Benefits of SLT in intraocular hypertension

Open angle glaucoma (OAG) is a disease of the optic nerve, with characteristic changes in the appearance of the optic nerve, typically with corresponding changes in the visual field.  It can occur with or without an increase in eye pressure. In white Europeans, OAG affects 2% of those over 40 years of age, increasing to 10% for those aged 75 years or older. Rates are higher in people of black African or black Caribbean origin. OAG is potentially blinding; 10% of UK blind registrations are due to glaucoma.(1)

On a global scale, glaucoma is the leading cause of irreversible blindness worldwide. Many risk factors have been identified for OAG, with raised intraocular pressure (IOP) being consistently the most important.(2) Ocular hypertension (OHT) is a condition where the IOP is consistently elevated above 21mmHg without changes in either the optic nerve or visual field. OAG is usually asymptomatic until the disease in quite advanced and, once diagnosed lifelong monitoring and or treatment is essential. Once sight has been lost due to glaucoma, it cannot be restored.(1)

Currently, the most common treatment pathway is eye drops to lower the eye pressure and, if they are unsuccessful, incisional filtration surgery. While eye drops are effective in lowering eye pressure, they need to be taken daily and can have both local and systemic side effects. They are a prescription-only medication, requiring the preparation and administration of repeat prescriptions by the health service, coupled with the inconvenience of having to collect and organise repeat prescriptions by the patient. Daily, lifelong administration of eye drops is required. It can be difficult to remember to use the drops every day and they can be difficult to administer, especially for the elderly, in whom glaucoma is most prevalent. Eye drops have both local side effects, for example, red eye, dry eye, chronic inflammation, change in eye colour, increased eyelash growth or change in periocular skin colour, as well as systemic side effects, for example, shortness of breath, bradycardia, change in mood or impotence.(3)

Selective laser trabeculoplasty (SLT) is a non-invasive outpatient procedure that has been shown to be as effective as eye drops in lowering eye pressure after a single treatment.

Aqueous humour is produced by the ciliary body within the eye to nourish the internal structures of the eye and drains from the eye via the trabecular meshwork (TM). In OAG, there is increased resistance to aqueous outflow at the TM.

Applying laser to the TM to lower eye pressure, argon laser trabeculoplasty (ALT) was described by Wise and Witter in the 1970s.(4) Several large trials, such as the Glaucoma Laser Trial (ALT versus Timolol), showed the long-term effectiveness of ALT in controlling eye pressure.(5)

In 1983, Anderson and Parish described selective photothermolysis, a process by which pigment selectively absorbs light energy, causing destruction of the pigment but not the surrounding non-pigmented tissues.(6) This property has been exploited in many laser treatments, for example, in laser tattoo removal. The TM has a pigmented and a non-pigmented section, and Latina and Park used the principal of selective photothermolysis to selectively target the pigmented TM with a Q switched, frequency-doubled Nd:YAG laser.(7)

SLT has been shown to be as effective as ALT in lowering eye pressure.(8) However, because it uses 1000-times less energy than ALT, it does not cause structural damage to the TM and is therefore potentially repeatable.

Exactly how SLT produces a reduction in IOP is the subject of ongoing research. SLT has been shown to increase trabecular outflow. Laser trabeculoplasty induces cell division and causes the release of cytokines (inflammatory mediators), which may restore function to the TM. These biochemical changes take approximately four to six weeks after laser therapy to occur.(9)

SLT is an outpatient procedure. Five minutes before the start of the treatment, drops are instilled into the eye to prevent the eye pressure becoming transiently raised after the procedure and to anaesthetise the ocular surface. A contact lens is placed on the surface of the eye and the laser is delivered to the TM. One hundred laser shots are applied to the whole 360 degrees of the TM. The laser spot size and duration are fixed, and only the energy setting needs to be adjusted by the laser operator.  The procedure is typically painless and takes approximately 20 minutes. The eye pressure is then checked one hour after the laser and non- steroidal anti-inflammatory drops, for example, kerorolac 0.5%, are used three times daily for three days. The patient is then seen at one week and again at four weeks.

The laser procedure has a good safety profile. The IOP may increase after the laser, but this is transient and can be managed with additional eye drops in the short term. Patients may experience transient blurring of vision and mild ocular pain. Rarely, hyphaema (blood in the anterior chamber of the eye), keratitis (inflammation of the cornea) and cystoid macular oedema (a transient collection of fluid in the retina) can occur.

SLT is not recommended for use in neovascular glaucoma, angle closure or heavily pigmented angles.

It can take up to eight weeks to see the full treatment effect of SLT. Response rates range from 94% to 58% at one year, reducing over time to 59% to 31% at five years.(8)

Prostaglandin analogues, for example, latanaprost 0.005% (one drop at night), are the current first-line treatment for OAG. Several studies have compared SLT to prostaglandin analogues and have found them to be equivalent.9 However, there are a number of weaknesses in these studies. The LIGHT (Laser in Glaucoma and Ocular Hypertension) trial is a very large National Institute for Health Research-funded study that has been designed to determine whether initial laser treatment with SLT leads to a better quality of life. It is a six-centre pragmatic randomised, controlled trial run by Moorfields Eye Hospital that will recruit 718 subjects and follow them for three years.

SLT is clinically proven to treat glaucoma by safely and effectively reducing intraocular pressure in a single, outpatient procedure. It be an effective adjunct to medication therapy or used as a primary treatment to reduce or eliminate the need for topical glaucoma medications. SLT protects the TM and can increase patient compliance.

1. National Institute for Health and Care Excellence. CG85 Glaucoma: NICE guideline. www.nice.org.uk/guidance/CG85.
2. Rolim de Moura CR et al. Laser trabeculoplasty for open angle glaucoma. Cochrane Database Sys Rev 2007;Issue 4:CD003919.
3. Barkana Y, Belkin M. Selective laser trabeculoplasty. Surv Ophthalmol 2007;52:634–54.
4. Wise, JG, Witter SL. Argon Laser therapy for open-angle glaucoma. A pilot study. Arch Ophthalmol 1979;97:319–22.
5. The Glaucoma Laser Trial Research Group. The Glaucoma Laser Trial (GLT) and glaucoma laser trial follow up study: Results. Glaucoma Laser Trial Research Group. Am J Ophthalmol 1995;120:718–31.
6. Anderson RR, Parish J. Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation. Science1983;220(4596):524–7.
7. Latina M, de Leon JM. Selective laser trabeculoplasty. Ophthalmol Clin N Am 2005;18:409–19.
8. Realini T. Selective laser trabeculoplasty. J Glaucoma 2008;17:497–502.
9. Samples JR et al. Laser trabeculoplasty for open angle glaucoma. A report by the American Academy of Ophthalmology. Ophthalmology 2011;118:2296–302.