Abnormal change in foveal cone ERG with increasing duration of light exposure in diabetic retinopathy

Purpose. To study the change in foveal cone electroretinogram (ERG) amplitudes with increasing duration of light exposure in patients with nonproliferative diabetic retinopathy (NPDR), with and without clinically significant macular edema (CSME). Methods. Several consecutive 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). The ERGs were evaluated to determine whether increasing duration of light exposure results in an increase in amplitude, as had been previously found in normal subjects. Results. The amplitudes measured at the 3rd averaged consecutive recording were not significantly different from those measured at the 1st recording in eyes with NPDR, and the mean change was -7%. The absence of a significant rise in amplitude was found both in eyes with and without CSME. The -7% mean change is significantly different from that previously found in normal subjects using the same protocol, instruments and tester (p=0.004). Conclusions. These findings may reflect a foveal cone abnormality in eyes with NPDR, with or without CSME, that may be consistent with the subnormal visual acuity found in some eyes with NPDR without CSME, and in some eyes with CSME even following the recommended laser photocoagulation treatment.