The Impact of SLT on Glaucoma Treatment

Laser Trabeculoplasty (LTP) has been a recognised treatment for glaucoma for more than 30 years. The landmark study by Wise and Witter established this technique.¹ Although LTP was primarily performed with Argon laser (ALT), lasers of other wavelengths such as Diode, Krypton and continuous wave Nd:YAG laser can also be used to create the same thermal effect. This effect is independent of the wavelength and is related to the energy delivered.²

The uptake of thermal LTP into clinical practice had not been huge despite clinical trials that showed safety, equal efficacy to timolol in intraocular pressure (IOP) lowering, better long-term preservation of optic disc and visual field when compared to using initial eye drops. The Glaucoma Laser Trial (GLT) showed this with initial treatment of glaucoma.³ The Advanced Glaucoma Intervention Study (AGIS) showed long-term benefit in medically, maximally treated eyes for two years in white people and up to ten years in black people.⁴

Although a safe treatment, there were reports of uveitis, peripheral anterior synechiae (PAS) formation and vision loss. LTP, in large studies, was performed without side effects, however, in the real world there is much more variation in clinical outcome with LTP. Histologically, coagulative damage was seen at the trabecular meshwork with loss of the normal trabecular beams and architecture. In addition, experimental glaucoma in laboratories is produced in primates with the liberal use of thermal LTP. These were the main reasons LTP was left for those patients where maximum medical therapy has failed.

“SLT has given us choice in glaucoma and ocular hypertensive treatment, another prong in our armamentarium of IOP management

Selective laser trabeculoplasty (SLT) with a frequency-doubled Q-switched Nd:YAG laser has a selective photothermolysis effect; it lysis the pigmented cells selectively without causing collateral damage to the surrounding trabecular meshwork architecture. In essence, it improves trabecular outflow without scarring the trabeculum. With this appeal, we can use laser with greater confidence that no long-term damage is being done to trabecular outflow. There are many problems with eye drops: local and systemic side effects, non-compliance, non-response, on-going expense and sometimes, a general reduction in quality of life. For many patients there is huge appeal in achieving lower IOP without eye drops or with fewer eye drops.

When SLT was first introduced almost a decade ago, it was used by most practitioners in place of LTP. In other words, it was offered to patients who needed lower IOP when they were on the maximally tolerated medical therapy (MTMT). With increased experience with SLT, particularly the safety profile, many of us are offering it as an alternative to eye drops at all therapy points: initial therapy, as the first adjunctive therapy and in eyes on MTMT failing to reach target IOP (see diagram).

Some studies show that SLT is more effective in treatment-naïve eyes and those eyes with higher IOP (more likely in non-treated eyes). In my experience, many patients still opt for initial medication, although, a smaller, but significant group will opt for initial SLT. This is particularly the busy young professional group. I offer SLT to my patients every time treatment needs to be commenced. That is, with initial therapy and with each adjunctive eye drop; SLT is offered as an alternative.

In most eyes, an IOP reduction similar to latanoprost can be expected from 360 degrees SLT treatment.² A reduction of about 20-30 per cent can be expected in responders to treatment. A responder rate of up to 80 per cent has been reported in previously untreated eyes. A reduction of 20-25 per cent may be achieved in eyes on treatment.

Predicting who will respond to SLT is not easy. Some eyes respond more than others and this does not seem to be related to the amount of trabecular pigment, glaucoma risk factors, the different sub-types of glaucoma or the presence of pseudophakia. Higher pre-treatment IOP is associated with greater response.

The degree of trabecular pigmentation does not seem to influence the outcome but does influence the laser energy needed. The treatment protocol varies between practitioners. Some opt for 180 degrees of treatment; others perform 360 degrees in one or two sessions. Due to the larger spot size (400 μm), SLT is technically easier to perform than LTP (50 μm) and takes only a few minutes (see images).

There are very few side effects apart from mild discomfort and a ‘pink’ eye for a day or two. In the past, anti-inflammatory agents were used post-laser but many, including myself, do not routinely use a post-procedure anti-inflammatory eye drop. Serious side effects have not been reported apart from one case of hyphaema. Apraclonidine or brimonidine are used before and after laser to prevent an IOP rise. All pre-laser eye drops are continued; if the IOP response is very good, a trial of stopping some eye drop therapy can be considered.

We do not know if the same contraindications should be applied to SLT as LTP. LTP is contraindicated in angle closure, angle recession, uveitic glaucoma, aphakic glaucoma, congenital glaucoma and complex glaucomas such as silicone oil glaucoma. Further studies are needed to answer this question. SLT can be used in post-intravitreal triamcinolone steroid response. SLT can be used safely and effectively in following iridotomy in angle closure eyes when angle not closed by PAS is treated. SLT can be used in eyes that have had previous LTP, with success. In eyes that are heavily pigmented and eyes that have had previous LTP, caution must be exercised as a sustained elevation requiring surgery has been reported.⁵ Clinical experience with SLT re-treatments is increasing and has been shown to be effective.

SLT has given us greater choice in glaucoma and ocular hypertensive treatment, another prong in our armamentarium of IOP management. It has given our patients a safe, relatively painless, one-off treatment that can take the place of eye drops, additional eye drops or defer the need for surgery. While it is not effective in all patients, we should feel comfortable in presenting it as an early option in IOP-lowering therapy.

Dr. Ridia Lim is a cataract and glaucoma specialist in private practice at Hunter Street Eye Specialists, Parramatta, Sydney and a consultant glaucoma surgeon at Sydney Eye Hospital.

References:

1. Wise JB, Witter SL. Argon laser therapy for open-angle glaucoma. A pilot study. Arch Ophthalmol 1979;97(2):319-22.
2. Barkana Y, Belkin M. Selective laser trabeculoplasty. Surv Ophthalmol 2007;52(6):634-54.
3. The Glaucoma Laser Trial (GLT) and glaucoma laser trial follow-up study: 7. Results. Glaucoma Laser Trial Research Group. Am J Ophthalmol 1995;120(6):718-31.
4. Ederer F, Gaasterland DA, Dally LG, et al. The Advanced Glaucoma Intervention Study (AGIS): 13. Comparison of treatment outcomes within race: 10-year results. Ophthalmology 2004;111(4):651-64.
5. Harasymowycz PJ, Papamatheakis DG, Latina M, et al. Selective laser trabeculoplasty (SLT) complicated by intraocular pressure elevation in eyes with heavily pigmented trabecular meshworks. Am J Ophthalmol 2005;139(6):1110-3.