David J Browning, MD, PhD
Focal laser treatment is still the mainstay of treatment for diabetic macular edema (DME), but the technique has some shortcomings, including poorly defined treatment endpoints, retreatment intervals, and its use relative to other treatments.
The endpoint for laser treatment has never been defined. The Early Treatment Diabetic Retinopathy Study (ETDRS) advocated white burns. An example of this older style of laser treatment can be seen at http://www.theretinaexchange.com/viewphoto.php?phototitle=tooheavyfocal.jpg. Current technique uses a dusting of 50-micron, barely visible burns, with mild blanching of the subjacent retinal pigment epithelium. Thus, “maximal laser treatment” may mean different things to different doctors.
The conventional retreatment interval, per the ETDRS, is 4 months. It must be stressed that this interval was based on an agreed protocol, not upon the likelihood of having obtained the maximal result from the laser. However, our group has data showing that the thinning effect of focal laser may not plateau until 10 months after the procedure. Since each focal laser treatment session decreases the sensitivity of the central retina, and since burn scars can expand in size over time, it is desirable to avoid retreatment. The Diabetic Retinopathy Clinical Research network (DRCR.net) is currently designing a study to determine the optimal retreatment interval, but until those results are available, it may be prudent to postpone retreatment as long as progressive thinning of at least 11% relative to the last observation is observed. (11% is the 95% confidence threshold for true macular thinning measured by optical coherence tomography [OCT].)
The traditional sequence of treatments for persistent DME has been multiple sessions of focal laser, intravitreal triamcinolone injection, and vitrectomy. This sequence is not based on current evidence of superiority. It may be worthwhile to change this sequence based on the degree and durability of macular thinning to each treatment. A single session of focal laser yields a durable but modest thinning of approximately 30 microns, although the response is variable. A single injection of intravitreal triamcinolone yields dramatic macular thinning (on average, over 100 microns) within days, but the response wanes with time. The response duration is dose-dependent; a 4-mg dose lasts approximately 3-4 months. Vitrectomy yields dramatic macular thinning with a long duration of response, which may still be building at 6 months postoperatively.
Based on the foregoing data, and pending the results of the DRCR.net research, here is the treatment algorithm I use in my practice:
- In eyes with 20/20 visual acuity and milder edema, focal laser remains an excellent choice with the best proven response and side effect profile. Focal laser can be repeated, but at longer retreatment intervals.
- In eyes with more severe edema and 20/25-20/50 visual acuity, combination therapy with an intravitreal injection of triamcinolone plus focal laser, or in phakic patients, a nonsteroidal anti-VEGF drug plus focal laser.
In eyes with severe thickening and 20/60 visual acuity or worse, combination therapy with vitrectomy, intravitreal injection, and laser could be administered in a single procedure.
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