The popularity of selective laser trabeculoplasty has risen in recent years owing to its significantly lower laser fluency and enhanced predictability compared to argon laser trabeculoplasty (ALT). Selective laser trabeculoplasty (SLT) has also been found to be equal in efficacy to topical prostaglandin monotherapy at lowering IOP.1 One of the advantages of SLT compared to ALT has been minimal thermal and collateral tissue damage, with the theoretical benefit of repeatability.
SLT still rarely may cause pressure spikes after treatment, however, in addition to mild and transient inflammation and pain. Growing knowledge about heat stress proteins and cellular response emphasizes that lethal damage to cells is not necessary to provide a therapeutic benefit in retinovascular applications,2,3 and the same is likely true with glaucoma.
Micropulse technology (Iridex) breaks a continuouswave laser beam into short, repetitive pulses that allow cooling between laser applications, thereby reducing or preventing thermal damage,4 as is true for SLT. After setting a level for laser power and a spot size, the surgeon controls thermal exposure with the duration and duty cycle. Duration refers to the total length of time that laser energy is delivered, including the rest periods, and duty cycle denotes the percentage of time that the laser is delivering energy versus resting. For example, for a pulse duration of 2 milliseconds with a duty cycle of 15%, laser energy would be applied for 0.3 milliseconds followed by a 1.7-millisecond rest period. This pattern would be repeated for the indicated duration.
By using short bursts of laser energy, micropulse laser trabeculoplasty (MLT) has been shown to cause no anatomical effects compared to ALT burns on the trabecular meshwork when evaluated by electron microscopy. Essentially, MLT-treated tissue was similar in appearance to control tissue.4 MLT seems to be a very gentle treatment for glaucoma, and I look forward to more highquality peer-reviewed research.
MLT lowered IOP in a phase 2 clinical study,5 and over the past year, my colleagues’ and my early results with modified laser settings have been promising. I have been performing the procedure for a year. My current technique is to use a 532-nm wavelength, 1,000 mW of power, a spot size of 300 μm, a total exposure duration of 300 milliseconds, and a duty cycle of 15%. After a 360º treatment, I have seen approximately a 25% decrease in IOP so far in more than 50 patients. Although I do not yet know how long the effect will last, considering the minimal alteration of tissue with treatment, MLT may be repeatable.
Certainly, one distinguishing feature of MLT is evident: the lack of any sign of treatment during or after the procedure. Typically, blanching of pigment or microcavitational bubbles are evident during laser trabeculoplasty (LTP). This is not the case with the extremely low level of micropulsed energy delivered with MLT. Eyes are generally very quiet postoperatively and so far seem to tolerate MLT extremely well. Importantly, my colleagues and I performing this procedure have seen no IOP spikes after treatment thus far, although, again, our experience is limited to a small number of eyes over the course of 1 year.
MLT is a relatively new method of LTP that uses advances in the delivery of laser energy. Although the published data are relatively early, the procedure seems to hold promise as a viable option for treating openangle glaucoma. Certainly evident is the value of a safe therapy not involving topical medical drops, as has been clear with SLT and ALT. LTP has a role as first-, second-, and third- or fourth-line therapy. MLT may provide a versatile platform for multiple applications, and micropulse laser therapy appears to offer additional safety. This may be particularly important for patients who are at risk of an IOP spike or for whom inflammation could be problematic. I look forward to gaining more experience and seeing more data.
Iqbal Ike K. Ahmed, MD, FRCSC, is an assistant professor and director of the Glaucoma and Advanced Anterior Surgical Fellowship at the University of Toronto. Dr. Ahmed is also a clinical assistant professor at the University of Utah in Salt Lake City. He is a consultant to Iridex. Dr. Ahmed may be reached at (905) 820-3937 ext. 161; firstname.lastname@example.org.
- Katz LJ, Steinmann WC, Kabir A, et al. Selective laser trabeculoplasty versus medical therapy as initial treatment of glaucoma: a prospective, randomized trial. J Glaucoma. 2012;21:460-468.
- Binz N, Graham CE, Simpson K, et al. Long-term effect of therapeutic laser photocoagulation on gene expression in the eye. FASEB J. 2006;20(2):383-385.
- Yu AK, Merrill KD, Truong SN, et al. 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.
- Fudemberg SJ, Myers JS, Katz LJ. Trabecular meshwork tissue examination with scanning electron microscopy: a comparison of micropulse diode laser (MLT), selective laser (SLT), and argon laser (ALT) trabeculoplasty in human cadaver tissue. Invest Ophthalmol Vis Sci. 2008;49(5):ARVO e-abstract 1236.
- Fea AM, Bosone A, Rolle T, et al. Micropulse diode laser trabeculoplasty (MDLT): a phase II clinical study with 12 months follow-up. Clin Ophthalmol. 2008;2(2):247-252.