MicroPulse Laser Trabeculoplasty
Myranda R. Partin
Steven R. Sarkisian Jr.
The clinical management of glaucoma continuously evolves with more efficacious topical medications, less invasive surgeries, and safer laser procedures. Topical antiglaucoma medications are commonly used as first-line treatments before surgery. In the past, argon laser trabeculoplasty (ALT) and selective laser trabeculoplasty (SLT) have demonstrated safe alternatives to eye drops. Recently, MicroPulse laser trabeculoplasty (MLT) offers another safe, effective alternative to pharmacotherapy in treating glaucoma patients. The procedure is performed with the Iridex IQ laser which is offered in various wavelengths of 532, 577, and 810 nm (Iridex Corporation, Mountain View, CA).1 The platform has many other applications for ophthalmology, including transscleral cyclophotocoagulation, panretinal photocoagulation, iridotomy, and laser suture lysis.2,3 The multifaceted Iridex IQ 577 nm is reserved for treatment zones within the macula. There is very little absorption within the macular xanthophyll and higher transmission through dense ocular media, while still targeting melanin in retinal pigment epithelium (RPE).4 Singh has demonstrated efficacy using the IQ 577 nm continuous wave mode in conjunction with anti-VEGF injections for diabetic macular edema, panretinal photocoagulation, and retinopexy procedures.5 Moreover, he has shown that pretreating patients with darker colored irides prior to Nd:YAG laser peripheral iridotomy may help decrease patient discomfort. The IQ 577 nm can be switched to MicroPulse mode for laser trabeculoplasty; however, the IQ 532 nm is the standard mode for laser trabeculoplasty because it is the equivalent to the SLT.6 The 810 nm is reserved for laser cyclophotocoagulation using the MicroPulse P3 glaucoma device powered by Cyclo G6 Glaucoma Laser System.5
MicroPulse Technology is an advanced laser technology that breaks up a continuous-wave laser beam into very small, repetitive micropulses, which allow energy to be delivered with intermittent periods. The cooling periods in between the micropulses reduce thermal buildup and tissue damage while inducing beneficial biological effects. MLT is at work 15% of the time while the laser is at rest 85% of the time, effectively eliminating burn complications.7,8 Each application delivers a 300-ms envelope of 150 2-ms micropulses. Each micropulse consists of 0.3 ms of laser on time (pulse width) and 1.7 ms of laser off time (pulse interval).2 The permeability of the trabecular meshwork (TM) is increased after the laser due to the release of inflammatory cytokines, similar to how the SLT works but without the thermal damage.9
 FIGURE 13-1 Electron microscopy showing how MLT thermally effects but does not destroy pigmented trabecular meshwork cells.1 (From Gawęcki M. Micropulse laser treatment of retinal diseases. J Clin Med. 2019;8(2):242. Published 2019 Feb 13. doi: 10.3390/jcm8020242) ALT, argon laser trabeculoplasty; continuous-wave; MLT, MicroPulse laser trabeculoplasty. |
Where MLT differs from SLT is that MLT thermally affects, not destroys, pigmented trabecular meshwork cells without thermal or collateral damage. Cycling the application on and off minimizes the rise in temperature; therefore, the tissue has time to cool off. Microscopic photos have proven that MLT generates enough thermal energy to injure but not alter the trabecular meshwork cells compared to the SLT and ALT lasers.7 As with any trabeculoplasty, there appears to be an immediate release of cytokines followed by monocytic recruitment that facilitates greater permeability. The principle of laser trabeculoplasty may be explained by several mechanisms, including the mechanical pulling open of the uveoscleral trabecular meshwork and Schlemm’s canal, cellular mechanisms that stimulate cell division, and biochemical mechanisms that alter cytokines and stimulate the macrophage-like capacity of trabecular-lining cells (Fig. 13.1).7
INDICATIONS
Key Indications
Primary open-angle glaucoma (POAG)
Ocular hypertension
Normal-tension glaucoma (NTG)
Pigment dispersion syndrome (PDS) or pigmentary glaucoma
Pseudoexfoliation syndrome (PXE) or pseudoexfoliative glaucoma
Primary angle closure (PAC) or primary angle closure glaucoma (PACG) after successful angle-closure treatment.
