A New Option for Trabeculoplasty

Studies show MLT has equivalent efficacy at a lower cost with less risk of complications.

BY ANTHONY ECONOMOU, D.O.

Micropulse laser trabeculoplasty (MLT) provides the same IOP-lowering effects as argon laser trabeculoplasty (ALT) and selective laser trabeculoplasty (SLT) with less energy and inflammation than the earlier procedures. For this reason, I now use MLT exclusively on my glaucoma patients for whom trabeculoplasty is indicated. Let me explain the procedure and outline its benefits.

Performing MLT

MLT is performed with the 810-μm IQ 810 photocoagulator (Iridex, Mountain View, Calif.), a multipurpose ophthalmic laser that can be used for transpupillary, transcleral and intraocular treatments. The device treats the deeply pigmented cells of the trabecular meshwork (TM) through laser-induced thermal elevation.

MLT employs a series of low power pulses (2 watts over a 300-μm spot) for a short duration, or "micropulse" of 0.3 milliseconds (ms), followed by a pause in laser delivery for 1.7 ms repeated over a 200 ms exposure (100 micropulses are delivered per laser application). We perform MLT using 66 confluent laser applications covering the whole height the trabecular meshwork over 180°.

An advantage for the surgeon is that MLT is relatively simple to perform. The large spot is easy to aim and focus, and the laser features a continuous foot pedal for ease of operation. (See Figure 1 for more detail).

Evolution of Laser Trabeculoplasty

More than 15 years ago, the Glaucoma Laser Trial (GLT)^1,2 established that laser trabeculoplasty was at least as effective as timolol for initial therapy for primary open-angle glaucoma (POAG). Despite this, laser trabeculoplasty has never caught on to the degree one might expect. This is due in part to some of the limitations of ALT, including the potential for post-treatment IOP spikes and peripheral anterior synechia (PAS), which causes coagulation damage to the TM. Also, ALT retreatment has limited efficacy due to the thermal effects of the laser resulting in coagulation and subsequent scarring of the tissue.

SLT was an improvement, allowing surgeons to perform a more repeatable, more comfortable procedure that was just as effective as ALT with clinical utility earlier in the treatment paradigm. The high power creates micro-explosions, which can lead to anterior chamber (AC) reaction and postoperative IOP spikes. These are less common than with ALT, but can be more severe. Approximately 9% of SLT patients had an IOP spike of more than 8 mm Hg,³ compared to 34% of ALT patients who had a 5-mm Hg IOP rise in the GLT.

The biggest drawback to SLT is that it requires a dedicated, expensive laser. Many of us who have been advocates for laser trabeculoplasty for a long time have been eager to find a way to perform it with our existing diode lasers.

MLT Delivers

MLT appears to have improved upon both of the other laser procedures. The eyes remain quiet with minimal postoperative inflammation, which is very important. With SLT and ALT, patients often require nonsteroidal anti-inflammatory drugs (NSAIDs) or steroidal medications for pain control, which can almost defeat the purpose of the procedure. In my experience with MLT, patients have not required postoperative relief and have had no pressure spikes.

Figure 1. MLT's mechanism of action.

We all know that patients who take prostaglandins, now the most common first-line therapy for POAG, are the most prone to post-laser pressure spikes. In my experience, these patients can experience a 20%-25% IOP increase following SLT, but I have not seen pressure spikes at all with MLT.

MLT interacts with, but does not destroy, the TM or the pigmented endothelial cells in the TM. There is no pigment clumping, as we used to see with ALT. The MLT procedure is capable of reaching the thresholds of cytokine activation and MMP-3 up-regulation without champagne bubble formation, tissue blanching or other visible signs of photothermal damage and inflammation.

Figure 2. The results of Dr. Ingvolstad's 2005 study, presented at ARVO, and my own results.

Studies Demonstrate Efficacy

A study presented by Dr. Ingvoldstad at the 2005 Association for Research in Vision and Ophthalmology (ARVO) meeting compared the short-term IOP-lowering effects of MLT and ALT.⁴ The study included 21 eyes of 21 patients randomized to either MLT or ALT. All eyes had preoperative IOP of more than 21 mm Hg. At 1 month and 3 months, MLT lowered IOP as well as or better than ALT. In addition, 91% of the ALT eyes, but only 20% of the MLT eyes, showed cell and flare 1 hour after treatment.

My colleagues and I conducted our own study in our office using the same MLT parameters as the Ingvoldstad study: 2000 mW laser energy, 200 ms pulse "envelope" consisting of 100 pulses, each of 0.3 ms with a 1.7-ms interval. We performed MLT on 54 eyes of 34 patients, with similar results. At 8 weeks and 3 months, MLT lowered IOP as well as or better than the previously shown reductions following ALT. Figure 2 summarizes the results of both studies.

We then examined the effect of the patients' medication regimens on these results. In our study, 27 eyes were on a beta-blocker alone; 14 eyes received fixed combination of dorzolamide hydrochloride/timolol maleate ophthalmic solution (Cosopt, Merck) and a prostaglandin (latanoprost, bimatoprost, or travoprost); and nine eyes received more than two medications, but were not on a prostaglandin. At 8 weeks, the eyes that received more than two medications had an IOP decrease of more than 30%, while the other two groups of patients had an IOP decrease of approximately 20%. However, by 3 months, the eyes that received the prostaglandin experienced a total IOP decrease of almost 25%, while the other two groups had a total IOP decrease of less than 20% (Figure 3).

Figure 3. In this study, we sought to examine the effects of patients' medication regimens on MLT's results.

This is in contrast to our experience with SLT, in which patients taking prostaglandin drugs sometimes didn't fare as well as others — certainly a concern, given the prevalence of prostaglandin therapy. In my opinion, MLT is ideal for these patients.

Too Many Benefits to Ignore

In summary, MLT is an easy procedure to perform clinically. It is equivalent to other laser trabeculoplasty procedures in lowering IOP, but it does so with less AC inflammation, less disruption of the blood-aqueous barrier, less risk of IOP spikes, no pain and no PAS. Additionally, MLT delivers less energy with superior confinement of the photothermal effects as compared with ALT or SLT. OM

References

1. The Glaucoma Laser Trial Research Group. The glaucoma laser trial: 2. Results of argon laser trabeculoplasty vs. topical medicines. Ophthalmology. 1990;97:1403-1413.
2. The Glaucoma Laser Trial Research Group. The glaucoma laser trial and glaucoma laser trial gollow-up Study: 7, results. Am J Ophthalmol. 1995;120:718-731.
3. Cvenkel B. One-year follow-up of selective laser trabeculoplasty in open-angle glaucoma. Ophthalmologica. 2004;218:20-25.
4. Ingvoldstad DD, Krishna R, Willoughby L. Micropulse diode laser trabeculoplasty vs. argon laser trabeculoplasty in the treatment of open-angle glaucoma. Paper presented at: the Annual Meeting of the Association for Research in Vision and Ophthalmology; May 2005; Fort Lauderdale, Fla.