Subthreshold diode micropulse laser for DME

Jun 01, 2012

The era of invisible retinal photography

In his seminal book on modern education, Closing of the American Mind, Allan Bloom observed that oftentimes the things that disturb us most about prior civilizations are the very things that they took most for granted: Child sacrifice. Gladiatorial games. Bell-bottom pants. Modified ETDRS macular photocoagulation for diabetic macular oedema (DME)? Any retinal-destructive laser procedure for retinal vascular disease?1

What does this have to do with laser treatment for DME? Nothing. But it has much to do with the humans who think about it and do it. Information theory states that the amount of new information we can learn from a given event is directly proportional to the improbability of that event occurring.4 How much do we learn from yet another study confirming that ranibizumab is effective against choroidal neovascularization from yet another cause? So when something really new and unexpected comes along offering a wealth of new insights and the prospect of significant advancement, it must, by definition, strike us as very unlikely - if not impossible, and even absurd. Realization that the ridiculous and impossible may also be the truth makes us, at best, uncomfortable. And if there is one thing that humans value and defend, it is their comfort. Wars are fought over less.

The discovery that laserinduced thermal retinal burns could effectively treat retinal disease represents one of the great advances in the history of ophthalmology. The necessity of laser-induced retinal damage in the treatment of DME has been universally accepted for almost 50 years, and remains the prevailing notion today.5 So what are we to make of the idea that those very same retinal burns - or any laser-induced retinal damage, for that matter - are actually unnecessary and simply represent an adverse side-effect of treatment? See 'uncomfortable' above.

In April 2000 a pilot study was started involving laser treatment for DME with, for the first time, the explicit intent of avoiding laser-induced retinal damage of any kind by use of a low-duty cycled micropulsed 810 nm diode laser (IRIDEX Corp., Mountain View, California, USA). At same time, the modest clinical effects anticipated from this invisible ('low-intensity') laser treatment were sought to be magnified by a fundamental change in treatment technique, permitted by the unique safety of the micropulsed laser: that of confluent ('high-density') treatment of the target tissue (DME) with contiguous small laser spot applications.

The pilot study was born of the base desire to avoid replacing a spent argon laser. It succeeded through a little bit of good intuition and masses of good luck, not to mention sufficient ignorance as to the impossibility of the proposition to give it a try.6 Reported in 2005, this study stated that 'low-intensity/high-density' subthreshold diode micropulse photocoagulation (SDM) appeared to be as effective as conventional suprathreshold photocoagulation for DME but without any clinical or angiographic evidence of laser-induced retinal damage at any point during or after treatment. Without laserinduced retinal damage there were no complications or adverse treatment effects. SDM appeared to be, in fact, absolutely harmless.7,8 These findings have been corroborated by subsequent studies including randomized clinical trials.10–14 Other reports suggest SDM may also be effective for the treatment of macular oedema due to branch retinal vein occlusion, central serous chorioretinopathy and proliferative diabetic retinopathy.