The mesopic negative response (MeNR): a novel approach to assess retinal ganglion cell function within the rod...

The photopic negative response (PhNR) and pattern ERG, both established electrophysiological measures of retinal ganglion cell (RGC) function, are recorded under photopic conditions. The purpose of this study was to describe the mesopic negative response (MeNR), a novel marker of RGC function within the rod pathway.

Ten visually-normal controls (mean age ± SD 54.6 ± 5.6 years) and 12 patients with severe primary open-angle glaucoma (mean age ± SD 58.4 ± 4.9 years) participated. Light-adapted, full-field ERGs were elicited by a rod-isolating pulse generated on the principle of silent substitution (0.46 scot. cd/m²; 55% contrast; 40 ms) presented against a steady background (0.30 scot. cd/m²). In addition, (1) the PhNR was recorded; (2) subjects were dark-adapted for 20 min and the ISCEV DA 0.01 was recorded.

The normal rod-isolated pulse response was characterized by a positive potential at 85 ms followed by a slow negative potential (MeNR) at 175 ms following the pulse. The mean (± SEM) amplitude of the positive potential was similar for the control (13.4 ± 1.2 µV) and glaucoma (11.6 ± 1.35 µV) groups (p = 0.33), and was correlated with the DA 0.01 amplitude (r = 0.71, p < 0.001). The amplitude of the MeNR was significantly (p < 0.001) attenuated for the glaucoma group (4.5 ± 0.7 µV) compared to the controls (9.7 ± 1.25 µV), and was correlated with the PhNR amplitude (r = 0.86, p < 0.001).

Rod-isolated ERGs can be obtained without dark-adaptation using silent-substitution. The positive potential and MeNR of the rod-isolated response appear to be generated by rod bipolar cells and RGCs, respectively. In severe glaucoma, the positive (bipolar cell) potential was not significantly affected, whereas the MeNR was significantly reduced. MeNR analysis may be useful for studying RGC function within the rod pathway.