Selective laser trabeculoplasty: An updated narrative review

Abstract

Selective laser trabeculoplasty (SLT) has experienced a resurgence in interest, primarily driven by promising findings from the Laser in Glaucoma and Ocular Hypertension Trial. By offering SLT as an initial drug-free treatment option, we may be able to thwart issues such as adherence and persistence that plague our current medical management protocols. In this comprehensive narrative review, we delve into the current body of literature that explores the utility of SLT across a wide spectrum of scenarios and glaucoma subtypes. We present evidence that provides valuable insight into the efficacy and benefits of SLT, positioning it as a viable option in the management of glaucoma. Careful consideration of the associated risks and challenges is also necessary for successful adoption into clinical practice. Despite the ample evidence supporting SLT’s efficacy, some questions remain regarding its long-term effects and the potential need for retreatment. This review aims to shed light on these aspects to guide clinicians in making informed decisions and tailoring treatment plans to individual patient needs. This review also provides the readers with a bird’s eye view of the potential impact of SLT and adds clarity to the various therapeutic protocols that one can follow to ensure optimal clinical outcomes for our patients.

Glaucoma management underwent a seismic shift when prostaglandin analogs (PGAs) were approved two decades ago. They demonstrated superior efficacy, greater ease of use, and better adherence and persistence with minimal systemic side effects, propelling them as the first-line drug of choice for disease management.[1,2,3] Though superior in comparison to alternative therapeutic options, persistence and adherence rates for PGAs hover around 25%–45%, which are insufficient to reduce disease morbidity.[4,5] Furthermore, PGA-related side-effects and the occurrence of prostaglandin-associated peri-orbitopathy syndrome (PAPS) have concerning cosmetic issues[6,7,8,9] and significant long-term impact on glaucoma management, both in terms of disease monitoring and poor surgical outcomes.[10,11,12]

The abovementioned reasons led to the re-evaluation of selective laser trabeculoplasty (SLT), which always had the potential but was not widely adopted as a therapeutic choice in glaucoma management due to the lack of robust evidence.[13,14] This approach has changed dramatically after the landmark Laser in Glaucoma and Ocular Hypertension Trial (LIGHT).[15] The LIGHT trial filled in this lacuna and provided 6-year follow-up data, allowing for a prominent shift in glaucoma practice patterns.[16] The UKNICE guidelines now recommend that patients with ocular hypertension (OHT) and primary open-angle glaucoma (POAG) should be initially offered SLT as first-line treatment.[17] This evidence potentiates a scenario wherein we are likely to see a wider acceptance of SLT as a first-line therapeutic choice for POAG, OHT, and some specific secondary glaucomas. With this situation unraveling, an updated review of SLT would be immensely useful for ophthalmologists embarking on shifting glaucoma practice patterns. This review aims to encompass and highlight existing literature that impact intraocular pressure (IOP) outcomes post SLT and provide the readers with a bird’s eye view to help make the appropriate therapeutic choice for their patients.

SLT and primary open-angle glaucoma

The role of SLT in the management of POAG has been extensively studied. Multiple peer-reviewed reports along with seven meta-analyses vouch for the efficacy of SLT for IOP control in patients with POAG[18,19,20,21,22,23,24] [Table 1]. SLT results in an average 6.9%–35.9% IOP reduction in patients with OAG.[21] When compared with argon laser trabeculoplasty, SLT has better efficacy with fewer medications required for IOP control at 12 months.[24] The adverse events and side effects are rare and comparable across different types of laser trabeculoplasties.[21,24] A recent Cochrane review has also recommended the use of SLT for controlling IOP at a lower cost than conventional medications.[14]

Table 1.

