Subthreshold diode micropulse laser photocoagulation (SDM) as invisible retinal phototherapy for diabetic macular edema: a review

Abstract

Purpose: To present the state-of-the-art of subthreshold diode laser micropulse photocoagulation (SDM) as invisible retinal phototherapy for diabetic macular edema (DME).

Method: To review the role and evolution of retinal laser treatment for DME.

Results: Thermal laser retinal photocoagulation has been the cornerstone of treatment for diabetic macular edema for over four decades. Throughout, laser induced retinal damage produced by conventional photocoagulation has been universally accepted as necessary to produce a therapeutic benefit, despite the inherent risks, adverse effects and limitations of thermally destructive treatment. Recently, SDM, performed as invisible retinal phototherapy for DME, has been found to be effective in the absence of any retinal damage or adverse effect, fundamentally altering our understanding of laser treatment for retinal disease.

Summary: The discovery of clinically effective and harmless SDM treatment for DME offers exciting new information that will improve our understanding of laser treatment for retinal disease, expand treatment indications, and improve patient outcomes.

Fig. (1)

(A) Monochrome fundus photograph and (B) intravenous fundus fluorescein angiogram (FFA) of eye following conventional suprathreshold ETRDS-style macular photocoagulation for DME. Note full-thickness chorioretinal scars easily visible clinically and angiographically (arrows). (C) Monochrome fundus photograph and (D) FFA following “classical” continuous wave laser “subthreshold” macular photocoagulation. Note less obvious clinically visible retinal scars easily seen by FFA (arrows). (E) Color fundus photograph and (F) FFA following “clinical” “subthreshold” macular photocoagulation. Note absence of notable laser-induced retinal scarring on fundus photography that is easily seen by FFA (arrows). (G) Monochrome fundus photograph, (H) FFA pre- and (I) postoperatively and Spectral-domain OCT before (J) and 4 months after (K) “true” subthreshold macular laser treatment for DME (arrow) with SDM. Treatment consisted of 483 confluent SDM applications using a 131um retinal spot, 5% duty cycle, 0.3 second pulse duration, 0.9 Watt power placed throughout the area of macular thickening, including the nasal fovea, demonstrated by SD-OCT preoperatively. Note complete absence of laser-induced retinal damage with resolution of DME. Preoperative VA 20/30. Postoperative 20/25.

Fig. (2)

A-F. Graphic representation of the “Effective Surface Area” of various modes of retinal laser treatment for retinal vascular disease. Vermillion = Retina unaffected by laser treatment. Brown = Area of retina destroyed by laser and inactive with respect to ability to produce extracellular cytokines. Yellow = Area of retina affected by the laser but not destroyed, able to contribute to the therapeutic effects of laser treatment via laser-induced alteration / normalization of cytokine expression. PASCAL = pattern scanning laser. MP = diode micropulse laser. SDM = “High density / low-intensity” subthreshold / subvisible diode micropulsed laser.