Foveal cone ERG in diabetic macular edema

Purpose. To study the foveal cone electroretinograms (ERGs) in patients with nonproliferative diabetic retinopathy (NPDR), with and without clinically significant macular edema (CSME). Methods. Foveal cone ERGs were elicited with a stimulator-ophthalmoscope from 16 eyes of 8 patients with NPDR, of which 9 eyes had CSME, with a 4° stimulus (retinal illuminance: 4.8 log td) flickering at 42 Hz and centered within a 12° steady surround (retinal illuminance: 5.5 log td). Results. On average, eyes with NPDR had amplitudes that were significantly lower compared with normal subjects (p<0.001). Their visual acuity was directly correlated with their amplitudes (p<0.01), and inversely correlated with their implicit times (p<0.001). However, foveal cone ERG amplitudes in eyes with CSME were significantly lower than, and their implicit times were significantly longer than, those from eyes without CSME (p<0.05 and p<0.001, respectively). Furthermore, while all 9 eyes with CSME had implicit times that were delayed (100%), only 2 of the 7 eyes without CSME (29%) had delayed implicit times. In contrast, visual acuity did not differ significantly between the two groups. Conclusions. Our findings suggest that in patients with NPDR the foveal cone ERG may be more sensitive an indicator of the stage of maculopathy than a change in visual acuity, and that increasing delays, and or decreasing amplitudes, of the foveal cone ERGs in patients with NPDR may be a sign of developing CSME.