George Hogarth

Introduction

Intraocular pressure (IOP) is the only modifiable risk factor in the development and progression of glaucoma and thus lowering IOP is the main goal of glaucoma care. SLT was first introduced in 1995. Prior to this ALT (argon laser trabeculoplasty) was used. ALT uses thermal energy to cause scar formation which leads to mechanical stretching of the surrounding untreated regions of the trabecular meshwork, subsequently leading to a reduction in IOP. In contrast to this SLT selectively targets pigmented cells without damaging adjacent healthy tissue (1).

SLT mechanism of action

SLT targets only pigmented trabecular cells and causes no structural damage to the trabecular meshwork. The process is known as selective photothermolysis. In order for this to occur there must be pigment in a cell so that it will absorb the laser energy better than the surrounding tissues. The laser uses a brief pulse duration (3 ns) confining heat to the pigmented cells (2).

Physical Examination

The preoperative examination must include gonioscopy to evaluate the angle. It is crucial to note whether or not the trabecular meshwork is visible as this is the structure that is treated by SLT. The amount of trabecular meshwork pigmentation should be noted, heavily pigmented eyes can respond better to the procedure.

Indications

SLT can be used to treat the following conditions:

• Primary open-angle glaucoma (POAG)
• Ocular hypertension (OHT)
• Secondary open-angle glaucomas: Including pseudoexfoliation glaucoma and pigmentary glaucoma (with lower energy settings)
• Normal tension glaucoma

SLT is particularly suitable for patients who cannot use or are intolerant to glaucoma medications. It can also be used as an adjunct therapy with glaucoma medication to enhance the overall IOP-lowering effect.

Following the results of the LiGHT trial, SLT is offered as a first-line treatment for open-angle glaucoma and ocular hypertension. The trial showed that SLT is clinically effective and cost-effective as a treatment for both POAG and OHT at three years (3,4).

The Procedure

SLT is performed under topical anaesthetic with a goniolens. A frequency-doubled q-switched 532 nm Nd:YAG laser is used with pulse duration of 3 ns and spot size of 400 µm. The laser is focussed on the trabecular meshwork, bubble formation is seen once the treatment has been carried out. Generally, 180 or 360 degrees are treated in a session.

Postoperative Treatment

Studies have been carried out to evaluate the impact of postoperative treatment on the efficacy and safety of SLT.

The “Steroid After Laser Trabeculoplasty (SALT)” trial evaluated the impact of post-SLT medication on IOP reduction in POAG, exfoliative glaucoma (XFG) and OHT patients (5). Participants were  randomized into three groups  with 0.5% ketorolac, 1.0% prednisolone or saline tears four times a day for 5 days after SLT. There were statistically different results between the placebo and treatment group with the group treated with prednisolone or ketorolac having a greater reduction in IOP after SLT.

A similar study by Jinapriya et al found no statistically significant difference between groups treated with 1.0% prednisolone acetate, 0.5% ketorolac tromethamine and artificial tears at the one-month follow-up (6).

In the UK, the majority of patients are not prescribed any eye drops to use post SLT. If they are prescribed some, then it is in the form of anti-inflammatory.

Complications

SLT has a very low complication rate. The most frequent side effects are IOP spikes, anterior chamber inflammation, eye pain or discomfort and peripheral anterior synechiae (1). The frequency of IOP spikes is reduced by two-thirds when prophylactic alpha-adrenergics are used.

IOP Spikes

In the LIGHT trial, in which 776 SLTs were performed, only 6 IOP spikes were observed.

Anterior Chamber Inflammation

The West Indies Glaucoma Laser Study (WIGLS) found cells and flare in the anterior chamber in 40.3% and 9.7% of eyes, respectively.

Peripheral Anterior Synechiae

Baser et al. reported two cases of PAS after repeated SLT. PAS did not progress and IOP remained stable in both instances (7).

Repeatability

The energy applied in SLT is relatively low and there is limited tissue damage meaning it has a high repeatability as a procedure. There are several trials which have evaluated the safety of repeated SLT however due to different study designs, patient characteristics, follow-up periods and sample sizes it is difficult to compare the results.

Conclusion

The results of the LiGHT study has shown SLT to be an effective first line treatment for the diagnosis of open angle glaucoma and ocular hypertension. It is particularly useful for patients who have difficulty instilling eye drops. Furthermore, the ability to be able to repeat SLT if necessary means patients are able to achieve relatively long periods of drop-free glaucoma treatment. SLT can also be used to control high IOP in patients on maximal tolerable medication. Other important advantages of SLT include having only minor, transient, self-limiting side effects.

Response rates can vary and at one year postoperative have shown to be between 59-96%. The take-home message should be that despite its many benefits SLT is not a cure and regular follow-up appointments with a glaucoma specialist are necessary.

References

1. Zgryźniak A, Przeździecka-Dołyk J, Szaliński M, Turno-Kręcicka A. Selective Laser Trabeculoplasty in the Treatment of Ocular Hypertension and Open-Angle Glaucoma: Clinical Review. Journal of Clinical Medicine. 2021 Jul 27;10(15):3307.
2. ‌https://fyra.io. How SLT Works [Internet]. Glaucoma Today. Available from: https://glaucomatoday.com/articles/2005-mar-apr/0305_01.html
3. Gazzard G, Konstantakopoulou E, Garway-Heath D, 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. 2023 Feb;130(2):139-151.
4. ‌Stephenson M. First-Line Treatment: SLT or Medication? [Internet]. Reviewofophthalmology.com. 2024. Available from: https://www.reviewofophthalmology.com/article/firstline-treatment-slt-or-medication
5. 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 Nov;126(11):1511-1516. doi: 10.1016/j.ophtha.2019.05.032. Epub 2019 Jun 6. PMID: 31444008; PMCID: PMC6810843.
6. Jinapriya D, D’Souza M, Hollands H, El-Defrawy SR, Irrcher I, Smallman D, Farmer JP, Cheung J, Urton T, Day A, Sun X, Campbell RJ. Anti-inflammatory therapy after selective laser trabeculoplasty: a randomized, double-masked, placebo-controlled clinical trial. Ophthalmology. 2014 Dec;121(12):2356-61. doi: 10.1016/j.ophtha.2014.07.017. Epub 2014 Sep 16. PMID: 25234015.
7. Baser, E.F.; Akbulut, D. Significant peripheral anterior synechiae after repeat selectıve laser trabeculoplasty. Can. J. Ophthalmol. 2015, 50, e36–e38.