MLT offers safe, well-tolerated approach to lower IOP, reduce need for medication

Take-home

Nathan M. Radcliffe, MD, discusses use of laser technology for treating patients with early-stage glaucoma as an approach to decrease IOP or to reduce the need for IOP-lowering medications.

Dr. Radcliffe

By Nathan M. Radcliffe, MD, Special to Ophthalmology Times

The concept of laser trabeculoplasty has evolved in recent years.

Argon laser trabeculoplasty (ALT), described in 1974, was the first use of laser trabeculoplasty therapy.¹ It was initially developed to have a mechanical effect on the trabecular meshwork (TM) by causing thermal damage to shrink the trabecular ring. The idea was that thermal coagulation and contraction would stretch out the TM in areas that were not affected by the laser, which would improve outflow.

More in Special Report: MLT offers safe, well-tolerated approach to lower IOP, reduce need for medication

Scientists learned that a laser also may have a biological effect on the TM as well as a mechanical effect. It is conceptually appealing to avoid coagulation or damage of the TM. A biological effect applies energy to the TM to stimulate the body’s immune system by introducing macrophages and inflammatory mediators to clean out, and potentially to repopulate sick TM cells with healthy ones.

As a result, selective laser trabeculoplasty (SLT) was developed. SLT employs a Q-switched Nd:YAG laser. Its large spot size makes it easy to target the TM, and it generally causes minimal intraocular inflammation, but it does cause some. Clinical data suggest that SLT and ALT have similar efficacy.²

Furthermore, studies have shown that SLT laser decreases the IOP in the fellow eye as well as the treated eye, which may indicate that the laser induces a bilateral biological effect to reduce IOP.³

NEXT: Understanding the technology + Photo

Understanding the technology

Micropulse laser technology (MLT) was first developed by Iridex Corp. It delivers energy in short microbursts that allow cooling between applications.

Imagine holding a hand over a candle for 5 seconds. The heat from the candle will build and eventually burn the hand (thermal spread). However, one could wave a hand over a candle for 30 seconds without getting burned. Although cumulatively the hand has spent 5 seconds over the flame, no burn occurred because thermal spread was inhibited.

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MLT operates with a similar principle. Instead of applying 100 ms of continuous laser, it breaks up the laser application into small packets of repetitive short pulses followed by a rest period, allowing tissue to cool between each pulse and reduce thermal buildup. This process is described as a duty cycle, which is the amount of time the laser is firing and the amount of time it is resting within each application.

MLT was initially applied to treating diabetic macular edema (DME). Studies have shown that subthreshold MicroPulse laser therapy is clinically effective in treating DME^4-6 and other retinal pathologies, such as retinal vein occlusions^7,8 and central serous retinopathy,^9,10 with the benefits of no tissue damage detectable at any time point postoperatively. This same technology is now being applied to the treatment of glaucoma.

While MLT has not been as well studied as SLT or ALT, data are on the way. Initially, the procedure was available only with the 810-nm laser. However, MLT now can be performed with the IQ 532-nm laser (Iridex), which is my preference. In one phase II clinical study, MLT with the 810-nm laser was effective in reducing IOP in 75% of patients with open-angle glaucoma without significant complications, with 60% of patients achieving greater than 20% IOP lowering.¹¹

Performing MLT laser is very similar to other forms of trabeculoplasty. A standard trabeculoplasty lens may be used. However, since MLT does not blanch the TM, I have enjoyed using the Ocular Latina 5 Bar Indexing SLT lens. This lens allows the surgeon to keep track of the number and position of laser applications in the absence of a visible effect on the TM and will help the surgeon apply 100 evenly spaced applications.

I use a 300-µm spot size and apply the laser for 300 ms duration with 600 mW to 800 mW of power. The duty cycle for MLT is typically 15%, indicating that during the 300-ms application, the laser will be applied for 15% of that time, delivered in evenly spaced micropulses. I treat 360° with 100 laser applications.

NEXT: Positive results

Positive results

With MLT, I have seen positive results in patients. While the anecdotes below do not substitute for prospective clinical trials, they are offered as examples of successful applications of MLT therapy.

A 65-year-old male patient with ocular hypertension presented with a best-corrected visual acuity of 20/60 and corneal epithelial staining. He had an IOP of 20 mm Hg and corneal thickness of 550 µm with early glaucomatous optic nerve and visual field changes. He was taking a prostaglandin analogue and two adjunctive agents.

After a trial of many different medications, including preservative-free formulations, he remained intolerant of drops. His pressure was 26 mm Hg off of his drops and 20 mm Hg to 21 mm Hg on no drops after MLT in both eyes. His symptoms are now gone.

Another patient, who had difficulty remembering to take his drops, had an IOP of 18 mm Hg on a prostaglandin analogue. He wanted to discontinue applying drops. I performed MLT on his left eye and asked him to return in 2 weeks for follow-up. He missed that appointment and returned in 6 weeks on no drops in either eye. His IOPs were 24 mm Hg in the right eye (untreated) and 18 mm Hg in the left eye that underwent MLT. We proceeded with MLT in the right eye.

In my experience, after MLT, eyes are perfectly “quiet” and I have not encountered any postoperative pressure spikes. This makes it an appealing option for patients in whom such a spike might be problematic. When firing an ALT laser, the energy reaction on the TM is evident. There is a visible thermal effect occurring on the TM, and there is damage and scarring afterward. Depending on the energy used with SLT, cavitation bubbles may be visualized. With MLT, there is no evidence of blanching, scar formation, or cavitation bubble creation.