Summary of meta-analyses that evaluate efficacy of SLT for treatment of open-angle glaucoma (OAG)

Authors	No. of studies	No. of eyes/patients	Groups compared	Main results
Zhou et al.[24]	22 RCTs	2859/2704	ALT, medications, 180° SLT, 270° SLT, 360° SLT, new LT, transscleral 360° SLT with SLT conducted without gonioscopy and low energy 360° SLT	All types of LT effective Recommendation for RCTs with larger sample sizes. Fewer medications at 12 months with SLT vs. ALT (WMD: 0.28, 95% CI=0.06–0.50; P=0.014)
Chi et al.[23]	8 RCTs	1501/1229	Medications	SLT similar to medications for IOP reduction and control SLT can be first-line therapy for OAG
Li et al.[22]	4 RCTs, 1 clinical trial	492/366	Medications	SLT similar to medications for IOP reduction
Wong et al.[21]	4 + 4 RCTs*	SLT (166 patients) vs. meds (107 patients) SLT (150 eyes) vs. ALT (140 eyes)	Medications, ALT	SLT similar to medications and ALT for IOP reduction Complications are rare but include IOP spike requiring surgery, persistent macular edema, transient corneal haze, and transient corneal thinning.
Peng et al.[20]	3 RCTs	231/161	Medication†	SLT similar to prostaglandin analogs as primary therapy IOP reduction more in the prostaglandin group (WMD: −0.85, 95% CI=−1.43 to−0.27) SLT reduces the need for additional antiglaucoma interventions (WMD: 1.11, 95% CI=0.60–2.06)
Wang et al.[19]	6 RCTs	482/442	ALT	SLT similar to ALT for all evaluated outcomes SLT similar to ALT for IOP reduction at 1 hour, 1 week, 1 month, 6 months, 1 year, 2 years, 3 years, 4 years, and 5 years. SLT similar to ALT for reduction in the number of medications, success rate, adverse events, or side effects
Wang et al.[18]	6 RCTs	463/456	ALT	SLT similar to ALT for therapeutic IOP response and adverse events such as IOP spike and anterior chamber flare SLT better than ALT in patients with previous failed laser treatment. (WMD: 1.48, 95% CI=0.75–2.21) Patients with SLT required fewer medications for IOP control than ALT (WMD: 0.29, 95% CI=0.01–0.56)

*4 RCTs for SLT vs. medication and 4 RCTs for SLT vs. ALT. ^†Specifically, prostaglandin analogues (article in Chinese). ALT: argon laser trabeculoplasty, CI: confidence interval, IOP: intraocular pressure LT: laser trabeculoplasty, RCT: randomized controlled trial, SLT: selective laser trabeculoplasty, WMD: weighted mean difference

Long-term outcomes of primary SLT in treatment-naive early POAG indicate a sustained control of IOP over time. The reported treatment success rate (IOP reduction by 20% from baseline and IOP <19 mmHg) was 98.0% at year 1, 89.0% at year 5, and 72.0% at year 10. However, it is imperative to note that 60% of patients required re-treatments.[25] Moreover, although initial and repeat SLTs lead to comparable long-term IOP reduction, repeat SLTs have longer effect duration, indicating an incremental increase in effect; however, this needs further validation.[26,27,28] Khawaja et al.,[29] using EMR records of 831 SLT-treated eyes, reported the probability of success as 70% at 6 months, 45% at year 1, and only 27% at year 2. The variation in results may be due to different definitions of success and failure adopted by different studies.

SLT has been recommended as primary treatment for POAG and OHT, primarily due to the encouraging results from the LIGHT trial.[15,16] This was a prospective randomized controlled trial (RCT) designed to compare health-related quality of life (HRQoL) in newly diagnosed, treatment-naive patients with POAG or OHT, treated with either topical IOP-lowering medication from the outset (Medicine 1^st) or primary SLT followed by topical medications as required (Laser 1^st). Secondary outcomes were cost-effectiveness, disease-specific HRQoL, clinical effectiveness, and safety. The trial recruited 718 subjects (356 Laser and 362 Medication arm), and at 3 years, SLT provided a stable, drop-free IOP control to 69.0% of POAG patients, with a reduced need for surgery, lower cost, and comparable HRQoL. At all-time points, drop-free disease control was achieved in a higher percentage of OHT and mild POAG eyes compared with moderate and severe POAG eyes. This was further substantiated by the authors publishing the 6-year outcomes recently.[16] They noted in eyes that received SLT, 69.8% remained at or less than the target IOP without the need for medical or surgical treatment. Eyes with POAG that had received SLT showed a lower rate of progression compared to eyes in the medication arm (92 vs. 125; P < 0.01). They also reported a good safety outcome post SLT, with no serious laser-related adverse events over 6 years.

