SLT Blog by Dr. Savak Teymoorian

03 September 2018

Laser Trabeculoplasty – Isn’t It All the Same? No!

The field of glaucoma is changing so rapidly that sometimes it is hard to understand what exactly is completely new as opposed to another iteration of something old. This issue holds true in the category of laser trabeculoplasty. Providers tend to lump together all forms including ALT, SLT, and even MLT. The perceived thought process is that they are all different forms of laser trabeculoplasty but essentially the same. The reality, however, is that each are quite different; it warrants an appreciation for their subtle differences. This presents a challenge to physicians to decide which to use in practice. The forward-thinking physician recognizes this situation and views it as an opportunity to better understand and leverage this technology in patient care.

The field of glaucoma is changing so rapidly that sometimes it is hard to understand what exactly is completely new as opposed to another iteration of something old. This issue holds true in the category of laser trabeculoplasty. Providers tend to lump together all forms including ALT, SLT, and even MLT. The perceived thought process is that they are all different forms of laser trabeculoplasty but essentially the same. The reality, however, is that each are quite different; it warrants an appreciation for their subtle differences. This presents a challenge to physicians to decide which to use in practice. The forward-thinking physician recognizes this situation and views it as an opportunity to better understand and leverage this technology in patient care.

Let’s begin with ALT (argon laser trabeculoplasty) since it was invented first. Introduced back in the late 1970’s, ALT was performed by an argon laser which emits light in the green and blue-green spectrum that is well absorbed by melanin present in the trabecular meshwork. Argon lasers have now been replaced in ophthalmology by frequency-doubled solid-state lasers which emit green only light but the procedure is still called ALT. The small spot size which is used, of 50 microns, along with an exposure time of 0.1 seconds, needs more energy to have an effect and creates subsequent heat at and near the site of application. This results in scarring of the trabecular meshwork. Because of its thermal effects and resulting permanent architectural changes, the use of ALT is limited to a single application over one area. It is not effective when needed to be repeated including if a patient’s IOP increased over time.

Another consideration to contemplate is the possible effect of interfering with subsequent glaucoma surgery. This originally was not a problem since the trabecular meshwork and Schlemm’s Canal were not affected by  the gold-standard bypass surgeries of trabeculectomy and tube shunts. However, the introduction of MIGS has changed the landscape. We now must take into account the fact that  MIGS options are reduced if ALT is used as the laser trabeculoplasty modality. The decision to utilize ALT in a patient likely eliminates the use of trabecular bypass stents like iStent and of canaloplasty like ABiC. This latter subclass of MIGS represents the best benefit-to-risk (B/R) group by targeting and rejuvenating the natural physiological outflow system. Loss of this category forces the physician to select other MIGS options with less desirable B/R ratios.

The next option in this category is SLT (selective laser trabeculoplasty) which has been around for over 20 years. This involves utilizing a green light with 532nm wavelength produced by frequency-doubling an Nd:YAG laser. It uses a larger spot size of 400 microns with a duration of 3 nanoseconds. The result is a very focused targeting of melanin needing less energy, all without damaging surrounding tissue because there is no creation of heat. This becomes critical because it does not create the scarring as seen with ALT. The downstream effects including the ability to successfully retreat patients through repeat SLT applications later down the line of patient care. This also means that SLT doe not prevent the use of canal-based MIGS. The decision to either choose SLT first or convert from ALT to SLT comes quite clear for both patients and providers.

Another consideration is MLT (micropulse laser trabeculoplasty) which can  use  a diode laser (with a longer wavelength of 810nm as compared to ALT and SLT) or the same green wavelength. 810 nm diode results in deeper tissue penetration and less absorption by the targeted melanin structures. This wavelength is now mostly used for transscleral coagulation of the ciliary body. There is still some confusion though regarding green wavelength MLT. Basically it is a mild form of ALT because the duration for MLT is 0.2 seconds that is divided into 100 smaller millisecond pulses. These smaller units allow for cycling to occur with this laser that permits cooling between periods. The result is to prevent the strong thermal damage seen previously with ALT. But the difficulty is to heat the tissue sufficiently to cause any effect. Although there is strong evidence to support SLT and MLT over ALT, the differentiation between SLT and MLT is not always understood, in that both treatments are invisible on the TM. Both stimulate an effect. But no study as yet has shown that the mild temperature rise provided by MLT lasts any longer than a few months, whereas the biochemical stimulation of SLT has been repeatedly shown to be effective in some patient populations for many years, which may justify the extra investment in a dedicated system.

In real life practice, providers will need to select one version of laser trabeculoplasty or have compelling reasons to change from their current platform to a newer one. The underlying factors that guide in this decision-making process are very similar to the procedure we use to determine the next course of action in the medical and surgical care for patients. Specifically, do the pros outweigh the cons among options and justify the financial implications. The decision to either primarily choose or change to SLT becomes apparent based on the profiles and applications provided above. It provides the best profile – successful and low risk therapy to patients while doing so in a manner that is both cost-efficient and -effective to providers. Specifically, providers with ALT and MLT would be much better suited to and should convert to SLT. Remember, the goal in glaucoma care is the highest quality of life for our patients. This reasoning is what drives our decisions to upgrade and utilize new technology.

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