As a result, I do not need to prescribe any postoperative drops. With SLT, I generally prescribe prednisolone or a nonsteroidal anti-inflammatory drug about half of the time. Because I have not encountered pressure spikes when performing MLT, I typically feel comfortable stopping the patient’s glaucoma medications right away (depending on the clinical scenario).

Patients are generally receptive to receiving MLT, but the psychology of the physician-patient conversation surrounding laser therapy is fascinating. Most ophthalmologists would choose laser trabeculoplasty as a primary therapy for their own eyes, but do not always offer it first to patients. This is partially due to habit, and partially the result of patients’ skepticism toward laser therapy.

However, the success of MLT laser demonstrates that both physicians and patients take comfort in knowing that there will be no observable damage from the therapy.

NEXT: MLT in the exam lane

MLT in the exam lane

One unique aspect of the laser used for MLT is that, unlike SLT, the laser is small and can be placed on a standard slit lamp. In my office, the laser is located in one of my exam rooms, and is employed for laser trabeculoplasty directly after receiving consent from the patient. Where I practice in New York City, office space is very limited. Due to the compact size of the laser, it can be kept in a standard examination lane rather than requiring a separate laser room.

Additionally, the laser can be used for all other standard “argon” applications, including retinal photocoagulation (at a slit lamp or with an indirect ophthalmoscope) and argon laser iridotomy. Patients take some comfort in seeing what the laser device actually looks like while they are making their decision to undergo the procedure. For some patients who have physical limitations, it is nice not to have to move them from one room to the next in order to perform preoperative evaluation, laser treatment, and postoperative evaluation.

NEXT: First, do no harm

First, do no harm

The concept of MLT is to transition the efficacy of ALT without any thermal damage. It should be noted that ALT and SLT are well-proven, and they have their place in any glaucoma practice. SLT laser is still my preferred therapy for more complicated or advanced patients who are on several drops and are potentially in need of incisional glaucoma surgery.

However, I use MLT on early-stage glaucoma patients who are interested in reducing their drops or avoiding the need of a second eye drop. I have had success with this approach, which often allows a safe and very well-tolerated approach to decrease IOP or to reduce the need for IOP-lowering medications.

References

1. Worthen DM, Wickham MG. Argon laser trabeculotomy. Trans Am Acad Ophthalmol Otolaryngol. 1974;78:OP371-OP375.

2. Juzych MS, Chopra V, Banitt MR, Hughes BA, Kim C, Goulas MT, Shin DH. Comparison of long-term outcomes of selective laser trabeculoplasty versus argon laser trabeculoplasty in open-angle glaucoma. Ophthalmology. 2004;111:1853-1859.

3. Rhodes KM, Weinstein R, Saltzmann RM, Aggarwal N, Kooner KS, Petroll WM, Whitson JT. Intraocular pressure reduction in the untreated fellow eye after selective laser trabeculoplasty. Curr Med Res Opin. 2009;25:787-796.

4. Vujosevic S, Bottega E, Casciano M, Pilotto E, Convento E, Midena E. Microperimetry and fundus autofluorescence in diabetic macular edema: Subthreshold micropulse diode laser versus modified early treatment diabetic retinopathy study laser photocoagulation. Retina. 2010;30:908-916.

5. Lavinsky D, Cardillo JA, Melo LA Jr., Dare A, Farah ME, Belfort R Jr. Randomized clinical trial evaluating meters versus normal or high-density micropulse photocoagulation for diabetic macular edema. Invest Ophthalmol Vis Sci. 2011;52:4314-4323.

6. Luttrull JK, Sramek C, Palanker D, Spink CJ, Musch DC. Long-term safety, high-resolution imaging, and tissue temperature modeling of subvisible diode micropulse photocoagulation for retinovascular macular edema. Retina. 2012;32:375-386.

7. Parodi MB, Spasse S, Iacono P, Di Stefano G, Canziani T, Ravalico G. Subthreshold grid laser treatment of macular edema secondary to branch retinal vein occlusion with micropulse infrared (810 nanometer) diode laser. Ophthalmology. 2006;113:2237-2242.

8. Parodi MB, Iacono P, Ravalico G. Intravitreal triamcinolone acetonide combined with subthreshold grid laser treatment for macular edema in branch retinal vein occlusion: A pilot study. Br J Ophthalmol. 2008;92:1046-1050.

9. Koss MJ, Beger I, Koch FH. Subthreshold diode laser micropulse photocoagulation versus intravitreal injections of bevacizumab in the treatment of central serous chorioretinopathy. Eye (Lond). 2012;26:307-314.

10. Lanzetta P, Furlan F, Morgante L, Veritti D, Bandello F. Nonvisible subthreshold micropulse diode laser (810 nm) treatment of central serous chorioretinopathy. A Pilot Study. Eur J Ophthalmol. 2008;18:934-940.

11. Fea AM, Bosone A, Rolle T, Brogliatti B, Grignolo FM. Micropulse diode laser trabeculoplasty (MDLT): A phase II clinical study with 12 months follow-up. Clin Ophthalmol. 2008;2:247-252.

Nathan M. Radcliffe, MD, is assistant professor of ophthalmology, Weill Cornell Medical College, New York-Presbyterian Hospital, New York. Dr. Radcliffe may be reached at 646/962-2020 or drradcliffe@gmail.com. Dr. Radcliffe has served as a consultant for Iridex Corp.