Neurovisual abnormalities preceding the retinopathy in patients with long-term type 1 diabetes mellitus

Background: Changes in the retina caused by diabetes may lead to visual imp airment in dim light, even with good visual acuity and visual fields. To ev aluate the visual abnormalities preceding the retinopathy in patients with type 1 diabetes mellitus (DM), we applied electrophysiological methods. Met hods: The visual evoked responses were recorded with sinusoidally modulated vertical gratings at 10 spatial frequencies presented sequentially on a hi gh-resolution monitor in patients with type 1 DM and in normal volunteers. Similarly, the contrast visual evoked responses of 10 contrast levels were recorded at five spatial frequencies. Both amplitudes at the second harmoni c were calculated by discrete Fourier transform. The visual acuity and cont rast thresholds were determined objectively. Results: There was dissociatio n between the Snellen and the estimated visual evoked response acuity measu rements in patients with diabetes (r(2)=0.077, P=0.44). The saturation phen omena were observed at lower levels of contrast stimuli than in normal indi viduals at. 1.0, 2.0, 4.0 and 8.0 cycles per degree (P=0.0001). The contras t sensitivity function was deeply abnormal in all tested patients despite t he metabolic control. The values of the area under the curve of the visual evoked response amplitude-contrast level function at five spatial frequenci es were smaller in patients with DM than in the control group (P <0.05) at all spatial frequencies tested. Conclusions: Patients with type 1 DM withou t retinopathy showed significant lower amplitude of the visual evoked respo nses at all spatial frequencies tested, with the saturation phenomena obser ved at lower level of contrast stimuli. In addition, there was a dissociati on between the sweep visual evoked responses and the Snellen acuity measure ments. A significant and nonselective neuronal visual loss involving the vi sual pathway precedes the ophthalmoscopically detectable retinopathy in pat ients with type 1 DM.