SLT as a first-line therapeutic option for POAG has propelled into several healthcare systems. The logistics and costs may not work for every healthcare system, but a gradual change appears inevitable.[30] Long-term progression of glaucoma often requires additional interventions post SLT, and this has been well documented.[28,29] SLT is not a magic bullet, and practitioners have to educate patients and the community about the need for continued monitoring of glaucoma status with periodic follow-ups.

SLT and ocular hypertension

The Ocular Hypertension Treatment Study (OHTS) has demonstrated that early medical treatment of OHT patients reduces the 5-year incidence of POAG by 60%.[31] The LIGHT trial showed that IOP reduction with SLT was similar in OHT and POAG eyes. They reported a mean initial IOP (mmHg) lowering at two months of 8 ± 4.0 in OHT eyes and 6.5 ± 4.3 in POAG eyes. The mean percentage IOP reduction was 29.7 ± 13.1% in OHT eyes and 26.1 ± 14.7% in POAG eyes. The authors noted a clear trend toward increasing absolute IOP reduction, with higher baseline IOP in both OHT and POAG eyes. Drop-free disease control was achieved in 88.6% (140/195 eyes) of OHT eyes after one or two SLT procedures at 3 years, which was higher than in eyes with mild (76.6%), moderate (56.1%), or severe glaucoma (42.3%).[32] These results indicate that SLT can be considered an effective and safe first-line treatment for OHT patients at risk of progression.

SLT and normal tension glaucoma

IOP reduction remains the cornerstone of managing normal tension glaucoma (NTG). SLT has been evaluated as a therapeutic option in patients with NTG.[1,33,34] Lee et al.[34] demonstrated that a single session of 360° SLT in NTG patients can reduce an additional 15.0% IOP reduction while using 27.0% less medication at 1 year. Over the course of 2 years following SLT, significant reductions in IOP and medication use were observed, with 11.0% of the eyes no longer requiring medication.[35] There was, however, a gradual reduction in the absolute success rate, from 61.0% at 6 months, 22.0% at 12 months, and 11.0% at 24 months. They also demonstrated that a higher intraocular IOP before SLT (coefficient = 1.1, OR = 3.1, P = 0.05) and a lower IOP at 1 week post SLT (coefficient = −0.8, OR = 0.5, P = 0.04) were associated with treatment success.[36]

Although the impact of SLT on NTG might not be as remarkable as in individuals with POAG, it remains a valuable tool, particularly in cases where poor adherence to glaucoma medications or in patients intolerant to medications. However, SLT primarily addresses the pressure-related aspect of the disease and does not directly target the hypoperfusion associated with NTG, which can play a crucial role in disease progression. Thus, with appropriate case selection, we may be able to pass on the therapeutic benefits of SLT to NTG patients.

Impact of demographics on outcomes

Conventional patient factors such as age, gender, TM pigmentation, lens status, and central corneal thickness status have been investigated and not found to be predictive of SLT success.[37,38] Moreover, the safety and efficacy of SLT have been demonstrated across different ethnic groups.[15,24] This includes Caucasian,[39] Hispanic,[40] Brazilian,[28] Asian,[41] Indian,[42] and African[43,44,45] patients. In studies with mixed ethnicity, Africans have been shown to have a sustained IOP reduction (20.0%) in 90.0% of eyes as compared to 50.0% of Indian eyes at 12 months follow-up.[46] Similarly, the mean decrease in medications after secondary SLT for Blacks and Whites has been reported to be significantly different, with Blacks having a larger mean decrease in medications than Whites (P < 0.01).[47] Peripheral anterior synechiae (PAS) development post SLT appears to be more common in Chinese patients, with a reported 4-year incidence of 13.6%.[48] Thus, there are subtle differences between different ethnicities and SLT outcomes, which may be driven by factors such as angle configuration, angle pigmentation, and differences in baseline IOP.

