Selective laser trabeculoplasty (SLT) was introduced in the United States in 2001 as a novel laser surgery to lower intraocular pressure (IOP). The laser used is a frequency-doubled Q-switched Nd:YAG that treats the trabecular meshwork (TM) based on the principle of selective photothermolysis, in which a short laser pulse targets pigmented TM cells in a relatively nondestructive manner. Prior to the advent of SLT, argon laser trabeculoplasty (ALT) was the most common form of trabeculoplasty. ALT is thought to lower IOP by leveraging thermal energy to contract TM tissue, making the overall structure more porous and increasing aqueous outflow.^1,2

The mechanism by which SLT lowers IOP, however, is poorly understood. It is thought that SLT leads to increased aqueous outflow through the TM, either through a mechanical change, remodeling of the extracellular matrix due to cytokine release, or stimulation of TM cell proliferation. Ultimately, the true mechanism of action may be multifactorial.

The spot size and pulse duration for SLT are fixed at 400 micrometers and 3 nanoseconds, respectively. The optimal power setting, however, is not well understood despite more than a decade of clinical use. The data regarding the ideal power to use for SLT is limited and variable in its conclusions. One study of 74 patients concluded that lower power SLT (defined as 0.3-0.5 mJ) is equivalent to standard power (0.6-1.0 mJ) and may be safer.³ A separate study of 49 patients found that higher total power used is associated with a greater IOP-lowering response.⁴

Reviewing the literature, almost all studies of SLT use the formation of visible cavitation or “champagne bubbles” as an endpoint to determine power, either choosing the lowest power when cavitation bubbles are visible, or decreasing the power once cavitation bubbles are seen to arrive at a power where bubbles are not formed. There are, however, no data that specifically support either of these methods to arrive at the ideal treatment power.^3-7

Given that the mechanism of action by which SLT lowers IOP is poorly understood, it is not surprising that there is also debate over precisely what power settings or treatment endpoints should be used. The original histologic analysis of cadaver eyes treated with ALT and SLT showed crater-like tissue destruction after ALT, yet no visible damage after SLT.⁷ In subsequent work using a wider range of treatment powers, our group has demonstrated that SLT can cause visible tissue damage, albeit at treatment powers higher than commonly used in clinical practice.⁶ The idea, however, that SLT is completely nondestructive is likely incorrect. Samples in this latter study also showed no difference in histologic appearance whether cavitation bubbles were seen during treatment or not.

TM pigmentation is variable between patients and even throughout the 360 degrees of angle in individual eyes. Because SLT is purported to selectively target pigmented cells in the TM, it seems that the optimal treatment parameters would likely vary among patients. Further, while eyes with increased pigmentation are thought to respond better to SLT compared to eyes with light or absent pigmentation, clinical studies have failed to confirm a relationship between treatment success and angle pigmentation. As of yet, no data exist to further guide these decisions.

Ultimately, while commonly taught and used as a treatment endpoint in clinical studies, the use of visible cavitation bubbles to titrate treatment power during SLT has no data underpinning its use. Given that the mechanism of action of SLT is likely multifactorial, it is unclear how one should go about choosing and titrating the most appropriate power settings for each patient. In our practice, we titrate power down by 0.1 mJ when cavitation bubbles are first seen, and some titrate down again if bubbles continue to be seen. This has been effective in our practice, and is one method that could be considered until we have further knowledge to guide our treatment. GP

References

1. Latina MA, Park C. Selective targeting of trabecular meshwork cells: in vitro studies of pulsed and CW laser interactions. Exp Eye Res. 1995;60(4):359-371.
2. Latina MA, Sibayan SA, Shin DH, Noecker RJ, Marcellino G. Q-switched 532-nm Nd:YAG laser trabeculoplasty (selective laser trabeculoplasty): a multicenter, pilot, clinical study. Ophthalmology. 1998;105(11):2082-2088.
3. Kennedy JB, SooHoo JR, Kahook MY, Seibold LK. Selective laser trabeculoplasty: an update. Asia Pac J Ophthalmol (Phila). 2016;5(1):63-69.
4. Lee JW, Wong MO, Liu CC, Lai JS. Optimal selective laser trabeculoplasty energy for maximal intraocular pressure reduction in open-angle glaucoma. J Glaucoma. 2015;24(5):e128-e131.
5. Habib L, Lin J, Berezina T, Holland B, Fechtner RD, Khouri AS. Selective laser trabeculoplasty: Does energy dosage predict response? Oman J Ophthalmol. 2013;6(2):92-95.
6. SooHoo JR, Seibold LK, Ammar DA, Kahook MY. Ultrastructural changes in human trabecular meshwork tissue after laser trabeculoplasty. J Ophthalmol. 2015;2015:476138.
7. Kramer TR, Noecker RJ. Comparison of the morphologic changes after selective laser trabeculoplasty and argon laser trabeculoplasty in human eye bank eyes. Ophthalmology. 2001;108(4):773-779.

Jeffrey R. SooHoo, MD, is an assistant professor in ophthalmology at the University of Colorado School of Medicine in Denver, Colorado. He reports no related disclosures. Reach him at jeffrey.soohoo@ucdenver.edu.

Leonard K. Seibold, MD, is an associate professor and codirector of the glaucoma fellowship at the University of Colorado School of Medicine. He reports no related disclosures.

Malik Y. Kahook, MD, is the Slater Family Endowed Chair in Ophthalmology, vice chair of clinical and translational research, professor and chief of the glaucoma service and codirector of the glaucoma fellowship in the department of ophthalmology at the University of Colorado Anschutz Medical Campus in Denver, Colorado. He reports no related disclosures.