Laser trabeculoplasty (LT) is making a comeback as a treatment for open-angle glaucoma. It was first introduced as argon laser trabeculoplasty (ALT) in the 1970s. In the 1990s, selective laser trabeculoplasty (SLT) was developed to provide a similar intraocular pressure (IOP) reduction by only heating pigmented cells with a short pulse duration, providing a more targeted treatment and causing less tissue destruction. Especially since the LiGHT trial¹ supported its first-line use, SLT has been gaining momentum as a primary intervention in glaucoma treatment with proven efficacy and the potential for a drop-free life.

Lasers for SLT currently on the market in the United States include Solo, Tango, Tango reflex, Optimis Fusion, and Solutis SLT (Ellex); Selecta Duet and Selecta Trio (Lumenis); YC-200 S Plus (Nidek); and Lightlas SLT Solo (Lightmed). Pattern scanning laser trabeculoplasty, used by Pascal (Iridex), is a closely related computer-guided treatment method that applies a sequence of patterns onto the trabecular meshwork (TM). Direct SLT is an automated, rapid, noncontact procedure performed directly through the limbus (without gonioscopy) that is in development (Belkin Vision).

The latest iteration of LT is micropulse laser trabeculoplasty (MLT), IQ 532 (Iridex), which was introduced in 2008. The treatment delivers less than 1% of the energy of continuous-wave 532-nm green laser, with a similar IOP lowering capacity to SLT.^2-4 Micropulse technology divides the laser beam into microsecond bursts interspersed with longer resting intervals that allow the tissue to cool between pulses and reduce thermal buildup within the tissue directly targeted by the laser. The laser does not cause thermal necrosis.^5,6 Instead, it creates a stress response that induces a biological effect similar to SLT.⁷

Effectiveness and Safety

Most patients respond very well to both MLT and SLT. Anecdotally, about 80% of the patients we treat at our center with LT have an IOP reduction of 20% to 30%. Patients freshly diagnosed with mild glaucoma or ocular hypertension are those who respond best to LT.

Our research found that patients responded equally well to MLT regardless of baseline IOP status.⁸ Our 252-eye study on the predictive factors for SLT found that baseline IOP significantly predicted outcome.⁹ Those with lower IOPs did not experience as significant a reduction in IOP or the same rates of remaining drop free. Any patient with a baseline IOP <18 mmHg tended to receive less benefit.

Of the procedures we have performed over the last several years, we have not experienced a 1-hour IOP spike with MLT. We also have not seen postprocedure inflammation. We therefore do not give medications before or after MLT.

With SLT, there is a risk of an IOP spike. Our 1-hour IOP spike rate is approximately 5%. Many glaucoma specialists give patients an aqueous suppressant prophylactically before the procedure, while some only do so if needed. It is common to provide an anti-inflammatory agent after the procedure.

Candidates and Timing

Glaucoma specialists have begun offering LT as an option earlier and earlier in the treatment algorithm. For many patients first diagnosed with glaucoma, SLT or MLT treatment will be offered as a first-line treatment instead of first putting the patient on medication. Even if the effects of LT diminish over time, buying the patient even 1 or 2 years without drops can substantially impact the quality of life and symptoms of ocular surface disease. Before deciding to treat with SLT, however, a physician must assess whether a patient is at high risk for an IOP spike or inflammation.

Previous angle surgery is not an exclusion criterion for LT. In our experience, patients respond to LT regardless of the success or failure of angle surgery. The opposite is also true: Previous treatment with LT does not appear to affect the outcome of subsequent angle surgery.¹⁰

The Procedures

When performing SLT, some surgeons opt to treat 180° of the angle per session to avoid an IOP spike and inflammation. The standard spot size for SLT is 400 µm.

Micropulse is typically performed over 360° of the angle. We often treat with 120-140 shots when performing MLT to maximize its IOP-lowering effect and treat bilaterally in the same session if indicated. The standard spot size for MLT is 300 µm. A smaller spot size can facilitate targeting the pigmented TM.

For both MLT and SLT, a practical approach is to start from the inferior angle and move clockwise. The laser should be directed at the pigmented, filtering part of the TM. In a tight angle, instilling brimonidine or pilocarpine about 10 to 15 min before the procedure or shining a bright light helps open the angle. The laser should be aimed slightly biased anteriorly to avoid the iris.

Sometimes it is difficult to identify the filtering portion of the pale-pigmented TM, and the ciliary body or Schwalbe’s line can easily be mistaken for TM. In this case, gentle compression with the goniolens can elicit blood reflux to delineate it.

Power Setting

For SLT, surgeons generally titrate the power, increasing until microbubbles (“champagne bubbles”) become visible and then slightly reducing the power. Generally, SLT power settings range from 0.6 mJ to 1.4 mJ.