SLT and angle closure disease

Management of angle closure disease (ACD) requires a laser peripheral iridotomy (LPI) followed by IOP control.[49] Typical practice protocols would include a drug regimen similar to the management of POAG leading to drug-induced side effects and poor adherence.[1] Several investigators have evaluated SLT in eyes wherein the angle is sufficiently open to treat the trabecular meshwork (TM).[50,51,52,53]

One of the earliest studies done by Ho et al.[50] reported an IOP reduction of 4 mmHg in 72% of eyes at 6 months without a change in the number of medications at the end of the study. In another prospective RCT that compared the efficacy of SLT (49 eyes) against PGA (47 eyes) in post-iridotomized treatment-naive eyes with primary angle closure (PAC) and PACG at 6 months, the authors reported an IOP reduction of 4.0 mmHg (95% CI = 3.2–4.8) in the SLT group (P < 0.001) and by 4.2 mmHg (95%CI = 3.5–4.9) in the PGA group (P < 0.001).[51] They reported no differences between the SLT and PGA groups in the absolute mean reduction of IOP (4.0 vs. 4.2 mmHg, respectively; P = 0.78). The complete success rates (IOP ≤21 mmHg without medications) were significantly higher in the PGA group (P = 0.008). The mean endothelial cell count showed a significant decrease from baseline in the SLT arm (4.8% decrease; P = 0.001). No other events such as persistent uveitis or increase in PAS were noted in eyes that underwent SLT. The authors concluded that though SLT has a hypotensive effect in eyes with ACD, the therapeutic effectiveness of PGA is superior. In a case-control study involving 59 eyes diagnosed with ACD, subjects were treated with SLT after an iridotomy and compared with POAG eyes. The authors reported an IOP reduction of 20% or more from baseline, or discontinuation of one or more glaucoma medications to be 84.7% in the PAC/PACG group and 79.6% in the POAG group (P = 0.47).[52] Overall, the efficacy of SLT was equivocal in both PACD and OAG. Further data from a long-term observational study spanning over 6 years reflected similar findings of efficacy in both diagnostic subgroups, with an efficacy of 84.0% at 1 year and dropping to 6.0% at 6 years.[53] Thus, available evidence indicates that SLT can be a reasonable alternative to medical therapy in PAC and PACG with the potential to harness its effects with appropriate case selection and systematic follow-up.

SLT in secondary glaucoma

Secondary glaucomas represent a wide spectrum of causes that can elevate IOP. Typically, the primary outflow obstruction is at the TM, and SLT could be an acceptable therapeutic modality except in the presence of active trabeculitis. Shazly et al. compared the efficacy of SLT in a small group of eyes with PXFG and POAG.[54] At 30 months of follow-up and after 180° of SLT therapy, the POAG group showed a mean IOP of 17.6 ± 2.8 mmHg and a mean IOP reduction of 5.7 ± 2.1 mmHg, and the PXFG group showed a mean IOP of 18.3 ± 4.7 and a mean IOP reduction of 5.3 ± 3.0 mmHg. They reported the cumulative probability of success for patients with either POAG or PXFG to remain off medications for 2.5 years following SLT to be approximately 75.0%. In another prospective study, Ayala et al. evaluated IOP reduction and inflammation following SLT in a group of 60 patients diagnosed with POAG/PXFG and reported similar IOP reduction (6.0 mmHg in both groups; P = 0.27) and the inflammation parameters between the subgroups at various time points.[55]

Subjects with pigmentary glaucoma (PG) have been reported to respond favorably to SLT over the short term.[56] There are concerns of complications such as IOP spikes due to the excessive absorption of laser by the pigmented TM, and the guidelines recommend titrating laser energy and area of treatment appropriately in this subset of patients.[57] Koucheki and Hashemi,[58] in a prospective, nonrandomized, interventional study of patients with OAG unresponsive to maximum tolerable antiglaucoma medication, assessed the efficacy of 360° SLT in a cohort of patients with PG, POAG, and PXFG. They reported a mean IOP reduction of 14.5% in the PG group with a similar profile of IOP reduction among the POAG (16.7%) and PXFG (16.6%) subgroups. Complications such as IOP spikes and inflammation were recorded to be more common in the PG group, and as discussed earlier, this could be explained by the excessive laser absorbed by the pigmented TM. Long-term outcomes have been reported by Ayala et al. from their retrospective analysis.[56] They reported a cumulative drop in success rates from 85.0% (first year) to 14.0% over four years post 180° SLT, with an average time to failure documented as 27.4 months post SLT.