Laser treatments for glaucoma have been gaining popularity as first-line treatments because hypotensive drops cause disruption to the ocular surface, are costly to patients, and often are associated with poor compliance. Hyperemia and lash growth due to topical medications are generally considered minor cosmetic problems. However, hyperemia can be a burden for patients who are still working or have an active social life. Surgery may achieve lower intraocular pressure (IOP), but it is associated with greater risks and complications not seen with laser therapies. The concept behind pulsed laser delivery is to minimize thermal energy and physiological damage to ocular tissue. MLT offers advantages over other forms of trabeculoplasty and can be a great tool for any glaucoma practice.
The similarities between SLT and MLT are beneficial due to the significant research already done with the SLT laser. Ocular hypertension and POAG are well known to have the best response to laser therapy compared to other types of glaucoma. Several studies have shown MLT to reduce IOP of POAG patients by 12.2% to 21.3%.10,11,12 In the past, success rates with SLT have been correlated with higher starting levels in IOP; however, NTG has also seen beneficial outcomes with laser therapies.13,14 It would be reasonable to believe that MLT would also have similar outcomes although further research is still needed.
Laser trabeculoplasty, such as ALT and SLT, has been a successful option in the past for pigmentary and pseudoexfoliation glaucoma patients who need adjunctive therapy or wish to lessen their need for drops. The biochemical theory in which MLT mimics SLT would conjecture that it also would be a fair option; however, to this date, the authors know of no studies having been performed. Similar to SLT, the MLT laser settings likely should be titrated down for pigmentary glaucoma or those with heavier pigmented TM. Studies have shown a higher occurrence rate for postlaser IOP spikes in those with darker pigmented angles.14,15 However, in patients with PXE, the success and adverse event rate compare similarly with POAG, and the presence of pseudoexfoliation does not increase the rate of a post-laser IOP spike.14,16
CONTRAINDICATIONS
Key Contraindications
Any type of angle-closure glaucoma or angle closure where adequate TM is not visible
A relatively high preoperative IOP (>30 mm Hg) in conjunction with advanced optic nerve damage, in which case a possible postoperative transient IOP elevation is thought to pose a significant risk for the patient’s vision
Glaucoma associated with uveitis, trauma, ischemia/neovascularization, juvenile open-angle glaucoma, or angle dysgenesis
A history of previous MLT failure in the same or fellow eye
Congenital glaucoma
Significant corneal endothelial disease
Glaucoma associated with inflammatory conditions or neovascularization is contraindicated due to the potential of the laser worsening the condition. Numerous studies have been done on the ALT and SLT laser for such conditions but not yet the MLT. However, one can assume the outcome would be similar. One study done on uveitic patients with glaucoma who underwent SLT showed only 8% of quiet eyes (no inflammation for at least 90 days) had flare compared with 13% of controlled eyes (inflammation within 90 days but quiet on the day of the procedure). It was concluded that SLT was still considered a viable treatment modality.17
Those without any corneal endothelial disease have shown in previous studies an incidence of corneal edema post-SLT as approximately 0.8%.18 At one-week postoperatively, thinning was documented and thought to be contributed by the laser heat dissipation causing stromal contractions. Normal corneal thickness readings were again found by one-month post-laser and postulated to be from keratocyte migration and stromal remodeling.19 In compromised corneas, the effects of SLT may be transient but they have a higher long-term risk, especially after repeated SLT.20 MLT likely follows a similar contraindication profile as SLT.
INFORMED CONSENT CONSIDERATIONS
Key Informed Consent Adverse Events Listed as Follows:
IOP spike
Inflammation
Pain/discomfort during and/or after the procedure
Macular edema
Corneal haze/edema
Transient blurring of vision
Bleeding/hyphema
Lack of efficacy
Loss of efficacy over time
Informed consent is important not only for liability purposes but also to help the patient understand more about the procedure. A brief summary in laymen terms should include how the laser works on the pigmented region of the “drain” in their eye and that their body uses immune cells to help clean out the drain. In return, this lowers the eye pressure which is currently the only modality of treatment. Although laser trabeculoplasty is considered a minor procedure, it still needs to be taken seriously. While the incidence of an adverse event is low, the patient needs to be well educated by the attending physician.
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