Titrating power for MLT has been common for retina specialists, with the goal of using sufficient energy to elicit a biological response without causing any structural scarring. Most glaucoma specialists have not done this but instead have used standard MLT settings of 1,000 mW of power and a 300-ms duration, with a 15% duty cycle.

Although there are no published data yet to support the practice, some surgeons, including those in our center, choose to increase energy settings to between 1,200 mW and 1,500 mW for MLT, after titrating power upwards from 1,000 mW in 100-mW increments until the blanching of the TM is seen in the pigmented area, then reducing the power by 100 mW to maximize the effect.

Follow-up

After SLT, a one-hour postoperative IOP check is recommended, and the use of an NSAID drop for a week is customary. After MLT, patients can go home right after the procedure without additional medication. For both SLT and MLT, there is no activity or driving restrictions after the procedure.

After both types of LT, it usually takes about 6 to 8 weeks for maximum pressure reduction, so there is generally a follow-up visit at about 2 months. Surgeons may opt to see select high-risk SLT patients after 2 weeks to ensure there has not been a rise in IOP.

The IOP reduction that results from LT treatment usually lasts a year or two. In some patients, it will hold for 4 or 5 years. When IOP begins to increase, either procedure can easily be repeated.

Laser Choice

Whether surgeons choose to perform MLT or SLT treatment is often determined by the type of laser they have in their practice. However, when it comes to buying a new laser, the micropulse laser can be a more versatile option since it can also be used as a continuous-wave argon laser when the micropulse function is turned off. Many retina specialists also use the MLT technology for subthreshold treatment of macular pathology.

Conclusion

Laser trabeculoplasty is evolving. Selective laser trabeculoplasty has helped patients reduce or eliminate drops with a better safety profile than ALT, and now MLT provides similar results without concern about IOP spikes and inflammation. Laser trabeculoplasty is establishing itself as a safe and effective first-line therapy for open-angle glaucoma for most adult patients.

References

1. Gazzard G, Konstantakopoulou E, Garway-Heath D, et al. Selective laser trabeculoplasty versus eye drops for first-line treatment of ocular hypertension and glaucoma (LiGHT): a multicentre randomised controlled trial [published correction appears in Lancet. 2019 Jul 6;394(10192):e1]. Lancet. 2019;393(10180):1505-1516. doi:10.1016/S0140-6736(18)32213-X
2. Abramowitz B, Chadha N, Kouchouk A, Alhabshan R, Belyea DA, Lamba T. Selective laser trabeculoplasty vs micropulse laser trabeculoplasty in open-angle glaucoma. Clin Ophthalmol. 2018;12:1599-1604. doi:10.2147/OPTH.S167102
3. Pate C, Deokule S. Review of current lasers in the management of glaucoma. J Curr Glaucoma Pract. 2008;2(3):4-7. doi:10.5005/jp- journals-10008-1034
4. De León M, Ortega Santana JF, García López A. Selective laser trabeculoplasty vs. micropulse laser trabeculoplasty for the treatment of open angle glaucoma and ocular hypertension. Int J Res Med Health Sci. 2017;14(1):30-34.
5. Yu AK, Merrill KD, Truong SN, Forward KM, Morse LS, Telander DG. The comparative histologic effects of subthreshold 532- and 810-nm diode micropulse laser on the retina. Invest Ophthalmol Vis Sci. 2013;54(3):2216-2224. doi:10.1167/iovs.12-11382
6. Inagaki K, Shuo T, Katakura K, Ebihara N, Murakami A, Ohkoshi K. Sublethal photothermal stimulation with a micropulse laser induces heat shock protein expression in ARPE-19 cells. J Ophthalmol. 2015;2015:729792. doi:10.1155/2015/729792
7. Sun CQ, Chen TA, Deiner MS, Ou Y. Clinical outcomes of micropulse laser trabeculoplasty compared to selective laser trabeculoplasty at one year in open-angle glaucoma. Clin Ophthalmol. 2021;15:243-251. doi:10.2147/OPTH.S285136
8. Hirabayashi MT, Rosenlof TL, An JA. Comparison of successful outcome predictors for MicroPulse laser trabeculoplasty and selective laser trabeculoplasty at 6 months. Clin Ophthalmol. 2019;13:1001-1009. doi:10.2147/OPTH.S205977
9. Hirabayashi M, Ponnusamy V, An J. Predictive factors for outcomes of selective laser trabeculoplasty. Sci Rep. 2020;10,9428. https://doi.org/10.1038/s41598-020-66473-0
10. King J, Lee D, An J. Surgical outcomes of ab-interno Schlemm’s canal procedures in open-angle glaucoma patients with previous selective laser trabeculoplasty. Poster presented at: Annual meeting of the American Glaucoma Society Meeting; March 14, 2019; San Francisco, CA.