The uveitic spectrum of patients has raised IOP due to the inflammatory process as well as to the steroid-induced process. Overall, disease pathology is complex, and the inflammatory component could lead to worsened outcomes with SLT. Zhou et al.[59] reported similar IOP reduction profiles post SLT in their uveitic and POAG patients at 18 months. Though the mean medications increased from 2.7 at baseline to 3.5 at 18 months, they did not note any difference in terms of the complication rates in uveitic eyes compared to POAG or PXFG eyes.

Steroid-induced glaucoma is typically encountered during the course of management of uveitic eyes and following intravitreal steroid injections wherein withdrawal is tricky or not possible. Rubin et al. reported the efficacy of SLT from a retrospective review of eyes with steroid-induced glaucoma following intravitreal injections.[60] In their series, the IOP decreased to 23.9 ± 10.6 at 3 months (P < 0.006) and 15.7 ± 2.2 at 6 months (P < 0.001) from a baseline of 38.4 ± 7.3 mmHg. Maleki et al.[61] also reported the efficacy of SLT in uveitic eyes that had received intravitreal steroids. Mean IOP in their series lowered to 13.42 mmHg (55.7% reduction) at 6 months and 15.14 mmHg (50.4% reduction) at 12 months from a baseline of 30.57 mmHg.

To summarize, secondary glaucoma is often amenable to SLT, and with appropriate case selection, invasive surgery can be avoided in a significant proportion.

SLT in treatment-naïve eyes and as adjunct therapy

The effect of SLT as first-line therapy in treatment-naive eyes has been reported in fewer studies than outcomes in previously treated eyes. This can be attributed to the real-world scenario where medications are the clinically preferred first line of treatment and factors such as expertise, resources, need for additional sittings, and physician preferences interfere with SLT service delivery.

Nagar et al.[62] compared the efficacy of SLT against latanoprost and reported a similar percentage of success in both arms (75.0% in SLT and 73.0% in the latanoprost arm). However, they reported IOP fluctuations to be more effectively reduced by latanoprost (64.0% vs. 41.0%). Recently, compelling evidence for the efficacy of primary SLT in POAG and OHT was provided by the LIGHT trial, wherein 75% of participants in the SLT arm maintained drop-free disease control at 3 years.[15] It was further supported by subsequent 6-year follow-up data, which reported medication-free status and reduced need for incisional glaucoma and cataract surgery in 70% of participants from the SLT arm.[16]

Ansari et al.[33] in their retrospective study involving 54 subjects (108 treatment-naïve eyes) with POAG evaluated the long-term efficacy of SLT. They defined success of treatment as achieving at least a 20% reduction in IOP and IOP <19 mmHg, and with that cutoff reported a success rate of 98.0% at year 1, 89.0% at year 5, and 72.0% at year 10. Failure is most common after the third year. The median time to re-treatment was 81 months (CI: 60–100 months), with 60% needing re-treatment by 10 years. Higher baseline IOP was associated with an increased risk of re-treatment in their study. The correlation between any previous antiglaucoma medication and SLT outcome has been evaluated in some studies. While some suggest reduced efficacy,[63,64] others reported no correlation between them.[29,65] The varying duration of antiglaucoma medication and the inadequate washout duration might explain the different outcomes from these studies. In particular, topical therapy with PGA has been associated with decreased IOP response due to a similar mechanism of action,[66] and carbonic anhydrase inhibitors are associated with better response.[67]

Extent of treatment and energy

SLT can be administered in 1–4 quadrants of the eye (90°, 180°, 270°, and 360°), and multiple studies have compared the variability in IOP outcomes based on the extent of treatment. 360° of SLT has been associated with better efficacy in comparison to 180° at 6 months,[68] 12 months,[69] 18 months,[70] and 24 months[71] of follow-up. Prasad et al.[72] also demonstrated a lower range of IOP fluctuations with 360° SLT in comparison to 180° at 2 years of follow-up. Some reports contradict the above and have reported a lack of statistically significant benefit of 360° over 90° at 2 years[73] or 180° at 6 months.[74]

