

• Patients with mild glaucoma or ocular hypertension where cataract surgery alone may provide adequate pressure reduction
• Patients where the degree of additional pressure reduction needed is uncertain and a staged approach allows reassessment
• Patients where the surgical complexity of combined cataract plus iStent is not appropriate

Summary: an honest assessment

The LiGHT trial six-year data showing 70 percent of patients drop-free after a single SLT is the strongest evidence base available for any minimally invasive glaucoma treatment. Combined cataract plus iStent inject is also well-supported in its specific indication: patients with coexisting cataract and mild to moderate open angle glaucoma. Neither approach replaces filtration surgery for advanced disease requiring very low target pressures.

The evidence for SLT as a first-line alternative to drops is now strong enough that the LiGHT trial investigators and multiple glaucoma guidelines have recommended it should be offered as an alternative to drops for newly diagnosed open angle glaucoma. The six-year LiGHT data showing 70 percent of patients drop-free and surgery-free is the most compelling evidence base for any minimally invasive glaucoma intervention.

The evidence for combined cataract plus iStent inject is also strong in its specific indication: patients with coexisting cataract and mild to moderate open angle glaucoma who want better IOP control and medication reduction at the time of cataract surgery. The combined procedure consistently outperforms cataract surgery alone for both IOP reduction and medication reduction.

The honest comparison between cataract then SLT versus combined cataract plus iStent is that both approaches can achieve meaningful IOP control in appropriate patients. The combined approach offers more IOP reduction in a single operation. The staged approach preserves more options and avoids a permanent implant for patients who may not ultimately need it.

Neither approach eliminates the need for ongoing glaucoma monitoring. Both are most appropriate for mild to moderate open angle glaucoma. Neither replaces filtration surgery for patients with advanced glaucoma requiring very low target pressures.

For related reading, see our guides to cataract surgery in patients with glaucoma, dry eye from glaucoma drops and its effect on biometry, and cataract surgery in Melbourne.

Arranging an assessment

If you have been diagnosed with glaucoma or ocular hypertension and are interested in discussing drop-free or drop-reducing treatment options, I consult at Northern Eye Consultants, Northpark Private Hospital, Bundoora, and at Bass Coast Eye Centre, Wonthaggi. A referral from your GP or optometrist is required.

Contact Northern Eye Consultants to arrange an assessment. For referral information for GPs and optometrists, visit the Northern Eye Consultants for-referrers page. Further information on cataract surgery Melbourne including a published five-year surgical outcomes audit is available at drmacintyre.com. For referral information for optometrists and GPs, see the dedicated referrers page. For information on glaucoma services at Northern Eye Consultants, visit the practice website. For a general overview of arrange an assessment at Northern Eye Consultants.

References

1. Gazzard G, et al. LiGHT Trial Study Group. Laser in Glaucoma and Ocular Hypertension (LiGHT) trial: six-year results of primary selective laser trabeculoplasty versus eye drops. Ophthalmology. 2023;130(2):139-151. https://pubmed.ncbi.nlm.nih.gov/36252702/
2. Selective Laser Trabeculoplasty. PMC review. PMC10662953. https://pmc.ncbi.nlm.nih.gov/articles/PMC10662953/
3. Efficacy and safety of first-line or second-line SLT for normal-tension glaucoma. PMC11029453. https://pmc.ncbi.nlm.nih.gov/articles/PMC11029453/
4. Hedengran A, et al. The use of benzalkonium chloride in topical glaucoma treatment. Acta Ophthalmologica. 2023;101 Suppl 278:3-21. https://pubmed.ncbi.nlm.nih.gov/38037546/
5. Kolko M, et al. Impact of glaucoma medications on the ocular surface and how ocular surface disease can influence glaucoma treatment. Ocular Surface. 2023;29:456-468. https://pubmed.ncbi.nlm.nih.gov/37302545/
6. iStent with Phacoemulsification versus Phacoemulsification Alone: A Meta-Analysis. PMC4492499. https://pmc.ncbi.nlm.nih.gov/articles/PMC4492499/
7. Five-Year Outcomes of iStent inject Implantation With or Without Phacoemulsification. PubMed 40193042. https://pubmed.ncbi.nlm.nih.gov/40193042/
8. Clinical outcomes of combined cataract surgery with iStent inject W versus cataract surgery alone. PMC12675655. https://pmc.ncbi.nlm.nih.gov/articles/PMC12675655/
9. The Effect of Phacoemulsification on IOP of Patients With Open Angle Glaucoma: A Systematic Review. Journal of Glaucoma. 2024;33(8):576-586. https://doi.org/10.1097/IJG.0000000000002386
10. Effect of phacoemulsification on IOP in early glaucoma: A prospective multi-site study. PubMed 32031310. https://pubmed.ncbi.nlm.nih.gov/32031310/