What is the difference between Argon Laser Trabeculoplasty (ALT) and Selective Laser Trabeculoplasty (SLT)?
ALT and SLT have been two of the most common laser surgery options offered to glaucoma patients in recent decades.
ALT is the original procedure for Laser Trabeculoplasty but has been largely replaced by the newer SLT procedure. ALT is still used rarely. There is also a new micro-pulsed version named MLT (not discussed in this answer).
There are also laser procedures for iridotomy and iridoplasty, but those are outside of the scope of this question too. Therefore, in the rest of this answer I will focus mainly on SLT (and mention its efficacy -- see the section Effectiveness below). I will also offer my opinion on current trends that could impact patient decisions around SLT.
As mentioned above, SLT (Selective laser trabeculoplasty) is the newer procedure and it has fewer side effects. With the older ALT, it is possible to cause thermal injury and coagulative damage to the trabecular meshwork, and this makes repeat procedures difficult. However, with SLT the treatment can be repeated by targeting more degrees of the trabular circumference. Selective laser trabeculoplasty can be a primary treatment of open angle glaucoma or it can be an adjunct to medication therapy. But it is contraindicated for closed angle glaucoma and should not be performed on patients who did not respond to initial treatment.
There are three trends that may impact one's decision to use SLT. First is the development and introduction of two new types of glaucoma medications:
The second part of this trend is the rapid development of MIGS procedures and devices. MIGS is Minimally-Invasive Glaucoma Surgery.
Third is the availability of a new cyclophotocoagulation procedure called Micropulse transscleral cyclophotocoagulation (MP-TSCPC)
Some glaucoma organizations claim, "MIGS procedures will not replace or eliminate traditional glaucoma surgery such as trabeculectomy and tubes)." However, that may not be true in the long term. The new MIGS procedures (possibly used in combination) will provide IOP reduction on par with trabeculectomy in up to moderately severe cases (and far more than what can typically be achieved with SLT). Micropulse transscleral cyclophotocoagulation can also provide significant IOP reduction and can be used in severe cases too.
In my opinion, anyone considering SLT should also be aware of these new trends. With that stated, let's continue the discussion of SLT.
In an article, “Comparison of the Morphologic Changes after Selective Laser Trabeculoplasty and Argon Laser Trabeculoplasty in Human Eye Bank Eyes” researchers Theresa R. Kramer, M.D. and Robert J. Noecker, M.D. Concluded that “SLT applied in vitro to the trabecular meshwork of human eye bank eyes seemed to cause no coagulative damage and less structural damage to the human trabecular meshwork when compared with ALT and, therefore, it is a safer and more repeatable procedure”.
Rarely encountered risks of SLT include corneal damage and swelling of the central retina (macula), both of which could result in loss of vision.
The Minimally Invasive Glaucoma Surgeries procedures I alluded to earlier may not work as well (or may not be possible to perform) when there is scarring of the angle structures. Angle scarring is seen in less than 3% of eyes treated with Selective Laser Trabeculoplasty, but it is still a risk to consider if you think you may want to or need to take advantage of the latest MIGS options in the future.
Dr. David Richardson, who performs both SLT and MIGS procedures, feels SLT has a great safety profile. He has said that in some cases, "it’s actually a safer first option than many of the medications."
The newer SLT selectively targets only the pigmented cells.The doctor will use a slit lamp microscope and gonio lens to guide the laser to the target drainage cells of the trabecula. Lowering the intraocular pressure is achieved by precisely targeting intracellular melanin granules to activate individual cells while not disturbing adjacent non-pigmented cells. This process is called selective photothermolysis. The macrophages triggered by the cytokines released from activated cells reactivate the meshwork thereby reducing fluid outflow resistance and lowering intraocular pressure.
Unlike Argon laser trabeculoplasty, in selective laser trabeculoplasty the melanin granules within individual trabecular meshwork cells receives 6000 times less energy density (fluence). The lower fluence is a result of the larger spot size and the lower energy required. With SLT the light energy is provided by a specially designed Q-switched, frequency doubled Nd:YAG laser operating at 532nm green with an output of from 0.3 to 1.5 millijoules. Using a specially developed gonio lens, a large 400 micron spot is directed into the angle to minimize beam distortion and provide good visualization of the angle. Unlike ALT, focus is not critical since the large spot has a greater depth of focus than the 50 micron ALT spot. A red aiming beam the same size as the treatment beam shows the area to be treated.
Approximately 80% of people experience lower eye pressure after SLT with the pressure reduction being in the range of only 10-20%. This relatively small reduction in IOP is the most common reason for not using SLT.
The effectiveness of this laser treatment will depend somewhat on the pigment content of the trabecular meshwork, as these are the cells which are targeted for pressure lowering. The effect of the laser is not permanent, usually wearing off within a few years.
Depending on response, the laser may be repeated more than once by targeting different degrees of the trabecular circumference. If taking eye drops for glaucoma, one will most likely still need them after this laser treatment, and if the initial treatment is not successful, the doctor may choose to repeat it.
Note: Many respondents to our FitEyes email group have indicated that they either did not receive much of a pressure lowering effect, or that it needed to be redone soon after.
The entire routine will take approximately one and a half hours, but the actual procedure will last only a few minutes. Numbing drops are used and then a special contact gonio lens will be used to help focus the laser light. The SLT involves sitting at a slit lamp similar to those used for eye exams. People rarely experience discomfort during this procedure. Once it is over and the lens is removed, there can be some temporary blurriness for about an hour, due to the ointment used with the contact lens. Depending on the circumstances, you may need to use special eye drops for several days to prevent inflammation.
Though serious acute side effects are very rare, there may be a transient rise in intraocular pressure after the treatment which would require additional eye drops. And there may be some temporary inflammation requiring additional anti-inflammory drops. Finally, there may be some slight irritation from scratching of the eye by the lens used in the procedure.
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