Paula et al.[75] suggested that 180° SLT performed initially in the nasal sector was associated with better outcomes in comparison to temporal at 6 months. This finding could be attributed to the distribution and density of collector channels.[76]

Pehkonen and Välimäki[77] showed that the average IOP reduction with 270° SLT at 6 months was similar to that of 180°. Wong et al.[78] demonstrated that increasing the application of spots from 120 to 160 over 360° resulted in better IOP outcomes at 1 year with no increase in IOP spikes. Chen et al.[79] demonstrated that 90° SLT with 25 spots had similar outcomes in comparison to 180° with 50 spots.

The optimal energy dose response for SLT has not been established.[80] Overlapping of SLT spots in the same location in repeat applications appears to decrease IOP response.[81] This could be due to a ceiling effect of TM modification achieved by repeat procedures beyond which damage from laser and disease progression prevent further therapeutic response. Tang et al.[82] demonstrated that low-energy (1/2 of conventional energy) 360° SLT reduced IOP with lesser complications in comparison to standard energy protocol. Danielson et al.,[83] reported that fixed high-energy (1.2 mJ) has similar results to standard titrated energy (starting at 0.8 mJ and titrating using champagne bubble effect) at 36 months post 360° SLT. The Clarify the Optimal Application of SLT (COAST) trials are underway to further evaluate the role of low-energy SLT, repeated annually, in reducing/delaying the need for ocular hypotensive agents, surgical intervention, and improving quality of life.[84]

The LIGHT trial presented robust IOP control data with minimal IOP spikes over 6 years.[16] They used a 360° SLT with 100 non-overlapping shots (25 per quadrant, 3-ns duration, 400-μm size). The energy delivered varied from 0.3 to 1.4 mJ. The desired endpoint was the production of a few fine champagne bubbles. Reaction to laser with large gas bubbles and TM blanching were not acceptable, and if noted, the power was titrated in steps of 0.1 mJ. Pigmented TM was expected to require lower energy (0.3–1.2 mJ) than non-pigmented, and it was advisable to start treatment at 0.4 mJ.[30]

Post-SLT steroid vs. NSAIDs

Conventionally, topical anti-inflammatory medications were given post SLT to reduce inflammation in the immediate post-SLT period. However, this inflammation is usually transient and self-resolving in nature. In addition, it is hypothesized that such topical therapy may interfere with the immune-mediated mechanism of IOP reduction post SLT.[85] Realini et al.[43] reported good IOP outcomes in Afro–Caribbean POAG participants without the administration of topical anti-inflammatories, and the potential concern of enhanced inflammation in these eyes with dark irides and excessively pigmented TM was reported to be low. Gracner suggested that short-term use of 0.1% dexamethasone had no influence on post-SLT outcomes in eyes with POAG at 24 months.[86] Rebenitsch et al.[87] reported that loteprednol did not appear to have an effect on IOP outcomes at 1 year post SLT. The SALT trial, published in 2019, compared the role of postoperative NSAIDs (ketorolac 0.5%), steroids (prednisolone 1%), and saline in IOP outcomes at 12 weeks post SLT and concluded that short-term steroids or NSAIDs may improve post-SLT IOP outcomes.[85] During the LIGHT trial, SLT treatment did not include routine use of anti-inflammatory drops post laser, but participants were provided NSAIDs for topical use in case of significant discomfort, despite also providing routine oral analgesics (e.g., paracetamol). This has been reported to be a common practice in most centers worldwide.[30]

SLT and outflow facility

Different types of imaging techniques have been used to evaluate the impact of SLT on the outflow facility.[88,89,90,91] These methods include horizontal enhanced depth imaging (EDI) optical coherence tomography (OCT) B-scans, pneumatonography, fluorophotometry, and electronic Schiøtz tonography.

Post-SLT expansion of the Schlemm canal has been demonstrated using horizontal enhanced depth imaging (EDI) optical coherence tomography (OCT) B-scans.[89] Gulati et al. used pneumatonography and fluorophotometry to report that baseline higher aqueous flow and lower outflow facility may be predictive of better response to SLT.[90] Goyal et al. evaluated the effect of 180° versus 360° primary SLT on the outflow facility by using electronic Schiøtz tonography.[88] They reported that while SLT significantly increased the outflow facility in both the 180° group (37.5%) and 360° group (41%), the difference between the two was not statistically significant (P = 0.23).

