Affecting one in 200 people up to the age of 50 – and rising to one in 10 between the ages of 50 and 80 – glaucoma is one of the most frequent causes of blindness if left untreated. Glaucoma occurs when an imbalance in production and drainage of fluid in the eye increases eye pressure to unhealthy levels. The drainage canals between the iris and cornea (the ‘angle’) become clogged, blocked or covered, resulting in high pressure that can irreparably damage the optic nerve. 

Traditionally, eye drops have been used to reduce the pressure; however, advanced laser technologies are now an invaluable mainstay in the surgical treatment of glaucoma for a subset of patients who are unable to comply with medical treatment or for whom eye drops are insufficient to control the intraocular pressure (IOP). The most common laser-based treatment for glaucoma is trabeculoplasty, in which the laser is used to reduce IOP caused by open-angle glaucoma – the most common form of the condition. 

Trabeculoplasty: ALT and SLT

“The type of laser surgery we use depends on the form of glaucoma and its severity,” says Ms Aoife Doyle, consultant ophthalmologist at the Royal Victoria Eye and Ear and St James’s Hospitals. “With open-angle glaucoma there is a painless, slow rise of pressure that results in a gradual loss of peripheral vision. This is the most common form, affecting over 75% of people with the condition. Acute-angle closure glaucoma has a more spectacular presentation with sudden attacks of very high pressure in the eye. It’s extremely painful and patients come in with headaches, vomiting, blurred vision and so on.

“Eye drops are commonly used to control open-angle glaucoma but this can be very expensive and a nuisance since they must be taken for the rest of the patient’s life. Some patients have poor compliance: they may find it difficult to administer the drops daily, perhaps they have arthritis or are forgetful, and for some there are unwanted side-effects. So that’s where laser trabeculoplasty comes in. It is increasingly being used as a first-line treatment also.”

Laser trabeculoplasty uses a very focused beam of light to treat the drainage angle of the eye. This surgery makes it easier for fluid to flow out of the front part of the eye, decreasing pressure in the eye. There are two types of laser trabeculoplasty: argon laser trabeculoplasty (ALT), which uses a thermal argon laser to open up the drainage angle of the eye; and selective laser trabeculoplasty (SLT), which uses a non-thermal YAG laser to do the same job.

ALT was first introduced as a successful treatment modality for open-angle glaucoma by Dr James Wise in 1979. The advent of SLT came much later, in the late 1990s. Both procedures have proved equally effective in reducing IOP; however, control over the pressure inside the eye may decrease as time passes and laser surgery may need to be repeated.

“A large study dating back to the 1990s demonstrated that ALT is beneficial in the treatment of open-angle glaucoma. Even when used instead of drops, it appeared to be quite effective. However, the benefit wore off after a few years,” says Aoife. 

Long-term studies have shown that ALT maintained IOP control in 67-80% of the eyes one year after the procedure and in 35-50% of the eyes five years after the procedure.

Aoife explains that, during ALT, a thermal laser is directed at the trabecular meshwork – the eye’s drainage system located at the junction of the iris and cornea in the front of the eye – producing a focused beam of light that makes a very small burn or opening in the eye tissue, helping the drainage system to work better. This is a brief day procedure and, usually, half the fluid channels are treated first. If necessary, the other fluid channels can be treated in a separate session another time. This method prevents over-correction and lowers the risk of increased pressure following surgery.

“The argon laser has a burning effect, it’s called coagulative, in that way it’s supposed to make little holes in the draining angle of the eye but actually the burn itself may eventually cause scarring and close over. Whereas SLT uses a different, very short pulse of energy – a few nanoseconds of high energy – and a different type of laser, called a YAG laser,” Aoife elaborates. “This laser gives a high burst of energy at a particular frequency, apparently disrupting certain cells containing pigment, and that effect sets off an inflammatory process. The inflammatory mediators facilitate outflow of fluid through the trabecular meshwork, so basically the inflammation set off by the laser widens the inter-trabecular spaces, thereby facilitating outflow. SLT uses a lower energy so it doesn’t cause burns and scarring and it appears to be as effective as ALT.”

Proponents of SLT claim that because of the lack of thermal damage to the trabecular meshwork, it can be repeated over time with renewed effectiveness and without damage to the eye’s draining system. There are numerous reports of cases where patients with previous ALT whose pressure had come back up were treated successfully with SLT. 

“Although SLT was first introduced in the late 1990s, it has only really become popular since about 2004/2005. The Eye and Ear was the first hospital in Ireland to introduce this technology in 2006. There are a few private clinics and individuals around the country who have it now, but as far as I know we’re the only public hospital that offers SLT.”

She says SLT is technically easier to perform and arguably a safer procedure than ALT. “Selective laser trabeculoplasty is the most common laser procedure I do – argon is good but there’s no particular benefit to it over SLT. Both are day procedures and are minimally invasive. With SLT, the patient is awake and sitting upright, their eye is anaesthetised with drops. We use a lens to direct the laser and hold the lids open. It only takes about five to 10 minutes to treat each eye and we do it in the outpatient clinic. In fact, when I see the patient, if I think SLT is indicated I will do it straight away, where possible. Afterwards, we’ll monitor the patient for about an hour to make sure they don’t have any complications or inflammation or rise in their eye pressure. And then they’re free to go home.”

In general, patients can resume normal daily activities the next day after laser surgery.

Both ALT and SLT can be used as first-line treatment in patients with open-angle glaucoma. It has been shown to be as effective in lowering IOP as topical drops, and new research, published in April this year in the Archives of Ophthalmology, suggests that laser trabeculoplasty may be a more cost-effective alternative to eye drops, due primarily to reduced levels of medication adherence.

Aoife points out that some patients treated with laser trabeculoplasty may still need to take eye drops, albeit fewer than before. Patients will also be monitored at biannual check-ups for any increase in IOP.

Other laser treatment options

The second most common laser treatment for glaucoma is laser iridotomy, which is used to lower elevated IOP caused by angle-closure glaucoma, after the pressure in the affected eye has been reduced with medicine or when medicines fail.

“Angle closure was for many years only treated by a surgical procedure called peripheral iridectomy, in which a small section of peripheral iris is excised through a limbal incision. Since the advent of the YAG laser about two decades ago, most of these patients are now treated with a YAG laser iridotomy,” says Aoife. 

“Another laser approach is cycloablation, which is used in complex glaucoma cases such as those that are difficult to treat or those performed in conjunction with cataract surgery. Cycloablation basically involves destruction of part of the ciliary body of the eye in order to reduce the amount of fluid introduced by the eye.

“Cycloablation is very effective but it does have potential complications so we save that for people who already have impaired vision and are trying to hold on to the remaining vision they have for as long as possible.” 

Laser technology is developing all the time and promises enhanced surgical approaches in the treatment of glaucoma. The latest micropulse laser trabeculoplasty (MLT), for example, provides the same pressure-lowering effects as ALT and SLT with less energy and inflammation. 

Future developments

Last year, 115 people with glaucoma joined the National Council for the Blind in Ireland (NCBI) – which provides support and services to people experiencing sight loss – bringing to more than 1,000 the number of people using NCBI services as a result of having glaucoma. Experts warn that the incidence of glaucoma is set to increase by 33% over the coming decade, linked to the expected rise in the population between 2011 and 2021, as projected by the CSO.

“The best way to tackle glaucoma is to diagnose it early, so it’s important to encourage people over 50 to get their eyes tested every couple of years,” Aoife urges. “Glaucoma treatments are very effective, especially if caught early, and there is every chance that treatment with eye drops or laser surgery, or a combination, will prevent progression of the disease.”