Micropulse laser for managing DME

Despite the evidence demonstrating its efficacy and safety, subthreshold micropulse laser has faced several obstacles to adoption.

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Gerardo Garcia MD
Even in the current era of anti-VEGF therapy, laser continues to have a role in the management of diabetic macular edema (DME). When choosing laser, however, retina specialists should consider the subthreshold micropulse approach rather than performing conventional laser photocoagulation, said Gerardo Garcia MD, speaking at WOC 2016 in Guadalajara, Mexico. His recommendation is based on the relative efficacy and safety of the two modalities. “Results from several randomised trials show that subthreshold micropulse laser is at least as effective as conventional laser for treating DME, but subthreshold micropulse laser is less invasive,” said Dr. Garcia, professor of ophthalmology, Tecnológico de Monterrey, Mexico City, Mexico. “There is still a need for a randomised clinical trial that compares anti-VEGF treatment alone, subthreshold micropulse laser, and the combination of the two. In the meantime, however, I think it is reasonable to use subthreshold micropulse laser for treating DME that is resistant to anti-VEGF therapy or as initial treatment for non-center involving DME.” Subthreshold micropulse capability is available for commercially available lasers emitting at three different wavelengths—532 nm (green), 577 nm (yellow), and 810 nm (infrared). Unlike conventional laser treatment for DME that is done using continuous mode delivery and causes photocoagulation, subthreshold micropulse laser treatment involves emission of a separated stream of microsecond pulses and works via a photostimulatory mechanism of action. With its shortened exposure times interspersed with off periods, subthreshold micropulse laser treatment only slightly elevates temperature of the RPE. Instead of causing thermal damage, the treatment induces heat shock proteins in the RPE cells, leading to the production of cytokines that decrease edema. “The efficacy of burning the retina with conventional laser treatment for DME was established 25 years ago in the Early Treatment Diabetic Retinopathy Study (ETDRS), but causing burns too close to the fovea results in reduction of visual acuity. Therefore, it may be better not to burn the retina,” Dr. Garcia said. Results of several randomised trials comparing conventional laser photocoagulation and subthreshold micropulse laser with an 810 nm device for treatment of DME show similar morphologic and functional outcomes for the two treatment groups in analyses of changes in macular thickness and visual acuity. One study, however, also included microperimetry measurements, and its results support the use of the less invasive subthreshold micropulse laser treatment. In the latter study, 62 eyes of 50 patients with center-involving, clinically significant DME were randomised to subthreshold micropulse diode laser or modified ETDRS photocoagulation (Retina. 2010;30(6):908-916). At 12 months, BCVA was stable and similar in the two treatment groups, and mean central retinal thickness was significantly decreased in both groups and to a similar extent. However, mean central 4 degrees and 12 degrees retinal sensitivity had increased significantly in eyes treated with the micropulse laser, but it had decreased in eyes treated with conventional laser. And, the differences between the two groups were statistically significant.
Retina specialists who perform subthreshold micropulse laser treatment may feel as if they are performing ‘retinal homeopathy'
IMPEDIMENTS TO GREATER USE Dr. Gerardo suggested that despite the evidence demonstrating its efficacy and safety, subthreshold micropulse laser has faced several obstacles to adoption. First, laser in general is considered less effective than anti-VEGF therapy, and subthreshold micropulse laser treatment is a latecomer to the treatment armamentarium compared with anti-VEGF injections. In addition, the fact that the subthreshold micropulse modality does not produce visible lesions may create skepticism about whether the treatment is really doing anything. “Retina specialists who perform subthreshold micropulse laser treatment may feel as if they are performing ‘retinal homeopathy’,” Dr. Garcia said. Although we do not have a lot of tools to prove it, findings from imaging with scanning laser opthalmoscopy and spectral domain optical coherence tomography provide evidence that subthreshold micropulse laser does cause a biologic effect. (AM J Opthalmolol 2010:150:856-862)  The absence of visible lesions with subthreshold micropulse laser treatment also made it difficult to track treated areas using lasers delivering single spots. This issue, however, has been addressed by the introduction of pattern laser delivery systems that provide better spot coverage without overlap while also increasing treatment efficiency.