Recent advances such as hemoglobin video imaging (HVI) offer the noninvasive potential to quantify human aqueous outflow in real time.[91] Khatib TZ et al. were able to demonstrate a significant increase in the aqueous column after the administration of SLT and found that it correlated with the degree of IOP reduction. Emerging techniques such as HVI hold promise for enhancing our understanding of aqueous outflow dynamics in glaucoma management and may allow us to screen and treat patients who may benefit the most.

Direct SLT

Direct SLT (DSLT) is a new method of laser trabeculoplasty where laser energy is delivered directly to TM through the perilimbal ocular area, thereby eliminating the need for gonioscopic laser delivery.[92] It uses the same Q-switched Fd-NDYag laser but aims to address three major limitations of SLT: the need for surface contact with gonioscope, skilled expertise in gonioscopy, and long duration of procedure. The average duration of the DSLT procedure is 2–3 s, which is significantly shorter than SLT.

The first prospective clinical trial involving DSLT was conducted in 15 eyes: ten with POAG, four with OHT, and one with PXFG. The mean baseline IOP (mmHg) in all eyes was 26.7 ± 2.3. At 1, 3, and 6 months, this value significantly reduced to 21.7 ± 4.2 (by 18.1%), 20.8 ± 2.5 (by 21.4%), and 21.5 ± 4.1 (by 18.8%), respectively, thereby indicating the efficacy of the procedure.[93]

The technique offers immense advantages that could allow wider use of the technology with potential administration by general ophthalmologists and advanced therapeutic practitioners. This could then prove to be a promising tool to manage the immense burden of glaucoma, which is estimated to hit 111.8 million by 2040.[94] Currently, the GLAUrious study, a large-scale multicenter RCT, is underway to study the safety and efficacy of DSLT in reducing IOP in OAG patients.[92]

SLT and long-term safety

SLT is a minimally invasive intervention and typically does not cause significant inflammation.[24] Most evidence for this is provided by studies conducted in eyes with open angles.[15,16] However, it has been observed that SLT can stimulate the release of prostaglandins, cytokines, and free oxygen radicals in the anterior segment.[95,96] These inflammatory agents can result in increased permeability of the corneal endothelium.[97] In addition, the formation of excessive and large bubbles during the procedure may lead to endothelial cell damage.[97,98] However, long-term studies such as the LIGHT trial have reported no sight-threatening complications of SLT as well as no clinically identifiable corneal changes at the end of a 6-year follow-up period.[16]

There is some evidence to show that eyes with POAG may behave differently with SLT application compared to eyes with PACG.[51,98] Kurysheva et al. reported that lower baseline endothelial cell count and older age correlated with increased corneal endothelial damage. The endothelium, however, recovered in the POAG patients at 1 month, whereas in PACG patients, the endothelium showed a sustained loss with no recovery noted at 6 months.[98] The biological plausibility of the endothelial cell loss to be progressive over the years after an SLT is minuscule; however, one should exercise caution in eyes with shallow ACD that would need repeat therapy.

Summary

SLT has demonstrated immense potential to keep a patient drug-free. The 6-year data from the LIGHT trial reported that 70.0% of the subjects with POAG and OHT remained drop-free. The laser arm subjects also demonstrated a lower rate of disease progression and a reduced need for cataract and glaucoma procedures. The safety profile has been reported to be robust, and it is now recommended as the first-line treatment for OAG and OHT by the UKNICE.[17] The ease of effective laser delivery through the DSLT has opened up an exciting era for glaucoma management, and there exists a potential of blunting the disease morbidity significantly without the dependence on daily dosed drugs. The costs and logistics of including SLT as a therapeutic option may not be feasible for every healthcare system, but this is likely to change, and acceptance on a wider scale should soon be a reality with lowered equipment costs and appropriate training strategies.

Financial support and sponsorship:

Nil.

Conflicts of interest:

There are no conflicts of interest.