—As Dr Grand's thoughtful letter points out, the following specifics of the Early Treatment Diabetic Retinopathy Study (ETDRS)1 are critical to interpreting its results and applying these results to clinical practice:
The ETDRS demonstrated the beneficial effects of focal photocoagulation for macular edema in eyes which had either mild or moderate nonproliferative diabetic retinopathy. Similar treatment effects were also observed for a small number of eyes with more advanced retinopathy.
Beneficial effects of focal photocoagulation were most prominent in eyes with "clinically significant macular edema," which is defined as retinal thickening that involves or threatens the center of the macula.
Stereo slit-lamp biomicroscopy with a contact lens is often necessary to detect clinically significant macular edema.
The beneficial treatment effect was demonstrated for photocoagulation as performed in ETDRS. For most patients this treatment was direct focal photocoagulation with only a few eyes
Photocoagulation for Diabetic Macular Edema-Reply. Arch Ophthalmol. 1986;104(8):1116. doi:10.1001/archopht.1986.01050200021014